Atomic absorption continuum studies at wavelengths below 320

Atomic Absorption Continuum Studies at Wavelengths below 320 Nanometers Peter N. Keliher’ and Charles C. Wohlers‘ Chemistry Department, Villanova University, Villanova, Pa. 19085

A direct comparison Is made, for elements having resonance lines below ca. 320 nm, between atomic absorptlon using a 200-W mercury-xenon lamp contlnuum source and atomic absorption using spectral line hollow cathode lamp sources. A high resolution echelle grating monochromator is used for both contlnuum and line sources. The results indicate that the contlnuum source allows the determination of almost all of the elements normally done with line sources with only a sllght sacrifice in sensitivity. Comparlson data for magnesium in hydraullc fluid are shown.

Atomic absorption using a continuum source (AAC) offers a number of advantages over atomic absorption using a line source (AAL), the most prominent of which is the savings of time and money realized through using only one continuum source instead of a separate spectral source, such as a hollow cathode lamp (HCL) or electrodeless discharge lamp (EDL), for each element to be determined. However, AAC has not been extensively investigated in the past, primarily because it is difficult if not impossible for the typical medium resolution monochromator used in analytical atomic spectrometry to completely isolate the spectral line from its background. This means that, even with narrowest slits, a large amount of unabsorbed light will always reach the detector and sensitivities (defined as the concentration necessary to give 1%absorption of the incident light) will be relatively poor. Nevertheless, several authors have performed AAC using medium resolution instrumentation (1-3), and have obtained reasonable results, although sensitivities were generally not reported. Some attempts have been made to overcome the limitations of medium resolution monochromators through the use of wavelength scanning (4-6), Fabry-Perot interferometry (7), and high resolution echelle spectrometry (8).With these last two methods, sensitivities similar to those obtained by AAL were realized. Using the echelle spectrometer, it was possible to use spectral bandwidths equal to or less than the width of the atomic spectral lines in flames with greater luminosity (or throughput) than a conventional monochromator with such small spectral bandwidths would have. The wavelength range covered was much wider than is possible with a single pair of Fabry-Perot etalons, and the echelle monochromator has the further advantage of being more mechanically and thermally stable than a Fabry-Perot. The echelle monochromator (SpectraMetrics, Inc., Andover, Mass.) which we used previously (8) did, however, have one serious drawback when compared to AAL. Light losses were too great below approximately 300 nm for adequate detection of the xenon continuum used; this was due primarily to the poor quality of the grating used in the early SpectraMetrics prototype system which we were using at that time. In early 1973, however, the echelle specPresent address, Jarrell-Ash Division, 590 Lincoln Street, Waltham, Mass. 02154. 140

ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976

trometer system was completely redesigned by SpectraMetrics and was replaced by a new model which uses gratings from a different manufacturer and has a more conventional Czerny-Turner mounting which eliminates three mirrors. Although this new system is still referred to as a “SpectraSpan”, it must be emphasized that the design is quite different from the early prototype system which we used for our previous studies (8-11). With the new monochromator, the signal intensity observed from a xenon continuum using narrow slits was adequate for AAC down to a wavelength of approximately 225 nm. Accordingly, AAC of those elements which were not previously reported because of luminosity problems was attempted and is reported in this publication.

