CA1084731A - Method for high resolution gas chromatography - Google Patents

Abstract

ABSTRACT
A sensitive high resolution gas chromatography method for elution of volatile sample mixtures at controlled speed including intermittent stop flow operation. The method utilizes chromatographic exit column pressure above one at-mosphere absolute up to and higher than 50 atmospheres absolute concurrently with controlled low velocity carrier gas movement within the column, The method facilitates stop flow operation for on line spectral analysis of eluted samples, and facilitates matching the speed of high resolu-tion chromatograph operation to other on line analysis processes.

Description

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The present inventiol~ relates in general to a methocl o~ high re~olution gas chromatocJraphy and in particular rel~-tes to an improvcd method for conductincJ ~as chromatographic separation o~ components of volatile sample mixtures and con-venient concentration of eluted samples~ The method utilizes elevated column pressure, controlled column flow rate and in-cludes intermittent stop flow operation.
Gas chromatograph ins-truments and the gas chromato-graph methods comprise a class of extremely sensitive devices and methods for the séparation of components ~rom a volatile sample mixture. Usual practice has been to mix the volatile sample with an inert carrier gas, the mixture o~ which is then percolated through a column containing granulated particles pro-viding a large surface area. Depending upon the choice oE the granulated packing material, a solid or liquid stationary phase is interfaced with the mobil carrier and sample mixture gaseous Phase- The sample mixture components are, in the percolating process, partitioned between the mobil gaseous phase and the stationary liquid or solid phase. Each component or, if poorly resolved by the process, each class of component compounds will exhibit a unique~rate oE travel through a given column reEerred to as the retention timo for the oompound in the oolumn.

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Heretofore extensive investigative work has been conducted with temperature programming of chromatographic columns and with various packing materials for columns with a purpose to increase solubility of the sample mix-ture in one or both the stationary phase and mobil gase-ous phase. Comparable investigation work has not been con-~
ducted to date to examine the effect of increased low and intermediate range column exit pressure, i,e., gauge pres- ` ;
sures measured at the coLumn outlet in the range of one to fifty atmospheres gauge pressure on the efficiency and operation of chromato-graph columns. Some earlier work at , high pres9u~es, i,e., pressures in the L000 to 2000 at-s mospheres (absolute) range~ has been conducted to separate i, large molecular weight molecuIes, the earlier very high ~1 ` ! .
~,; pressure investigations depended upon the altered near liquid like density of the carrier gas at extreme pressures to heighten sampLe solubility in the gaseous phase and facilitate separation of high molecular weight compounds.
From the a~oresaid high~pre9sure invs9tigations no readily usable laboratory device or method for gensral purpose ~;
; chromatographic procedures within the pressure range of one to fif~y atmospheres gauge was disclosed, Usual practice has been to operate conventional gas ~;J~ ~ chromatographs at the column gas veloc~ity which optimizes S~ the resolutlon and speed of operation for a specific sample mixture. This ~procedure has been achieved by operating , ~ the output opening of the column at atmospheric pressure, '3 ~ and regulating the flow rate of carrier gas injected into ~, :~ , 1~8gL731 the column input opening from a pressurized tank.
Commonly, the pressuri7.ed cylindrical tank reservoir of carrier gas is held at pressures above 200 atmospheres.
In conventional gas chromatograph laboratory practice dur~
ing operation~ the pressure within the column in the vi-cinity of the carrier gas input opening is between one and ~hree atmospheres gauge pressure, By adjusting the carrier gas inpu~ flow rate, the carrier gas velocity with-in a specific coLumn may be adjusted to achieve the reso-lution and sample separation desired. The pressure drop ,~
between the input and output regions of conventional chromatograph column usage ranges between one and three atmospheres. Accordingly, the gas velocity in the column is substantial due to the pressure drop forces aLong the i,length of the column. Under these conditions, any sudden . . . .
change in the gas pressure or gas velocity at the column outlet opening such as may be induced in conventional , . .
sample collection procedures may produce mixing within the column and reduce the resoIution and separation of follow~
ing separa~ed but still entrained components, Conventional gas chromatograph laboratory practice, in order to achieve rapid highly resolved separations, utilizes '~
higher as distinct from lower carrier gas velocities with-in the column to reduce time the purified sample entrained in the lower portion of the column may be exposed to dif-~- fusion. Only wlth relatively high carrier gas velocities within the column can less mixing in the lower portions of - ;~
the column in the vicinity of the column outlet opening be achieved under conventional column pressure ranges. By '' ~' - ~ ~

