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ANALYTICAL CHEMISTRY, VOL. 50, NO. 8, JULY 1978
out and will be reported on in due course.
(9) H. H. Kiet, L. P. Blanchard, and S.L. Malhotra, Sep. Sci., 12, 607 (1977). (IO) R. B. Williams, “Symposium on Composition of Petroleum Oil”, ASTM
ACKNOWLEDGMENT The authors are indebted to T o Xuan Ky for many helpful discussions in carrying out the computer analyses.
Spec. Tech. Publ., 224, 168 (1958). (1 1) R. M. Calla, “Introductionto Numerical Method and Fortran Programming”, John Wiley, New York, N.Y., 1967. (12) C. Lanzos, “Applied Analysis”, Prentice-Hall, New York, N.Y., 1956.
LITERATURE CITED
RECEIVED for review December 27, 1977. Accepted April 24, 1978. The authors gratefully acknowledge the financial assistance received from B. P. Canada Limited, from the National Research Council of Canada, and from the Department of Education of the Government of Quebec. The work described in this paper forms part of the general research program undertaken by the “Groupe de Recherches en Sciences Macromol6culaires” a t Lava1 University.
(1) J. P. Dickie and T. F. Yen, Anal. Chem., 39, 1847 (1967). (2) J. G. Speight, Fuel, 50, 102 (1971). (3) E. Hirsch and K. H. Altgelt, Anal. Chem., 42, 1330 (1970). (4) G . A. Haley, Anal. Chem., 44, 580 (1972). (5) E. J. Dickinson, Proc. Assoc. Asphalt Paving Techno/.,43, 132 (1974). (6) L. Zalka and T. Mandy, Acta Chim. Acad. Sci. Hung., 79, 375 (1973). (7) T. F. Yen, Energy Sources, 1, 447 (1974). (8) Y. Katayama, T. Hosoi, and G. Takeya, Nippon Kagaku Kaishi, I , 127 (1975).
CORRESPONDENCE Comparison of Radiant Power of the Eimac Xenon Arc Lamp and Hollow Cathode Lamp Sources Sir: Lately there has been considerable interest in the use of the Eimac pre-focused xenon arc lamp as a source in atomic absorption in (1-3),atomic fluorescence ( 4 , 5 ) ,and molecular luminescence spectrometry (6). Recently, this lamp has been characterized with respect to its power output, noise spectrum, and optical properties (7). Most workers find that the Eimac lamp provides significantly higher radiant power than other commonly used continuum sources of comparable power input (6, 7). We and others (2, 3, 8 ) have been interested in this lamp as a primary source in atomic absorption. In this application, an obvious question which arises is how the radiant power of the Eimac lamp compares with the hollow cathode lamps ordinarily used in commercial atomic absorption instrumentation. The purpose of this paper is to provide some data to answer this question. At the outset we must decide exactly which intensity measure we wish to compare, i.e. radiant power (watts), radiance (watts sr-l m-2), irradiance (watts m-2), etc. The objective of such a comparison is to tell which source would be “more intense” for a particular application, in this case atomic absorption spectrometry. Greater intensity is desirable in this application for three reasons; first, to reduce the influence of thermal emission signals originating from analyte emission, flame background, or graphite furnace blackbody emission; second, to reduce the effect of photon shot noise; and third, to avoid difficulties from photomultiplier dark noise. The first of these benefits requires greater source radiance, while the latter two require greater radiant power a t the photodetector. Thus, we wish to compare the two sources both on the basis of radiance, which is a function of the sources themselves, and on the basis of radiant power a t the detector, which is also a function of the spectrometer and entrance optics. What one actually measures, of course, is the photoanodic current of the detector, which is a measure of, and directly proportional to, the radiant power emerging from the exit slit. Moreover, the photocurrent is proportional to the radiance of the source, as well as to the detector sensitivity, the transmission factor of the spectrometer and the entrance optics, the width and height of the slits, and the solid angle of radiation collected from the source by the spectrometer. Thus, one may also compare the relative radiances of two 0003-2700/78/0350-1218$01.00/0
sources by keeping constant all terms associated with the entrance optics, spectrometer, and photodetector; thus, the ratio of measured photoanodic current would be equal to the ratio of radiances. Although the comparison of radiant power a t the detector is straightforward, a comparison of radiance is complicated by the fact that the Eimac lamp contains an internal parabolic reflector which is an integral part of the lamp. Other sources, i.e., conventional Xenon arc lamps and hollow cathode lamps, do not have such a reflector. The problem is, do we consider the reflector as a part of the lamp itself or as a part of the entrance optics of a spectrometer? If the latter, then the radiance comparison is not straightforward. We will try to avoid this problem by measuring only the relative radiant powers a t the detector, but we will do so for two different experimental designs, In the first design, the photocurrents will be compared