Resonance Flame Atomic Fluorescence Spectrometry with Continuous Wave Dye Laser Excitation Robert B. Green,’ and John C. Travis* Analytical Chemistry Division, Institute for Materials Research, National Bureau of Standards, Washington, D. C.20234
Richard A. Keller2 Physical Chemistry Division, Institute for Materials Research, National Bureau of Standards, Washington, D.C.20234
The potential contribution of continuous wave (CW) dye laser excitation of resonance atomlc fluorescence for analytlcal flame spectrometry was evaluated. Noise sources which were related to fluctuations In the flame and in the laser scatter were responsible for the observed detectlon llmlts which were an improvement over those achieved with previously used excltation sources for flame atomlc fluorescence and comparable to results for flame emlsslon spectrometry. Based on experimental data, It was estimated that continuous wave excltatlon powers of 1-10 watts would be sufficient to bring the slgnalto-noise ratio into a constant range. With the exceptlon 0f nonresonant detection and saturation contrlbutions to reduced quenching, higher laser powers common to pulsed sources are of no advantage and, in fact, are detrimental to Sensitivity. Other aspects of CW dye laser excltatlon are presented and discussed.
Recently, laser-excited fluorescence has been shown to be the most sensitive technique for the determination of atomic sodium (1-3). Concentrations as low as 100 atoms/cm3 were detected by Fairbank et al. using continuous wave (CW) dye laser excitation ( 3 ) .This paper investigates the use of tunable CW dye laser emission for the excitation of resonance flame fluorescence for the analytical determination of atomic species. The results reported here for barium are comparable to sensitivities for flame atomic emission spectrometry (FAES) and better than those reported for flame atomic absorption spectrometry (FAAS) and conventional source pr pulsed laser-excited flame atomic fluorescence spectrometry (FAFS) ( 4 ) . Until now, published detection limits for FAAS and FAFS have generally not been competitive with FAES for elements whose analytical lines are above 500 nm because of their relativity high excited state populations in flames. Significantly, this study also demonstrates that the observed detection limits with CW dye laser excitation are not imposed by any inherent deficiencies of the fluorescence technique but by the inadequacy of the flame as an atom reservoir. The practical, analytical potential of flame atomic fluorescence spectrometry was first demonstrated in J. D. Winefordner’s laboratory in 1964 (5, 6 ) . Certain intrinsic advantages for the analysis of metals have been well documented (7) but the technique has gained limited popular acceptance, partially because of the inadequacy of conventional excitation sources. The development of high intensity, tunable laser sources in the past decade offered the possibility that their use would yield a significant improvement in sensitivity and detection limits. Extensive work with pulsed systems (8-13) -both laser pumped and flashlamp pumped-has demonPresent address, Department of Chemistry, West Virginia University, Morgantown, W. Va. 26506. Present address, CNC-2, Los Alamos Scientific Laboratory, Los Alamos, N.M. 87545. 1954
strated that, indeed, laser excitation is a significant improvement over conventional excitation, but the improvement was far less than initially expected. In fact, in most cases, the lowest flame detection limits were still obtained by FAES and FAAS. Some of the problems associated with pulsed systems may be the cause of the less-than-anticipated results, e.g., radiofrequency (rf) noise, poor pulse-to-pulse repeatability, difficulty in attaining and maintaining a narrow bandwidth and accurate wavelength,and data handling of low duty cycle, pulsed outputs. It is true that careful attention to experimental details of complex laser technology and data acquisition may reduce these problems, but in practice they have contributed to the results reported in the literature. In contrast