554
Anal. Chem. 1983, 55, 554-557
Pulsed Signal Collection for Laser-Enhanced Ionization Spectrometry Mark A. Nippoldt‘ and Robert 6 . Green* Department of Chemisfry, Universi?y of Arkansas, Fayeffeville, Arkansas 7270 1
Potentlal and fleld relatlonships have been examlned for the nonintrusive split cathode/burner head anode electrode configuration. Pulsed laser-enhanced Ionization slgnal collection was investigated as a remedy for electrical Interferences. After voltage was applled to the cathodes, formation of the collectlng fleld requlred 1.5 ms. RC time constants for aspirated water and aqueous 100 I.c.g/mL lndlum were determined to be 1.0 f 0.1 ms and 1.3 f 0.1 ms, respectlvely. These resuns show that there Is no advantage to pulsed signal collection with external cathodes when compared with dc applled voltages. Time-resolved signal recovery was demonstrated as an Instrumental method for discrimlnatlng agalnst electrical Interferences.
In laser-enhanced ionization (LEI), thermal ionization of an analyte atom in a flame is enhanced by a pulsed dye laser tuned to an absorption transition. The laser-related current pulse which is detected with electrodes is a quantitive measure of the concentration of the absorbing species. LEI, a special case of the optogalvanic effect ( I ) , is being investigated for analytical flame spectrometry (2-13). Detection limits which are superior to those obtained with existing flame spectrometric methods have been reported for many metals (3-8). Since the LEI signal depends on both the analyte transition probability and the energy difference between the populated excited state and the ionization potential, many transitions which are not suitable for purely optical techniques have been used to great advantage. Stepwise (6-8) and two-photon (5) excitation processes have produced increased sensitivity and selectivity for many elements. A four-level atomic model has been developed to describe the LEI signal production mechanism by a combination of optical and collisional processes (9). Electrical interferences associated with high ion concentrations in the flame have complicated the determination of analytes in samples which contain large concentrations of low ionization potential concomitants. Although electrical interferences are important only for samples containing 1A and some 2A elements, this interference makes the analysis of some real samples more difficult. For this reason, a portion of LEI research has been directed toward understanding electrical interferences and mitigating their effects (10-13). Electrical interferences have been attributed to an ion space-charge a t the cathodes (3-5,10,11). This charge layer reduces the effective field which is responsible for signal collection. Plate electrodes were shown to have a considerably improved tolerance to signal suppression compared with rods (11). This was due to a reduction in the density of the ion sheath surrounding the cathode. Recently Turk has reported the use of a water-cooled, immersed cathode for LEI spectrometry (13). The improved resistance to electrical interferences was explained by use of a model system with two plane parallel electrodes. The ne-
’Present address: Riker Laboratories, 19901 Nordhoff St., Northridge, CA 91324.
cessity of a nonzero field for signal collection was emphasized. By excitation near the surface of the immersed cathode, the LEI signal for a 50 pg/mL iron analyte was recovered in the presence of several thousand micrograms per milliliter of sodium without suppression, but signal enhancement was still observed. The present work discusses the potential and field relationships for the split cathode/burner head anode configuration and illustrates the proportionality of the LEI signal and the electric field strength. The objective was to understand the potential and field relationships for the split cathode configuration so that pulsed signal collection could be investigated as a remedy for electrical interferences. If the voltage could be applied a t the appropriate time during the laser pulse, perhaps the LEI signal could be collected in the absence of interferences. The split cathode configuration is preferable to the immersed cathode because the former is nonintrusive. Also it may be used effectively for those samples without low ionization potential concomitants.
