Anal. Chem. 1982, 5 4 , 2443-2447
of retention data of convergent homologous series gives very similar void volumes which are, moreover, in rather close agreement with those obtained by using injections of D20. Although this is not a thermodynamic proof for the use of convergent homologom series, it is however a remarkable result. It should also be' noted that the V ovalues determined with all homologous seiriies under study give linear log k ' vs. n, plots of the same slope. According to our results, this is not the case when Vois taken as the total volume of liquid in the column.
ACKPJO WLEDGMENT A.M.K. gratefully acknowledges the scientific exchange agreement for financial support. LITERATURE CITED (1) Berendsen, G. E.; Schoenmakers, P. J.; de Galan, L.; Vigh, G.; VargaPuchony, 2. J . Llq. Chromatogr. 1980, 3 , 1669-1686. (2) Wells, M. J. M.; Clark, C . R. Anal. Chem. 1981, 53, 1341-1345. (3) Slaats. E. H.; Markovski, W.; Fekete, J.; Poppe, H. J . Chromatogr. 1981, 207, 299-323. (4) Riedo, F.; KovLts, E. s;?.J . Chromatogr. 1982, 239, 1-28. (5) HorvLth, Cs.; Melander, W. R. "Book of Abstracts", 183rd National Meeting, of the American Chemical Society, Las Vegas, NV, March
2443
28-April 2, 1982; American Chemical Society: Washington, DC, 1982; ANYL 072. (6) Horvlth, Cs.; Lin, H.J. J . Chromatogr. 1976, 726, 401-420. (7) McCormick, R. M.; Karger, B. L. Anal. Chem. 1980, 5 2 , 2249-2257. (8) Knox, J. H.; Kawszan, R., Kennedy, G. J. Symp. Faraday SOC. 1980, 15, 113-125. (9) Johnson, J. K., Jr.; Cernosek, S. F., Jr.; Gutierrez-Cernosek, R. M. J . Chromatogr. 1979, 777, 277-31 1. (10) Karger, B. L.; a n t , J. R.; Hartkopf, A.; Weiner, P. H. J . Chromatogr. 1976, 128, 65-78. (11) Unger, S. H.; Feuerman, T. F. J . Chromatogr. 1979, 176, 426-429. (12) NeMhart, B.; Krlnge, K. P.; Brockmann, W. J . Liq. Chromatogr. 1981, 4 , 1875-1886. (13) Vigh, G. Y.; Varga-Puchony, Z . J . Chromatogr. 1980, 796, 1-9. (14) Haken, J. K.; Wainwright, M. S.;Smith, R. J. J . Chromatogr. 1977, 733,1-6. (15) Wainwright, M. S.;Haken, J. K. J . Chromatogr. Chromatogr. Rev. 1980, 784, 1-20. (16) Colin, H.; KrstulovlE, A. M.; Guiochon, G., manuscript in preparation. (17) Scott, R. P. W.; Simpson, C. F. J . Chromatogr. 1980, 797, 11-20. (18) Colin, H.; Dlez-Masa, J. C.; Gulochon, G.; Czajkowska, T.; Miedziak, I. J . Chromatogr. 1978, 767, 41-65. (19) KovLts, E. sz., prlvate communication. (20) Poppe, H., private communication
RECEIVED for review April 22,1982. Accepted September 7, 1982.
Capillary Gals Chromatography/Fourier Transform Mass Spect romet ry Robert L. Whlte and Charles L. Wllklns" Department of Chemistty, Universi@ of California -Riverside,
Riverside, California 9252 1
The coupling of capillary gas chromatography to two different Fourier transform mass spectrometers (FTMS) is descrlbed. By use of a jet separator gas chromatography/mass spectrometry (GC/MS) Interface, It is shown that the technlque Is capable of rapid scannilng as well as high-resolution mass analysis when combined with support-coated open tubular (SCOT) Capillary columns. With the appropriate software, It Is possible to peak swlitch at high resolution over arbitrarily wlde mass ranges (a feat which cannot be accomplished by conventional mass spectrometry). I n additlon, with no changes in instrument,atlon, FTMS can be operated in a chemical lonlzatlon (Cli) mode, for GC/MS.
