Anal. Chem. 1992, 64, 1710-1720
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Background Correction by Wavelength Modulation for Pulsed-Laser-Excited Atomic Fluorescence Spectrometry Evelyn G. Su, Richard L. Irwin: Zhongwen Liang, and Robert G. Michel’ Department of Chemistry, University of Connecticut, 215 Glenbrook Road, Storrs, Connecticut 06269-3060
Instrumentation was constructed to modulate the dye laser wavelength for background correctlon In laser-excltedatomlc fluorescence spectrometry (LEAFS). To achleve wavelength modulation a pleroelectrlc pusher was used to drlve the wavelength tuning mlrror In a laboratory-constructed grazlng Incidence dye laser. The laser pulses were synchronlred wlth the pleroeiectrlc pusher movement so that alternate laser pulses measured the atomic fluorescence slgnal at the analytlcal atomlc spectral llne (on-line) and the background signal at a wavelength dirplaced to one slde of the atomic llne (off-line). The backgroundcorrectedsignal was obtalned by subtractlngthe off-llne “background” from the on-llne “signal plus background”. The spectral llne wldth (fwhm) of the dye laser was 0.009 nm, whlle the wavelength modulatlon Interval was controllable over the rangefrom 0 to 0.2 nmwlth a spectral resolution llmlted only by the spectral llne wldth of the laser. Thls type of background correctlon could, In prlnclple, be applled to other types of tunable lasers such as pulsed TI: sapphlre lasers. The performanceof background correction by wavelength modulatlon (WM) was demonstrated by measurement of sodlum resonance fluorescence In an alracetylene flame and by thalllum nonresonance fluorescence In a graphlte furnace. The experlmental data lndlcated that the wavelength modulation corrected, effectlvely and quantltatlvely, for flame background, blackbody emlssion from a graphlte furnace, and scatter of laser radlatlon off aluminum chlorlde (1 mg/mL as AI) matrlx partlcles In both the furnace and the flame. Analytlcal results were In good agreement wlth certllledvaluesfor the determlnatlonof s o d h Instandard reference materlals by the use of modulated LEAFS.
INTRODUCTION The primary sources of background in laser-excited atomic fluorescence spectrometry (LEAFS) depend on the atom cell in use. In a graphite furnace electrothermal atomizer (ETA), the main backgrounds are stray laser radiation and blackbody emission from the heated furnace, although laser radiation scattered off sample matrix particles, and possibly molecular fluorescence, have been observed.’ Compared to the similar technique of atomic absorption with a hollow cathode lamp, background correction in LEAFS is relatively trivial, because it can usually be done by tuning the laser wavelength slightly away from the atomic spectral line so that a background signal can be measured without the atomic fluorescencesignal. This is not possible with a hollow cathode lamp with ita fixed analytical wavelength. Bolshov et al.* used this technique by calculation of the average background signal from several separate furnace atomizations, while the
* To whom all correspondence should be sent.
+ Pfuer
Inc., Eastern Point Rd., Groton, CT 06340. (1)Butcher, D. J.; Dougherty, J. P.; Walton, A. P.; Wei, G.-T.; Irwin, R. L.; Michel, R. G. J. Anal. At. Spectrom. 1988,3, 1059. ( 2 ) Bolshov, M. A.; Zybin, A. V.; Smirenkina, I. I. Spectrochim. Acta 1981,36B, 1143. 0003-2700/92/0364-1710$03.00/0
dye laser was detuned 0.2-0.3 nm away from the analytical wavelength. We have reported the use of Zeeman effect background correction for ETA LEAFS?+ which has the advantage that it is a rapid, automatic method of correction, in addition to ita ability to make background measurements exactly at the analytical wavelength. Whether this latter ability is of practical significance for ETA LEAFS has yet to be shown.6 For flame LEAFS, the backgrounds are similar to those in a furnace, although the flame background tends to be 1 or 2 orders of magnitude smaller than furnace blackbody background. Quite a few background correction techniques have been investigated for flame LEAFS including intermodulated fluorescence,harmonic saturation spectroscopy,optical polarization techniques, two-channel boxcar detection, timeresolved fluorescence, and an early form of continuous-wave (CW) laser wavelength modulation. We have already published brief comparisons of these techniques.’-3 Wavelength modulation (WM) is an automatic version of the manually tuned procedure of Bolshov et aL2 An early wavelength modulation paper, for a CW laser, employed a piezoelectric micrometer to tilt an etalon at 1.5 Hz, which modulated the laser output over a 0.01-nm band-pass.’ A lock-in amplifier was used to measure the modulated fluorescence component of the photomultiplier current and thus to discriminate against the background signal. Later, Brassington8reportedamodification, to a Chromatix CMX-4pulsed dye laser, which enabled the laser pulses to be tuned alternately between two different wavelengths. This was useful for remote measurement of atmospheric pollution (e.g. SOz),as the use of two lasers could be avoided. The alternate laser pulse wavelength switchingwas realized by rocking both a birefringent filter and a frequency-doubling crystal in synchronism with the laser pulses. The wavelength was shifted by 0.7 nm at a laser repetition rate of 25 Hz, for the detection of SO2 at 300.1 nm with background correction at 299.4 nm. The wavelength drift for that system, while not in operation, was about 0.06 nm/h for both wavelengths. In another scheme for flame LEAFS, the addition of an etalon inside the cavity of a CW dye laser limited the laser output to 0.05-nm wavelength bands, each separated by 1.0 nm, that corresponded to the free spectral range of the etaThe lasing wavelength was further limited to only one of the bands by the appliedvoltage from a commercial,voltage driven, electroscan tuner. The magnitude of the applied voltage determined which particular band would lase. Consequently, a square-wave modulation of the applied voltage ~
~~~~~
(3) Dougherty, J. P.; Preli, F. R., Jr.; McCaffrey, J. T.; Seltzer, M. D.; Michel, R. G . A w l . Chem. 1987,59, 1112. (4)Preli, F. R., Jr.; Dougherty, J. P.; Michel, R. G. Spectrochim. Acta 1988,43E, 501. (5)Dougherty, J. P.; Preli, F. R., Jr.; Michel, R. G. Talanta 1989,36,
151.
(6) Irwin,R. L.;Butcher,D. J.; Wei,G.-T.; Miche1,R. G.Spectrochim. Acta, in press. (7) Fairbank, W. M., Jr.; Hansch, T. W.; Schawlow, A. L. J. Opt. SOC. Am. 1975,65, 199. (8)Brassington, D. J. J.Phys. E 1978, 11, 119. (9)Goff, D. A.;Yeung, E.S. Anal. Chem. 1978,50, 625.
