(10) E. Gelpi, E., Peralta, and J. Segura, J. Chromatogr.Sci., 12, 701 (1974). (11) C-G. Hammar, and R. Hessling, Anal. Chem., 43, 298 (1971). (12) W. F. Holmes, W. H. Holland, B. L. Shore, D. M. Bier, and W. R. Sherman, Anal. Chem., 45, 2063 (1973). (13) L. D. Gruenke, J. C. Craig, and D. M. Bier, 23rd Annual Conference on Mass Spectrometry and Allied Topics. Houston, Texas, May 25-30, 1975. (14) R. W. Kelly, J. Chromatogr.,71, 337-339 (1972). (15) A. A. Boulton, S. R. Philips, and D. A. Durden, J. Chromatogr., 82, 137 (1973). (16) J. Rosello, J. Tusell, and E. Gelpi, J. Chromatogr..in press.
(17) M. C. Sdnchez, J. Colome, and E. Gelpi, J. Chromatogr., 126, 601 (1976). (18) C. Sutiol and E. Gelpi, unpublished results.
RECEIVEDfor review March 11, 1976. Accepted December 9,1976. T h e authors thank the Comision Asesora de Investigaci6n Cientifica y TBcnica for Research Grant 100.01.721/ 74.
Opto-Acoustic Trace Analysis in Liquids with the FrequencyModulated Beam of an Argon Ion Laser W. Lahmann,* H. J. Ludewig, and H. Welling lnstitut fur Angewandte Physik, Technische Universitat, Hannover, Welfengarten 1, Federal Republic of Germany
The opto-acoustic determination of low absorptlons has been appiled to the assay of small amounts of nonfluorescent absorbers In liquids. The output of an argon ion laser at 488 and 514 nm was used for an excitation of the sample at a perlodicaily alternating wavelength. This dual-wavelength operation enabled a considerable reduction of the background signal. A detection limit of 9 X 1O’O molecules per cm3 (12 ppt) has been achieved.
Lasers have proven to be a powerful analytical tool, e.g., in atomic fluorescence flame spectroscopy by replacing the hollow cathode or electrodeless discharge lamp excitation by a nitrogen-laser-pumped dye laser, as reported by Omenetto e t al. (1). The most dramatic improvement in detection sensitivity by use of lasers was attained in non-emitting homogenous gas media. The highest sensitivity reported so far has been achieved by Fairbank, Hansch, and Schawlow (2) with a CW dye laser resulting in a detection limit of only 100 Na atoms per cm3. Despite these outstanding achievements in the gas phase, laser fluorimetry in condensed media may turn out to be an even more important analytical application of fluorescence as most analytical problems involve matter in the condensed phase. Zare has given a review of the corresponding experiments performed so far ( 3 ) ,e.g., concerning a combination of laser fluorescence detection with chromatographic separation. In his review a detection limit of 7.5 X lo8 molecules of Rhodamine 6G per cm3, corresponding to 0.0006 ng per cm3 is reported. However, in trace analysis the analytical procedure quite often has to apply a nonfluorescent, highly absorbing indicator for the substance to be detected. T h e concentration of this indicator is usually determined by absorption spectrometry with conventional spectrophotometers. Here the use of lasers, as for fluorimetry, should allow a considerable improvement in the detection limit, as it is possible t o measure substantially lower absorptions by use of lasers in conjunction with optoacoustic absorption spectrometry. In opto-acoustic absorption measurements a periodically interrupted laser beam is directed into a sample cell. The absorbed radiation is converted into heat via nonradiative processes thus producing periodic pressure fluctuations which are detected with a sensitive microphone. This opto-acoustic determination has been successfully applied to the detection
of ultralow gas concentrations; a comprehensive review is given by Dewey ( 4 ) . So far the opto-acoustic trace analysis has been confined to the gas phase. This article describes the opto-acoustic method utilized for liquid media. This method may find widespread application because of the ubiquity of liquids in trace analysis. In contrast to opto-acoustic absorption measurements in the gas phase, the background absorption of the chief constituent (solvent via air) is considerable, producing a substantial opto-acoustic signal. For an elimination of this background a dual-wavelength excitation with a laser beam is applied where the absorption by the solvent is equal for both exciting wavelengths, but highly different for the dissolved substance to be detected.
