(17) Y . Yamamoto, J. Kumamaru, T. Hayashi. and M. Kanke. Tabnta, 19, 953 (1972). (18) J. B. Headridge and D. R . Smith, Talanta. 18, 247 (1971). (19) H. L . Kahn, Amer. Lab., 3 ( E ) , 35 (1972). (20) J. Aggett and T. S.West, Anal. Chim. Acta, 57, 15 (1971). (21) T . R. Hauser. T. A. Himmers, and J. L. Kent, Ana). Chem., 44, 1819 (1972). (22) C. R. Parker, J. Rowe, and D. Sandoz, Amer. Lab.. 5 (8). 53 (1973). (23) J. Lehner and 6 . Behr. J. Agric. food Chem., 19, 1011 (1971). (24) K . G. Brodie and J. P. Matousek, Anal. Chem., 43, 1557 (1971). (25) W. K. Robbins, Anal. Chem. 46, 2177 (1974). (26) I. Rubeska and B. Moldan. "Atomic Absorption Spectrophotometry", CRC Press, Cleveland, 1971, pp 134-5.
(27) T. Gorsuch, "Destruction of Organic Matter", Pergamon Press, London, 1970, pp 77-79. (28) 0. L. Davies. Ed.. "Statistical Methods in Research and Production", 01iver and Bovd. London. 1958. (29) V. V. Nalimbv, "The Application of Mathematical Statistics to Chemical Analysis", Pergamon Press, London, 1963. (30) ASTM Special Tech. Publ. 443, "Atomic Absorption Spectroscopy", Philadelphia, 1969, p 20.
RECEIVED for review November 14, 1974. Accepted April 3, 1975.
Spatial Differentiation of Optical Emission in Q-Switched LaserInduced Plasmas and Effects on Spectral Line Analytical Sensitivity William J. Treytl,' Kenneth W. Marich,2 and David Giick3 Division of Histochemistry, Department of Pathology, Stanford University School of Medicine, Stanford, Calif. 94305, and the Divisions of Information Science and Engineering, and Life Sciences, Stanford Research Institute. Menlo Park, Calif. 94025
A comparison was made of the relative signal-to-nolse ratios ( S/N) of time-differentiated photoelectric and photographic-densitometric recording of element optical emlsslon from selected regions of laser-generated plasmas. A newly designed 0-spoiled ruby laser microprobe delivering -35 mJ per pulse, and equipped with a six-channel dlrect-reading time-differentiated photoelectric system, was used to analyze samples containing P, Mg, Fe, Cu, and Zn In organic matrices. Signal from a region displaced 0.6 mm laterally from the vertical midline of the plasma and extending from 0.5 to 3.5 mm above the laser focal plane gave -2.5-fold increases of SIN relative to the on-axis measurement. The improvement in analytical response of the photoelectric over the photographic-densitometric technique was signlflcantly less than would be expected based on the relative quantum efficiencies of the photomultiplier tube and the photographic plate.
Recent studies from this laboratory of factors affecting the analytical spectroscopic quality of optical emission of laser-induced plasmas from biological samples have dealt with such matters as sample matrix effects ( I ) , atmospheric influences (2), temporal resolution (3, 4), and statistical correlations of signal and background ( 5 ) . This paper reports the effects of vertical and lateral spatial differentiation on the emission spectra of laser-generated plumes, and also presents a comparative evaluation of photoelectric and photographic detection. The formation of a plasma is a complex, non-equilibrium process. Gradients of pressure, temperature, and electron density exist across the plasma volume (6-8)and the spectral emission response to these factors varies among elements and molecules (9-11). There is evidence of nonhomPresent address, FMC Corporation, Engineered Systems Division, Santa Clara, Calif. 95052. Present address, Stanford Research Institute, Information Science and Engineering Division, Menlo Park, Calif. 94025. T o whom reprint requests should be sent. Present address, Stanford Research Institute, Life Sciences Division, Menlo Park, Calif. 94025.
