U.S. patent application number 12/681022 was filed with the patent office on 2010-12-23 for method and system to measure the concentration of constituent elements in an inhomogeneous material using libs.
This patent application is currently assigned to INSTITUT NATIONAL D'OPTIQUE. Invention is credited to Alain Cournoyer, Marc Levesque.
Application Number | 20100324832 12/681022 |
Document ID | / |
Family ID | 40522968 |
Filed Date | 2010-12-23 |
United States Patent
Application |
20100324832 |
Kind Code |
A1 |
Levesque; Marc ; et
al. |
December 23, 2010 |
METHOD AND SYSTEM TO MEASURE THE CONCENTRATION OF CONSTITUENT
ELEMENTS IN AN INHOMOGENEOUS MATERIAL USING LIBS
Abstract
A system and method to improve the accuracy of the measure of
constituent element(s) in a sample containing domains potentially
including the constituent element(s) are described herein. For each
domain, the volume of the domain is estimated and the concentration
of the constituent element(s) in the domain is determined using
LIBS. When all the domains have been analyzed, the volumetric
concentration of the domains is summed and divided by the total
volume of the sample. Accordingly, by limiting the concentration
analysis to separate domains, it is possible to improve the
accuracy of the concentration analysis.
Inventors: |
Levesque; Marc;
(Saint-Augustin-de-Desmaures, CA) ; Cournoyer; Alain;
(Quebec, CA) |
Correspondence
Address: |
BCF LLP
1100 RENE'-LE'VESQUE BLVD. WEST, 25TH FLOOR
MONTREAL
QC
H3B-5C9
CA
|
Assignee: |
INSTITUT NATIONAL D'OPTIQUE
Sainte-Foy
QC
|
Family ID: |
40522968 |
Appl. No.: |
12/681022 |
Filed: |
April 4, 2008 |
PCT Filed: |
April 4, 2008 |
PCT NO: |
PCT/CA08/00648 |
371 Date: |
August 3, 2010 |
Current U.S.
Class: |
702/23 ;
356/318 |
Current CPC
Class: |
G01N 21/718
20130101 |
Class at
Publication: |
702/23 ;
356/318 |
International
Class: |
G06F 19/00 20060101
G06F019/00; G01J 3/30 20060101 G01J003/30 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 4, 2007 |
US |
11973009 |
Claims
1. A method for measuring the concentration of a constituent
element in an inhomogeneous sample having a relatively regular
surface, the method comprising: scanning at least a portion of the
relatively regular surface of the sample to obtain a plurality of
pixels representing the surface; grouping the pixels in a
predetermined number of classes according to a pixel
characteristic; grouping contiguous pixels belonging to the same
class into domains; and, for each domain: determining the surface
occupied by the domain; measuring, using LIBS, the concentration of
the constituent element in at least one measurement site of the
domain; determining a total quantity of the constituent element in
the domain; and, determining a concentration of the constituent
element of the sample by dividing an added total quantity of the
constituent element of each domain by a volume of the sample.
2. The concentration measurement method of claim 1, wherein the
pixel characteristic is selected from the group consisting of shade
of grey, color, fluorescence and intensity.
3. The concentration measurement method of claim 1, wherein the
regular surface is selected from the group consisting of a flat
surface and a cylindrical surface.
4. The concentration measurement method of claim 1, wherein the
concentration of the constituent element in the sample is
determined by: Concentration Sample = Domains ( Volume Domain
.times. Concentration Domain ) Volume Sample ##EQU00004## where
Volume Sample = Surface Sample * .sigma. ##EQU00004.2## Volume
Domain = Surface Domain * .sigma. ##EQU00004.3## .sigma. being a
thickness so determined that the boundary of each domain may be
assumed perpendicular to the relatively regular surface of the
sample.
5. A method for measuring the concentration of a constituent
element in an inhomogeneous sample having a relatively regular
surface, the method comprising: scanning at least a portion of the
relatively regular surface of the sample to obtain a plurality of
pixels representing the surface; grouping contiguous pixels having
the same pixel characteristic into domains; grouping domains
containing pixels having the same pixel characteristic into groups
of domains; determining the surface occupied by each group of
domains on the relatively regular surface; selecting a portion of
the scanned surface that includes at least one domain from each
group of domains; determining a measurement matrix having
equidistant measurement sites covering the selected portion of the
scanned surface; taking measurements, using LIBS, at every
measurement site; determining, for each group of domains, the
concentration of the constituent element in the group of domains by
considering the LIBS measurements taken in measurement sites
belonging to the group of domains; determining a total quantity of
constituent element of each group of domains; and, determining a
concentration of the constituent element of the sample by dividing
an added total quantity of the constituent element of each group of
domains by a volume of the sample.
6. The concentration measurement method of claim 5 wherein the
determination of the concentration of the constituent element in a
particular group of domains includes averaging the LIBS
measurements taken at measurement sites belonging to the particular
group of domains.
7. The concentration measurement method of claim 5, wherein the
distance between the equidistant measurement sites is slightly
smaller than the smallest of the domains.