0 1992 Amerlcan Chemical Soclety
ANALYTICAL CHEMISTRY, VOL. 64, NO. 15, AUGUST 1, 1992
resulted in alternation between two laser wavelengths separated by a multiple of 1nm. It was shown that the use of this wavelength modulation allowed for correction of light scatter and extended the barium limit of detection down to 10 ppb, but the minimum modulation interval of 1nm was 2 orders of magnitude larger than a typical atomic spectral line width in a flame or furnace. Background this far away from the atomic line may not always be a true measure of the background at the atomic line. Recently,two other background correction methods related to wavelength modulation in. LEAFS have been reported. Apatin et al.loimplemented background correction for ETA LEAFS by computer-controlled adjustment of the dye laser wavelength between groups of laser pulses. During atomization, measurementswere made with a fiied number of pulses on the analytical line, then a fixed number of pulses off the analytical line and so on. The change of the dye laser wavelength took about 40 ms, which limited the laser repetition rate to 20 Hz. The computer program reconstructed, by linear interpolation, the time profiles of the signal and background, and subtracted the two to give the corrected signal. This technique allowed a measurement of the background within one sample atomization cycle. However, the authors pointed out that it worked well only for sample atomization times that exceeded 4-5 s and that shorter times, for example for volatile elements, would probably lead to significant distortion of the temporal profile of the signal. This temporal resolution problem can have a significant effect on accuracy of furnace measurements, as demonstrated by graphite furnace atomic absorption experience.’l Sj6str6m12reported a multichannel approach to background correction for ETA LEAFS. Simultaneous measurements at three different wavelengths, one at the analyte line, the other two on either side, were made possible by mounting three optical fibers in a row at the exit slit of the monochromator. The optical fiber in the middle forwarded the light at the analyte line wavelength, A, to one photomultiplier tube (PMT1) and the other two fibers forwarded the light, at the other two wavelengths, X + AX and X - AX,to a second photomultiplier tube (PMT2). The background-correctedsignal was obtained by subtraction of the PMTl signal from that of PMT2. The advantage of this technique was that it measured background temporally simultaneous with the signal. However, because the optical fibers had a relatively large diameter of 1 mm, the spectral resolution of the background correction was limited. Sj6strdm achieved a background measurement 3-4 nm away from the analyte line wavelength, because the monochromator had a linear dispersion of 2 nm/mm, which resulted in a fwhm (full width at half-maximum) spectral transmission of about 1.5 nm. This low spectral resolution could be a disadvantage because for some real samples, background levels at the analyte wavelength may be different from those 3-4 nm away, which means that spectral interferences become more likely. For example, the system would not be able to distinguish between the two sodium D lines which are 0.6 nm apart. Although light losses in the fibers, of about 25 % , were present in Sj6str6m’s experiment, they were not of significance for measurements limited by the furnace background, because the light losses applied equally to both signal and background. The present paper is concerned with the development of background correction by wavelength modulation that can (10)Apatin, V. M.; Arkhangel’skii, B. V.; Bolshov, M. A.; Ermolov, V. V.; Kolwhnikov, V. G.; Kompanetz, 0. N.; Kuznetaov, N. I.; Mikhailov, E. L.; Boutron, C. F.; Shishkovskii, V. S. Spectrochim. Acta 1989, ME, 253. (11)Hardy, J. M.; Holcombe, J. A. J. Anal. A t . Spectrom. 1987, 2, 105. (12)SjhtrBm, S.J. Anal. A t . Spectrom. 1990, 5, 261.
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operate at any desired modulation frequency up to the limit of the excimer laser repetition rate, with both high temporal and spectral resolution. By temporal resolution, we mean the time lag between the measurement of “the signal plus background” and that of “background only”. By spectral resolution, we mean the difference in wavelength between “on-line signal + background measurement” and “off-line background measurement”. In order to achieve these aims, fluorescence and background were repetitively measured by alternate laser pulses, by use of a piezoelectric pusher to drive the wavelength tuning mirror in a laboratory-constructed grazing incidence dye laser. In principle, this type of background correction could be applied, in exactly the same way as described here, to other types of tunable lasers such as pulsed Ti:sapphire lasers. This is important because dye lasers are generally impractical for routine analytical instrumentation but pulsed Tisapphire lasers show promise of easier tunability and greater practicality, which may allow ETA LEAFS to be feasible as a routine analytical technique. The wavelength modulation approach for background correction of LEAF’S was tested both in the visible region, represented by sodium at 589.0 nm in the flame, and in the ultraviolet region, represented by thallium at 276.8 nm in the graphite furnace. All the experiments for sodium were performed at a laser excitation and fluorescence detection wavelength of 589.0 nm, with a laser repetition rate of 40 Hz. Thallium was excited at 276.8 nm and fluorescencedetected at 353 nm with a laser repetition rate of 50 Hz.
THEORY A laboratory-constructed grazing incidence dye laser was used in this work because modification of our commercial dye lasers was less straightforward. The grazing incidence design was introduced independently by Shoshan et al.13 and by Littman and Metcalf.14 Unlike the Hansch16 dye laser design, a relatively narrow bandwidth laser, with line widths of 0.1-0.2 cm-I, can easily be constructed and optically aligned without use of an intracavity beam expander or etalon. To achieve a narrow line width with the Hansch design, highquality intracavity optical elementsare needed,and alignment is quite difficult. Wavelength Modulation in a Grazing Incidence Dye Laser. A grazing incidence dye laser consists of a grating, a dye cell, a tuning mirror, and an output mirror (see Figure la). The output wavelength of the dye laser is determined by the grating equation X = x(sin 8,
+ sin 4)/m
(1)
where x is the grating period, m is the diffraction order, Bo is the angle of incidence, and 4 is the diffraction angle. From eq 1,it can be seen that the laser wavelength can be modulated by a change of the grating diffraction angle. In this work, wavelength modulation was achieved by a piezoelectric pusher which was mounted on the back of the tuning mirror. The tuning mirror was driven back and forth, pivoted at the mount (see Figure 11,by avoltage applied to the piezoelectric pusher. The change in the grating diffraction angle was directlyrelated to the movement of the piezoelectric pusher. In order to understand the relationship between the movement of the piezoelectric device and the modulation of the laser wavelength,consider the simple geometry shown in Figure 1. In Figure la, MA and MN are the directions normal to the tuning mirror and the grating, at the grating center point M. BC is the normal to the surface of the tuning mirror (13)Shoshan, I.; Danon, N.; Oppenheim, U. J. Appl. Phye. 1977,48, 4495. (14)Littman, M. C.;Metcalf, H. Appl. Opt. 1978,17, 2224. (15)Hansch, T.W.Appl. Opt. 1972, 11, 895.
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 15, AUGUST 1, 1992 ~
PUMP LIGHT,,,,,
T U N I N G MIRROR
For fist-order diffraction (between 416 and 833 nm), m = 1, and in our case, l l x = 2400 lines/”:
L
d4ldX = 0.0024001~0s4 (radlnm) = 0.13761~0s4 (deglnm) ( 5 ) Combination of eqs 5 and 3 with ALP, in micrometers, gives U
?
+
-
y
DIFFRACTION
DYE
ALJAX = 0.002400PBIcos 4 (rmlnm) For sodium at 589.0 n, 4 = 24.4’, and PB = 25 mm
+
+
GRATI
AXIAL, = 0.0152 nmlpm (7) If it is assumed that the piezoelectric device16responds linearly to the applied voltage, then
TUNING MIRROR
\
(6)
v,
U $ A V = 15 pmIl50 V = 0.1 pm/V AXlAV = 0.001 52 nm/V
(8)
Equation 6 is the main equation which establishes the relationshipbetween the movement of the piezoelectricdevice and the modulation of the dye laser wavelength. From eq 4 Flgwo 1. Schematic of the grazing incidence dye laser oscillator. IL is an intracavity lens and CL is a cyllndrical lens, all other symbols are defined in the text. (a, top) shows the whole dye oscillator, and (b, bottom) is an enlargement of the geometry of the pusher movement.
Table I. Theoretical Change in Diffraction Angle and the Piezoelectric Pusher Movement Required for a Wavelength-Modulation Interval of Ah = kO.06 nm* wavelength, X (nm) diffraction angle, $ (deg) d$/dX (deg/nm) change in diffraction angle, d$ (deg) pusher movement, ALP (rm) a
416.7 0 0.138 hO.0069
589.0 24.4 0.140 0.151 k0.0070 f0.0076
800 66.9 0.351 f0.0175
k3.0
f3.1
k7.7
500 11.5
k3.3
All the symbols in the table are defied in the text.
