- Email: [email protected]
Wavelength modulation diode laser atomic absorption spectrometry in analytical flames
Specrrochrmrco
Acra, Vol. 498. Nos 12-14. PP 1463-1472. 1994 Copyright 0 1994 Elsevier Saence Ltd Pnnted in Great Britarn. All rights reserved 0x4-8.547/94 57 00 + .cn
0584-8547(94)ooo80
Wavelength
modulation
diode laser atomic absorption analytical flames*
spectrometry
in
H. GROLL,~ CH. SCHN~~RER-PATSCHAN.? YIJ. KURITSYN$and K. NIEMAX@ Institut fur Spektrochemie und Angewandte Spektroskopie an der Universitlt Dortmund, BunsenKirchhoff-Str. (Received
11, D-44139 Dortmund,
3 May 1994; accepted
Germany
19 July 1994)
Abstract-The potential of wavelength modulation laser atomic absorption spectrometry in analytical flames is demonstrated by the measurement of titanium, cesium and chromium applying fundamental and frequency doubled radiation of commercially available semiconductor diode lasers. In dependence on the radiation power, absorbances of the order of 10-4-10-6 are measured, which reveal very low detection limits even when weak absorption lines are used.
1. INTRODUCTION ATOMIC absorption spectrometry (AAS) in flames is a relatively old technique for element analysis. It is still popular and widely used in spectrochemistry (see, for example, Ref.[l] and references therein). The popularity of flame-AAS would be even higher if lower detection limits could be obtained with this technique. In routine analysis with commercial instruments, the detection limits are typically given by the characteristic concentration (1% absorption which corresponds to 4.4 x lop3 absorbance). The performance can be improved, with some effort, by optimization and stabilization of the instrumental parameters and by measurements with long integration times [l]. However, if low concentrations have to be determined, the more sensitive graphite furnace-AAS rather than flame-AAS is preferred in routine analysis. This paper will show that there is a powerful method which yields detection limits in flame-AAS as low as in conventional graphite furnace-AAS. It is the technique of wavelength modulation-laser atomic absorption spectrometry (WM-LAAS) with semiconductor diode lasers. As discussed extensively in the literature (see, for example, the review by SILVER [2]), wavelength modulation spectrometry can eliminate the l/f noise in absorption measurements and reduce the total noise to the shot noise limit. This means that, with optimum conditions, absorbances of the order of 10-7-10-8 can be detected in wavelength modulation spectrometry. Recently, the analytical potential of WM-LAAS with diode lasers has been shown by our group [3, 41. In the first paper, rubidium, lanthanum and aluminium were measured in commercial graphite tube atomizers [3]. While rubidium and lanthanum were studied by absorption of fundamental diode laser radiation at 780.03 and 670.95 nm, aluminium was measured with deep blue light at 396.15 nm produced by second harmonic generation (SHG) in a non-linear crystal. The aluminium experiment at 396.15 nm in particular demonstrated the possibility of measuring about 50 elements in absorption [5] with blue and near-uv radiation generated by commercial laser diodes. The 3a-detection limits for WM-LAAS in a graphite furnace were 1 pg/ml,
* This paper was published in the Special Honor Issue of Spectrochimica Acfa Part B, dedicated to J. D. Winefordner. t Present address: Institut fiir Physik, Universitlt Hohenheim, Garbenstrasse 30, D-70599 Stuttgart, Germany. $ Institute of Spectroscopy of the Russian Academy of Sciences, 142092 Troitsk, Moscow Region, Russia. B Author to whom correspondence should be addressed. 1463
1464
H. GROLL etal. SHG dde
Fig. 1. Experimental
analytIca photodlodr 01 photomultqd~rr
r----------
arrangement with and without second harmonic generation box). Ll, L2 and L3 are lenses.