8. The concentration measurement method of claim 5, wherein the
concentration of the constituent element in a group of domains is
determined by: Concentration Group = Domains ( Volume Domain
.times. Concentration Domain ) Volume Group ##EQU00005## where
Volume Domain = Surface Domain * .sigma. ##EQU00005.2## and Volume
Group = Surface Group * .sigma. ##EQU00005.3## .sigma. being a
predetermined thickness so determined that the boundary of each
domain are assumed perpendicular to the relatively regular surface
of the sample.
9. The concentration measurement method of claim 8, wherein the
concentration of the constituent element in the sample is
determined by: Concentration Sample = Group ( Volume Group .times.
Concentration Group ) Volume Sample ##EQU00006## where Volume
Sample = Surface Sample * .sigma. . ##EQU00006.2##
10. A method for measuring the concentration of a constituent
element in an inhomogeneous sample having a relatively regular
surface, the method comprising: determining a number, size and
position of domains on the relatively regular surface; determining
a surface occupied by each domain on the relatively regular
surface; and, for each domain, measuring, using LIBS, the
concentration of the constituent element in at least one
measurement site of the domain; determining a total quantity of
constituent element of each domain; and, determining the
concentration of the constituent element of the sample by dividing
an added total quantity of the constituent element of each domain
by a volume of the sample.
11. The concentration measurement method of claim 10, wherein the
concentration of the constituent element in the sample is
determined by: Concentration Sample = Domains ( Volume Domain
.times. Concentration Domain ) Volume Sample ##EQU00007## where
Volume Sample = Surface Sample * .sigma. ##EQU00007.2## Volume
Domain = Surface Domain * .sigma. ##EQU00007.3## .sigma. being a
thickness so determined that the boundary of each domain are may be
assumed perpendicular to the relatively regular surface of the
sample.
12. A system for measuring the concentration of a constituent
element in an inhomogeneous sample having a relatively regular
surface, the system comprising: a controller; a three-axis
positioning assembly configured to hold the sample and controlled
by the controller; a pulsed laser generally aimed at the relatively
regular surface of the sample and controlled by the controller; a
camera generally aimed at the relatively regular surface of the
sample and configured to supply a scanned image of at least a
portion of the relatively regular surface to the controller; a
plasma emission collection assembly configured to collect light
generated by a plasma on the relatively regular surface of the
sample; a spectrometer having an input configured to receive the
light collected by the plasma emission collection assembly; the
spectrometer having an output and being controlled by the
controller; an iCCD camera so mounted to the output of the
spectrometer as to receive an image therefrom and supply the image
to the controller; wherein the controller may control the
three-axis positioning assembly to place a desired measurement site
at the aimed position of the pulse laser, control the pulsed laser
as to generate a laser pulse that generates a plasma at the desired
measurement site, receive the spectrum from the iCCD camera and
determine the concentration of the constituent element from the
received spectrum and the controller being further configured as to
make such measurements at predetermined measurement sites
determined by examination of the scanned image and to calculate the
total concentration of the constituent element from the determined
concentrations of the constituent element at the predetermined
measurement sites.
13. A concentration measuring system as recited in claim 12 wherein
the controller is a computer provided with an input device and a
display.
14. A concentration measuring system as recited in claim 12,
further including a second camera aimed at the relatively regular
surface of the sample, the second camera being so configured as to
supply an image of the aimed position of the pulse laser to ensure
that the aimed position registers with the desired measurement
site.
15. A concentration measuring system as recited in claim 14,
further comprising a 50/50 beamsplitter to allow both cameras to
aim at the same position on the relatively regular surface of the
sample.
16. A concentration measuring system as recited in claim 12 wherein
the pulse laser is selected from the group consisting of short
pulse lasers and ultrashort pulse lasers.
17. A concentration measuring system as recited in claim 12,
wherein the pulse laser is so controlled as to emit two pulse with
a predetermined delay therebetween.
18. A concentration measuring system as recited in claim 12,
wherein the three-axis positioning assembly is so configured as to
define an enclosed space where an argon atmosphere may be
maintained.
19. A concentration measuring system as recited in claim 12,
wherein the three-axis positioning assembly is so configured as to
define an enclosed space where a low pressure atmosphere may be
maintained
20. A concentration measuring system as recited in claim 12,
wherein the plasma emission collection assembly includes first and
second parabolic mirrors and a fiber optic provided with proximate
and distal ends; the first parabolic mirror being so configured and
positioned as to collect plasma light from the surface of the
sample and the second minor being so configured and positioned as
to supply the collected plasma light to the proximate end of the
fiber optic; the distal end of the fiber optic being connected to
the input of the spectrometer to supply the collected plasma light
thereto.
21. A concentration measuring system as recited in claim 20,
wherein the optic fiber is a UV/VIS optic fiber.
22. A concentration measuring system as recited in claim 20,
wherein the plasma emission collection assembly further includes a
three-axis positioning assembly controlled by the controller.
23. A concentration measuring system as recited in claim 12,
wherein the spectrometer is an Echelle spectrometer.
24. A concentration measuring system as recited in claim 14,
further comprising a third camera aimed at the relatively regular
surface of the sample to supply plasma information to the
controller.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] None.
