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Anal. Chern. 1989, 6 1 , 1694-1697
Fourier Transform Atomic Absorption Flame Spectrometry with Continuum Source Excitation Mark R. Glick, Bradley T. Jones, Benjamin W. Smith, and James D. Winefordner* Department of Chemistry, University of Florida, Gainesville, Florida 3261 1
The design and performance of a Fourler transform atomic absorption flame spectrometer (FT-AAS) is presented. A 800-W xenon arc continuum source and a Michelson interferometer are used. A signal to noise disadvantage arising from the multiplex feature of FT-AAS Is demonstrated by varying the photon flux at the detector without changing the exclting radiation. A gratlng is used for dispersion of the radiation before the interferometer to reduce the spectral window at the photomultiplier tube. Detection iimlts for several elements are generally an order of magnitude poorer than those obtained by contkwrum atomic absorption methodsuskrg echelle-gratlng spectrometers. Line profiles and absorption spectra, within the r e g h of the spectral window selected by the grating, can be obtained with this method. Standard curves for sodium were constructed to extend the linear calibration range, by using absorbances measured at the absorptlon maximum and 0.022 nm off-line.
advantage is not realized. For dense spectra, such as molecular fluorescence and absorption spectra, a multiplex disadvantage can even appear (22,23). T o continue our investigation of Fourier transform spectrometry in the visible and ultraviolet regions, we have used a commercially available Michelson interferometer for atomic absorption measurements in a flame. Absorption spectra can be obtained with the Fourier transform atomic absorption spectrometer (FT/AAS), and multielement determinations can be performed for elements within the selected spectral window. Detection limits for several elements with analytical lines in the visible and ultraviolet regions are given. The advantages of continuum source AAS that have been realized in other systems-automatic background correction, multielement analysis, single source, extended calibration curves-can also be found in this system. The detection limits are a t least 1order of magnitude poorer than other continuum source, atomic absorption spectrometers.
EXPERIMENTAL SECTION INTRODUCTION Atomic absorption spectrometers (AAS), although commercially available only with line sources, have been constructed with continuum sources since their first analytical use. Because the sensitivity of the absorption method is dependent on the effective spectral bandwidth of the spectrometer (1-3),the use of continuum sources has been limited. When line sources are used, the effective bandwidth is determined by the width of the atomic emission line in the source. When continuum sources are used, the monochromator determines the effective bandwidth; only when the effective bandwidth approaches that of the analyk absorption line can the sensitivity of continuum source AAS equal that of line source AAS. For this reason, much work on continuum source AAS dwells on the development of high-resolution spectrometers. T o achieve the resolution needed for AAS, several different approaches have been taken. High-resolution Fabry-Perot interferometers have been demonstrated with continuum source AAS ( 4 , 5 ) and, the more practical, echelle-grating spectrometers (6-8). Modulation to achieve resolution of the analytical line has been illustrated, including spectral line modulation (9) and sample modulation (IO, 11). Resonance monochromators have been used to decrease the effective spectral bandwidth (12-14). Of all these methods, the use of echelle-grating monochromators has been the most successful and is the subject of several reviews (15-17). Other interferometers have been suggested as possible components in an atomic absorption spectrometer (18,19). Fourier transform spectrometry (FTS)has the advantages of high spectral resolution capability, wavelength accuracy, and high throughput. The disadvantages expected of FTS in the visible and ultraviolet regions have also been well documented (18,20,21).In the photon shot noise limited cases, a multiplex