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EXPERIMENTAL Instrumentation. The system used for AAC is shown in Figure 1.The lower portion of the photograph shows a general overview of the system while the upper portion shows a close-up of the entrance optics used for AAC. As we obtained the echelle monochromator from SpectraMetrics without the usual exit slit cassette assembly and photomultiplier tube housing, modifications were necessary. A slit holder, similar to the one used for the entrance slit, was positioned a t the point where the exit slit cassette would normally be placed. A 2-mW Helium-Neon Metrologic Model ML-680 Laser (Metrologic Inc., Bellmawr, N.J.) was used for optical alignment. The slit holder was followed by a Hamamatsu R446 P M T with housing side mounted. This assembly was then enclosed in a laboratory constructed light-tight box attached to the monochromator ( B in Figure 1). The entrance optics, as seen in Figure 1, are identical to those commonly employed in AAL, with a source focused through a lens onto a flame which is then focused through a second lens onto the entrance slit. The distance from the source to the entrance slit is approximately 70 cm. The burner system (C in Figure 1) used was taken from a Varian Techtron AA-4, with Techtron AB-41 or AB-51 10-cm absorption burners used for air-acetylene and an AB-50 6-cm absorption burner used for nitrous oxide-acetylene. A Perkin-Elmer variable nebulizer was used, and the gas flows were controlled by a Techtron G.C.U. 2 Gas Control Unit, of the type used on the AA-4. The mechanical chopper (D in Figure 1) used was constructed for us by the Spectrogram Corporation, North Haven, Conn., and was operated a t 400 Hz, in phase with the photometric amplifier. The continuum source used for AAC measurements was a Hanovia 200-W mercury-xenon lamp ( E in Figure 1) powered by a Hanovia Model 27799 Lamp Power Supply (Engelhard Hanovia, Inc., Newark, N.J.), which was normally run a t 7 A. The lamp was enclosed in a water cooling jacket and was mounted on a micrometer positioning device for easy adjustment. The micrometer device was taken from the Techtron AA-4 and is the device that is normally used for burner adjustments. I t should be noted that there exists a danger of explosion from mercury-xenon lamps and that unenclosed operation (as is shown for purposes of illustration in Figure 1) would not generally be recommended. A simple shield can easily be placed around the continuum source. The photometric amplifier used was the Spectrogram Model LPA-1; the amplifier also contains the P M T power supply. A Spectrogram Model IDV-1 Integrating Digital Voltmeter was used with the photometric amplifier for integration and more convenient readout. For AAL measurements, the Hanovia lamp and mechanical chopper were replaced by the appropriate HCL powered by a Spectrogram Model LPS-1 Hollow Cathode Lamp Power Supply. The HCL’s were standard commercially available ones manufac-

tured hy Perkin-Elmer, Atomic Spectral Lamps, Pty., or Westinghouse, and were modulated at 400 Hz hy the Spectrogram Photometric Amplifier. A residual dc current, usually about 0.2 mA, was superimposed on the 400-Hz square wave to improve stahility (12). A Varian Teehtron Model AA-4 was used for some comparison measurements. All components were standard and no modifications were made, except that the burner system from an AA-5 was used. The slits used for AAC were 10 wm wide for both entrance and exit slits and were provided by Baird-Atomic, Bedford, Mass. These slits were mounted on SpectraMetrics slit holders and were masked to 300-um height. For comparison AAL, 500 X 1000 urn SpectraMetrics slits were used. These large slits ensure that the total line from the HCL passes through the monochromator ( I 1j. Procedure. Essentially the same procedure was used BS has been reported previously (8). Working solutions were made from stock solutions prepared from analytical grade reagents. The ahsorption maxima are found exactly as in emission work, i.e., the monochromator is "peaked" on the proper wavelength using a high concentration of the appropriate element. The monochromator does not drift from the wavelength once it is set. Measurements were made by integrating blank (distilled water) and sample solutions for 10 see each, and repeating this three or four times. All blank and all sample readings were then averaged, divided to obtain B percent transmittance, and then logarithms taken to obtain the ahsorbanee.

RESULTS AND DISCUSSION Line width data for several of the elements investigated are given in Table I. As has been noted by Kirkhright and Troccoli (13),absorption line profiles become considerably broadened as the concentration is increased so the absorption data presented in Table I should be regarded only as minimum values observed at low concentrations. In contrast to the situation for lines above ca. 320 nm, very few data are available ?concerning line widths at these low wavelengths. This is certainly due to the difficulty of using a Fahry-Perot interferometer at wavelengths below 320 nm. The single experimental value for an HCL spectral line width, that of magnesium 285.2 nm (14),is probably large as no correction was made for instrumental factors or for self-absorption broadening. The actual widths are probably closer to the theoretical values, as there should be little hyperfine splitting (hfs) in any of these lines, with the exceptions of cadmium 228.8 nm and manganese 279.5 nm. T h e manganese lines at 403.1, 403.3, and 403.4 nm exhibit extensive hfs (19), so it is quite possible that this line does also. Comparison of the theoretical and measured line widths with the spectral slit widths used indicates that sensitive results for AAC comparable to AAL should be readily attainable. Calibration curves for iron 302.1 nm, magnesium 285.2 nm, manganese 279.5 nm, and iron 248.3 nm, are shown in Figures 2-5. These curves indicate that, in spite of expectations, the AAC results were not as good as those for AAL. However, they are linear over a significant range and give