FIELD OF INVENTION
[0002] The present invention generally relates to Laser-Induced
Breakdown Spectroscopy (LIBS). More specifically, the present
invention is concerned with a method and system to improve the
precision of measured concentrations of constituent elements in
inhomogeneous materials using LIBS.
BACKGROUND OF THE INVENTION
[0003] Quantitative and real time analysis of the elementary
composition of materials is of great interest in many fields
including geological survey, industrial production, environment
diagnostics and on-line control of product quality, for
example.
[0004] The LIBS technique is often used for the quick analysis of a
sample's elementary constituents since it can be applied in situ
and give results in real time.
[0005] While the LIBS technique is believed well known in the art,
it will be summarized hereinbelow.
[0006] LIBS is a spark spectrochemical technique that uses a
short-pulsed laser (nanoseconds) or an ultrashort pulse laser
(picoseconds and femtoseconds) that is focused on a sample to
create a microplasma near the surface thereof. The microplasma is a
transient event having a peak temperature reaching 10,000 to 20,000
K.
[0007] In this environment, a portion of the sample is converted
into plasma and the chemical bonds are broken to produce
electronically excited atoms and ions. These excited species give
off resonant and sharp radiation at specific wavelengths that
depend on the constituent element.
[0008] By analysing the light emitted by the microplasma within a
narrow range (generally from about 200 to about 980 nm) it is
possible to identify the constituent elements by their specific
emission wavelengths and to measure the concentration of the
identified constituent elements by measuring the intensity of the
light at their specific emission wavelengths.
[0009] LIBS may be considered a real-time procedure since its
response time is generally less than a second.
[0010] The LIDS event generates a tremendous amount of data and,
interestingly, virtually every laser shot produces a usable
spectrum. Furthermore, LIBS is very good at analysing small
particles.
[0011] LIBS can operate at atmospheric pressure while producing
useful plasma emission intensities. The actual plasma emission is
generally characterized both by a continuum spectrum (generally
referred to as bremsstrahlung emission) and by discrete emission
lines.
[0012] The continuum emission and the discrete emission, from both
atoms and ions, decay at different rates. While the continuum
emission decays usually within a few microseconds, the discrete
emission persists strongly for tens of microseconds. The discrete
plasma emission can, therefore, be resolved both spectrally and
temporally to yield spectra containing the atomic emission lines
corresponding to the atoms present in the plasma volume.
[0013] It is generally known that many variables can negatively
influence the precision of LIDS measurements. Some of these
variables such as the laser properties (wavelength, pulse duration,
focusing spot size, etc.) and the detection window (delay time and
gate width) can be taken into consideration when the measurements
are taken. However, the physical properties of the sample are more
difficult to take into account. This problem called "matrix
effects" is well known in the art and is a factor that limits LIBS
accuracy. Accordingly, the precision of LIBS measurement are
generally relatively poor for many applications.
[0014] There is therefore a need for improvements in methods and
systems for LIBS measurements.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] In the appended drawings:
[0016] FIG. 1 is a schematic view of a concentration measuring
system according to an illustrative embodiment of the present
invention;
[0017] FIG. 2 is a schematic side view of a portion of the
concentration measuring system of FIG. 1;
[0018] FIG. 3 is a block diagram of a first illustrative method for
measuring the concentration of a constituent element according to
an aspect of the present invention;
[0019] FIG. 4 is a block diagram of a second illustrative method
for measuring the concentration of a constituent element according
to an aspect of the present invention;
[0020] FIG. 5 is a block diagram of a third illustrative method for
measuring the concentration of a constituent element according to
an aspect of the present invention; and
[0021] FIGS. 6A-6C are images illustrating the analysis portion of
the measuring method.
DETAILED DESCRIPTION
[0022] In accordance with an illustrative embodiment of the present
invention, there is provided a method for measuring the
concentration of a constituent element in an inhomogeneous sample
having a relatively regular surface, the method comprising:
[0023] scanning at least a portion the relatively regular surface
of the sample to obtain a plurality of pixels representing the
surface;
[0024] grouping the pixels in a predetermined number of classes
according to a pixel characteristic;
[0025] grouping contiguous pixels belonging to the same class into
domains; for each domain: [0026] determining the surface occupied
by the domain; measuring, using LIBS, the concentration of the
constituent element in at least one measurement site of the domain;
[0027] determining the total quantity of the constituent element in
the domain;
[0028] determining the concentration of the constituent element of
the sample by dividing the added total quantity of the constituent
element of each domain by the volume of the sample.
[0029] According to another illustrative aspect of the present
invention, there is provided a method for measuring the
concentration of a constituent element in an inhomogeneous sample
having a relatively regular surface, the method comprising:
[0030] scanning at least a portion of the relatively regular
surface of the sample to obtain a plurality of pixels representing
the surface;
[0031] grouping contiguous pixels having the same pixel
characteristic into domains;
[0032] grouping domains containing pixels having the same pixel
characteristic into group of domains;
[0033] determining the surface occupied by each group of domains on
the relatively regular surface;
[0034] selecting a portion of the scanned surface that includes at
least one domain from each group of domains;
[0035] determining a measurement matrix having equidistant
measurement sites covering the selected portion of the scanned
image;
[0036] taking measurements, using LIBS, at every measurement
sites;
[0037] determining, for each group of domains, the concentration of
the constituent element in the group of domains by considering the
LIBS measurements taken in measurement sites belonging to the group
of domains;
[0038] determining the total quantity of constituent element of
each group of domains;
[0039] determining the concentration of the constituent element of
the sample by dividing the added total quantity of the constituent
element of each group of domains by the volume of the sample.
