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Anal. Chem. 1985, 57,2740-2743
shown in Figure 2. The slopes that are plotted correspond to data obtained in the t 1 / 2domain of 10-15.8 ms1I2. The marked nonlinearity may be due to competition between K+ and Li+ for sites in the film. When the LiN03 concentration is decreased from 0.1 M to 0.01 M, the value of the slope for M K+ in Figure 2 increases from 0.75 to 1.1 pC s-1/2. The sensitivity of the method depended on both the film thickness and the anodic potential that was applied. The Figure 2 data were obtained by using the most positive potential excursion that did not result in the below-mentioned loss of electroactivity. The electrode is very stable when the film is in the form of Ag(I)-Mo(CN)8"r, but oxidizing it, presumably to the Mo(V) state, causes a gradual loss of the deposit. The sensitivity therefore decreases as several cyclic voltammetry experiments are performed. For example, the anodic peak current in Figure 1 decreases from 7.96 pA to 6.42 pA after 100 cycles a t 50 mV s-l. Moreover, a potential excursion positive of 950 mV causes the development of a sharp anodic peak, for which a related cathodic process is not observed; a total loss of the film within a few cycles is observed. This paper demonstrates the possible extension of voltammetric methodology toward selective processes and to the determination of analytes that are not electroactive. A more
stable film will need to be devised, however, to make this approach practical.
Registry No. [Mo(CN),I4-, 17923-49-8; K, 7440-09-7; Ag, 7440-22-4; carbon, 7440-44-0. LITERATURE CITED Guadalupe, A. R.; AbruAa, H. D. Anal. Chem. 1985, 5 7 , 142-149. Cox, J. A.; Kulesza, P. J. Anal. Chem. 1984, 56, 1021-1025. Cox, J. A.; Kulesza, P. J. Anal. Chim. Acta 1983, 154, 71-78. Ellis, D.; Eckhoff, M.; Neff, V. D. J. fhys. Chem. 1981, 85,
1225-1231. Furman, N. H.; Miller, C. 0. I n "Inorganic Syntheses, Vol. 111"; Audrleth, L. F., Ed.; McGraw-Hill: New York, 1950;p 160. Engstrom, R. C. Anal. Chem. 1982, 5 4 , 2310-2314. Bucknall, W. R.;Wardlaw, W. J. Chem. SOC. 1927, 2981-2992.
James A. Cox* Basudev K. Das Department of Chemistry and Biochemistry Southern Illinois University Carbondale, Illinois 62901
RECEIVED for review May 9,1985. Accepted July 1,1985. This work was supported by the National Science Foundation under Grant CHE-8215371. It was reported in part at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, February 1985, Paper No. 1052.
AIDS FOR ANALYTICAL CHEMISTS Enhancement of Wavelength Measurement Accuracy Using a Linear Photodiode Array as a Detector for Inductively Coupled Plasma Atomic Emission Spectrometry Scott W. McGeorge' and Eric D. Salin* Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, Quebec, Canada H3A 2K6 Accurate wavelength calibration and a high degree of spatial resolution are required of a multichannel detector for atomic emission spectrometry. While our experience has been with inductively coupled plasmas (ICP), the energetic nature of many emission sources produces complex spectra, and it is imperative that an unknown line be correctly identified. I t is often the case that lines are observed that are not listed in common wavelength tables (I,2), and a comprehensive listing like the MIT Wavelength Tables (3)must be consulted. The MIT tables list virtually all of the known atomic lines of significant intensity in the UV-visible region; however, the relative intensity data for each line was obtained from arc and spark experiments and is therefore not necessarily the same as the relative intensity observed for the ICP. The MIT tables often list 10 or more lines within a 0.02-nm range, so it would seem advantageous if the wavelength measurement accuracy of a photodiode array (PDA) based spectrometer system was better than hO.01 nm. Spectrometer systems incorporating a PDA as a multichannel detector for atomic emission (4-9) and absorption (10, Present address: PRA International, 45 Meg Drive, London, Ontario, Canada N6E 2V2.