There is little doubt that the combination of gas chromatography with mass spectrometry is one of the most useful tools available to analytical chemists for complex mixture analysis. Current literature reflects the usefulness of this versatile technique in many areas of research including agricultural ( I ) , biomedical (2), energy (3), environmental (4, 5), and industrial (6) applications. Because GC/MS is such an important application of mass spectrometry, it follows that characterization of new analytical mass spectrometric techniques should include ain evaluation of t,he GC/MS combination. This paper deals with the evaluation of GC/MS using SCOT capillary columns and Fourier transform mass spectrometry. In recent years Fourier transform mass spectrometry (FTMS) has become a new and viable analytical mass spectrometric technique (7-25). Previous studies have shown that this type of instrument is capable of rapid scanning and 0003-2700/82/0354-2443$0 1.25/0
ultrahigh mass resolution and would be expected to interface well with capillary gas chromatography (11). Interfacing a packed column gas chromatograph to an ion cyclotron resonance spectrometer has been reported as a means of determining momentum transfer collision frequencies but was not advocated for routine GC/MS applications (16). However, our preliminary experiments demonstrated that GC/FTMS is feasible and that SCOT capillary column peak profiles can easily be obtained (13). In other work, it was determined that exact mass assignments could be made on fragment ions using low-resolution (ca. 1000 full width at half height (fwhh)) FTMS spectra and that, if isobaric peaks are suspected, high resolving power may be employed to distinguish them (12). Thus, three different GC/FTMS operation modes are available: low-resolution wide mass range scanning and both low- and high-resolution selected ion monitoring. All of these modes employ rapid scan times (e1s) and as a result are compatible with narrow capillary column peak widths (5-10 9).
EXPERIMENTAL SECTION Two Fourier transform mass spectrometers were used in this work. One of these instruments was constructed at the University of Nebraska and has been described in detail previously (13). This system was constructed for use with a Varian V-7300 electromagnet and was interfaced to a dual column Perkin-Elmer Sigma I1 gas chromatograph. The other FTMS was manufactured by Nicolet Analytical Instruments (FT/MS-1000) and uses a horizontal bore superconducting magnet. This instrument was interfaced with a Varian Vista 6000 gas chromatograph via a probe inlet vacuum lock. Both instruments operated using 0.0254-m cubic trapped ion analyzer cells. The electromagnet-based instrument was operated at a magnetic field strength of 1.2 T whereas the FT/MS-1000 field strength was 1.9 T. Both in@ 1982 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 14, DECEMBER 1982
GLASS JET SEPARATOR
/
U I
\ALUMINUM
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Figure 1. GCIFTMS probe 1000 mass spectrometer.
OVEN
0
25
50
FLOW INTO SEPARATOR (ml/minl
GLASSIMETAL SEAL
mounted interface design for the FT/MS-
struments were interfaced to their respective gas chromatographs with molecular jet separators manufactured by R. H. Allen Co., Boulder, CO. Three different capillary columns were employed for the separations reported in this work they were a 15.2 m X 0.5 mm OV-101 SCOT capillary column, a 35 m X 0.44 mm Carbowax 20M SCOT capillary column, and a 30 m X 0.323 mm DB-1 bonded phase fused silica capillary column. The electromagnet-based system was controlled with a Nicolet 1180 minicomputer and the FT/MS-1000 employed a faster Nicolet 1280 computer. The operating principles of FTMS have been described in detail elsewhere (14).For GC/FTMS experiments using electron impact ionization, a 3-llA, 50-eV electron beam was used. The beam duration was typically 10-75 ms and 1-Vtrapping potentials were used. In the CI mode, a 3-pA, 12-eV electron beam was used. The partial pressure of the ionizing gas, methane, was 5 X 10" torr. A continuous electron beam was used and measurements were made in the "quench off" mode to maintain a steady-state concentration of CI reagent ions in the cell (14).