[0040] According to another illustrative aspect of the present
invention, there is provided a method for measuring the
concentration of a constituent element in an inhomogeneous sample
having a relatively regular surface, the method comprising:
[0041] determining the number, size and position of the domains on
the relatively regular surface;
[0042] determining the surface occupied by each domain on the
relatively regular surface;
[0043] for each domain, measuring, using LIBS, the concentration of
the constituent element in at least one measurement site of the
domain;
[0044] determining the total quantity of constituent element of
each domain;
[0045] determining the concentration of the constituent element of
the sample by dividing the added total quantity of the constituent
element of each domain by the volume of the sample.
[0046] According to yet another illustrative aspect of the present
invention, there is provided a system for measuring the
concentration of a constituent element in an inhomogeneous sample
having a relatively regular surface, the system comprising:
[0047] a controller;
[0048] a three-axis positioning assembly so configured as to hold
the sample; the three-axis positioning assembly being controlled by
the controller;
[0049] a pulsed laser generally aimed at the relatively regular
surface of the sample, the pulse laser being controlled by the
controller;
[0050] a camera generally aimed at the relatively regular surface
of the sample; the camera being so configured as to supply a
scanned image of at least a portion of the relatively regular
surface to the controller;
[0051] a plasma emission collection assembly so configured as to
collect light generated by a plasma on the relatively regular
surface of the sample;
[0052] a spectrometer having an input so configured as to receive
the light collected by the plasma emission collection assembly; the
spectrometer having an output; the spectrometer being controlled by
the controller;
[0053] an iCCD camera so mounted to the output of the spectrometer
as to receive an image therefrom; the iCCD camera being so
configured as to supply the image to the controller;
[0054] wherein the controller may control the three-axis
positioning assembly so as to place a desired measurement site at
the aimed position of the pulse laser, control the pulsed laser as
to generate a laser pulse that generates a plasma at the desired
measurement site, receive the spectrum from the iCCD camera and
determine the concentration of the constituent element from the
received spectrum; the controller being further configured as to
make such measurements at predetermined measurement sites
determined by examination of the scanned image and to calculate the
total concentration of the constituent element from the determined
concentrations of the constituent element at the predetermined
measurement sites.
[0055] It is to be noted that the term "domain" is to be construed,
herein and in the appended claims, as any finite three-dimensional
portion of a body that is statistically homogeneous on the scale of
the domain.
[0056] It is also to be noted that the expression "inhomogeneous
material" is to be construed, herein and in the appended claims, as
any solid material that contains at least one domain of generally
homogeneous material that is different from other portions of the
solid material.
[0057] It is also to be noted that the term "sample" is to be
construed, herein and in the appended claims, as the volume of
material for which a measurements are actually taken.
[0058] It is also to be noted that the present description refers
to different documents. These documents are hereby included herein
by reference.
[0059] Other objects, advantages and features of the present
invention will become more apparent upon reading of the following
non-restrictive description of illustrative embodiments thereof,
given by way of example only with reference to the accompanying
drawings.
[0060] Generally stated, illustrative embodiments of the present
invention concern a system and method to improve the accuracy of
the measurement of constituent elements in a sample containing
discrete domains of homogeneous material potentially including the
constituent elements. In specific embodiments of the present
invention, each domain is considered as being part of a group
determined according to characteristics of the domain. For each
group, the concentration of the constituent elements is determined
using LIBS. When all the groups of the sample have been analyzed,
the quantity of the constituent elements of the groups is summed
and divided by the total volume of the sample to yield the
concentration of the constituent elements in the sample.
Accordingly, by limiting the concentration analysis to various
discrete domains of the sample, it is possible to improve the
accuracy of the concentration analysis.
[0061] Turning now to FIG. 1 of the appended drawings a LIBS system
20 for the determination of the concentration of the constituents
of a sample according to an illustrative embodiment of the
invention will be described.
[0062] The LIBS system 20 includes a three-axis positioning
assembly 22 designed to removably hold a sample 24, a pulsed laser
26 which is generally aimed at the sample 24, a first camera 30
provided with a 0.25 zoom 32, a second camera 34 provided with a
4.times. zoom 36. Both cameras 30 and 34 are aimed at the sample 24
via a 50/50 beamsplitter 38. The LIBS system 20 also includes a
plasma emission collection assembly 40 supplying light generated by
plasma to a spectrometer 42 provided with an iCCD (intensified
Charged-Coupled-Device) camera 43 at its output.
[0063] The three-axis positioning assembly 22, the laser 26, the
cameras 30 and 34 and the spectrometer 42 are controlled by a
controller 50 provided with a display 52 and an input device 54.