ogeneity of composition within the plume (12). There is, consequently, reason to expect significant qualitative and quantitative differences in the spectra emerging from specific spatial regions of the plasma. Emission from specific sections has been studied and striking differences have been reported (12-14). The effect of such differences on analytical sensitivity will be reported here. Although photoelectric detectors have much higher quantum efficiencies than photographic plates and can be incorporated into rapid systems for measurement of spectral line intensity, the photographic technique offers the advantage of displaying lines with higher resolution over a broader spectral range than is now possible by the use of electronic detection. The increase of detectivity obtained by photoelectric measurement has not been adequately defined for the laser microprobe. In this paper, comparison is made of the optical emission of P, Mg, Fe, Cu, and Zn in organic matrices by photoelectric and photographic measurements.
EXPERIMENTAL Materials. Stock solutions of the following reagent grade salts were prepared: Cu(N03)2.3H20, Mg(N03)~6H20, and FeC13. 6H&. Stock solution I consisted of the final mixture (mM):7.88 Cu, 15.3 Zn, and 12.3 Mg. Stock solution 11 was 1.85M Fe. Dilutions of 5, 10,50, 100,500, and 1000 were made from each stock solution. A 2-ml aliquot of each diluted solution was mixed with 18 ml of hot 5% gelatin (USP) and gelled a t 7 "C for 24 hr. Frozen microtome sections, l o p thick, were cut from pieces of the frozen (Dry Ice) gelatin and mounted on 1- X 3-inch plastic slides. Phosphorous samples were 40-k thick sections of normal liver tissue which had been previously shown to have a high phosphorous content. Apparatus. The improved Q-switched ruby laser microprobe ( 1 5 ) used for analysis of the samples was equipped with a time-differentiated photoelectric detection system ( 3 ) .The polychromator consisted of six photomultiplier tubes (C31005C, RCA, Harrison, N.J.), with their mu-metal shields and divider networks, mounted in a hexagonal close-packed configuration. A specially designed aluminum light guide was mounted on the front of each phototube assembly. The aluminum device consists of an offset four-sided hollow wedge whose internal mirrored surfaces were diamond polished. The apex of the hollow wedge was machined off to form an entrance slit 150 Gm wide and 12 mm long. The base of the wedgeshaped light pipe forms a square aperture 14 mm on a side opening ANALYTICALCHEMISTRY, VOL. 47, NO. 8, JULY 1975
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onto the photomultiplier tube. At 3000 A, approximately 80% of the light entering the light pipe entrance slit reaches the photocathode through multiple reflections. These light pipes were designed so that their entrance slits lie in the spectral plane of the 0.75-m Czerny-Turner spectrometer (Model 78-490, Jarrell-Ash). The dispersion of the spectrometer was approximately 10 A per mm. The laser energy delivered to the sample was -35 mJ in a pulse of -30 nsec focused from a 3-mm diameter beam through a microscope objective with a 11-mm focal length. The lens system used to couple the plasma light to the spectrometer was designed for maximal coupling efficiency matching the f h . 5 collection aperture with the f/6.0spectrometer entrance aperture and focusing a 4X magnified image on the entrance slit. Procedure. The six-channel polychromator was set to measure the following lines (nm): P, 253.5; Mg, 285.2; Fe, 302.0; Cu, 324.7; Zn, 334.5. The sixth channel, normally reserved for background measurements was not operational and background was measured after grating rotation as will be described. Hollow cathode lamps (Perkin-Elmer, Norwalk, Conn.) were used to position the respective tubes. In order to minimize chromatic aberration effects, the lens coupling system was focused a t 289.3 nm. Positioning of the spectrometer optical axis with respect to the laser-induced plasma was accomplished by use of a reverse illuminating He-Ne laser. By reverse illumination through the exit slit of the spectrometer, the continuous laser light emerging from the entrance slit was focused by