Laser-Excited Atomic Fluorescence Flame Spectrometry L. M. Fraser and J. D. Winefordner' Department of Chemistrv, University of Florida, Gainesville, Fla. 32601 ~ T LINE H AND CONTINUUM sources of excitation have been used for atomic fluorescence spectrometry ( I , 2). These sources are usually either operated continuously (CW), and the radiation is modulated by mechanical choppers, or is modulated electrically. Mechanical or electrical modulation of exciting radiation and ac (often phase sensitive detectors) detection of the modulated photodetector signals due to modulated fluorescence is utilized to eliminate measurement of emission signals resulting from the flame gases as well as other constant (dc) signals. However, sinusoidal (or square wave) modulation of source radiation does not usually result in any significant increase in the atomic fluorescence signal-tonoise ratio and, in fact, can even result in a decrease under certain circumstances (3). O n the other hand, a stable, repetitively-pulsed source of excitation with a small duty cycle, i.e., small ratio of on-to-off-time, could result in a significant increase in the fluorescence signal-to-noise ratio due primarily to the decreased noise; during the short ontime, the signal can be of the same order as the average signal resulting from CW sources, whereas the noise will be small because of the small number of random photodetector pulses due to most noise sources commonly present in atomic fluorescence flame spectrometry, e.g., dark current, flame background, analyte emission, etc. (4). According to the above discussion, a n ideal source would be a high power source with a small duty cycle. Because of practical problems associated with exchanging line sources, the ideal source should also produce a continuum from below 200 nm to about 800 nm. Pulsed high pressure inert gas continuum sources, however, are very expensive, bulky, and rather inconvenient to use. In addition, with high pressure discharge lamps, stray light can be considerable, pulse duration can be quite long, the spectral radiance per pulse can be quite variable, and the spectral radiance per pulse at pulse repetition rates of 10 Hz or greater can be quite low. A possible alternative to the pulsed continuum high pressure source is given in the present investigation. A stable, pulsed, tunable dye laser pumped with a N 2 laser is used to excite atomic fluorescence in flames. The dye laser system used here has a peak power of greater than 10 KW at all wavelengths, a pulse repetition rate of about 1-25 Hz, a spectral half width of about 0.1-1 nm, and a pulse half width of about 2-8 nsec. By proper choice of dye and grating angle, any wavelength region (0.1-1 nm wide) between 360 and 650 nm can be selected to excite atomic fluorescence. By use of a fast response multiplier phototube and a boxcar integrator (gated amplifier) capable of aperture (sampling) gate widths of the order of 10 nsec wide, all noises except scatter of laser radiation within the relatively turbulent flame gases can be essentially eliminated. Some initial atomic fluorescence measurements, including detection limits,
Author to whom reprint requests should be sent. (1) J. D. Winefordner and T. J. Vickers, ANAL.CHEM., 42, 206R
(1970). (2) J. D. Winefordner and R. C. Elser, ibid., 43 (3), 24A (1971). (3) J . D. Winefordner, M. L. Parsons, J. M. Mansfield, and W. J. McCarthy, ibid., 39, 436 (1967). (4) J. D. Winefordner, Accorrrzts Chem. Res., 2, 361 (1969).