EXPERIMENTAL SECTION Apparatus. The basic experimental apparatus has been described previously (11). The excitation source, a CMX-4 linear flashlamp pumped dye laser with frequency doubling capability (Chromatix, Inc., Sunnyvale, CA), was operated at 10 Hz with a rhodamine 590 laser dye (Exciton Chemical Co., Dayton, OH). The laser pulse width was approximately 1ps. The laser was tuned to 303.9 nm, an indium resonance line. The acetylene/air flame was supported on a commercial atomic absorption premix burner (Model 370, Perkin-Elmer Corp., Norwalk, CT) with a 10-cm slot burner head. Molybdenum plate electrodes (1mm X 20 mm X 70 mm) were positioned with the bottom edge 10 mm above the burner head, 15 mm apart and parallel to the flame. The laser beam was positioned 18 mm above the burner and centered between the cathodes. The LEI signal was separated from the dc background with a 5 - ~time s constant RC filter. The unamplified signals were displayed on a Hewlett-Packard 1741A oscilloscope (Palo Alto, CA) and the amplitude was measured from the base line. A 5-mV background was observed and subtracted from the signals. This background signal was due to stray capacitive effects from the high voltage discharge that initiated the laser pulse. The applied voltage from a Sorenson Model 1003-200 power supply (South Norwalk, CT) was switched between 0 and -750 V with a rise time of less than 1 p s for all pulsed voltage experiments (see Figure la). For those experiments which required a dc applied voltage, the switching circuit was bypassed. The switching circuit shown in Figure l a was triggered with a Hewlett-Packard 8011A pulse generator which was triggered by the laser “sync out”. The pulse generator provided the proper pulse polarity and a variable pulse width from 25 ns to 0.1 s. A voltage pulse width of 4 ms was used for experiments reported here. To determine the relationship of the LEI pulse to the applied voltage pulse, we delayed the laser pulse with respect to the “sync out” pulse by inserting a delay circuit in the laser head between T P 8 on the timing and temperature control board and FL 2 (Figure lb). (Detailed circuit diagrams of the laser are available in the Chromatix CMX-4 operating manual.) The first one-shot provides a delay from 0 to 2 ms and the second reproduces the TP 8 pulse which is responsible for firing the flashlamp. In some experiments, the potential in the flame was monitored by placing a 1mm diameter tungsten rod in the flame at the laser beam position. The burner head functioned as the anode for these measurements.
0003-2700/83/0355-0554$01.50/0 0 1983 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 3, MARCH 1983 a
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Flgure 1. Circuits for pulsed signal collection experiments: (a) negative applied voltage pulser, (b) laser pulse delay circuit wlth tlming diagram.
Reagents. Aqueous standards were prepared from 99.97% indium metal (Matheson, Coleman and Bell, Northwood, OH) and reagent grade potassium chloride and magnesium chloride (Fisher Scientific Co., Fairlawn, NJ) according to the procedure described in ref 14.
RESULTS AND DISCUSSION As Turk illustrated, the variation of the electric potential and field between the cathode and the anode depends upon the volume ionization rate (13). In practice, the volume ionization rate is proportional to the ion concentration when the flame temperature and composition remain constant. In the usual experimental situation, concomitants with low ionization potentials aclcount for the bulk of the total ion concentration in the flame (10, 11). According to Turk's diagrams for plane parallel electrodes, a small addition of concomitant ions may increase the electric field strength midway between the electrodes where the laser beam would normally be positioned resulting in increased LEI signal (Le., enhancement). Larger increases in ion concentration result in the reduction of the field strength a t the midpoint causing LEI signal suppression. Beyond a characteristic ion concentration, the electric field a t the site of laser enhancement is zero. At these high ion concentrations a field exists near the cathode surface and only ions produced by laser enhancement in this region will be collected. This description of the potential and field contours for plane parallel electrodes implies the proportionality of the LEI signal and the field strength. Before pulsed signal collection experiments were undertaken, the potential and field relationships for the split cathode/burner head anode configuration were examined. One approach to understanding electrostatic fields involves a consideration of point charges, equipotential contours, and field force lines (15). Even though it is not entirely accurate to represent the electrodes1 as point charges, electric field maps have been derived by using this approach which can be readily translated to the simple two-electrode case described by Turk (13). Separating the cathode and using the burner head as the anode further complicate the analysis of potential and field relationships but an intuitive description of the electric field between the split cathode (andthe burner head anode (ground) may be suggested. Equipotential contours near an electrode are rings that are concentric with the electrode. Contours farther from an electrode are distorted concentric rings. Field lines between the cathodes and the burner head anode will be perpendicular to the equipotential contours. This will result in an electric field which extends from both cathodes to the anode with force linies which can be represented as arcs,
Laser Delay ( m s k
Flgure 2. LEI signal as; a function of the laser delay after application of voltage to the cathodes: (a) 100 pg/mL In, (b) 100 pg/mL I n "Ith 1 pg/mL K, (c) 100 pg/mL I n with 3 pg/mL K, (d) 100 pg/mL I n with 5 pg/mL K. The error bars associated wlth Individual measurements fall within the plotting circles.