(in dashed
7.5 @ml and 0.1 ng/ml for rubidium, lanthanum and aluminium, respectively. The second paper reported on WM-LAAS measurements of metastable chlorine, fluorine and oxygen atoms in a low-pressure dc plasma [4]. It is the credit of the group of Prof. Winefordner to have performed the first absorption measurement with a semiconductor diode laser in analytical flames in 1990 [6]. They used a multimode laser diode for the measurement of rubidium. Multielement measurements in flames by simultaneous operation of six diode lasers and the extension of the dynamic range to high concentrations by wavelength tuning to the wing of the absorption lines have been demonstrated by our group [7]. This present paper will show that the technique of WM-LAAS can also extend the dynamic range of flame absorption spectrometry to lower element concentrations. We will show, as in the paper on graphite furnace WM-LAAS [3], that cesium and titanium can be measured successfully by absorption of fundamental radiation of commercial laser diodes at the resonance line and a weak absorption line, respectively. On the other hand, the use of SHG will be demonstrated in measurements of strong absorption lines in chromium and titanium. Furthermore, we will compare the 3a-detection limits obtained by WM-LAAS with the characteristic concentrations (1% absorption) reported for hollow cathode AAS in flames applying the most sensitive absorption lines, since in routine measurements the typical detection limits of hollow cathode AAS are of the order of the characteristic concentrations. It is known that the detection limits of hollow cathode AAS in analytical flames can be better than the characteristic concentration if long integration times are applied. However, very stable flames and sample introduction are required in such measurements which are difficult to realize in analytical routine laboratories.
2. EXPERIMENT The experimental arrangement for WM-LAAS, presented schematically in Fig. 1, has been already described in our recent paper on WM-LAAS in graphite furnaces [3]. Therefore, it will be only described shortly. However, the modifications necessary for WM-LAAS measurements in analytical flames are given here in more detail. Two different GaAlAs laser diodes were used, a Hitachi HL 8318 (power: 50 mW) and a Sharp LTO 16 (power: 30 mW). A commercial power supply (LDC 400 by Profile) provided stable wavelength tuning of the laser diodes by temperature and current. At constant temperature and current, the stability of the optical laser frequency was better than 50 MHz over several hours. Taking into account the widths of the pressure and Doppler broadened lines in the flame, the experimental wavelength stability was sufficient to avoid systematic errors by wavelength drifts. Cesium and titanium were measured by the D2 resonance line at 852.11 nm (oscillator strength
WM-LAAS
1465
in analytical flames
wavelength
-
Fig. 2. 2f profiles of the cesium DZ-line measured in an air-propane flame with three different modulation amplitudes. The cesium concentration was 1 @ml.
f = 0.71 [8]) and the weak absorption line at 842.65 nm (f = 0.0096 [9]), respectively. The initial level Ei of the titanium absorption line (Ei = 6661 cm-l) is a thermally populated level. The HL 8318 laser diode was used for both transitions. The HL 8318 was also applied in the measurements of chromium. Here, the radiation was frequencydoubled by focussing the laser light by a large aperture lens system with astigmatism compensation into a LiI03 crystal. This simple procedure provided up to about 100 nW at the strong chromium absorption line at 425.44 nm (f = 0.11 [lo]) applying about 50 mW fundamental power. Furthermore, titanium was not only measured by the weak near-ir line, but also by the strong transition at 399.86 nm (f = 0.093 [lo]). Radiation of this wavelength was generated in a shorter LiI03 crystal by the laser diode LTO 16. The SHG-power was about 50 nW. Different laminar burners by Techtron with pneumatic nebulization operated by a Varian flame controller were used. An air-propane burner (length 1 = 10 cm) was applied in the cesium experiment, while chromium and titanium were measured with an air-acetylene flame (I = 10 cm) and a N#-acetylene flame (I = 5 cm), respectively. The aqueous samples were prepared with deionized water from standard solutions (Cs: 1 mg/ml from CsCl in Hz0 by Riedel-de Haen, Cr: 10 mg/ml from CrCl, in dilute HCl by Merck, and Ti: 10 mg/ml from Tic& in 18% HCl). Sinusoidal current modulation generated by a Wavetek power supply (FG-5000) was added to the diode current. The amplitudes of the current modulation determined the maximum wavelength modulation. Modulation frequencies up to about 100 kHz were applied. The absorption of cesium and titanium in the near-ir spectral range was detected by a photodiode (Hamamatsu S 2386-5K), while a photomultiplier (Hamamatsu R 212) was used to measure the frequency-doubled radiation in the chromium and titanium experiment. In the latter case, an appropriate glass filter blocked the fundamental radiation of the diode laser. The detector signals were processed by a lock-in amplifier (Ithaca 3961B) in the 2f mode. The amplified signals were displayed by a storage oscilloscope (Hameg HM 205-3) with 8 bit resolution. For further processing the data were transferred to a personal computer.