at the pusher mounting point. P’ is the pivot point, and PP’ is the normal to the tuning mirror surface and passes through the pivot point P’ to intersect the surface at point P. PC is parallel to the surface of the grating and intersects with the tuning mirror normal at point 0 and with the grating normal at point N. Angle BPC = angle AMN = 4. The pusher was mounted in such a way that it was able to extend along BC freely. Assume that the original resting position of the pusher is at B, after a small extension it moves to point B’ (see Figure lb), which results in a change in grating diffraction angle A4. We then have ALP = BB’ = PB tan A4
(2)
where BB’ and ALPis the extensionof the piezoelectricpusher and PB is the distance between the pusher mounting point and the pivot point, which was 25 mm in our system. Since BB’is extremely small (in micrometers) and so is A$, eq 2 can be simplified to ALP = BB’ = PB sin A4 = PBAd
For the grazing incidence scheme, Bo zz 90°, so sin Bo therefore
mX = x(1 + sin 4) Differentiation of eq 4 gives dX = x cos 4 d4lm
(3)
= 1,
sin 4 = -416.667 This equation determines the diffraction angle for the corresponding wavelength. By combination of this with eq 5, the value for d4ldX can be obtained. The value of d41dX can be considered as the same as that of A4/AX for a small change in diffraction angle and laser wavelength. Generallyspeaking, a wavelength modulation interval of 0.0541 nm (AX = 0.050.1 nm) is big enough to move the wavelength of the laser completely away from an analyte atomic line, in an atmospheric pressure atom cell. The required extension distances of our piezoelectric pusher, needed to generate a dye laser wavelength change, AX, of 0.05 nm, are listed in Table I, for several wavelengths between 417 and 800 nm. If a second harmonic generator is used to generate ultraviolet (UV) radiation, then twice the extension distances listed in the table (Us) are required for a modulation interval of 0.05 nm of UV radiation. The piezoelectric device that we chose was capable of movement of the tuning mirror by a maximum of 15 rm, which, from Table I, is adequate for most analytical lines that are likely to be encountered in flames and furnaces. INSTRUMENTATION
Grazing Incidence Dye Laser with Wavelength Modulation Capability. The grazing incidence dye laser oscillator system constructed in our laboratory was a closed cavity laser with an intracavity lens (Figure 1). The grating was a holographic grating. The manufacturers and descriptions of all major parta and components used in the instrumentation are listed in Table 11. The tuning mirror had 100% reflectance, and the output mirror had 10% reflectance. The intracavity lens was achromatic. It was used to improve the dye laser power and further narrow the spectral line width by refocusingthe light beam back over itaelf.17J8 A cylindrical lens was employed to focus the pump laser onto the dye cell. The dye cell used was a commercially available flow through cell, with a teardrop flow pattern. The laser dye was pumped through the cell by a pump with adjustable flow rates up to 4 Llmin. (16)Low voltage piezoelectric manual; Burleigh Instruments, Inc.:
(4)
---.
New 1989. ~. Yark. ----
(17)Yodh, A. G.; Bai, Y.; Golub, J. E.; Mossberg, T. W. Appl. Opt.
1984, 23, 2040.
(18)Lisboa, J. A,; Teixeira, S. R.; Cunha, 5.L. S.; Francke, R. E. Opt. Commun. 1983,44, 393.
ANALYTICAL CHEMISTRY, VOL. 64, NO. 15, AUQUST 1, 1992
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Table 11. Instrumentation description/model no. grating/2400 lines/", 50 mm long tuning mirror/50 mm X 13 mm, 10 mm thick output mirror/from Molectron dye laser DL-19P (10-mm diameter) intracavity lens/Achromatic, 25-mm diameter, f = 90 mm cylindrical lend50 mm X 60 mm, f = 4 in. oscillator dye cell/DL261 amplifier dye cell/DL266 dye pump/K33MYFY-233 optical breadboarddl ft X 1ft, 2 ft X 4 ft kinematic mirror mounts and translational stages piezoelectric pusher/PZL415 and low-voltage amplifier driver/PZ-l5OM excimer laser/800XR (mas repetition rate/100 Hz, typical power 40 mJ) 45O UV beam-turning mirrors/laser grade, R > 98% UV beam splitter/lO%-90%, 10% transmittance plano-convex lens, PLl/f = 515 mm a t 308 nm, 2-in. diameter plano-convex lens, PL2/f = 1030 mm a t 308 nm, 24x1. diameter monochromator/H-10 (f/3.5,0.1-m focal length, 8 nm/mm linear dispersion) photodiode array/RLlO24G digital oscilloscope/9400A frequency doubler/Inrad 5-12 opto-transmitter and receiver graphite furnace/HGA-BOO flame premixing chamber capillary burner (1-cm diameter) photomultiplier tube/R212 UH wide-band preamplifier/VV100 BTB pyroelectric joulemeter/J3-05DW boxcar integrator module/SR250 (gate width/lO ns) computer interface module/SR254 and software package/SR265 AT compatible microcomputer/system 200 computer interface/DT2801-A data processing software
The laser oscillator (Figure 1)was mounted on a 1-ft X 1-ft breadboard, which had mounting holes at 1-in. centers to mount optical stages. This small breadboard was further mounted on a 2-ft X 4-ft optical breadboard. The tuning mirror and the grating were affixed to kinematic mirror mounts, which allowed fine adjustment of the tilt and position of both optics. The tuning mirror mount was attached to a g-in.-diameter stainless steel ring mounted on a high precision, 0.1 arcsecond resolution, rotational stage, which had a 2.68in. clear center aperture. The grating was attached to a specially designed stationary mount that sat in the center of the rotational stage. In this way, the grating sat stationary in the center of the rotational stage, and the tuning mirror was rotated about the face of the grating. The dye cell was attached to a kinematic mirror mount, and mounted on a translational stage, which allowed the dye cell position to be optimized. The output mirror was mounted in a highprecision kinematic mirror mount to allow fine angle and tilt adjustments. The intracavity lens was placed between the grating and the dye cell, at one focal length from the dye cell, according to Yodh et al.17 The cylindrical lens was held at a distance of approximately one focal length away from the dye cell, and placed on a translational stage for the purpose of adjustment. To obtain wavelength modulation capability, a piezoelectric pusher with 15-pm maximum extension, was attached to one end of the tuning mirror and was pivoted at a distance of 13 mm away from the other end of the mirror. The distance between the pivot point and the mounting point of the pusher was 25 111111. The pusher was able to move 0-15 pm, dependent upon the voltage applied to the pusher, which was controlled by a low-voltage amplifier driver. The pusher was capable of operation up to a modulation frequency of 1kHz, and modulation of the pusher was accomplished by a sine wave, of the desired frequency, applied to the amplifier driver. The sine wave, after amplification by the amplifier driver, was adjusted to an amplitude suitable for the desired modulation interval. Here, a sine wave was used to drive the
manufacturer American Holographic, Inc., Littleton, MA Lumonics, Inc., Kanata, Ontario, Canada Laser Photonics, Orlando, FL Edmund Scientific, Barrington, NJ Oriel, Stratford, CT Laser Photonics, Orlando, FL Laser Photonics, Orlando, FL Emerson, St. Louis, MO Technical Manufacturing Co., Peabody, MA Oriel Co., Stratford, CT, or Melles Griot, Irving, CA Burleigh Instruments, Burleigh Park, NY Tachisto, Needham, MA Acton Research Corp., Acton, MA CVI Laser Corp., Putnam, CT Acton Research Corp., Acton, MA Acton Research Corp., Acton, MA Instruments SA, Metuchen, NJ Reticon Corp., Sunnyvale, CA LeCroy Corp., Chestnut Ridge, NY Interactive Radiation Inc., Northvale, NJ Interoptics, Burlingame, CA Perkin-Elmer, Norwalk, CT Perkin-Elmer, Norwalk, CT laboratory constructed Hamamatsu, Middlesex, NJ LeCroy Corp., Spring Valley, NY Molectron, Sunnyvale, CA Stanford Research Institute, Palo Alto,CA Stanford Research Institute, Palo Alto, CA Dell, Austin, T X Data Translation, Marlboro, MA user-written by use of ASYST software
