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Anal. Chem. 1983, 55, 1432-1434
Response Time in Electrochemical Cells Containing Ion-Selective Electrodes Sir: Recently, several papers have appeared dealing with the response time of ion-selectiveelectrodes (ISEs) in solutions having only potential-determining ions. Reports have also been published about the response time of ISEs in the two ionic range where both potential-determining and interfering ions are present (1-4). Response time of electrochemical cells with ISEs becomes important to every user when the time scale of the combined measurement is comparable to the response time itself. This happens in the dynamic measuring system or in various applications such as flow injections, flow titrations, and continuous dilution techniques. It is not explained very exactly in every case what is meant by response time. What we generally measure are the timedependent potential values of the cell after change of the activity of the appropriate ions. The measured values are therefore response times of the cell itself. The cell contains two electrodes, and in the case of the cell with the transference, a solution interface is also present. Therefore, response time of the cell may incorporate some of the changes of all these components. A very important question is whether we can distinguish among the response times of these potential sources. It is clear that if the ISE response is faster than the other potential sources, then it is possible to distinguish between the response time of the ISE and that of the other sources by measuring the fast part of the potential response in time. This can happen in some precipitate-based electrodes because the response time of such electrodes is very fast. The other extreme possibility is if the ISE responds very slowly. In this case the influence of other sources becomes negligible. In all cases between the two extreme cases, it cannot be properly distinguished how much is the response time of the ISE. In the literature (5-7)) many kinds of response curves can be found. In some cases, such curves were reported which cannot be described by a unique equation (1). This is understandable in terms of a superposition of two or more reactions with different processes. In these cases, conventional definitions of response time (8,9),meaning tM),tW,tg5,tw and t*, cannot give meaningful information. A previous correspondence (4)pointed out some logical paradoxes involved in these conventional definitions of response time and gave the differential quotient of the potential-time as a technically useful parameter. For the measurement of the ISE response time, it is nec-
essary to use systems in which the response time cannot be confused with a mixing time of the solution and in which the adhering layer of the electrode surface can be diminished as thin as possible. Only in these cases, can we hope to find a fingerprint of the reactions corresponding to the ionic transport to and/or from the surface of the electrode. In the literature, there are such types of systems (2, 3, 10). They can be constructed with either a wall-jet assembly or capillary systems, etc. There is a group of electrodes where the potential response is below 1s. In these cases, only the response time can probably be described with the help of the first-order reaction and we can use these rate constants as the response time. In cases where ISE response is slow and comparable with other processes, it is of no value to determine the response time for the ISE alone, but those for the electrochemical cells are valid.
ACKNOWLEDGMENT The authors express their sincere thanks to the Japan Society for the Promotion of Science and the Hungarian Academy of Sciences.
LITERATURE CITED Shatkay, A. Anal. Chem. 1976, 48, 1039-1050. Llndner, E.; Toth, K.; Pungor, E. Anal. Chem. 1082, 5 4 , 72-76. Llndner, E.; Toth, K.; Pungor, E. Anal. Chem. 1082, 54. 202-207. Uemasu, I.; Umezawa, Y. Anal. Chem. 1082, 5 4 , 1198-1200. Rechnitz, G. A.; Kresz, M. R. Anal. Chem. 1966, 3 8 , 1786-1768. Fleet, 6.; Ryan, T. H.; Brand, M. J. D. Anal. Chem. 1974, 4 6 , 12-15, Blaedel, W. J.; Dlnwlddle, D. E. Anal. Chem. 1975, 47, 1070-1073. (8) Pure Appl. Chem. 1076, 48, 127-132. (9) IUPAC I n f . Bull. 1978, No. I , 69-74. ( I O ) Llndner, E.; Toth, K.; Pungor, E. Bunsekl Kagaku 1981, 3 0 , S67-S91 (In English).
(1) (2) (3) (4) (5) (6) (7)
Ern0 Pungor Institute for General and Analytical Chemistry Technical University 1521 Budapest, Hungary
Yoshio Umezawa* Department of Chemistry Faculty of Science The University of Tokyo Hongo, Tokyo 113, Japan
RECEIVED for review January 12,1983. Accepted March 22, 1983.
AIDS FOR ANALYTICAL CHEMISTS Reduction of Electronic Noise in Inductively Coupled Plasma Atomic Emission and Fluorescence Spectrometric Measurements G. L. Long, E. G. Voigtman, M. A. Kosinski, and J. D. Wlnefordner" Department of Chemistry, Unlverslty of Florida, Galnesville, Florida 326 1 1
The magnitude of the noise associated with an analytical measurement greatly affects the limit of detection for an analytical method. In most cases, the limit of detection, cL,
is calculated as
cL = 3sb/m
0003-2700~8~/0355-1432$0~ .50/0 0 1983 American Chemlcal Society
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Figure 1. Diagram of the 1104 filter unit using one Corcom RFI power line filter (20R6). The metal oxide varistors, MOV, used are (a) power MOV (General Electric V130PAlOA) and (b) MOV (General Electric V130LAlOA).