to pulsed systems, continuous wave (CW) dye lasers possess none of the undesirable characteristics described above. The experiment of Fairbank et al. ( 3 ) ,reporting fluorescence from 100 sodium atoms/cm3, utilized properties difficult or impossible to achieve with pulsed lasers. The laser was operated a t a bandwidth of -0.005 nm and frequency modulated. The narrow bandwidth maximized the ratio of fluorescence signal to scatter, and frequency modulationcoupled with synchronous detection-provided electronic discrimination against wavelength independent scattering. In the experiment discussed above ( 3 ) , the sample was contained in a long absorption cell a t reduced pressure, eliminating quenching and collisional broadehing and producing a relatively scatter-free environment and well equilibrated, low noise, sample concentration. Although such a sample environment is not compatible with the types of solution analysis generally performed by flame methods, it would be appropriate for such analytical applications as isotope ratio determination and air pollution analysis at reduced pressure. It may be estimated, based on the results of Fairbank et al., that linewidths associated with atmospheric pressure and high temperature plus collisional quenching will result in a sensitivity loss of 102 to lo4 for flame or furnace applications assuming “ideal” atomization. One remaining advantage of pulsed dye lasers over CW dye lasers, at present, is the (expanded fundamental and frequency-doubled) wavelength range available. The CW system utilized in this study is limited to a range of -540 to 680 nm by the 5-watt argon ion pump laser. However, with commercial lasers currently available, dye laser tuning ranges of 420-800 nm have been reported. In the mid- to near-ultravioletwhere most atoms have strong resonance transitions and atomic fluorescence has decided advantages over emissionpulsed lasers have the advantage in frequency doubling efficiency. On the other hand, CW frequency doubling has been widely demonstrated, and usable doubled output powers in the milliwatt range are expected in the near future. Although such power levels seem low when contrasted to kilowatts of pulsed uv, the average power, or the integrated energy per second, is comparable, because of the low duty cycle of pulsed systems. Furthermore, the Fairbank et al. (3) experiment, discussed above, utilized a greatly attenuated beam yielding
ANALYTICAL CHEMISTRY, VOL. 48, NO. 13, NOVEMBER 1976
MONOCHROMATOR
n II
9 c w iAsER
CW DYE LASER
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LASER OFF
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Data collection format for a typical data point with on-resonance, fixed wavelength operation. The flame used for this point was
Figure 2.
H2-02-Ar
Figure 1. Block
diagram of experimental configuration. Laser beam indicated by heavy solid line
Table I. Flame Composition
Flow rates. l./min
Components 3 pW, to avoid biasing the sodium ground state distribution a t low temperatures and pressures. This and the work reported here indicate that the importance of laser power is generally over-rated in relation to other parameters such as bandwidth, frequency modulation, and amplitude stability. Laser excitation offers other advantages over conventional sources, in addition to those of sensitivity and detection limit. The continuous wavelength scan capability of both pulsed and CW dye lasers combines the wavelength scan advantage of the conventional continuous source (14)with the high intensity, narrow spectral bandwidth properties of the conventional line source, but with higher intensity and better spectral purity. A high resolution laser-excited line profile may be used to monitor sample linewidth and to check for spectral interferences and self-reversal. Because of its narrow bandwidth and high intensity, frequency selective detection may often be avoided altogether for low background resonance fluorescence studies with the CW laser. Interference filters suffice for high background flames and/or nonresonance fluorescence studies.
