Anal. Chem. 1989, 67, 1310-1312
1310 l.0y
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set function that is to be used to smooth the raw data. Next, the sample interval is specified and the basis set matrix, such as V, is calculated. The pseudoinverse is multiplied by the basis matrix to result in the matrix of rowwise impulse-response vector functions. For an odd smoothing interval of n = 2 N + 1, the first N 1 row vectors are used to estimate the values of the first N 1points. The interval of estimation is the first 2 N 1 raw data. These are not moving averages. Next, the middle, or ( N + 1)th row is convoluted with the middle raw data in a moving average algorithm. This convolution is performed up to the last point able to be estimated with this vector, N index units prior to the end of the data. The last N points are smoothed by using the last N row vectors in the matrix. As with the first data, the same interval is used for all N estimations. The effect of smoothing by using this type of filter is illustrated in Figure 2. The raw data was obtained by adding synthetic white noise to points described by a second-order polynomial. Although there are only 100 data points shown, a third-order filter with a smoothing interval length of 25 was used. Notice in particular the initial and final points of the smoothed data. Multipass smoothing can be performed by either successive application of the algorithm or by self-convolution of the impulse-response vector functions prior to a single-pass smooth (7). The latter is only useful for estimation of the data, not derivatives.
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Flgure 1 . Filter output variance ratio as a function of the location of
the estimate within the smoothing interval (offset index on horizontal axis) and the order of the polynomial (separate curves). The relative variance is symmetrical about the middle index point notated here as zero offset Thus only the first 13 out of a total of offset points are shown. The polynomial order ranges from 1, the bottom curve, to 7 at the top. The variance ratio lines do not cross but do coincide at certain points.
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ACKNOWLEDGMENT The author in indebted to one reviewer for pointing out that ref 8-10 report methods that are similar to those of this report. LITERATURE CITED
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SEQUENCE INDEX Figure 2. Output of a third-order poiynomial fitter of wklth 25 smoothing 100 synthetic raw data points. Raw data are shown as points and the
(1) Bialkowski, S. E. Anal. Chem. 1988, 6 0 , 355A-361A. (2) Bevington, P. R . Data Reduction and Error Analysis for the Physical Sciences: McGraw-HIII: New York, 1969. (3) Johnson, L. W.; Riess, R. D. Numerical Analysis. 2nd ed.; AddisonWesley Publishing: Reading, MA, 1982. (4) Golub, G. H.; VanLoan, C. F. Matrix Computations; Johns Hopkins University Press: Baltimore. MD, 1983. (5) Savitsky, A,; Golay, M. J. Anal. Chem. 1964, 3 6 , 1627-1639. (6) Enke, C. G.; Nieman, T. A. Anal. Chem. 1976, 4 8 , 705A-712A. (7) Bromba, M. U. A.; Zlegler. H. Anal. Chem. 1983, 55, 1299-1302. (8) Proctor, A,; Sherwood, P. M. Anal. Chem. 1980, 52, 2315-2321. (9) Leach, R . A,; Carter, C. A,; Harris, J. M. Anal. Chem. 1984, 5 6 , 2304-2307. (10) Wentzell, P. D.; Doherty, T. P.; Crouch, S. R. Anal. Chem. 1987, 5 9 , 367-371.
smoothed data as a continuous line. center" filters is adequate for the particular data, then better signal to noise ratios will be obtained by using these. The algorithm for single pass smoothing using the matrix computation scheme is apparent. One first selects the basis
RECEIVED for review July 15, 1988. Resubmitted January 24, 1989. Accepted March 9, 1989. This work was supported in part by CHE-8520050 awarded by the National Science Foundation.
Continuous Flow Fast Atom Bombardment Mass Spectrometry Using a Modified Electron Impact Source Colin S. Creaser* and Susan Crosland Srhool of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ, U.K
Continuous flow fast atom bombardment mass spectrometry (FAB-MS) (1-3) has been the subject of considerable interest recently because the range of solvents that may be used as matrix materials at room temperature is extended to include more volatile liquids, so enabling the technique to be used as a high-performance liquid chromatography (HPLC) interface ( 3 ) . The continuous flow of solvent may also overcorne sonie of' the quantitation problems experienced in 0003-2700/89/0361-13 lO$O 1.50/0
conventional FAB-MS (1) by reducing the effect of highly surface-active components of complex samples that concentrate at the liquid surface and dominate the spectrum. In order to use FAB for the qualitative and quantitative analysis of organophosphorus compounds (4-6), we required a continuous flow system that could be used regularly without causing undue disturbance to the normal operation of the spectrometer. In existing commercial instruments changing 1989 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 61, NO. 11, JUNE 1, 1989 la1 CONTINUOUS FLOW FAB
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1311
Schematic diagram of the continuous flow FAB system.
from electron impact (EI) to FAB conditions usually involves replacing the spectrometer source, thus limiting applications that include frequent comparisons of ionization procedures. In this paper we report the modification of a conventional E1 source for combined continuous flow FAB and E1 operation.
