U.S. patent application number 09/764963 was filed with the patent office on 2002-09-19 for apparatus and methods for infrared calorimetric measurements.
This patent application is currently assigned to FLIR Systems Boston, Inc.. Invention is credited to Neilson, Andy C., Teich, Jay S..
Application Number | 20020132360 09/764963 |
Document ID | / |
Family ID | 26940472 |
Filed Date | 2002-09-19 |
United States Patent
Application |
20020132360 |
Kind Code |
A1 |
Neilson, Andy C. ; et
al. |
September 19, 2002 |
Apparatus and methods for infrared calorimetric measurements
Abstract
Apparatus and methods for performing calorimetry. The apparatus
include optical devices for detecting thermal processes and
multiwell sample plates for supporting samples for use with such
optical devices. The methods include measurement strategies and
data processing techniques for reducing noise in measurements of
thermal processes. The apparatus and methods may be particularly
suitable for extracting thermal data from small differential
measurements made using an infrared camera and for monitoring
chemical and physiological processes.
Inventors: |
Neilson, Andy C.; (Groton,
MA) ; Teich, Jay S.; (Weston, MA) |
Correspondence
Address: |
James R. Abney
Kolisch, Hartwell, Dickinson, McCormack & Heuser
200 Pacific Building
520 S.W. Yamhill Steet
Portland
OR
97204
US
|
Assignee: |
FLIR Systems Boston, Inc.
|
Family ID: |
26940472 |
Appl. No.: |
09/764963 |
Filed: |
January 17, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60249931 |
Nov 17, 2000 |
|
|
|
Current U.S.
Class: |
436/147 ;
422/400 |
Current CPC
Class: |
G01N 25/482 20130101;
G01N 25/72 20130101; G01N 25/4846 20130101 |
Class at
Publication: |
436/147 ; 422/55;
422/102 |
International
Class: |
B01L 003/00; G01N
025/20 |
Claims
We claim:
1. A plate for holding a plurality of samples, comprising: a frame;
and a plurality of sample wells disposed in the frame for holding a
corresponding plurality of samples, each sample well having at
least one infrared-transmissive surface configured to transmit
thermal infrared radiation emitted by a sample positioned within
the well.
2. The plate of claim 1, the frame being substantially rectangular,
where the length of the frame ranges between about 125 mm and about
130 mm, and where the width of the frame ranges between about 80 mm
and about 90 mm.
3. The plate of claim 1, where the number of sample wells in the
plate is selected from the group consisting of 96, 384, 768, 1536,
3456, and 9600.
4. The plate of claim 1, where the density of sample wells in the
plate is at least about 1 well per 81 mm.sup.2.
5. The plate of claim 1, where the volume of each sample well in
the plate is less than about 500 microliters.
6. The plate of claim 1, where the infrared-transmissive surface is
composed at least in part of an infrared-transmissive polymer.
7. The plate of claim 6, where the infrared-transmissive polymer is
polyethylene.
8. The plate of claim 1, where at least a portion of the
infrared-transmissive surface transmits at least about half of
incident thermal infrared radiation having wavelengths between
about 3 micrometers and about 5 micrometers.
9. The plate of claim 8, where at least a portion of the
infrared-transmissive surface transmits at least about 80% of
incident thermal infrared radiation having wavelengths between
about 3 micrometers and about 5 micrometers.
10. The plate of claim 1, where at least a portion of the
infrared-transmissive surface transmits at least about half of
incident thermal infrared radiation having wavelengths between
about 7 micrometers and about 14 micrometers.
11. The plate of claim 10, where at least a portion of the
infrared-transmissive surface transmits at least about 80% of
incident thermal infrared radiation having wavelengths between
about 7 micrometers and about 14 micrometers.
12. The plate of claim 1, each sample well having a bottom and a
side, where the infrared-transmissive surface includes at least a
portion of the bottom.
13. The plate of claim 12, where at least a portion of the bottom
is substantially planar.
14. The plate of claim 12, where the sample wells are frustoconical
in shape.
15. The plate of claim 1, where the frame and the sample wells each
include an optically transmissive surface configured so that the
optical reader can detect electromagnetic radiation transmitted
from a sample through both the optically transmissive surface of
the corresponding sample well and the optically transmissive
surface of the frame.
16. The plate of claim 1, further comprising a cover configured to
cover the sample wells, reducing evaporation from samples contained
within the sample wells.
17. The plate of claim 1, further comprising a thermal reference
region disposed about the sample wells in the frame, where thermal
infrared radiation detected from a sample positioned in at least
one of the sample wells may be calibrated using thermal infrared
radiation detected from an adjacent thermal reference region.
18. The plate of claim 17, the sample wells having a central axis,
where the thermal reference region includes an annular emissive
reference surface positioned about the central axis of each sample
well.
19. A plate for holding a plurality of samples, comprising: a
frame; and a plurality of sample wells disposed in the frame for
holding a corresponding plurality of samples; where the frame and
the sample wells each include an optically transmissive surface
configured so that an optical reader can detect electromagnetic
radiation transmitted from a sample through both the optically
transmissive surface of the corresponding sample well and the
optically transmissive surface of the frame.
20. A method of detecting thermal infrared radiation, comprising:
providing a sample plate having a plurality of sample wells
containing a corresponding plurality of samples, each sample well
having at least one infrared-transmissive surface configured to
transmit thermal infrared radiation emitted by a sample positioned
within the well; providing an optical device configured
preferentially to detect thermal infrared radiation; and detecting
thermal infrared radiation transmitted from a sample through the
infrared-transmissive surface of at least one of the sample wells
in the sample plate using the optical device.
21. The method of claim 20, further comprising correlating the
detected radiation with the progress of a chemical or physiological
reaction occurring within the sample.
22. The method of claim 20, the frame being substantially
rectangular, where the length of the frame ranges between about 125
mm and about 130 mm, and where the width of the frame ranges
between about 80 mm and about 90 mm.
23. The method of claim 20, where the number of sample wells in the
sample plate is selected from the group consisting of 96, 384, 768,
1536, 3456, and 9600.
24. The method of claim 20, where the density of sample wells in
the sample plate is at least about 1 well per 81 mm.sup.2.
25. The method of claim 20, where the volume of each sample well in
the sample plate is less than about 500 microliters.
26. The method of claim 20, the sample plate comprising an insert
portion containing the sample wells and a frame portion for
supporting the insert, further comprising forming the sample plate
by mating the insert portion with the frame portion.
27. The method of claim 20, where the optical device comprises: an
examination site; and a detector configured to receive and
preferentially to detect thermal infrared radiation transmitted
from a sample positioned within a sample well at the examination
site.
28. The method of claim 20, the sample wells having a central axis,
the optical device having an optical axis, further comprising
aligning the central axis and the optical axis prior to the steps
of detecting thermal infrared radiation.
29. The method of claim 20, further comprising shielding the sample
from incident radiation to reduce the proportion of the sample
signal arising from transmission, reflection, and/or
photoluminescence from the sample.
30. The method of claim 20, further comprising filtering the
radiation transmitted from the sample to extract thermal infrared
radiation prior to the step of detecting thermal infrared
radiation.
31. The method of claim 20, where at least about half of the
thermal infrared radiation detected by the optical device has a
wavelength between about 3 micrometers and about 5 micrometers.
32. The method of claim 20, where at least about half of the
thermal infrared radiation detected by the optical device has a
wavelength between about 7 micrometers and about 14
micrometers.
33. The method of claim 20, each sample well having a bottom that
transmits thermal infrared radiation, where the step of detecting
thermal infrared radiation includes the step of detecting the
thermal infrared radiation through the bottom of the sample
well.
34. The method of claim 33, where at least a portion of the bottom
is substantially planar.
35. The method of claim 20, further comprising: detecting thermal
infrared radiation transmitted from a reference region adjacent the
sample; and constructing a sample signal characteristic of the
thermal infrared radiation detected from the sample based on the
thermal infrared radiation detected from the sample and the
adjacent reference region.
36. The method of claim 35, the sample wells having a central axis,
where the thermal reference region includes an annular emissive
reference surface positioned about the central axis of each sample
well.
37. The method of claim 20, further comprising detecting thermal
infrared radiation transmitted from a plurality of samples
contained in a corresponding plurality of sample wells in the
sample plate using the optical device.
38. The method of claim 37, where the thermal infrared radiation is
detected simultaneously from the plurality of samples.
39. The method of claim 37, where the thermal infrared radiation is
detected sequentially from the plurality of samples.
40. The method of claim 20, further comprising computing a quantity
related to a characteristic of the thermal infrared radiation
transmitted from the sample.
41. The method of claim 40, where the quantity is representative of
the temperature of the sample.
42. The method of claim 40, further comprising: computing the
quantity for a plurality of samples; and displaying the quantities
graphically in a manner representative of the arrangement of the
corresponding sample wells in the sample plate.
