Gas Chromatograph-Mass Spectrometer with Dual Electron ImpacVHigh Pressure Ion Source Ragnar Ryhage Laboratory for Mass Spectrometry, Karolinska Institute, S- 104 0 I Stockholm SO, Sweden
Many applications of chemical ionization have been described iaince Munson and Field published their first paper in 1966 (1, 2). The construction of ion sources has changed slightly riince that time, but experiments with many different reagent gases have been made by chemists working in the field of mass i$pectrometry,which has resulted in a more versatile and comprehensive application of this method for ionization. Magnetic sector instruments, in which the ion source is a t high potential, were mainly used in the first work with chemical ionization and the sample was introduced by the direct insertion probe or via the heated inlet system (3-5). However, during recent years, several articles have been published where gas chromatography-chemical ionizationmass spectrometry has been used (6, 7). Quadrupole mass spectrometers have frequently been used both for direct insertion probe and GC-CI-MS operation (8,9). Some authors have used single ion sources designed for CI operation to produce E1 spectra where helium was used as the reagent gas. These spectra show similar fragmentation as normal E1 spectra (IO).Because of the small aperture of the exit slit and of the electron beam entrance hole, E1 mass spectra of optimal sensitivity cannot be obtained using CI ion source conditions. Other publications describe a fast changing EI/CI mode of operatioin (11). In this case, two ionization chambers with different slit widths were used to obtain optimal sensivity. The work reported here illustrates how a GC-MS instrument for E1 is modified to produce optimal conditions both for the E.[ and CI mode and to show the advantage in using two different reagent gases when the instrument is operating in the CI mode.
EXPERIMENTAL Instrumentation. All experiments were performed with an LKB 2091 GC-MS instrument. The ion source and inlet system were modified as follows: A new ion source was constructed (patent applied for) so that the apertures of the ion exit slit, the electron beam entrance, and the exit to trap, which are included on a movable, goldplated stainless steel band (8 X 0.03 mm), could be changed to permit either the electron impact or chemical ionization mode of operation. The slit size for the ion exit in the E1 mode is 5 X 0.3 mm; for the filament electron entrance, 2 X 1mm; and for the electron exit to trap, 2 X 2 mm In the CI mode, the dimension of the exit slit is 3 X 0.03 mm; for the electron entrance holes, 0.3 mm in diameter; and there is no exit to the trap. Figure 1shows the operational principle of the new ion source in the CI mode. By moving the band 21 mm to the other slit dimension, the ion source is ready for operation in the E1 mode. It ir; important that the ion source is gas tight and that leaks around the movable band are small as compared to the apertures in the band. For this reason, the ionization box has been given suitable radii and the surface is well polished. A differential pumping system is used which employs Edwards high vacuum pumps Ed 300 and Ed 150 at the ion source housing and the analyzer, respectively. The pumping speed around the ion source is about 200 l/s for a pressure of about Torr and the pressure in the analyzer is about Torr for an ion source pressure of about 0.5 Torr. By installing a cold trap a t the top of the ion source housing and by using liquid nitrogen, the pumping speed will increase and the pressure for isobutane in the ion source housing drops to 10-5 Torr for an ion source pressure of 0.5 Torr. The pressure in the ion source is measured by a Pirani gauge connected to a special probe made of glass which can be used in the same position as the direct probe inlet. The emission control unit for the dual EI/CI source consists of two
regulating circuits, one for the trap current (25 to 200 PA) and one for the total electron emission burrent (0.25-4 mA) in the E1 and CI modes, respectively. The ion source has two extraction plates and two repellers. The voltage potentials on the extraction plates and the two repellers are adjusted separately for E1 and CI and are switched, together with the electron energy, between the two modes of operation. The electron voltage can be continuously changed from 10 to 100 eV for E1 and by steps from 50 to 600 eV in the CI mode. An integrator is connected to the galvanometer amplifier at the collector of the mass spectrometer. The circuit has start/stop integrate option programmed to sum up the ion intensities within a chosen mass range. The scan of the mass spectrometer can operate independent of the integrator and the output from the integrator produces the ion current gas chromatogram. Operation. Figure 2. shows a schematic diagram of the combined gas chromatograph-mass spectrometer for EI/CI operation. Magnetic valves are