Techniques in Gas Chromatography/Chemical Ionization Mass Spectrometry Frank Hatch and Burnaby Munson' Department of Chemistry, University of Delaware, Newark, Del. 197 1
A gas chromatograph has been directly coupled to a magnetic mass spectrometer wlthoui a separator. With different source configurations both low pressure (El) and high pressure (Ci) mass spectra can be obtained. The direct coupling causes little loss in chromatographicresolution. Operating parameters of the instrument are discussed and examples of GCICIMS are glven. Flows as high as 20 milmin (He, STP)or 10 ml/min (NP, CH4,STP)have been taken into the m a s spectrometer without degradation of performance.
The combination of a gas chromatograph (GC) and a mass spectrometer (MS) has aptly demonstrated its utility as a powerful analytical tool. Many combinations of different types of instruments with different types of interfaces have been reported and these combinations have been thoroughly reviewed (1-11). The interface between the gas chromatograph and the mass spectrometer is the critical point in the combination and the majority of all GC/MS instruments contain some type of separator for pressure reduction and sample concentration. The types of separators have been reviewed in detail (1-3, 9). Although the separator perform well over a wide range of operating conditions, none functions as a universal interface because of restrictions in allowed carrier gas, temperature range, sample hold up, necessary flow rates, etc. The simplest interface is a direct flow connection without a separator and all, or part of, the GC effluent is taken into the mass spectrometer. This type of interface has been used in several laboratories in conjunction with capillary columns with flows of 0.4 to 4 ml/min (STP) of He being taken directly into an open mass spectrometer source (12-15). One instrument with a very high capacity pumping system has been reported to handle 20 ml/min (STP) of He (16).In all of these systems the pressures in the mass spectrometers are reported to be very low, essentially comparable to conventional electron impact conditions, and the spectra are considered to be produced by electron impact (EI) only. It should be noted, however, that the pressures which are reported are those which are measured by an ionization gauge attached to the source housing chamber and no direct measurements have been reported of the pressures within the source or ionization chamber of the mass spectrometer. Even with an open design of the electron impact source, there is a significant pressure drop across the openings of the source or ionization chamber. Consequently, the pressures in the ionization region will be much higher in GC/EIMS instruments than in conventional E1 mass spectrometers. Although the majority of the material in the source is He and not sample, ion-molecule reactions involving sample molecules should be considered. The technique of chemical ionization (CI) mass spectrometry has become very popular during the past decade ( 17-22). Since this technique requires high source pressures (0.2 to 2 Torr), the development of GC/CIMS was obvious. Several laboratories have reported GC/CIMS combinations (12, 23-28) and commercial instruments are now available with GC/CIMS capability. Only TOF and quadrupole instruments
have been used in direct flow GC/CIMS because of possible discharges in magnetic instruments from the high voltage mass spectrometer source to the grounded gas chromatograph at high source pressures. The magnetic instruments which have been adapted for GC/CIMS have contained a separator in the interface and the CI reagent gas has been introduced through a separate inlet. Direct flow GC/CIMS offers the advantages of simple construction, low dead volume, lack of mass discrimination of sample, and the use of any carrier gas in the gas chromatograph. In this paper we present a simple direct flow interface that allows the operation of either GC/CIMS and GCIEIMS with a magnetic instrument and discuss the performance of the instrument and techniques in GC/CIMS operation.