For clarity purpose, the interconnections between these various
elements and the controller 50 have not been illustrated in FIG.
1.
[0064] The three-axis positioning assembly 22 is so controlled by
the controller 50 that it may position a desired portion of the
sample 24 in the line of sight of the pulsed laser 26 for scanning
reasons or for LIBS measurement reasons as will be described
hereinbelow. As a non-limitating example, it has been found that a
three-axis positioning assembly manufactured by Physik Instrumente
model M-126.DG is adequate for the present application.
[0065] The pulsed laser 26 is used to create a plasma for LIBS
measurement as is generally known in the art.
[0066] The pulsed laser 26 may be a short pulse laser (nanoseconds)
or an ultrashort pulse laser (femtoseconds and picoseconds). It has
been found advantageous to use a ultrashort pulse laser for many
reasons: [0067] The plasma generated is more reproducible and the
resonant and sharp radiation at specific wavelengths that depend on
the constituent element is more stable, improving the precision and
the sensitivity of the measure; [0068] Since there is less power
involved in the plasma, the size of the plasma is decreased, which
limits the effects of optical self-absorption of the plasma to
thereby improve the precision of the concentration measurement;
[0069] The emission continuum is weaker and decreases more rapidly;
[0070] Since it is less of a thermal reaction, the "matrix effects"
are decreased which leads to increase precision; [0071] Since the
heat-affected zone is smaller, it is possible to take LIBS
measurement closer to one another; and, [0072] It can be applied to
more material since it is less sensitive to the nature of the
material.
[0073] As a non limitating example, it has been found that the
ultrashort pulsed laser manufactured by Quantronix under model
Integra C 1.0 is adequate to be used as the pulsed laser 26.
[0074] The camera 30, provided with a 0.25.times. zoom 32 is used
to scan the prepared surface of the sample 24 as will be described
hereinbelow. The controller 50 therefore controls the camera 30 and
the positioning assembly 22 accordingly. The data from the camera
30 is transferred to the controller 50 and may be displayed on the
display 52. The camera 30 may be a monochrome, a color or a
hyperspectral camera. As a non limitating example, it has been
found that the camera 30 could be a CCD camera manufactured by Sony
under model number DFW-SX910. It is to be noted that other types of
camera could be used.
[0075] The camera 34, provided with a 4.times. zoom 36, is used to
ensure that the correct location, as determined by the controller
50 or by the user, is aimed at by the pulsed laser 26 for the LIBS
measurement as will be described hereinbelow. The camera 34 is
similar to the camera 30.
[0076] The plasma emission collection assembly 40 is designed to
transmit the light generated by the pulsed laser 26 emission onto
the sample 24 to the spectrometer 42. The collection assembly 40
should be as efficient as possible in the entire spectrum
considered while being substantially free of chromatic aberration.
It is well known that reflective optics is well suited to this
task. The plasma emission collection assembly 40 includes two
parabolic mirrors 44 and 46 and a UV/VIS (Ultraviolet-Visible)
optic fiber 48 connecting the collection assembly 40 to the
spectrometer 42. The parabolic mirrors 44 and 46 are mounted
off-axis. The mirror 44 collects the light emitted by the plasma
created on the sample by the laser 26 and the mirror 46 focalizes
the collected light onto the optic fiber 48. To adequately position
the plasma emission collection assembly 40, a three axis
positioning assembly (not shown) is provided. It is therefore
possible to move the collection assembly depending on the position
of the plasma that the user desires to measure. Of course, the
three axis positioning assembly may be connected to the controller
50 to be controlled thereby.
[0077] As a non limitating example, it has been found that the
parabolic mirrors manufactured by Edmund under model number M47-087
are adequate to be used as the mirrors 44 and 46 and that the
UVIVIS optic fiber supplied by Ocean Optics under model number
QP50-2-UV-VIS is adequate to be used as the optic fiber 48.
[0078] The spectrometer 42 is used to make the LIBS measurements
and it receives the plasma generated light from the optic fiber 48.
To improve accuracy, it is interesting to use a spectrometer that
has a wide bandwidth so as to acquire the entire spectrum of
elements at the same time. Accordingly an Echelle spectrometer is
interesting. While many spectrometers can be used, it has been
found that the spectrometer manufactured by Andor under model
number Mechelle 5000 is adequate to be used as the spectrometer
42.
[0079] The iCCD camera 43 is mounted to the output of the
spectrometer 42. It is interesting to use time-gating to delay the
beginning of the signal acquisition and to circumscribe an
acquisition window to thereby limit the influence of the emission
continuum which decreases with time. It is however to be noted that
the use of an ultra-short pulse laser as the ablation source
creates an emission continuum that influences less the LIBS
measurements. Accordingly, the time-gating might be optional and
the iCCD camera 43 could be replaced by a conventional camera (not
shown).
