mosphere. The boat is subsequently pushed slowly into a n 800 “C inlet. The results of this latter technique on the fuel oil samples exhibited much greater precision (+2,0% relative standard deviation) and good accuracy, so that for routine analysis the latter procedure is considered the more reliable. T h e precision of the reductive sulfur technique for industrial petroleum samples is summarized in Table VIII. All samples with sulfur greater than 500 ppm were diluted in toluene to approximately 50 ppm for the coulometric data shown here. The number of duplicate analyses per man-day
is on the order of twenty (40 determinations), exclusive of dilution time. ACKNOWLEDGMENT The,authors thank Dean Hoggan of A R C 0 Chemical Co., Anaheim, Calif., for permission to use the ASTM results presented and for valuable help in coordinating this study.
RECEIVED for review September 22, 1969. Accepted January 9, 1970.
Flame Emission Spectrometry with Repetitive Optical Scanning in the Derivative Mode W. Snelleman,’ T. C. Rains, K. W. Yee,* H. D. Cook,2and 0. Menis Analytical Chemistry Diuision, National Bureau af Standards, Washingtori, D. C.
A flame emission spectrometer using a rapid repetitive scan of a narrow wavelen th region has been developed. By this method o f wavelength scanning the second derivative of the output intensity is measured. The use of this approach to minimizing spectral interference in matrices and the use of microsamples greatly enhance the potentialities for flame emission spectrometry, and minimize the need for a monochromator of high resolving power. A quartz plate, made to vibrate at 145 Hz, is mounted behind the entrance slit of the monochromator. The ac amplifier is synchronized with the oscillations of the quartz plate. When the amplifier is tuned to twice the frequency of vibration, the second derivative of the spectrum is obtained. This permits the measurement of weak line spectra nested in or on a broad band or continuum. It is demonstrated that spectral interference due to CaOH bands and/or a continuum are minimized in the measurement of barium. The elimination of interferences from bands and flame structure led to an improvement in detection limits of alkali and alkaline earth elements in the presence of many matrix ions. An analysis can be performed with 50 pl of solution which makes it applicable to biochemical and air pollution studies.
THEMEASUREMENT of the radiant intensity of an atomic line which is located on a bandhead arising from the concomitant or flame gases is of concern to the flame emission spectroscopist. Frequently, an atomic line cannot be used because of interferences from some overlapping band structure. Two typical examples are the CaOH bands (5430-6220 A), which interfere with the Na 5890 A and Ba 5536 A lines, and the MgOH bands (3600-4000 A) which contribute to high background for the Fe 3720 A and Ru 3727 A lines. Also, the O H band system, covering the region of 2800 to 4000 A, limits the measurement of the radiant intensity of many atomic lines in flame emission spectrometry. These interferences have discouraged many workers in the field of flame emission. However, Buell ( I ) in using a high resolution monochromator has shown that many atomic lines in this region can be used.
A technique which permits the measurement of weak line spectra nested in or on a broad band or a continuum resulted in a n improvement in detection limits. In addition, microliter samples can conveniently be taken for analyses. The newly designed optical system permits a rapid repetitive scan of a narrow wavelength region. This scan is synchronized with the ac amplifier. With the ac amplifier tuned to the same frequency as the vibrating quartz plate, the signal produced is the first derivative of the emission intensity with respect to wavelength, At twice the frequency, the second derivative is obtained, This mode of operation permits the measurement of weak line spectra without the interference from background radiation or broad band spectra. When operating at high frequencies (145 Hz), the apparent signal-tonoise ratio is improved by eliminating the low frequency flicker noise of the flame. This technique was applied to the determination of lithium in 50 p1 of solution containing high concentrations of sodium, and to the determination of barium in a calcium matrix. Derivative spectroscopy has been described by several authors (2-4). Giese and French (5) used it to detect low intensity bands by measuring the first derivative of the transmission curve with respect t o wavelength. Under certain conditions, a recording of the first or second derivative of an absorption spectrum can give increased resolution over a normal spectrum. In magnetic resonance spectroscopy the first derivative and occasionally the second derivative are recorded to increase resolution of the spectrum (6). There are various ways for obtaining the derivative of absorption lines or bands with respect to wavelength. Balslev (7) vibrated the exit slit of his spectrometer and synchronously detected the signal at the vibrating frequency while Gilgore et al. used optical wobblers (8). Snelleman ( 9 ) was able to in-
Present address, Fysisch Laboratorium, Rijks-Universiteit Utrecht, The Netherlands. Measurement Engineering Division, National Bureau of Standards, Washington, D. C. “Flame Emission and Atomic Absorption Spectrometery,” J. A . Dean and T. C. Rains, Eds., Dekker, New York, N. Y . , 1969.