43. The method of claim 20, further comprising covering the sample
wells to reduce evaporation from the samples.
44. The method of claim 20, further comprising: placing the samples
in the sample wells such that the samples occupy at least about
half of the volume of the sample wells; and covering the sample
wells to reduce evaporation from the samples.
45. The method of claim 20, further comprising: converting the
detected thermal infrared radiation to a signal; and processing the
signal to reduce the proportion of the signal that is attributable
to noise.
46. The method of claim 45, where the step of processing the signal
includes the step of computing a quantity based on distinguishable
components of the signal representing thermal infrared radiation
detected from the same sample at different times.
47. The method of claim 45, where the step of processing the signal
includes the step of computing a quantity based on distinguishable
components of the signal representing thermal infrared radiation
detected from different portions of the same sample.
48. The method of claim 20, further comprising: detecting thermal
infrared radiation transmitted from a plurality of samples
contained in the sample wells using the optical device; converting
the thermal infrared radiation detected from each sample to a
corresponding signal; and adjusting the signals so that each has
the same preselected value at the same preselected time.
49. The method of claim 48, where the preselected value is
zero.
50. The method of claim 48, where the preselected time is zero.
51. A system for detecting thermal infrared radiation, comprising:
a sample plate having a plurality of sample wells for holding a
corresponding plurality of samples, each sample well having at
least one infrared-transmissive surface configured to transmit
thermal infrared radiation emitted by a sample positioned within
the well; and an optical device configured preferentially to detect
thermal infrared radiation transmitted from a sample through the
infrared-transmissive surface of the corresponding sample well.
Description
CROSS-REFERENCES
[0001] This application is based upon and claims benefit under 35
U.S.C. .sctn.119 of the following U.S. Provisional Patent
Applications, each of which is incorporated herein by reference:
Ser. No. 60/249,931, filed Nov. 17, 2000; and Ser. No. ______,
filed Dec. 19, 2000, titled APPARATUS AND METHODS FOR INFRARED
CALORIMETRIC MEASUREMENTS, and naming Andy C. Neilson, Jay S.
Teich, Michael R. Sweeney, James D. Orrell III, Marc Samson, John
M. Hopkins, and Michael W. Oster as inventors.
FIELD OF THE INVENTION
[0002] The invention relates to calorimetry. More particularly, the
invention relates to apparatus and methods for performing
calorimetry that use optical devices to detect thermal processes
and/or multiwell sample plates to support samples for use with such
optical devices.
BACKGROUND OF THE INVENTION
[0003] Thermodynamics has established the interrelationship between
various forms of energy, including heat and work. Moreover,
thermodynamics has quantified this interrelationship, showing, for
example, that in chemical and physiological processes the
difference between the energy of the products and the energy of the
reactants is equal to the heat gained or lost by the system. In an
"exothermic" process, this difference is negative, so that the
process releases heat to the environment. Conversely, in an
"endothermic" process, this difference is positive, so that the
process absorbs heat from the environment. Thus, "calorimetry," or
the measurement of heat production and/or heat transfer, can be
used to determine if a chemical or physiological process is
exothermic or endothermic and to estimate the energy produced or
consumed.
[0004] The measurement of heat production and/or heat transfer in
chemical and physiological processes can be quite complicated.
Standardly, such measurements are made using a device known as a
"bomb calorimeter." This device typically includes a sturdy steel
container with a tight lid, immersed in a water bath and provided
with electrical leads to detonate a reaction of interest inside the
calorimeter. The heat evolved in the reaction is determined by
measuring the increase in temperature of the water bath.
[0005] Unfortunately, bomb calorimeters are inadequate for the
measurement of heat production and/or heat transfer in many areas
of chemistry and physiology. For example, the study of processes
involving uncommon and/or expensive components may require analysis
of samples too small for bomb calorimetry. Similarly, the
high-throughput screening of pharmaceutical drug candidate
libraries for drug activity may require analysis of too many
samples for bomb calorimetry.
[0006] The analysis of small samples is especially problematic due
to their small heat capacities and large surface-to-volume ratios.
Many chemical and physiological processes lead to very small
changes in temperature (<0.05.degree. C.), making their analysis
susceptible to environmental contamination. In particular, whenever
there is a temperature difference between a sample and the
environment, heat can be exchanged between the sample and the
environment, for example, by conduction, convection, and/or
radiation, among others. Such heat exchange may quickly alter the
temperature of a small sample and thereby obscure any temperature
change associated with a reaction. Moreover, fluid samples such as
those typically used in studies of chemical and physiological
processes may initiate secondary reactions with the environment,
such as evaporation. Evaporation, by definition, is an exchange of
energy (moisture is added to the air, while chemical volume is
reduced). This process takes place on the surface of the sample,
where the sample is exposed to the environment, and so may be
especially problematic for small samples due to their relatively
large surface-to-volume ratios. Evaporation not only removes energy
from the sample, contaminating the measurement, but also may
increase measurement noise due to surface instability as the fluid
phase changes to a gas phase.
SUMMARY OF THE INVENTION
[0007] The invention provides apparatus and methods for performing
calorimetry. The apparatus include optical devices for detecting
thermal processes and multiwell sample plates for supporting
samples for use with such optical devices. The methods include
measurement strategies and data processing techniques for reducing
noise in measurements of thermal processes. The apparatus and
methods may be particularly suitable for extracting thermal data
from small differential measurements made using an infrared camera
and for monitoring chemical and physiological processes.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is a partially schematic cross-sectional view of a
system for detecting thermal processes.
[0009] FIG. 2 is a top view of a multiwell sample holder for use
with an optical device for detecting thermal processes.
[0010] FIG. 3 is a cross-sectional view of the multiwell sample
holder of FIG. 2, taken generally alone line 3-3 in FIG. 2.
[0011] FIG. 4 is a graph showing the infrared transmissivity of a
preferred sample well material as a function of the thickness of
the material.
[0012] FIG. 5 is a pseudo-color image showing the extent and effect
of thermal cross talk in sample wells in (A) the microplate of
FIGS. 2 and 3 and (B) a standard commercial microplate.
[0013] FIG. 6 is a graph showing noise envelopes associated with
measurements of sample temperature taken from the (A) top and (B)
bottom of a sample well after application of common-mode noise
subtraction, area averaging, and frame averaging. The noise
envelope associated with the top-read data is significantly larger
than the noise envelope associated with the bottom-read data due to
evaporation.
[0014] FIG. 7 is a graph showing the size of the noise envelope as
a function of the number of frames average in an image-averaging
experiment.
[0015] FIG. 8 is a graph showing the effects of common-mode noise
and drift on thermal data collected using an infrared camera.
[0016] FIG. 9 is a graph showing the effects of offset subtraction
on thermal data.
[0017] FIG. 10 is a graph showing the effects of removing
common-mode noise on thermal data.
[0018] FIG. 11 is a software screen for use in the display of
thermal data, including temperatures and temperature
differentials.
[0019] FIG. 12 is a software screen for use in the collection
and/or analysis of data from measurement and reference regions.
[0020] FIG. 13 is a software screen for use in defining selected
characteristics of the reference region.
DEFINITIONS
[0021] Technical terms used in this application have the meanings
that are commonly recognized by those skilled in the art. The
following terms may have additional meanings, as described
below:
[0022] Common-mode noise. Typically low-frequency (<1 Hz) noise
caused when internal control loops, such as the servo on a
cryogenic cooler, create response changes in the detector. In an
infrared camera, these noise sources may be common to each sensor
element and may be geometrically displaced across the sensor array.
For example, at a given time, a thermal wave from the expansion of
helium gas in a sensor cooler may cause slight gain changes in the
sensor that cause a group of sensor elements in one geometric
region of the array to respond differently, or out of phase with,
another group of sensor elements in another region of the array. In
most applications, common-rode noise is insignificant; however, in
high-sensitivity (<0.05.degree. C.) applications, common-mode
noise may become a limiting factor.
[0023] Heat. A form of energy associated with the motion of atoms
or molecules. Heat is capable of being transmitted by (1)
conduction through solid and fluid media, (2) convection through
fluid media, and (3) radiation through empty space.
[0024] Infrared (IR) radiation. Invisible electromagnetic radiation
having wavelengths from about 700 nanometers, just longer than red
in the visible spectrum, to about 1 millimeter, just shorter than
microwave radiation. Infrared radiation includes (A) near IR (from
about 700 nm to about 1,300 nm), (B) middle IR (from about 1,300 nm
to about 3,000 nm), and (C) far or thermal IR (from about 3,000 nm
to about 1 mm). Near and middle IR is infrared radiation that
typically is caused by vibrations and low-level electronic
transitions in molecules and that is only peripherally related to
heat. In contrast, thermal IR (or thermal infrared radiation) is
infrared radiation that is caused or produced by heat and that is
emitted by an object in proportion to the temperature and
emissivity of the object.