[0080] As can be better seen from FIG. 2, which is a side view of a
portion of the system 20 of FIG. 1, the system 20 also includes a
camera 56 provided with a 10.times. zoom 58 and an illumination
assembly 60. The camera 56 is used to obtain an image of the plasma
generated on the sample 24 and to supply this image to the
controller 50 to allow the controller to localize the plasma,
determine the dimensions, shape and other characteristics of the
plasma and to obtain an energy density profile thereof. To do this,
the camera 56 is synchronized with the pulse laser 26. The camera
56 may be a monochrome or color camera and is mounted away from the
plasma to limit the number of elements in the proximity of the
sample and to protect the lens from the generated plasma. The data
obtained is used to optimize the system 20 set-up, for example by
calibrating the positioning assembly 22 and by helping during the
alignment of the different optical elements.
[0081] The camera 56 is mounted to a three-axis positioning
assembly (not shown) to allow the adequate positioning and focus of
the camera with respect to the plasma. Of course, the three-axis
positioning assembly may be connected to the controller 50 to be
controlled thereby.
[0082] It is to be noted that the relative energy density emitted
by the plasma gives an approximation of the temperature of the
plasma and that the dimensions of the plasma gives an approximation
of the weight of the matter that is ablated by the plasma. These
data could optionally be used to normalize the emission spectrum of
the plasma to allow the correction of the LIBS measurement
characteristics between laser pulses.
[0083] It is to be noted that the use of the camera 56 is optional
should it be only used to determine the size of the plasma and to
optimize the set-up system 20. Indeed, this information could be
determined during a set-up phase of the system 20 and not be
required afterwards.
[0084] It has been found that a CCD camera manufactured by Pixelink
under model number PL-B781F is adequate to be used as the camera
56. Of course, other types of camera could be used.
[0085] The controller 50 may be a computer provided with an
adequate acquisition card.
[0086] Generally stated, methods according to illustrative
embodiments of the present invention are concerned with the
measurement of the concentration of constituent element(s) in
discrete domains and, by estimating or measuring the volume of the
discrete domains in the sample, determining the concentration of
the constituent element(s) for the entire sample.
[0087] Turning now to FIG. 3 of the appended drawings, a LIBS
measurement method 70 according to an illustrative embodiment of an
aspect of the present invention will be described.
[0088] The first step 72 of the method 70 generally consists in
preparing the sample. The sample 24 may be, for example, a surface
rock or a core sample. The preparation of the sample 24 includes
the creation of a regular surface thereon and the polishing of this
surface. This regular surface will be preferably flat or
cylindrical. For example, a diamond saw can be use to create the
regular surface and emery paper can be used to polish the regular
surface to yield a prepared surface.
[0089] The determination of the size and position of the various
domains present on the prepared surface is done in step 74. As
mentioned hereinabove, a domain is a portion of the sample that is
contiguous and has a homogeneous composition of components. This
determination can be done manually, for example by a geologist
using a scanned image of the surface where the domains are manually
selected so as to cover the entire surface of the prepared surface.
It is to be noted that the domains are so determined that any point
on the prepared surface is part of a single domain.
[0090] Next, in step 76, the concentration of the constituent
elements is determined, using LIBS, for each domain determined in
step 74.
[0091] It is to be noted that many LIBS technique can be used to
make the LIBS measurement. For example, a Calibration Free LIBS
(CF-LIBS) technique developed by Ciucci et al. ["New procedure for
quantitative elemental analysis by laser-induced plasma
spectroscopy", Applied Spectroscopy, vol 53, no 8, 1999, pp.
960-964] can be used.
[0092] Once all the LIBS measurements have been taken and the
concentration results for each domain determined in step 74 are
determined by the controller, the method 70 then statistically
determines the concentration of the constituent elements for the
entire sample in step 78.
[0093] To achieve this, the volume occupied by each domain must be
determined. First, the surface area of each domain is first
determined, for example by using a scanned image. Since the LIBS
technique is generally a surface technique, i.e. creating craters
having depth that range from about 1 to about 100 micrometers, it
may be considered that the boundaries of each domain are
perpendicular to the prepared surface of the sample for these small
depths. Accordingly, the volume occupied by a domain may be
determined as follows:
Volume.sub.Domain=Surface.sub.Domain*.sigma.
[0094] Where .sigma. is a predetermined thickness in the micrometer
range.
[0095] The total volume of the sample for further calculations is
determined as follows:
Volume.sub.sample=Surface.sub.sample*.sigma.
[0096] Accordingly, the total concentration for the entire sample
is given by:
Concentration Sample = Domain ( Volume Domain .times. Concentration
Domain ) Volume Sample ##EQU00001##
[0097] Where Concentration.sub.Domain is the concentration of a
constituent element in a particular domain. In other words, the
quantity of the constituent element in each domain is calculated
and summed and the result is divided by the volume of the sample to
yield the concentration of the constituent element in the
sample.
[0098] Turning now to FIG. 4 of the appended drawings, a LIBS
measurement method 100 according to an illustrative embodiment of
an aspect of the present invention will be described. It is to be
noted that while the method 100 will be described with reference to
the elements of the system 20 illustrated in FIGS. 1 and 2, other
systems (not shown) could be designed or assembled to carry out
this illustrative method 100.