* Author
to whom correspondence should be sent. 0003-2700/89/0361-1694$01.50/0
A diagram of the Fourier transform atomic absorption spectrometer is shown in Figure 1. Radiation from the continuum source was passed through the flame without any spectral dispersion. A lens was used to focus the radiation from the lamp on an iris diaphragm, where it was apertured and collimated by a second lens. A second iris diaphragm allowed only a portion of the collimated radiation from the less turbulent region of the lamp image to pass through the flame (24). After the flame, a grating was used to disperse the radiation before the interferometer. A quartz lens focused a portion of the dispersed radiation onto the entrance aperture of the interferometer. The Michelson interferometer (DA3.02, Bomem, Vanier, Quebec) in this system was commercially available and used without modification. A photomultiplier tube (R647, Hamamatsu, Bridgewater, NJ) was used for detection of the interferogram. The source of radiation was an unfiltered, 300-W xenon arc lamp (Cermax, ILC Technologies, Sunnyvale, CA). Predispersion of the radiation was accomplished by a 2400 groove/mm, plane-ruled grating, blazed at 300 nm (SLM-Aminco,Urbana, IL).The grating could be rotated to select the radiation window of interest, which was focused onto the entrance aperture of the interferometer. The spectral half-width of the source radiation entering the interferometer was approximately 5 nm. The fuel-lean air/acetylene flame was produced by a 10-cm slot burner (Perkin-Elmer, Norwalk, CT). Collimated white light, apertured to 1 cm diameter, passed through the flame to the grating. A second aperture was used to reduce the collection of flame emission. Even at concentrations as high as 10 mg/mL, no analyte emission could be detected with the optical configuration shown in Figure 1. Each absorption measurement was made by recording a reference spectrum with l.OO-cm-' resolution, unless indicated otherwise. One hundred interferograms were coadded for both reference and absorption spectra. The scan rate of the interferometer was 0.15 cm/s, for an average time of spectrum acquisition of 10 min.
RESULTS AND DISCUSSION A grating was used for predispersion of the radiation entering the interferometer to limit the window of radiation striking the detector (25). In the photon shot noise limited 0 1989 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 61, NO. 15, AUGUST 1, 1989 CASE
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MICHEL S O N INTERFEROMETER
CASE
I 1
BURNER
Schematic diagram of the Fourier transform atomic absorption spectrometer using a continuum source.
Figure 1.
CASE
I 1
LL +
16938. 0
I1
C
-
17008. 0
W a v e n u m b e r f c rn-l)
Comparison of signal to noise of the sodium doublet for case I and case 11. Instrumental bandwidth was 1.00 cm-'.
Flgure 3.
II II
Q1
m
I
I amp
n lI / \ I
100,
CASE
I
/ \ I O
Table I element
wavelength, nm
Ag cu Mn Na
328.068 324.754 279.482 588.995
detection limit, rg/mL FT/continuum continuum" 0.2 0.1 0.2 0.02
0.007 0.01 0.01 0.003
" Reference 17.
L 16'710. E
Wavenumber
(cm-1)
1 ' 8 4 8 ' 8
Relative intensities of HeNe laser radiation and excitation radiation from the xenon arc lamp. In case I1 the laser radiation reaching the photomultiplier tube was approximately 200 times that in case I. Figure 2.