[0025] Radiosity. The radiation emanating from an object is
determined by the following parameters: (1) emissivity, i.e., the
amount of radiation the object emits, (2) reflectivity, i.e., the
amount of externally derived radiation the object reflects, and (3)
transmissivity, i.e., the amount of externally derived radiation
the object transmits. For example, the thermal power P radiated by
an object may be described by the equation P=.di-elect
cons..sigma.AT.sup.4, where .di-elect cons. is the emissivity of
the object, .sigma. is the Stefan-Boltzmann constant, A is the area
of the object, and T is the temperature. Emissivity, reflectivity,
and transmissivity are dimensionless parameters with values that
range between 0 and 1. For a given material, the sum of these
parameters should equal unity, so that each parameter is inversely
correlated with the sum of the other parameters. A material with an
opaque surface has a transmissivity of zero, so its emissivity
equals one minus its reflectivity. Materials that radiate very well
and absorb a large percentage of the radiation that strikes them
have high emissivities.
[0026] Parasitic noise. Typically low-frequency (<0.1 Hz) noise
caused by stray radiation incident on the detector from within the
detector housing, creating an offset in output that results in
measurement error. The stray radiation may be caused by slight
temperature changes internal and/or external to the detector. In an
infrared camera, the error may be geometrically displaced across
the camera array, as determined by the efficiency of the cold
shield for the camera sensor and the baffling within the Dewar.
Most infrared cameras attempt to correct for parasitic noise using
some form of internal calibration mechanism, such as a
uniform-temperature shutter that periodically drops in front of the
sensor to perform an offset compensation. These calibration
mechanisms inherently interrupt measurements and can lead to
measurement errors if the uniform-temperature shutter is not
actually perfectly uniform in temperature. All infrared radiometers
have some form of parasitic noise.
[0027] Spatial Noise. Typically lower-frequency (<60 Hz) highly
nonlinear noise reflecting detector artifacts caused by variations
in the manufacturing process, for example, during metal oxide vapor
deposition (MOVD). In an infrared camera, these artifacts may cause
slight differences in the gain or response characteristics,
spectral characteristics, and/or stability characteristics of the
various elements, columns, and/or rows of the sensor. Spatial noise
may result in low-frequency noise or a drift component, which may
still remain even after performing a calibration for pixel gain and
offset in the camera.
[0028] Temporal noise. Typically high-frequency (>60 Hz) random
noise caused by (radiated or conducted) electronic noise, A/D
quantization, 1/f noise, microphonics, and/or a low electronic
signal-to-noise ratio from the detector.
[0029] Thermal conductivity. The quantity of heat transmitted, due
to a unit temperature gradient, in unit time under steady
conditions in a direction normal to a surface of unit area, when
the heat transfer is dependent only on the temperature
gradient.
[0030] Thermodynamic noise. Noise caused by thermodynamic
instabilities in a medium, such as a fluid-to-gas phase transition.
Surface measurements of most fluids, including water, show
significant instability due to thermodynamic noise caused by
evaporation.
DETAILED DESCRIPTION
[0031] The invention provides apparatus and methods for performing
calorimetry (or thermogenic analysis). The apparatus include
optical devices for detecting thermal processes and multiwell
sample plates for supporting samples for use with such optical
devices. The methods include measurement strategies and data
processing techniques for reducing noise in measurements of thermal
processes. The apparatus and methods may be particularly suitable
for (1) extracting thermal data from small differential
measurements made using an infrared camera, and (2) for monitoring
chemical and physiological processes.
[0032] FIG. 1 shows a system 100 for detecting thermal processes in
accordance with aspects of the invention. The system includes an
optical device 102 configured to detect thermal radiation 104 and a
sample plate 106 having a sample well 107 configured to support a
sample 108 for use with the optical device. The system may be used
to monitor thermal processes in the sample or samples by detecting
temperature changes correlated with heat production (e.g., from a
chemical or physiological reaction) and/or heat transfer in the
samples. This correlation may be performed using any suitable
method, such as those described in the following U.S. provisional
patent application, which is incorporated herein by reference: Ser.
No. ______, filed Dec. 19, 2000, titled APPARATUS AND METHODS FOR
INFRARED CALORIMETRIC MEASUREMENTS, and naming Andy C. Neilson, Jay
S. Teich, Michael R. Sweeney, James D. Orrell III, Marc Samson,
John M. Hopkins, and Michael W. Oster as inventors. If there are
multiple samples, the radiation transmitted from the samples may be
detected sequentially from each sample, for example, by point
reading, or simultaneously from some or all of the samples, for
example, by image reading. The optical device may include a
detector 110 such as an infrared optical sensor configured
preferentially to detect thermal infrared radiation. The detector
measures thermal energy radiated from a sample (or samples)
supported by the sample plate and converts the measured energy to a
signal such as an electrical signal that can be converted into a
temperature, for example, using a blackbody or graybody
approximation. In a preferred embodiment, the detector includes an
imaging device such as an infrared focal plane array (FPA)
configured to obtain a time-dependent series of two-dimensional
infrared images of processes occurring in samples in a multiwell
sample plate, permitting measurement of temperature and temperature
changes in each process, as a function of time, geometrically
across the plate. The series of images typically is collected at a
preselected frequency (typically >1 Hz) for a preselected period
significant relative to a characteristic time of any time-dependent
process being monitored. The data subsequently may be processed
and/or reported at a lower frequency. The data may be used to
monitor, screen, rank, and/or otherwise analyze thermal processes
occurring in the sample. The thermal analysis may be used alone or
together with other measurements to assess the presence,
concentration, physical properties, and/or activity of a compound
or compounds in the sample. Thus, the system provides a noncontact,
noninvasive method for measuring thermal properties such as
temperature, in contrast to bomb calorimeters, thermometers, and
capacitive and resistive circuits.
[0033] The system and its components may be configured to improve
the accuracy and/or sensitivity of thermal measurements,
particularly thermal measurements involving small samples and/or
small temperature changes. The optical device may be configured to
reduce measurement errors associated with noise, such as
common-mode, parasitic, spatial, temporal, and/or thermodynamic
noise, among others. The sample plate may be configured to
facilitate detection of thermal radiation through a surface of the
plate and/or to reduce measurement-contaminating heat transfer
between the samples and the environment (including other samples).
The optical device and sample plate may together be configured to
reduce noise associated with evaporation, for example, by using a
bottom-read detector and a sample plate having an
infrared-transmissive bottom surface and in some cases a cover.
[0034] The remainder of the Detailed Description is divided into
four sections: (A) optical devices, (B) noise reduction, (C) sample
holders, and (D) examples.
A. Optical Devices
[0035] The optical device generally comprises any device capable of
preferentially detecting thermal infrared radiation and using the
detected radiation to analyze thermal processes in a sample. The
phrase "capable of preferentially detecting thermal infrared
radiation" means that the device is configured and/or operated so
that it detects more thermal infrared radiation than any other form
of radiation (i.e., so that at least about half of the radiation
detected is thermal infrared radiation). The phrase excludes any
device that detects thermal infrared radiation only incidentally,
as might occur in an optical device configured to detect visible
light if thermal radiation leaked into the detector. The capability
for preferentially detecting thermal infrared radiation may reflect
use of one or more of the following mechanisms, among others: (A)
use of spectral filters preferentially to "extract" thermal
infrared radiating by blocking radiation other than thermal
infrared radiation, including visible, near IR, or middle IR
radiation, (B) use of detectors having enhanced sensitivity for
thermal infrared radiation, and/or (C) postprocessing of a detector
signal to reduce and/or compensate for any component of the signal
not resulting from detection of thermal infrared radiation.
[0036] FIG. 1 shows an optical device 102 constructed in accordance
with aspects of the invention in use as a part of a system 100 for
detecting thermal processes. The device includes a detector 110
configured to detect thermal infrared radiation emitted by a
sample, a stage 112 configured to support a sample in a sample
holder for thermal analysis by the detector, and a processor 114
configured to analyze radiation detected by the detector. The
detector and stage are positioned such that at least a portion of
the thermal infrared radiation emitted by the sample is incident,
indirectly or preferably directly, on the detector. This may be
accomplished by ensuring that a central axis CAL of the sample
wells is aligned with an optical axis OA of the instrument prior to
detection of thermal infrared radiation. Alignment simply means
that the two axes are sufficiently close to parallel that radiation
from a central portion of the sample well is detectable by the
instrument. For example, in FIG. 1, the central axis of each sample
well is aligned with the optical axis of the instrument. The
detector may be positioned below the stage to form a "bottom-read"
instrument, above the stage to form a "top-read" instrument, or in
other positions to form other instruments. The instrument of FIG. 1
is a bottom-read instrument, and the instrument of FIG. 1 with
components of the optical device 102 inverted and positioned above
the stage is a top-read instrument.