EXPERIMENTAL A block diagram of the experimental configuration is shown in Figure 1.The burner was the capillary type similar to those described by Aldous e t al. (15). The head consisted of 86 stainless steel capillaries (0.70-mm id.) close packed in an 11-mm diameter cylinder. The diameter of the capillary was chosen to provide a burner head which would support a variety of flames and therefore its performance was slightly compromised for some fuel-oxidant combinations. A sheath for inert gas was also constructed from the same capillaries to fit the burner head and was used in conjunction with a Pyrex chimney to prevent entrainment of laboratory air for the sodium experiments. The sheath alone resulted in no significant improvement in data. The burner head was incorporated into a pre-mixing chamber with sample aspiration through a variable flow nebulizer. A fully adjustable mount was employed to optimize the burner position for each fuel-oxidant mixture. The fuel-rich hydrogen-oxidant (air or argon-oxygen) flame is particularly sensitive to the position of the inner cone in relation to the exciting laser beam. (The smaller cones produced by the individual capillaries of the burner head coalesce under fuel-rich conditions to produce a single inner cone.) As the viewing position in the flame is lowered, after the optimum position is passed, the scattered light from the inner cone increases relative to the fluorescence. At the optimum position, the fluorescence-to-scattering ratio is a maximum. The viewing height was 32 mm above the burner head which translates to approximately 6 mm above the tip of the inner cone for the stoichiometries listed in Table I for the hydrogen-oxidant flames. This
Ha-air H2-Ar-02 CzHz-air
Fuel
Oxidant
13.5 4 3.5
7.1 8.4 (91%Ar, 9% 02) 8.4
Sheath Ar
1.8
ratio was found to have a less critical spatial dependence for the acetylene-air flame. The gas handling system consisted of a panel manifold with rotameters for gas flow metering and valves to allow the use of a variety of fuel-oxidant combinations. Argon was used exclusively in the sheath. Table I gives specific information about gas combinations and flow rates. The excitation source was a commercial CW dye laser which was longitudinally pumped with powers up to 5 watts by an all-line argon ion, CW laser. The dye laser uses an intra-cavity birefringent filter for coarse-tuning the dye laser, with a resultant bandwidth of 0.03 nm, and a 0.5-mm etalon to further narrow the output to 0.003 nm. The etalon tilting mechanism was driven by a stepping motor to wavelength-scan the dye laser output over a -0.05-nm range. A PIN photodiode was calibrated vs. a calorimetric power meter and positioned to monitor the power. Laser-induced fluorescence was detected in both nondispersive and dispersive modes. Nondispersive experiments were done for sodium with and without a 1-nm bandwidth interference filter using an Oriel 7060 photomultiplier (S-5 photocathode) of the common nine-stage, side-window variety. For barium experiments, a 0.5-m monochromator was used in conjunction with a thermoelectrically cooled detector housing and an EM1 6256RF photomultiplier (S-ll).The fluorescence was focused on the monochromator slit with a 10-cm focal length quartz lens symmetrically placed between the flame cell and monochromator. The dispersion of the monochromator was 1.6 nm/mm and the bandpass was 0.32 nm with the 200-fim slits used. The laser beam was mechanically chopped a t 2 kHz and the fluorescence synchronously detected to discriminate against flame background and emission. The analytical signal was normalized by ratioing vs. the CW laser power and the resulting signal converted to frequency. The normalization was employed to negate fluctuations in the dye laser power. The pulses were counted in a multichannel analyzer, multiscaling a t 1 s per channel. The data were then processed by computer to give analytical curves, line profiles, and statistical analysis. Figure 2 shows the data collection format for a typical data point with on-resonance, fixed wavelength operation. Means and estimated standard errors from 25 inner data channels (each representing a 1-s integration) were obtained for the 30-channel segment with sample aspirated and the similar segment with the blank solution aspirated. The difference and propagated error represent the analytical signal. For frequency scanned operation (see Figures 5 and 6), peak heights and uncertainties were obtained by fitting near reso-
ANALYTICAL CHEMISTRY, VOL.48, NO. 13, NOVEMBER 1976
1955
I
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Ba CONCENTRATION Ingirnll
Figure 3. Barium fluorescence at low concentrations. The outer vertical axis relates to the data for fluorescence from barium in a H2-02-Ar flame I
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Figure 5. Wavelength scan of barium fluorescence at low concentrationsh a H2-02-Ar flame, X- 0 ng/ml, 0 20 ng/ml, 0 50 ng/ml, A 75 ng/ml, 100 ng/ml
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+-
Table 11. Fluorescence/Emission Ratios, 100 mW Incident Power Element Ba Ba Na
.01
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10
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Ea CONCENTRATION liiglmll
Figure 4. Barium fluorescence showing the dynamic range of the technique. The displacement of the two curves is arbitrary due to different experimental conditions nance channels to a parabola for comparison with a similar analysis for the blank. Commercially available,purified laser dyes were dissolved in ethylene glycol. Rhodamine 110 had a wavelength range from 535 to 620 nm with a tuned power of 100 mW at 553.5 nm. Rhodamine 6G had a wavelength range from 570 to 650 nm with a tuned power of 300 mW at 589 nm. All chemicals used were reagent grade. Sodium standards were prepared with highly purified water from a fused silica nonboiling still and stored in Teflon bottles. All other standards were prepared with distilled water which had been passed through a mixed-bed,ion exchange resin.