the coupling optics to a selected point on a screen placed a t the plasma site. Precise vertical and horizontal positioning of this focal point was made by calibrated movement of the spectrometer and the coupling optics. For lateral displacement, a plastic slide containing a series of 10-p diameter craters a t 500-pm intervals was placed on the microscope stage with the line of craters perpendicular to the spectrometer input axis. The grating was rotated to zero order and the collection lens was positioned to focus the emerging He-Ne laser light a t the crater line. The end crater was positioned a t the desired location and the rear spectrometer foot was adjusted for maximum light scattering from the appropriate crater. The procedure for vertical positioning is similar. In this case, a white object with a sharp edge (opaque glass) was placed on the stage and the microscope was focused on the edge. The stage was then raised the desired amount and the height of the rear spectrometer foot was adjusted so that the edge of the opaque glass bisected the focused spot of the rear illuminating laser light. Optimization of photoelectric time parameters followed the procedure described earlier (3, 4 ) with the spectrometer entrance axis set for maximum light collection. Reasonably optimal response could be obtained using a common timing parameter of 15-psec integration time and 12-,usecdelay for all elements but phosphorous, which was best measured with a 3.5-psec delay. After time optimization, the effect of lateral offset, and then of vertical displacement were determined. Backgrounds for each spectral line were obtained by rotating the grating so that the respective channels were 0.7 nm above the line (0.7 nm below the line for P). For comparison of detection methods, photoelectric measurements were performed at the best photoelectric timing and off-axis position, and corresponding photographic data were taken under identical spatial conditions and with identical samples and sampling conditions. Kodak Royal-X Pan photographic plates (Eastman Kodak, Rochester, N.Y.) were developed for 10 min a t 68 OF which effectively increased their speed to an ASA of 2400 and the spectral lines of interest were analyzed with a scanning microphotometer (Model 23-100 Jarrell-Ash, Waltham, Mass.).
RESULTS AND DISCUSSION The effect of lateral displacement on photographic spectra from laser-induced plasmas of gelatin samples doped with Cu, Zn, and Mg is shown in Figure 1. With the internal camera mirror in position for focusing spectra in the exit port of the spectrometer, it was observed that light from the back illuminating laser was also imaged in the focal plane (Figure l a ) . This procedure allowed definition of the vertical position of the spectrometer’s optical axis with respect to the photographed spectra. Thus, it was inferred that if the light pipes were in the focal plane of the exit port, the vertical optical axis of the spectrometer was 3.0 mm below the center of the light pipe slit. It follows that the 4 X magnification of the coupling system, maxi1276
ANALYTICAL CHEMISTRY, VOL. 47, NO. 8, JULY 1975
Mg (285.2nm)
cu
(324.7 nm)
I
I
Figure 1. Typical spectra of laser-induced plasmas from gelatin samples
(a) Spectrometer optical axis coaxial with Q-switched laser axis and 1.3 mm above laser focal plane, dots from back illuminating He-Ne laser define the position of the vertical optical axis of the spectrometer. (b). (c).(d).and ( e ) No back illumination, optical axis of spectrometer entrance laterally displaced 0, 0.25. 0.5. and 0.75 mm, respectively, from the vertical mid-line of the plasma
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LATERAL DiSPLACEMENT O F OPTiCAL AXIS FROM PLASMA MiDLlNE - mm
Figure 2. Effects of lateral displacement, without vertical displacement of optical axis on response of measurement of emission of elements in gelatin (Mg, Cu, Zn) and liver (P) matrices
mum light collection was obtained at a vertical elevation of 0.8 mm above the microscope focal plane and no lateral offset, under which conditions the center of the bottom of the light pipe entrance slit was imaged at the laser focal point for the wavelength selected.