piezoelectric device in order to give it the smoothest movement, which we considered to be desirable in order to minimize vibrations and possible mechanical drift in the dye laser. In addition, the sine wave was synchronizedwith the laser pulses. Wavelength modulation can work in two ways. First, the laser output can alternate between the analytical wavelength (A), at which the total fluorescence signal + background is measured, and a wavelength (X + AX or X - AX) slightly away from the analytical wavelength at which only background is measured. The net fluorescence signal is obtained by subtraction of the background from the total signal. In this way, the background is measured only to one side of the analyte line, either at X + AX or at X - AX, which gives rise to one-directionalwavelength modulation. Second,the dye laser can alternate between three different wavelengths, A, X + AX, and X - AX. The total signal is still measured at the analytical line A, while the measured background can be the average at two different wavelengths X + AX and X - AX, which gives rise to two-directional wavelength modulation. For one-directional wavelength modulation, the minimum voltage (0 V) of the sine wave corresponded to the on-line measurement, and the maximum voltage of the sine wave corresponded to the off-line measurement. For two-directional wavelength modulation, the minimum voltage (0 V) of the sine wave would correspond to the laser output at X - AX, the maximum to X + AX, and an intermediate voltage to A. In this paper, all the background correction experiments were done by onedirectional wavelength modulation. The dye laser system described above was suitable for background correctionin the visible region. For the ultraviolet region, where useful analytical lines exist for most elements, some modifications to the dye laser system, Figure 2, were needed in order to allow second harmonic generation capability. One dye amplifier stage was added to increase the dye laser output. The dye laser was pumped by a pulsed excimer laser. Xenon chloride gas was used in the excimer laser which emits at 308 nm. A 45O UV beam-turning mirror was used to direct the excimer beam onto the grazing incidence
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ANALYTICAL CHOMISTFIY, VOL. 64,NO. 15, AUWST 1, 1992 OSCILLATOR
WLIFIER 011
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>
dye laser system, as shown in Figure 2. The excimer laser beam waa focused, by use of cylindrical leneea, onto the oecillator dye cell and the amplifier dye cell, after being split into two portions by an W beam splitter. The two laser grade, W,spherical, plano-convex le", PL1 and PL2, were placed so that the horizontal excimer beamsize matched that of the dye cell since the cylindrical leneea focus only in the vertical direction. The pumping rate of the amplifier stage has a dramatic effect on the integrated gain of the ampwier, while the input signal from the dye cmcillator hae a much smaller effect.'@ Aarordingly, the beam splitter, with a splitting ratio of lo%+% was chosen to split the excimer PUBlP beam into the two p O h O M , with 10% g o b the oecillator and 90% going into the amplifier. The excimer pump beam for the amplifier waa delayed about 3 ne by use of three excimer laeer beam-turning mirrors, aa delay line mirrors, to "iaethe amplified spontaneous emiaeion.m Chucrcteriuth of the Dye Lnser. For initial teete of the grazing incidence dye laser system, as well M for the bacLgioundcorrectfon for d u m in the air-acetylene flame, thelaser dye Rhodamine 60 (Exciton, Inc.,Dayton, OH) was employed, for the oecillator, at a concentration of 5 mM dhohed in methanol. The amplifier dye solution waa more dilute by a factor of 4. About 500 mL of dye solution was needed for one fill of either of the oecillator or amplifier circulation system. A clearing ratio of 31 for the oecillator and 3 for the amplifier, at an excimer laser repetition rate of 80 Hz,waa empluyed and was more than eufticient to provide a fresh sample of dye solution between each pump laser pulee.l9l The total dye laser efficiency WM 4-6 96, with or without the ampliAer stage, except that without the amplifier, only about 25% &the excimer energy was u88d to pump the o d l a t o r , hi ordm to avoid damage to the dye cell. The maximum oecilkwoutputwaa 3OOpJ. The maximumamplifier output, o b t a h b l e after careful optimizatione of all the optical components, at 571 nm, which was not the peak of the dye R8G, waa 2.3 mJ a t an excimer pump energy of 40 mJ. The maximameecond harmonic generation (SHG) output wlte 66 pJ, which was an SHG efRciency of 2.7%. The wavelength Was catibrated approximately by use of a monochromator. Similar resulta were obtained for Rhoda" * eMO(E.citon, Inc., Dayton,OH), which waa utilized for teste of background correction of the thallium fluorescence signal, excited at 277
nm, in the graphite furnace. The dye amplifier output of R6G aa a function of wavelength demonetrated that the dye R6G provided a laeing range of about 15 nm (fullwidth at half-maximum,fwhm), with a maximum at 579 nm. The dye laeer spectral line width wss m w u r e d by use of a Fabry-Perot interferometer.A photodiode uray was utilized as the detector, and a digital d w p e was wed to store the interferogram and to manipulate the data. The line widtha of the millator and amplifier output were measured, at 571 nm, under identical experimentalconditione. They were both about 0.003 nm. The line width of the SHG radiation was not meaaured, because of the lack of the appropriate etalon. In this work, sodium,excited and detected at 589 nm,and thallium,excited at 276.8 nm,detected a t 363 nm, were chosen to teat the wavelength modulation system both with and without a second harmonicgeneration system. The oecillator dye laser output waa employed directly for the excitation of d u m a t o m in the akacetylene flame, aftar being expanded to about 4" diameter by a prism. The o d l a t o r output energy waa more than enough to saturate the sodium energy levels. Only the minimum laser intensity that is requiredto saturate the transition, should be used, in order to optimize signal-to-noise ratio.u Therefore, a typical dye laser energy of about 5-7 & / p u b was employed for the majority of the sadium work. For thethalliumwork, a eecond harmonic generation system waa employed to convert the vieible dye amplifier output into ultraviolet light. The W radiation was then expanded to 4-5 mm in diameter, by a telescope system, and directed into the electrothermal atomizer. A typical SHG energy of 2-3 pJ/pulse waa used for the majority of the thallium w o r t This energy was known to saturate the thallium atomic transition.%* Pulrre-to-Pulre Vuiation with and without Wave length Modulation. The pulse-to-pulse variation in energy was measured to aaaeaa the quality of the pulsed-laaer output. The pulee-to-pulse variation was defined as relative standard deviation (RSD)of the laser energy per pube. It was measured by use of a Molectron pyroelectric joulemeter in conjunction with the Stanford boxcar integrator module and a computer intarface module with a Stanford software package.= The pulse-to-pub variation of the excimer laser output was approximately 10 9% for all repetition rates, while that of the dye amplifier output, without modulation, ranged from 13 to 17% and that of the SHG ranged from 16 to 24% for the repetition rates from 5 to 60Hz. With modulation,the p u b t o - p u b variation of the dye output WIU the m e ae that without modulation, becauw the dye output did not change within asmall modulation interval. For the second harmonic generation with modulation, the SHG crystal had to be adjusted 80 that the SHG pulae energy at both on-line and off-line wavelengthe was equalized. The Dame pulse-to-pulse variation (17-23 9% on the SHG output waa obtained with and without modulation at the coat of a compromise in the average SHG pulse energy. (22) Morris, M. B.; McIlrath, T.J. Appl. Opt. 1W9,24,4146. (23) S d t a r , M.D. Laser excited atomic fluorawnce in a carbon tube furnace and microwave excited dectrodelscle dbcharge lamp for flame atomic fluarc*amce rpectromefzy. PhD. The&, Univedty of Connecticut,1886. (24) Weekn,S. J.;Hareguchi,H.;Winefordner,J.D.Anol. Chem. 1978, 60.380. .. (26)Butcher, D. J.; Irwin,R. L.; Tatrh.rhi, J.; Su,G.; Wei,0.-T.; Michel, R 0. Awl. Spectmac. 19)o. 44, 1621. (28) Irwin. R L;Butcher, D. J.; Tot.hnnhi.d.; Wei G.-T.;Micha R G. J. A d . At. Spectrom. 1990,5,603. (27)Manual for -Fast Gated Integraton and Boxcar Averogm", SR!W package; Standford Research Systemr: Palo Alto, CA, 1987, Appmdix B, pp 91-102. I
(18) Hnilo, A. A.; Mmmno, F.A; Marlinea, 0.E J. Opt. Soe. Am. E
1987,1,6B.
(20) Mclrw. T.J.; Lobin, J.; Young,W.A. Appl. Opt. 1982,21,726. (21) Irwin, R L. TransVerne Zeemnn background correction for b r excited atomic f l u a a w " in a graphite h c e . Ph.D. "io, Department of Chemistry, University of Connecticut, 1991.