--
I
DYE LASER
I
FLAME
I PLANE MIRROR
-+TI
I
e
l
-
W T O M U L.TIFuER TUBE
*
p
T
POWER SUPPLY
T
Figure 1. Block diagram of experimental system for laserexcited atomic fluorescence flame spectrometry Instrumental components are: Model 1000-N2 laser with power supply and trigger circuits and Dial-a-Line dye laser, AVCO Everett Research Laboratory, Everett, Mass. 02149; RCA 1P28A photomultiplier tube; Model 4-8400 scanning 0.25 m Czerny-Turner grating monochromator, American Instrument Co., Inc. Silver Spring, Md. 20910; Model 160 Boxcar Integrator, Princeton Applied Research Corp., Princeton, N. J. 08540; Servoriter I1 Potentiometric Recorder, Texas Instruments, Inc., Houston, Texas 77006; Model 412B high voltage power supply, John Fluke Manufacturing Co., Inc., Seattle, Wash. 98133; Modified Jarrell-Ash Triflame nebulizer burner (see Figure 2) and capillary burner mounted on Perkin-Elmer chamber-nebulizer (see text); 2-stage regulators on gas cylinders and burner regulator with flow meters for Perkin-Elmer Model 303 atomic absorption flame spectrometer, Perkin-Elmer, Inc., Norwalk, Conn. 06852 analytical curves, and spectral resolution, for AI, Ca, Cr, Fe, G a , In, Mn, Sr, and Ti in either Hn/air or C2H2/N20 flames are reported here. EXPERIMENTAL A block diagram of the experimental setup is shown in Figure 1. The specific components of the experimental setup are also designated in Figure 1 . Two nebulizer-burner systems were used for the present studies. A modified JarrellAsh Triflame burner system was used for Hn/air flames. The modified nebulizer-burner system, shown in Figure 2, consists of the Hetco nebulizer-burner, a cylindrical chimney, and a special Alkemade-type burner head ( 5 ) containing a matrix of 73 0.75-mm diameter holes (2 X 0.5 cm burner area). The Hl/air flame produced with the modified JarrellAsh Triflame burner was very laminar compared to totalconsumption burner flames. The nebulizer-burner used for C2H2/N20flame in the present studies was of the capillary type described by Aldous et a f . (6). This nebulizer-burner utilized a Perkin-Elmer aspiration chamber assembly for the Model 290 atomic absorption spectrometer and a circular (IO-mm diameter) burner head containing a bundle of 75 capillaries (0.69-mm i d . ) . A simple scan drive system was constructed t o vary the grating angle in the dye laser and therefore t o allow a continuous variation of the dyeolaser wavelength over a small wavelength range (say 10-50 A). RESULTS AND DISCUSSION The spectral distribution (obtained by varying the dye laser wavelength) in approximately a 1.O-nm wavelength region ( 5 ) T. Hollander, Ph.D. Thesis, University of Utrecht, The Nether-
lands, 1964. (6) K M. Aldous, R. F. Browner, R. M. Dagnall, andT. S. West, ANAL.CHEM., 42,939 (1970).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 12, OCTOBER 1971
1693
-
HoLE PITTERN
...... ..... ...... Of
396.1 m C&/I$O
I
1
A'.'.'.'.
Flame
0.sm
s;m ;.m
C - JARRELL ASH TRI-FLAME ASPIRATION CWMEER
Time
I- I
Figure 4. Flame background with laser sourfe off and flame background with laser source on at 3961 A
C JARRELL ASH HETCO NEBULIZERBURNER
In both cases, water is being introduced at a rate of 4.0 mljmin into the Perkin-Elmer chamber. Experimental conditions of boxcar integrator: 10-nsec aperture gate width; 3-nsec aperture time constant; IO-Hz repetition rate 403.3nrn
Figure 2. Modified Jarrell-Ash Triflame nebulizer burner for use with H2/air flame
Blank
-
Hp/Air
-
Flame Loser On
Mn (5 p#rnl)
Blank
15-
-t
t
2r
10-
-1
>
01 Time
Figure 5. Signal-to-noise ratio for Mn at 5 pg ml-l and a water blank in an H2/airflame
'"r
-t E -
800
d
-.c
Experimental conditions are as in Table I. Note that the noise level for blank with laser on is similar to the noise level with laser on for C2Ht/N20 flame Shown in Figure 4. Also note the slightly different dc offset for the H2/air flame in Figure 5 as compared to C2H2/N20 flame in Figure 4
c
600
-
0
D r .