[0037] The stage may be movable, so that samples may be deposited
at a first position, moved to a second position for fluid
dispensing, moved to a third position for thermal equilibration,
moved to a fourth position for thermal detection, and moved to a
fifth position for pickup. These positions may be the same or
different, and any given position (except the detection position)
may be present or absent. The stage may move sample holders
translationally and/or rotationally, among others.
[0038] The sample generally comprises any object or system of
objects intended for thermal analysis. The sample may include
compounds, mixtures, surfaces, solutions, emulsions, suspensions,
cell cultures, fermentation cultures, cells, tissues, secretions,
and/or derivatives and/or extracts thereof. The sample also may
include the contents of a single sample site, or several sample
sites, depending on the assay.
[0039] The detector generally comprises any device for
preferentially detecting thermal infrared radiation and converting
the detected radiation into a signal representative of the detected
radiation. A preferred detector is an imaging detector, such as an
infrared camera, that is capable of simultaneously viewing part or
all of a sample holder.
[0040] The stage generally comprises any mechanism for supporting a
sample in a sample holder at an examination site for thermal
analysis by the device. A preferred stage is a transporter capable
of moving the sample holder vertically and/or horizontally between
the examination site and one or more transfer sites where the
sample holder can be loaded onto and/or unloaded from the
stage.
[0041] The processor generally comprises any mechanism for
analyzing the signal from the detector, for example, to conduct a
thermal analysis. The analysis may include conversion of a signal
representative of intensity and/or wavelength into a signal
representative of temperature and/or differential temperature,
among others. The analysis also may include performing calculations
to reduce noise and/or to facilitate data reporting, as described
below. The processor may be intrinsic to the detector, extrinsic to
the detector, or both.
[0042] The optical device also may include a housing 116 to support
and protect the detector. The housing may include, among others, an
optics tube 118, an infrared-transmissive window 120, and/or a
baffle 122 having an aperture 124. The optics tube may support the
detector and/or components of the housing, such as the window and
baffles. The optics tube also may reduce the amount of unintended
thermal infrared radiation entering the detector. The window may
permit thermal infrared radiation to enter the housing for
detection, while also permitting the housing to be sealed to reduce
contamination and/or (partially) evacuated to reduce absorption
and/or scattering of thermal radiation prior to detection. The
window may include a portion formed of zinc selenide (ZnSe) and/or
polyethylene, among others. The baffles may block stray thermal
infrared radiation. The optical device may be configured to shield
the sample from incident radiation to reduce the proportion of the
sample signal arising from transmission, reflection, and/or
photoluminescence from the sample.
[0043] The optical device also may include a data output mechanism
such as a view screen or printer for reporting results of any
thermal analysis. Data generally may be reported using any suitable
method, physical and/or electronic, digital and/or analog, and
static and/or time varying, among others. Suitable methods include
tables, graphs, and/or images, among others. Data may include
temperatures and/or temperature differentials, among others, at a
fixed time or as a function of time.
B. Noise Reduction
[0044] Noise is almost invariably a problem in measurements of
thermal processes. However, noise may be especially problematic in
measurements of thermal processes involving small samples and/or
small temperature changes, where even minor noise can obscure or
overwhelm any temperature change associated with the thermal
process.
[0045] The effects of noise on measurements of thermal processes
can be reduced using noise reduction techniques. Noise reduction
involves identifying sources of noise (e.g., measurement noise,
camera noise, etc.) and then developing methods for reducing or
eliminating the noise or its effects. Unfortunately, thermal
detectors such as infrared cameras are susceptible to several types
of noise, including common-mode, parasitic, spatial, and temporal
noise, among others. In addition, fluid samples are susceptible to
other types of noise, including thermodynamic noise, which is
caused by thermodynamic transformations, such as evaporation, in
which the sample changes from a fluid to a gas. Thus, noise
reduction in measurements of thermal processes may involve the
application of one or more different methods.
[0046] The present "state of the art" for infrared, radiometric
cameras defines measurement performance in terms of noise-related
parameters, specifically, accuracy and sensitivity. Here, accuracy
is a relative absence of error or mistake, and sensitivity is an
ability to detect or measure an input, especially a weak input. The
sensitivity of infrared cameras normally is quantified in terms of
noise equivalent temperature difference (NETD), which is the RMS
noise/response at a given temperature, f#, and operating frequency
(normally 30.degree. C., f/1, and 60 Hz). NETD typically is the
limiting factor in determining measurement sensitivity. A typical,
state-of-the-art, high-performance infrared camera, such as the
FLIR SC3000, using Quantum Well (QWIP) sensor technology, specifies
an absolute accuracy of about 2.degree. C. and a sensitivity (NETD)
of about 0.03.degree. C. Unfortunately, this sensitivity may be
inadequate in measurements of small temperature changes.
[0047] The invention provides methods for reducing noise and/or
enhancing accuracy and/or sensitivity in thermal measurements,
particularly measurements involving small samples and/or small
temperature changes. These methods may be implemented using any
suitable apparatus, such as any processor associated intrinsically
or extrinsically with the optical device. These methods may improve
upon the previous state-of-the-art sensitivity described above,
potentially providing sensitivities of <0.01.degree. C. and RMS
noise levels of <0.005.degree. C., at least when the methods are
used on data collected with preferred sample holders. The methods
may involve application of one or more of the following techniques,
among others:
[0048] 1. Low-pass Filtering
[0049] Data may be collected at a relatively high frame rate and
passed through a low-pass (time-domain) filter to reduce
high-frequency "temporal noise." For example, using an infrared
camera as described above, full-field radiometric data may be
collected at a 60 Hz frame rate and passed through a low-pass
filter internal to the camera electronics to yield filtered data
corresponding to a lower frame rate.
[0050] 2. Frame Averaging
[0051] Data may be averaged (or otherwise smoothed) over a series
of frames to remove any residual high-frequency temporal noise. For
example, frame averaging may be performed pixel-by-pixel by summing
the values T.sub.ij associated with a given pixel ij in each frame
k and then dividing the sum by the total number N of frames used in
the average: 1 T ij FA = 1 N k = 1 N T ij ( k ) ( 1 )
[0052] Here, < >denotes averaging. Typically, the middle
frame in the set of N frames is replaced by the average frame. The
preferred number of frames to use in the frame average is
determined by competing factors. Generally, it is better to use a
larger number of frames because frame averaging and low-pass
filtering typically reduce high-frequency random temporal noise by
the square root of the number of frames averaged. However, the
overall time corresponding to the number of frames used in the
average should be small relative to the time scale of thermal
changes in the system to avoid averaging frames that differ due to
actual differences in the temperature of the sample rather than due
merely to noise. In the data presented below under Examples, the
optimum number of frames for frame averaging was between about 4
and 16. Frame averaging may be performed separately for measurement
and reference areas.
[0053] 3. Area Averaging
[0054] Data may be spatially averaged to return a reduced number of
(average) values or a single (average) value for each area as a
function of time. For example, area averaging may be performed by
summing the values T.sub.ij associated with some or all of the
pixels in a given area A of a frame and then dividing the sum by
the total number M of pixels used in the average: 2 T AA ( k ) = 1
M i , j A T ij ( k ) ( 2 )
[0055] Thus, in a sample holder having 96 sample wells each having
a measurement area and a reference area, area averaging may be used
to reduce the data set to as few as 96 measurement values and 96
reference values by independently averaging pixel values over all
or part of each measurement and reference area. The measurement and
reference areas may be distinguished using application software
implemented in the processor. Area averaging typically involves
>4 pixel elements and preferably involves >9 pixel elements.
Area averaging may reduce the effects of geometric, spatial noise
common to most FPA detectors. Such noise may reflect defective or
nonlinear detector elements and/or slight differences in amplifier
characteristics.
[0056] 4. Reference Calibration
[0057] Data may be calibrated using a reference standard, such as
an adjacent local (e.g., perimeter) reference standard, for
example, by subtracting a reference value from a corresponding
measurement value to return a differential measurement for each
sample well as a function of time.
T.sub.RC=T.sub.Meas-T.sub.Ref (3)
[0058] Here, the measured and reference values may be properties of
the thermal radiation detected from the measurement and reference
regions, respectively, such as intensities, or they may be
quantities derived from such properties, such as temperatures. The
method may be applied pixel-by-pixel or area-by-area, among others.
Subtracting reference values from measurement values may reduce or
eliminate common-mode noise, internal drift, and/or parasitic noise
local to the region of the detector array used in the measurements.