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L A T E R A L DISPLACEMENT OF OPTICAL AXIS FROM PLASMA MIDLINE rnm
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Figure 3. Effects of vertical displacement at fixed lateral displacement on response of measurement of emission of elements in biological matrices (Figure 2)
Figure 4. Off-axiSregion Of plasma (hashed area) common to the elements (Mg, c u , Zn, p) that yields optimal response of measurements-this region derived from points of measurement of maximum response and standard error for each of the elements
The photoelectric datapresented in Figures 2 and 3 show the effects of lateral and vertical displacement of the optical coupling axis with respect to the plasma center and the microscope focal plane. Regions of maximum response were located for each element, expressed as background corrected signal divided by the standard deviation of the mean (standard error) of the background noise ( S I N ) .The backgrounds include optical and non-optical noise components and are measurements of the total non-signal, or noise, component. The error bars in these figures are the standard errors of the background corrected signals for 10 sample populations, and give an indication of the precision obtainable with the laser microprobe. Optimal spectral response from off-axis regions of laser-induced plasmas relative to base-line conditions (focal point of Q-switched laser imaged a t bottom center of the light pipe entrance slit) are given in Table I. In all cases, significant increases were obtained. The behavior shown in Table I is not unexpected. I t has been well documented that a distinct region approximately defined by the cone formed by the focusing laser light exists in the plasma center which is of significantly higher
temperature, pressure, and electron density than in the outer parts of the plume. This zone is the principal source of continuum emission (12, 13, 16). Emission lines from the central plasma zone should undergo the greatest broadening, and the higher opacity of the central plasma should reduce the effective depth within the plume a t which emitting elements can be detected. As a consequence, although the greatest intensity of light is emitted from the center of the plasma, the greater intensity of continuum emission results in decreased SIN. In some cases, actual decreases in discrete signal emission from the plasma center were observed. Continuum emission decreases much faster with time than discrete emission (3, 4, 13), reflecting the rapid dissipation of the locally severe conditions at the center of the plasma during the laser pulse. A time-differentiated detector, therefore, measures a much cooler, diffuse, and perhaps a more homogeneous plasma, and so a reduction of spatial effects is to be expected. However, even with timedifferentiation, SIN increases of a t least factors of 2 were found. Figure 4 indicates that an optimal off-axis region was found that encompassed all elements studied. This region
Table I. Response Gain by Measurement of Spectral Emission from Off-Axis Regions of Laser-Induced Plasmas Optimum Responsea Obtained b) Lateral and V e r t i c a l Displacement
!rom On-Anis Position of Plasma Region Measured Vcrtical Displacement Lotcral Displacement Reference Element
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positiond
Off-anis, m m
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Pf 18 i 4 54 * 5 (0.5 i 0.2) 3 . 0 i 0.7 57 * 12 88 i 1 7 ( 0 . 3 i 0 . 1 ) 1.5 0.4 4.5 i 1.7 Mg' 310 z 25 820 i lgO(0.7 i 0.1) 2.7 i 0.7 350 i 30 370 i 5 0 ( 0 . 3 i 0.2) 1.0 i 0.2 2.8 i 0.8 Cup 340 i 30 460 i 40 (0.5 i 0.2) 1.4 i 0.2 160 i 15 330 i 30 (0.3 * 0.2) 2.0 0.3 2.7 i: 0.5 Zng 130 i 15 370 i 50(0.5 5 0.1) 2.9 0.5 140 i 15 210 + 2 5 ( 0 . 4 i 0.2) 1.6 i 0.2 4.4 i 1.0 ( I Response = (Signal-Background (Arbitrary Units))/Std Error of Mean of Background (Arbitrary Units) f Std Error, 10 samples per element. Measurement on 3- x 0.038-mm cross section area. Focal point of laser beam coincides with image of bottom of entrance slit of phototube snorkle. On-axis vertically, 0.6 mm off-axis laterally. e Lateral gain X vertical gain. Air-dried, 4 0 - p sections of fresh-frozen human liver. g Air-dried, 10-p sections of 5% gelatin containing respective salts of undiluted stock solutions (see under Materials).