iii
3 400200-
0
-1
1
0
Wavelength
L -
Figure 3. a. Spectrometer slit function obtained by wavelength scanning the dye laser radiation scattered from a Teflon sheet used in place of the flames (monochromator wavelength is set at 422.7 nm) b. Spectral distribution of fluorescence radiation of Ca (1000 pg/ml) obtained by varying the dye laser wavelength over about 1 nm surrounding 422.7 nm (monochromator wavelength set a t 422.7 nm). Solution aspiration rate is 4 ml/min. All other experimental conditions are as in Table I c. Spectral distribution of scattered radiation by the flame gases obtained by varying the dye laser wavelength over about 1 nm surrounding 422.7 nm (monochromator wavelength set a t 422.7 nm) 1694
e
surrounding 422.7 nm and the relative magnitudes of signals obtained by spraying water (background)-blank-and 1000 p g ml-1 of Ca into a H?/air(15.2 1. min-' H? and 7.5 1. min-l air) flame using the modified Jarrell-Ash Triflame burner system (solution flow rate was 4.0 ml min-l) are shown in Figures 3c and 3b, respectively. Also shown in Figure 3a is the spectrometer slit function for the monochromator obtained by scattering dye laser radiation off a Teflon (Du Pont) sheet while varying the dye laser wavelength. It is evident that the spectral distribution of the blank follows the spectrometer slit distribution. Also, it is evident that the blank signal is many times smaller than the atomic fluorescence signal obtained at 422.7 nm with 1000 pg ml-1 of C a ; the blank signal is probably due to scatter from refractive index variations within the flame (i.e,, the same response was observed whether water, 500 ppm of diverse salts, or nothing was being aspirated). Also solvent and solute vaporization are quite complete in both flame-nebulizer-burner systems used in these studies. The blank signal (and noise) is nearly the same at all wavelengths (i.e., for all dyes and all wavelengths) and depends primarily upon the flame gases ( / . e . ,the scatter signal and noise are independent of thermal emission from the flame gases, from the analyte, and from the matrix and are nearly independent of the flow rate of solution into the spray chamber).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 12, OCTOBER 1971
The flame background with laser source off and with the laser source o n (in both cases, water is being introduced into a n C2H2/N20-6.5 1. min"' C 2 H 2and 14.5 1. min-' N20) is shown in Figure 4. The noise level with the laser source o n is several times greater than with the laser source off. Also the noise level with water being aspirated into the C2H2/N20 flame is very similar to the noise level with water being aspirated into the H?/air flame; however, it should be noted by comparing Figures 4 and 5 that although the peak-to-peak noises with water being aspirated are similar, the dc offset differs between C2H2/NpO and &/air. Of course, the dc offset can be suppressed. The signal-to-noise ratio obtained for M n (403.3 nm) at a concentration of 5 pg mI--l is shown in Figure 5. The limit of detection for M n is about 0.3 pg ml-' under similar experimental conditions. Other elements give similar signalto-noise ratios near the limit of detection. As long as a sufficiently wide monochromator spectral bandpass is used, it is possible to scan the laser wavelength over a small wavelength region and thereby record the fluorescence spectra of closely-spaced fluorescence lines, Le., a fluorescence excitation spectrum. In Figure 6 is shown the fluorescence excitation spectra of C r in the wavelength region of 351.0-361.0 nm. Within this region Cr has a multiplet of 3 lines (357.8, 359.3, and 360.5 nm). The relative rnagnitudes of the three lines is modified to some extent by the spectrometer slit function. The resolution of the recorded fluorescence spectra is determined only by the spectral halfwidth of the laser emission band. Analytical curves for Ca, Cr, Fe, G a , In, Mn, and Sr in Hn/air flames and AI and Ti in a n G H ~ / N Z flame O are shown in Figures 7 and 8, respectively. The analytical curves (log-log plots) are linear (slopes of unity) from the detection limits to concentrations at least l o 3 greater than the detection limits. The linearity is as expected from theoretical considerations (7). The relative standard deviations of fluorescence signals 10-fold, 100-fold, and 1000-fold above the detection limits are about 8 %, 5 %, and 3 %, respectively. Detection limits (concentrations producing a signal-tonoise ratio of 2) for AI and Ti in a n C2H2/N20flame and for (7) J. D. Winefordner, V. Svoboda, and L. J. Cline, CRC. Crir' Rev. Anal. Chem., 1, 233 (1970).