EXPERIMENTAL Two microcapillary valves (Precision Sampling Corp.) are connected in series between the gas chromatograph and the mass spectrometer. One valve can be adjusted to control the flow of GC effluent into the mass spectrometer and the other valve functions as an off-on isolation valve to prevent solvent or unwanted samples from entering the mass spectrometer during a GC run or to allow independent operation of either the GC or mass spectrometer. The valve assembly is electrically insulated from the gas chromatograph by a piece of quartz tubing, 5 cm X 1.0 mm i.d. The quartz tubing is connected to an effluent splitter at the end of the chromatographic column by a 90-cm section of 0.25-mm i.d. stainless steel tubing. The tubing was passed through the oven of the thermal conductivity detector. Small leaks and excessive dead volume prevented the use of the thermal conductivity detector as an acceptable pathway. To reduce the dead volume all connections were made with stainless steel Swagelok fittings which had been filled with silver solder and then drilled out. The volume of the interface is about 50 pl. Vespel ferrules (heatable to 350 "C) were used in the connections to the quartz tubing. The connections were treated with dimethyldichlorosilae prior to the installation of the interface. The major portion of the pressure drop occurs across the microvalve assembly. Consequently, the exit of the gas chromatograph is essentially a t atmospheric pressure and discharges will not occur between the valve assembly and the grounded gas chromatograph. Because the valve assembly is electrically insulated from ground discharges do not occur from the source of the mass spectrometer to the valve assembly through the intermediate pressures in the transfer lines. The valves, however, are not a t a well defined potential and are fitted with plastic caps for safe adjustment when the instrument is in operation. The valve assembly may be opened or closed during an experiment without significantly altering the chromatographic separation. We generally close the interface during an experiment to prevent the passage of large amounts of solvent into the mass spectrometer. Because of geometric constraints in our laboratory, we were obliged to connect the gas chromatograph to the mass spectrometer through a long glass line (100 cm X 6 mm 0.d.) with one bend. There are no advantages from using a long transfer line and its elimination would be desirable; however, as we will demonstrate later, no significant deterioration of GC performance was noted with this long transfer line. The glass line is connected to the valve assembly by a flexible stainless steel bellows and a glass-to-metal seal. No discharges were observed to the grounded gas chromatograph under normal CI conditions a t room temperature. At elevated temperatures Pyrex conducts sufficiently to allow discharges through the glass walls to insulated heating tapes (29). The metal valve assembly presented similar problems. T o eliminate the discharges, the glass lines were coated with Ceramacast 505 (Aremco). These lines and the valve assembly were then wrapped with asbestos tape and mica tape ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
169
T a b l e I. C r i t i c a l Dimensions a n d A p p r o x i m a t e Pressures for GC/EIMS a n d GC/CIMS GC/EIMS (E1 source) Electron entrance: 3.0 X 1.5 = 4.5 mm2; Conductance 1 2 . 7 / m , 1.1s Ion exit: 6.4 X 0.25 = 1.6 mm2; Conductance 1 u r n , 1.h 5 ml(STP)/min, He: PS 1 0.05 Torr; PSH2 5 X Torr; P A 2 1 X 10-6 Torr GC/CIMS (CI source) Electron entrance: 0.51 d = 0.20 mm2; Conductance 1 0 . 1 2 / m , 1.1s Ion exit: 1.5 X 0.13 = 0.19 mm2; Conductance 1 0 . 1 1 / m , l./s 4 ml(STP)/min, PS 1 1 Torr; PSH Torr; P I 10-6 Torr CH4, N2: 10 ml(STP)/min, Ps 2 1 Torr: PSH2 1 X Torr; P A 2 1X Torr He: N
-
and then wrapped with glass insulated heating tapes which were wrapped with asbestos tape. The thermocouples were placed between the mica tape and asbestos tape. With this additional insulation, GCICIMS operation was satisfactory at interface temperatures up to 250 "C with the source at a potential of 6 kV and a t temperatures of 300 'C with the source a t 4 kV. The mass spectrometer is a Du Pont (CEC) 21-llOB mass spectrometer which has been modified previously for high pressure operation (30,31).Minor modifications have been made during the past few years; consequently, a brief description of the present system is given here. The source housing is evacuated by a baffled 4-inchTM oil diffusion pump with a rated speed of 680 l./s (air, 10-5 Torr) and Torr). a 2-inch diffusion pump with a rated speed of 150 1.1s (air, The pumping speed at the source was calculated and measured to be 250-280 l./s (air, Torr). The analyzer (or electric sector) is evacuated with two pumps, a 