when all optical parameters external to the lamps are held constant, i.e., same detector sensitivity, spectrometer slit widths, external entrance optics, and position of source. This essentially provides a comparison of the relative irradiance (watts m-2) a t the entrance slit, providing that the fraction of light passing through the entrance slit which is actually collected by the collimating mirror in the spectrometer is the same for both sources. In any case, this experiment will allow a comparison of the sources on the basis of the first of the aforementioned three benefits of greater intensity, i.e., reduced effect of atomizer emission. In the second experiment design, we acknowledge the fact that in atomic absorption measurements, a continuum source like the Eimac would ordinarily be used with a very high resolution spectrometer (like the echelle spectrometer used in this work), while a line source could be used with a conventional medium-resolution monochromator with much larger slit area and possibly greater acceptance angle. Thus it would be of practical interest to compare the radiant power emerging from the exit slits of these two lamp-spectrometer systems, because it has a bearing on the relative influence of photon shot noise and dark current noise of the detector. The comparison of the radiant powers of line and continuum sources must of course take into account the very different spectral distributions of the two sources. In conventional atomic absorption instruments, the hollow cathode 0 1978 American
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 8, JULY 1978
line source is used in conjunction with a medium-resolution monochromator which isolates the desired spectral line from others emitted by the lamp. Since the width of the line profile from the lamp is much less than the spectral bandpass of the monochromator, the photodetector measures the integrated power distribution over the entire line profile. Therefore, in measuring the radiant power of a continuum source for comparison, it is desirable to measure the integrated power of the continuum source over the same spectral interval as the width of the line source. This could be done by utilizing a very high resolution spectrometer system, such as an interferometer-monochromator system or an echelle spectrometer. In this work we have used an echelle spectrometer which provides spectral bandpasses from 0.002 to 0.004 nm. Even this is not quite as small as the profile widths of most hollow cathode lamps, however, so it is necessary to correct the radiant powers measured for the difference between the obtainable spectral bandpass a t each wavelength and the width of the hollow cathode lamp line. We have assumed that the instrument function of the echelle monochromator is Gaussian so that the profile is given by:
G(X) = Go exp
1
where AXG is the full-width at half-maximum intensity (FWHM) and Go is the value of the function a t bandwidth center. The profiles of the hollow cathode lamp (HCL) emission lines are nearly of Gaussian shape (9) and are given by:
I(X) = I o exp
1
where AXL is the FWHM of the emission line, and Io is the spectral irradiance of the source a t ho, the line center. The spectral irradiance of the continuum source is assumed to be constant with respect to wavelength.
(3) T h e observed spectral power distribution a t the exit slit, is given by the convolution of Equations 1 and 2, or 1 and 3:
(4) It is not necessary to numerically compute ai since taking the ratio of aifor both line and continuum sources gives (IO):
T h e correction factor to the measured anodic current, which is proportional to ai,is (AXG/AX~);or just the ratio of the calculated spectral bandpass to the tabulated emission profile width at a particular wavelength (IO).
EXPERIMENTAL The hollow cathode lamps were the standard [16.5 cm (6.5-in.) long, 3.8 cm (1.5-in.) body-diameter 2.54 cm (1.0-in.) windowdiameter] lamps obtained from either Westinghouse, Varian, or Instrumentation Laboratories. The 300-W Eimac VIX-UV Xe arc lamp (Varian Eimac Division, San Carlos, Calif.) has had a total of about 200 h of use. A new Spectrometrics Model 102 echelle spectrometer (Spectrometrics, Inc., Andover, Mass.) was used with a Hamamatsu photomultiplier tube and conventional dc electronics. In order to compare the lamps themselves, the same optical and detector system were used for both lamps. The lamps were