*
ANALYTICALCHEMISTRY, VOL. 47, NO. 8 , JULY 1975
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-
____.._
Table 11. Comparison of Photoelectric a n d Photographic-Densitometric Measurements of Signal Intensity from Certain Elements in Organic Matrices Photoelectric
Element
Pb
C o n c n mJi
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(Arbitraq units)a
Photographic
*
64 i 6.5 4.0 1.5 58 9.3 2.4 1.4 63 i 8.2 3 . 0 i 1.4 1 9 + 2.3' 24 3.0 MgC 12.3 1300 -t 190 6.0 i 1.6 2.5 380 i 60 5.4 i 1.7 2.4 350 60 7.8 i 1.6 2.4 340 i 80 5.0 1.5 2.4 380 i 35 2.4 250 i 25 6.4 1.5 54 i 3.0d FeC 1850 8090 970 140 i 15 370 2900 + 400 53 + 5.2 185 1000 + 150 11 i 2.9 37 420 i 43 4.7 i 1.7 18.5 160 i 23 2.2 + 1.3 3.7 7.4 i 3.0 0 1.3 58 + 1.3' cuc 7.88 340 i 4 5 25 i 4.4 1.58 90 12 3.9 + 1 . 7 0.79 32 i 5.2 1 . 8 1.4 0.16 5.5 i 1.7 1.0 1.1 0.08 40 6.0 1.0 1.1 0.02 2.0 1.1 0.7 -t 1.2 13.9 i 0.8d ZnC 15.3 200 i 20 13 + 3.6 3.06 52 i 8.0 2.4 i 1.6 16 + 3.0 1.0 i 0.7 1.53 0.31 3.8 * 2.0 0 i 1.5 0.15 1.8 i 1.5 0.2 1.1 15.6 i- 0.6' a Ten samples per element, measured cross-section areas in plasma, 3- X 0.038-mm (Photoelectric), 0.5- X 0.038-mm (photographic), at optimum regions 0.35 and 0.50 mm above laser focal plane, respectively, and 0.6-mm lateral displacement from laser vertical axis. Each pair of values for photoelectric and photographic measurements obtained from same sample. Air-dried, 40-p sections of fresh-frozen human liver. Air-dried, 10-p sections of 570gelatin containing respective salts at 6 dilutions of stock s o h . (see under Materials). Ratio of photoelectric to photographic measurements f std error calculated from least squares fit of the measurements. Zero constraint employed after verifying that no significant distortion of the fit resulted.
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Figure 5. Variation of ratio of photoelectric to photographic sensitivities as a function of wavelength for elements in organic matrices (Figure 2), entrance coupling optics focused at 290 nm
*
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*
was derived from points representing the positions a t which maximum spectral response was measured for each element. Standard errors were calculated from the observed variations of the measured SIN at their optimal positions as shown in Figures 2 and 3. Since the collection optics have a 4 X magnification, the light pipe entrance slits correspond to a 38-pm X 3-mm rectangular cross-section of the plasma. The light pipe entrance slit positionings relative to the spectrometer vertical axis are such that the 0.8-mm vertical displacement sets the image of the microscope focal plane at the bottom of the slits. Therefore, the optimal region in terms of actual plasma dimension observed under these conditions, is a rectangular cross-section 38 pm wide, 3 mm high, displaced 0.6 mm laterally from the plasma center and 0.35 mm vertically above the microscope focal plane. It has been demonstrated that sufficient sample mixing does not occur in the plasma (12). Localized sample distributions could account for apparent differences in the observed maxima. 1278
ANALYTICAL CHEMISTRY, VOL. 47, NO. 8, JULY 1975
Photographic data were taken at the optimal lateral offset value of 0.6 mm. Since the microphotometer had a maximum scanning slit height of 2 mm, the optimal vertical scanning position had to be determined. A 2-mm slit positioned 2.5 mm above the spectral base gave the best element response values, expressed as background-corrected signals divided by the standard error of the noise. This corresponds to a 38-pm X 0.5-mm cross-sectional area of the plasma, displaced 0.6 mm laterally from the laser axis and 0.5 mm vertically from the focal plane of the laser, when optical magnification is taken into account. Response values were measured for different concentrations, and correlated with the