ANALYTICAL CHEMISTRY, VOL. 64, NO. 15, AUQUST 1, 1992
4' FUNCTION %F GENERATOR
PIEZOELECTRIC PUSHER
TTL(F)
1715
CRYSTAL INPUT TTL
F
2F
I
2 F OR
4: LASER TRIGGER
Flgure 3. Block diagram for the laser trigger circuit described in the text.
S y n c h r o n i z a t i o n of L a s e r P u l s e s a n d P u s h e r Movement. An electronic trigger circuit was designed to synchronizethe laser pulses with the movement of the tuning mirror and with both the boxcar trigger and the computer data acquisition. In addition, the circuit was designed to work for both one-directional and two-directional wavelength modulation. A block diagram of the circuit is shown in Figure 3. A sine wave from a function generator with a frequency,F, was used to drive the piezoelectric device. A frequency of 2Fwas needed to trigger the laser to do one-directional wavelength modulation, and a frequency of @was needed to do two-directional wavelength modulation. A TTL square wave from the same function generator was used for the input to the trigger circuit, with the same frequency and the same phase as the sine wave. A digital frequency doubler gave pulses, of frequency "W, that corresponded to all the falling and rising edges of the square-wave input. These pulses were then inverted and sent to a digital phase shifter, which was configured to provide a phase shift capability between 0 and 360"with respect to the input of the trigger circuit. A pulse shaper was used to produce pulses of 2.5-ps pulse width. A second phase shifter was used to shift these pulses 180' and also to give 2.5-ps pulses. These two streams of 2.5-ps pulses, one from the second phase shifter, one from the first pulse shaper, were 180" out of phase and were added together to give a frequency of '4F" by use of an OR gate. The 2F or 4F pulse train, chosen dependent upon whether one-directional or two-directional wavelength modulation was to be studied, was lengthened by the second pulse shaper, to an adjustable range of 5-20 ms, required to meet the external trigger requirements of the excimer laser. The output from the second pulse shaper was current-amplifiedby a transistor pair and sent to an optotransmitter, where the 2F or 4F train of pulses was changed into hght pulses. These light pulses were further sent, by an optical fiber to avoid rf pickup, to a point close to the excimer laser's high-voltagepower supply, where a circuit was located that converted the light pulses back to electrical pulses by use of an optoreceiver to trigger the laser. The excimer laser was triggered at each falling edge of the trigger pulses (Figure 4). The timing diagram (Figure 4)shows, for one-directional wavelength modulation,the phase and frequency relationships between all the outputs at different stages of the trigger circuit and the synchronization of the pusher movement with the laser pulses. The X-axis represents time, and the Y-axis represents voltage output or light output in arbitrary units. The output pulses from the frequency doubler were of the same phase with respect to the source, either the sine wave or the synchronizedTTL square wave. The first phase shifter phase-shiftedthe laser trigger pulses, by an adjustable amount between 0 and 360" with respect to the source, which ensured the synchronization of the laser light pulses with the
EXCIMER LASER TRIGGER LASER
ON-,LINE BOXCAR OUTPUT SIGNAL
W F OFF-LINE
Flgure 4. Timlng diagram for the synchronizationbetween the pusher movement, laser trigger, boxcar trigger, and data acquisttion. F is the frequency of the sine wave that was used to drlve the piezoelectric pusher. The X-axis representstime and the Y-axis representsvoltage output, or light output, in arbitrary units.
movement of the piezoelectric pusher. The 2F pulses from the output of the first phase shifter were then shaped, to meet the requirements of the excimer laser trigger. The excimer pump laser radiation pulses were generated at each falling edge of the laser trigger pulses. In Figure 4,the excimer laser trigger pulses are drawn, for simplicity, with about 50% duty cycle. The exact duty cycle depends on the pulse repetition frequency and the external trigger pulse width requirements of the excimer laser. This flexibility was built into the dye laser to allow for the use of either of our two excimer lasers. The dye laser radiation pulses were used to trigger the gated boxcar integrator through an optical fiber and a photomultiplier tube.28 The optical fiber picked up the light pulse at the dye cell without blocking the laser output. In this way, the boxcar trigger was synchronized with the laser pulses and thus the pusher movement. The synchronization of the boxcar trigger, with the data points taken by the computer, is treated in a later section of this paper. Detection System. A monochromator, with a band-pass of 4 nm, was used. The resonance fluorescenceof sodium in the air-acetylene flame was detected at a right angle with respect to the laser beam (transverse illumination), while the front surface illumination approach%was utilized for detection of the nonresonance fluorescence of thallium in the graphite furnace electrothermal atomizer. The air-acetylene flame, or the graphite furnace, was imaged onto the entrance slit of the monochromator through twofll lenses of 50-mmdiameter. The fluorescence signal was deteded by a photomultiplier tube and amplified by a wide-band (200-MHz) preamplifier with a gain of 10 and an input impedance of 1kQ. The signal from the preamplifier was sent to the boxcar integrator used in last sample mode and with a 10-ns gate width. Both background and fluorescence data points were collected from one channel of the boxcar by the computer. Data Acquisition and Processing. The computer took data from a single channel of the boxcar, which gave an output data set that alternated between signal + background and background only. A user-written subroutine in ASYST was developed to separate these two sets of data. In order to do this, the collection of the data points was synchronizedwith (28) Seltzer, M. D.; Hendrick, M. S.; Michel, R. G. Anal. Chem. 1985, 57,1096.
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 15, AUGUST 1, 1992
TRIGGERING CIRCUITRY
FUNCTION GENERATOR
I
I
I
EXCIMER LASER
2nn/
I
PUSHER?
DYE LASER
ATOM
CELL
PMT MONOCHROMATOR
COMPUTER
BOXCAR
Figure 5. Block dlagram of the experlmental setup for wavelengthmodulated LEAFS. For details of individual componentssee Table 11.
the laser pulses and the boxcar trigger pulses, by the use of an external clock to drive the computer A/D conversion. The 2F pulses from the output of the first phase shifter were directly employed as the external clock. Figure 4 shows the phase and frequency relationships among the sine wave, the output of the first phase shifter, the excimer laser trigger pulses, laser radiation pulses, and the boxcar output signal. The output of the first phase shifter allowed a phase match with the boxcar output signal, which, triggered by the dye laser light pulses, alternated between signal + background and background (on-line and off-line). In other words, the computer took one data point for every single laser pulse, and the data points alternated between signal + background and background only. This synchronization ensured that, no matter what was the laser frequency, the computer could separate these two sets of data and plot the total signal, the background signal, and the backgroundcorrected fluorescence signal. If desired, smoothing could also be done before the data were plotted. EXPERIMENTAL SECTION Standard Solutions. Standard solutionswere prepared daily on a class 100 clean air bench, by serial dilution of a 1 mg/mL stock solution with a0.5% nitricacid solution (Ultrex,J. T. Baker Chemicals, Phillipsburg, NJ). All the ‘standard solutions were stored in polyethylene bottles. The stock solutions were made from high-purity metals or metal salts (SPEX Industries, Metuchen, NJ). Procedure. A block diagram of the experimental setup is shown in Figure 5. For flame LEAFS, asodium standard solution was continuously aspirated into the air-acetylene flame, while for ETA LEAFS, a 20-pL aliquiot of thallium solution was deposited onto a L’vov platform, inside the graphite furnace,to be dried and charred before atomization. A typical furnace program used for thallium is summarized as follows: dry, 150 “C, ramp 20 s/hold 20 s; char, 300 “C, ramp 25 s/hold 20 s; cool, 20 O C , ramp, 1s/hold 10 s; atomize, 1800 O C , ramp 0 s (maximum heating rate)/hold 5 s; clean, 2500 “C, ramp 1s/hold 5 s. In the atomization step the internal argon gas flow was stopped for maximum sensitivity.