RESULTS Two elements (barium and sodium) that were amenable to fluorescence excitation with a CW dye laser were chosen for study. The gf values a t their excitation wavelengths are given as a measure of their respective absorption cross-sections. Barium. Excitation wavelength 553.5 nm, gf = 1.5. The fluorescence intensity as a function of barium concentration is shown in Figure 3 for two different flames. The dynamic range of this technique is shown by the log-log plot in Figure 4. The fluorescence excitation profiles a t several concentrations are shown in Figure 5 . The ratio of the fluorescence intensity to the flame emission was measured in a dc, nonchopped mode. The results are tabulated in Table 11. Flame emission had a negligible conribution to the observed fluorescence signal when the excitation beam was chopped and the emission synchronously detected. 1956
Flame HZ-Oz-Ar CzHz-air HZ-Oz-Ar
Fluorescence/ emission 3000 35 730
A monochromator was used for the collection of the data described above. Lack of an appropriate filter occasioned the use of the monochromator but there were no indications that the fluorescence could not be treated in the nondispersive mode (see below). The solutions for the CzHz-air flame contained 1000 pg/ml (ppm) of potassium to suppress ionization of the barium. Ionization buffers did not increase signal-tonoise ratios in the Hz-Oz-Ar flame and were not used in this case. Figures 3 and 4 show that a linear relation exists between the fluorescence intensity and the concentration of barium. The sensitivity compares well with the results of other flame analysis techniques (16-18). In all cases, the laser induced fluorescence was easily visible above the flame emission. Sodium. Excitation wavelength 589.0 nm, gf = 1.3. The fluorescence intensity as a function of sodium concentration is shown in Figures 6 and 7 for a H2-02-Ar flame. Figure 6 displays line profile peak values, uncorrected for background contamination. The nonzero intercept on the ordinate represents the presence of an approximately 9-11 ng/ml contamination in all solutions. Fluorescence line profiles for the HZ-Oz-Ar flame a t high concentrations are shown in Figure 8. In these measurements, the sodium fluorescence was focused directly onto a photomultiplier tube. It was found that no intervening frequency selective devices were necessary (nondispersive mode). The data for Figure 7 were collected with the laser held a t the resonance wavelength (as in Figure 2), while the data for Figure 6 were obtained by scanning the laser wavelength. No significant difference in the results was observed between the fixed frequency technique and an analysis of the line profiles although the former permitted a better analysis of the noise contributions. The line profiles in Figure 8 are particularly interesting. They clearly show the effects of self reversal and demonstrate the value of line profile analysis.
ANALYTICAL CHEMISTRY, VOL. 48, NO. 13, NOVEMBER 1976
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Figure 6. Analytical curve for sodium fluorescence at low concentrations in a H2-02-Ar flame
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Flgure 7. Analytical curve for sodium fluorescence showing the dynamic range of the technique
Table
Naa a
CWFAFS 8 (CzHz-Air) 2 (Hz-OZ-Ar) 2 (Hi-Oi-Ar)
FAES (16) FAAS (17) 30 (C~HZ-02) 50 (C~HZ-N~O)
...
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WAVELENGTH lnrnl
Figure 8. Wavelength scans of sodium fluorescence showing the onset of self-absorption. ( a ) 100 pg/ml, (6) 50 pg/ml, ( c ) 20 pg/ml, (d) 10 pg/ml. All four concentrations are shown to the same scale
111. Limits of Detection, ng/ml
Element Ba
588.98
...
