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Anal. Chern. 1086, 58,250-251
X and can be solved numerically by the Newton-Raphson method, for example. LITERATURE CITED (1) Strong, F. C. Anal. Chem. 1979, 57, 298-299. (2) Strong, F. C. Anal. Chem. 1980, 52, 1152-1153. (3) Leary, J. J.; Messick, E. E. Anal. Chem. 1985, 57, 956-957. (4) Ellerton, R. R. W. Anal. Chem. 1980, 52, 1152-1153. (5) Schwartz, L. M. Anal. Chem. 1877, 49, 2062-2068. (6) Brownlee, K. A. "Statistical Theory and Methodology I n Science and
Engineering", 2nd ed.; Wiiey: New York, 1965; Section 11.9.
Lowell M. Schwartz Department of Chemistry University of Massachusetts Boston, Massachusetts 02125 RECEIVED for review June 3, 1985. Accepted September 5, 1985.
AIDS FOR ANALYTICAL CHEMISTS Calibration Procedure for Spark-Source Mass Spectrometer Photoplates Leslie S. Dale
CSIRO, Division of Energy Chemistry, Mail Bag 7, P.O. Sutherland, NSW, 2232, Australia Quantitative analysis by spark-source mass spectrometry using photoplate detection depends on the accurate determination of the percent transmittance-relative intensity relationship (the characteristic curve) of the emulsion. Usually this relationship is based on either the Seidel or Hull transformation ( I ) . The Hull function, which is preferred in this laboratory, is described by the equation
-)
100 - T 'Iy
IH =
k(
where IHis the corresponding Hull intensity, k is a proportionality constant, T is the measured percent transmittance, T, is the saturation transmittance, and y is the slope of the transmittance-intensity plot. To establish y and T,, it is necessary to obtain a suitable set of data by selecting an element present in the sample having isotopes with a wide range of abundances. The selection is somewhat limited since only a few elements such as tin, barium, and ytterbium meet these requirements. If one of these elements is present in the sample its concentration must be high enough to enable measurement of its isotopes over a range of exposures. Because of variations in the response of ion-sensitive emulsions, even within batches, each plate must be calibrated by using the above procedure. In this laboratory, a wide range of materials is analyzed and, because of the inherent problems associated with obtaining a suitable emulsion calibration, a different approach to establishing the characteristic curve was necessary. During the analysis of germanium metal samples, inspection of the photoplate showed that the Ge2+ion cluster occurring between m/e 140 and 152 (11 masses) had a wide gradation in intensity and appeared to be suitable for calibration purposes. Photoplate calibrations were made with excellent results by using this group of masses.
EXPERIMENTAL SECTION The instrument used as a JEOL JMS-OlBM2 spark-source mass spectrometer. Germanium metal electrodes (2-mm square section, 10 mm long, and with wedge-shaped tips) were used. Exposures of 1,2 , 4 , 8 , and 10 nC were made on Ilford Q2 plates. The plates were developed in Ilford Phenisol developer for 4 min at 20 "C. Measurements of the percent transmittance of the 11 Gez+lines were made. The data were processed by computer using a nonlinear regression program that fitted the data to the Hull equation and calculated the
Table I. Calculated Relative Intensities of Gez+Ions mass
intensity, arbitrary units
140 142 143 144 145 146
177 472 134 995 179 1000
mass
intensity, arbitrary units
147 148 149 150 152
238 739 50 238 25
slope and saturation transmittance.
RESULTS AND DISCUSSION The relative intensities of the Ge2+ion cluster are given in Table I. Each exposure provides 11 data points, with intensities ranging from 25 to 1000. Values of y and T, obtained by computer analysis of a typical data set gave y = 1.14 f 0.02 and T, = 5.0 f 0.6% (la) with a root mean square deviation of 2.2% for the calculated intensities. In practice, data are obtained from those exposures for which the transmittance value for mass 146 is greater than the average T, value of the batch of plates in use. The average values of y and T, are then used for subsequent calculations of sample intensities. With the availability of 11 measurements and a wide gradation in intensities the suitability of the emulsion calibration is demonstrated by the regression analysis of the data. The use of this calibration procedure accounts for plate-to-plate variations in y and T,. The extent of these variations may be judged from data from 10 plates processes consecutively. The mean of the y was 1.16 with a standard deviation of 0.081 (relative standard deviation (RSD) = 7.2%). The range of values was between 1.03 and 1.29. Similarly, for the T , the mean was 3.6 with a standard deviation of 0.50 (14 % RSD) with the range between 2.8 and 4.0. If the mean values of y and T, were assumed for each plate, up to a 14% error in intensity would result for a measurement of 50% transmittance. With respect to the high and low y's and their associated T, values, the error would be 22%. This demonstrates the improvement in quantitative analysis that can be achieved when reliable y and T, values are used. The calibrations obtained by using the Gez+ion cluster are similar to those obtained when using atomic ions. For example, on a typical plate where barium isotopes could be measured, the y and T, values obtained by the use of these isotopes were 1.24 and 3.9%, respectively. The Gez+ cali-
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Anal. Chem. 1986, 58,251-256
bration gave y = 1.21 and T,= 3.7%. Although some space on the photoplate is required for recording the germanium exposures, it is compensated by the provision of suitable plate calibration data, irrespective of the composition of the sample. A possible disadvantage is that, using Ge2+,the y and T,values obtained specify the calibration over a narrow region of the mass range. However, in practice, it is rarely possible to obtain adequate calibration data over the entire mass range using internal elements. The inconvenience of having to include separate germanium exposures on each plate is partly overcome by first recording, in turn,
the exposures on each photoplate located in the plate storage chamber. When a sample is run, the plate has already recorded on it the required calibration data. No memory effects for Ge occur. Registry No. Gez+,64177-43-1. LITERATURE CITED (1) Hull, C. W. Presented at Tenth Annual Conference on Mass Spectrom-
etry, New Orleans, 1962,404.