RESULTS AND DISCUSSION GC/FTMS Interface. In order to obtain high-resolution mass spectrometric data using FTMS, low operating pressures within the analyzer cell must be maintained. For GC/FTMS, this means that the flow of gas into the vacuum system from the gas chromatograph should be minimized and the conductance of the vacuum system should be maximized. The vacuum system conductance is a quantity related to the pumping efficiency of the instrument and is fixed by the design of the spectrometer. Conductance measurements were made for the electromagnet-based spectrometer and the FT/MS-1000 by introducing a known flow rate of helium gas into the vacuum system and observing the pressure maintained. The conductance (C) of the system was calculated by using eq 1where Q is the gas throughput in units of (torr L)/s and P is the vacuum system pressure in torr. Conductance
C = Q/P values of 360 L/s and 120 L/s were calculated for the electromagnet-based and FT/MS-1000 instruments, respectively. For minimum flow of gas from the gas chromatograph to the FTMS, a jet separator interface was employed. A separator interface was chosen instead of an open split interface because the enrichment properties inherent in separator operation would provide some degree of preferential transport of separated mixture component molecules to the FTMS which ultimately would result in increased mass spectrometric sensitivity. The GC/MS interface selected for these studies was a glass molecular jet separator which was specially designed to attain low FTMS pressures (ca. lV8 torr). A diagram of the interface developed for the FT/MS-1000 is shown in Figure 1. The interface was probe-mounted to provide for
Figure 2. Variation of FT/MS-1000 vacuum system pressure as a function of gas flow rate into the jet separator.
easy installation and removal. Similar construction was used for the electromagnet-based instrument (17). It was discovered that rough pump vibrations picked up by the glass separator of the FT/MS-1000 were so intense that the arms of the separator would snap off when anchored in the oven using glass-lined stainless steel transfer lines (GLT). To avoid this problem, we relieved stress on the glass jet by using flexible fused silica capillary tubing for transfer lines instead of the rigid GLT. The interface oven was constructed from an aluminum box in which Variac controlled strip heaters were mounted. Stable temperatures as high as 350 OC were easily maintained by using this configuration. Figure 2 illustrates the variation of vacuum system pressure as a function of flow rate entering the jet separator for the FT/MS-1000 interface. Typical flow rates for experiments presented here consisted of a 2 mL/min He column flow rate combined with 10 mL/min He makeup gas for a total of 12 mL/min He entering the jet separator. At these flow rates, an FTMS operating pressure of 4 X torr was easily maintained. This allowed mass resolution of 40 000 (fwhh) for m / z 78 from benzene. The high-resolution sensitivity of the GC/FTMS combination was tested by measuring the on-column detection limit for benzene molecular ion ( m / z 78) while monitoring a 1amu mass range from m / z 77.5 to m / z 78.5. A 50-ng sample of benzene mixed with toluene solvent was injected onto a 30-m DB-1 bonded phase fused silica capillary column, and time domain data were signal averaged during the benzene peak elution. After background subtraction, the mass spectral SIN for m/z 78 was 23:l for 100 signal averaged scans. For a single scan (ca. 100 ms) a SIN of 2:l is calculated for a 44-ng oncolumn injection of benzene. Software Considerations. The rapid scanning capability of FTMS can result in a data storage problem if each scan during the separation is to be saved. For example, at a scan rate of 100 ms/file (acquiring 8192 data points for each file) a data storage device with a capacity of 5 X lo9 bits would be required to store the information generated during a 50-min capillary column separation. For comparison, the total storage capacity of the disk storage unit supplied with the FT/MS1000 is only 9.14 X lo7 bits which is 4.9 X lo9 bits short of the required capacity. To avoid this data storage problem, a means of chromatographic detection is needed so that only mixture component spectral information is stored while background-containing files are excluded. The ideal method of GC detection using the FTMS would be to Fourier transform the time domain data and use the resulting mass spectra to detect peak elution. However, current speeds for 4-8K Fourier transforms are not rapid enough to permit using this approach. If a rapid hardware FT device were incorporated, this technique would become feasible. However, in the absence of such hardware, we developed another approach not requiring transformation. Alternately, small 512 or 1024 point transforms similar to the Nicolet "chemigram" technique used
ANALYTICAL CHEMISTRY, VOL. 54. NO. 14, DECEMBER 1982
2445
50 REAL-TIME
CHROMATOGRAM
m h
1-
F g u r 4. WmMS mepration of a sample prepared by mtxlng equal volumes of benzene, toluene, p-xylene, chlorobenzene, and bromcbenzene. One mlcroliier was lnjacted and split 2 5 1 . An OV-101 column was temperature programmed (110-190 “C at 15 ‘Clmin) using a column flow rate of 3 mLlm1n. Each spectrum is separated from tlm next by a 1.2-5 Interval. The FTMS operating pressure was 5
x lod
tOR.