These noise sources have a tendency to be geometrically dispersed
across the sample holder or sample wells, so that other
noise-reduction techniques, such as single-point reference or
Fourier transform characterization and subtraction have limited
success. These other methods have a tendency to amplify noise where
it shifts out of phase relative to adjacent areas, whereas the
local reference compensates for geometric shifting.
[0059] 5. Offset Subtraction
[0060] Data may be adjusted by subtracting one or more offsets from
each measurement.
T.sub.OS=T-T.sub.Offset (4)
[0061] The offset may be used to set the initial-time differential
measurements for each sample well at t (time)=0, so that there is a
common starting point from which to measure changes in temperature.
Offset subtraction effectively creates a zero reference at the
beginning of the experiment and adjusts the difference in
temperature between the measurement region and associated reference
region to zero. Adjusting the offset to zero may compensate for
field nonuniformity resulting from camera drift prior to the start
of data collection.
[0062] 6. Bottom Reading
[0063] Reading through the bottom of an infrared-transmissive
sample well may reduce thermodynamic noise created at the interface
of dry air and the sample. In particular, evaporation at sample
surfaces exposed to dry air may create a saturated gas layer
adjacent the sample surface. This layer may be opaque or nearly
opaque to the thennal detector and show significant instability
(measured to be >0.05.degree. C.). Measurement noise created by
evaporation may be fivefold or more greater than measurement noise
associated with reading through the bottom of the sample well or
from an independent black body reference. Additionally, evaporation
at the surface may lower the surface temperatures measured by the
camera by as much as 2.degree. C. This 2.degree. C. difference is a
heat sink for the reaction being measured. Bottom reading allows
the top surface of the sample well to be sealed so that the space
above the sample becomes saturated with moisture, reducing
evaporation noise and heat loss.
[0064] The application of these noise-reduction methods generally
is quite flexible. For example, each method generally may be
applied separately, alone or in combination with any number of
other methods. Moreover, each method generally may be applied in
any order.
B. Sample Holders
[0065] The sample holder or sample plate generally comprises any
substrate or material capable of supporting a sample for thermal
analysis. Suitable sample holders may include microplates, PCR
plates, biochips, chromatography plates, and microscope slides,
among others, where microplate wells and biochip array sites
comprise assay or measurement sites.
[0066] The sample holder may include a thermal isolation structure
disposed between the sample wells to reduce thermal transfer
between the wells and the environment and thus between adjacent
wells. The thermal isolation structure may include a thermal
buffer, thermal barrier, and/or isolation well, among others, as
described below. The thermal isolation structure may be composed at
least in part of a different material than the sample wells. The
thermal isolation structure may substantially surround a central or
optical axis of each sample well, isolating the wells without
obstructing transmission of thermal infrared radiation along the
central axis. The thermal isolation structure also may be disposed
such that any straight line below a plane formed by the tops of the
sample wells connecting a portion of one sample well to a portion
of an adjacent sample well intersects the isolation structure.
[0067] The sample holder also may include an insert member defining
an array of sample wells and a support member having a thermal
isolation framework in a configuration corresponding to the array
of sample wells. The sample wells each may have a central axis, and
the insert may engage the support member such that each sample well
is thermally isolated from adjacent sample wells without
obstructing the transmission of thermal infrared radiation along
the central axis. The thermal isolation framework may include a
thermal buffer, thermal barrier, and/or isolation well, among
others, as described below.
[0068] The sample holder also may include an insert having a
plurality of sample wells, and a thermal isolation member for
supporting the insert so that each sample well can be precisely
positioned along an optical path, where the thermal isolation
member provides a thermally controlled thermal reference surface
adjacent each well as viewed along the optical path. The reference
surface may define an aperture that frames the associated optical
path.
[0069] A preferred sample holder is configured as a microplate
having a frame and a plurality of sample wells disposed in the
frame for holding a corresponding plurality of samples for
analysis. This format may combine small-volume samples and a
high-density holder, permitting automated analysis of large numbers
of samples. This format also may be configured to reduce unintended
heat exchange between the samples and the environment (including
between the sample and other samples) and/or to permit an optical
detector to measure thermal infrared radiation transmitted through
a surface of the sample holder.
[0070] The sample holder may include one or more of the following
features, among others:
[0071] 1. Thin Surface
[0072] A sample well having at least one surface having a thickness
of less than about 0.005 inches, and preferably less than about
0.001 inches, and most preferably less than about 0.0005 inches. A
thin surface may be important for at least two reasons: (1)
increased infrared transmissivity, and (2) decreased thermal
conductivity. These two criteria preferably may be met using a
single material, such as a polymeric polyethylene blend having a
high infrared transmissivity (e.g., greater than about 50% or about
80%) and a low thermal conductivity (e.g., less than about 1 W/m-K
or about 0.6 W/m-K).
[0073] A thin (i.e., reduced-thickness) surface may increase
transmissivity. A thin surface may be less likely to absorb thermal
energy being radiated by the sample due to its shorter path length
and more likely to have an outer (i.e., non-sample-contacting)
surface at the same temperature as the sample, facilitating
calorimetric analysis through the surface. A preferred thin surface
has a high transmissivity (e.g., >80%) for thermal infrared
radiation, particularly thermal infrared radiation having
wavelengths between about 3 and 5 micrometers and between about 7
and 14 micrometers. (These wavelength ranges may be especially
useful in thermal imaging, because they correspond to minima in
atmospheric absorption.) A thin more transmissive surface
preferably is located at least at the bottom of the sample well to
permit detection from the underside of the sample holder using a
bottom-read analyzer. The surface may be substantially (e.g.,
optically) flat to reduce optical aberrations during analysis
through the surface.
[0074] A thin (i.e., reduced-thickness) surface also may decrease
thermal conductivity. There are three primary mechanisms for heat
transfer in the plate: conduction, convection, and radiation.
Typically, conduction is the most significant mechanism, and
radiation is the least significant mechanism. Conduction may be
described by the equation P=KA.gradient.T, where K is the thermal
conductivity, A is the surface area, and .gradient.T is the
temperature gradient. Thus, reducing surface area may reduce
conduction. A primary path for conduction is through the walls of
the sample well to contact points on the associated frame. This
path may be reduced using thin-walled sample wells. Moreover,
because the thermal conductivity of air (.about.0.02 W/m-K) is less
than that of the preferred well material (0.6 W/m-K), it is
important to use the air as much as possible for a conduction path.
Thus, the wells hold heat much like a thermos. Finally, the thermal
capacitance of a thin material is lower, so that there is less
change in temperature due to the initial .DELTA.T in the system. In
particular, the material may be selected such that the thermal mass
of the sample wells is no more than about half the thermal mass of
an aqueous sample positioned in the sample well, even when the
sample well is completely full. A thin less conductive surface
preferably is located at least at the sides of the sample well.
[0075] 2. Thermal Buffer
[0076] A thermal buffer disposed between the sample wells to resist
thermal transfer between sample wells, or between the environment
and the sample wells. The thermal buffer generally comprises any
mechanism for resisting a change in temperature. In this sense, the
thermal buffer resembles a pH buffer, which resists a change in pH
when an acid or base is added to a solution by binding to the added
species, or an electrical capacitor, which resists a change in
voltage by storing or releasing charge. The thermal buffer may be
used to buffer (or keep relatively constant) the temperature of any
structure adjacent the sample well, such as the trapped volume
described below. The thermal buffer may include a structure having
a high thermal mass (or heat capacity), which can absorb heat
without undergoing a significant change in temperature. This high
thermal mass structure may, for example, have a substantially
higher thermal mass (or heat capacity) than the sample wells and/or
corresponding samples, for example, three, five, or even ten times
higher. The high thermal mass structure may include a metal such as
aluminum and/or a high thermal capacitance plastic, among
others.
[0077] 3. Thermal Barrier
[0078] A thermal barrier disposed between the sample wells to block
thermal transfer between sample wells. The thermal barrier
generally comprises any mechanism for blocking the transfer of heat
into or out of the sample wells or the vicinity of the sample
wells, such as an adjacent trapped volume. The thermal barrier may
include a material having a low emissivity and/or a high
reflectivity for infrared radiation. For example, the thermal
barrier may include a material that reflects at least about half of
the infrared radiation that otherwise would be incident upon
surfaces of the sample well. Generally, emissivity and reflectivity
are inversely related; thus, shiny, metallic materials tend to have
low emissivities and high reflectivities, whereas matte,
dark-colored materials tend to have high emissivities and low
reflectivities. In a preferred embodiment, the thermal barrier
includes a material having a reflectivity of at least about 0.8 and
an emissivity of at most about 0.2.
[0079] 4. Double-walled Sample Wells
[0080] A double-walled sample well, formed, for example, by
positioning a sample well in a corresponding isolation well. In a
preferred embodiment, a plurality of isolation wells are disposed
in a frame, a corresponding plurality of sample wells are disposed
in the isolation wells, and none of the sample or isolation wells
is in fluid contact with another of the sample or isolation wells.