11)spectrometry have generally been configured with reciprocal dispersions (Rd)of 0.8-2 nm/mm. This corresponds to a spectral window of 20-50 nm in width using the 25.6 mm wide, 1024-element PDA manufactured by EG&G Reticon Corp., which was specifically designed for spectrometric applications. Grabau and Talmi have shown (12) that the Rd should be limited to about 0.4 nm/mm if the resolution of a PDA with a 25-pm diode spacing is to match that of a conventional entrance/exit slit spectrometer system. I t is clear that a compromise between resolution and wavelength coverage must be made in a linear dispersion system. Spectral lines are randomly spaced which means that lines can fall between diodes or on several diodes with a subsequent ambiguity as to their true position. In a 20-nm spectral window system such as ours, a measurement accuracy exceeding hO.01 nm can only be achieved if subdiode image positioning can be obtained. It would be most advantageous if only one known line was required for the identification of unknown wavelengths.
EXPERIMENTAL SECTION Apparatus. The radiation source for all measurements was an ICP 2500 (Plasma Therm, Kresson, NJ). The sample intro-
0003-2700/85/0357-2740$01.50/00 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 13, NOVEMBER 1985
ducti6n system consisted of a MAI( 200 fixed cross-flowpneumatic nebulizer (Sheritt Research Analytical Services, Fort Saskatchewan, Alberta) and a MAK low flow torch. A 1:l image of the plasma was focused on the 2 5 - ~ mentrance slit of a Jarrell-Ash 78-462 1-m Czerny-Turner spectrometer (Allied Analytical, Waltham, MA) using a 20-cm focal length lens. A laboratory constructed detection system (13) based on an RL1024S PDA (EG&G Reticon Corp., Sunnyvale, CA) was situated in the exit focal plane of the spectrometer. The detector was controlled by an AIM-65 single board computer (Rockwell International, Anaheim, CA), and data collected by the AIM-65 were transferred to a 280 based computer system running under the CP/M operating system. A system interface was written in Pascal/MT+ for the 280 computer and incorporated the wavelength calibration software and other routines for data manipulation. Reagents. Solutions of the elements used for the calibration of the spectrometer/detection system and for the performance evaluation were prepared from reagent grade salts in deionized/distilled water. Procedure. A series of nine 20-nm spectral windows, each containing several uniformly dispersed lines, were collected ranging from 200 to 400 nm. To minimize the possibility of spectral overlap, as few elements as possible were used to provide prominent lines across a given window. The concentrations of analyte solutions were adjusted such that all calibration lines were sufficiently intense for the integration period employed. The resulting information was incorporated into the Pascal system interface allowing spectrum calibration by simple menu selection. R E S U L T S AND DISCUSSION System Calibration, The reciprocal dispersion for a conventional grating spectrometer is wavelength dependent as indicated by the relationship (14)
where Rd is the reciprocal dispersion, d is the groove spacing of the grating, fi is the angle of diffraction with respect to the grating normal (a function of wavelength), m is the order, and F is the focal length of the optical system. The calibration must be accurate to better than *0.01 nm, and the parameters of the above equation were not known to this degree of accuracy. Therefore an empirical approach was used to determine R d as a function of wavelength. The lines used for the calibration procedure are listed in Table I. Each calibration line fell on approximately five diodes (pixels) with the 25-pm entrance slit width. For each line of each window the most intense pixels were fit using a second-order polynomial routine coded in Pascal. This provided the “subdiode” peak position. The reciprocal dispersion between an arbitrarily chosen reference line, near the center of the PDA if possible, and each calibration line were then calculated in nm/diode using eq 2.
In eq 2 A, and A, are the true wavelengths of the central reference line and a calibration line and D,and D,are the subdiode peak positions of the reference line and a calibration line. Initially the average R d per window was used to calculate unknown wavelengths; however, a monotonically increasing error was observed as the distance between the reference line and the “unknownnline increased. This was due to the change in R d within a window, so a modified procedure was developed. T o account for the change in R d within a window an algorithm was required which would be able to calculate the Rd between the reference line and each unknown line knowing only the wavelength of the reference line and the subdiode peak positions of the reference and unknown lines. This was
2741
Table I. Calibration Windows and Lines for Calibration (All Wavelengths in nm)” window range
reference line
200-220
Zn 213.856
calibration lines Zn 202.551 Zn 206.191
Pb 216.999 Pb 220.351 220-240
Ba 230.424
Pb 220.351 Pb 224.689 Ba 234.758 Pb 240.195
240-260
Pb 247.638
Ag 241.319 Ag 243.779
Zr 257.139
Pb 261.418 260-280
Cr 267.879
Mn 260.569 Cr 266.342 Mg 279.553 Mg 280.270
280-300
V 290.882
Mg 279.553 Mg 280.270 Mg 285.213 V 292.402 V 292.464 Hg 296.728
310-330
Ca 317.933
V 309.311 V 310.230 V 311.071 Ca 315.887 Ag 328.068
330-350
Zr 343.823
Na 330.232 Zr 339.198 Zr 349.621
350-370
Zr 357.685
Zr 350.567 Zr 355.195
Pb 363.958 Pb 368.347 390-410
Ca 396.847
Ca 393.367 Mn 403.076 Sr 407.786
“Note: wavelengths are from ref 3. 1
0
-400
l
l
-200
I
l
I
0
I
200
I
l
I
400
Peak Position(Diode)
Figure 1. Plot of R , vs. distance between calibration lines and reference line for the 280-300 nm spectral window.