Le., a t any point, the electric field for the split cathode configuration must be viewed as a vector. A vector sum of the forces a t a point exactly equidistant between the cathode plates has no component in the horizontal direction. In theory, cations produced by laser enhancement of thermal ionization a t this location should not be collected. (The effect of the flame velocity is not considered although it should contribute to the "loss" of cations.) Fortunately, the LEI signal recovered in the time domain of this experiment is due to the collection of electrons (16,17) which go to the burner head. The origin of the LEI signal was1 confirmed by measuring the LEI signal while translating the laser beam horizontally between the cathode plates. The LEI signal remained constant within the flame as one would expect if electron collection was responsible for the measured LEI signal. There was a loss of signal near the cathode surface which was due to the air gap between the flame and the cathodes. Photoelectric emission from the cathode surface was apparently responsible for a very small signal when the laser beam impinged on either cathode. Potential measurements made with a tungsten rod inserted in the flame a t the laser beam position, using the burner head as the anode, demonstrated the formation of the cation sheath (10). When 1 pg/mL potassium was added to a solution of 100 hg/mL indium, the potential drop between the tungsten rod, representing the laser beam, and the burner head was increased. This implied a decrease in the potential drop between the split cathodes and the tungsten rod, indicating withdrawal of the sheath toward the cathode surfaces. As thie potential measurement suggests, this level of potassium coritaminant suppressed the indium LEI signal. In some case,3, the addition of readily-ionized concomitants has been shown to enhance the LEI signd (10,11). When 6 pg/mL magnesium was added to the 100 ng/mL indium analyte, the potential drop between the tungsten rod, representing the laser beam, and the burner head (decreased. This implied an increase in field strength a t the laser beam position, which would result in an enhanced LEI signal for indium. Pulsing the applied voltage was investigated as a means of reducing the effects of the ion sheath a t the cathodes and improving the resistance of the split cathode configuration to electrical interferences. The results which are shown in Figure 2 illustrate the formation of the collecting field and the effect of potassium on it. The maximum LEI signal could be recovered only if the laser pulse was delayed a t least 1.5 ms after initiation of the applied voltage pulse. For delays greater than 1.5 ms and less than 4 ms, the LEI signal obtained with the pulsed applied voltage was identical with the dc experiment. Addition of potassium to the solution containing the indium analyte produced LEI signal suppression similai~
556
ANALYTICAL CHEMISTRY, VOL. 55, NO. 3, MARCH 1983
n
0 Figure 3. Discharge time response curves in the flame measured at the laser beam position: (a) 100 pg/mL In, (b) 100 pg/mL In with 1 pg/mL K, (c) 100 pg/mL In with cathodes moved into the flame.
to that observed previously with dc signal collection ( 1 0 , I I ) . Further investigation revealed that the delay in establishing the collection field was primarily due to charging of the electrical double layer a t the flame-air interface. Figure 3 shows the results of potential measurements made with a tungsten rod replacing the laser beam while pulsing the applied voltage on the split cathodes. These discharge time response curves indicate that, in the pulsed applied voltage mode, the measured potential between the tungsten rod (i.e., the laser beam position) and the burner head decreased with time. Therefore the potential between the signal collection cathodes, where the voltage was applied, and the burner head anode increased. This resulted in a time-dependent increase in the signal collection field. Therefore, when the laser was turned on near the beginning of the applied voltage pulse, the maximum LEI signal was not collected because the field had not reached its maximum value. The addition of a low ionization potential concomitant further retarded field development (see Figure 3b). If the level of concomitant concentration reduced the field below the threshold value for signal collection, no LEI signal was observed. With the following relationship, the RC time constant for field formation was calculated for the external split cathode configuration under several conditions. If Vo equals the initial potential of the tungsten rod at time zero and V equals the potential at time t , then
v = Voe-t/RC
(1)
By use of simple algebra and rearrangement of the terms, the equation may be rewritten as