corresponding photoelectric data from the same samples. These data are presented in Table 11. In an overall analytical comparison of different types of systems, the quantum efficiency and noise characteristics of the detectors are important factors, but they may not impose the principal limitation. For example, as in the present case, the source (plasma) could emit a large amount of optical noise that cannot be spectrally, temporally, or spatially resolved from the useful signal. If sensitivity is defined as the change of signal response per unit change of analyte concentration, Le., slope of the analytical curve, and the signal response is defined as the ratio of the background corrected signal to the standard error of the background, then the ratios of photoelectric to photographic measurements given in Table I1 are measurements of the relative sensitivities of the two methods. Photoelectric detection gave only a 15- to 58-fold improvement over photographic detection, even though the photomultiplier tubes had a detectivity approximately 1000 times greater. The quality of optical emission is therefore the limiting factor rather than detector response. In fact, if observed increases due to time-differentiation are taken into account (3, 4 ) , non-time-differentiated photoelectric and photographic results are comparable. The principal limiting factors are optical continuum and signal variations caused by differences in laser energy, sample vaporization, plasma formation, and plasma composition. There is considerable variance in the response ratios for each element. Plotting the ratios vs. wavelength gives a well defined curve with a maximum at 290 nm (Figure 5 ) , the wavelength a t which the collecting lens system was focused, suggesting chromatic aberration effect. A mirror coupling
system currently under test, shows promise of correcting this problem.
termination of the best conditions should be made for each specific application.
CONCLUSIONS
LITERATURE CITED
Advantage can be taken of the non-homogeneity of the optical emission from laser-induced plasmas to optimize analytical sensitivity. Greater than twofold increases in response, relative to the on-axis optical coupling conditions, were obtained from gelatin and liver sections sampled by a single-spike Q-spoiled laser delivering 35 mJ, by employing spatial differentiation. A single optimal position could be found common to all elements tested. Since time-differentiation was employed in all measurements, these increases are probably conservative relative to a non-time-differentiated system, although measurements with the latter were not made. The optical noise in the emission signal of the plasma constitutes primarily a continuum radiation. Both qualitative and quantitative variations in the optical emission of the plasma have been found to be the principal factors affecting signal response and detection limits. As a result, even though the photomultiplier tubes were about 1000 times more sensitive than the photographic film used, the response recorded photoelectrically was less than 60 times that of the photographic-densitometric method. This increase resulted mostly from use of the time-differentiated photoelectric recording. Optimal effective emission from specific regions in the plasma will vary with laser energy, atmospheric conditions, and probably the nature of the sample. Consequently, de-
