U.S. patent number 3,896,661 [Application Number 05/431,938] was granted by the patent office on 1975-07-29 for method of coupling thin layer chromatograph with mass spectrometer.
This patent grant is currently assigned to Stanford Research Institute. Invention is credited to James H. McReynolds, Robert M. Parkhurst.
United States Patent |
3,896,661 |
Parkhurst , et al. |
July 29, 1975 |
Method of coupling thin layer chromatograph with mass
spectrometer
Abstract
A method of analyzing a mixture of chemical substance in which a
thin layer chromatograph is formed by placing a quantity of the
mixture or a solution of the mixture at one end of a
chromatographic medium of a sorptive material and through
differential migration of the various chemical substances in the
mixture separated zones of absorbed chemical substances are created
on the chromatographic medium. The chromatographic medium itself is
then placed in a vacuum immediately adjacent the ion source of a
mass spectrometer. The various zones of absorbed chemical
substances are then heated one at a time. As each zone is heated
the absorbed chemical substance sublimes directly into the ion
source of the mass spectrometer, and a mass spectrograph of that
particular zone of chemical substance is formed.
Inventors: |
Parkhurst; Robert M. (Redwood
City, CA), McReynolds; James H. (Palo Alto, CA) |
Assignee: |
Stanford Research Institute
(Menlo Park, CA)
|
Family
ID: |
23714064 |
Appl.
No.: |
05/431,938 |
Filed: |
January 9, 1974 |
Current U.S.
Class: |
73/61.54;
250/288; 250/425 |
Current CPC
Class: |
G01N
30/95 (20130101); H01J 49/16 (20130101) |
Current International
Class: |
G01N
30/95 (20060101); G01N 30/00 (20060101); H01J
49/10 (20060101); H01J 49/16 (20060101); H01j
039/34 (); G01h 031/08 () |
Field of
Search: |
;73/23.1,61.1C,422GC
;250/288,425 ;210/31C,198 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Swisher; S. Clement
Claims
What is claimed is:
1. A method for analyzing a mixture of chemical substances
comprising the steps of forming a thin layer chromatograph of the
mixture whereby differential migration of substances in the mixture
forms a succession of absorbed zones of the substances along a
chromatograph medium to define the thin layer chromatograph,
positioning the thus formed thin layer chromatograph in relatively
high vacuum adjacent to and communicating with the ion source of a
mass spectrometer, successively heating each of the absorbed zones
whereby as each zone is heated the absorbed substance sublimes
directly into the ion source, and forming a mass spectrograph of
each of the thus subliming substances.
2. A method in accordance with claim 1 wherein selective heating of
the absorbed zones is accomplished by selectively directing a beam
of light upon each of said absorbed zones.
3. A method in accordance with claim 1 wherein selective heating of
the absorbed zones is accomplished by providing an electric
resistance heater element adjacent each of the absorbed zones and
passing current through the heater element.
4. A method in accordance with claim 1 wherein the thin layer
chromatograph is formed on the inside surface of a cylindrical
support which is open at both ends with one of its ends adjacent
the ion source, and wherein selective heating of each of the
absorbed zones is accomplished by passing an electrical resistance
heating element down the length of the cylindrical support on its
exterior, and including the step of providing a flow of an inert
gas into the opposite end of the cylindrical support to carry
subliming zones of substances into the ion source.
5. A method in accordance with claim 1 wherein a support member has
electrical resistance heating elements formed thereon, with a
chromatographic sorptive medium formed on top of a portion of the
electrical resistance heating elements and the thin layer
chromatograph formed along the sorptive medium, and wherein
successive heating of the absorbed zones is accomplished by
successively passing an electric current through the resistance
heating elements underneath each of the absorbed zones.
6. A method in accordance with claim 5 including mechanical
advancing means to successively displace the thin layer
chromatograph with respect to the ion source so as to successively
position each of the absorbed zones in close proximity to the ion
source.
Description
BACKGROUND OF THE INVENTION
This invention pertains to a method for analyzing mixtures of
chemical substances and in particular pertains to such a method
involving a combination of thin layer chromatography and mass
spectroscopy.