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conventional columl~ pressure we mean column pressure, measured at the column outlet opening, equal to atmospheric absolu~e pressure or zero gauge pressure.
The high gas velocity in the chromatograph column causes some difficulty when the emergent separated compo-nents must be confined in a container ~or later analy9is or use Separated and entrained sample components within the column moving at rapid velocities l'pile up" if the column is stopped or restricted in a transient manner for purposes of segregating a sample component as it leaves '' the column Moreover, in chromatograph columns as conven- ``
tionally operated, a pressure of approximately one at-mosphere (absolute) prevails in the vicinity of the outlet opening. When the column is operated in a stop flow mode, .~ . . ~ . .
` the'rate of gaseous diffusion is sufficiently high at the ~ ~
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relatively low one atmosphere (absolute~ exit pressure ~hat '~ the separated and entrained components of the sample mix-.. ~ ~
~i ture hel'd in any extension of the column Eor storage are , rapidly mixed by diffusion and the resolution or separation `` ' ' .,~ .
''' ' between them is degraded. That is~ a separate and pure , !,,' ` sample component becomes contaminated with diffusion of -other components of the sample from within the column or `'' ' any extension of the column.
'` It is desirable to perform spectral and other analysis I ~ on eluted pure sampIes emitted from a gas chromatograph. An on line" arrangement is most convenient for the operator.
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'~' However, in such arrangement, the time for cycling the ~, spectral analysis procedure must be coordinated with the ., .

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clifference in retention times oE varlous sample compo- !~
nents leaving the chromatograph column. Spectral analysis procedures such as infrared spectroscopy normally require more ti~e than the dif~erence in retention times of several ~;
separated compounds passing through a chromatograph column, The same condition characterizes other spec-tral analysis methods such as, for instance, mass spectrometry, ultra violet and visible band spectrometry, Raman spectrometry, and nuclear magnetic resonance; also a similar condition characterizes analytic procedures that are not spectral, such as electromechanica~, polarographic and coulometric analysis. Present laboratory practice reconciles the di~-ferences on the one hand of the time oE response of chro-matographic separation processes, and on the other hand, ` ' ~., .
the normally much slower response time of spectral and ~' certain other analysis processes by one of two methods, neither of,which is completely satisEactory.
First, some spectral analysis are done on the fly, ,, The fly scan method, necessarily only one rapid scan, fails, to extract the optimum spectral analysis data'Erom the mov- ' ing purified sample. Valuable data is often lost. The fly scan method, however, avoids disturbing the gas flow emitted from the column outlet opening and the att,en,dant mixing in the gas stream due to pressure disturbance that ~' may be reflected back into the column which stop flow oper- ' ation is conventional existing devices would certainly cause. ~ `
The second method in current practice is that of stor~
~, , ing in a detachable container a pure sample of the eluted '~
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` ~L0~473:31 material~ issuing from the outlet of the chromatograph column. This latter approach minimi~.es but does not avoid the disturbance of the gas flow in the colutnn with its attendant mixing wi~hin ~he operating columrl It may expose the pure samples to outside contamination sources When the sample is stored, it is possible ~o scan repeatedly with the spectral analysis device and obtain the optimal spectral -analysis data.
Conventional practice furnishes the purified sample to the storage chamber at a pressure of one atmosphere. Some spectral analysis procedures, such as infrared spectroscopy requires that ~he sample be concentrated before analysis, thus requiring still another step with the attendant added costs, time and contamination risks.
The most convenient manner of achieving the required time coordination between a chromatograph and a spectral analysis device is to operate the chromatograph column in a stop flow mode, provided this can be achieved without reduc-ing the resolution and separation o~ subsequent entrained components.
Hereto~ore, no chromatograph methocl or required clevice , has been available which provided low velocity control of the : ~.
., ~
: carrier gas moving through a gas chromatograph column while superior resolution of separated sample components was maintained. Similarly, heretofore, no chromatograph method and required device has been available with which stop . ~ . ~ . .
flow mode of operation could be conducted and good separa- ~
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tion of eluted pure materials maintained . .