done by calculating the Rd for each reference/calibration line pair using eq 2 and the lines listed in Table I. Then these Rd values were plotted as a function of the distance between each calibration line and the reference line. The plot obtained for the 280-300 nm window is illustrated in Figure 1. A line near the center of a given window was chosen as the reference giving rise to positive and negative diode displacements for the “unknown” lines. A linear least-squares regression was performed for each of the nine plots (spectral windows), and
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 13, NOVEMBER 1985
Table 11. Calibration Results (Wavelengths and Calibration Errors in nm)
window range
calibration line
200-220
220-240
240-260
260-280
280-300
310-330
Zn 213.856
Ba 230.424
Fe 248.327
Cr 267.716
Mn 285.213
Ca 317.933
2.030
unknown line
error
Zn 202.551 Zn 206.191 Pb 216.999 Pb 220.351
-0.010 0.007 -0.001
Pb 220.351 Pb 224.689 Ba 233.527 Ba 234.758 Pb 239.379 Pb 240.195
-0.005 -0.008 -0.003 -0.001
Fe 238.863 Fe 239.563 Fe 239.933 Fe 240.488 Fe 248.815 Fe 252.282 Fe 258.588
-0.007 -0.004
Mn 260.569 Cr 266.342 Cr 266.602 Cr 267.879 Cr 276.259 Cr 276.654 Mn 279.482 Mn 279.827
-0.007 0.003 0.002
Mn 279.482 Mn 279.827 Mg 280.270 Mn 293.306 Mn 294.921 Hg 296.728
-0.003 -0.001 0.002 0.003 -0.003 -0.004
V 309.311 V 310.230 V 311.071 V 311.838 V 312.528 Ca 315.887 Cu 324.754 Cu 327.396 Ag 328.068
-0.004 -0.003 -0.004 -0.004 -0.003 -0.004 -0.001
0.003
0
-0.003
-0.011
-0.004 -0.001 0.003 0
0
0.002 0.004 0.003 0.003
0.001
-0.001
330-350
Zr 339.198
Zr 330.628 Zr 335.609 Zr 343.053 Zr 343.823 Zr 347.939 Zr 348.115 Zr 349.621 Zr 350.567
0.003 0.004 0.004 0.002 0.004 0.003 0.006 -0.001
350-370
Zr 357.685
Zr 349.621 Zr 350.567 Zr 355.195 Zr 355.660 Pb 363.958 Pb 368.347
0 -0.007
380-400
Ca 393.367
Mg 383.231 Mg 383.826 Ba 389.179 Ca 396.847 Mn 403.076
-0.003 0.009 -0.001 0
0.005 0.006 0.001 -0.001
0.002
the resulting slopes and intercepts were recorded as a function of the wavelength of the reference line. A plot of the intercept vs. reference wavelength data is shown in Figure 2. These data were fit by using a second-order least-squares polynomial yielding the equation
b = (2.06515 X
- X(6.85 X lo-’) - A2(4.78
X
(3)
-
2.016
g
2.002
8 P
N
P
1
.g
c
1.988
1.974
1 200
1
250
‘0
J
1
1
1
300
350
400
Wavelength(nm)
Flgure 2. Plot of the intercepts of the data typified by Figure 1 vs. reference wavelength for nine calibration windows.