In the field of analysis of complicated chemical mixtures, there
have been methods developed in which different structure-finding
techniques have been coupled together. As an example, there are
described in an article entitled "TLC In Direct Coupling with GC
and MS", Kaiser, R., Chemistry in Britain, 5(No. 2) 54 (1969),
various proposed methods for coupling together analytical
techniques. One method involves the coupling together of gas
chromatography with a mass spectrometer. The use of gas
chromatography requires that the compounds to be separated be
volatile at a temperature suitable for gas chromatography. Certain
complicated organic mixtures such as natural products, drug
metabolites and clinical samples are not sufficiently volatile to
be separated by gas chromatography. Furthermore, the use of gas
chromatography directly coupled to a mass spectrometer requires the
use of gas separators (helium separates, for example) to remove the
large amounts of carrier gas. The article referred to above also
has proposed a combination of thin layer chromatography with gas
chromatography and a mass spectrometer. In this arrangement a thin
layer chromatography is formed on a plate. A flame torch is
utilized on the back of the thin layer chromatograph plate which is
not in close proximity to the ion source to successively evaporate
the zones thereon. These zones are successively evaporated at
atmospheric pressure into a nitrogen or helium gas stream for flow
into a mass spectrometer. Again, this technique requires that the
compounds forming the various zones have a rather high vapor
pressure since they must evaporate into the gas stream.
These techniques in the prior art have been somewhat deficient in
not being applicable to complex non-volatile mixtures which require
analytical determination.
OBJECTS AND SUMMARY OF THE INVENTION
Accordingly, it is an object of this invention to provide a method
for analyzing complex non-volatile mixtures.
It is another object of this invention to provide a method for
analyzing complex mixtures which directly couples thin layer
chromatography with mass spectroscopy.
Briefly, in accordance with one embodiment of the invention, a thin
layer chromatograph of the mixture to be analyzed is formed wherein
differential migration of substances in the mixture forms
physically separate absorbed zones of the substances on the
chromatographic medium. The thus formed thin layer chromatograph is
then placed in a vacuum in close proximity to an ion source of a
mass spectrometer. The physically separated zones of the thin layer
chromatograph are then heated, one at a time, to sublime the
substance in a zone directly into the ion source of the mass
spectrometer. A mass spectrograph is then formed of each of the
substances .
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagrammatic illustration of one means of practising
the invention by directly combining a thin layer chromatograph
adjacent to the ion source of a mass spectrometer.
FIG. 2 is a schematic illustration of one embodiment of a thin
layer chromatograph in accordance with the invention adjacent to an
ion source of a mass spectrometer.
FIG. 3 is similar to FIG. 2 and shows another embodiment of a thin
layer chromatograph.
FIG. 3a is an end view of the thin layer chromatograph of FIG.
3.
FIGS. 4 and 5 are illustrations of still other embodiments of thin
layer chromatographs for practising the method of the
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The basis of this invention is a method of coupling thin layer
chromatography with a mass spectrometer detection system. The
method is generally applicable to analyzing virtually any kind of
mixtures which can be chromatographically separated and is
especially suitable for analyzing complicated organic mixtures such
as natural products, drug metabolites and clinical samples which
are not sufficiently volatile to be analyzed by gas chromatography
- mass spectrometer techniques.
Referring now to FIG. 1, there is shown a diagrammatic illustration
of a typical arrangement for practising the method of the
invention. A mass spectrometer 11 of any of the conventional types
is suitable for use in practising the invention. The specific
arrangement (magnetic sector instrument) shown in FIG. 1 is
intended only by way of an example. Other mass analyzers such as
the quadrupole or time of flight are equally applicable. The mass
spectrometer 11 includes an ion source portion 12, a tube 13,
magnetic structure 14, collector portion 15 and suitable
electronics 16. The ion source portion 12 and the tube 13, as well
as the collector portion 15, are all in a vacuum with an
appropriate vacuum connection 17 to the vacuum pump (not shown).
The ion source portion 12 includes an inlet 18 into which molecules
of a sample to be analyzed are introduced. There is also provided a
suitable electrode structure 19 wherein a stream of electrons
generated by a filament in the conventional manner strikes the
molecules of the sample causing ionization and fragmentation in a
conventional manner. A pair of electrodes 20 and 21 are provided
with a voltage source V.sub.1 maintaining a small potential of a
few volts for directing the ions through an aperture 21a in the
electrode 21. An additional electrode 22 is provided having an
aperture 22a therein and a relatively large potential is maintained
between the electrodes 21 and 22 by a voltage source V.sub.2, on
the order of several thousand volts. The ions and fragmented ions
are thus accelerated through aperture 22a through the spectrometer
11 and those ions which pass through an exit aperture 23 are
collected by an electrode 24 which is connected to suitable
electronics for generating the mass spectrum of the sample, all in
a conventional manner.