2 (CzHn-Air)
Profile data.
The ratio of fluorescence intensity to the flame emission for a range of concentrations from 0.01 to 50 wg/ml for multiple samples is included in Table 11. It should be pointed out that the data in Table I1 are meant only to indicate a gross relationship between laser-excited fluorescence and thermallyexcited emission. The data in Table I1 were corrected for contributions from dark current and flame background. The apparent large discrepancybetween the fluorescence/emission ratios shown in Table I1 for sodium and barium in the same flame may be accounted for by the narrower barium linewidth, the diminished barium emission due to the Boltzmann population of its relatively higher energy level (18 060 cm-l vs. 16 973 cm-' for sodium), and the branching dilution via fast collisional equilibration of the D1 and D2 states of the sodium fluorescence. There is sodium contamination in a normal laboratory atmosphere that is a significant complication for trace analysis. Entrained laboratory air showed visible fluorescence at a flame-sheath interface under laser excitation without sample aspiration. The addition of a Pyrex chimney with the argon sheath improved the situation, but it was still difficult to maintain standards free of low sodium contamination (-10 ng/ml). The scatter of the data points of Figure 6, which is greater than the calculated short term errors of the individual points, reflects variation in the contamination. This con-
tamination variability was responsible for the relatively large minimum detectable quantity for sodium (see Table HI), considering the relative sodium/barium free atom fractions -1.00/0.002 (19). Noise. Determination of the noise source responsible for the ultimate limit of detection required both a detailed statistical analysis of the data presented above and a separate set of experiments in which the laser power was varied. For barium in the low background, high fluorescence yield H2-02-Ar flame, the noise was found to vary as the square root of the sample concentration up to about 150 pglml, with a linear concentration dependence becoming predominant at higher concentrations. In the high background, low fluorescence yield CzHz-air flame, the noise level was relatively constant until a concentration of about 15pg/ml of barium was reached, varied as the square root from about 15 to 90 pg/ml, and was linear at higher concentrations.Both sets of data were obtained with approximately 100 mW of laser power at 553.5 nm. A t 100 mW of laser power, laser scatter was scarcely evident above the flame background contribution to the signal for the H2-02-Ar (or H2-air) flame, and not evident at all in the C~Hz-airflame. In order to investigate the potential advantage of higher power dye lasers, scattering experiments were performed with the 514.5-nm line of a 5-watt argon ion laser and the H2-air flame. (No significant power dependence was observed for the CaHz-air flames.) Below about 300 mW of laser power, the noise level was relatively constant. A brief square root dependence on laser power was evident from 300 to about 400 mW, with a linear dependence dominating at higher powers. No attempt was made to eliminate the possi-
ANALYTICAL CHEMISTRY, VOL. 48, NO. 13, NOVEMBER 1976
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bility of accidental coincidences with molecular bands of flame species, or to compare the Hpair flame with the H2-02-Ar flame.