When -In (V/ Vo) was plotted vs. t , a straight line with a zero intercept resulted. The inverse of the RC time constant is given by the slope. By use of a least-squares fit of the data, time constants for the aspiration of deionized water and aqueous 100 pg/mL indium were determined as 1.0 f 0.1 ms and 1.3 f 0.1 ms, respectively. The presence of additional ions in the flame contributed to a longer time constant as would be expected. These results show that there is no advantage to pulsed signal collection with external cathode configurations when compared with using dc applied voltages. The capacitance of the split cathode configuration was essentially eliminated by positioning the cathodes in contact with the flame (see Figure 3c). Experiments using pulsed applied voltage were repeated with the cathodes in contact with the flame. When the split cathodes were separated by 12 mm with the laser beam midway between them, the maximum LEI signal for indium was recovered when the laser pulse was delayed 200 ps after the applied voltage pulse. A LEI signal pulse was observed
Figure 4. Oscilloscope traces of sequential LEI signals for 100 pg/mL In with 5 pg/mL K. Note that while the transient signal increased to a maximum (1-3) and then decreased to zero with time (4, 5), the
background increased linearly. with as little as 70 ps delay. When a water-cooled, immersed cathode replaced the split cathodes and the laser beam was positioned near the cathode surface, the largest LEI signal for indium was recovered after an 8-ps laser delay. These experiments illustrate that although the capacitance of the signal collection system can be largely eliminated by placing the cathodes in contact with the flame, excitation near the cathode surface also appears to be essential for LEI signal recovery within the time domain of a laser pulse. Exciting near the surface of an immersed electrode has already been shown to reduce sodium interferences for less complicated dc signal collection (13). Since LEI signal enhancement has not been eliminated by using an immersed cathode (13),pulsing the applied voltage may still be useful. It is not clear if an immersed electrode will compensate for electrical interferences from concomitants with higher ion fractions than sodium in an acetylene/air flame. The use of higher temperature acetylene/nitrous oxide flames may also aggravate electrical interferences. Observation of the LEI signal at laser delays less than 8 ps was not possible because the signal was obscured by a large voltage derivative spike originating from the capacitor used to separate the signal from the background. The derivative spike is a practical limitation to further reductions in signal recovery times when a simple RC network is used for signal processing. During this work, an interesting example of the sensitivity of LEI was observed and an instrumental approach for discriminating against electrical interferences was demonstrated. When a sample is aspirated into a premix burner, it is diluted in the mixing chamber prior to introduction into the flame. The concentration in the flame reaches a steady-state value after a relatively short time. The sequence is reversed when sample aspiration is terminated. The progress of the sample concentration and dilution in the flame was followed by use of the oscilloscope in the storage mode. The LEI signal for 100 pg/mL indium reached a steady state 500 ms after sample aspiration was initiated. Figure 4 illustrates the sequence of events when potassium was a concomitant. The indium LEI signal increased until about 300 ms elapsed and then it decreased as the concentration of the thermally ionized concomitant began to suppress the signal. Table I illustrates the origin of the transient LEI pulse with data taken from previous steady-state work on electrical interferences ( I O ) . Potassium concentrations were chosen at five different times after the initiation of aspiration. Time 5 was assigned the potassium concentration which produced complete suppression of the LEI signal for 100 ng/mL indium with -800 V applied in a dc mode. As indicated in Table I, other indium concentrations maintain the same ratio between the analyte and concomitant after each time increment. The percent signal recovery was interpolated from the graph in Figure 5, ref 10. The LEI signal recovered was calculated based on one arbitrary signal unit per concentration unit. The example confirms that the LEI
Anal. Chem. 1983, 55, 557-564
--___
Registry No. In, 7440-74-6;K, 7440-09-7.
Table I. Origin of Transient LEI Pulse Derived from Steady-State Data ( 2 0)
LITERATURE CITED
time after aspiration (arbitrary units) concn of K (pg/mL) concnof In(ng/mL) %signal recovery LEI signala a
1
2
0.05
0.5
1
99 1
3
4
2.5 50
3.5
10
86 8.6
60 30.0
25 17.5
One signal unit/concentration unit.
_____I___
70
557
5 5 100 0 0
-_
signal for indium should reach a maximum and then return to a value which corresponds to the steady-state signal for indium with a potassiurn matrix as the experiment suggested. The maximum amplitude of the transient LEI signal was linear with indium concentration. Since it was also linear with potassium concentration, a standard addition technique would be necessary to quantify the indium. This approach permits the determination of an analyte in a matrix that would otherwise completely suppress the analyte signal. Observation of the transient LEI signal for the analyte is the instrumental analogue of sample dilution to reduce matrix interferences.