The double-walled wells may include a trapped volume formed between
an outer surface of the sample wells and an inner surface of the
corresponding isolation wells, further reducing thermal transfer to
and from samples positioned in the sample wells. The trapped volume
may enclose air and/or an inert gas, and/or be partially or fully
evacuated relative to standard atmospheric pressure. The trapped
volume also may enclose or be lined along its perimeter with a
thermal barrier, i.e., a material having a low emissivity and/or a
high reflectivity for infrared radiation to reduce radiation
thermal transfer to and from the sample well.
[0081] 5. Plural Optically Transmissive Surfaces
[0082] A plurality of optically transmissive surfaces, at least one
associated with the frame and at least one associated with the
sample well, where the surfaces are configured so that an optical
reader can detect electromagnetic radiation such as infrared
radiation transmitted from a sample through both the optically
transmissive surface of the corresponding sample well and the
optically transmissive surface of the frame. For example, a
plurality of optically transmissive surfaces may be formed by
corresponding surfaces of a sample well and isolation well in a
double-walled well, as described above.
[0083] 6. Measurement and Reference Regions
[0084] A combination of a measurement region and a reference
region. The measurement region may be a portion of a sample well,
and the reference region may be an adjacent portion of the frame,
isolated from the sample well, particularly a high-thermal-mass
and/or high emissivity (>0.5 and preferably >0.8) surface
portion capable of acting as a blackbody or graybody reference. The
reference region may be composed at least in part of a different
material than the sample wells and may include a metal such as
aluminum. The reference region(s) may be disposed adjacent (e.g.,
about or between) the sample wells, so that each measurement region
is near a corresponding reference region, reducing artifacts that
reflect temperature drift across the sample plate. Thus, an
M.times.N array of measurement regions might be complemented by an
M.times.N of reference regions disposed about the measurement
regions, or an (M-1).times.(N-1) array of reference regions
disposed between the measurement regions. The reference region may
be configured as a ring or annulus distributed about or adjacent a
perimeter of the sample well and/or about and preferably
symmetrically about a central axis of the sample well. The
reference region may be positioned about or above the top of a
corresponding sample well, and/or about or below a corresponding
sample well. The reference regions and the corresponding sample
wells may be separated by a gap such as an air gap along a line
connecting each portion of the thermal reference regions and the
corresponding sample wells to reduce heat transfer between the
sample wells and the thermal reference regions. Thermal
characteristics of the measurement region may be calibrated using
thermal characteristics of the reference region, for example, by
subtracting the reference characteristic from the measurement
characteristic. This calibration may reduce geometrically dispersed
common-mode noise, including the effects of internal parasitic
radiation and camera drift. The use of dedicated reference regions
may free up all of the sample wells for data analysis, because none
of the wells needs to be used as a reference well.
[0085] 7. Consumable Sample Well Inserts
[0086] A combination of a reusable frame and a consumable sample
well insert (or a set of consumable sample well inserts) configured
to fit within or mate with the frame. The combination may
facilitate reuse of portions of the sample holder that are
expensive, such as the thermal buffer and/or thermal barrier. The
combination also may facilitate disposal of portions of the sample
holder that contact the sample by reducing the amount of such
materials that must be discarded. The combination may be
constructed so that the insert is substantially supported by the
frame yet substantially thermally insulated or isolated from the
frame. The frame and the sample wells may be composed of the same
or preferably different materials. Here, consumable may be defined
as more likely to be discarded than reused, typically because it is
more convenient and/or less expensive to be discarded than reused.
For example, a consumable sample well insert may obviate the need
to clean sample wells between samples.
[0087] 8. Cover
[0088] A cover configured for use with the sample holder. The cover
generally comprises any mechanism for covering the sample holder,
or a portion of the sample holder, to reduce contamination of the
samples and/or to reduce evaporation from the samples (e.g., by
reducing exposure to dry air and/or convective air currents), among
others. The cover generally may be formed of any suitable material,
such as a rigid plastic and/or a thin layer of oil or other less
evaporative material layered over the sample. The cover may be
infrared transmissive, so that samples may be analyzed through the
cover using a top-read analyzer. The cover may be configured to
touch the top surface of the sample. Alternatively, the cover may
be configured to leave an air gap between the top surface and the
cover, particularly a small air gap that may quickly saturate with
fluid vapor after fluid samples are positioned in the wells and
before reactant or catalyst are delivered to reduce evaporative
cooling during analysis. Generally, evaporation may be reduced by
reducing the size of the air gap, for example, by using shallow
wells and/or by substantially filling the wells (for example, until
the samples occupy at least about half or even about eight-tenths
or nine-tenths of the volume of the sample wells). Alternatively,
or in addition, evaporation may be reduced by increasing the
humidity of the air adjacent the sample well or sample holder. The
cover may include an aperture so that a fluid delivery system such
as a pipette can pierce the cover and deliver reactant fluids.
D. Examples
[0089] The following examples describe without limitation further
aspects of the invention. These examples show that thermal cross
talk between sample wells can be reduced by thermally isolating the
sample wells and that thermal resolution and the accuracy and
sensitivity of thermal measurements can be enhanced by reducing
noise, including common-mode, parasitic, spatial, temporal, and/or
thermodynamic noise, among others. Additional examples including
color drawings showing pseudocolor methods for displaying thermal
imaging data are described in the following U.S. provisional patent
application, which is incorporated herein by reference: Ser. No.
______, filed Dec. 19, 2000, titled APPARATUS AND METHODS FOR
INFRARED CALORIMETRIC MEASUREMENTS, and naming Andy C. Neilson, Jay
S. Teich, Michael R. Sweeney, James D. Orrell III, Marc Samson,
John M. Hopkins, and Michael W. Oster as inventors.
EXAMPLE 1
[0090] This example describes a preferred sample holder for use in
measurements of thermal processes, including chemical and
physiological processes.
[0091] FIGS. 2 and 3 show a sample holder 200 constructed in
accordance with aspects of the invention. The sample holder
includes a high thermal mass frame or base 202, a plurality of
sample wells 204 and a corresponding plurality of windows 206,
trapped volumes 208, reference regions 209, and opaque coatings
210, and a cover 212.
[0092] Frame 202 is the main structural component of sample holder
200. The frame generally may be sized and shaped as desired, for
both convenience and utility. Frame 202 is sized and shaped to form
a microplate, enabling the sample holder to be used with standard
microplate equipment, such as handlers, washers, and/or readers,
among others. A preferred frame is substantially rectangular, with
a major dimension X of about 125-130 mm, a minor dimension Y of
about 80-90 mm, and a height Z of about 5-15 mm, although other
dimensions are possible. Frame 202 may include a base 214
configured to facilitate handling and/or stacking, a notch 216
configured to facilitate receiving the cover, and/or a plurality of
apertures 218 configured to receive and support a corresponding
plurality of sample wells. The apertures provide clearance around
the sample wells, creating an air gap that may provide thermal
isolation between the base and the sample well. The apertures may
be formed using any suitable method, including machining and/or
casting the frame to include the apertures. The inner surface of
each aperture may be polished and/or lined with an opaque (i.e.,
low transmissivity) coating 210 such as AlSiO or gold to reflect
infrared radiation and thus to form a thermal barrier to heat
conduction to and from the sample wells. In this embodiment,
adjacent sample wells may be separated by two thermal barriers and
a portion of the frame disposed between the two thermal barriers.
The apertures and/or the sample wells may be tapered, such that the
separation between the sample wells and the walls of the
corresponding apertures increases from the top to the bottom of the
sample wells, further reducing conduction between the sample wells
and the walls of the apertures.
[0093] The frame generally may be constructed of any suitable
material. For example, frame 202 is constructed using a material
having a high thermal mass (heat capacity) and high thermal
conductivity, such as aluminum and/or other metals. Preferred
materials such as aluminum may reduce the time required for thermal
stabilization within the test chamber while being sturdy enough for
repeated, rugged use. In particular, a high thermal mass base
(and/or an adjacent structure) may function as a thermal buffer,
helping to maintain a constant temperature around sample wells
positioned in apertures 218.
[0094] Sample wells 204 are used to support and separate samples
220 for calorimetric analysis. These sample wells may vary in size,
shape, number, and arrangement, generally as desired, as long as
the wells fit in the frame, and more particularly fit within the
corresponding apertures in the frame. Exemplary sizes range between
about 1 .mu.L and about 500 .mu.L, and more preferably between
about 1 .mu.L and about 200 .mu.L. Exemplary shapes include cones,
frustums of cones, cylinders, and parallelepipeds, among others.