DISCUSSION Detection Limits. The concentrations corresponding to a signal-to-noiseratio of 2 are listed in Table I11for the present experiment (CWFAFS) as well as the best literature values for FAES and FAAS. A signal-to-noise ratio of 2 corresponds to the detection limit defined by St. John et al. (20) and represents a 95% confidence level. The barium results are perhaps more representative of the potential of the technique because of the sodium contamination problems. It is difficult to find comparable values for other FAFS measurements because, in this wavelength region, atomic emission has been much better than other methods and excitation sources have been unsatisfactory for barium and sodium (18).It is impressive that the present FAFS detection limits are comparable to FAES detection limits. This is reasonable because it is flame properties which ultimately limit both methods. In particular, the laser induced fluorescence results could be significantly improved by using other atomization techniques (see below). The improvement of CW laser induced fluorescence measurements over conventionally induced fluorescence measurements is not surprising since the intensity of this CW dye laser (-5 W/cmz) is lo4 times greater than the intensity of a typical sodium line source ( 5 X W/cm2) with normal focusing optics (0.05 sr) (21). It is interesting to note that the detection limits reported in Table 111 correspond to flame concentrations of approximately IO7 atoms/cm3 for sodium and barium (22). Reproducibility. Inspection of the data in Figures 3,4,6, and 7 shows that some individual points deviate from the fitted curve by slightly more than would be expected from the statistical analysis of their short term fluctuations. These larger deviations are due to long term drifts of laser wavelength which could be eliminated by stabilization through feedback mechanisms and to changes in laser intensity which are difficult to compensate for. In the case of sodium, an additional source of scatter in the data is variations in the contamination level. The results reported here were obtained on a prototype instrument; therefore, it would be reasonable to expect improved results for a carefully designed, dedicated CW laser spectrometer. Noise Analysis. The lack of significant improvement over flame emission is initially somewhat surprising since our measurements (Table 11) showed the fluorescence intensity to be much larger than the emission intensity for sodium and barium. These results prompted the detailed analysis of the sources of noise in resonance flame fluorescence. A t normal laser power (100 mW), the limiting noise at low concentrations was found to be principally shot noise due to the low level of the fluorescence from the sample for the H2-0z-Ar flame and flame background for the CaHz-air flame, although both components made some contribution in both flames. The flame background noise is composed of shot noise and flicker. The relative importance of these two components was not determined and does not affect our conclusions. In the CzHz-air flame, at concentrations -20 pg/ml, and normal laser powers, concentration-related shot noise contributions appear. The linear contribution to the noise becomes evident at concentrations R 150 hg/ml in both flames. The dye laser had a tuned output of about 100 mW. It was of interest to see if the effect of the limiting shot noise, flame noise, and laser scatter noise components on the signal-tonoise ratio could be reduced by going to higher laser powers and to find the power above which a further increase would 1958
not result in an improvement. The 514.5-nm emission from an argon ion laser was used to determine the dependence of the noise in the laser scatter from the Hz-air flame as a function of laser power. Since the short term laser amplitude stability is better than 1%,this noise is composed of scatter-related shot noise and fluctuations (“flicker”) in the scattering medium ( o t l ) . It was found that a t -500 mW, the shot noise component is equivalent to the scatter-related flicker. Above -3 watts, the shot noise term is less than 15% of the flicker component and the noise is due to just the fluctuations in the scattering medium (aspirated H20 into the flame). A t this point, a further increase in laser power will not improve signal-to-noise ratios. Although the actual power at which flicker noise takes over depends upon the detection optics and electronics, it is important to note that this power will be in the 1-to 10-W range and there is no advantage in signal-to-noise ratio by going to power levels in the kW range. There are two important exceptions to this conclusion. When the emission intensity is largely determined by quenching collisions in the flame, high excitation powers cause saturation of the atomic transition and a reduction of the effects of quenching