4-inch oil diffusion pump rated a t 500 l./s (air, Torr) and a 2-inch TM oil diffusion pump (180 l./sec, air, Torr). The pumping speed at the electric sector was estimated to be 150 l./s Torr, air). For all GC/MS experiments, the mass spectrometer was operated at low resolution, but no degradation of mass spectrometric resolution was noted until the highest flowrates and source pressures (>2 Torr) were used. Two different ionization chambers are used, one for GC/CIMS and the other for GC/EIMS. We do not have the capability of rapid changeover from one type of operation to the other without breaking the vacuum (32). The two sources can be interchanged in half a day for comparison of performance. Table I lists the critical dimensions and approximate pressures for both modes of operation. Source pressures, P,, are measured with a Texas Instruments quartz spiral Bourdon gage and are accurate to f0.005 Torr or better. The source housing pressures, P S H ,and analyzer pressures, PA, are estimated from ionization gauges, uncorrected for the nature of the gases. Since there are large pressure gradients within the source housing and the analyzer, these pressures are only rough estimates of the pressures in the flight path of the ions. The gas flows were measured with a bubble flowmeter which could be attached to the outlet of the gas chromatograph (flame ionization detector). The precision of the measurements was f 0 . 3 ml/min. The values which are given in the paper are the experimental results measured at room temperature ( 4 5 "C) and atmospheric pressure (-760 Torr). The gas flows to the mass spectrometer were obtained by difference between the flows at the outlet of the GC with the valve to the MS closed and opened to give the desired source pressure. Figure 1shows data for source pressure and flow rate into the mass spectrometer under different conditions. The different shaped points on each curve correspond to different experiments. The source pressure vs. flow rate curve was relatively reproducible from one source installation to another. Therefore, approximate values for the flow rates into the mass spectrometer could b e obtained from the measured source pressures and vice versa. The flow rate vs. pressure curves depend strongly on the nature of the gas, as shown in Figure 1 for He and Nz. The ratio of pressures at the same flow rate is about 3; = 2.6. The source pressure in the open, E1 source is much lower for the same flow rate than the source pressure in the tight CI source. The gas chromatograph was a Hewlett-Packard 7620 A with a flame ionization detector. The majority of the separations were obtained 170
ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
M Y TO ME8 SPIRRCt!ZTII.
Figure 1. Flow rate
ml/min
to mass spectrometer vs. source pressure
DELAY TIME
(seconds)
1
2
3
4
5
6
7
8
9
1 0 1 1 1 2
rnl/rnin FWW RATE TO MASS SPECTROMETER
Figure 2.
Delay time vs. flow rate to mass spectrometer
with a 4-m column (3.2-mm 0.d.) of 10% SE-30 80-100 mesh WHP. The flame ionization detector and the mass spectrometer source were connected in parallel so that both methods of detection could be used. Samples were generally 1.0 ~1 of solutions of 1or 10 fig/pl for studies on the delay time and loss of resolution. The aryl ketones were separated isothermally a t 190 OC or with a temperature program of 160 to 250 "C a t 4O/min. He was used as the carrier gas for all GC/EIMS experiments, and either He or Nz was used for GC/CIMS experiments. Other chemical ionization reactant gases were mixed in the transfer line prior to the source. The interface, mass spectrometer source, and injection port were generally maintained a t 200 "C and the flame ionization detector at 250 "C. Equal lengths of 0.25-mm ss tubing connect the splitter to the flame ionization detector and to the interface to the mass Spectrometer. This choice allowed optimal control of the flow into the mass spectrometer by the valve assembly. The upper limits of flow of GC effluent gas into the mass spectrometer were about 10 ml/min (STP) for Nz and CH4 or 20 ml/min
$1
CI SOURCE
44-
I
C
1
l
l
2
3
l
4
l
1
5
.
6
1
7
8
1
9
1
1
0
:
ml/min .FLOW RATE
1.0
2.0
I
3.0
I
4.0
To W S S SPECTROMETER
5.0
ml/min
Flgure 4.
FLOW RATE TO MASS SPECTROMETER
Figure 3.
Resolution loss vs. flow rate to mass spectrometer, CI
n
SOLVENT
Resolution loss vs. flow rate to mass spectrometer, El
(STP) for He. Above these flowrates the pressures in the source housing and analyzer became sufficiently large to produce significant
BUTYROPHENONE
decreases in mass spectrometricsensitivityand resolution. Very good performance of the GCiMS combination could be obtained by taking one third or one fourth of the GC effluent into the mass spectrometer; consequently,GC flows of 30-40 mlimin of Nz or 60-80 mlimin of He could be used.