EXPERIMENTAL Figure 1 shows the experimental setup. The output beam of an argon ion laser operating in the “all lines mode” is separated by the prism P1 into its spectral components. The perforated plate S transmits the two most powerful beams at 488 and 514 nm and screens off all the other beams of the argon ion laser. (In Figure 1 these beams are already omitted for lucidity and the angle between the two remaining beams at 488 and 514.4 nm is somewhat exaggerated for the graphic representation). The two beams are spatially reunited with the lens L and the second prism P2, and the recombined beams are directed via mirror M into the sample cell. The chopping wheel driven by a small synchronous motor which is fed by a frequency-stabilized sine wave generator interrupts the two transmitted beams in such a way that only the main intensity of one beam at a time is transmitted. The laser output power is adjusted to a level where both beams show the same output power (-700 mW); thus as a net effect, a laser beam with slightly varying intensity (at the frequency 2 Y , if Y is the chopping frequency for each beam), but with a switching wavelength impinges on the cell. The cell is a piezoelectric ceramic tube (PZT-5H, supplied by Vernitron; length: 76.2 mm, inner diameter 19.0 mm) which is sealed off at its faces: at one face with a quartz window, at the other with a ceramic plate incorporating a small quartz window and an opening to fill the cylinder. The piezoelectric ceramic acts simultaneously as a sample cell and as a pressure sensor for the pressure fluctuations induced in the sample liquid by absorbed radiation. This pressure sensor is advantageous for the detection of small pressure fluctuations in liquids as it is a live sound recorder and hence adjusted to liquids with their high acoustic stiffness. Alternatively, a crystal microphone might be used as well. Condenser microphones however, usually employed for opto-acoustic measurements in gaseous media are inadequate as they are sensitive primarily for the motion of its membrane. The amplitude of motion Ax is rather small in liquids for a given pressure fluctuation Ap according to Ax = Ap/2.nupc (where u = frequency, p = density, c = velocity of sound), as the density of ANALYTICAL CHEMISTRY, VOL. 49, NO. 4, APRIL 1977
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frequency counter
'
reference unit
chart recorder!
-1--j preompl.
cell
Figure 1. Experimental arrangement
liquids is approximately a thousand times larger than the density of gases at atmospheric pressures. The cell is placed into an airtight chamber for a reduction of the ambient acoustic noise. The pressure fluctuations are recorded with a phase sensitive detection system. Its reference signal is produced by the light split off with the glass plate P onto the photodiode D. The light-induced pressure fluctuations Ap in the opto-acoustic cell depend on some material constants of the solvent according to Kohanzadeh, Whinnery, and Carroll ( 5 ) :
where K = bulk modulus, O(T= thermal expansion coefficient, p = density, c = velocity of sound, c,, = specific heat, cy = absorption coefficient, and P = average laser output power. The ratio R of the pressure fluctuations of chloroform and carbon tetrachloride in comparison with water is 21.0:l and 23.2:1, respectively, whereas methanol and ethylene glycol are less suitable with R = 9.5:l and R = 6.21. In addition, chloroform and carbon tetrachloride show very low absorption coefficients in the visible spectral region and are frequently used as solvents in colorimetric determinations so that their use is advantageous for the opto-acoustic trace analysis. The dual-wavelength operation enables a substantial reduction of the background signal which is considerable even with pure low-loss liquids as chloroform or carbon tetrachloride. The absorption coefficients of these solvents in the 500-nm region are somewhere between lo-:< and lo-* cm-' according to purity, causing a background absorption of nearly 0.1-1% in the sample cell. If the absorption coefficients of a solvent are equal for both wavelengths (488 and 514.5 nm) and the powers of the two beams are equal as well, the opto-acoustic signal at the chopping frequency P should be zero and there should be a small signal at 2u. Slight differences of the absorption coefficients at the two wavelengths as they occur for low-loss solvents can be compensated for by a proper adjustment of the two beam powers by changing their relative intensities which is feasible here by simply altering the total output power. Thus a complete reduction of the background signal should be possible. In practice the reduction is limited by the ambient acoustic noise and the electrical noise introduced by the piezoelectric transducer and the subsequent amplification and signal processing,causing a background signal drift around zero. This background drift corresponds to a residual absorption coefficient of -IO-" cm-I for chloroform. The opto-acoustic cell is excited far off its resonance frequencies, at 700 Hz, in contrast to most opto-acoustic measurements in the gas phase ( 4 ) .Because of heating of the solvent by the laser beam, the resonance frequency would drift and cause severe changes in the output signal.