[0099] The first step 102 is similar to the sample preparing step
72 described hereinabove and generally consists in preparing the
sample for LIBS. Again, the sample 24 may be, for example, a
surface rock or a core sample.
[0100] Once the prepared surface is ready, the sample 24 may be
mounted to the three-axis positioning assembly 22 so that the
prepared surface faces the camera 34.
[0101] The next step 102 is to scan an image of at least a portion
of the prepared surface of the sample 24 via the camera 30. If a
small enough portion of the surface is to be scanned, it can be
done without displacement of the three-axis positioning assembly 22
and sent to the controller 50. If a larger portion of the prepared
surface has to be scanned, multiple contiguous images of the
surface can be obtained by the camera 30 while the three-axis
positioning assembly 22 is moved by the controller 50. The
controller 50 may then assemble the multiple images according to
known techniques to obtain an image of the desired portion of the
prepared surface. FIG. 6A illustrates a grey-level scan of a
portion of a sample.
[0102] The scanned image of the prepared surface is then analyzed
in step 106 to determine the number, size and position of the
discrete domains. As mentioned hereinabove, a domain is a portion
of the sample that is contiguous and has a generally homogeneous
composition of components. It has been assumed herein that a finite
contiguous portion of the surface that appears homogeneous on the
scanned image is made of a composition of elements that is also
homogeneous and therefore constitutes a domain.
[0103] Accordingly, according to known image analysis techniques,
the analysis of the scanned image includes the grouping of pixels
of this image that are generally homogeneous and contiguous into
domains. Since there is a large number of possible grey levels that
a pixel may take, all the pixels of the scanned image are divided
among a given number of "classes". Pixels of the same class are
considered to have similar characteristics. When pixels of the same
class are contiguous and create a finite portion of the image, they
define a domain. If many different domains are made of pixels that
belong to the same class, these domains are considered from the
same domain group and the composition of those domains may be
considered to be the same.
[0104] FIG. 6B illustrates the analysis of the scanned image of
FIG. 6A when the given number of classes used in the image analysis
is two (2). As can be seen from the comparison of FIGS. 6A and 6B,
many pixels that appear different to the human eye have been
considered in the same class by the image analysis.
[0105] In sharp contrast, FIG. 6C illustrates the analysis of the
same scanned image of FIG. 6A when the given number of classes is
seven (7). As can be seen from this figure, the variations of the
appearance of the pixels in the scanned image are well reproduced
in the analyzed image.
[0106] It is also to be noted that the scanned image analysis can
be done solely by the controller running an appropriate program or
can be done by the controller in collaboration with an operator
having geological experience. For example, supervised
classification using the Hidden Markov Model could be used. The
Hidden Markov Model is explained in "Hidden Markov Models:
Estimation and Control (Stochastic Modelling and Applied
Probability)", Robert J. Elliot et al., Publisher: Springer; 1
edition (Jan. 9, 1997), ISBN-10: 0387943641, ISBN-13:
978-0387943640.
[0107] Alternatively, the scan image analysis could be done solely
by an operator.
[0108] Of course, while the description of the scanned image
analysis hereinabove is concerned with the grey level as a means to
determine the visual homogeneity of pixels, and thereby determine
the class to which each pixel belongs to, other features such as
the intensity, the color or the fluorescence of the pixel could be
used, depending on the technology used for the camera 30. Of
course, other features of the pixel could be used.
[0109] Once the number, size and position of the domains are
obtained, the LIBS measurement (step 108) may take place. For each
group of domains, i.e. domains having material that have the same
characteristics, one could decide a) to take one LIBS measurement
in a domain of the group, b) to take many measurement in one
particular domain of this group to determine mean concentration
values for this class, c) to take many measurements from different
domains of this group to determine mean concentration values for
this class, d) to take a measurement for each domain of this group
to determine mean concentration values for this class. The
precision of the results and the speed at which results must be
obtain dictate the number of measurements to be taken.
[0110] For each LIBS measurement, the three-axis positioning system
22 positions the desired portion of the sample in the correct
position for the creation of the plasma in the domain of
interest.
[0111] Once the correct positioning is confirmed by the camera 34,
the pulsed laser 26 generates the plasma and the light collected by
the collection system 40 is transferred to the spectrometer 42 and
the iCCD camera 43 supplies this data to the controller 50.
[0112] Again, it is to be noted that many LIBS technique can be
used to make the LIBS measurement. For example, a Calibration Free
LIBS (CF-LIBS) technique developed by Ciucci et al. ["New procedure
for quantitative elemental analysis by laser-induced plasma
spectroscopy", Applied Spectroscopy, vol 53, no 8, 1999, pp.
960-964] can be used.
[0113] One skilled in the art will understand that the LIBS
measurements, including the correct positioning of the sample prior
to the plasma generation, may be automated by the controller
50.
[0114] Once all the LIBS measurements have been taken and the
concentration results for each group of domains are determined by
the controller, the method 100 then statistically determines the
concentration of the constituent elements for the entire sample in
step 110.