In accordance with the invention the sample molecules are
introduced into the ion source portion 12 of the mass spectrometer
from a thin layer chromatograph in very close proximity to the ion
source portion and in a vacuum. The physically separated zones of
the thin layer chromatograph are successively heated, one at a
time, as described more fully hereinafter and the substances
forming the various zones of the thin layer chromatograph need only
sublime a short distance in a vacuum directly into the ion source
of the mass spectrometer. Thus substances having a very low vapor
pressure can be easily introduced from a thin layer chromatograph
into the mass spectrometer by a direct coupling of the two.
Referring again to FIG. 1, the thin layer chromatograph 25
(examples of which are discussed hereinafter) is suitably mounted
in a vacuum enclosure 26 which communicates directly with the ion
source through the inlet 18. Means are provided to heat the various
zones of the thin layer chromatograph 25 one at a time to cause the
substances to sublime into the ion source. Various techniques are
suitable for applying heat to the thin layer chromatograph as
discussed hereinafter. One suitable method is electrical heating
which has been diagrammatically illustrated in FIG. 1 as a heating
coil 27 surrounding the thin layer chromatograph 25 and having
electrical connections 28 and 29 to which a voltage source can be
connected.
The remaining part of the description will be devoted to a
description of various examples of methods of preparing suitable
thin layer chromatographs in accordance with the invention and
introducing the separated substances from the chromatograph into
the ion source of the mass spectrometer.
Turning now to a consideration of FIG. 2, there is shown an example
of practising the method of this invention in accordance with one
embodiment. In this embodiment there is provided means such as a
glass tube 31 on the inside of which there is formed a layer of a
sorbent suitable for use in forming a thin layer chromatograph.
This may be a silica-gel slurry, for example, which is allowed to
dry. The chromatograph is run in the conventional manner by
depositing a sample of the mixture to be analyzed at one end of the
tube (indicated by region i in FIG. 1). A mobile phase or solvent
is passed through the inside of the tube 31 over the region i
through the inside of the tube. Owing to their selective sorption,
the constituents of the mixture migrate with the solvent or mobile
phase at different rates and separate from one another as a series
of zones z1, z2, z3, etc., is absorbed in the silica-gel and
physically separated from one another along the length of the glass
tube. Then the thus formed thin layer chromatograph is placed in a
vacuum enclosure communicating with and in close proximity with the
ion source of the mass spectrometer. Next, in accordance with the
principles of the invention heat is applied to the series of zones
z1, z2, etc., one at a time, to cause the absorbed substances in
each of the zones to sublime into the ion source of the mass
spectrometer. As illustrated in FIG. 1, this can be conveniently
done by means of a heating coil 32 which is selectively moved along
the outside of the tube 31 from zone to zone. As illustrated in
FIG. 2, a flow of a gas such as helium may be introduced into the
tube 31 to carry the sublimed substances into the ion source 33 of
the mass spectrometer. Mass spectrographs are run in a conventional
manner of each of the individual substances which are sublimed from
the zones z1, z2, etc., into the ion source of the mass
spectrometer.
Turning now to a consideration of FIGS. 3 and 3a, there is shown a
schematic illustration of another embodiment of practising the
method of this invention. In accordance with this embodiment a
silica-gel or other suitable chromatograph medium is deposited in a
groove 34 formed in a glass rod 35. The chromatograph is then run
in a customary manner such as discussed before in connection with
FIG. 2, with differential migration of the various substances in
the sample producing a series of physically separated absorbed
zones of substances in the silica-gel medium along the length of
the rod 35. The rod 35 with the separated zones of absorbed
substances along its length is then placed in a vacuum in close
proximity to the ion source 36 of a mass spectrometer. Suitable
means such as the mechanical advancing mechanism 37 are provided
extending within the vacuum enclosure for selectively advancing the
glass rod 35 past the inlet 36a to the ion source so as to
successively position each of the zones of absorbed substances
adjacent the inlet 36a. In accordance with the principles of the
invention, as each of the zones of absorbed substances is
positioned in front of the inlet 36a, heat is applied thereto. In
the embodiment of FIG. 3 the heat is applied by means of a light
source 38 focusing a light beam through the glass rod onto the
silica-gel having the absorbed substance thereon. In the embodiment
of FIG. 3 the light source can be an ordinary focused source of
infrared or visible light or can be, for example, a laser. Each of
the absorbed substances is in this manner evaporated from the
silica-gel on the glass rod into the ion source of the mass
spectrometer. As each substance is thus evaporated, a mass
spectrograph is run in the conventional manner.