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It is a irst object of the present invention to pro-vide an improved high resolution general purpose gas chromato-graph method~
It is another object of the present invention to pro-vide an improved general purpose gas chromatograph method in which the velocity of the carrier gas moving through the column may be controlled and caused to move at very low velocities without loss of separation and resolution of the purified and eluted components.
It is still another object of the present invention to provide an improved general purpose gas chromatograph method suitable for direct in line operation of a suitably pressurized ;
gas chromatograph with any of a variety of spectral analysis devices wherein the rate of emission of purified compounds from the chromatograph may be adjusted to the rate of operation o a spectral analysis device.
Still another object of the present invention is to - provide an improved general purpose sensitive gas chromatograph !:
method which facllitates stop flow operation.
And still another object of our invention is to pro-; vide an improved general purpose gas chromatograph me-thod in which the pressure and concentration of purified sample compounds, emitted from a suitably pressurized gas chromato-graph column, is sufficiently high that no intermediate step to concentrate the purified samples is required prior to i~
performing spectral analysis procedures on the samples.
According to the present invention, there is provided a method for high resolution gas chromatography comprising the ,1 steps of passing a stream of carrier gas combined with a vapor sample~mixture through a column at normal operating pressures :! .
and velocities, and intermittently stopping the flow of gases - 7 ~ ~ `~

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with valye means jus~ax~ ed to the column outlet, wherewith eluted but en-trained sample vapors, are held ~or brief intervals during stopped flow within the packed column, whereby subsequent eluted samples entrained within the column may be intermi-ttently briefly retained during sto~ flow without appreciable diffusi~n and remixing.
These and other objects and advantages of'the prese~-t invention w-ll appear from the following description having re-ference the attached nosl limitative drawings, wherein:
Figure 1 : shows a gas chromatograph accordiny to the present invention;
Figure 2 : is a schematic graph of a van Deemter plot;
i Figure 3 : is a schematic chart illustrating an im-; proved chromatograph peak shape according -to the present inven-tion;
Figures : 4, 5 and 6 show chromatogram of a stop flow operation at various pressure conditions.
Referring to Figure 2 a schematic graph of a van 20 Deemter plot is shown wh;ch displays on the vertical axis the inverse of the number of theoretical plates and on -the horizon-tal axis the gas velocity in a chromatoyraph column~ 'rhe van ,~ Deemter plot is commonly used in a gas chromatography art and is described in numerous publica-tions. The quantity N, or number of theoretical plates, is a widely known figure of merit for ~ ' comparison of the efficiency of chromatograph column performance. '' ', We have observed that with successively higher column ' pressure a greater value of N, number of theoretical plates for ` a given chromatograph column,may be achieved. This is illus-, . .
trated by the family of curves shown in the chart of Figure 2.

Each curve represents performance of a given column at the in- ' . ' '.

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dicated pressurc as ~as velocity within the columrl is varied.
It is evident that the best efficiency,that is tlle la~-~est value oE N, is achieved with increasing pressure with simulta-neous reduction in column yas velocity.
We also observed that the resolution o the samples tested improved with increased column pressure and increased retention time, Tr. Figure 3 is a schematic chart illustratlve of the improved chromatograph peak shape with increased pres-sure and increased time of retention. The time of retention is inversely proportional to column gas velocity. The highér resolution peaks or improved peak shapes are narrower at the base.

