2594
Anal. Chem. 1984, 56,2504-2596
coefficients of -3,12,17,12, and -3 with norm of 35-and is superimposed on the equivalent of Figure la.) The SIN ratio shown in logarithmic form in curve Ib indicates the variation of the pure quantization noise during the prescribed autoranging between 12- and 10-bit acquisition accuracy. Curve 211a shows the effect of adding the constant amplitude bidirectional random noise before digitizing the simulated signal. The magnitude of the constant amplitude was selected so as to cause essentially no random noise at short times (when S is large) and up to 5% random noise a t long times (when S is small). While the brevity of this correspondence does not permit a detailed statistical correlation of the variance due to noise with the computed S I N ratio, it is quite clear from Figure 211 that an easily recognized pattern may be obtained (after Savitzky-Golay smoothing) even in the presence of as much as 1% noise. This observation, in turn, suggests that for the application described herein, full 12-bit A/D conversion accuracy may not be necessary with appropriate digital filtering. CONCLUSION The feasibility of using the digitally computed function A(ln i)/A(ln t ) in the elucidation of the rate-limiting mechanism of an electrochemical process has been demonstrated. In that it employs digital filtering to minimize the effect of noise, this method shares some of the advantages of conventional chronocoulometry where this filtering is performed via analogue integration; however, the primary disadvantage of conventional chronocoulometry-the inability to distinguish instantaneously among several time-dependent eventa- is overcome by using this method, and an instantaneous response to variations in the current caused by several different electrode processes may be readily obtained. Since a considerable amount of free computational time is available during the later stages of data acquisition if this strategy is employed, it also appears that on-line computer manipulation and processing of the data would be made more feasible in this manner. Moreover, as this work clearly demonstrates, it is possible to detect variations in d(ln i)/d(ln t ) of the order that would be observed in many electroanalytical experiments. Thus, this work suggests that decision concerning the control of data acquisitions and/or the on-line interpretation of acquired electrochemical data may be based upon the evaluation of this function. Since each rate-limiting process encountered in electroanalysis may be expected to exhibit its own unique d(ln i)/d(ln t ) characteristic, we believe that the experimental evaluation of A(ln i)/A(ln t ) and comparison with known characteristics that may be stored within limited memory as
relatively simple mathematical functions (to thereby minimize memory requirements) will prove to be most useful in the on-line elucidation of the mechanism. This comparison might be made by simplex fitting to a library of results as suggested by Ridgway ( I I ) , or by pattern recognition techniques as suggested by Perone (12),or by deviation-pattern recognition techniques as suggested by Meites (13)or Rusling (14). Regardless of the method of comparison, however, we believe that the computation of this function will prove to be most useful in the identification of the rate-limiting steps in any electrode process, whether that rate be controlled by homogeneous kinetics as in this somewhat trivial illustration or, more germane to electroanalysis, by mass transport, heterogeneous electrode kinetics, or capacitive/adsorptive effects such as double-layer charging. Investigations of the discriminatory capabilities of the function d(ln i)/d(ln t ) in the elucidation of electrode mechanisms under the control of each of these processes either alone or in combination (and the utilization of this information in the complete interpretation of the digitized current transient) are currently under way. LITERATURE C I T E D He, Pleixin; Avery, James, P.; Faulkner, Larry R. Anal. Chem. 1982, 54, 1313A-1326A. Soong, F. C.; Maloy, J. T. J . Electroanal. Chem. 1983, 153,29-41. Alberts, G. S.; Shain, I.Anal. Chem. 1983, 35, 1859-1866. Adams, Ralph N. “Electrochemistry at Solld Electrodes”; Marcel Dekker: New York, 1969; Chapter 8. Feldberg, S. W. Electroanal. Chem. 1989, 3 , 199-296. Lawson, R. J.; Maloy, J . T. Anal. Chem. 1974, 46,559-562. Bezilla, 8 . M.; Maioy, J. T. J . Electrochem. Soc 1979, 126, 579-583. Maloy, J. T. I n “Laboratory Techniques in Electroanalytical Chemistry”; Kissinger, P. T.,Heineman, W. R., Eds.; Marcel Dekker: New York, 1984; Chapter 16. Kelly, P. C.; Horlick, G. Anal. Chem. 1973, 45,518-527. Savltzky, A.; Golay, M. J . E. Anal. Chem. 1964, 36, 1627-1639. Hanafey, M. K.; Scott, R. L.; Ridgway, T. H.; Rellley, C. N. Anal. Chem. 1978, 50, 116-137. Schachterle, S.D.; Perone, S . P. Anal. Chem. 1981, 53, 1672-1678. Meites, L.; Shia, G. A. I n “Chemometrics”; Kowalski, B. R., Ed.; American Chemical Society: Washington DC 1977; pp 127-152. Rusling, J. F. Anal. Chem. 1983, 55, 1713-1718.