Fgure 3. Flow chart of the software used In OClFTMS operatlcn.
in Fourier transform infrared s p e c h m e t q muld he used. The real-time chromatogram plotted in this paper were generated with the ‘raw” time domain data by adding together the ahsolute values of the digitized data points and plotting this sum as a function of scan number. With carrier gas alone entering the FTMS, the time domain signal contained prim d y hackground noise, as the operating conditions did not permit detection of helium (mlz 4). When mixture components eluted, the time domain signal was enhanced due to the detection of fragment ion resonance signals. As a result, a marked increase in the value of the time domain summation was obse~edwhen mixture components entered the analyzer cell of the ETMS. As these moledes were pumped away (by turbomolecular pumps for both instruments) the summation value returned to the hackground level. The chromatogram SIN was enhanced hy A“/*where applicable by signal averaging N time-domain transients. Disk storage of time-domain data was selectively controlled by setting a chromatogram threshold, which determined which data fdea were saved. All chromatogram intensities were plotted, whether the data file was stored or not. A flow chart of the process for GC/FTMS operation is depicted in Figure 3. To start, a pulse sequence was executed and time domain data were digitized. If the heterodyne mode was selected, peak switching could he ohtained hy moving the excitation bandwidth to the next mass. This was done by changing the rf excitation starting frequency while keeping the rf bandwidth constant. Peak switching masses were preselected before the chromatographicseparation was hegun. A real-time chromatogram was produced (vide supra) by adding the absolute values of the digitized data and plotting the result on a digital plotter under interrupt control. When a preset threshold was exceeded, the data file was assumed to contain spectral information and was stored on magnetic disk. This cycle was repeated until the program was stopped by the operator or the available disk storage space was exceeded. A “raw generated chromatogramfor an equal volume mixture of benzene, toluene, p-xylene, chlorobenzene, and hromobenzene using the electromagnet-based ETMS is shown in Figure 4 along with the frequency domain mass spectra ohtained from the individual files. The mass spectra were ohtained hy Fourier trxnsrormation of the stored data after
-
500 t i -
Fgure 5. OC/FTMS peak swlchlng from mlz 78 to m l r 156 at a scan rate of 285 mslflkt. Chomatograph conditions and sample slze were Mentical with those reported in Figure 4. me FTMS operating torr. pressure was 5 X the chromatographic separation. As can be seen, the chromatographic detection scheme produces a plot which represents the total ion intensity for the mass range covered. In this example, the chromatogram threshold was set to zero so that all of the data were collected. Each data file consisted of 100 signal-averaged scans and fides were collected a t a rate of one per second. A mass range from m / z 38 to m l z 200 was plotted. Mass spectral resolution of lo00 (fwhh)was ohtained a t m l r 18. High-Resolution Peak Switching. Selected ion monitoring, or peak switching, in conventional GC/MS is based on voltage switching on magnetic instruments and has been restricted to relatively narrow mass ranges due to sensitivity reduction when the voltage is dropped. The recent development of fully laminated magnets has permitted wide range peak switching with scan cycle times on the order of 300 ma. However, this technique is limited to low-resolution mass analysis. In FTMS, unlike conventional mam spectrometry, ion detection is based on a computer-controlled radio frequency sweep. I t is a trivial matter to alter the sweep parameters with the appropriate computer software. The peak switching rate in FTMS is therefore only limited by the scan time of the instrument because the actual switching is accomplished hy the computer in microseconds. By operating in a heterodyne mode with a narrow bandwidth, it is possible to obtain high-resolution mass spectral data with data acquisition times on the order of hundreds of milliseconds (11).