RESULTS AND DISCUSSION Effects of Modulation on the Performance of the Laser System. It is crucial to maintain equal average pulse energy a t both on-line and off-line wavelengths in order to modulate the atomic fluorescence signal only, without modulation of background signals such as stray light or concomitant scatter, which are proportional to laser pulse energy. This requirement is automatically met in the visible region because the visible dye laser output is the same within a small wavelength modulation interval (0.014.1 nm) due to the broad dye wavelength tuning curve. For the SHG radiation, the SHG
energy per pulse in the “autotrack”mode of our INRAD second harmonic generation system (Table 11)drifted with time, while it stayed constant in the “manual” mode. This was because the INRAD device senses the UV output of the SHG crystal and dynamically adjusts the phase match angle in order to maintain maximum conversion at any chosen wavelengthz9 and to compensate for environmental temperature changes that might cause the SHG output to drift. When the SHG crystal was in the autotrack mode, it only followed one wavelength and maximized the SHG output at that wavelength, while the SHG output at the other wavelength dropped. In the case of the manual mode, no change in the phase match angle was made by the autotracker, therefore, the SHG energy at both wavelengths could be manually set and maintained at about the same level within their relative standard deviations, which is normally about 20 ?6 Even though the SHG system was used in manual mode, the output pulse energy was sufficientlystable with time to allow excellent analytical measurements during the course of day, although some small adjustments were needed each hour or so. The mechanical stability of the grazing incidence dye laser was tested in terms of the mechanical drift in wavelength as a function of time. The mechanical drift was measured by adjusting the laser to the sodium wavelength a t 589.0 nm before and after a certain time interval, and the wavelength difference was inferred from the movement of the tuning mirror rotational stage that was necessary to compensate for the drift. It was found that the dye laser wavelength drifted by0.014 nm, overnight for 12 h, which was about 0.001 nm/h, with the piezoelectric device pusher stationary. With the device running, under the same conditions used for wavelength-modulated LEAFS, the drift was 0.028 nm, also overnight for 12 h, which was about 0.002 nmlh. Six repeated spectral scans of the sodium excitation profile were done on the same day, within 12 h, with different wavelength modulation intervals. The maximum fluorescence was found to be within an excitation wavelength of 588.995 f 0.002 nm for the six scans. Pusher Movement as a Function of Applied Voltage. The movement of the piezoelectric pusher, as a function of the applied voltage, was tested by measurements of sodium fluorescence in the flame and thallium fluorescence in the graphite furnace. To test the wavelength change, as a function of the extension of the piezoelectric pusher, the tuning mirror was first set at a position so that the laser wavelength was at the analyte line with modulation off; then the fluorescence of a 1ppm sodium solution was measured as a function of the dc voltage applied to the piezoelectric device. This was done by a manual stepwise increase of the voltage, followed by a stepwise decrease of the voltage. The results (Figure 6a) indicated that there was noticeable hysteresis16of the piezoelectricpusher. Similar results were found for a 10 ppb thallium aqueous solution (Figure 6b) although the initial laser wavelength with modulation off (the 0-V point) was not set quite at the analytical peak. This hysteresis was not of concern, because only two points, on-line peak and off-line background were used for wavelength modulation. The region that displayed the most hysteresis was not used. The laser pulses only corresponded to the point at the peak of the atomic line and the first point displaced completely off the atomic line, so that a good, reproducible,modulated, fluorescence signalcould be recovered as long as a sufficient modulation interval was used. It can be seen (Figure 6a) that a minimum of 70 V, which corresponded to a wavelength modulation interval of 84 pm,
.
(29) Znrad Manual for ‘Model 5-12 AUTOTRACKER, SHG servo tuning system”; Interactive Radiation, Inc., Northvale, NJ, 1982.
ANALYTICAL CHEMISTRY, VOL. 64, NO. 15, AUGUST 1, 1992
0 0
20 24
40 40
80
IK)
loa
72
91
120
120 144
140(vob) lM(pm)
“OT
3.0-
(b)
DC BIAS vo LTAGE/EQ uIVALENT sHG WAVELENGTH c HANGE Flgwe 6. Effect of voltage, applled to the piezoelectric pusher, on the flame and furnace fluorescence signals. The Y-axis represents the fluorescence signal, while the X-axis represents the dc bias voltage apptiedtothepusher. ThesecondrowofX-axklabelsisthewavelength change that correspondedto the applled dc bias voltage. (a)Excltatlon spectral scan of 1 ppm Na aqueous solution in a flame by stepwlse increase and decrease in the dc bias voltage of the amplifier driver: 0 V = 588.995 nm (at the peak): filled triangle, 0-140 V: fllled circle, 140-0 V. (b) Excitation spectral scan of 10 ppb TI aqueous solution In a graphite furnace by stepwlse Increase and decrease In the dc bias voltage of the amplifier driver. Each point In the figure represents the average signal of three or more furnace firings at the corresponding wavelength: 0 V = 276.778 nm (thalllum peak = 276.787 nm); open clrcle, 0-100 Volts; filled circle, 100-0 Volts.
was required to tune the laser wavelength completely off the sodium line from ita peak. The wavelength value on the Xaxis was derived according to the experimental value of Ax/ A Vobtained by calibration against the sodium D line spacing, which is described below. It should be noted here that some shift could occur in Figure 6,between the wavelength scales for increasing and decreasing voltages, due to some, albeit small, backlash in the rotational stage. The full width at half-maximum (fwhm) for the sodium excitation profile was about 26 pm. Accordingly, a modulation interval of about 3 times the fwhm of the atomic excitation profile was adequate to move the laser wavelength completely off the sodium analytical line. The full width at half-maximum of the thallium atomic excitation profile was 0.012 nm. Figure 6b shows that a minimum of 60 V, which corresponded to a dye laser wavelength interval of 0.072 nm or a UV wavelength interval of 0.036 nm after frequency doubling, was needed to detune the SHG radiation completely off the thallium line from its peak. A wavelength modulation interval of about 3 times the fwhm of the atomic excitation profile was also needed to go completely off the thallium analytical line. The wavelength modulation interval is defined here as the difference in laser wavelength between on-line and off-line measurements. In general, the choice of WM interval must
1717
depend on the atomic spectral line width of the analyte line. For most elements, the fwhm of the atomic excitation profile ranges from 0.001 to 0.02 nm. Therefore, a modulation intervalof 0.01-0.1 nm, dependent on the element of interest, is enough for one-directional background measurements to be completely off the analyte line. Ideally, the wavelength modulation interval should be big enough for 100% of the fluorescence signal to be recovered, but small enough that the possibility of spectral interferences is minimized. If the modulation interval is chosen to be too small, the wings of the analyte line will appear in the background channel and will be subtracted from the total signal to give a net loss in fluorescence signal. The experimental ratio of wavelength changeto the voltage change (AAlAv) was derived from the laser excitation spectrum of sodium by use of the known wavelength separation between the two sodium 2D lines. An average AX/AV of 0.001 18 f O.OO0 03 nm/V was obtained for three repeated scans. A theoretical value of AAlAV was calculated in the above theory section to be 0.001 52 nm/V for sodium, based on the assumption that the piezoelectric device responded nearly linearly to the voltage applied.’6 The experimental result and the theoretical value agreed within 20% for sodium. Precision and Detection Limit with and without WM. Precision and detection limit with and without modulation were experimentally obtained under identical conditions to compare the effect of modulation on the analytical performance. The relative standard deviation (RSD) for 16 measurements of a 1 ppm sodium and a 10 ppb thallium aqueous solution were respectively 1.2% and 4.4% with WM off and 1.7% and 6.5% with WM on. A simple F-test shows that the precision with and without modulation did not differ significantly at a confidence level of 95% The detection limits were determined by extrapolation of the calibration curves to a signal level equal to 3 times the standard deviation of 16 measurements of the blank noise. With the wavelength modulation off, the measurement of blank noise was performed 16 times with the laser tuned to the analytical wavelength, and another 16 times with the laser tuned completely off the analytical wavelength. With the wavelength modulation on, the on-line and off-line blank signal measurements could be obtained from the same data set. The on-line detection limit of sodium was 0.6 ppb with WM on and 0.5 ppb with WM off. The off-line detection limit of sodium was 0.4 ppb for both with and without WM. This result indicates that the use of WM did not degrade the detection limit in the visible region. The detection limit of thallium was not determined. It can be seen from the above comparison that modulatedand unmodulatedLEAFS provide the same precision and detection limit. CalibrationCurve for Sodium and Its Linear Dynamic Range with vodulation. A calibration curve for sodium was constructed by use of aqueous solutions and with wavelength-modulatedLEAFS. Neutral-density filters were used in front of the monochromator to cut down fluorescence signal, for high sodium concentrations, to ensure a linear response of the photomultiplier tube. The calibration curve was linear up to 10 ppm, and it reached a plateau after 100 ppm. The linear dynamic range by wavelength-modulated LEAFS was about 5 orders of magnitude, which was the same as that without modulation. Background Correction in the Flame. Figure 7a shows the correction, at the sodium wavelength, of the air-acetylene flame background, which included stray laser radiation and flame emission. The upper trace (A) is the flame emission plus laser stray radiation with a relative signal size of 2.51. The lower trace (A - B) is the background-corrected signal with a relative signal size of 0.10, which was the result of
.
