Anal. Chem. 1903, 5 5 , 488-492 Bahr, U.; Schulten, H. R. "Topics in Current Chemistry"; Sprlnger: New York, 1981; Vol. 95. Coulllaud, B.; Bloomfield, L. A,; Lawler, J. E.; Siegel, A,; Hansch, T. W. Opt. COmm~fl.1980, 35, 359-362. Coulllaud, B.; Dabkiewicz, Ph.; Bloomfieid, L. A,; Hansch, T. W. Opt. Lett. 1982, 7 , 265-267. Webster, C. R.; Woste, L.; Zare, R. N. Opt. Commufl. 1980, 35, 435-440. Bjorkland, G. C. Opt. Left. 1980, 5 , 15-17. Gallagher, T. F. Kachru, R.; Gounand, F.; Bjorklend, G. C.; Lenth, W.
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RECEIVEDfor review September 10,1982. Accepted November 9, 1982. Support of the Department of Energy, under the auspices of Los Alamos National Laboratory, is gratefully acknowledged.
Sectored Wheel Wavelength Modulation for Flame Atomic Fluorescence Spectrometry John 1. McCaffrey and R. G. Mlchel" Department of Chemistry, University of Connecticut, Storrs, Connectlcut 06268
Further studles of the sectored wheel (rotating quartz mechanical chopper) are descrlbed. The sectored wheel allows for hlgh-frequency, squarswave-form, wavelength modulatlon for background correction of contlnuum source exclted flame atomlc fluorescence spectrometry (AFC) and provides efficient square-wave wavelength moduiaflon over the large wavelength intervals necessary for AFC. Results of studles to determine the magnltude of transmisslon losses, reflection losses, and defocusing at large moduiatlon intervals (1.0 nm or more) are described. Efflcient square-wave modulatlon at 160 Hz has been demonstrated, and the system has potentlal for higher modulation frequencies. The sectored wheel Is potentlally useful for both atomlc emlssion and atomic absorption spectrometrles as well as for AFC.
The sectored wheel has been described previously ( I ) and proposed as an alternative to oscillating refractor plate wavelength modulation for atomic spectrometry. The most significant advantage of the sectored wheel is its ability to provide square-wave-form wavelength modulation at high modulation frequencies. Square wave forms have been shown both theoretically (2) and experimentally (3, 4 ) to lead to improved signal-to-noise ratios (SNRs) in atomic spectrometry. A second advantage of the sectored wheel is its ability to modulate over larger wavelength intervals than the oscillating refractor plate while still maintaining the square wave form at all modulation frequencies. This advantage is particularly significant for background correction of continuum source excited atomic fluorescence spectrometry (AFC) where short focal length monochromators with wide slit widths are used to maximize the collection of fluorescence from the atom cell. Such instrumentation necessitates large modulation intervals in order to measure the spectral background a t each side of the analyte atomic fluorescence wavelength of interest. For atomic emission and atomic absorption, where required modulation intervals are usually much smaller, the advantages of the sectored wheel are primarily in the square modulation wave form and higher modulation frequencies. The use of the sectored wheel has already been demonstrated for atomic emission with an echelle monochromator (5), and Harnly (4) has suggested that the construction of a sectored wheel with more than four sectors will allow extension of the linear dynamic range of calibration curves of continuum source excited atomic absorption spectrometry (AAC). Such extension has
already been shown to be possible when using oscillating refractor plates (6). High-frequency modulation allows discrimination against low-frequency flicker noise inherent in the instrument. The effect of modulation frequency on the rise and fall times of the square wave form was studied together with some preliminary studies of the relationship between SNR and modulation frequency. Placing the sectored wheel in the monochromator defocuses the image a t the exit slit and leads to light losses at the wheel by reflection and absorption at the quartz sectors. In addition, these losses differ for the different thicknesses of the sectors. This causes some incidental source intensity modulation. These effects were studied to determine their effect on the SNR and accuracy of AFC measurements. Such effects are usually regarded as insignificant when oscillating refractor plates are used, because of the use of small modulation intervals. Here, large intervals were being used thus necessitating detailed study.
EXPERIMENTAL SECTION The components used in the instrument system were similar to those used in an instrument described previously (1, 7). The performances of the two instruments were identical in all respects. The sectored wheel, an improved version of that described previously (I), was constructed from four quadrants of optical quartz plate, arranged as in ref 1. The two diametrically opposite quadrants were each 'I8 in. thick, the third and fourth quadrants were '/I6 in. and 3/16 in. thick, respectively. They were held in place with a circular metal collar grooved to hold each piece securely. A chain drive was used t o connect the sectored wheel to a dc motor (TRW Globe, Dayton, OH, Model 403A117-3). A set of photographs of the whole assembly is shown in Figure 1. The assembly was placed just after the middle slit of the double monochromator that was used (Jobin Yvon DHZOA; linear dispersion, 2 nm/mm). The position of the wheel was arranged such that the image of the middle slit passed through the bottom half of the wheel and was aligned along the vertical diameter. In order to provide a frequency reference signal, a small two-blade mechanical chopper, mounted directly on the motor shaft, chopped the beam from a miniature infrared light-emitting diode. A phototransistor directly across from the light emitting diode was used to detect the chopped IR radiation and provide the reference signal for the photon counter. A handle was used to pivot the whole sectored wheel assembly. Mechanical stops limited the travel of the assembly to 60". The housing was marked with the angle of incidence from 0 to 60' at 0.5" intervals. The arrangement of the sectors of the wheel (1) provided primarily for 2F detection although pseudo-1F detection ( I ) was
0003-2700/83/0355-0488$01.50/0 0 1983 American Chemical Society
ANALYTICAL CHEMISTRY. VOL. 55.
NO. 3, MARCH
1983
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Table I. General Experimental Conditions for Wavelength Modulated Continuum Source Excited Atomic Fluorescence Spectrometry (AFC) monochromator (double monochromator, additive dispersion, Jobin Yvon DH20A) entrance and exit slit widths spectral band-pass middle slit width focal lenath source (Vmian eimac, Xenon arc Model VIX 300 uv) current power wavelength modulation frequency angle of incidence of light beam on wheel source modulation frequency atom cell ~
1.0
.