region, predispersion resulted in an increase in the signal to noise ratio (S/N), because the photon flux a t the detector is reduced. This limits the spectral region that can be used for multielement analysis to 5 nm. The entire absorption spectrum in that window can be obtained by the spectrometer, but unlike echelle-grating systems, multiple lines a t discontinuous spectral regions cannot be simultaneously measured. Gratings with poorer dispersion could be used to obtain a larger spectral window, which would permit the acquisition of absorption spectra over an even wider range. The larger spectral window, however, would also increase the radiation a t the detector and degrade S/N. To clearly demonstrate the multiplex disadvantage, the effect of photon flux a t the detector on the S / N of sodium absorption lines was qualitatively investigated. The Bomem interferometer uses an internal HeNe laser for alignment, which unavoidably strikes the photomultiplier tube. To attain a spectrum with high S/N, the HeNe laser radiation was spatially blocked a t the exit port of the interferometer. In Figure 2, case I shows the relative intensity of the HeNe laser radiation that could n o t be blocked, in comparison to the excitation radiation from the continuum source. For case 11,
the internal laser radiation was not blocked, and an external HeNe laser was also used to increase the photon flux striking the detector. In case I1 of Figure 2, the laser radiation has a peak intensity almost 200 times that of case I. The excitation radiation was not changed. The effect on S/N of the absorption spectrum of the sodium doublet is shown in Figure 3. The poorest signal to noise is obtained in case 11, when the HeNe laser radiation is much more intense than the excitation radiation. The noise from the HeNe is distributed to the analytical lines. Unfortunately, the radiation from the internal laser cannot be blocked entirely, and inevitably it reaches the photomultiplier tube. In the shot noise limited region, this contributes to a multiplex disadvantage. A solar-blind photomultiplier tube would avoid this particular problem. Detection limits for several elements with lines in the ultraviolet and visible region are shown in Table I. Generally, the detection limits are at least an order of magnitude poorer than those that have been obtained by an echelle-grating spectrometer and continuum source. The same trend of poorer detection limits as the analytical line moves to shorter wavelengths that is observed in other continuum source AAS methods was observed here. In an effort to determine the cause of the poorer detection limits, the sensitivity and noise of the system were investigated. The sensitivity is related to the effective spectral resolution and in this system is determined by the mirror
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 15, AUGUST 1, 1989 0.3
I
0.81
1
i*
I
m n L 0 v1
i 001
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I 10
1
1
1000
1 0 0
R e s o l u t i o n
(pm)
Figure 4. Dependence of observed absorbance on instrumental bandwidth. Copper absorbance was measured at 327.396 nm.
30450.P
Wavenumber
( c m - l )
30800. 0
Flgure 6. Absorption spectrum of a three-component aqueous soknion, 100 pg/mL: 1.OO cm-' resolution and 10-min acquisition time. 0.5
5 8 9 . 0 1 7 nm 588.995 nm
al 0
fi d
e L 0 67
e I
1
I
1
1Io
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1000
Concentration (ppm)
4
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Figure 5. Calibration curves for sodium, using absorbance measurements at the peak, 588.995 nm, and 0.022 nm off the peak, 589.017 nm: 1.00 cm-' resolution and 10-min acquisition time.
movement of the interferometer. Figure 4 shows the effect of instrumental resolution on the measured absorbance at the 327.396-nm line of copper. As the spectral resolution increases, the observed peak absorbance increases, until the line is completely resolved. Below an instrumental resolution of 3 pm, the absorbance does not change. All absorbance measurements made for the determination of the detection limits in Table I were made with an instrumental resolution of 1.00 cm-'; at the copper line this corresponded to 11 pm. Although this resolution did not result in maximum absorption, it was chosen as a compromise. Much longer spectrum acquisition times would have resulted if better resolution was selected. An instrumental resolution of 3 pm yielded a higher absorbance for copper, but the time of acquisition became prohibitive. One advantage that is realized to some extent is the multichannel capability of FT/ AAS. Atomic absorption spectra can be obtained over the profile of the line, as long as the line falls within the selected window of radiation entering the interferometer. This affords the possibility of wing the profile of the absorption line for diagnostic purposes, background correction, and for extension of the calibration curve. The capability of extending the linear range of the calibration curve has been demonstrated with continuum source AAS (26,27). The automatic acquisition of the absorption spectrum that is obtained in FT-AAS allows this type of extension without foresight and preparation. Figure 5 shows the extension of the linear portion of a calibration curve for sodium. Absorption measurements were taken from the spectrum at the
.
0 1
35000. 0
Wavenumber
.
,..