where sb is the standard deviation of the blank readings and rn is the analytical sensitivity (1).Analytical methods which are fairly noise free and have a high analytical sensitivity should give low cL values while noisy, less sensitive techniques give higher cL values. The miajor sources of noise in spectrometric determinations can be attributed to analyte flicker noise, source noise, dark current noise, and electronic noise (2). In inductively coupled plasma, ICP, emission measurements and ICP excited ICP fluorescence measurements, a noise source which can severely plague the analyst is the radio frequency, rf, electronic noise. Strong rf‘ signals can affect sensitive electronic systems if the ICP unit or the detection electronics are not properly shielded and grounded. Shielding problems with the ICP torch “box” can arise if large or numerous observation windows have been cut into the “box” in order to perform plasma diagnostic studies. Also, ILCP excited ICP fluorescence measurements can require drastic changes in the ICP torch box to accommodate the necessary optics (3). Even if the detection electronics are properly shielded and grounded, the power and earth ground lines for the detection electronics may then affect the stability of the detection system. Anothler rf contamination source can be the rf generating unit. In most ICP measurements, powers from 500 to 2000 W are used. If any rf leakage occurs within the generating unit, the power and ground lines can conduct rf signals. These rf signal13 can then couple with the detection electronics through the power and ground lines (4)and affect the stability of the analytical measurement. This report describes one solution to reducing the rf electronic noise in ICP measurements. A reduction may be achieved through the use of two rf power line filtering units, which are simple to construct. One unit is designed to filter rf signals from the 110-V power lines for the detection electronics. ‘The other unit is used to prevent rf signals on the power lines of the generator from coupling with the power liries of the detection electronics. Through the use of these filtering units, a considerable reduction in the electronic noise may be achieved; thus, lower limits of detection for ICP emission and ICP excited ICP fluorescence measurements can be attained. EXPERIMENTAL SECTION Filter units were constructed for the 110-V and 220-V power lines and rf signals were minimized by using Corcom RFI poweir line filters (20R6). These filters are rated at 20 A and operate from 50 to 400 Hz.They may be obtained from Newark Electronics (12F2479). 110 V Filter Unit. The filter unit used to remove the rf from the 110-Vpower lines for the detection electronics consists of one RFI fiiter ‘and four metal oxide varistors, MOV. The 110-V power lines were attached to the line input of the RFI filter in the following manner as shown in Figure 1. The filtered output was taken from the load output of the RFI filter and was supplied to the detection electronics by using a conventional three-pronged 110-V safety receptacle. To eliminate voltage transients which also may occur on the 118-V power line, four MOVs were added to the circuit. The filter unit and receptacles were shielded in a metal box. Filter Units.
b
1
C
I
O.lnA
il
Figure 3. Tracings of the output signals of the detection electroniics.
ICP was operated at 2 kW and detection electronics had a 1 nA sensitivlty. Filtering conditions were (a) no power line filtering wed, (b) only 1 1 0 4 power line filter used, and (c) both 110-V and 2 2 0 4 power line filters used.
220 V Filter Unit. The filter unit used to reduce the rf signals on the power lines for the rf generator is illustrated in Figurie 2. Since a 220-V three-phase power line was used, two RFI filters were required. The filter unit was hard wired into the rf genertitor cabinet at the entry point of the power lines. The power lines were attached to the line input of the RFI filters and the filtered outputs were taken from the load terminal as shown in Figure
2. I n s t r u m e n t a t i o n . The basic setup and operating conditions for the ICP system artd the detection system have been previoudy described (3). The only change involved the use of a PAR lh6A lock-in amplifier. M e a s u r e m e n t of Noise. The effect of the rf electronic noise on the background signals was observed by monitoring the output of the lock-in amplifier. The detection system was operated with -1000 V applied to the photomultiplier tube, a lo4 A/V setting for the current to voltage converter, and a 1 mV sensitivity on the lock-in amplifier. To measure the electronic and dark current noise, the shutter of the photomultiplier housing was closed. The output of the lock-in amplifier using a 0.3-s time constant was displayed on a strip chart recorder. The reduction in the amount of rf electronic noise on the background signal was observed by operating the ICP and the detection electronics with and without the power line filter units. R E S U L T S AND DISCUSSION The effect of rf electronic noise on the background signal can be seen in Figure 3. In tracing a, one ICP unit was operated at 2 kW without the use of the 220-V filter unit, and the detection electronics (full scale sensitivity of 1nA) were also operated without the 110-V filter unit. The shutter of the photomultiplier housing was closed so that the electronic and dark current noise could be measured. The effect of the insertion of the 110-V power line filter unit on the output
Anal. Chem. 1983. 55, 1434-1437
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signal can be seen in tracing b. Here, the ICP conditions and the sensitivity of the detection electronics remained unchanged. The noise of the output signal is reduced by a factor of 3 when this filter unit is used. A further reduction in the electronic noise of the output signal can be obtained if the 220-V power line filter unit is added to the rf generator. Tracing c is the output signal obtained when the 110-V power line filter unit is used for the detection electronics and the 220-V power line filter is used in the rf generator. For this tracing, the same ICP conditions and detection electronic sensitivity were used. The noise of the output signal has been reduced by a factor of 20 in this tracing as compared to tracing a. Clearly, the use of these filtering units allows a significant reduction in the rf electronic noise associated with this ICP system. Since the torch “box” used in these measurements has been modified for diagnostic studies and for ICP excited ICP fluorescence measurements, the rf leakage may be greater than normally expected for an ICP system. Hence, other systems may not exhibit as great of a reduction in the rf
electronic noise with the insertion of these filter units. If, however, the rf electronic noise makes up a smaller percentage of the noise associated with ICP measurements, the use of these filter units can reduce the amount of background noise. For the system described in this report, the use of rf filters significantly reduces the electronic noise and allows the attainment of lower LOD values for ICP emission and ICP excited ICP fluorescence measurements.