(1) K. W. Marich, P. W. Carr, W. J. Treytl, and D. Glick. Anal. Chem., 42, 1775 (1970). (2) W. J. Treytl, K. W. Marich, J. B. Orenberg, P. W. Carr. D. C. Miller, and D. Glick, Anal. Chem., 43, 1452(1971). (3) W. J. Treytl, J. B. Orenberg, K. W. Marich, and D. Glick, Appl. Spectrosc., 25, 376 (1971). (4) W. J. Treytl, J. 6. Orenberg, K. W. Marich, A. J. Saffir, and D. Glick, Anal. Chem., 44, 1903 (1972). (5) A. J. Saffir, K. W. Marich, J. B. Orenberg, and W. J. Treytl. Appi. Spectrosc., 26, 469 (1972). (6) A. W. Ehlen, J. Appi. Phys., 37, 4962 (1966). (7) A. J. Alcock, C. Demichelis, K. Hamal, and B. Tozer. Phys. Rev. Lett., 20, 1095 (1968). (8) N. G. Basov, 0.N/ Krokhin, and G. V. Sklizkov, Appi. Opt., 6, 1814 (1967). (9) E. Archbald, D. W. Harper, and T. P. Hughes, Brit. J. Appi. Phys., 15, 1321 (1964). (10) D. D. Burgess, B. C. Fawcett, N. J. Peacock, Proc. Phys. Soc., 92, 805 (1967). (1 1) E. H. Piepmeir and H. V. Malmstadt, Anal. Chem., 41, 700 (1969). (12) E. H. Piepmeir and D. E. Osten, Appi. Spectrosc., 25, 642 (1971). (13) C. D. Allemand, Spectrochim. Acta, Part E, 27, 185 (1972). (14) R. H. Scott, and A. Strasheim, Spectrochim. Acta, Part E, 25, 311 (1970). (15) K. W. Marich. W. J. Treytl, J. G. Hawley, N. A. Peppers, R. E. Myers, and D. Glick, J. Phys. E,7, 830 (1974). (16) E. W. Sucov, J. L. Pack, A. V. Phelps, and A. G. Engelhardt, Phys. Fluids, I O , 2035 (1967).
RECEIVEDfor review February 4, 1974. Accepted April 14, 1975. Supported by Research Grant GM 16181 and Research Career Award 5K6AM18,513 (to D.G.) from the National Institutes of Health, U S . Public Health Service and the Stanford Research Institute.
Exploding Wires as an Intense Ultraviolet Continuum Excitation Source with Preliminary Application to Atomic Fluorescence Spectrometry D. W. Brinkman and R. D. Sacks’ Department of Chemistry, University of Michigan, Ann Arbor, MI 48 104
Preliminary studies are presented which show that thin metallic wires exploded by capacitive discharge can produce very intense continuum radiation extending far into the ultraviolet spectral region. A dense, high dielectric strength ambient atmosphere yields the most intense and reproducible continuum emission. The irradiance of this emission increases linearly with the energy initially stored on the capacitor bank. Although the wire material seems to have little effect, decreasing the wire diameter from 0.25 mm to 0.08 mm yields increasing irradiance. The radiation from a 54-mm long, 0.08-mm diameter Chromel-A wire exploded with 720 J in argon at 1 atm is both intense and reproducible, with a YO relative standard deviation from shot to shot of 5.4%. At 220 nm, the peak irradiance of this continuum is more than five orders of magnitude greater than that obtained from a 1600 W Xe arc lamp. This controlled, intense continuum radiation source then is used in conjunction with a premixed nitrous oxide-acetylene flame, using a watercooled, capillary tube burner, to demonstrate the potential application of this system for atomic fluorescence spec-
’ Author to whom correspondence should be directed.
trometry. Analytical curves are presented for Mn, Co, and Cd, along with detection limits for these elements in the low ppm range.
Since the first analytical applications of atomic fluorescence were proposed ( I ) and demonstrated ( 2 ) barely over a decade ago, the interest shown by the considerable amount of work in the area is an indication of the method’s potential. With the introduction of high intensity thermostated electrodeless discharge tubes and tunable dye lasers, very low detection limits have been obtained for most elements normally analyzed by atomic spectrometry ( 3 ) . However, various deficiencies have proved a hindrance to any general application and practical use of the method. Both of the above sources are relatively expensive and rather complicated to operate, while applicable to only one element a t a time. In order to do multi-element analyses simply, an intense continuum source is needed whose emission remains high far into the ultraviolet spectral region. Xenon arc lamps are useable down to about 250 nm, although they are not always intense enough for optimum application, and their intensity falls off rapidly below this region ( 4 ) . ANALYTICALCHEMISTRY, VOL. 47, NO. 8 , JULY 1975
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