(cm-l)
3 5 8 2 4 . 0
Figure 7. Absorption spectrum of a two-component aqueous solution. 100 pg/mL: 1.00 cm-' resolution and 10-min acquisition time.
peak of the line profile and on the edge of the line profile. The limited multielement capability is demonstrated by the absorption spectra of Figure 6 and Figure 7. Four absorption peaks corresponding to 100 pg/mL of three elements, Cu, In, and Ag, are shown in Figure 6. A t shorter wavelengths, four absorption peaks corresponding to 100 pg/mL of two elements, Mn and Mg, are shown in Figure 7. Clearly the analytical use as a simultaneous method is limited, because only those elements that happen to have absorption lines within the 5-nm spectral window will appear in the spectrum. The use of a grating with poorer dispersion would allow the simultaneous determination of more elements, but S / N would decrease due to the increased total light flux reaching the photomultiplier tube.
CONCLUSION
IT-AASwith a continuum source is limited as an analytical technique since the spectral range, sensitivity, detection power, and acquisition time are interdependent and limited. Nevertheless, the technique may have limited use in specialized analytical applications, especially where ease of background correction and line identification are important and where several selected elements must be measured simultaneously. FT-AAS with a continuum source has had and will continue to have significant use as a diagnostical technique, particularly
Anal. Chem. 1989, 6 1 , 1697-1701
for line profile and broadening studies.
LITERATURE CITED (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) 112) (13) (14) (15)
Waish, A. Spectrochlm. Acta 1955, 7 , 108-117. Winefordner, J. D. Appl. Spectrosc. 1963, 2 7 , 109-111. Fassel, V. A.; Mossotti, V. G. Anal. Chem. 1963, 35, 252-253. Velllon, C.; Merchant, P., Jr. Appl. Spectrosc. 1973, 2 7 , 381-365. Nkis, G. J.; Svoboda, V.; Winefordner, J. D. Spectrochim. Acta 1972, 276, 345-363. Kellher, P. N.; Wohlers, C. C. Anal. Chem. 1974, 46, 682-687. Keliher, P. N.; Wohlers, C. C. Anal. Chem. 1976, 4 8 , 140-143. Zander, A. T.; O'kver, T. C.; Kellher, P. N. Anal. Chem. 1976, 48, 1166-1 175. Cochran, R. L.; Hieftje, G. M. Anal. Chem. 1978, 50, 791-600. Marinkovic, M.; Vlckers, T. J. Anal. Chem. 1970, 42, 1613-1618. Mossotti, V. G.; Abercrombie, F. N.; Eakin, J. A. Appl. Spectrosc. 1971, 2 5 , 331-341. Paiermo. E. F.: Crouch. S. R. Anal. Chem. 1973. 45. 1594-1602. Blackburn, M. B.; Winefordner, J. D. Can. J . Spectrosc. 1982, 2 7 , 137- 140. Larkins. P. L.; Radziuk, B. Van Loon, J. C. Spectrochim. Acta 1983, 388,473-460. O'Haver, T. C.; Messman, J. D. Rag. Anal. Spectrosc. 1986, 9 , 483-503.
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(16) Harnly, J. M. Anal. Chem. 1986, 58. 933A-943A. (17) OHaver, T. C. Analyst 1984, 709, 211-217. (18) Winefordner, J. D.; Avni. R.; Chester, T. L.; Fkgerald, J. J.; Hart, L. P.; Johnson, D. J.; Plankey, F. W. Spectrocfdm. Acta 1976. 318, 1-19. (19) Thorne, A. J . Anal. At. Spectrom. 1987, 2 , 227-232. (20) Hirschfeld, T. Appl. Spectrosc. 1976, 30, 68-89. (21) Merh. Lawrence TransformaMonsin OpmcS; Wiley 8, Sons: New York, 1965. (22) Jones, B. T.; Olick, M. R.; Smith, 8. W.; Winefordner, J. D. Spectroc h h . Acta, in press. (23) Giick, M. R.; Jones, B. T.; Smlth. B. W.; Winefordner, J. D. Appl. Spsctrosc 1986, 43, 342-344. (24) Cochran, R. L.; Hleftje. G. M. Anal. Chem. 1977, 49, 2040-2043. (25) Stubley. E. A.; Horlick, G. Appl. Spectrosc. 1985, 39, 811-817. (26) O'Haver, T. C.; Harnly, J. M.; Marshall, J.; Carroll, J.; Littlejohn. D.; Ottaway, J. M. Analyst 1985, 170, 451-458. (27) Harnly, J. M.; OHaver, T. C. Anal. Chem. 1981, 53, 1291-1298.