RESULTS AND DISCUSSION For a first opto-acoustic trace analysis in liquids, t h e determination of @-carotene in chloroform was investigated. @-Caroteneis a highly absorbing nonfluorescent dye whose molar absorptivity (in chloroform) is 102 000 L mol-' cm-l for 488 n m and 37 000 L mol-I cm-l for 514.5 nm. With the application of the dual-wavelength operation described above, the background absorption (absorption by chloroform -9 X cm-l) was suppressed whereas the different absorption 550
ANALYTICAL CHEMISTRY, VOL. 49, NO. 4, APRIL 1977
coefficients of /%carotene at 488 and 514.5 n m caused a strong opto-acoustic signal. Defining the detection limit as a 2:l ratio of the optoacoustic signal-to-background noise level, a detection limit of 9 X 1O1O molecules of 6-carotene per cm3 or 0.08 ng per c m 3 (corresponding t o a n absorption coefficient of 2.2 X cm-l) was found. This concentration is approximately 130 times higher than the minimum concentration for the fluorimetric determination of Rhodamine 6G reported by Zare ( 3 ) ,which represents the lowest detection limit obtained so far for fluorimetric determinations in liquids. (Rhodamine 6G shows a high molar absorptivity, tmax= 130 000 L mol-l cm-I at 515 n m and its quantum efficiency exceeds 0.9 ( 6 ) ,so t h a t it is a n ideal substance for a fluorimetric detection.) Despite t h e difference of two orders of magnitude, t h e opto-acoustic detection limit obtained here compares favorably with the fluorimetric detection limit of Rhodamine 6G, remembering the high opto-acoustic background of the solvent in comparison with the fluorimetric background of t h e solvent which should be negligible. For a second example of a n opto-acoustic trace analysis, a determination with more practical importance was chosen, the detection of selenium. T h e most widely used reagents for a fluorimetric or photometric determination of selenium are 3,3'-diaminobenzidine and 2,3-diaminonaphthalene, but t o obtain sufficiently reproducible results the recommended analytical procedure has to be observed strictly, since changes in temperature and light strongly affect the development of the indicating dye ( 7 ) . A different photometric determination with less stringent experimental restrictions, b u t with high selectivity and reproducibility has been proposed by Feigl and Demant (8)and Murashowa and Sushkowa (9),the detection of selenium with asymmetric diphenylhydrazine. Selenium(1V) oxidizes diphenylhydrazine to form a red dye (diphenylhydrazine quinoneanyl) whose absorption maximum is around 530 n m with a molar absorptivity in terms of selenium(1V) of -1000 L mol-l cm-l. As described by Murashowa and Sushkowa (9) the optimum conditions for the color reaction were chosen on using a selenious acid solution whose acidity was 1.5 N with respect to HC1 and a 5% solution of diphenylhydrazine in ethanol. The colored reaction product was extracted with chloroform by shaking the solution with chloroform in a separatory funnel. After phase separation, the chloroform extract was poured into the opto-acoustic cell which was filled up with pure chloroform. T h e ethanolic solution of diphenylhydrazine shows a slight brown color itself, resulting in an absorption of -2% in the sample cell. This background absorption was eliminated with the dual-wavelength operation at a proper power level of the argon ion laser as described above. So a detection of 15 ng Se per crn:j, corresponding to a n absorption of 3.5 X cm-', was attained. Oxidizing agent ions, copper(II), iron(III), vanadium(IV), and tungsten(V1) interfere with this selenium determination. Masking agents such as sodium oxalate, ammonium fluoride, and ethylenediaminetetraacetic acid, disodium salt are inadequate as t h e acidity of the solution prevents their normal function, i.e., t o form highly insoluble compounds with t h e interfering ions. So t h e total selenium should be separated beforehand by extraction, destillation, or chromatographic methods as described, e.g., by Nazarenko and Ermakov ( 7 ) . In both opto-acoustic absorption measurements, the minimum detectable absorbed fraction was -0.03%. This represents a n improvement by a factor of at least 30 in comparison with conventional spectrophotometers, as even with highprecision double-beam spectrophotometers the absorbed
fraction has to exceed 1%,considering t h a t the minimum uncertainty of these instruments with respect to the absorbed fraction is 0.2 to 0.4% (IO). The opto-acoustic absorption measurement can easily be extended to other spectral regions, in particular to the blue and ultraviolet regime too which is used in most colorimetric determinations. Argon ion lasers and dye lasers and the application of frequency doubling in conjunction with these lasers can provide the required light power for this spectral region.
LITERATURE CITED N. Omenetto, N. N. Hatch, L. M. Fraser, and J. D. Winefordner,, Anal. Chem., 45, 195 (1973).