Figure 1. Photograph of instrumental setup Lower ponlon shows O~erallsetup. Note coarse and line order and wavelength dials for direct WaYelenQthmeasurement. Upper portion 01 photograph Show6 (A) echelle entrance Slit and heigM mask arrangement, (B)light tight PMT houslng, (C)Varian Techtron pre-mixed burner system, (D) Spectrogram mechanical chopper. and (E) Water Cooled mercury-xenon lamp with Techvan micrometer device for easy adjustment. Distance from lamp to entrance Slit is approximately 70 cm

reasonable sensitivity, and thus should he analytically useful, as will he shown below. This lack of sensitivity of the AAC results, beyond what might be expected, is probably due to some loss of resolution due to a widening of the slit function beyond the value obtained for the spectral slit width. The slit function width, or spectral bandpass, is comprised of three factors, one of which is the spectral slit width derived from the geometrical width of the entrance and exit slits. The other factors are the diffraction width of the slit image and broadening factors due to optical effects in the monochromator such as aberrations from the mirrors, nonparallelism of the slits with each other and with the grating, etc. T h e diffraction width, or critical slit width, is found by AXd =

X f/W COS 6

where f is the monochromator focal length, W is the grating width, and 6 is the blaze angle of the grating (20). For the monochromator used here, A b varies from 3.6 pm at 200 nm to 5.4 pm at 300 nm. This is a significant portion of the IO-pm slit width used, so the diffraction width can be expected to contribute to the total spectral bandwidth. One might estimate the magnitude of the apparent widening of the spectral bandpass by comparing Figures 2-5

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____-__

~__.

Table I. Spectral Line Width Data Element and line,

nm

Iron, 302.1 Magnesium 285.2

Hyperfine splitting pattern, a

a 0.0000 (15)

0.0019 0.0038

Manganese 279.5 Iron 248.3 Cadmium 228.1

f

n

g

Relative tntensity

79 10 11

HCL

line

width,

A

spectra1 bandwidth 10-prn slits, A

Absorption line width in flame

0.00656 0.00936 0.017 ( I 4 ) d

0.0088 0.0083

0.0060b 0.0053b 0.0035

0.0082 0.0072

0.014-0.018c 0.012-0.015e

0.0066

0.008-0.011c

0.015-0.02OC 0.021-0.026C

0.0274 ( I 7)e

0.0110 ( 1 8 ) P . h i

a92% '6Fe,6% "Fe, 2% I'Fe, 0.3% "Fe. b Assuming pure Doppler hroadening and Doppler temperature of 500 K (19). Theoretical range (16). d By Zeeman scanning. eBy curve of growth measurement. f100% 15Mn.81.2% lo6Cd,0.9% 10gCd, 12% "'Cd, 24% " T d , 29% "'Cd, 7.6% "'Cd. hoxy-acetylene flame.

ANALYTiCAL CHEMISTRY,

VOL. 48,

NO. 1, JANUARY 1976 * 141

LO.

I

O1.

om,

1.W

1O.W

C9nmlR11.l W"

Figure 2. Calibration curves for iron 302.1nm

aoi

loo

IO

ID00

C a ~ l r r t , . " .ppm

( A ) Hollow cathode lamp, (6) Continuum. Air-acetylene flame

Figure 4. Calibration curves for manganese 279.5 nm ( A ) Hollow cathode lamp, ( 6 ) Continuum. Air-acetylene flame

0014 01

I

10

100 loo

10

cencmtr.llD", ppm

CMmt,,lrn,

Figure 3. calibration curves for magnesium 285.2nm

( A ) Hollow cathode lamp, (B)Continuum. Air-acetylene flame

Table 11. Comparison of Sensitivities Using Continuum and Line Sources Sensitivity

AAC, ppm Sensitivity, AAL, ppma

Ref. 2 2

Aluminum 309.2 Aluminum 308.2C Antimony 231.5C Boron 249.7 Beryllium 234.9 Bismuth 306.8 Cadmium 228.8 Cobalt 240.7 Gallium 287.4 Indium 303.9 Iron 248.3 Iron 302.1C Lead 283.3C Magnesium 285.2 Manganese 279.5 Molybdenum 313.3 Nickel 232.0 Thallium 276.8 Tin 286.3C Tin 235.5C Vanadium 318.5d

1.0 1.6 1.05 40 0.025 1.5 0.025 0.15 2.5 0.7 0.12 0.44 0.5 0.007 0.055 0.5 0.15 0.5 4.3 3.8 1.7

this work

Flameb

This work

2.7 4.5 300

0.15 0.5 1.1 0.016 0.09

2.7

4.1 7.7 4.5 49 0.07 8 0.17 0.5 12.5 2.1 0.3 1.5 1.5 0.02 0.14 1.6 0.7 1.o 17 27 8.4

n-a n-a a-a n-a

n-a a-a a-a a-a a-a a-a a-a a-a a-a a-a a-a

n-a a-a a-a a-a a-a n-a

Sensitivity is defined as the concentration necessary t o give 1% absorption (0.0044 absorbance) of the incident light. b a-a, air-acetylene flame; n-a, nitrous oxide-acetylene flame. C Not most sensitive line. d Multiplet, only one line of which is used in AAC.