EXPERIMENTAL SECTION Conventional (static) FAB ionization was carried out by using a direct insertion probe (DIP) and a combined capillatron gun and sample target (Phrasor Scientific, Ltd.) (7). The gun/target is introduced through the existing E1 probe lock of a Kratos MS25 or MS30 spectrometer, which is temporarily extended to accomodate the longer FAB probe, a modification that takes only 2 or 3 min to perform. A schematic diagram of the continuous flow FAB system fitted to the source of a Kratos MS25 spectrometer is shown in Figure 1. The existing E1 source was modified for continuous flow FAB as follows: The guides from the gas chromatography (GC) and chemical ionization (CI) source inlets were removed, and a short section of fused-silicacapillary restrictor column (90 cm X 60 pm i.d.) was introduced into the spectrometer source, through the CI inlet, to carry the degassed matrix and sample. The end of this column was fitted into a 0.5 mm 0.d. syringe needle which was itself held inside a short length of narrow-bore stainless steel tubing (1/16-in.0.d. X 0.5-mm i.d. X 15 mm) and positioned in the source inlet by a section of l/s-in. copper sleeving. The target was formed by cutting one end of the 1/16-in.tubing to an angle of approximately 30" and placing the tip of the capillary and the needle flush with this face. Silcoset (IC1 plc) was used to form a seal between the column and the back of the tubing, to prevent sample and matrix from flowing back down the tube away from the target and also to hold the column in position. The final 2 mm of polyimide coating was removed from the end of the capillary. The target was positioned in the source by removing the ion repeller and feedng the free end of the capillary column through the source into the CI glass inlet; a '/4- to 1/16-in.reducing union (S.G.E., Ltd.) with a Kalrez ferrule (Du Pont) was used to maintain the vacuum. When the column and the capillaritron gun were in position, the geometry (Figure 1)was similar to that for the conventional DIP FAB gun/target alignment. The gun was operated by using xenon at 6 kV and 40 pA for all measurements, but the target tip was removed to allow bombardment of the capillary target in the continuous FAB mode. The spectra reported here were obtained at a source temperature of 50 "C and flow rates of 0.5-5 pL m i d . Samples were introduced into the capillary either by placing the end of the column directly in the sample or matrix or by injection from a Rheodyne Model 4010 valve with a 1-WLloop. The matrix flowed through the valve under a head pressure of up to 70 kP of helium. RESULTS AND DISCUSSION FAB spectra were readily obtained by using the modified source with a 5% glycero1:water matrix a t flow rates in the range 0.5-5 p L min-'. Over the period of each working day, up to 10 h, stability was good even a t operating pressures of Torr (measured by the source ion gauge) with a flow rate of 5 p L min-'. Varying the source temperature between room temperature and 50 "C appeared to have little effect on the stability of the signal but increased the rate of evaporation of the matrix from the target. Residual matrix and sample
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(a) Continuous flow FAB spectrum of a-hydroxypropyl diethylphosphonate. (b) Conventional FAB spectrum. Flgure 2.
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Figure 3. Ion current stability of the continuous flow FAB system for the protonated molecular ion of glycerol ( m / z 93).
6 Flgure 4.
12 MINUTES
Elution profile for a-hydroxypropyl diethylphosphonate.
was baked off overnight a t 50 "C. An example of the application of the continuous flow FAB system to the study of organophosphorus compounds is shown in Figure 2, in which the conventional and continuous flow FAB spectra of a-hydroxypropyl diethylphosphonate are compared. The two spectra correspond well, both showing the protonated molecular ion m / z 197 as base peak and major fragments a t m / z 169, 141, 121, and 93. Samples of other compounds (e.g., tetrabutylammonium chloride) also produced the expected spectra. Figure 3 shows the excellent ion current stability for the protonated molecular ion of glycerol ( m / z 93) in a 5% glycero1:water matrix for a period of over 90 s. This stability contrasts with earlier attempts to set up a continuous flow system in which the capillary was isolated from the source pottential and the resulting ion beam was found to be extremely unstable. The elution profile for a-hydroxypropyl diethylphosphonate shown in Figure 4 was obtained by monitoring the protonated molecular ion as the sample eluted from the column. Most of the sample eluted within 3 min, and the profile shows a very steep initial rise in ion current with a long tail. This eluant was introduced into the system by taking the end of the column out of the matrix, placing it into the prepared sample for a few seconds, and then returning the capillary to the matrix reservoir. The spikes on the profile are due to air that was introduced into the capillary with the sample. These spikes were less prominent for samples introduced via a Rheodyne valve connected to a matrix reservoir held under a head pressure of helium, but the memory effects remained. This is attributed to the accumulation of sample and matrix on the target, which is fully illuminated by the atom beam. Alternative target designs are being investigated to overcome these memory effects. A central feature of the modified source is its ability to perform electron impact as well as FAB ionization. If the capillaritron gun is withdrawn and the column capped after