[0115] To achieve this, the volume occupied by each group of
domains must be determined. First, the surface area of each group
is first determined using the scanned image as analyzed in step
106. More specifically, the surfaces of the domains of a particular
group are added. Since the LIBS technique is generally a surface
technique, i.e. creating craters having depth that range from about
1 to about 100 micrometers, it may be considered that the
boundaries of each domain are perpendicular to the prepared surface
of the sample for these small depths. Accordingly, the volume
occupied by a group of domains may be determined as follows:
Volume.sub.Group=Surface.sub.Group*.sigma.
[0116] Where .sigma. is a predetermined thickness in the micrometer
range.
[0117] The total volume of the sample for further calculations is
determined as follows:
Volume.sub.sampie=Surface.sub.sample*.sigma.
[0118] Accordingly, the total concentration of an element for the
entire sample is given by:
Concentration Sample = Group ( Volume Group .times. Concentration
Group ) Volume Sample ##EQU00002##
[0119] Where Concentration.sub.Group is the concentration of a
constituent element in the particular group. In other words, the
quantity of the constituent element in each group is calculated and
summed and the result is divided by the volume of the sample to
yield the concentration of the constituent element in the
sample.
[0120] As mentioned hereinabove, the Concentration.sub.group value
may be determined by a single LIBS measurement of a particular
domain of a group or by many LIBS measurements of the same or
different domains of the group. If, for example, all the domains of
a group are sampled to obtain a mean value of the concentration,
the following equation may be used:
Concentration Group = domains ( Volume domain .times. Concentration
domain ) Volume Group ##EQU00003##
[0121] Turning now to FIG. 5 of the appended drawings, a method 200
according to another illustrative embodiment of the present
invention will be described. It is to be noted that since some of
the steps are identical to the steps of the method 100 described
hereinabove, only the different steps will be described
hereinbelow.
[0122] The surface of the sample is prepared (step 202), scanned
(step 204) and analyzed to determine the number size and position
of the domains (step 206) as it is done in the method 100.
[0123] Then, at step 208, a portion of the analyzed scanned image
is selected. This section advantageously contains at least one
domain of each group of domains present in the sample.
[0124] A LIBS measurement matrix is then determined in step 210.
This matrix defines equidistant measurement sites. The distance
between the measurement sites is such that each domain contained in
the selected portion includes a measurement site.
[0125] Step 212 consists in sequentially taking LIBS measurements
at every measurement sites defined in step 210. It is to be noted
that this can be done automatically via the controller that
controls the three-axis positioning system.
[0126] To determine the concentration of the constituent elements
in the selected portion of the sample, the controller determines to
which domain, therefore to which group of domains, each LIBS
measurement belongs. Then, the concentration calculation may take
place as described hereinabove in step 110 where the selected
portion of the sample is considered the entire sample.
[0127] The results thus obtained for the selected portion may then
be extended to the entire scanned surface of the sample.
[0128] It is also possible, in step 214 to obtain the concentration
of the constituent element separately for each group of domains and
to then generalize the results for the entire sample surface.
[0129] The use of systems and methods according to illustrative
embodiments of the present invention has many advantages, such as:
[0130] In the geological survey for the mining industry, it is
possible to obtain more accurate results, particularly when the
concentration of the constituent element is low and contained in a
few domains since only a small number of localized LIES
measurements are then taken; [0131] It is possible to analyze
simultaneously all the elements of the periodic chart; and [0132]
The technique does not require a complicated sample preparation
procedure.
[0133] Many modifications can optionally be done to the system and
method to reinforce the LIBS signal to thereby improve the
sensibility of the LIBS measurement. Example of these modifications
include: [0134] Placing the sample in an Argon gas environment;
[0135] Reducing the gas pressure in the vicinity of the sample;
[0136] Configuring the pulsed laser so as to generate two pulses at
very close intervals (microseconds); the first laser pulse causes a
shockwave that reduces the gas pressure at the plasma site and the
second laser pulse can therefore generate a more intense plasma in
the lower pressure atmosphere; [0137] Using microwaves to increase
the temperature at the plasma site; [0138] Configuring the pulse
laser so as to generate two pulses at extremely close intervals
(hundreds of picoseconds); the temperature of the plasma is then
higher since the second pulse heats the plasma produced by the
first laser pulse
[0139] It is to be noted that the above mentioned modifications are
not required to obtain good concentration results. It is also to be
noted that the sample and the three-axis positioning assembly
should be somehow enclosed should one decide to provide an argon or
low pressure atmosphere.
[0140] One skilled in the art will understand that while the above
description of illustrative embodiments of the present invention
has been described hereinabove in a geological survey environment,
this should not be construed as to limit the present invention.
Indeed, industrial production, environment diagnostics and on-line
control of product quality, for example, could benefit from
embodiments of the present invention.
[0141] It is to be understood that the invention is not limited in
its application to the details of construction and parts
illustrated in the accompanying drawings and described hereinabove.
The invention is capable of other embodiments and of being
practiced in various ways. It is also to be understood that the
phraseology or terminology used herein is for the purpose of
description and not limitation. Hence, although the present
invention has been described hereinabove by way of illustrative
embodiments thereof, it can be modified, without departing from the
spirit, scope and nature of the subject invention as defined in the
appended claims.
* * * * *