The slopes of the intercept vs. reference wavelength plots were found to fluctuate randomly so the average value of 5.05 X lo-* was used. Wavelength Measurement. The measurement of an unknown lines’ wavelength is a simple matter using the slope and intercept data. First, all of the line positions are determined by using the three-point fit. Then a known line in the window is selected and eq 3 is used to calculate the intercept of the Rd vs. diode position curve for a window centered on the known line. The R d between the known line and all of the unknown lines is calculated by using
Rd = b
+ m(D,- D,)
(4)
where b is the intercept from eq 3, m is the slope of 5.05 x lo4, and D, and D, are the subdiode positions of the unknown line and the reference line, respectively. The unknown wavelength is determined from the relationship
A, = A,
+ Rd(D, - D,)
With this approach the R d between a known line and each unknown line is individually determined minimizing the error induced by assuming a constant Rd as a function of wavelength. The results using this approach are shown in Table 11. The average absolute error is 0.003 nm with a standard deviation of 0.003 nm. Therefore, this technique yields an average wavelength measurement accuracy approximately a factor of 10 better than a method utilizing the pixel spacing as the limiting spatial resolution. The worst case errors range from -0.011 nm to 0.009 nm. Registry No. Zn, 7440-66-6;Pb, 7439-92-1; Ba, 7440-39-3;Fe, 7439-89-6; Mn, 7439-96-5; Cr, 7440-47-3; Mg, 7439-95-4; Hg, 7439-97-6; V, 7440-62-2; Ca, 7440-70-2; Cu, 7440-50-8; Ag, 744022-4; Zr, 7440-67-7; Na, 7440-23-5; Sr, 7440-24-6.
LITERATURE CITED Wlnge, R. K.; Peterson, V. J.; Fassel, V. A. Appl. Spectrosc. 1979, 3 3 , 206. Parsons, M. L.; Forster, A.; Anderson, D. “An Atlas of Spectral Interferences In ICP Spectroscopy”; Plenum Press: New York, 1980. Harrlson, G. R. “MIT Wavelength Tables”; Wiley: New York, 1939. Codding, E. G.; Horlick, G. Spectrosc. Lett. 1974, 7 , 33. Horlick, G.:Codding, E. G.; Leung, S.T. Appl. Spectrosc. 1975, 2 9 , 48. Horlick, G. Appl. Spectrosc. 1978, 3 0 , 113. Betty. K. R.; Horlick, G. Appl. Spectrosc. 1978, 3 2 , 31. Talmi, Y.; Sieper, H. P.; Moenke-Bankenburg, L. Anal. Chim. Acta 1981, 127, 71. Kubota. M.: Fullshiro, Y.: Ishida. R. Smctrochlm. Acta, Part 8 1982, 3 7 8 , 849. Chuang, F. S.; Natusch, D. F. S.; O’Keefe, K. R. Anal. Chem. 1978, 50, 525.
Anal. Chem. 1985, 57,2743-2745 (11) Codding, E. G.; Ingle, J. D., Jr.; Stratton, A. J. Anal. Chem. 1980, 52, 2133. (12) Grabau, F.; Talmi, Y. "Multichannel Image Detectors"; Y. Talmi, Ed.; American Chemical Society: Washington, DC, 1983; Vol. 2, Chapter 4. (13) McGeorge, S. W. Ph.D. Thesis, McGill University, Montreal, Quebec,
2743
1985; Chapter 2,; (14) Ingle, J. D., Jr. Notes on Basics on Spectrometric Basics"; Oregon State University: Corvallis, OR, 1979; Chapter 4.
Received for review May 3, 1985. Accepted June 17, 1985.