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The experimental results,illustrated in Figures 2 and 3,demonstrate the feasibility of our improved method wherein we pass a sample to be eluted through a-chromato-graph coLumn, the col.umn being maintained throughout its length at a pressure above one atmosphere absolute,and we maintain a sufficiently small pressure drop along the column to limit the column gas velocity to a small value The point of the mo~t efficient separation takes place in the Figure 2 illustrations at the minimum values of the curves shown point A for the 250 psi curve, B at the 200 psi curve and D at the 45 psi curve.
A device Ln which chromatographic separations may be conducted at eleva~ed pressures using our me~hod is shown in a schematic illustration in Figure L. A pressure re- ;`
: sistant chr~omatograph column 10 is shown having an inlet opening 12 and an outlet openin~ 14. The column 10 may be packed with any of a variety of granular stationary phase :
~ materials 16~ many o~ which are known and available to .~ those familiar with chromatographic laboratory procedures.
~he column 10 is normally operated ln a thermally insulated chamber or oven 18 in which the temperature of the column may be closely controlled ;;: An injection port 20 is mounted to the column inlet opening L2. The injection port is comprised of a heavily walled chamber 22 which may be heated from an external source not shown in the illustratLon to adjust the carrier gas temperature before introducing it into the column 10.
An inner chamber 24 is mounted within the heavily walled 9 ~

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chamber 22. The outlet end of the inner chamber 24 is connected to the inlet opening 12 o:E the column 10. The ~.
inner chamber 24 is provided near its upper end with a plurality o~ small apertures 26 which communicate between ;
the interior of heavily walled chamber 22 and the interior .: - .
of the inner chamber 24 through which carrier gas may be ~ ~-caused to flow through the inner chamber and be introduced .~
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through the inlet opening 12 into the column 10. '~
The upper end of the inner chàmber 24 is sealed wlth a septum 30. The septum is a self-sealing body, through ..~
which smalL samples of gaseous or readily volitized mixtures . may be injected into the system for elution or separation.
.: The septum 30 is firml~ sealed in place with a threaded cover 32, ~n aperture 34 is provided in the septum seal .
. cover 32 through which analysis samples may be inserted, The carrier gas is conveniently retained at relatively , high pressure in a cylindrical tank reservoir 40, Pres-sures in commèrciàlly available cylinders of compressed :~
gases such as purified nitrogen~ carbon dioxide, heLium, - ~, argon and other commonly used chromatograph carrier gases i9 normally available up to 200 atmospheres, The carrier gas reservoir 40 i$ connecte~ through pressure resistant /
tubing 42 to the interior of the injection port heavily walLed cham~er 22. A flow rate meter 44 and a pressure ~.
gauge 46 is connected to the pressure line 42. ~An adjust~
able flow regulation valve 48 controls the quantity of carrier gas passed into the injection.port and into the chromatograph column. A pressure reduction valve 49 . .

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reduces ~he pressure o~ the carrier gas to a predetermined value as it is passed into line 42.
A thermal conductivity detector 50 having a tempera-ture controlled chamber 52, input.and output openings 54 and 56, respectively, and an electronic sensing means 58.
A thermistor connected through an appropriate circuit, not ~
shown in the illustrations, will sensitively detect any .; :
change in thermal conductivity of the gas present in the chamber 52. Conversely stated,.the detector will detect :~
a change in composition o~ gas flowing through the detector chamber 52 due to different thermal conductivity oE the varied gas composition, The sensor output voltage may be displayed graphicalLy in a moving chart recorder 62, The .;~
input opening 54 of khe detector is connected through a ':',., . ~ .
, pressure resistant line 64 to outlet opening 14 of column.
; . , , stop valve 65 is inserted in the line 64, preferable ;~
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juxtaposed to the column outLet opening. In all events, the stop vaLve 65 is mounted so that negligible open volume remains.within the conduit or tube connected between the column outlet 1~ or the solid/liquid stationary phase packing within the column, and the stop valve 65, The stop valve 65 is preferably a quick response valve, That iS9 ', rapid closing and rapid~opening action is achieved with ~ .
small rotation of the valve stem, The valve 65 may be.oper- :
ated at times under substantial pressure conditions, there~
fore, a pressure resistant valve design is required, ' An adjustable flow restrictive valve 70 is connected : ;
on the first or inlet side~to the output opening 56 of the ~