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 14, DECEMBER 1982
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Figure 6. GC/FTMS reconstructed chromatograms of a lacquer thinner separation using (a) electron impact ionization and (b) methane chemical ionization. A 0.2 pL oncolumn sample size was injected onto a DB-1 bonded phase column which was temperature programmed after a 5-min isothermal period at 30 OC (30-180 O C at 4 "C/min). Column flow rate was 3 mL/min.
If high resolution is not necessary, much faster scan rates are possible. Figure 5 shows the result of peak switching to monitor the molecular ions of benzene and bromobenzene during a GC separation. The benzene molecular ion peak appears on the right side of the plot and the bromobenzene molecular ion is on the left. In this example, peak switching was performed over a 78 amu mass range from m/z 78 to m/z 156. A 500-Hz bandwidth was covered and a scan time of 285 ms per file was measured with the electromagnet-based FTMS instrument. Each file was a single FTMS scan. A resolution of 40000 (fwhh) for m/z 78 was obtained. Since the mass spectral peak widths for the m / z 78 and m/z 156 ions were identical, the mass resolution at m / z 156 in this experiment was 20000 (fwhh). Although the chromatographic peak width for the benzene elution was only 3 s, five scans containing m / z 78 information were obtained. Even though the bromobenzene trace exhibited some tailing, the elution time of this chromatographic peak was only 6 s. Although this example illustrates selected ion monitoring of only two mass spectral peaks, the technique could just as easily be applied to several masses. However, as the number of selected ions increases, the number of scans that profile an individual component elution decreases a t a given mass resolution. Obviously, the degree of digital peak profiling of eluting components could be improved at the expense of mass resolution by detecting ions for a shorter period of time. CI-GC/FTMS. Simplified mass spectra and, in some cases, increased specificity in GC/MS can be obtained by use of chemical ionization (CI) instead of electron impact ionization (EI) Chemical ionization GC/FTMS is straightforward and requires no instrumental changes. The CI reagent gas was added to the vacuum system via a gas/liquid inlet and a time delay inserted in the pulse sequence between ion formation and ion detection to allow for ion-molecule reactions to occur between CI reagent ions and mixture component neutrals. A reaction delay period was necessary because of the requirement to maintain low-pressure FTMS conditions (ca. lo4 torr). Because excessively long reaction delay times could greatly reduce the FTMS scan rate, it was desirable to operate the spectrometer in a continuous or "quench off" mode. In I
this mode, the quench pulse was not employed and the electron beam allowed to operate continuously. This had the effect of increasing the ion density in the analyzer cell while also permitting increased ion sampling rates as compared with the conventional "quenched" pulse sequence (18). A comparison of E1 and CI-GC/FTMS chromatograms of a commercial lacquer thinner generated using the FT/MS1000 superconducting magnet is shown in Figure 6. Since total ion intensity was measured, some of the GC peaks in Figure 6 appear to be very broad because of overlapping component elutions (e.g., see Figure 7). The CI reagent gas was methane and the reagent ion was CH6+. Because many of the components did not readily undergo methane CI, numerous GC peaks found in the E1 chromatogram were not observed in the CI plot. Also, the relative chromatogram peak heights for the CI-GC/FTMS separation were different from those obtained with EI. These differences reflect the relative reactivities of the individual mixture components with the CH6+reagent ions. Figure 7 is a plot of some of the mass spectra obtained at the beginning of the CI-GC/FTMS separation, as a function of time. Gaps in the plot resulted because the time domain data acquired at these times were not saved as they did not satisfy the chromatogram threshold condition. The indicated peaks in the plot arose as a result of protonation of the molecular ions of the eluting components and were not observed in the corresponding EI-GC/FTMS mass spectra. The identities of the components in the lacquer thinner sample were determined with a combined GC/ FTIR/FTMS system described elsewhere (19). Significant E1 fragmentation was observed for 2-propanol for the CIGC/FTMS separation even though low-energy (12 eV) electron impact was used for ion formation. Mixed EI/CI fragmentation patterns of this sort are often obtained in lowpressure FTMS chemical ionization (14).