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placed at the same position on the optical rail so that the entrance optics (a single double-convex quartz lens) would form an image of the source on the entrance slit. Slits 25 pm wide and 300 pm tall were used in all cases. Photomultiplier voltage was of course the same for both measurement. The hollow cathode lamps were operated at the manufacturers recommended currents. The Eimac lamp was operated at the rated current of 20 A. Care was taken to optimize the position of each lamp to maximize the observed photoanodic current. In addition to the comparison of the lamps themselves, it is also of interest to compare the photon flux (radiant power) emerging from the exit slit of the Eimac-echelle spectrometer system to that emerging from the exit slit of a system composed of a hollow cathode lamp and medium-resolution monochromator system typical of commercial AA instrumentation. It is expected that the throughput of the echelle spectrometer will be significantly lower than that of a conventional medium resolution monochromator because of the smaller slit width and height of the echelle. In order to obtain at least a rough idea of this comparison, a Jarrell-Ash half-meter Ebert monochromator was used as representative of a typical commercial AA monochromator, An interchangeable phototube assembly was employed so that both monochromators could use the same photodetector. The slits of the half-meter Ebert were set to 100-wm width and maximum height. This width was suggested by the 80-pm widths recommended by the manufacturers for the Instrumentation Laboratories 353 atomic absorption spectrometer, the line-source instrument used in our previously published comparisons (2). RESULTS AND CONCLUSIONS Table I compares the intensities of the sources expressed in terms of the ratio of observed photoanodic current measured with identical optical systems, corrected for the difference between the spectrometer spectral bandwidth and the line source half width. As can be seen from the second-to-last column, the Eimac lamp is significantly more intense than every hollow cathode lamp tested. In many cases, the intensity advantage of the Eimac lamp is more than two orders of magnitude. The last column in Table I compares the relative radiant power emerging from the exit slit of the Eimac-echelle combination to that of the Ebert monochromator with hollow cathode lamps. Due to its smaller slit width and height (and lower optical speed), the throughout of the echelle spectrometer is much less than that of the half-meter Ebert. As a result, the Eimac-echelle system exhibits greater radiant power output only at wavelengths above about 300 nm. I t is clear that this can be expected to be only a very approximate comparison because of the large number of instrumental variables which could shift the results one way or the other. For example, a line-source instrument with sufficient dispersion could easily employ slit widths larger than 100 pm for many elements. On the other hand, a practical line-source instrument must employ a number of additional optical components (mirrors, beam splitters, choppers, etc.) in order t o achieve double-beam background-corrected performance. A significant light loss is usually associated with those extra components. The continuum-source system uses only a single refractor plate wavelength modulator (>90% transmission). It should be pointed out that the Eimac-echelle system was adopted not because it offered a potential intensity advantage, but rather because of its superior background correction capability (8) and its potential for extension to simultaneous multielement analysis. Indeed, we had thought that the continuum lamp might not be able to compete in intensity with sharp line sources within their very narrow line widths. I t now seems that is not the case. From a practical point of view, the radiant power differences at the exit slits are often not enough to have a large effect on the detection limits of the two systems. However, a significant advantage of a system with higher source intensity, even if the monochromator throughput is correspondingly lower, is that thermal emission
1220
ANALYTICAL CHEMISTRY, VOL. 50, NO. 8, JULY 1978
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from the atomizer (flame or furnace) will have a small effect on the system signal-to-noise ratio. In fact, we have never found atomizer emission to be a significant factor in the Eimac-echelle system.
ACKNOWLEDGMENT We thank Michael Epstein for his help in obtaining some of the experimental data. LITERATURE CITED (1) R. C. Eiser and J. D. Wlnefordner, Anal. Chem., 44, 698 (1972). (2) A. T. Zander, T. C. O’Haver, and P. N. Keliher, Anal. Chem., 40, 838 (1977). (3) T. C. O’Haver, J. M. Harniy, and A . T. Zander, Anal. Chem., 49, 662 (1977). (4) W. K. Fowler, D.0.Knapp, and J. D. Winefordner, Anal. Chem., 46, 601 (1974). (5) F. Llpari and F. W. Piankey, Anal. Chem,, in press. ( 6 ) R. J. Perchaiski, J. D. Winefordner, and B. J. Wilder, Anal. Chem., 47, 1993 (1975). (7) R. L. Cochran and G. M. Hleftje, Anal. Chem., in press. (8) J. M. Harnly and T. C. O’Haver, Anal. Chem., 49, 2187 (1977). (9) H. 0. Wagenaar, I. Novotny, and L. deGalan, Spectrochim, Acta, Pari B , 29, 301 (1974). (IO) L. deGalan and J. D. Winefordner, Spectrochlm. Acta, Part 8 , 23, 277 f 1966). (11) D. 0.Cooke, R. M. Dagnaii, and T. S. West, Talanta, 19, 1309 (1972). \ - - - I
NO. 8,
JULY 1978
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(12) P. N. Keliher and C. C. Wohlers, Anal. Chem., 46, 682 (1974). (13) P. N. Keiiher and C. C. Wohiers, Anal. Chem., 48, 140 (1976). (14) H. G. Wagenaar and L. deGaian, Spectrochim. Acta, P a r t s , 28, 157 (1973).