(2) W. M. Fairbank, T. W. Hansch, and A. L. Schawiow, J. Opt. Soc. Am., 65, 199 (1975). (3) R. N. Zare, “Laser Spectroscopy”, Proc. Sec. Int. Conf., Megeve, June 23-27, 1975, Springer, Berlin, 1975. (4) C. F. Dewey, Opt. Eng., 13, 483 (1974). (5) Y. Kohanzadeh, J. R. Whinnery, and M. M. Carroll, J. Acoust. SOC.Am., 57,67 (1975). (6) K. H. Drexhage, in “Dye Lasers”, F. P. Schafer, Ed.. Springer, Berth, 1973. (7) I. I. Nazarenko and A. N. Ermakov, “Analytical Chemistry of Selenium and Tellurium”, Halsted Press, Jerusalem, 1972. (8) F. Feigi and V. Demant, Microchim. Acta, 1, 322 (1937). (9) V. I. Murashowa and S. G. Sushkova. J. Anal. Chem. USSR, 19, 1396 (1964). (IO) G. F. Lothian, “Absorption Spectrophotometry”, Adam Hilger, London, 1969.
RECEIVEDfor review October 26,1976. Accepted December 20,1976.
Preparation of Electrodeless Discharge Lamps for Elements Forming Gaseous Covalent Hydrides G. E. Bentley‘ and M. L. Parsons* Department of Chemistry, Arizona State University, Tempe, Ark. 8528 1
A general procedure has been developed to prepare electrodeless discharge lamps for elements whlch form gaseous covalent hydrides. Lamps have been prepared for antimony, germanium, and selenium. The lamps have been characterized with respect to spectra, Intenslty, stabillty, Ilfetlme, and production reproduclbillty. Atomic fluorescence measurements were used to indicate intensity at the line center as well as analytical feaslbllity.
Analytical atomic absorption spectrometry (AAS) and atomic fluorescence spectrometry (AFS) require the use of excitation sources t h a t have high intensity, narrow spectral line widths, short and long term stability, and long lifetimes. Hollow cathode lamps (HCL) have been applied successfully to AAS. However, conventional HCL’s do not provide sufficient intensity for general use in AFS. Further, it is not possible to make suitable HCL’s ( I ) for many of the metals and metalloids of groups 5A and 6A. Microwave-powered electrodeless discharge lamps (EDL) have been used successfully as spectral line sources for many of these elements. Unfortunately, EDL’s produced by placing milligram quantities of the analyte into a quartz tube with 1-10 Torr of inert gas pressure have demonstrated problems of instability, lack of reproducibility in manufacture, and short lamp lifetimes (2). One of the most promising methods of improving EDL performance is by precisely pipetting microgram quantities of a metal salt into a lamp blank using dilute aqueous solutions of the analyte. By using a known volume of the solution and evaporating all traces of the water, one knows precisely how much analyte remains in the tube ( 3 ) .This method of introducing the analyte is restricted to compounds that form simple aqueous salts. T o prevent the emission of intense band spectra, it is important t h a t all water be removed and t h a t no oxygenated species remain. These limitations preclude elements Present address, Los Alamos Scientific Laboratories, Group
CNC11, Los Alamos, N.M. 87544.
that form volatile hydrated cations or ones that exist predominantly in solution as oxyanions. I t was desired to make EDL’s reproducibly and reliably for the group 4A, 5A, and 6A elements t h a t exist as oxyanions in solution. These elements are generally volatile leading to out-gassing difficulty if introduced in the metallic form. The elements in the groups of interest form gaseous covalent hydrides when their oxides are treated with a sufficiently strong reducing agent ( 4 ) . T h e use of a gaseous form of the analyte allows precise knowledge of the amount of compound present in the lamp bulb before sealing. This is accomplished by controlling the pressure of the metal hydride when introducing it into the lamp blank. For a lamp bulb 1 cm in diameter and 3 cm long, with an analyte gas pressure of 1 Torr, approximately 10 bg of the element are present in the bulb before sealing. Germanium (group 4A), antimony (group 5A), and selenium (group 6A) lamps were prepared by using the hydrides for each element. The lamps were characterized by the following: 1) reproducibility of preparation (measured by determining the relative atomic fluorescence produced by each lamp using a standardized set of conditions), 2) stability as a function of temperature, 3) “useful” intensity (useful intensity was determined by measuring the intensity of fluorescence induced by each lamp), and 4) lamp lifetime. Busch and Vickers ( 5 )have proposed a mechanism of excitation in EDL’s t h a t involves Penning ionization. This mechanism dictates that a high electron concentration exists in the lamp plasma. Thus, it is desirable to add a quantity of easily ionized alkali metal to the lamp prior to sealing. At the relatively low temperature of operation of EDL’s, it is also desirable t o have a volatile halide compound of the analyte present in the lamp t o ensure that a sufficient concentration of analyte is in the vapor state to provide adequate intensity. (The metal hydride bond is weak and probably dissociates when the lamp is sealed. This is discussed below.) T o meet both of these requirements, a quantity of either potassium iodide or potassium bromide solution was added for lamps prepared by the hydride method. ANALYTICAL CHEMISTRY, VOL. 49, NO. 4, APRIL 1977
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