RESULTS AND DISCUSSION The effects of the long transfer lines on the performance of the GC/MS were investigated. Figure 2 shows the variation in the delay time, the time difference between the peak maxima on the GC trace and on the MS trace, as a function of gas flow into the mass spectrometer. The points show the repeatabilities of the measurements, fl s and f 0 . 3 ml/min. Delay times were measured for a series of aryl ketones for isothermal and variable temperature GC experiments. The delay times were independent of the molecular weight and retention time of the compound at the same flow rate of the GC effluent into the mass spectrometer. The temperature of the interface is maintained sufficiently high to minimize absorption problems and is seldom changed. No effect of this temperature on the delay time was noted for the range used in our experiments. The delay time is caused primarily by the long glass transfer line of the interface. From these data, we estimate that the average linear gas velocity is about 6 cm/s a t a flow of 1ml/min and about 50 cm/s at a flow of 5 ml/min. A long delay time is not detrimental to GC/MS operation, but an estimate is needed, particularly in GC/CIMS, in order to initiate the scans a t the proper time. The flow rates to the mass spectrometer are generally 3 ml/min in low pressure E1 operation and 6 ml/min in high pressure CI operation; hence, the delay times are generally about 5-8 s in E1 operation and 0-3 s in CI operation, using He as the carrier gas. Comparisons were made of GC retention times, determined with the FID, with the valve to the mass spectrometer opened and closed to see if splitting the flow into the mass spectrometer altered the GC separation. A small effect was noted: a decrease in retention time with increasing split ratio to the mass spectrometer. This change can be attributed to the decrease in the restriction to gas flow a t the column outlet. There
Comparison of GC and MS traces: Aryl ketones, defocused made for M S operation (. . .); (-), FID
Figure 5.
was no difference in performance when the column effluent was split between the FID and the mass spectrometer or between the FID and the atmosphere. The effect does not alter the chromatographic pattern to any significant extent. A much more significant parameter is the loss in gas chromatographic resolution through the interface. There are several ways to report this loss in resolution or holdup: R is the number of theoretical plates of the column determined from the peak width of FID trace divided by the number of theoretical plates determined from the peak width of the MS trace (2).A value of 1 corresponds to no loss in resolution in the interface. Figure 3 shows a plot of R vs. flow rate to the mass spectrometer for GC/EIMS using He as the carrier gas. Estimates ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
171
1. CHLOROBENZENE
3 . BENZONITRILE
4. DICHLOROBENZENE 5. ACETOPHENONE
6. METHYL BENZOATE
I
3
1
Figure 7. Reactant ion
L)
monitoring: Ion current at m/e = 17 vs. time in
GC analysis of simple mixture
I
Figure 6. GCKIMS of
synthetic mixture of containing aryl ketones
Dual parallel operation; ( a )FID trace, all compounds; (b) (105) from N2/H20 reagent gas,P = 1.3 Torr, aryl ketones only. Mixture = Cz-Cg aryl ketones plus aliphatic and aromatic hydrocarbons
of repeatability of the measurements are shown. These values are the averages for seven aryl ketones for each experiment. No significant variation was observed in R for these compounds, C&5COCH3 through C6H5COC8H17. At flow rates above 3 ml/min, there is essentially no loss in resolution despite the long transfer line. These results were obtained with a packed column of approximately 6000 theoretical plates. At flow rates to the mass spectrometer of 1-3 ml/min, there is some deterioration of gas chromatographic performance as indicated by R values of 1-1.5. This loss in resolution is comparable to that reported for membrane separators ( I , 3 )and routine GC/MS operation is not impaired. Figure 4 shows similar data for GC/CIMS operation with two carrier gases, He and Nz.The points without error limits are values from Figure 2 for He a t low pressures in GC/EIMS operation. The two sets of data for He are very close. For flows above 3 ml/min, no significant decrease in resolution is noted. At low flow rates, it appears that