142

10,oOo

Figure 5. Calibration curves for iron 248.3 nm

( A ) Hollow cathode lamp, (6) Continuum, Air-acetylene flame

Element and l i n e , nm

1.ooO

pwn

ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976

with the corresponding curves for copper a t 324.7 nm given previously (Ref. 8, Figure 5). In this case, one observed slightly better results than those given in Figures 2-5 for a situation where the spectral bandpass and spectral line widths are approximately the same (8, 21). In Table I, it can be seen that the spectral slit width is generally half the spectral line width in the flame; thus one can estimate that the effective spectral bandpass is approximately 2 to 2.5 times the spectral slit width, indicating that the slit function broadening from both diffraction and optical effects is present. These broadening effects could be reduced through the use of a slightly higher resolution monochromator or by redesigning the echelle monochromator specifically for AAC. I t should be noted that the SpectraMetrics system is normally used with a dc argon plasma and was designed primarily as an emission instrument. Table I1 shows sensitivities obtained on this instrument for AAC and AAL compared to commercially published values (22). The AAC sensitivities are generally about three to five times higher (i.e., poorer) than those commercially published, which is similar to those given previously (8). When comparing AAC with AAL performed on the same instrument and with identical solutions, flame stoichiometry, etc., AAC gives results about 1.5 times poorer than those for AAL. That indicates that, with the instrumentation used here, AAC and AAL are directly comparable for nearly all wavelengths used in flame spectrometry. The apparently anomalous results for AAL for boron can perhaps be explained by the possibility of wide lines resulting from the use of a Perkin-Elmer Intensitron boron HCL operated a t high current, as is recommended by the manufacturer.

Table 111. Determination of Magnesium in MIL-H-560B Hydraulic Fluid Sample No.

AAL results, p p m

314 31 7 356 363 378

0.29 0.29 0.29 0.25 0.21

AAC r e s u l t s , p p m

0.31 0.27

0.30 0.24 0.21

With the results reported here, atomic absorption using a continuum source and a high resolution echelle monochromator has been extended to all elements having sensitive absorption lines above 225 nm. Thus, the only elements which cannot be determined a t their most sensitive lines are: bismuth, tin, antimony, arsenic, selenium, tellurium, lead, and zinc. Of these, antimony, bismuth, tin, and lead can be determined a t other, less sensitive, lines. Using the mercury-xenon continuum, it is not possible to detect mercury because of the low source radiation around the mercury 253.7-nm line. This is probably caused by absorption of mercury vapor in the continuum source; use of a pure xenon continuum would eliminate this problem. AAC could be extended to lower wavelengths by any one of a number of means: use of a higher luminosity monochromator, a higher power xenon lamp such as the Eimac recommended by Winefordner for continuum atomic fluorescence measurements (23), or perhaps a high power hydrogen or deuterium continuum source. We have attempted to construct a continuum EDL source that would be reasonably intense and stable between 200 and 300 nm but our results, to date, have not been very successful. Nevertheless, this might be a possibility. Detection limits were not specifically determined but those for AAC would probably compare less favorably with respect to AAL detection limits. It is felt, however, that sensitivities give a better comparison of the fundamental nature of AAC vs. AAL, as they are not so heavily dependent upon source noise. Detection limits for AAC could be improved by the use of a higher luminosity and/or resolution monochromator, or a magnetic lamp stabilizer, such as is used in spectrofluorimetry. I t should be noted that our mercury-xenon lamp is about four years old and is gradually becoming less and less stable. We would expect a new Hanovia mercury-xenon lamp to have greater stability and this, in turn, would lead to lower detection limits. In other words, detection limits in AAC represent a "state of the art" whereas sensitivities represent a more fundamental AAC concept. Finally, the application of AAC to some real samples was attempted through the determination of magnesium in hydraulic fluid. The samples used were obtained from U.S. Navy jets as part of a program to predict failures in the hydraulic system (24). The concentrations were determined for both AAC and AAL by constructing a best-fit straight line calibration curve from three standards. AAL was performed in the usual way on the Varian Techtron AA-4 and AAC was performed on the AAC-echelle system, the results obtained are compared in Table 111. It is apparent, and not surprising, that the two methods give quite similar results.