Arunee Therdteppitak J. T. Maloy* Department of Chemistry Seton Hall University South Orange, New Jersey 07079
RECEIVED for review March 26,1984. Accepted July 9,1984. This paper was presented, in part, at the National Meeting of the American Chemical Society, St. Louis, April 1984, as part of the ACS Award Symposium in Analytical Chemistry honoring Professor Allen J. Bard (paper ANYL 26).
Liquid Secondary Ion Time-of-Flight Mass Spectrometry Sir: We have been interested for some time in developing an approach to SIMS-TOF mass spectrometry which uses primary beams with fluxes of the order used in scanning sector instruments ( I ) and desorption of samples from the liquid phase (2). The time-of-flight mass analyzer has the advantage that high mass ranges can be observed without degrading the basic transmission of the analyzer which accompanies lowered accelerating potentials in the sector instruments. For many analyses in biochemistry for which unit resolution is not required, such an instrument may offer a low-cost alternative for molecular weight and sequence ion determination. This paper describes some preliminary investigations of this approach.
EXPERIMENTAL SECTION The instrumental configuration which we have used to test the feasibility of the approach uses a commercially available timeof-flight mass spectrometer (CVC-2000, Rochester, NY) of the type developed by Wiley and McLaren (3) and a Kratos (Ramsey, NJ) Minibeam I ion gun (Figure 1). Modifications to these instruments are minimal. A second 4-in. diffusion pumping system is mounted onto one of the unused ports of the source housing via a vacuum “tee”. The ion gun is mounted on the other side of the “tee” in such a way that it sits directly over the pump. With this additional pump, torr can be used, while the pressures in the ion gun up t o torr. analyzer and source regions are maintained at about 3 X No additional slits are required to maintain differential pressure,
0003-2700/84/0356-2594$01.50/00 1984 Amerlcan Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984
2595
HIGH FLUX PARTICLE BOMBARDMENT TIME OF FLIGHT MASS SPECTROMETER
ELECTRON
I"
I
I
POSITION
QRlD
ION DRAWOUT PULSE
PULSE
ION DRAWOUT PULSE
lsl0N
I
J/
MGNETIC ELECTRON Mlll T l P l IFR
I
-
SAMPLE PROBE
C I l ANALOG INTEGPATI& SCANNER
Flgure 1. Block dlagram of the liquid SIMS-TOF instrument. ECSlY0
,
CS'
GRID
m1%
133
I
CESIUM IODIDE
39.1~
n PULSE
4 6bJM 6KV
FILAMENT
(Eo- E,) 0 (5KV - 50V)
EFIL
Flgure 2. Schematic of modification of Minibeam I ion gun supply for pulsing by the time-of-flight circuitry. since the ion gun itself is designed to support 3-4 orders of magnitude differential pressure. The electron grid pulse from the CVC time-of-flight mass spectrometer, normally used to pulse the electron beam in the E1 mode, is used instead to pulse the gun. Since the gun itself is at 5 kV with respect to ground, this 30-V pulse is transmitted to the emission regulator through a high-voltage capacitor and the power amplified (as shown in Figure 2) in order to support high emission currents. The filament-to-grid voltage is switched from 10 to 40 V. Since the ionization potential of argon is 15.8 eV, this effectively turns the argon ion beam on and off to produce ion bursts of between 1 and 10 ps. In the experiments reported here, the pulse has an instantaneous current of 1p A for 5 ps, which corresponds to -3 X 10' primary ions/pulse. This pulse width is identical with that used in E1 operation of the instrument so that focusing is accomplished in the same manner by using a drawout pulse (3). The drawout pulse also triggers an oscilloscope connected to the anode of the magnetic electron multiplier so that secondary ion spectra can be observed as repetitive (10 kHz) traces. Samples were deposited from distilled water solution on glycerol on the surface of a (2 mm diameter) copper probe. When the ion gun is turned on, beam size, focus, and positioning controls are adjusted to maximize the primary ion current impinging on the sample probe, m measured by a Kiethley 410 B electrometer. The
AR+
40
li)
20
30
40
50
60
70
MICROSECONDS
Figure 3. "Scanned" time-of-flight mass spectra of cesium iodide. beam size ranges from 0.2 to 1mm. Spectra are observed on the oscilloscope and then recorded on an X-Y recorder using the analogue integrating scanner unit on a 10-8-A (full scale) range. Scanning gate width was about 100 ns, and the rate was about 2 min over the 100-ps range. (While this method of recording is somewhat primitive, it is sufficient for the experiment reported here, and a system which records all masses during each timeof-flight cycle is planned.)
RESULTS Spectra of CsI (without glycerol) and tubocurarine hydrochloride are shown in Figures 3 and 4,respectively. The latter spectrum was obtained after 15 min of irradiation, indicating the stability of the secondary ion beam. Estimates of the ion currents (based upon a multiplier gain of 104-105) indicate that the molecular ion current is about 5-50 ions/cycle and
Anal. Chem. 1984,56,2596-2598 TUBOCURARINE HYDROCHLORIDE :h SLYCEROL
ZCI
10
LJ
30
43
53
bC
70
-
80
MIC3OSECOVDS
Flgure 4. “Scanned” time-of-fllght mass spectrum of tubocurarine hydrochloride.
that total ion currents exceed
lo2 ions/cycle.