357.8 nm
-
Wavelength
Figure 6. Atomic fluorescence spectra of Cr (1000 pg m1-I) in wavelengthrange of 357-361 nm Monochromator conditions: wavelength set at 359 nm; spectral bandwidth about 4 nm. Other experimental conditions are as in Table I Ca, Cr, Fe, G a , In, Mn, and Sr in H2/air flame are given in Table I. The pertinent experimental conditions for Figures 7 and 8 and for the detection limits in Table I are given in Table I as footnotes. The laser excited atomic fluorescence detection limits for AI, Ca, Cr, In, and Sr are of the same order of magnitude or better than any previous atomic fluorescence flame spectrometric values obtained with any source type. The values for Ga, Fe, and Mn are definitely inferior to previously reported values obtained with line sources; no explanation can be given for these diverse values. The value for Ti is the first reported atomic fluorescence limit of detection. All detection limits obtained with laser excitation are lower than previously reported values obtained with continuum high pressure gas discharge lamps. The laser excited atomic fluorescence limit of detection for Ti is
Figure 7. Analytical curves for laser excited atomic fluorescence of Ca, Cr, Fe, Ca, In, Mn, and Sr in "/air flames Experimental conditionsare same as in Table I o Chromium m Strontium 0
Golllum
a Iron
o Monpanrrr
ANALYTE CONCENTRATION (pg/ml)
ANALYTICAL CHEMISTRY, VOL. 43, NO. 12, OCTOBER 1971
1695
Figure 8. Analytical curves for laser excited atomic fluorescence of AI and Ti in an C2H2/ NzO flame Experimental conditions are same as in Table I
10'1
1
1
10-1
10-2
ANALYTE
I
I
I
IO
I
I02
I
lo3
I
CONCENTRATION @g/ml)
Table I. Detection Limits for Several Elements by Laser Excited Atomic Fluorescence Flame Spectrometry Element-line (nm) Al-396,l Ca-422.7 Cr-359.3 Fe-372.0 Ga-403.2 In-410.4 Mn403.1 Sr-460.1 Ti-399.8
Flame conditions (gas flow rates I./min). N*O-C*H* Air-H, 14.5-6.5 ... 7.5-15.2 ... 7.5-1 1 . 8 ... 7.5-1 1 . 8 ... 7.5-20.0 ... 7,5-18.3 ,.. 7.5-20.0 , . . 7,5-15.2 14.5-6.5 ... 0
.
Monochromator slit width: nm 1.44 2.40 1.44 2.40 1.44 2.40 1.44 1.44 1.44
Laser Dye No. 17166-B 17/52 17/66-D 17166-D 17166-B 17166-B 17166-B 17/53 17166-B
Detection limit; rg/ml 0.03 0.01 0.03 0.3 0.3 0.01 0.3 0.03 0.1
All measurements taken at 2.5-3.0 cm above burner top. Other experimental conditions: photomultiplier voltage - 900 V; boxcar integrator conditions: 50-ohm input; >50-MHz bandwidth; dc coupling; 10-nsec aperture gate width; 10-nsec aperture time constant; AVCO dial-a-line laser: IO-Hz repetition rate, 17 kV. c Concentration resulting in signal-to-noise ratio of 2. a
b
Table 11. Comparison of Detection Limits by Atomic Flame Spectrometry Element
AFa (laser)
AI Ca
0.03 0.01
Cr
0.03
Fe Ga In
0.3 0.3
Mn Sr
0.01 0.3 0.03
Ti
0.1
AFLb
AFCb
AEC
AALc
(2)
(7)
( 2 , 7)
(2, 7)
0.005 o.Ooo1 0.005
0.04
0.1
.
..
0.02 0.05 0.008 0.01 0.1
0.1 10.0
0.006
...
0.03
...
...
1.0
0.05
5.0 2.0
0.01 0.005 0.005 o.Ooo2 0.02
,
..
o.Ooo5 0.005 0.005
0.07 0.05 0.002
0.004 0.1
this study. These values are obtained under good but not necessarily optimized conditions. b AFL = Atomic fluorescence flame spectrometry excited with narrow line sources. AFC = Atomic fluorescence flame spectrometry excited with continuum sources. Detection limits are best reported values by a number of workers and the specific references can be found in the general references listed after the methods. AE = Atomic emission flame spectrometry. AAL = Atomic absorption flame spectrometry with narrow line sources. Detection limits are best reported values by a number of workers and the specific references can be found in the general references listed after the methods. I(
1696
0
comparable with the best previously reported value by atomic absorption and atomic emission flame spectrometry. The laser excited atomic fluorescence flame spectrometric detection limits for Al, Cr, and In are within about 10-fold and for Ca, G a , Mn, and Sr are within about 100-fold of the best previously reported detection limits by either atomic absorption or atomic emission flame spectrometry (see Table 11). The laser excited atomic fluorescence limits of detection could certainly be improved by IO- t o 100-fold by a more extensive optimization of experimental conditions. These initial experimental results indicate possible use of dye lasers as sources of excitation in atomic fluorescence flame (or nonflame) spectrometry. With the dye laser source, it is now possible t o obtain low detection limits, freedom from flame background noise, and long linear analytical curves for many elements (most elements can be excited by the dye Iaser--i.e., relatively few elements have no sensitive spectral absorption lines above 360 nm). By frequency doubling, it may be possible t o extend the wavelength range down t o nearly 200 nm. RECEIVED for review April 12, 1971. Accepted June 11, 1971. Research sponsored by AFOSR (AFSC), U.S.A.F.Grant NO. 70-1880B.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 12, OCTOBER 1971