at a cost of increased laser scatter (23). An even more important exception occurs when nonresonance detection of the fluorescence is used. In this case, the effect of laser scatter is greatly reduced and higher laser powers may be advantageous to overcome the limiting noise sources-depending upon their origin. Although some atomic species can be advantageously analyzed by nonresonance fluorescence techniques, resonance fluorescence remains the most common and relevant for analytical purposes. The appearance of fluctuation-related noise factors in both the concentration and power dependence noise studies underscores the inherent limitation of the flame as an effective atom reservoir for analytical flame spectrometry. Large improvements would be expected from more controlled atomization sources, e.g., furnaces and sputtering sources. Similar, fluctuation-related noise sources ultimately determine the detection limits in flame emission and, therefore, it is reasonable that the detection limits are similar for both techniques. Comparison to Pulsed Laser Excitation. It is instructive to compare the use of medium power (100 mW-1 W) CW tunable lasers with high power (1-100 kW) tunable, pulsed lasers. The difficulties in processing pulsed signals, eliminating rf noise associated with the pulse, frequency narrowing the laser output, and pulse-to-pulse intensity variations inherent in pulsed systems have plagued investigations in this area. These problems may be minimized by careful laser and detector design, but they impose significant experimental difficulties. A point often made by proponents of high power excitation is that, in the presence of saturation of the analytical transition, laser intensity fluctuations become unimportant (23). This is true, but it only cures a problem introduced by the instability of that particular source and at the same time grossly increases the amount of intensity dependent scattered light present in the system. Saturation does decrease the effect of quenching and, in some cases, this might be an important consideration. None of these detrimental characteristics are possessed by CW systems, and in cases where the two techniques can be compared, significantly lower detection limits are found with CW excitation. An important limitation of the CW system is the wavelength range currently available (420-800 nm). Only 14 elements have commonly used analytical lines within this wavelength region. This limitation is only temporary. Frequency doubling techniques for tunable CW systems are under development and soon tunable CW radiation will be available down to 220 nm a t power levels on the order of milliwatts. Dynamic Range. The above discussion has primarily
ANALYTICAL CHEMISTRY, VOL. 48, NO. 13, NOVEMBER 1976
treated detection limit aspects of laser excited atomic flame fluorescence. A fairly obvious advantage of laser excitation is the improvement in dynamic range over conventional FAFS sources which accompanies the improvement in detection limit. The high concentration end is limited in both cases by self-absorption, but the ability to observe line profiles removes the ambiguity associated with double-valued working curves (see Figure 7) and allows the use of the full double-valued fluorescence curve, with the line shape (see Figure 8) indicating which side of the maximum should be used. A limited shift of the linear region of the working curve to higher concentrations can be attained by irradiation in the absorption wings. Resolution. The intensity and spectral purity of the CW dye laser extends the well known property of line source FAFS of requiring little, if any, dispersive analysis of the fluorescence radiation. The sodium data presented here were acquired with no wavelength selective elements in the detection optics. In general, use of a filter or monochromator is desirable for eliminating laser scatter when nonresonance fluorescence may be observed and reducing the flame background contribution to the limiting noise. It must be stressed that the selectivity, or freedom from spectral interferences, of the method is provided by the laser, not by the wavelength selectivity of the detector. A laser, operating at 600 nm with a 0.003-nm bandwidth, offers a spectral resolution of 2 X lo5, comparable to the very best spectrographs. The additional resolution possible by operating the laser single mode (bandwidth nm) provides resolution far in excess of that required for normal analytical analysis but is potentially useful for special analytical problems, e.g., isotope analysis.
ACKNOWLEDGMENT The authors thank T. J. Murphy and J. R. Moody of the Analytical Spectrometry Section for providing the highly purified water.
LITERATURE CITED (1)D. A. Jennings and R. A. Keiler, J. Am. Chem. Soc., 94,9249 (1972). (2) S.Mayo, R. A. Keller, J. C. Travis, and R. B. Green, J. Appl. Phys., in press.