1718
ANALYTICAL CHEMISTRY, VOL. 64, NO. 15, AUGUST 1, 1992 Table 111. Correction for Scatter off a 0.5 ppm Sodium Solution in an Aluminum Chloride Matrix (1 mg/mL as Al) by Wavelength-ModulatedFlame LEAFS.
A OS
sample 0.5 ppm aqueous solution lo00 ppm as A1 lo00 ppm Al + 0.5 ppm sodium
A-B
0.0.
background signal background-corrected (arbitrary units) signal 2.64f 0.2 12.9 f 0.6
14.3 f 0.8 13.9 f 1.0
14.3 f 0.6 26.4 f 0.9
The data are expressed as mean f standard deviation for five repeated measurements. Recovery of sodium from the aluminum chloride matrix was 95 f 6%
.
A
4.0
A-B 2.0
B _._ .
0
2.0
4.0
6.0
610
10.0
TIME /SECONDS
Flgwe 7. (a)Air/acetylene flame background correction by wavelength modulation. A fivepoint smooth was used after the original “on-line” and “off-line”data sets were separated: trace A, flame emission stray laser radiation; trace A - B, background-corrected signal. (b) Correction of the background, generated by aluminum chloride matrix, of 0.5 ppm Na in the matrix (1 mg/mL as AI) in the air/CnH*flame: trace A, Na fluorescence 4- AIC13 flame background: trace A - B,
+
+
background-corrected fluorescence signal.
subtraction of the off-line measurement from the on-line measurement. This background-corrected signal size was within the precision of the measurements at such a low signal level and clearlyindicated that the background was effectively corrected. It is noticeable that the noise on the backgroundcorrected signal was much smaller than the noise on the background signal. Both the on-line and off-line laser pulses measured the same flame background almost simultaneously (with only 20-ms time lag). It is logical that the noises on these two measurements were well enough correlated that the corrected signal resulted in smaller noise levels. The off-line background of a 0.5 ppm Na aqueous standard solution was also studied. It was found that the background was the same as the normal flame background as presented above. In other words, the sodium standard solution did not cause an appreciable amount of laser scatter. Aluminum chloride is known to produce strong nonspecific background signalsin the graphite furnace.21 This nonspecific background signal could be either concomitant scatter or broad-band molecular fluorescence. We consider it more likely to be concomitant scatter rather than molecular fluorescence, although no rigorous studies have been carried out. A matrix of 1mg/mL aluminum chloride, as Al, was used to generate a scatter signal to allow a more rigorous test of the WM background correction system. Figure 7b shows background correction of flame sodium fluorescence from a 0.5 ppm sodium standard in an aluminum chloride matrix (1 mg/mL as Al). Trace A is the total signal or on-line measurement, which included the sodium fluorescence signal, concomitant laser scatter off the aluminum chloride, flame background emission, and stray laser radiation; sodium atomic emission was insignificant. Trace B is the background signal or off-line measurement, which included only concomitant laser scatter off the aluminum chloride matrix, flame emission, and stray laser radiation. Trace A - B, is the result of the
on-line measurement (A) minus the off-line measurement (B), or background corrected signal. It can be seen that the background signal from the aluminum chloride matrix was about one-third of the total signal from the 0.5 ppm sodium solution and could be corrected easily by the wavelength modulation approach. In fact, there should be no limit to the magnitude of the background that can be corrected, due to the long linear dynamic range of the calibration curves and a lack of any restrictions on the size of the background that can be measured by the off-line laser pulses. The only likely limit is the normal increase in noise with background signal size, which will degrade precision and detection limit. It also can be seen from Figure 7b that the noise on the background (trace B) was significantly smaller than the noise on the total signal (trace A) and there was little correlation between these two noises. The noise on the total signal was a combination of fluorescence signal noise, matrix scatter noise, and the various background noise of Figure 7a. The fluorescence signal was about twice as big as the background so that significant fluorescence signal noise can be expected. As a result, the total signal noise was significantly greater than the background noise and the noise on the backgroundcorrected signal (A - B) was similar to the noise on the total signal (A). Little correlation existed between these two types of noises because of their different sources. Table I11 shows the experimental results of a quantitative test of the WM background correction. Listed in Table I11 are the values of background signal and background-corrected fluorescence signal for a 0.5 ppm sodium aqueous solution, an aluminum chloride matrix solution, and a 0.5 ppm sodium in the matrix solution, respectively. The scatter background from the matrix was as big as the 0.5 ppm Na fluorescence signal. The aluminum chloride matrix itself contained some sodium as an impurity. The experimental results (Table 111) showed that the wavelength modulation corrected the backgrounds completely. The sodium recovery from the matrix was calculated to be 95 f 6% for five measurements. The relatively poor precision of this measurement was directly related to be noise on the high background scatter signal from the high concentration of the aluminum chloride matrix. Background Correctionin the Graphite Furnace. In graphite furnace LEAFS, the main backgrounds are similar to those in the flame, except that blackbody emission replaces the flame emission background. Similar experiments were done in the graphite furnace electrothermal atomizer for thallium measurements, excited at 276.8 nm and detected a t 353 nm. In order to modulate the fluorescence signal, without modulation of the background, the SHG crystal was carefully adjusted to ensure that the UV radiation from the second harmonic generator was the same at both on-line and off-line wavelengths, at the cost of 20-50% energy loss at each wavelength, dependent upon modulation interval. Figure 8 demonstrates the effectiveness of background correction by wavelength modulation in the graphite furnace.
ANALYTICAL CHEMISTRY, VOL. 64, NO. 15, AUGUST 1, 1992
171Q
W
0
6 u
fn W
rr
4 W
CK
d IY
I
. 0
.
.
.
1au3.0
.
I
4.os.o
bo
0
: : : : : , l ~ z o 3 . 0 o w s . o o A o
TIME/SECONDS Flgure 8. Background correction by wavelength modulation in the graphite furnace. (1) Top traces: A, furnace blackbody emission stray laser radiation; A - B, background-corrected signal; atomization temperature = 2300 OC. (2) Central traces: A, total background signal, e.g. aluminum chloride scatter stray laser radiation 4-furnace blackbody emission; A - B, backgroundcorrectedsignal; atomization temperature = 2000 OC. (3) Bottom traces: A, thallium fluorescence +aluminum chloride scatter furnace background; A - B, backgroundcorrected signal or net thallium fluorescence signal; atomizationt e m perature = 2000 OC.
+
+
+
-.050
-.oa
0.0
.OW
RELATIVE WAVELENGTH OF LASER EXCITATION/nm Figure B. Excitation spectral scan of 0.1 ppb thallium in the AiCi:, matrix (1 mg/mL as AI) with modulation. Each point represents the average signal from three or more furnace firings at the each wavelength: (a) spectral profile of the total signal; (b) spectral profile of the backgroundcorrectedsignal. The fwhm was 0.012 nm. The 0.0 point in the X-axis indicates the peak wavelength position of hilium at 276.787 nm.