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RECEIVED for review February 6,1989. Accepted May 12,1989. This work was supported by NIH-5R01-GM38434-02, Mark R* G1ick thanks BP America for support from a research fellowship.
Mixture Analysis and Quantitative Determination of Nitrogen-Containing Organic Molecules by Surface-Enhanced Raman Spectrometry J. J. Laserna,l A. D. Campiglia, and J. D. Winefordner* Department of Chemistry, University of Florida, Gainesville, Florida 3261 1
Surface-enhanced Raman spectrometry (SERS) on a slivercoated filter paper substrate of nitrogen-contalnlng organic molecules Is reported. A correction procedure for standardization of measurements is proposed and evaluated to solve the dmkutt proMem of quantitatbn of adsorbate in SERS. The relative standard devlation obtained through this procedure is around 15%. Linearity ( r = 0.99Q) was achieved up to 50 pg/mL aminoacridlne. A limlted dynamic range is observed, however, due to the ilmited number of SERS active sites In the substrate. Spectral fingerprinting of three-component mixtures by concentratlon-dependent selective molecular adsorption on the substrate is also reported.
Investigation of surface phenomena is a key factor for the understanding of subjects of technological interest, such as adhesion, catalysis, corrosion, semiconductor production and characterization, and ultrahigh-vacuum environments. The importance of many aspects of these interfacial phenomena often relies on chemical composition in such a way that techniques capable of providing analytical information on the surface are of concern. These techniques include Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), secondary-ion mass spectroscopy (SIMS), and ion-scattering spectroscopy (ISS). By the nature of the effect giving rise to the technique, surface-enhanced Raman spectroscopy (SERS) is a surface analytical technique. In comparison with other surface techniques, SERS can provide Present address: Department of Analytical Chemistry, Faculty of Sciences, University of Malaga, 29071 Malaga, Spain.
information on the chemical form and molecular structure of species situated on a variety of interfaces, including solidliquid (electrochemicaland colloid SEW) (I,2), solid-vacuum (island film SERS) (31, solidsas (silica and filter paper SERS) ( 4 , 5 ) and solid-solid (thin-film SERS) (6) interfaces. SERS allows studies of the interfacial and conformational behavior of biomolecules and thus enables one to characterize in situ the chemical identity, structure, and orientation of surface species in the adsorbed state (7,8).In addition, SERS has been shown to be capable of lateral spatial resolution down to the level of 1pm by means of Raman microprobe instrumentation (9)similar to the information available by AES and SIMS. However, in the latter techniques, the sensitivity is much lower (poorer) than the attomole mass sensitivity reported for SERS (9). Among the different materials reported to be SERS active (Ag, Au, Cu, Pd, Li, Na, K, Al, In, AgBr, AgCl, Ti02, etc.) the metallic form of silver has gained general acceptance for analytical measurements by combining several advantages. It has an appropriate dielectric function, in such a way that the enhancement factor on silver depends to a lesser extent on the excitation wavelength than in other SERS active metals such as gold and copper (IO). As a result, excitation must be in the red for Cu or Au ( I l ) , but visible wavelengths may be used with silver (12). The metallic form of silver can be easily handled at room temperature and ordinary pressure conditions, which is valuable from a practical standpoint. The analytical applications of SERS on a variety of silver active substrates are rapidly expanding. Sputter-deposited silver surfaces (13) have been evaluated for the thiocyanate anion. This substrate can be stored in air for long periods prior to Raman spectral examination in electrochemical and gas-phase
0003-2700/~9/0361-1897$01.50/0 0 1989 American Chemical Society
