Anal. Chem. 1980, 52, 2450-2451
2450
/
u 3.3 -
I
disk electrodes positioned as coplanar as possible by simple visual inspection, a collection efficiency-rotation rate calibration curve was prepared that proved t o be quite reproducible and reliable. Thus, the power and versatility of t h e rotating disk electrode for detecting and identifying intermediates formed a t the disk remain available despite the inconstant collection efficiency. ACKNOWLEDGMENT The numerous ideas and suggestions from the personnel in the departmental instrument shop as well as their skillful machining are a pleasure t o recognize. James Swartz was another reliable source of insightful comments and encouragement.
K.;I; 1 .
9 NC. CURRE',T CLRRE',T I
4
2c 3c 4'3 43 ( R 3 T A T l C h RCTEJ'"
I3
5c
EO
(rpm)'''
Flgure 4. Levich plots of limiting ring and disk currents vs. (rotation rate)"' prepared from the current potential curves in Figure 3.
T h e collection efficiency of t h e ring-disk electrode (12) was therefore not independent of rotation rate a t values above ca. 900 rpm. At lower rotation rates the collection efficiency became constant and very close to the value of 0.3 calculated from the geometrical dimensions of the disk, ring, and intervening gap (12). The decline in collection efficiency a t high rotation rates appeared not to result from lack of exact coplanarity of the disk and ring electrodes because intentional introduction of noncoplanarity produced only small changes in the collection efficiency. I t seems possible that the inevitable ridge present in the gap region where the two insulating materials abut each other may introduce perturbations in the hydrodynamic flow at the higher rotation rates that result in variations in collection efficiency. Nevertheless, with ring and
LITERATURE CITED
(1) Aibery, W. J.; Hitchman, M. L. "Ring-Disk Electrodes";Cbrendon Press: Oxford, 1971. (2) Coliman, J. P.; Marrocco, M.;Denisevich, P.; Koval, C.; Anson, F. C. J. Electroanal. Chem. 1979, 101, 117-122. (3) Collman, J. P.; Denisevich, P.; Konai, Y.; Manocco, M.;Koval, C.; Anson, F. C. J. Am. Chem. Soc., in press. (4) Yeager, E.; Zagal, J.; Nikolic, G. 2.; Adzic, R. R. Proc. Symp. Electrode Processes. 3rd 1979, 436. ( 5 ) Zagal, J.: Bindra, P.; Yeager, E. J . Electrochem. SOC. 1980, 727, 1506- 15 17. (6) Zhutaeva, G. V.; Shumiiova, N. A. Elektrokhimiya 1966, 2, 606-607. (7) Doronin, A. N. Elektrokhimlya 1968, 4 , 1193-1 194. (8) Harrington, G. W.; Laitinen, H. A.; Trendafilov, V. Anal. Chem. 1973, 45, 433-434. (9) Galus. 2.; Olson, C.; Lee, H. Y.; Adarns, R. N. Anal. Chem. 1962, 3 4 , 164- 166. (10) Oyama, N.; Anson, F. C. Anal. Chem. 1980, 52, 1192-1198. (1 1) Levich. V . G. "Physicochemical Hydrodynamics"; Prentice-Hall: Engle. . wood Cliffs, NJ, 1962. (12) Albery, W. J.; Bruckenstein, S. Trans. Faraday SOC. 1966, 62, 1920-1 93 1.
RECEIVED for review July 25, 1980. Accepted September 22, 1980. Contribution No. 6274. This work was supported by the National Science Foundation.
Coupling of Capillary Gas Chromatograph and Fourier Transform Mass Spectrometer Edward B. Ledford, Jr., Robert
L. White, Sahba Ghaderi, Charles L. Wilkins," and Michael L. Gross*
Department of Chemistty, University of Nebraska -Lincoln, Lincoln, Nebraska 68588
We report here the first demonstration that, with the fast scanning ability of Fourier transform mass spectrometry (FT/MS), mass spectral data can be acquired during a typical SCOT capillary column gas chromatography peak elution (5-10 s). Preliminary studies in our laboratory have shown that is is possible to scan fast enough to obtain the GC peak profiles of eluting components. I n addition, high-resolution mass spectral data over narrow mass ranges have been acquired during capillary column peak elutions. Thus, the F T / M S instrument can perform in a rapid lower resolution spectral scan mode or in a high-resolution multiple ion monitoring mode. EXPERIMENTAL SECTION The FT/MS instrument used for this study was specially designed with a high gas conductance vacuum chamber. This allows the mass analyzer to operate at low pressure, which is necessary for high mass resolution. The forward end of the vacuum chamber, which contains a cubic trapped ion analyzer cell (plate separation 0.0254 m), has a nominally rectangular shape and is constructed of 304 stainless steel. The width (internal 0.0667 m) is sufficient to permit insertion into the 0.0762-m air gap of a Varian V-7300 0.305-m electromagnet. That volume expands into a 0.152-m i.d. stainless steel tube which is outfitted with a 0.152-m Thermionics Model GS 6000 gate valve. The tube terminates at a Balzers TPU 0003-2700/80/0352-2450$01 .OO/O
500 turbomolecular pump which is backed by a Welch Model 1397 mechanical pump. Pressures were measured by using a Veeco Model RG-840 ionization gauge and controller. The pumping speed at the cell is 360 L/s which was determined by calculation and verified experimentally by introducing a known flow rate of helium and observing the pressure. The electromagnet (Varian, Model 7300) is equipped with one pair of 0.305-m cylindrical ring-shim pole caps with iron-cobalt ring for high homogeneity. Its maximum field strength is 1.37 T at the center of the air gap, and it was operated at 1.2 T for these experiments. It is outfitted with a Varian V-7800 13-kW basic power supply and a V-7550 current regulator. The vacuum system is interfaced to a Perkin-Elmer Sigma I1 gas chromatograph containing an OVlOl 15.2 m X 0.5 mm SCOT capillary column at 150 "C (isothermal). The interface consisted of either a direct coupled transfer line (0.5 mm i.d., 1.6 mm 0.d. glass-lined stainless steel obtained from Scientific Glass and Engineering) or a standard glass jet separator coupled to the transfer line. Both were held at 200 "C. When the separator was used, 20 mL/min of make-up helium gas was introduced prior to the jet separator. Ion formation and mass analysis were accomplished as previously described ( I ) . The ion excitation was provided by a Rockland System 5110 programmable frequency synthesizer under the control of a 40K word Nicolet 1180 minicomputer, equipped with a high-speed buffered digitizer capable of data acquisition 63 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 14, DECEMBER 1980
GC/FTMS 111
GC/CI - FTMS
- & -!y\I
8
2451
/Methyl Formate
/--
6
.u
time
m/z
50
100
Figure 1. GC-FT/MS spectra of a mixture of equal amounts of benzene, toluene, and xylene. Twenty-five signal averaged scans were taken per file.