CONCLUSION Trace analysis of complex mixtures by FTMS is best performed with GC/FTMS since FTMS dynamic range limitations (ca. 1000) severely hinder the direct mixture analysis approach (FTMS-CID). It is clear from analysis of the data presented here that GC/FTMS is still in its infancy. In the near future, improved FTMS sensitivity and the use of array processors for Fourier transformations should allow real time display of mass spectra of eluting components at rapid scan rates and should make the technique more competitive with conventional GC/MS combinations. A t present, GC/FTMS can be used in conjunction with SCOT capillary chromatography and provides the only available means of rapid peak switching at high mass resolution over arbitrarily wide mass ranges.
Anal. Chem. 1982, 5 4 , 2447-2456
LITIEiRATURE CITED Golan-Goldhirsh, A.; tiogg, A. M.; Wolfe, F. W. J . Agrlc. Food Chem. 1982, 3 0 , 320-323. Tserng, Kou-Yi; Kalhnn, Satlsh C. Anal. Chem. 1982, 54, 489-491. Felice, Lawrence J. Anal. Chem. 1982, 54, 869-872. Mltchum, R. K.; Korfnnacher, W. A,; Moler, G. F.; Stalling, D. L. Anal. Chem. 1982, 54. 719-722. Knuutinen, J.; Tarhanein, J.; Lahtlpera, M. Chromatographla 1982, 15, 9. Richardson, S. J.; Miller, D. E. Anal. Chem. 1982, 5 4 , 765-768. Comlsarow, M. B.; Marshall, A. G. J . Chem. Phys. 1975, 6 4 , 110-119. Marshall, A. G. Anal. Chem. 1979, 5 1 , 1710-1714. Comisarow, M. B. Int. J . Mass Soectrom. Ion Phys. 1981, 37, 251-257. McCrery, D. A.; Ledford, E. B., Jr.; Gross, M. L. Anal. Chem. 1982, 5 4 , 1435-1437. White, R. L.; Ledford, E. 8.. Jr.; Ghaderl, S.; Wllklns, C. L.; Gross, M. L. Anal. Chem. 1980, 52, 1525-1527. Ledford, E. B., Jr.; Gtiaderl, S.; Whlte, R. L.; Spencer, R. B.; Kulkarnl, P. S.; Wilkins, C. L.; Gross, M. L. Anal. Chem. 1980, 5 2 , 463-468.
2447
Ledford, E. B., Jr.; White, R. L.; Ghaderl, S.; Wllklns, C. L.; Gross, M. L. Anal. Chem. 1980, 5 2 , 2450-2451. (14) Ghaderl, S . ; Kulkarnl, P. S.; Ledford, E. B., Jr.; Wllklns, C. L.; Gross, M. L. Anal. Chem. 1981, 5 3 , 428-437. (15) Cody, R. B.; Furnler, R. C.; Frelser, B. S. Anal. Chem. 1982, 5 4 ,
-- .-.. PR-101
(16) Nguyen, M. T.; Wronka, J.; Starry, S.; Ridge, D. P. Int. J . Mass Spectrom. Ion Phys. 1981, 40, 195-210. (17) Wllklns, C. L.; Gross, M. L. Anal. Chem. 1981, 5 3 , 1661A-1676A. (18) Bartmess, J. E.; Caldwell, G. Int. J . Mass Spectrom. Ion Phys. 1981, 41. 125-134. (19) Wllklns, C. L.; Giss, G. N.; White, R. L.; Brissey, G. M.; Onylriuka, E. Anal. Chem. 1982, 5 4 , 2280-2264. I~
RECEIVED for review June 1, 1982. Accepted September 8, 1982. The support of the National Science Foundation under Grants CHE-77-03964 and CHE-80-18245 and the support of the Protection Agency under Grant R807251010 are gratefully acknowledged.