DISCUSSION Because of the large secondary ion currents produced from a “high-flux” primary ion beam, the experiments illustrate the femibility of using the TOF instrument in an “analogue mode”. That is, rapid analogue-to-digital conversion and signal-averaging techniques ( 4 ) can be used rather than ion-counting techniques as used in PDMS (5, 6) and other SIMS-TOF instruments (7). This approach provides an interesting alternative for samples which can take advantage of the solution chemistry available from the liquid matrix. For example, adjustment of the pH of glycerol or the use of p-toluenesulfonic acid can enhance protonation of some sample molecules (8). The use of tetraglyme as a matrix increases the desorption of higher mass clusters of cesium alkylsulfonates, which are used as high mass calibration standards (9). The use of glycerol is also compatible with an instrument which uses a grounded ion source and in which the sample/substrate is exposed only briefly (5 ps) to a low voltage (150 V) drawout field, rather than to the full accelerating potential. Spectra
of other compounds have been observed as well, such as the cyclic peptide cyclosporin (MW 1201) which can be observed on the oscilloscope but was not stable for the length of time (several minutes) required for analogue recording. Such spectra will in the future be recorded by fast (100 MHz) analogue-to-digital techniques similar to those describe earlier for laser desorption (4) but using faster repetition rates (100 Hz) which are possible with the pulsed ion gun. While this reduces the average flux, the instantaneous flux during the ionization period will continue to be of the order of microamperes. An important advantage of this approach is that it is based upon commerciallyavailable instrumentation, which we expect will speed its development in this and other laboratories as an acceptable and useful analytical instrument.
ACKNOWLEDGMENT We thank H. Saccho and Kratos Analytical Instruments for the loan of the Minibeam I ion gun. Registry No. Cesium iodide, 7789-17-5; tubocurarine hydrochloride, 57-94-3. LITERATURE CITED (1) Fenselau, C.; Yergey, J.; Heller, D. Int. J . Mass Spectrom. Ion Phys. 1983, 53, 5. (2) Surman, D. J.; Vickerman, J. C. J . Cbem. Research ( s ) 1981, 6 , 170. (3) Wiley, W. C.; McLaren, I. H. Rev. Sci. Instrum. 1955, 26, 1150. (4) Van Breemen, R. B.; Snow, M.; Cotter, R J. Int. J . Mass Spectrom. Ion Phvs. 1983, 49. 35. (5) MacFaLlane, R. D.; Torgerson, D. F. Science (Washington, D . C . ) 1976, 191, 920. (6) McNeal, C. J.; MacFarlane, R. D. J . Am. Chem. SOC. 1981, 103, I~DA
(7) C h i i ; B. T.; Standing, K. G. I n t . J . Mass Spectrom. Ion Phys. 1981, 4 0 , 185. (8) Fenselau, C.; Liberato, D.; Yergey, J.; Cotter, R. J.; Yergey, A. Anal. Chem., in press. (9) Heller, D. N.; Fenselau, C.; Cotter, R. J. Anal. Chem., submitted for publication.
Robert J. Cotter Department of Pharmacology The Johns Hopkins University Baltimore, Maryland 21205
RECEIVED for review May 23, 1984. Accepted July 9, 1984. This work was supported by Grants CHE 80-16440 and PCM 82-09954 from the National Science Foundation.
Microsample Liquid Analysis with a Wire-Loop Direct Sample Insertion Technique in an Inductively Coupled Plasma Sir: In several recent papers (1-5) various applications of the direct sample insertion device (DSID) as a sample introduction system for the inductively coupled plasma (ICP) have been discussed. Other sample introduction techniques for the ICP including furnaces, carbon cups, and tantalum strips have been used with some success (6-17). Most of the previous applications of the DSID have used a graphite electrode, though Horlick et al. (3) have reported using other materials for cups. Kirkbright and Walton ( 4 ) used a DSIDtype mechanism and made a brief mention of a tungsten wire loop; however, their investigation was extremely limited and suggested that the wire loop was of limited utility. We have developed a wire-loop system suitable for microsample liquid analysis using a unique DSID and have found the performance to be extremely good. We will report briefly on our prelim0003-2700/84/0356-2596$01.50/0
inary investigation of this system.
EXPERIMENTAL SECTION Most of the experimental apparatus is quite conventional and has been described previously (5). A Jarrell-Ash 1.0-m spectrometer with a 0.08-nm band-pass is used for all measurements. The PMT signal is amplified and offset by electronicswith a time constant of 0.01 s. Data are collected by an AIM-65 computer as a series of 512 12-bit data values at a rate of 750 Hz. Acquired data are background corrected for display by subtracting an averaged data set obtained with a bare wire. The data processing steps are illustrated in Figure 1. The DSID mechanism is changed from that reported originally (1) in one important aspect. The sample insertion system is a new design which is pneumatically powered by an independent argon gas cylinder. This is quite different from the stepper motor 0 1984 American Chemical Society