benzaldehyde’
‘-xylene
Figure 2. GC-FT/MS separation of the isobaric m / z 106 peaks from xylene and benzaldehyde.
at rates up t o 5 MHz (9-bit resolution) or 2.5 MHz (12-bit resolution). Ion image signals were amplified by a Tektronix 5A22N high-speed differential oscilloscope amplifier.
RESULTS AND DISCUSSION We have demonstrated in this preliminary study that the fast scanning capabilities of a Fourier transform mass spectrometer are sufficient to obtain full low-resolution mass spectra and, therefore, GC peak profiles of eluting components (Figure 1). These spectra can be obtained by using either a direct coupling or the jet separator interface. Furthermore, the use of a jet separator and make-up gas have allowed us t o obtain pressures as low as 5 x torr. Low-pressure operation is critical if high-resolution mass spectral data are desired (2-4). In fact, the addition of a jet separator has resulted in an increase in the attainable mass resolution for benzene m / z 78 from 8000 (FWHH, full width half-height definition) for direct couple operation to 23 OOO (FWHH) with the separator. As a further test of the resolving power of FT/MS in a GC mode, the isobaric doublet at m / z 106 derived from the molecular ions of xylene ( m / z 106.07825) and benzaldehyde (mlz 106.04187) was resolved (Figure 2). Since the two components had very different retention times on the OV-101 SCOT capillary column used, benzaldehyde was introduced into the spectrometer via an inlet system and xylene was injected into the GC column. Base line separation of the molecular ions was easily obtained with a mass resolution of 9000 (FWHH).
mlz
43
61
Figure 3. Low-pressure chemical ionization of a methyl formate GC peak by use of isobutane reagent gas.
Another analytical measurement method made possible is that of low-pressure chemical ionization mass spectrometry (5-7). By operating the F T / M S with a time delay between ion formation and excitation, it is possible to monitor the products of ion-molecule reactions. For example, isobutane reagent gas was introduced into the F T / M S vacuum can via an inlet system. Ionization resulted primarily in the formation of m/z 43. By injecting a more basic substance such as methyl formate into the GC column, it was possible to shift the dominant peak in the mass spectrum from m / z 43 to the quasimolecular M + H peak of methyl formate ( m / z 61). As the methyl formate eluted and was pumped out of the spectrometer, m / z 43 again became the dominant peak in the spectrum (Figure 3). Since the spectral bandwidth selection in F T / M S involves setting the frequency sweep of an rf sine wave excitation, it should be possible to alter both the size and position of a mass obsemation window simply by changing a few computer words. This could easily be done within microseconds. Switching of this type would allow for acquisition of both low-resolution, wide mass range data as well as high-resolution narrow mass range data during the elution of a single chromatographic peak. Thus, high-resolution multiple ion monitoring should be a practical possibility. We are currently developing this low-resolution scout-high-resolution check mode of operation along with high-resolution multiple ion monitoring over large mass ranges.
LITERATURE CITED Ledford, E. B., Jr.; Ghaderi, S.; White, R . L.: Spencer, R. B.; Kulkarni, P. S.; Wilkins. C. L.; Gross, M. L. Anal. Chem. 1980, 52, 463. Comisarow, M. 8.; Marshall, A . G. J . Chem. Phys. lQ78, 6 4 , 110. Marshall, A. G. Anal. Chem. 1979, 57, 1710. White, R. L.; Ledford, E. B., Jr.; Ghaderi, S.; Wilkins, C. L.; Gross, M. L. Anal. Chem. 1980, 52, 1525-1527. McIver. R. T., Jr.: Ledford, E. B., Jr.; Miiier, J . S. Anal. Chem. 1975. 4 7 , 692. Ghaderi, S.; Wilkins, C. L.; Gross, M. L.. submitted for publication in Anal.
Chem
.
Ghaderi, S.;Ledford, E. B., Jr.; Gross, M. L.; Wilkins. C . L., submitted for publication in Anal. Chem.
RECEIVED for review July 15, 1980. Accepted September 2, 1980. Support of this research by the National Science Foundation (Grant No. CHE-77-03964)and a grant from Gulf Oil Foundation are gratefully acknowledged.