Table IV. Background Correction for 0.1 ppb T1 in the Aluminum Chloride Matrix (1 mg/mL as Al) in Graphite Furnace with Wavelength Modulation
total signal
The top two traces show the correction for furnace background. No sample was added to the furnace, in order to investigate only the background signals, but the excitation and detection wavelengths used were those of thallium. The atomization temperature used was high, at 2300 "C,in order to obtain a reasonable amount of blackbody background signal. Trace A (top) was the background signal of a furnace blank, which included furnace blackbody emission and stray laser radiation with a peak area of 1.73(relative signal). Trace A - B (top) was the background-corrected signal with a peak area of 0.05, which was 3% of the background signal and dominated by the noise on the signal. The two traces in the middle of Figure 8 show sample matrix background correction by wavelength modulation. No analyte was added, but aluminum chloride (1mg/mL as Al) was added to the furnace, fired at 2000 "C, to generate a reasonable scatter signal. Trace A (middle) was the total background signal with a relative signal size of 0.91, which included laser scatter off the aluminum chloride matrix particles, stray laser radiation, and furnace blackbody emission. Trace A - B (middle) was the background-corrected signal with a relative signal size of 0.02, which clearly indicates that the total background was effectively corrected. The bottom two traces in Figure 8 show the sample matrix background correction by wavelength modulation with analyte added. Trace A (bottom) shows the total signal of 20 MLof 0.05 ppb T1 in the aluminum chloride matrix, fired at 2000 "C. The total signal included thallium fluorescence, background scatter signal generated by aluminum chloride, stray laser radiation, and furnace blackbody emission. Here, the background signal generated by aluminum chloride was dominant. The thallium fluorescence peak appeared before the background peak that was generated by aluminum
.025
(arbitrary background units) signal
sample 0.1 ppb T1 12.4 f 0.9 5.9 0.5 AlCl3 matrix" 0.1 ppb T1 15.5 f 0.7 in AlC13 matrix (1mg/mL aa Al)
backgroundcorrected signal
0.22 f 0.29 12.2 f 1.0 (n = 4) 5.2 f 0.4 0.68 f 0.8 (n= 5) 3.8 f 0.9 11.8 f 2.1 (n = 12)
Aluminum chloride matrix only, 1 mg/mL aa Al. Recovery of 0.1 matrix waa 97 & 19%.
ppb T1 from the aluminum chloride
chloride. Trace A - B (bottom) shows that the background peak generated by the aluminum chloride matrix was effectivelycorrected, which left only the thallium fluorescence peak. Figure 9 shows the excitation spectral scan, with modulation, for 0.1 ppb thallium in an aluminum chloride matrix (1mg/mL as Al). The peak-to-peak voltage of the sine wave applied to the pusher was 70 V. Figure 9a shows spectral profile of the total signal, e.g. fluorescence plus background, which was obtained from the on-line measurement. The background for thallium solution in the aluminum chloride matrix was significantly elevated. This elevated background was caused by the matrix. Figure 9b shows the backgroundcorrected spectral profile, which was obtained, point by point, by subtraction of the off-line background from the on-line total signal. Clearly, the elevated level due to the aluminum chloride matrix was effectively subtracted by application of wavelength modulation. Table IV shows the quantitative experimental results of the recovery of thallium from the aluminum chloride matrix, with the scatter background corrected by wavelength modulation. With modulation on, a 0.1 ppb aqueous standard solution of thallium gave a modulated average signal of 12.2,
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 15, AUGUST 1, 1992
Table V. Determination of Sodium in NIST. Standard Reference Materials with Wavelength-Modulated Flame LEAFS sample
value foundb
certified value
SRM 1577a,bovine liver 0.251 f 0.014% 0.243 f 0.013% SRM 1549,nonfat milk powder 0.454 f 0.047% 0.497 f 0.010% 160 f 20 ppm SRM 1572,citrus leaves 150 f 14 ppm a National Institute of Science and Technology. Average value for three parallel dissolutions.
while the average scatter-corrected signal of 0.1 ppb thallium in the matrix of 1mg/mL aluminum chloride as A1 was 11.8. A Student's T-test showed that these two numbers 12.2 f 1.2 (n = 4)and 11.8f 2.1 (n= 12)were not significantlydifferent. The recovery of the thallium signal from the matrix was 97 f 19% (average f standard deviation) at 0.1 ppb thallium level. The relative high standard deviation was due purely to the noise on the large scatter signal from the aluminum chloride matrix. Real Sample Analyses by Wavelength-Modulated LEAFS. Sodium in three National Institute of Standards and Technology standard reference materials (SRMs) was determined by wavelength-modulated LEAFS. These samples were dissolved by use of the same dissolution method described in a previous paper.6 Table V shows the average concentrations found for three parallel dissolutions of these samples by use of aqueous calibration. It can be seen that good agreement with the certified value was obtained for all samples.
rection also offers high spectral resolution. The wavelength modulation interval is easily controlled by application of an appropriate voltage to the piezoelectric pusher in the grazing incidence dye laser. High spectral resolution can be obtained by use of the smallest modulation interval necessary to move off the analyte atomic line. Wavelength modulation corrects all the major backgrounds such as scatter of laser radiation off concomitant species, stray light, flame emission in flame LEAFS, and blackbody emission in furnace LEAFS. It should correct broad-band molecular fluorescence background as well, although this type of background has not yet been conclusively demonstrated during real sample analysis. Twodirectional wavelength modulation should provide accurate correction for both nonstructured flat background and any sloping background at the analytical line. The only disadvantage of this background correction technique is that,like the multichannel background correction technique, it does not correct backgrounds exactly at the same wavelength as the analytical line, which may result in some error when structured background or spectral interferences exist near or within the analytical line. Fortunately, both structured background and spectral interferences seldom appear in LEAFS because of its inherent high spectral selectivity. Therefore, it still needs to be shown whether or not this disadvantage is a practical limitation.
ACKNOWLEDGMENT
This work was supported by the National Institutes of Health, Grant No. GM32002. Oriel Corp. (Stratford, CT) provided some of the optical components for the construction of the dye laser. R.G.M. was supported by a Research Career CONCLUSION Development Award from the National Institute of Environmental Health Sciences under Grant No. ES00130. E.G.S. Compared with other LEAFS background correction was supported by a Perkin-Elmer Research Scholarship,and techniques reported in the literature,3-1°J2 this type of R.L.I. was supported by a State of Connecticut High wavelength modulation offers several advantages. Technology Graduate Fellowship. A portion of this work It is very easy to obtain wavelength-modulated laser light was presented during a Plenary lecture, given by R.G.M., at with a piezoelectric pusher. There is no need to significantly the Fifth Biennial National Atomic Spectroscopy Symposium, modify the dye laser design in order to achieve wavelengthmodulated LEAFS. For Zeeman background c ~ r r e c t i o n , ~ ~ July 18-20,1990, Loughborough University of Technology, Loughborough, England, U.K. This work was also presented, it is physically difficult to fit a large magnet around a graphite in part, at several meetings as follows: 1991 FACSS XVIII, furnace. Background correction by wavelength modulation October 6-11,1991, Anaheim, CA, Paper No. 815, given by offsrs high temporal resolution. The commercial piezoelectric R.G.M.; X W FACSS meeting, October 7-12,1990, Cleveland, pusher can run at a frequency as high as several kilohertz, if OH, Paper No. 15, given by E.G.S.; Pittsburgh Conference, the excimer laser can operate at the same high frequency. as Paper No. 213, March 4-8,1991, Chicago, given by E.G.S. Commercially available excimer lasers can operate at up to a frequency of about lo00 Hz. The excimer laser used in this work was operated at a maximum frequency of 80 Hz. By RECEIVED for review February 3, 1992. Accepted May 4, contrast, the practical maximum frequency for Zeeman 1992. correction is limited to 240 Hz, which is 4 times the power Registry No. Sodium, 7440-23-5. line frequencyP+ Wavelength modulation background cor-