used for the selection of the reagent gas along with a needle valve and glass capillary to limit the gas flow. Each reagent gas container has a separate pressure regulator, and a gas flow of 1to 2 ml/min is sufficient to keep the pressure in the ion source between 0.4 and 0.8 Torr. In the E1 mode, magnetic valves 1and 2 are open and the other magnetic valves are closed. In the CI mode, valves 1and 2 are closed and one of the valves to the reagent gases, 5 , 6, or 7, is open. The pressure of the reagent gas in the ionization chamber can also be adjusted by needle valve 3. To reduce the time required to reach the operating pressure in this case, a magnetic valve, 4, can be opened for short periods which are determined by the setting of a timer. The exchange of reagent gases during continuous operation can be done as follows: As soon as a scan of a mass spectrum with the first reagent gas is completed, the magnetic valves 1and 2 are opened and the inlet line is evacuated in less than 0.5 s. Immediately afterwards, these valves are closed and another reagent gas is introduced into the ionization chamber and, after 0.5 I, when this gas has reached the operating pressure, a new mass spectrum can be taken in 1.5 s covering a mass range of m/e 5-500 (12).In this way, the GC-MS instrument can be operated in the CI mode and two complete 1.5-s scan spectra would then take about 5 s. T o obtain both E1 and CI mass spectra of the same GC peak, a proper timing of the required adjustments is necessary. The slits must be changed between scans which takes about 3 s and, a t the same time, a readjustment of the electrical parameters must be made. The required time for two complete 1.5-s scan spectra would them be 7 to 9 s. For the results presented here, methane and isobutane were used as reagent gases with helium as carrier gas through the gas chromatographic column with a flow rate of 10-20 ml/min for all experimental studies. The mass spectra were recorded on uv paper since no computer was available for this project.
C/ position
Figure 1. Schematic drawing of the combined EI/CI ion source
ANALYTICAL CHEMISTRY, VOL. 48, NO. 12, OCTOBER 1976
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c- Rcogcnt Gas +- Reagent Gas c- Reagent Gas
c
la FV Pump Figure 2. Schematic diagram of the combined gas chromatograph-mass spectrometer and reagent gas inlet system for EI/CI operation
’ 2
=
“isobutane
= C‘metClane
3 = Ei
Ei
f
e
d
c
b .~
a _-
Operation mode
Figure 3. Chromatogramsof the integrated ion current of pentobarbitone at different modes of operation a = El (only GC), b = Clkbm(only GC).Two complete mass spectra, one for each mode of operation were recorded for GC peaks c to f
RESULTS AND DISCUSSION As a demonstration of the switching time from the conventional E1 mode to the CI mode and vice-versa, and of the exchange of reagent gases in the CI mode, 2 pg of pentobarbitone were injected into the SE-30 column. The retention time for pentobarbitone was adjusted to be approximately 1 min. Figure 3 shows the GC peaks of the integrated ion current where a and b correspond to the E1 and CIi-but modes of operation. No mass spectra were recorded for these peaks. The 1830
width of the GC peaks at half-height are about 7 to 9 s and, during this time, two complete mass spectra, one for each mode of operation, could be recorded for each GC peak. GC peaks c to f were checked to determine if any cross-residual effect could be observed from one mode of operation to another. Mass spectra of GC peak c were recorded in the CI mode with isobutane as the reagent gas, C1, and in the E1 mode, C3. After the CI spectrum was recorded(Cl), the reagent gas was switched off and the electron energy, repeller, and extraction voltages were switched over to the E1 position as well as the movable band for a change to a larger exit slit and electron entrance hole into the ion source. After the E1 mass spectrum (C3) was recorded, the voltages and movable band were reset for the CI mode. GC peak d was recorded in reversed mode, E1 to C&.but, Two CI mass spectra were recorded for GC peaks e and f . The first mass spectrum of peak e was recorded with methane and the second with isobutane as a reagent gas. Peak f was first recorded with isobutane and then methane as the reagent gas. The mass spectra of GC peaks c and d in Figure 3 are shown in Figure 4.The E1 and CIL-but spectra show that even a fast change of the operating mode does not indicate any differences in the fragmentation pattern. In both cases, the E1 spectra have a base peak at mle 156 and in the CI mode mle 227 (M + 1) is the base peak. No cross contamination was observed in the mass spectra of GC peaks e and f in Figure 3. Figure 5 shows the switching time for GC-MS analysis for benzophenone between the E1 and CI,.bUt modes which was checked through the use of a multiple ion detector (MID) LKB 2091-710. The molecular ion (mle 182) and the (M H)+ion at mle 183 with the isotope peak at mle 184 were focused on the MID. As soon as the peak appeared for a few seconds in the E1 mode, the movable band and the ion source voltages were changed to the CI mode. This procedure was repeated during the time the GC peak eluted from the column. The mode of operation was changed from EI-CI-EI-CI in about 15 s. During this time, it should be possible to obtain