LITERATURE CITED (1) (2)
(3) (4)
Nomenclature, Symbols, Units and Their Usage In Spectrochemical Analysis-11. Data Interpretation. Pure Appl. Chern. 1976, 45, 99. Winefordner, J. D. “Trace Analysis: Spectroscopic Methods for Elements”: Why: New York, 1976: Vol. 46. Kosinski, M. A,: Uchida, H.; Winefordner, J. D. Anal. Chem. 1983, 55, 668. Ott, H. W. “Noise Reduction Techniques in Electronic Systems”: Wiley: New York, 1976.
RECEIVED for review January 24,1983. Accepted March 21, 1983. Work supported solely by Contract AF-AFOSRF49620-80-C-0005.
Characterization of Primary Beams in Fast Atom Bombardment and Liquid Secondary Ion Mass Spectrometry Sources Bryan L. Bentz* and P. Jane Gale RCA Laboratories, David Sarnoff Research Center, Princeton, New Jersey 08540
The successful application of secondary ion mass spectrometry to the study of large, thermally labile organic molecules dissolved in liquid matrices has been widely demonstrated (1-3). The novel means of sample preparation has resulted in long-lasting currents of molecular ions produced with high intensity. The increasing reliance of analytical chemists on the technique has been accompanied by interest in refining it through more careful specification of experimental conditions. With such information, findings among different laboratories engaged in similar research can more easily be compared ( 4 ) . More exact specification of experimental conditions can also make clearer the influence of each experimental parameter upon the production of secondary ions. These observations permit inferences about the mechanism of sputtering in the organic systems to be made, Chief among the parameters which must be specified are the characteristics of the primary atom or ion beam. These include the chemical identity and charge state of the particles, their energy, the size and shape of the beam as it impinges on the target (the beam spot), and the number of particles which reach the target per unit time. The latter two items taken together constitute the current density of the primary beam and strongly influence the absolute intensity of secondary ions produced, as well as the extent of beam damage induced in the uppermost surface layers. The characterization of primary beams has received much attention in the fields of sputtering and ion implantation (5). At least two general methods exist. If optical access t o the target is available, the target can be coated with a fluorescing material such as ZnS or KBr. The impact of the primary particles on the coated target causes it to fluoresce, allowing in situ visual inspection of the sue and shape of the beam spot. Very rough estimates of beam intensity can also be made from the brightness of the fluorescence. An electrical method for obtaining quantitative estimates of charged particle beam intensity employs a Faraday cup in place of the target (6). If
secondary electrons are properly collected, the ion current entering the cup can be measured. Perhaps the simplest, yet heretofore not widely adopted, method for obtaining estimates of beam size and shape takes advantage of an optical effect characteristic of certain refractory metal films (7) or oxides (8). As a result of multiple light-beam interferences, these materials appear colored when illuminated by visible light (9). The color perceived is dependent on the thickness of the oxide film; differences in color indicate differences in thickness. Bombarding such a f i i with a beam of either energetic ions or neutrals erodes the surface, resulting in a thinner f i i at the impact site. Thus, sputtering records in the film a permanent image of the position, size, and shape of the beam. If the duration of the exposure of the metal oxide film to the beam is known, the effective sputter rate (material removed per unit time) can be determined under analytical operating conditions. We report here the use of tantalum oxide, Ta205,to study the beam produced by a PHI Model 04-303 ion gun attached to an H P 5985B GC/MS modified for molecular secondary ion mass spectrometry (SIMS) (10). The method can easily be used to characterize the primary beams employed by any of the instruments currently being used for fast atom bombardment (FAB) or liquid SIMS.
EXPERIMENTAL SECTION Anodic tantalum oxide films were formed at room temperature following a simple procedure described by Pawel et al. (21). A polycrystalline tantalum strip, initially cleaned with methanol, was immersed in an electrolytic cell containing aqueous Na2S04 (0.5%) and a Ta cathode. The driving voltage for the reaction was provided by a Hewlett-Packard constant dc current power supply (Model 6186B). During anodization, current density was limited to several mA/cm2 to assure slow growth rates necessary for uniform film thickness. The oxidized metal strip was attached with silver paste to the sample probe tip used for sputtering (the FAB probe) (12). The
0003-2700/83/0355-1434$01.50/0 0 1983 American Chemical Society