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detector through a pressure resistant line 60 and through a sample collection chamber 68 and connected on the second -~ -or outlet side to a stop valve 76. The stop vaLve 76 is vented to atmospheric pressure A stop valve 61 is mounted on the line 60. The flow restrictive valve 70 may be a conventional pressure resistant needle valve comprised of an axially movable needle valve stem 72 which seats onto a beveled valve seat 74 The flow of gases through the flow restrictive valve may be adjusted to create any desired pressure in the vicinity of the column outlet opening greater than atmospheric pressure up to the upper pressure limits of the system. A pressure meter 66 connected to line 60 provides in~ormation of pressure at the column out-let 14. There is substantial resistance to gas ~lowing through the column between the inlet 12 and the outlet 14.
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'3' ~ ~ However, there is normally negligible resistance to gas flow between the column outlet 14 and the flow restrictive valve 70, therefore, the pressure within the system :~
measured at the gauge 66 adjacent to the flow restrictive valve 70, will be representative o~ the pressure within the column at the outlet opening 14. -Our invention achieves to a significant extent the .
im~roved advantages described above such as highe-r resolu-~ ~, l tion and stop flow operation without loss of resolution for `, ~ most volatile sample mixtures at column exit pressures as '.,', ~ . ~;
^~ low as 45 psi~absolute. Scme volatile sample mixtures are -more difficult to s~eparate and require a larger concentrated sample for later spectral analysis. Column exit pressures ,, :

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of 50 atmospheres gaug~ and higher may be required.
The gas chroma~ograph ilLustrated in the drawings has been constructecl to operate safely at up to 50 atmospheres gauge column exit pressures. Operation at higher ~han 50 atmospheres gauge pressure provided the carrier gas re-mains in the gaseous state would not be inconsis.tent with the lntent and purpose of our inven~ion.
Referring to the gas chromatograph system ilLustrated ;.
in Figure 1, typical operation utilizing our method is as follows: carrier gas is caused ~o flow through the injecti~n port 20 at a preselected pressure ranging, as shown on gauge 46, between one atmosphere and Eifty. The pressure reduction valve 49 readily permits the operator to establish a steady state pressure in the carrier gas flowing through .~ , .
pressure line 42. Any pressure, less than that present in the carrier gas reservoir tank 40, may be used. The quan- ~
; tity of car~ier gas flowing in line ~2, may be adjusted by ~ :
flow control valve 48 and measured by the flow rate meter The ~low resistance value 70 is then.ad~justed to fix the column exit pressure, as observed on gauge 66 to any pr.eselected valve between zero and fifty atmospheres or more. When the gauge 66 indicàtes zero gauge pressure, the chromatog~aph Ls being operated in the conventional ~: manner... Our invention relates to a gas chromatograph method for operation above zero column exit gauge pressure.
Velocity o~ flow throug.h the column 10 and detector `~:
~ 50 may be regulated at column exit pressures greater than .~ J3 ', ' . , . : . ; . . .