RECEIVED for review June 24,1985. Accepted August 12,1985.
Construction of a Microcomputer Controlled Near Normal Incidence Reflectance Spectroelectrochemical System and Its Performance Evaluation Chong-Hong P y u n and Su-Moon Park*
Department of Chemistry, University of New Mexico, Albuquerque, New Mexico 87131 Since Kuwana et al. (1) first used an optically transparent electrode (OTE) to monitor spectra of electrochemically generated products, many investigators have been using/improving/modifying the techniques. Several review articles have since appeared describing development of various spectroelectrochemical techniques (2-6). The in situ spectroelectrochemical techniques may perhaps be described in a few different types including: transmittance methods using an OTE, optically transparent thin layer electrodes (O'I"I'LE), or metal (gold) minigrids; reflectance methods in which the probing beam passes through the solution and reflects a t the reflective electrode; and an internal reflectance (multiple) technique where the probing beam is admitted from the backside of the electrode. Each of these methods has advantages and disadvantages. For example, the potential distribution and the ohmic voltage drop may often be problems for OTE and OTTLE methods due to their geometric restrictions in constructing an electrochemical cell. Electrode materials are limited also. The sensitivity of such techniques is limited by the diffusion layer thickness. The sensitivity was greatly improved by techniques reported by McCreery et al. (7) and Brewster and Anderson (8).Bewick et al. (9) employed the reflectance method in an infrared region with the electrode potential modulated between two proper values. In most of these spectroelectrochemical measurements, an electrochemical cell should have optical windows to accommodate the probing radiation to the space of interest in the vicinity of the working electrode. This imposes various restrictions on the cell geometry as well as working electrode materials. With the use of an optical fiber, these difficulties may be circumvented. The alignment of the optical path would be less crucial with the optical fiber probe (OFP). Due to these restrictions, spectroelectrochemical studies turned out to be difficult in an electrochemical system containing nonaqueous solvents and/or mercury pool electrodes. In these studies, solution preparation and handling were often difficult with such sophisticated cell designs. A spectroelectrochemical cell was designed with a study of nonaqueous electrochemical processes in mind (lo), but the cell had to be vacuum-tight. Also, most of these setups are more suited to studies of the solution species rather than films building up on the electrode surface. For studies of oxide films at metal electrodes, specular reflectance and/or ellipsometric methods
were used (3). The sensitivities of these techniques were so low that often polarization modulation had to be used. Ellipsometric measurements also may not provide spectral information. In our current communication, we describe construction of a computer controlled near normal incidence reflectance spectrometer (NNIRS), which makes spectroelectrochemical measurements straightforward. The spectrometer uses a bifurcated optical fiber probe and a reflective electrode. The system is suitable for studies of solution species and also of surface films on electrodes, including mercury pools. Data acquisition and system control were performed by an Apple 11+ microcomputer (11). EXPERIMENTAL SECTION Chemicals and Solution Preparation. Cobalt phthalocyanine (Eastman Organic, CoPC) was used as received for preparation of solutions in dimethyl sulfoxide (Me2SO). Me2S0 (Mallinckrodt) was used as a solvent after a fractional distillation with the reflux ratio of 5:l under the reduced pressure. Reagent grade p-benzoquinone (BQ) (Eastman Organic) was used after recrystallization in doubly distilled water. Methyl viologen hydrate (Aldrich) was used as received. Polarographic grade tetra-nbutylammonium perchlorate (TBAP; Southwestern Analytical, Austin, TX) was used after drying under vacuum at -95-100 "C for about 24 h. Ultrex sulfuric acid (J.T. Baker) was used as an aqueous electrolyte. Aqueous solutions were prepared by using doubly distilled deionized water. Instrumentation Details. The spectroelectrochemicalsystem consists of electrochemical, spectrophotometric, and computer subsystems as shown in the block diagram of Figure 1. The electrochemistry subsystem was made of a Princeton Applied Research (PAR) Model 173 potentiostat-galvanostat with a PAR 175 universal programmer, a Linseis LY 1800 x-y recorder, and an electrochemical cell of a special design to accommodate an optical fiber probe (OFP). Electrochemical cells (Figure 2) were designed so that electrochemical and spectroelectrochemical experiments may be performed under semiinfinite diffusion conditions. The cell has a highly reflective platinum electrode (Sargent Welch S-30101-20A, diameter 6.5 mm) facing up with the OFP placed -0.2-1 cm above it. The reference electrode was either Ag/AgCl in a saturated KCl solution for aqueous solutions or a silver wire pseudo reference for a nonaqueous solution. The reference electrode was shielded with a tiny Pyrex tubing whose tip was sealed with a small piece of platinum wire. An incomplete seal allows the necessary electrolyte transport at the junction. The counter electrode was
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