ANALYTICAL CHEMISTRY, VOL. 40, NO. 12, OCTOBER 1976
+
100
100
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Figure 4. M,ass spectra of pentobarbitone obtained from GC peaks c and d (Figure 3) c3 and d 3 were obtained in the El mode and c l and el were obtained in the CI mode with isobutone as a reagent gas
3
60
m/e 17
r
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,
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m/e 184
0
,
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,
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,
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,
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Figure 6. Intensities of the reactant ions CH5+ and C4H9+in the CI mode as a function of the recording and switching time when methane or isobutane, respectively, was used as a reagent gas
Lc*,
10 Seconds
I
Seconds
-
0
~
20
Figure 5. Intensities of ions at m/e 182, 183, and 184 of benzophenone as a function of the recording and switching time between the EI-CI-EI-CI modes three compllete 1.5-s scan mass spectra. To check the switching time of the reagent gases, the reactant ions mle 17 (CH5+)and mle 57 (C4~Hs+) were focused on the MID. The intensity of these ions was recorded when the reagent gases used were methane, isobutane, and methane, consecutively. Figure 6 shows the intensity of these ions as a function of the switching and recording time. After the first mass spectrum was taken, the reagent gas methane was switched off and a pumping time of 2 s was allowed before isobutane was introduced into the ion source. Thus, during a period of about 5 s, two CI mass spectra could be obtained. When methane was introduced a second time, a sharp peak of isobutane was seen, which means that the inlet line contained a small amount of isobutane. However, this did not influence the mass spectra obtained with methane as the reagent gas. From these experiments it could be concluded that, through a reduction in volume of the reagent gas inlet line, the influence of the remaining gas from the prior operation will be negligible. Furthermore, a reduction of the time required for two 1.5-s scan mass spectra using different reagent gases can be accomplished if proper synchronization of scan initiation and gas switching is made. Then the required time would be
about 4 s as compared to 5 s with the semiautomatic procedure reported above. Similarly, the required time to obtain two 1.5-s scan mass spectra can be reduced to 6 s as compared to 7 to 9 s if automatic control of EI/CI switching and scan initiation were employed. The moveable band which permits rapid changing of the operation mode can easily be exchanged for one having smaller or larger slits suitable for the type of samples to be studied and the reagent gas used. It is advantageous to run the mass spectrometer in the EI/CI mode of ionization for most of the compounds which show a low molecular ion intensity in the E1 mode. A fast change of the ionization mode is advisable when such samples are flashed from the direct probe inlet or when special components in a gas chromatographic analysis must be thoroughly investigated. Additional mass spectrometric data which can be obtained from such an instrument will greatly increase the certainty of identification. In particular, this could be of importance in molecule identification when computerized “library search” routines are used to assist analysts in obtaining correct answers. Since the GC-MS instrument used for this experiment is one of the first prototypes of LKB 2091, no time was spent on determing the maximum sensitivity and resolution. However, similar modifications which were made on this instrument have later been used for other LKB 2091 instruments. The specifications for these instruments regarding resolution and
ANALYTICAL CHEMISTRY, VOL. 48, NO. 12, OCTOBER 1976
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p
sensitivity are similar to the standard model in the E1 mode. In the CI mode, the resolution is the same as in the E1 mode but the sensitivity has not been established.
LITERATURE CITED (1)M. S.B. Munson and F. H. Field, J. Am. Chem. Soc., 88, 1621 (1966). (2)M. S.B. Munson, Anal. Chem., 43 (13),28A (1971). (3)H. M. Fales, G. W. A. Milne, and M. L. Vestal, J. Am. Chem. Soc., 91,3682 (1969). (4)H. M. Fales, G. W. A. Miine, and R. S. Nicholson, Anal. Chem., 43, 1785 (1971). (5) I. Dzldic, D. M. Desiderio, M. S. Wilson, P. F. Crain, and J. A. McCloskey, Anal. Chem., 43, 1677 (1971). (6) H. Miyazaki and Y. Hashimoto, J. Chromatogr., 99, 575 (1974).