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' zero gauge pressure by adjustments of the ~low rate valve 48. At higher column pressure, very low gas velo- - ;
city within the column may be attained without 109s 0 chromatographic resolution.
Intermi~ten-t stop flow operation of a chromatograph may be achieved with the closing of stop valve-65. ELuted -' . . .
but entrained sample vapors, when valve 65 is closed, are '~ ;
held within the lower portion of the packed column 10. It is important to avoid altering the "peaked" quality of the eluted samples, that the valve 65 be actuated with a quick" ,~
` ~ response. Some "stop flow" capability in a conventional . ~
~ chromatograph operated at conventional pressures and gas ;, ~' ', velocities may be demonstrated by utilizing a quick action ~
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valve mounted juxtaposed to the column outlet opening. ~ ' ,' , Stop Elow operatiOn is preferably achieved by u~
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,, zing a pressurized system equipped with a valved sample , chamber. Stop flow is implemented by simultaneously cLoslng the flow rate valve 48, the vent stop valve 76 ~ ' and the stop valve 65. Stop flow operation without 109s ~ :
; .
~ ~ of resoLut:ion o the remaining entrained'sample components " , "i may be readily attainecl if the valves 489 76 and 65 are closed on a slow moving rather than a rapidly moving gas r ~, ~ , flow. When the system is pressurized, the stop flow action , ,;
,, ' is more readily implemented; the samples are more,com- ~ ' , l .
,,~ pletely separated and according to our pressurized method ` ~ ~
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~ may be entrained for longer periods of time without'diffu- ~ ' , ' ~
~ sion while being held stationary within the chromatograph.' ~
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Samples may be concentratecl at whatever pressure the operator may desire by making appropriate adjustments in the pressure recluction valve 48 and simultaneously the flow restriction me~ns 70, The samp]e pressur~ chamber ' will collect selected samples with our arrangement at the pressure at which the chromatograph separation is conducted, Pressure within the sample chamber may be read on pressure gauge 66 which pressure will also be the pressure at the '' column exit 14, ' Speci~ic examples of application of our method to stop flow opera~ion of the chromatograph illustrated in Figure 1 at various pressure conditionsare ilLustrated in the graphs shown in Figures 4, 5 and 6, The graphs show detector voltage in millivolts on the verical axis9 and time in minutes on the ho,riæontal axis.
Figure 4 is a chromatogram of stop flow operation with the flow restriction means 70 open. The column exit pres- ~
sure was consequently zero gauge pressure as obs~rved on ~ ;
the gauge 66. The column had been stopped by closing stop ~
. , .
valve 65. When stop val.ve 65 is suddenly openecl~ the '~
eluted samples moved quickly past the detector 50 and left .'' ~, ~ a chromatogram wi~h poorly resolved crowded peaks. Poorly `~
; concentrated and crowded samples are difficult to scan with any spectral analysis device, The samples at conventional column pressure and column velocity are not fully separated.
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' , Figure 5 is a stop flow chromatogram prepared with the : , : .
column exit at 100 psi gauge pressure, Gas velocity in the column had been significantly slowed. The samples are well ,' ' '. ;

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separated. Sample concentration is significantly superior to that shown in Figure 4 prepared at zero gauge pressure at the column exit~
Notwithstanding the superior results achieved with our pressurlzed stop flow method, our stop flow method at normal operating pressure~ I -, that is, at atmospheric column outlet pressure is an advance over ~ present practices and is useful when pressur7zed equipment is not ; available.
Figure 6 is a stop flow chromatogram prepared at column exit ~
pressure of 180 psi gauge. The five components of the sample mixture ~.
are well separated; all samples are sufficiently concentrated so that the indicator was saturated. Additional concentration of the individual - ''l; ~
~samples prior to making a successful infrared spectroscopy test was not required.

The forego7ng descrlptions of preferred examples of our method ,invention are intended as belllg lllustrat7ve only; the scope of our InventTon Is set forth below 7n cla7ms. ~
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Claims (4)

The embodiments of the in vention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for high resolution gas chromatography comprising the steps of passing a stream of carrier gas combined with a vapor sample mixture through a column at normal operating pressures and velocities, and intermittently stopping the flow of gases with valve means juxtaposed to the column outlet, where-with eluted but entrained sample vapors are held for brief in-tervals during stopped flow within the packed column, whereby subsequent eluted samples entrained within the column may be in-termittently briefly retained during stop flow without apprecia-ble diffusion and remixing.

2. A method according to claim 1, wherein said stream of carrier gas combined with a vapor sample mixture for elution is passed through a pressurized gas chromatograph column, the column pressure being selected from a value between one atmos-phere and fifty atmospheres gauge pressure when measured at the column outlet, said gases being passed along the column at a slow velocity.

3. A method according to claim 1, wherein said flow of gases is intermittently stopped with quick action valve means.

4. A method according to claim 1, wherein said stream of carrier gas and said sample gaseous mixture for elu-tion is first passed through a pressurized gas chromatograph having a column, the column pressure being selected from a value between one atmosphere and fifty atmospheres gauge pressure, the column pressure being measured at the column out-let, then the eluted sample is collected in a multiple valved sample collection chamber, the chamber being connected through valve means to the column outlet, whereby the eluted gaseous sample is compressed and concentrated by means of the heighten-ed pressure within the column, the sample is then passed while concentrated and under pressure into the sample chamber where it is retained by actuation of the chamber valves without re-duction of concentration or dilution.