ANALYTICAL CHEMISTRY, VOL. 61, NO. 11, JUNE 1, 1989
1312
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Figure 5. E1 spectrum of 2-methylnaphthaleneobtained by using the capillary as a vapor-phase inlet. first allowing the matrix remaining inside to evaporate, then the E1 probe can be used as normal. Conventional E1 spectra are obtained under these conditions a few minutes after switching from FAB and show no interfering glycerol background. The sensitivity is comparable with that obtained by using an unmodified E1 source. However, more frequent source cleaning is required if the combined source is used extensively for FAB measurements. The capillary column can also be used as a vapor-phase inlet system for electron impact mass spectrometry. Figure 5 shows the E1 spectrum of 2-methylnaphthalene vapor sampled by placing the end of the column into the headspace above the liquid. The specrum matches well with the conventional E1 spectrum, and there is no background interference. The capillary system may be used in this way to sample volatile components of mixtures or to monitor temporal changes in sample vapor concentrations. While performing FAB ionization, using the modified source, one can operate the filament simultaneously under E1 or CI conditions. Figure 1 shows the position of the FAB system in relation to the filament in the source. The two beams (Le., electrons and xenon atoms) are aligned such that they cross in the center of the ion volume. With the filament operating under normal EI/CI conditions (5 A, 70-100 eV) the observed intensities of the FAB matrix and sample ions remain unchanged, or may be substantially reduced, as is the case for the ammonium ion of tetrabutylammonium chloride. If, however, the filament current is reduced, an optimum electron flux is reached, which produces an enhancement of the signal by an order of ,nagnitude or more in some cases. No emission current is observed under these conditions, and the enhancement is not observed if the filament current is increased to the point at which an emission current is first detected. Figure 6 shows the enhancement of the [M + H]+ ion (mlz 197) of a-hydroxypropyl diethylphosphonate with the filament operated a t 4.5 A. The greatly increased “filament-enhanced” signal observed is associated with an increase in the number of ion-molecule reactions taking place under the high-pressure CI conditions in the source, since little or no enhancement is observed a t the lower pressure mbar) associated with static FAB experiments. A similar enhancement of the abundance of [M + H]+ ions has been reported for neat liquid samples in a high-pressure CI/FAB source (8,9). Further work is being carried out to investigate the optimum conditions for this filament-enhanced FAB effect. The modified E1 source provides a simple, versatile, and effective method for rapid switching between EI, FAB, and
F I lament on Filament o f f Figure 6. [M + H]’ ion FAB peak, m l z 197, of a-hydroxypropyl diethyiphosphonate with the filament on and off.
filament-enhanced FAB operation. This modification is not specific to the particular instrument described in this investigation, but is also applicable t~ other instruments fitted with an E1 source. The existing E1 source requires only minor modification and allows for rapid changeover from FAB to E1 conditions without a noticeable decrease in sensitivity or increase in background in either mode. Flow rates for dynamic FAB in the range 0.5-5 p L min-’ are possible, the upper rate being limited by the pumping capacity of the spectrometer. The interface also provides a convenient vapor-phase inlet for E1 samples, as an alternative to the customary gas inlet systems, and a means of enhancing the intensity of FAB ion currents.
LITERATURE CITED (1) Caprioli, R. M.; Fan, T.; Cottrell, J. S. Anal. Chem. 1986, 5 8 , 2949-2954. (2) Barber, M.; Tetler, L. W.; Bell, D.: Ashcroft, A. E.; Brown, R. S.; Moore, C. Org. Mass Spectrom. 1987, 22, 647. (3) Ashcroft, A. E. Org. Mass Spectrom. 1987, 22, 754. (4) Cameron, D. G.;Creaser, C. S.;Hudson, H. R.; Pianka, M.; Wright, H. Chem. Ind. 1984, 774-776. (5) Creaser, C. S.;Crosland, S.; Bawa, F.; Cameron, D. G.; Hudson, H. R.; Pianka, M.; Shode, 0. 0.; Volckrnan, J. F. R o c . X V Meet., BMSS 1988, 179. (6) Creaser, C. S.;Crosland, S.;Hudson, H. R.; Kow, A.; Powrozynck, L.; Shode, 0. 0. Org. Mass Spectrom. 1888, 23, 148-150. (7) Perel, J.; Faull, K.; Mahoney, S. J.; Tyler, A. N.; Barchas, J. D. Am. Lab. 1984, 16(11) 94-100. (8) Campana, J. E.; Freas, R . B. J. Chem. Soc.. Chem. Commun. 1984. 1414-1 415. (9) Freas, R. B.; Ross, M. M.; Carnpana, J. E. J . Am. Chem. Soc. 1985, 107. 6195-6201.
RECEIVED for review September 1, 1988. Revised February 27,1989. Accepted March 7,1989. We thank the Science and Engineering Research Council for the award of a studentship (to S.C.).