(3)W. M. Fairbank, Jr., T. W. Hansch, and A. L. Schawlow, J. Opt. Soc. Am., 65, 199 (1975). (4)C. Th. J. Alkemade, "Nomenclature, Symbols, Units and their Usage in Spectrochemical Analysis. HI. Analytical Spectroscopy and Associated Procedures", IUPAC, 1972. (5)J. D. Winefordner and T. J. Vickers, Anal. Chem., 36, 161 (1964). (6)J. D. Winefordner and R. A. Staab, Anal. Chem., 36, 165 (1964). (7)R. F. Browner, Analyst(London), 99, 617 (1974). (8)M. 8.Denton and H. V. Malrnstadt, Appl. Phys. Lett., 18,485 (1971). (9) L. M. Fraser and J. D. Winefordner, Anal. Chem., 43,1693 (1971). (10)J. Kuhl and G. Marowsky, Opt. Commun., 4, 125 (1971). (11)J. Kuhi and H. Spitschan, Opt. Commun., 7, 256 (1973). (12) N. Omenetto, L. M. Fraser, and J. D. Winefordner, Appl. Spectrosc. Rev., 7, 147 (1973). (13) H. L. Brod and E. S.Yeung, Anal. Chem., 48,344(1976). (14)C. Veillon, J. M. Mansfield, M. L. Parsons, and J. D. Winefordner, Anal. Chem., 38, 204 (1966). (15)K. M.Aldous. R. F. Browner, R. M. Dagnall, and T. S. West, Anal. Chem., 42. 939 - - (1970). (16)V. A. Fassel and D. W. Golightiy, Anal. Chem., 39,466 (1967). (17)J. D. Wlnefordner, V. Svoboda, and L. J. Cline, Crlt. Rev. Anal. Chem., 233 (1970). (18)T. S.West and M. S.Cresser, Appl. Spectrosc. Rev., 7, 79 (1973). (19)M. L. Parsons, B. W. Smith, and G. E. Bentley, "Handbook of Flame Spectroscopy", Plenum Press, New York, N.Y., 1975,p 396. (20)P. A. St. John, W. J. McCarthy, and J. D. Winefordner, Anal. Chem., 39, 1495 (1967). (21)J. D.Winefordner, M. L. Parsons, J. M. Mansfield,and W. J. McCarthy, Anal. Chem., 39, 436 (1967). (22)J. D. Winefordner and T. J. Vickers. Anal. Chem.. 36, 1939 (1964). (23)E. H. Piepmeier, Spectrochim. Acta., Part B, 27, 431 (1972).
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RECEIVEDfor review April 9,1976. Accepted July 29,1976. This research was conducted under the auspices of the NBS Laser Chemistry Program. One of the authors (RBG) was supported by an NRC-NBS Postdoctoral Research Associateship. In no instance does the identification of commercial products imply recommendation or endorsement by the National Bureau of Standards. Presented in part at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 1976.
Detection of Lead via Lead-2O7m Using Cyclic Activation and a Modified Sum-Coincidence System Anthony Egan and Nicholas M. Spyrou" Department of Physics, University of Surrey, Guildford, Surrey, England
Samples containing lead were Irradiated in a reactor, counted on a detector, and the process repeated for an optimum number of cycles In order to determine the concentration of the element present. The 0.8s Isomer of 207mPbwas detected using two 7.5 cm X 7.5 cm Nal (TI) crystals operating in a modifled sum-colncldence system. The efflciency and resolution of the system were measured using a standard source. The sensltlvlty and detection limits for lead In various matrices of environmental interest were obtained. The sensitivity in an interference free matrix was found to be 5 pg.
Lead is well known as a toxic element and its distribution as a pollutant is widespread. Monitoring of lead levels in environmental and biological materials is carried out routinely using spectrophotometric methods which are necessarily destructive. Lead is not a good candidate for thermal neutron
activation analysis because of low reaction cross-sections and the nature of the radioisotopesproduced on irradiation, Table I ( 1 ) .However, it would be most convenient if lead could be detected and its concentration measured by instrumental neutron activation analysis (INAA) as is done for many other toxic elements of significance. The method of cyclic activation employed in this study was suggested by Givens et al. ( 2 ) for use with a pulsed neutron source and applied by Spyrou et al. ( 3 )with a fast transfer system on a reactor facility allowing repeated irradiation and counting of samples to be performed with a transit time of approximately l-s duration. It is therefore possible to detect short-lived isotopes using cyclic activation and for lead concentrations to be determined by measurement of the y rays emitted from 207mPb.The aim of the work was to provide and develop a system which would allow lead to be included in a nondestructive INAA scheme for environmental/biological samples ( 4 ) .
ANALYTICAL CHEMISTRY, VOL. 48, NO. 13, NOVEMBER 1976
1959