(1) B. E. Buell in
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ANALYTICAL CHEMISTRY, VOL. 42, NO. 3, MARCH 1970
( 2 ) A. Perregaux and G. Ascarelli, Appl. Opt., 7 , 2031 (1968). F. R. Stauffer and H. Sakai, ibid.,7,61 (1968). E. C. Olson and C. D. Alway, ANAL.CHEM., 32,370 (1960). A. T. Giese and C. S. French, Appl. Spectrosc., 9,78 (1955). C. H. Townes and A. L. Schwawlow, “Microwave Spectroscopy,” McGraw-Hill, New York, N. Y., 1955, Chapters 14 and
(3) (4) (5) (6)
17. ( 7 ) I. Balslev, Phys. Rec., 143, 636 (1966). (8) A. Gilgore. P. J. Stoller, and A. Fowler, Reu. Sci. Instrum., 38, ’1535 (1967). (9) W . Snelleman, Spectrochem. Acta, 23B, 403 (1968).
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Figure 1. Displacement of an oblique ray laterally and parallel to its initial direction crease the sensitivity of the measurement of copper by atomic absorption using a continuum source and optically scanning part of the continuum. I t was accomplished by placing in the optical path a vibrating mirror which was mounted o n the movement of a milliammeter. While this arrangement clearly demonstrated the feasibility of optical scanning in atamic absorption, it had certain limitations because of alignment difficulties. The current investigation led to the development of a system utilizing a n oscillating quartz plate which provided a stable and easy t o operate apparatus. Also, it was demonstrated that this technique had special applications in flame emission spectrometry.
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Apparatus. GENERAL INSTRUMENTATION. The instrumental system used in this study consists of a 0.75m CzernyTurner mount monochromator, fl6.5 aperture, p r v e d slits, exchangeable gratings blazed for 3000 and 7500 A with 1180 grooves/mm; a n oxygen-hydrogen flame with a total consumption burner and a n air-acetylene flame with a premixed nebulizer burner; gas flow meters and pressure regulators; a n end-on multiplier phototube, EM1 9558AQ, with 44-mm diameter photocathode and S-20 spectral response; a 0 to 2100 V multiplier phototube power supply, 0-30 mA, 0.001% regulation (line and load), *0.25% accuracy; a phase sensitive lock-in amplifier with full-scale sensitivity ranges of 100 nV to 500 mV, and a 10-mV strip-chart recorder, 0.2 sec full-scale response time. QUARTZ PLATE.A quartz plate 12 X 14 X 2 m m serving as the displacement element was mounted o n a rotating mechanical oscillator with a variable amplitude of up to 6" peak to peak. The light beam, in passing through the plate (as shown in Figure l), will be displaced laterally (perpendicular to the axis) and parallel to its initial direction when measured normal to the ray, and is given by the following relationship : Alp3Y = t . cos cy (tan CY - tan a') [tcy(n - l)/n] where t = the thickness of the plate, cy = radians in air, cy' = radians in the plate, and n = index of refraction of the material. The image shift is approximately proportional to the thickness of the quartz plate and the angle of rotation. In this experiment with a peak to peak amplitude of 5", the perpendicular shift was 1!2 p using cymean = 0" which corresponded to about 1.5 A of the spectrum. A larger displacement can be obtained by using a thicker quartz plate and by increasing the angle of rotation. However, to avoid N
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