Department of Chemistry University of Maryland College Park, Maryland 20742
T. C. O’Waver* J. M. Harnly
A. T. Zander Department of Chemistry Cleveland State University Cleveland, Ohio 44115
RECEIVED for review October 17, 1977. Accepted April 17, 1978. From a Dissertation to be submitted to the Graduate School, University of Maryland, by J. M. Harnly, in partial fulfillment of the requirements for the Ph.D. Degree in Chemistry. Presented in part a t the 29th Pittsburgh Conference on Analytical Chemistry and Applied spectroscopy, Cleveland, Ohio, 1978. Financial support provided by USDA, Agricultural Research Service, Nutrient Composition Laboratory.
Chelatometric Titration of Antimony(II1) with Piperidinecarbodithioate Sir: The salts of antimony(II1) are very easily hydrolized even in an acidic solution ( I ) , and in order to make a stable standard solution, it is convenient to use some organic solvent, such as acetone, N,N-dimethylformamide (DMF), etc. (2). When such a solution of antimony(II1) in some organic solvent should be standardized by a titration, a common technique using potassium bromate or iodometry ( 3 ) is not easily applicable, as it is difficult to remove the solvent without losing a part of antimony, and/or without changing its oxidation state. The complexometry using EDTA ( 4 , 5 ) ,or CYDTA (6) (1,2-cyclohexanediamine-N,N,N’,N’-tetraacetate) is useful for the purpose, as these solvents do not interfere with the titration. But the end point is not always clear in EDTA titrations, and the CYDTA titration needs a little complicated process. Therefore, it was desirable to find a new analytical technique. On this line, the authors had found a new chelatometric titration method for antimony(II1) in DMF, with some dialkylcarbamodithioates, such as dibenzylamine- or pyrrolidinecarbodithioate, using xylenol orange (XO) as an indicator. Although the end point of this titration was clear, as the titrant decomposed rapidly, it was necessary to check its concentration often, and the standardization process was not easy. Consequently, the authors have improved this technique, synthesizing the chelating reagent and coordinating it to antimony during the titration process.
Table I. Analyses of Antimony(III), % From titrations From titrations with piperidineb with CYDTAa Piperidine = 0.09920M SbC1, = 0.02450 M Titrant, Titrant, Titrant, Sb, Sb , mmol mmol mLC mLC mmol 0.0754 1.006 0.76 0.0251 0.0251 0.1488 1.50 0.0496 2.01 0.0492 0.2242 3.02 0.0747 0.0751 2.26 0.3026 0.1009 4.03 3.05 0.1008 0.3750 0.1250 0.1256 3.78 5.03 0.6061 0.2020 0.2012 8.05 6.11 0.7509 0.2503 0.2515 10.06 7.57 Piperidine = 0.04940 M SbCl, = 0.0150M Titrant, Sb, Titrant, Titrant, Sb mLC mmol mmol mLC mmol 0.0450 0.0150 0.91 1.006 0.0151 0.0899 0.0300 1.82 2.01 0.0301 0.1363 0.0454 2.76 3.02 0.0454 0.2258 0.0753 4.57 5.03 0.0754 0.3631 0.1210 7.36 8.05 0.1207 0.4525 0.1508 9.16 10.06 0.1509 Piperidine solution standardized with hya Ref, (6). drochloric acid. Pipetted volume corrected by the calibration.
EXPERIMENTAL Reagents. Reagents used were all GR grade from Wako Pure Chemicals Co. Ltd., and were used without any further purification. The DMF used may contain a trace of water but, as is shown later, it does not affect the titration, and any special dehydration process is not necessary. Standardization of antimony(II1) chloride in DMF was done where the chelating reagent by chelatometry using CYDTA (6), solution was standardized with copper(I1) sulfate at 70 “C, using Cu-PAN as an indicator just as in the same way as for EDTA. The concentration of the piperidine standard solution (in DMF) was determined by the acidimetry; it was diluted with water and was titrated with standard hydrochloric acid (0.1002M), using
methyl orange as an indicator. The end point was clearly observed. This amine solution was stable and the concentration of 0.05 M solution, for example, did not change even after one month. Procedure for Titration of Antimony(II1) Salt with Piperidine. The antimony(II1) chloride solution of about 0.014.1 M in DMF (containing 0.01-0.3mmol of antimony) was exactly pipetted into the mixed solution of 2C-30 mL of DMF, about 1-1.5 mL of 1 M trimethylamine in DMF, 0.5-1 mL of carbon disulfide, 1-1.5 mL of acetic acid, and 2 drops of XO indicator (0.2% aqueous solution of xylenol orange). The mixture was titrated with 0.05 or 0.1M DMF solution of piperidine. The color of the solution changed from violet to red near the end point, and from clearly red to yellow at the end point.
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