there is significantly better performance with Nz than with He. Perhaps this difference results from a lower diffusion of the sample in NP than in He. A less quantitative indication of loss of resolution in the interface is shown in Figure 5. This figure shows GC and MS traces of a simple mixture of aryl ketones. These data were obtained with a packed column of about 6000 theoretical plates at 190 "C and a t a mass spectrometer source pressure of 0.7 Torr. The broken line corresponds to the MS trace. A small constant delay time is noted, about 5 s. However, the peaks are only shifted slightly to longer times and there is no obvious peak broadening or tailing caused by the interface. Essentially no loss in resolution is indicated for this experiment, R = 1.0. The valves were closed to prevent the solvent from entering the mass spectrometer in this experiment. The loss in resolution at low flow rates results from diffusional peak broadening in the interface. However, because the interface is long, adsorption can readily create significant peak broad172
ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
ening. The temperature of the interface is maintained sufficiently high to reduce these effects to a minimum and the interface has been deactivated by standard procedures ( I ) . We feel that the dual parallel operation of the GC/MS in which both GC and MS traces can be obtained offers distinct advantages over an instrument in which the mass spectrometer is the only detector. In GC/EIMS, the beam monitor (which catches a fraction of the unresolved ion beam) can be used as a mass spectrometric total ion current monitor if the ion current of He+ is a small constant value and does not obscure the other compounds as they pass through the source. In GC/CIMS, however, the ion current from the reactant ions is always large and the beam monitor cannot be used as a detector for the individual compounds because there is little or no change in the total ion current (reactant plus sample ions) across a GC peak. It is possible to obtain a TIC chromatogram during GC/CIMS operation by employing a defocusing technique (1).The ion-focusing voltages and electrostatic field plate voltages are adjusted at a fixed (usually high) magnetic field to give a low value of the total ion current when only the low mass reactant ions are present. There is significantly less discrimination against the higher mass product ions from the sample; consequently, an increase in the TIC is noted as the samples come through the source of the mass spectrometer. This defocusing depends on the fringing field of the magnet and the TIC chromatogram is perturbed when a mass spectrum is recorded by changing the magnetic field. The technique has proved satisfactory for He, CH4, and Nz as reagent gases. Selected ion recording can be used to identify compounds with the same functional group in GC/EIMS. This procedure is less useful in GC/CIMS when one uses a weak, highly selective reagent gas that gives little fragmentation, because there is no common ion in the spectra of similar compounds. However, if one uses an energetic reagent gas that gives a common fragment ion (Hz or mixtures of He or Nz with small etc), then the selected amounts of CH4, HzO, i-CgHlo, "3, ion recording can be very useful in GC/CIMS for identifying compounds of the same class. Figure 6 shows comparative traces for a mixture of aryl ketones with other compounds. The GC trace (FID) shows all of the compounds and the MS trace at m/e = 105 with Nz/HzO reagent gas shows only those compounds containing the -CBH&O group. Essentially equivalent results are obtained by monitoring m/e = 105 in GC/EIMS experiments. For greater surety of the assignment, one would wish to use more than one ion. The advantage of the use of the high pressure technique of CI or CE for functional group identification is that few fragment ions are produced per compound; hence there is less chance of accidental overlap of different ions of the same masses.
Table 111.Spectra of Hexanophenone
Table 11. CI Spectra of Hexanophenone CHd/He Mixtures, % I / 2 I
E1 Mass
oven
GCEIMS, 70 eV, 0.05 Torr He
51 77 105 120 176 177
9.6 22.4 32.5 16.7 2.2 0.6
4.6 18.5 38.2 22.6 3.8 0.8
70 eV,
CHd/He Ion
Mass
C6H5 C5H11CO CBH~CO
77 99 105 120 175 176 177 205 217
M-H M M+H M+CzH5 M+CsHs
P , Torr
1:O
1:l
1:5
1:8
1:12
...
.,.