Turning now to a consideration of FIG. 4, there is shown another
form of practising the method of the invention. In the arrangement
shown in FIG. 4 there is provided a glass rod 39 which has a
continuous spiral of metal 41 formed thereon. The metal 41 is
preferably one having a relatively high resistance so that it can
function as a resistance heater, as hereinafter described.
Thereafter, one half of the glass rod 39 is coated with a suitable
chromatograph medium, such as a silica-gel 42 for example. A
mixture i which is to be analyzed is placed at one end of the glass
rod on the silica-gel and the chromatograph is run in a
conventional manner to produce the physically separated zones of
substances z1, z2, etc., along the length of the rod. The thus
formed thin layer chromatograph is then placed in a vacuum in close
proximity to an ion source 43 of a mass spectrometer. Suitable
means such as the mechanical advancing mechanism 44 are provided
extending within the vacuum enclosure for selectively advancing the
glass rod 39 past inlet 43a to the ion source, so as to
successively position each of the zones of absorbed substances
adjacent the inlet 43a. A pair of electrodes 46 and 47 are provided
adjacent the glass rod and on an opposite side thereof with respect
to the inlet 43a. The electrodes 46 and 47 are in electrical
contact with the metal spiral 41 on the glass rod 39. As mentioned
before, the metal spiral 41 is preferably formed of a metal having
a relatively high resistance so as to function as a resistance
heater. The electrodes 46 and 47 are connected to terminals 48 and
49 to which an electrical voltage source can be connected for
passing an electrical current through that portion of the metal
spiral 41 between electrodes 46 and 47. In this manner heat is
produced to selectively evaporate each of the zones of absorbed
substances z1, z2, etc., from the thin layer chromatograph through
inlet 43a into the ion source of the mass spectrometer. As each of
the zones is evaporated into the mass spectrometer ion source, a
mass spectrograph is run in the conventional manner.
In FIG. 5 there is shown another embodiment of a thin layer
chromatograph similar to the one shown in FIG. 4 and which can be
utilized in the same configuration as the chromatograph shown in
FIG. 4. In the embodiment of FIG. 5 a rectangular support member
51, which may be glass for example, has a plurality of metal strips
52 formed thereon. Thereafter, a suitable chromatograph medium 53,
such as silica-gel, for example, is formed in a strip down the
center of member 51 in a direction perpendicular to the strips 52.
A thin layer chromatograph is then run in the conventional manner
by depositing a sample i of the mixture to be analyzed at one end
of medium 53. Thereafter the sample i is chromatographed in the
conventional manner using a suitable mobile phase forming a
plurality of physically separated zones z1, z2, z3, z4 and z5 along
the length of the medium 53. The thus formed thin layer
chromatograph is placed in a vacuum enclosure in close proximity to
the ion source of a mass spectrometer. A pair of electrodes 54 and
55 are situated so as to contact those of the metal strips 52 which
are underneath the particular zone z1, z2, etc., which is in close
proximity to the ion source of the mass spectrometer. The
electrodes are connected to terminals 56 and 57 to which an
electric voltage source then passes an electric current through the
metal strips 52 which are contacted by the electrodes 54 and 55.
The metal strips 52 are preferably formed of a metal having a
relatively high resistance so that this current generates
sufficient heat to evaporate the substance z1, z2, etc., into the
ion source of the mass spectrometer. As was the case in the
embodiment of FIG. 4, mechanical means may be provided for
advancing the thin layer chromatograph step-by-step so as to
successively position each of the zones in close proximity to the
ion source. As each of the zones is thus positioned and evaporated,
a mass spectrograph is run in the conventional manner.
Thus what has been described is a method for analyzing mixtures of
substances in which a thin layer chromatograph is combined with a
mass spectrometer. In accordance with the invention the thin layer
chromatograph itself is placed in a vacuum in close proximity to
the ion source of a mass spectrometer. Heat is selectively applied
to the substances of the chromatograph to directly evaporate them
into the ion source of the mass spectrometer. Although the method
has been described with respect to a few illustrative examples,
obviously modifications may be made to the specific embodiments
disclosed herein without departing from the true spirit and scope
of the invention. For example, it is contemplated that any size
chromatographic apparatus such as a fine quartz fiber or tubular
fiber may replace the "glass rod", "tube", or "plates" specifically
referred to in the above description. Further, materials other than
glass, such as ceramic, silica, metal or other inert low vapor
pressure substrates may be used.
* * * * *