Exemplary numbers include 96, 384, 864, 1536, 3456, and 9600, among
others. Exemplary arrangements include rectangular and hexagonal
arrays, among others. Three preferred sample-well configurations
that will fit as rectangular arrays within a microplate-sized frame
are listed in the following table:
1 Number Arrangement Pitch (mm) Density (/mm.sup.2) of Wells of
Wells Between Wells of Wells 96 8 .times. 12 9 1/81 384 16 .times.
24 4.5 4/81 1536 32 .times. 48 2.25 16/81
[0095] Here, pitch is the center-to-center well-to-well spacing,
and density is the number of wells per unit area. In a preferred
embodiment, the frame will include a similarly spaced array of
apertures for receiving the sample wells. A preferred configuration
includes 96 frustoconical wells organized in an 8.times.12
rectangular array. Here, frustoconical refers to a well shape
having conical sides and a flat bottom, as shown in FIG. 3.
[0096] The sample wells may be formed as cups that may be inserted
into the frame to hold a sample for calorimetric analysis,
permitting an analyzer to measure temperature changes in the
sample, for example, resulting from chemical or physiological
processes. In this way, a single frame may be used to support cups
of different sizes and shapes, so long as the cup (or cups) will
fit within the corresponding apertures. The bottom surface 221 of
the cup may be flat and/or particularly thin in areas from which
infrared measurements are collected. A flat surface may serve to
reduce optical distortion and control reflections coming from the
surface. A thin surface may enhance infrared transmission, because
infrared properties generally are proportional to material
thickness. A thin surface also may help to ensure that the outer
surface remains at or very close to the temperature of the
fluid.
[0097] The cup inserts may be formed individually or joined to form
a sheet or sheets of cups for use in the frame. Individual cups and
small sheets of cups generally provide greater flexibility,
permitting cups to be mixed and matched (e.g., according to size,
shape, and/or infrared transmission properties, among others)
within a single plate. Large sheets of cups provide greater
structural stability and convenience, permitting many cups to be
changed at once. As mentioned above, cup inserts generally are
configured so that a single cup resides within a single aperture in
the supporting frame. However, cup inserts also may be configured
so that two or more cups reside within a single aperture,
permitting a single frame to support cups at two or more
significantly different sizes and/or densities.
[0098] The material properties of the cups are important for
thermal isolation and infrared transmissivity. FIG. 4 shows the
infrared transmissivity of a preferred cup material as a function
of material thickness. This preferred material is an
infrared-transmissive (polymeric) polyethylene blend sold under the
trademark Poly IR 2.TM. by Fresnel Technologies. The material has a
high infrared transmissivity that increases nonlinearly with
decreasing material thickness, showing a significant increase for
thicknesses below about 0.001 inches. Moreover, the material has a
low coefficient of thermal conductivity (.about.0.6
watts/meter-kelvin), which is about {fraction (1/10)} the thermal
conductivity of most other infrared-transmitting materials,
including Zn, Se, and Ge. In addition, the material has a low
thermal mass, so it should quickly assume the temperature of the
sample. The low thermal mass of the cup combined with its low
thermal conductivity and insulation from the base reduce heat loss
to the environment. The infrared transmission properties of the
material allow the detector to measure the temperature of the fluid
through the cup with less than 10% thermal contribution from the
cup. Further aspects of the preferred sample well material are
described in the following U.S. provisional patent application,
which is incorporated herein by reference: Ser. No. ______, filed
Dec. 19, 2000, titled APPARATUS AND METHODS FOR INFRARED
CALORIMETRIC MEASUREMENTS, and naming Andy C. Neilson, Jay S.
Teich, Michael R. Sweeney, James D. Orrell III, Marc Samson, John
M. Hopkins, and Michael W. Oster as inventors.
[0099] Window 206 is an environmental seal between the sample well
and the detector(located below the sample holder) when the sample
holder is used with a bottom-read analyzer. The window may be
formed of an infrared-transmissive membrane material selected to
enhance infrared transmission within the spectral sensitivity band
of the camera. A preferred material is zinc selenide, which
provides >97% transmission to the bottom surface of the sample
well.
[0100] Trapped volume 208 is formed between inner surfaces of frame
202, sample well 204, and window 206. The high thermal mass frame
that surrounds the sample well acts as a capacitor to maintain a
constant temperature within the trapped volume, which typically
contains air. The inner surfaces of the frame may be lined with an
opaque coating, as described above.
[0101] Reference region 209 is a source of a reference signal for
use in reference calibrations to reduce common-mode and parasitic
noise, among others, as described above. Generally, each sample
well includes a measurement region and a corresponding reference
region. The measurement region generally comprises a portion of the
sample or sample well, such as an infrared transmissive bottom
portion of the sample well for use with bottom-read instruments.
The reference region may comprise an adjacent portion of the frame,
such as an annular donut-shaped portion formed around a perimeter
and/or central axis of the measurement region. Here, the reference
region is positioned at an end of a support member formed by
portions of the frame disposed between the sample wells. The
thermal mass of the thermal reference region and associated support
member may be at least about the same as or greater than the
thermal mass of the corresponding sample well and/or sample. The
reference region may be formed of a high thermal mass and/or high
(>0.8) emissivity material such as a metal that acts as an
isolated, blackbody reference. The thermal reference region may
include a substantially flat emissive reference surface, where the
emissive surface is substantially parallel to a flat portion of the
bottom of the sample well and/or where the emissive surface is
within a factor of ten of the area of a flat portion of the bottom
of the sample well.
[0102] Cover 212 provides a mechanism for covering the sample
holder, or a portion of the sample holder, to protect samples from
evaporation and/or reduce the likelihood and amount of evaporation.
Cover 212 generally will leave a small air gap between the sample
and the cover that may saturate with fluid vapor to reduce
evaporation. Cover includes an aperture 222 so that a fluid
delivery system such as a pipette can pierce the cover and deliver
reactant fluids.
EXAMPLE 2
[0103] FIG. 5 shows results from an experiment designed to measure
thermal cross talk caused by heat conduction through the sample
plate. The experiment was performed using a top-read
thermal-imaging apparatus fitted with a quantum well (QWIP)
infrared radiometer from FLIR Systems. The figure shows thermal
images of two plates containing a fluid that evaporates when in
contact with dry ambient air. Here, relative temperature is denoted
by shading, where samples with relatively high temperatures have
increased shading, and samples with relatively low temperatures
have reduced shading. Plate 1 (left) is fabricated using a thin
polymer insert and a high thermal mass base, as shown in FIGS. 2
and 3. Plate 2 (right) is a standard commercially available 96-well
microplate fabricated from a polystyrene polymer (Costar 3628). The
thermal images show that plate 1 provides significantly better
thermal isolation than plate 2. In particular, the wells in plate 1
are insulated from the surrounding base material, reducing thermal
"cross talk" between adjacent wells, whereas the wells in plate 2
are poorly insulated, enhancing thermal "cross talk" and leading to
significant thermal gradients across the plate.
EXAMPLE 3
[0104] This example describes results of an experiment designed to
test the effects of evaporation on the apparent temperature of
samples positioned in wells in a multiwell plate.
[0105] The experiment was performed using the top-read
thermal-imaging apparatus and low cross-talk multiwell plate of
Example 2. In these experiments, sets of adjacent wells were filled
with fluids having various evaporation characteristics or else were
covered with a high-emissivity tape, as shown below.
2 T T T T T T T T T T T T T T T W W W A A A T T T T T T W W W A A A
T T T T T T W W W A A A T T T T T T O O O W W W T T T T T T O O O W
W W T T T T T T O O O W W W T T T T T T T T T T T T T T T
[0106] Here, A=alcohol, W=water, O=mineral oil, and T=tape. Alcohol
and water are prone to evaporation, whereas mineral oil and tape
are not. The apparent temperature in each well was measured at
fixed intervals during a 15-minute period.
[0107] The data show that evaporation affects the apparent
temperature of the samples. Specifically, the apparent temperature
of wells containing water or alcohol was about 27.degree. C.,
whereas the apparent temperature of wells containing mineral oil or
tape was about 29.degree. C., or about 2.degree. C. warmer.
Apparently, evaporation of water and alcohol cools the layer of gas
above these fluids, leading to lower measured temperatures.
[0108] The data also show that evaporation affects the apparent
temperature stability of the samples. Specifically, after
compensating for common-mode noise, wells containing water or
alcohol showed about 0.1.degree. C. peak-to-peak (PTP) temperature
variations, whereas wells containing mineral oil or tape showed
about 0.01.degree. C. PTP temperature variations, or about
one-tenth as large. Thus, evaporation may preclude accurate
measurement of small thermal processes within a well containing a
fluid prone to evaporation, at least if measured from above the
well. Conversely, reducing evaporation may improve thermal signal
and permit a more accurate measurement of thermal reactions within
the well.
EXAMPLE 4
[0109] This example describes results of an experiment designed to
determine whether temporal noise was caused by evaporation at the
surface of the fluid and whether temporal noise could be controlled
by reading the fluid through a transparent film.