Improvement of Speed of Separation in Packed Column Gas Chromatography Robert J. Jonker' aind Hans Poppe* Laboratory for Analytical Chemistty, Universl@ of Amsterdam, Nieuwe Achtergracht 766, 10 18 WV Amsterdam, The Netherlands
J. F. K. Huber Institute of Analytical Chemistry, University of Vienna, Wahrlnger Strasse 38, Vienna, Austria
The general strategy for the Improvement of speed of separation In packed colurnn gas chromatography is addressed. Starting from optimization equations for the dlsperslon In the gase phase, we dlscuss the varlous other ilmltlng factors such as sample capaclty, mass transfer in the statlonary phase, detection, and Injectbn. The packed column Is found to be superior to the open tirbuiar column when a high separatlon speed is to be combiiied wlth a reasonable slgnai to noise ratio and dynamic range. The optlmlzatlon leads to a signlflcant mlnlaturizatlon of the chromatographic system. I n the experimental part, columns between 32 and 250 mm in length, slurry packed with 10 hm siliceous partlcies, are shown to produce h-v curves with a mlnimum h value of 2.5-3.5 and a permeabliity factor of about 1.2 X lo-'. The fastest chromatograms show peak standard deviations of 2 ms, and more than 14 000 theoretical piatesls. The increased speed facilitates signall enhancement methods like ensemble averaging which Is demonstrated In an example.
The efficiency of the packed column in gas chromatography has been the subject of little innovative effort during the last decade. This is quite understandable because of the tremendous efforts which were rightly spent on the development and analytical applicaition of high efficiency open tubular column chromatography. Also the incentive for speeding up an analytical method already capable of yielding a result every 10-30 min has not beem very strong, as is often argued that sampling, sample pretreatment, and interpretation of the results etc. require times of comparable magnitude. 'Present address: Hewlett-Packard strasse, 7517 Waldbronn 2, GFR.
GmbH, Hewlett-Packard-
0003-2700/82/0354-2447$01.25/0
The latter argument does not apply in automatic control applications. Also the introduction of microcomputers in laboratory gas chromatographs, resulting in automation in all steps of the analytical process, leads to the demand and possibility of higher analysis rates. These higher rates, when available, in turn can be used for speeding up selectivity optimization procedures and routine analysis, in which many more samples per unit time can be analyzed with one chromatographic system. Also, the higher analysis rate probably will enlarge the field of averaging methods like ensemble averaging or correlation chromatography (1) both for decreasing the detection limit and for increasing the precision of the analysis. All of this suggests that an increase in the speed of gas chromatography is a very interesting topic. From the kinematic point of view the open tubular column is vastly superior to the packed column alternative, mainly because of the much higher permeability factor obtained in this geometry (2). However, as soon as the quantitation aspect of chromatography is brought in, the drawbacks of the open tubular geometry, associated with the connected low sample capacity, become manifest. We shall discuss this aspect in the following. This paper is devoted to the increase of speed and efficiency in packed column gas chromatography, when a few hundred to a few thousand theoretical plates are required. This leads to separation times of 0.1 to several seconds, applicable, e.g., in process control and monitoring systems, with or without the combination with signal enhancing methods. This work corroborates and extends efforts in the same direction carried out in previous work (3-8).
THEORETICAL SECTION Column Dynamics. Mobile Phase Effects. Know and Saleem (9) derived equations for optimum speed and resolution in column elution chromatography. For gas chroma@ 1982 American Chemical Society