7.4 6.6 3.3
8.1 2.1
14.5 11.0 6.6 3.0 1.6 45.0 4.5 1.8
0.6
0.6
1.4 11.2 11.8 9.5 4.0 2.5 40.9 5.3 2.5 0.6
1.2 9.5 10.2 9.2 3.7 3.1 43.1 5.2 3.7 0.9
1.4 10.3 10.6 12.4 3.4 3.2 35.8 5.3 4.6 1.3
4.0 0.2 59.9
Another trace that is the equivalent of a total ion current trace can be obtained only in high pressure (CI or CE) studies. Figure 7 shows preliminary work on Reactant Ion Monitoring, in which the ion current for one of the reactant ions, CH5+ in this case, is monitored during a GC separation (He carrier gas; CH4 added separately). The technique is experimentally very similar to the SIM technique, except that the ion current of a reactant ion rather than a product ion is being monitored. There is a decrease in the ion current of CH5+ (and all reactant ions) as each component of a mixtures passes through the source of the mass spectrometer. The decrease in the ion current of the reactant ion is related to the concentration of each compound and the rate constant for reaction of the reactant ion with that compound. It is a direct measure of the sensitivity of that compound with the reactant ion provided that there is no discrimination between compounds in the interface between the mass spectrometer and the gas chromatograph. The lack of discrimination by molecular weight or molecular type is characteristic of our interface without a separator, but discrimination is observed for separators (1). Additional work is being done to study the sehsitivity of this technique and also to evaluate i t as a means of determining rate constants for reaction with complex molecules. GC/CIMS can be done with carrier gases other than those which give essentially no flame ions, CH4 or i-CdH10vs. He or Nz. However, the advantages of dual recording are lost because the usual CI reagent gases cannot be used with a flame ionization detector and will have very low sensitivities with a thermal conductivity detector. One obvious solution to the problem which allows a great degree of flexibility in carrier gas type and carrier gas flow is to mix the desired CI reagent gas with the GC carrier gas after passing through the column but before entering the source. Table I1 shows typical CH4CI spectra obtained a t different CH4/He ratios. These results may be compared with a spectrum of C G H & O C ~ H obtained ~~ with pure CH4, also shown in Table 11.Although there is more fragmentation with the mixtures than with pure CH4, the major decomposition products are the same and there is a sufficient ion current a t (M H)+ and (M C&)+ to recognize the compound. Because the spectra obtained with the He/CH4 mixtures are relatively insensitive to the mixture composition, precise control of the mixture composition is not necessary to allow identification of the compounds from their spectra. Similar observations were made with He/i-C4Hlo mixtures. The pressure dependence of CI mass spectra is seldom reported, perhaps because most of the instruments do not contain a convenient direct pressure measuring device. We have reported previously that for single component reagent gases the spectra were independent of pressure from 0.2 to 2 Torr when the reactant ion distribution is independent of pressure (31,33).A similar invariance has been reported by
+
+
GC/CIMS, 500 eV, 0.6 Torr He 9.8 37.2 23.8 5.2 0.5 1.6
GC/CIMS, NZ,
0.8 Torr
... 7.8 48.3 11.2 1.0 15.7
other workers (34,35).From the general lack of comments in the literature about sensitivity of CI spectra to experimental parameters, it appears that there is no general problem about the pressure dependence of CI spectra, although there have been a few reports of a strong pressure dependence of CI spectra (36,37). With He as a carrier gas, one can obtain either low pressure spectra using the open E1 source or high pressure spectra with the tight CI source. Table I11 shows a comparison of data under these conditions. For C G H ~ C O C (Table ~ H ~ ~111) and several other compounds, the spectra obtained on our instrument under GC/EIMS conditions (source pressure of about 0.05 Torr of He) are very similar to those obtained under conventional electron impact conditions (oven or probe introduction, source pressure less than 0.001 Torr). The high pressure spectra with He as the carrier-reagent gas show significantly more fragmentation and less lower abundances of M+ ions. The observation that the GC/EIMS spectra do not contain significantly larger amounts of fragmentation than are noted in the conventional E1 spectra indicates that charge exchange reactions of He+ or Hez+ and other ion-molecule reactions are not significant under these conditions. In many cases, as indicated for the high pressure He and Nz spectra, (M H ) + rather than M+ ions are prominent, because of reactions of hydrogen-containing impurities (most probably H,O+); consequently isotope ratios for molecular ions from charge exchange spectra are likely to be unreliable. Studies on the relative sensitivities of GC/CIMS and GC/EIMS in our instrument are under way and will be reported later. Samples of several nanograms to the mass spectrometer can be readily detected. Our direct interface can be compared with other interfaces in the Yield = 100 (sample to MS)/(sample to GC) ( 3 ) .The maximum flow rates that can be handled by the mass spectrometer are approximately 20 ml/min of He or 10 ml/min of CH4 or Na ( P P 2 Torr). The yield of the interface is the split ratio of (Flow to MS)/(Flow from GC). If we consider a flow through the GC of 30 ml/min and a flow of 10-15 ml/min to the MS, then the yield is 3350%. This value compares quite favorably with the yields of several types of separators ( 2 , 3 ) .Enrichment is not a factor here since the high pressure is needed for sample ion-production. Similarly, in low pressure operation with the open E1 source, yields of about 15% can be achieved. For both low pressure and high pressure operation, the yield is constant for all compounds of a sample and does not depend on molecular weight or molecular type.