[0110] The experiment was performed using the top-read
thermal-imaging apparatus and multiwell plate of Examples 2 and 3.
However, here, each well was filled with water. Moreover, a first
set of data was collected as above, with the water exposed to
ambient air, and a second set of data was collecting after placing
a thin (.about.0.0005-inch thick) infrared-transparent film (Poly
IR II) over the wells, in direct contact with the water to simulate
a bottom-read design. The temperature in each well was again
measured at fixed intervals during a 15-minute period.
[0111] FIG. 6A ("top read") shows the range of measured
temperatures as a function of time, after subtraction of
common-mode noise, reading from the surface of the exposed water.
The data show significant thermodynamic noise, resulting from
evaporation at the surface of the water, with temperature
variations of greater than about 0.1.degree. C. (PTP). This noise
level would make it very difficult to derive small temperature
changes resulting from chemical or physiological processes beneath
the surface.
[0112] FIG. 6B ("bottom read") shows the range of measured
temperatures as a function of time, after subtraction of
common-mode noise, reading through the infrared-transparent film.
The data show significantly reduced temporal noise, with
temperature variations of less than about 0.025.degree. C. (PTP).
In this case, the film reduces or prevents evaporative cooling
because the fluid no is longer exposed to dry air. This reduction
in evaporative cooling reduces noise in the measurement, which
allows the system to record significantly smaller changes in
temperature. This measurement technique, particularly when combined
with the novel multiwell plate design of FIGS. 2 and 3, allows
accurate recording of small subsurface processes taking place in
the sample well, improving measurement resolution by about
fourfold.
EXAMPLE 5
[0113] FIG. 7 shows results of an experiment designed to determine
the preferred number of frames to average in the frame-averaging
noise-reduction technique. The experiment was performed using
top-read thermal-imaging apparatus and low cross-talk multiwell
plate of Examples 2-4. The experiments show the noise level at
several areas of the multiwell plate, averaged over 0, 2, 4, 8, and
16 frames at 60 Hz. The following chart summarizes the data for
selected areas within the image:
3 16.times. Normal 2.times. Frames 4.times. Frames 8.times. Frames
Frames Max 10656.00 10632.00 10632.00 10616.00 10600.00 (bit
counts) Min 10560.00 10576.00 10584.00 10576.00 10568.00 (bit
counts) Avg 10611.17 10604.05 10610.28 10592.24 10582.40 (bit
counts) Range 0.60 0.35 0.30 0.25 0.20 (Kelvin) Std Dev 0.10 0.07
0.05 0.04 0.04 (Kelvin)
[0114] Values are in 14 bit units. The "std dev" row is the
root-mean-squared (RMS) noise of a uniform target after application
of frame averaging.
EXAMPLE 6
[0115] FIG. 8 shows results of an experiment designed to determine
the level of common-mode noise typical of an infrared camera. The
experiments were performed using the apparatus and plate of
Examples 2-5. The figure shows a typical raw data set after frame
averaging (high-frequency noise reduction) and area averaging, but
before common-mode noise reduction and offset subtraction.
EXAMPLE 7
[0116] FIG. 9 shows results of an experiment designed to assess the
ability of offset subtraction to extract data for a 135-.mu.W
reaction in a multiwell plate. The experiments were performed using
the apparatus and plate of Examples 2-6.
[0117] FIG. 9A shows raw data for an experiment using four sample
wells in which one well received a "sample" comprising a constant
135-.mu.W input, while the other wells received a benign (i.e.,
non-reactive) sample. The data show the average temperature in the
measurement region as a function of time, after image averaging is
applied.
[0118] FIG. 9B shows the same data after offset subtraction, which
adjusts the data so that each measurement starts at zero at time
zero. There is a residual common-mode noise of approximately
0.05.degree. C. PTP, after offset subtraction. This common-mode
noise may be out of phase and may shift depending on the geometric
position of the cell, as shown in FIG. 8.
EXAMPLE 8
[0119] FIG. 10 shows results of an experiment designed to assess
the ability of offset subtraction and the reduction of common-mode
noise using a reference region local to the measurement region to
extract data for the 135-.mu.W reaction of Example 7 and FIG. 9.
The reduction of common-mode noise reduces the noise level for the
benign wells to an RMS level of about 0.004.degree. C. The presence
and thermal profile of the 135-.mu.W reaction is clearly visible
relative to the benign wells. The data compare favorably to a
thermal model that calculates the theoretical effect of a 135-.mu.W
input on a sample holder having the same fluid volume. Without
using the measurement and noise-reduction methods described here,
the reaction resulting from a 135-.mu.W input could not be detected
using an infrared camera.
EXAMPLE 9
[0120] This example describes software for performing and/or
evaluating calorimetric measurements.
[0121] FIG. 11 shows a software screen for the display of thermal
data. The software screen may include one or more data presentation
fields. These fields may be used to presentat data using any
suitable form, including tables, graphs, and pseudo-images, among
others. The fields may include a single display that includes or
summarizes data from multiple samples, and/or multiple displays
that each include or summarize data from one or a subset of the
multiple samples. If there are multiple displays, they may be
arranged in a manner representative of the layout of the
corresponding samples, such as an 8.times.12 array of mini-graphs
corresponding to the 8.times.12 array of samples in a standard
96-well microplate. The data displayed in the software screens may
include a characteristic of the thermal radiation detected, such as
the amount, intensity, and/or spectrum of the radiation. The data
also may include a computed quantity related to a characteristic of
the thermal radiation detected, such as a temperature. The data
also may include kinetic data, such as temperature versus time
(denoted "tick"). The software screen also may include software
switches for selecting the scale of the display, for example,
X-axis and Y-axis scales for graphical data and color schemes for
pseudocolor images, among others.
[0122] FIGS. 12 and 13 show software screens for collecting,
displaying, an/or calibrating data relating to the measurement and
reference regions. The top screen shows the application screen for
collecting data using the circular reference around the perimeter
of the measurement region. The bottom screen shot shows the setup
window for defining the characteristics of the reference. These
screens again may include one or more data presentation fields and
one or more software switches, among others. Moreover, these
screens may permit recording and/or reporting of system parameters,
such as emissivity, object distance, ambient temperature, relative
humidity, noise reduction methods, and/or sample holder format,
among others.
EXAMPLE 10
[0123] This example further describes noise-reduction methods
provided by the invention.
[0124] The noise-reduction methods may include (1) converting
detected thermal infrared radiation to a signal, and (2) processing
the signal to reduce the proportion of the signal that is
attributable to noise. The step of processing the signal may
include a step of temporally averaging the signal comprising
computing a quantity based on distinguishable components of the
signal representing thermal infrared radiation detected from the
same sample at different times. Alternatively, or in addition, the
step of processing the signal may include a step of spatially
averaging the signal comprising computing a quantity based on
distinguishable components of the signal representing thermal
infrared radiation detected from different portions of the same
sample.
[0125] The noise-reduction methods also may include (1) detecting
thermal infrared radiation transmitted from a plurality of samples
contained in the sample wells using an optical device, (2)
converting the thermal infrared radiation detected from each sample
to a corresponding signal, and (3) adjusting the signals so that
each has the same preselected value at the same preselected time.
The preselected value may be zero, among others, and/or the
preselected time may be zero, among others.
[0126] The noise-reduction methods also may include (1) detecting
thermal infrared radiation transmitted from a reference region
adjacent a sample, and (2) constructing a sample signal
characteristic of the thermal infrared radiation detected from the
sample based on the thermal infrared radiation detected from the
sample and the adjacent reference region. The reference region may
comprise an annular portion of the sample plate distributed
adjacent a perimeter and/or about a central or optical axis of the
sample well.
[0127] The disclosure set forth above encompasses multiple distinct
inventions with independent utility. Although each of these
inventions has been disclosed in its preferred form(s), the
specific embodiments thereof as disclosed and illustrated herein
are not to be considered in a limiting sense, because numerous
variations are possible. The subject matter of the inventions
includes all novel and nonobvious combinations and subcombinations
of the various elements, features, functions, and/or properties
disclosed herein. The following claims particularly point out
certain combinations and subcombinations regarded as novel and
nonobvious and directed to one of the inventions. These claims may
refer to "an" element or "a first" element or the equivalent
thereof, such claims should be understood to include incorporation
of one or more such elements, neither requiring nor excluding two
or more such elements. Inventions embodied in other combinations
and subcombinations of features, functions, elements, and/or
properties may be claimed through amendment of the present claims
or through presentation of new claims in this or a related
application. Such claims, whether directed to a different invention
or to the same invention, and whether broader, narrower, equal, or
different in scope to the original claims, also are regarded as
included within the subject matter of the inventions of the present
disclosure.
* * * * *