Spinning Sample Cup and Adjustable Angle Jet Flame for Molecular Emission Cavity Analysis S t a n Van Wagenen a n d Q u i n t u s Fernando* Department of Chemistry, University of Arizona, Tucson, Arizona 85721 Applications of the hydrogen diffusion flame in molecular emission cavity analysis, MECA, have been published extensively by Belcher and co-workers ( 1 ) and subsequently by Townshend and co-workers (2). In MECA, the emission from an analyte is generated and confined in a small cavity that is positioned,appropriatelyin a cool hydrogen diffusion flame. The intensity of the emission is monitored as a function of time with the aid of a flame emission spectrophotometer. The concentration of the analyte species that results in the emission is obtained from the peak height or peak area of the emission vs. time plots, and the molecular nature of the analyte species is very often deduced from the time at which the maximum emission intensity is observed. This versatile analytical technique has proved to be useful for a large number of analytical determinations ( 2 ) . The determination of sulfur-containing compounds by the MECA technique has attracted considerable attention especially since it is possible to determine mixtures of compounds containing sulfur in various oxidation states (3). For example, trace levels of sulfate ions in aqueous solutions can be determined by the addition of 1-5 p L of the solution into the MECA cavity that consists of a silica cup fitted into the hollow end of a metal rod. The cavity is positioned in a cool hydrogen diffusion flame and the intensity of the band emission from the Sz species generated is measured at 384 nm. Several variables have to be controlled in order to achieve an acceptable precision and maximum sensitivity. The production of Sz species in the cavity is facilitated by the addition of a predetermined amount of H3P04( 3 , 4 ) . The flow rates of the flame gases must be optimized; the downward tilt of the sample rod when it is positioned in the flame must be identical in every experiment, and the initial and final temperature of the metal rod must not vary from one determination to the next. In addition to these variables, the surface characteristics of the silica cup must not vary in the course of a series of determinations. This is one of the more difficult variables to control because the silica cup must be cleaned periodically to prevent the buildup of residues from a series of determinations. Consequently, variations can occur in the physical characteristics as well as the chemical composition of isolated areas on the surface of the silica cup. Another limitation of the silica cup is its size and shape that severely reduces the volume of the sample solution that can be added to the cup. If the sample volume is increased beyond a few microliters, there is a tendency for the sample to flow out of the sample cup and uneven heating of the sample solution in the MECA cavity results in poorly defined emission peaks. We have minimized these difficulties by (a) modifying the shape of the silica cup, (b) continuously spinning the metal rod and sample cup in the flame while the emission spectrum is recorded, and (c) changing the burner design. These modifications are described below and the results obtained 0003-2700/85/0357-2743$01.50/0
with the redesigned burner and the modified spinning sample cup, with a series of aqueuos solutions containing nanogram levels of sulfate and sulfite ions, are reported. EXPERIMENTAL SECTION Emission measurements were made with a modular flame emission system consisting of a slit and a scanning monochromator (GCA/McPherson Model EU-700-2) coupled with a photomultiplier module (GCA/McPherson Model EU-701-30). The output was digitized with the aid of a Keithley picoammeter, and the analog output was displayed on a Linear chart recorder. The sample rod assembly, the burner head, and the components of the emission spectrophotometer were mounted on an optical rail so that the emission generated in the silica sample cup could be lined up with the detection system. Gas flow rates were monitored and controlled with Matheson series R 7630 flowmeters. In previous work we had found that inhomogeneities on the silica surface caused random variations in the emission intensity. This effect can be minimized if the sample solution is spread into a thin film over the entire inside surface of the silica cup during the course of the measurement of the emission intensity. This can be accomplished by adding the solution containing the analyte into the sample cup and spinning the metal rod and sample cup, before it is introduced into the flame, and continuing to spin the sample cup while the emission intensity is recorded. An exploded view of the sample rod assembly and mount is shown in Figure 1. A beveled ring gear, G1, is attached to an axle into which the threaded sample rod containingthe silica cup is fitted at A. The axle rotates in the sealed roller bearings, R1 and Rz,that are fitted into two rectangular metal blocks attached to a rectangular plate, P. A hole is drilled in this rectangular plate to accommodate a bearing and an axle through which is passed the shaft, S, of a small electric motor, M. The end of the shaft is attached to a beveled pinion gear, G2,that meshes with the ring gear, G1, on the sample rod axle. The gears remain meshed while the sample rod axle is spinning and while the sample rod axle is being rotated into the appropriate position. This sample rod assembly is fitted as shown in Figure 2 on a mount that rides on an optical rail. The sample rod can be moved on this optical rail toward or away from the slit and the monochromator. The sample rod can also be moved in a direction at right angles to the optical rail. The slot and a setscrew in the plate on which the sample rod assembly is mounted enable the assembly to be moved laterally. The sample rod is continuously cooled with the aid of a copper cooling coil as shown in Figure 2B. The cooling coil is prevented from rotating by a stop that extends from the bottom of plate P. The silica cup that is fitted into the end of the sample rod is usually cylindrical. If a cylindrical cup containing the analyte solution is spun at about 1500 rpm, there is a tendency for the solution to be lost from the cup. This can be prevented by modifying the shape of the cup. In this work the cup that was used consisted of a hollow silica sphere, 6 mm in diameter, from which a segment, 2 mm in depth, was removed. The resulting sample cup resembled the cylindrical cup with its outer rim turned inward to form a lip. Loss of the analyte solution from a cup spinning at -1500 rpm was prevented by this configuration of the sample cup. A hollow spherical cavity with a 3.5 mm diameter 0 1985 American Chemical Society