+
LITERATURE CITED (1) W. H. McFadden, “Technique of Combined Gas ChromatographylMass Spectrometry: Applications in Organic Analysis”, Wiley-lnterscience, New York, 1973. (2) G. A. Junk, lnf. J. Mass Spectrom. /on Phys., 8, 1 (1972). (3) C. F. Simpson, CRC Crit. Rev. Anal. Chem., 3, 1 (1972). (4) C. Fenselau. Appl. Spectros., 28, 305 (1974). (5) A. L. Burlingame and G. A. Johnason, Anal. Chem., 44,337R (1972). (6) F. W. Karasek, Anal. Chem., 44 (4), 32A (1972). (7) J. T. Watson in “Ancillary Techniques of Gas Chromatography”, L. S. Ettre and W. H. McFadden, Ed., Wiley-lnterscience, New York, 1969. (8) D.I . Rees, Talanta, 16, 903 (1969). ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
173
(9) A. N. Freedman, Anal. Chim. Acta, 59 (1972), 19 Rev. (IO) C. Merritt, Appl. Spectrosc. Rev., 3, 263 (1970). (11) A. E. Gordon and A. Frigerio, J. Chromatogr, Chromatogr. Rev., 73, 401 (1972). (12) W. Blum and W. J. Richter, Tetrahedron Lett., 11, 835 (1973). (13) J. G. Leferink and P. A. Leclercq, J. Chromatogr., 91, 385 (1974). (14) W. H. McFadden. R. Teranishi, D. R. Black, and J. Day, J. FoodSci., 28, 316 (1963). (15) R. Teranishi, R. G. Buttery, W. H. McFadden, T. R. Mon, and Jan Wasserman, Anal. Chem., 36, 1509 (1964). (16) W. Henderson and G. Steel, Anal. Chem., 44, 2302 (1972). (17) M. S. B. Munson and F. H. Field, J. Am. Chem. Soc., 88, 2621 (1966). (18) A. L. Burlingame, R. E. Cox, and P. J. Derrick, Anal. Chem., 46, 248R (1974). (19) F. H. Field, "Ion Molecule Reactions", J. L. Franklin, Ed., Plenum Press, New York, 1972, pp 261-312. (20) M. S. B. Munson, Anal. Chem., 43 (13), 28A (1971). (21) D. F. Hunt, Progr. Anal. Chem., 6, 359 (1973). (22) G. W. A. Milne and M. J. Lacey, "Modern ionization Techniques in Mass Spectrometry", Crit. Rev. Anal. Chem., 45 (1974). (23) . , G. P. Arsenauit. J. J. Dolhun. and K. Biemann. Chem. Commun., 1542 (1970). (24) D. M. Schoengold and B. Munson, Anal. Chem., 42, 1811 (1970). (25) G. P. Arsenault, J. J. Doihun, and K. Beimann, Anal. Chem., 43, 1720 f\ l.9- 7 . 1. \I .
(26) E. 0. Oswald, L. Fishbein, B. J. Corbeth, and M. P. Waler, J. Chromatogr., 73, 43 (1972).
(27) M. G. Horning, J. Nowlin, K. Lertratanangkoon,R. N. Stillwell, W. G. Stillwell, and R. M. Hill, Clin. Chem. ( Winston-Salem, N.C.), 19, 845 (1973). (28) E. 0.Oswald. P. W. Albro, and J. S. McKinney, J. Chromatogr., 98, 363 (1974). (29) J. H. Futrell and Leonard H. Wojick, Rev. Sci. Instrum., 42, 244 (1971). (30) J. Michnowicz and B. Munson, Org. Mass Spectrom., 4, 481 (1970). (31) N. Einolf and B. Munson, lnt. J. Mass Spectrom /on fhys., 9, 141 (1972). (32) W.Kruger, N. Kuypers, and J. Michnowicz, Paper S-4, presented at 21st Conference on Mass Spectrometry, San Francisco, Calif., May 1973. (33) J. Michnowicz and B. Munson, Org. Mass Spectrom., 6, 283 (1972). (34) A. M. Hogg, Paper L3. presented at 21st Conference on Mass Spectrometry, San Francisco, Calif., May 1973. (35) D. P. Beggs, R. C. Dougherty, and W. H. Johnston, Paper J-6, presented at 19th Conference on Mass Spectrometry, May 1971. (36) J. Yinon and H. G. Boettger, Paper P-3, presented at 20th Conference on Mass Spectrometry, May 1972. (37) E. 0. Oswald, P. W. Albro, and J. D. McKinney, J. Chromatogr., 98, 363 (1974).