CONCLUSIONS The results indicate that it is possible on the echelle monochromator used here to determine by AAC nearly all of the elements normally done by AAL with only slight sacrifices in sensitivity. If a completely optimized (for atomic absorption) instrument were used, there is no reason why AAC should not give equal or better (i.e., lower) sensitivities and detection limits than AAL with considerably more convenience and less expense. We consider that further progress in this area will be in engineering development, with possible extensions to multielement atomic absorption spectrometry.

ACKNOWLEDGMENT We thank the Spectrogram Corporation, North Haven, Conn., for the loan of some of the equipment used in this study. We thank James Shea for his assistance with the construction of the photomultiplier housing assembly.

LITERATURE CITED N. P. lvanova and N. A. Korieva, Zh. Anal. Khim., 19, 1266 (1964). V. A. Fassel, V. G. Mossotti, W. E. L. Grossman, and R. N. Kniseley. Spectrochim. Acta, 22, 347 (1966). W. W. McGee and J. D. Winefordner, Anal. Chim. Acta, 37, 429 (1967). W. Snelleman, Spectrochim. Acta, Part 8,23, 403 (1968). R. C. Elser and J. D. Winefordner, Anal. Chem., 44, 698 (1972). G. J. Nitis. V. Svoboda, and J. D. Winefordner, Smctrochim. Acta, Pati 8,27, 345 (1972). C. Veillon and P. Merchant, Jr., Appl. Spectrosc., 27, 361 (1973). P. N. Keliher and C. C. Wohlers, Anal. Chem., 46, 682 (1974). M. S.Cresser, P. N. Keliher, and C. C. Wohlers, Anal. Chem., 45, 111 11973). M. S.'Cresser. P. N. Keliher, and C. C. Wohiers, Lab. Pract., 26, 335 (1975). P. N. Keliher and C. C. Wohlers, Appl. Spectrosc., 29, 198 (1975). G. M. Hieftje, E. B. Holder, A. S. Maddux, Jr., and Robert Lim, Anal. Chem.. 45, 238 (1973). G. F. Kirkbright and 0. E. Troccoli, Spectrochim. Acta. Part 8,28, 33 (1973). H. F. vanHeek, Spectrochim. Acta, Part 8, 25, 107 (1970). F. M. Kelly, Can. J. Phys.,35, 1220 (1957). M. L. Parsons, W. J. McCarthy, and J. D. Winefordner. Appl. Spectrosc., 20, 223 (1966). T. Hollander, PhD Thesis, University of Utrecht, The Netheriands, via. W. P. Townsend, D. S. Smyiy, P. J. T. Zeegers, V. Svoboda, and J. D. Winefordner, Spectrochim. Acta, Part 8,26, 595 (1971). W. W. McGee and J. D. Winefordner, J. Ouant. Spectrosc. Radiat. Transfer, 7, 261 (1967). H. C. Wagenaar and L. deGalan, Spectrochim. Acta, Part B, 25, 157 (1973). R . F. Jarrell, in "Encyclopedia of Spectroscopy", G. L. Clark, Ed., Reinhold. New York, 1960, p 243. H. C. Wagenaar, C. J. Pickford, and L. deGalan, Spectrochim. Acta. Part8, 29, 211 (1974). "The Cookbook", Analytical Methods for Atomic Absorption Sepctrophotometry, Perkin-Elmer Corporation, Norwalk, Conn., 1973. J. D. Winefordner, Chem Techno/., 5, 123 (1975). P. N. Keliher, "Determination of Wear Metals in Used Hydraulic (MIL-H560B) Fluid by Atomic Absorption Spectrometry", Paper presented at the 9th Middle Atlantic Regional Meeting (MARM), American Chemical Society, Wilkes-Barre, Pa., April 26, 1974.

RECEIVEDfor review June 5, 1975. Accepted October 17, 1975. Partial support of the U S . Navy, Naval Air Engineering Center, Ground Support Equipment Department, under Contract 72-C-1199, is gratefully acknowledged. This paper was presented at the 26th Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 4, 1975. Paper Number 162.

ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976

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