(7)F. D. Hlleman. T. A. Elwood, M. L. Vestal, and J. H. Futrell, 22nd Annual Conference on Mass Spectrometry and Allied Topics, Philadeiphia, Pa, May 19-24, 1974. (8) G. P. Arsenault, J. J. Dolhun, and K. Biemann, Anal. Chem., 43, 1720
(1971). (9)I. 0.Oswald, D. Parks, T. €ling, and B. J. Corbett, J. Chromatogr., 93, 47 119741. (10)D:M. Schoengold and M. S. B. Munson, Anal. Chem., 42, 1811 (1970). (11) W. Kruger, N. Kuypers, and J. Michnowicz. 21st Annual Conferenceon Mass Spectrometry and Allied Topics, San Francisco, Calif., May 20-25, 1973. 479 666 (12)B. Hedfiall and R. Ryhage9Anal.
RECEIVEDfor review May 20, 1976. Accepted July 7, 1976. This work was made possible by grants from the Swedish Board for Technical Development, Knut and Alice Wallenbergs' Foundation and the Hedlund's Foundation.
Self-Positioning Anti-Vortex Plug for Nuclear Magnetic Resonance Sample Tubes LeRoy F. Johnson Nicolet Technology Corporation, Mountain View, Calif. 9404 1
When it is desired to use a minimum volume of solution in an NMR sample tube, anti-vortex plugs are generally used to prevent formation of a vortex when the sample tube is spun. This is particularly important when sample tubes of diameter near 20 mm are used ( I ) . Commercial antiyortex plugs (such as made by Wilmad Glass Co.) are designed to be a press-fit through action of pliable fins on the central part of the plug. The plug, which has an axial hole for air escape, tapped a t one end, is positioned in the tube with the use of a threaded rod. When used with large diameter (15-25 mm), thin wall (0.5 mm) tubes, a press-fit plug can rather easily break the tube during insertion. Also, variable temperature operation is a problem because expansion of the plastic plug during high temperature operation can break the tube, while contraction of the plug during low temperature operation can cause the plug to slide to the bottom of the tube. An anti-vortex plug which overcomes the problems mentioned above is illustrated in Figure 1.A cylinder of Teflon of length about 35 mm is machined to a smooth diameter about 0.2 mm less than the inner diameter of the sample tube. The
NMR
SAMPLE
TUBE
top 10-mm section of the cylinder is turned-down to a diameter of about 5 mm and a small radial hole is drilled through this section. The bottom of the plug is made slightly convex. The sample tube is spot-heated using a torch with a very small flame at an appropriate distance up from the bottom, and dimpled using a small metal rod. In use, the sample tube is filled to a depth just higher than the dimpled spot. The plug, inserted in the sample tube, will float down to the point where it rests on the indented spot. Any trapped air bubbles can be moved out of the active sample volume by shaking the sample tube as if it were a clinical thermometer. To remove the plug, a long piano wire with a 90" hook at the end is inserted in the radial hole of the plug. In extended use, the anti-vortex plug described here has proved to be very convenient to use. Spin rates up to 100 rps have been used without formation of an air bubble below the plug. Dynamic balance of the loaded sample tube is not appreciably affected since the dimple region is quite small compared to the overall sample volume. Although initially conceived for use with 18-25 mm tubes, this plug design has also been quite practical with 12-mm tubes as well. Of course, with smaller diameter tubes, care must be exercised in generating the dimple in the tube so as to not distort the straightness of the tube.
LITERATURE CITED (1) A. Allerhard, R. F. Childers, and E. Oldfield, J Magn. Reson., 11, 272 (1973). ANTI-VORTEX
PLUG
RECEIVEDfor review July 16, 1976. Accepted August 9, 1976.
DIMPLE
CORRECTION
A C T I V E S A M P L E VOLUME
Figure 1. Diagram of 1832
anti-vortex plug in an NMR sample tube
Kinetically Asslsted Equilibrium Based Repetitive Determinations of Iron( II) with Ferrozlne In Flow-Through Systems
The authors of this article [V. V. S. Eswara Dutt, A. Eskander-Hanna, and H. A. Mottola, Anal. Chem., 48, 1207 (1976)] would like to point out the omission of a necessary assumption for Equations 7 and 8 to be valid. Conditions leading to these equations require that kz' >> k-1 as well as the stated h l >> k - 1 .
ANALYTICAL CHEMISTRY, VOL. 48, NO. 12, OCTOBER 1976