RECEIVED for review August 25,1976. Accepted October 15, 1976. This work was supported in part by a grant from the National Science Foundation, GP20231.
Defocused Metastable Scanning with a Mass Spectrometer and Laboratory Data System Alexander
M. Ferguson, Stephen A.
Gwyn, Lewis K. Pannell, and Graeme J. Wright*
Chemistry Department, University of Canterbury, Christchurch, New Zealand
The use of a laboratory data system with a double focusing mass spectrometer to permit rapid determination of metastable transitions to selected daughter ions by acceleratingvoltage scanning is described. The data system controls the accelerating voltage for tuning the daughter ion and scanning. The system provides four mass ranges ( m to 8 m, 4 m, 2 m, and 1.3 m for daughter ion mass m ) and three scan rates (30,90, and 240 s). A complete scan analysis including peak detection, operator selection of basellne and peak window parameters, and printout of precursor masses requires less than 2 min. Accuracy and precislon of the system are shown to be the same as manual scanning, giving mass assignment to better than 0.4 amu, and sensitivity is about eight times better than the manual mode.
Metastable ions, arising from ion decompositions outside the mass spectrometer ion source, are used routinely in the interpretation of mass spectra, and for the elucidation of ion structures, reaction pathways, and decomposition energetics in mass spectrometry. Their use has been the subject of a large number of papers, several recent reviews ( I ) , and a major book ( 2 ) , and needs no elaboration in this paper. Different mass spectrometers have different metastable characteristics, determined mainly by the geometry of their ion optics; the work described here was designed to take advantage of one of the metastable scanning modes of an AEI MS902 instrument with Nier-Johnson geometry. In this, as in most mass spectrometers, the largest number of metastable transitions usually occur in the first field-free region between the source and the electrostatic analyzer (ESA). Decompositions which take place in this region do not appear in the normal mass spectrum recorded a t the collector. However, two metastable defocusing techniques can be used to define all the transitions leading to a particular daughter ion which occur in the first field-free 174
ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
region. These are a) scanning the ion accelerating voltage at a fixed ESA voltage ( 3 ) and b) scanning the ESA voltage at fixed accelerating voltage ( 4 ) . Both techniques have advantages; we used the first because our instrument is equipped with an accelerating voltage supply which can be externally controlled, but our method would be equally applicable to the second technique. Manual metastable defocusing by accelerating voltage scanning on the MS902 and similar instruments requires that each daughter ion be focused magnetically a t a reduced accelerating voltage; Vis then ramped manually or electrically back to 8 kV. These methods use either meter or oscillographic recorder output and they are tedious and of limited sensitivity. An early paper ( 5 ) reported a semi-automatic system for metastable defocusing of single precursors, and a very sophisticated automatic system which scans all precursor masses has been reported recently (6). Our own requirements fell between these two; we wanted the speed and sensitivity of a computer-controlled system, without the need for costly hardware additions to our DEC Lab8/e data system. We also wanted flexible data manipulation and analysis facilities and visual display of the data. This paper describes a system which meets these requirements. Although designed for the MS902-LabWe pair, it could easily be adapted to most other mass spectrometer-computer combinations.
EXPERIMENTAL A schematic diagram of t h e mass spectrometer-computer system used in t h i s w o r k is shown in Figure 1. Mass Spectrometer. T h e MS902 mass spectrometer has been extensively modified, w i t h m o s t o f t h e original electronics replaced by solid state amplifiers and power supplies. T h e key changes relevant t o t h i s w o r k are a) a Spellman RHSR-1OP h i g h voltage power source for the accelerating voltage supply and b) a K e i t h l e y 301 electrometer amplifier in place o f t h e original m o n i t o r amplifier. T h e mass spectrometer accelerating voltage ( o u t p u t o f t h e Spellman RHSR power supply) can b e controlled either m a n u a l l y by means of a f r o n t p a n e l