Green, R. 8.; Keller, R. A.; Schenck, P. K.; Travis, J. C.; Luther, G. C. Appl. Phys. Lett. 1978, 29, 727-729. Green, R. B.; Kell'er, R. A.; Schenck, P. K.; Travis, J. C.; Luther, G,C. J . Am. Chem. SOC. 1978, 98,8517-6518. Turk, G. C.; Travis, J. C.; DeVoe, J. R.; O'Haver, T. C. Anal. Chem. 1978, 50,817-820. Travis, J. C.; Turk, G. C.; Green, R. B. ACS Symp. Ser. 1978, No. 85,91-101. Turk, G. C.; Travis, J. C.; DeVoe, J. R.; O'Haver, T. C. Anal. Chem. 1979, 57,1890-1696. Turk, G. C.; Mallard, W. G.; Schenck, P. K.; Smyth, K. C. Anal. C b m . 1979, 51, 2408-2410. Gonchakov, A. S ; Zorov, N. B.; Uuzyakov, Yu. Ya.; Maveev, 0.I. Anal. Lett. 1979, 72,1037-1048. Turk, G. C.; DeVoe, J. R.; Travis, J. C. Anal. Chem. 1982, 54, 643-645. Travis, J. C.; Schenck, P. K.; Turk, G. C.; Mallard, W. C. Anal. Chem. 1979, 51, 1516-'1520. Green, R. B.; Havrilla, G. J.; Trask, T. 0. Appl. Spectrosc. 1980, 34, 561-569. Havrilla, G. J.; Gruen, R. B. Anal. Chem. 1980, 52,2376-2383. Green, R. B. Anal. Chem. 1981, 53,320-324. Trask, T. 0.; Turk, G. C. Anal. Chem. 1981, 53, 1187-1190. Smith, B. W.; Parsons, M. L. J . Chem. Educ. 1973, 5 0 , 679-681. Corson, D. R.; Lorraln, P. "Introduction to Electromagnetic Fields and Waves"; W. H. Frtseman: San Francisco, CA, 1962; Chapter 4. Smith, K. C.; Mallard, W. G. Combust. Sci. Techno/. 1981, 26, 35-41. Mallard, W. G.; Smyth, K. C. Combust. Name 1982, 44,61-70.
ACKNOWLEDGMENT The authors thank Ken McElveen, David Paul, and Ted Beeler for their help with the electrical aspects of this project. The authors also acknowledge the helpful comments provided by E. H. Piepmeier.
RECEIVED for review July 27, 1982. Accepted December 6, 1982. This research was supported by the National Science Foundation under the Arkansas EPSCOR Grant and NSF Grants CHE-79186126 and CHE-810500.
nhancement and Restoration of Chemical Images from Secondary Ion Mass Spectrometry and Ion Scattering Spectrometry Bernard G. M. Vandeginstel and Bruce R. Kowalski" Laboratory for Chomometrcs, Department of Chemistry BG- 10, University of Washington, Seattle, Washington 98 195
Digital image processing methods are applied to chemical images of surfaces obtained by a computer-controlled Ion beam spectrometer. Various image restoratlon and image enhancement methods are tested according to their abllltles to remove the blurring e4fect imposed on the ion scattering spectrometry or secondary Ion mass spectrometry images by using a scannlng Ion beam wlth a finite beam dlameter. These methods provlde llmproved spatial resolutlon allowing much finer detail to be observed whlle, In some cases, also improving the slgnal to noise ratio. The methods tested are part of a large image proccesslng system applicable to images produced by a number of surface analytical Instruments. They are seen as the first step In complete Image processhg.
Resolution in the time (chromatography), wavelengtb (spectroscopy), and spatial domains (surface analysis) is an important .topicin analytical chemistry as it limits the amount 'Permanent address: llepartment of Analytical Chemistry, University of Nijmegen, Toernooiveld, 6525ED Nijmegen, The Netherlands. 0003-2700/83/0355-0557$0 1.50/0
of information that can be obtained during analysis. In many cases the search for improved resolution has been the motive for both progress in instrumentation and the introduction and development of novel mathematical tools, such as fact(or analysis (I,Z),multiple linear regression analysis (3, 4 ) and the Fourier convolution theorem (5). Most of all present-dtiy chemical applications of these mathematical methods have been on one-dimensional analytical methods, where a signal is recorded as a function of one variable (e.g., spectra, chrlomatograms). A new and logical development initiated in analytical chemistry is the augmentation of the dimensionality of the analytical data, realized by linking two one-dimension,sl methods, e.g., GC/MS, HPLC/UV, etc. Here, traditional univariate statistical methods fail to extract the full amount of analytical information and should be replaced by multivariate methods. A recent example (6) is the application of multivariate curve resolution to GC/MS data to determine the number of compoinents in a complex mixture and estimate the spectra of the pure compounds even when the chromatographic peaks are only partially resolved. Recently. multidimensional analytical methods have been developed in the area (of surface analysis that produce chemical 0 1983 American Chemical Society