3. EXPERIMENTALRESULTS AND DISCUSSION 3.1. Measurement of cesium Figure 2 shows 2f profiles of the cesium D2 line. The spectra different modulation amplitudes at a modulation frequency of concentration was 1 Fg/ml. The spectra were obtained by tuning a second Wavetek function generator continuously over the line.
were measured with 5 kHz. The cesium the diode laser with The splitting of the
1466
H.
GROLL et al.
modulatmn amphmde, GHz
Fig. 3. 2f WM-LAAS
signal intensity for cesium plotted against the modulation amplitude.
D2 line mainly due to hyperfine splitting of the ground state (9.192 GHz) is clearly visible. It is obvious that the spectral resolution is best if small modulation amplitudes are applied. However, to obtain large signals the amplitude has to be increased. The dependence of the signal on the modulation amplitude is plotted in Fig. 3. It can be seen that there is a broad maximum at about 18 GHz. Therefore, a modulation amplitude of about 18 GHz was used throughout the measurement of cesium in the flame. The laser diode was operated near to its maximum power of 50 mW to reduce the side modes and to have the small line widths [ll, 121. The radiation power was attenuated by a neutral density filter to about 100 l.r.W. A power of 100 p,W was just high enough to give optimum signal to noise ratios. This can be seen in Fig, 4, where the signal and the noise are plotted against radiation power. Since there was no saturation of the optical transition and of the photodiode, the analytical signal increased linearly with the radiation power. On the other hand, the noise was entirely due to the noise of photodiode and amplifier system below about 1 ~LW. Above 1 ~.LWthe noise increases monotonically with radiation power. Therefore, the signal to noise ratio increased for low powers and reached a constant value for laser powers larger than about 10 ~,LW(see Fig. 4). The cesium calibration curve, measured in the air-propane flame with a modulation frequency of 20 kHz and a modulation amplitude of 18 GHz, is presented in Fig. 5. The measurements were performed by driving the diode laser wavelength with 10 Hz between the minimum and maxium signal shown in Fig. 2. The time constant of the lock-in amplifier was 3 ms and the measuring time for each concentration 50 s. The
laser power, PW Fig. 4. Dependences of the WM-LAAS signal (S), the noise (N) and the signal to noise ratio (S/N) on laser power measured with a photodiode at the cesium D2 line.
WM-LAAS
in analyticalflames
1467
/
I ‘;/aabsorption
O.oJ
30 detection limit 1
Fig. 5. Calibration
1
I
10 100 1000 Cs concentration, @ml
curve for cesium measured by WM-LAAS 852.11 nm.
10000
in an air-propane
flame at
characteristic concentration of cesium (1% absorption) was found to be 200 @ml which is a factor of two higher than the value found by hollow cathode AAS in an air-acetylene flame [l]. The difference should be due to the different flame and nebulizer. The lowest detectable absorbance was 1 X lop5 in our WM-LAAS experiment. This datum corresponds to a detection limit of 0.5 @ml. Note, that this detection limit is of the same order of magnitude as the cesium detection limit by graphite furnace AAS with hollow cathode lamp (0.3 ng/ml [l]). 3.2. Measurement of titanium with fundamental radiation of diode lasers There are three titanium lines of different strengths in the wavelength range of the laser diode HL 8318 used. These lines are at 843.57 nm, 843.49 nm and 842.65 nm. The initial levels of the absorption lines are different, but they belong to the same multiplet with excitation energies of 6743 cm-’ (5F4 level), 6843 cm-’ (5Fg) and 6661 cm-’ (5F3), respectively. The oscillator strengths of the lines are 0.011, 0.019 and 0.0096, respectively [9]. Unfortunately, the two strongest lines were not suitable for measurement of titanium. Due to the N,O-acetylene flame, severe molecular line interferences were observed. The interferences can be seen in Fig. 6, where the 2f profiles of the three absorption lines are displayed for titanium concentrations of 1000 and 100 &ml and for the blank. Note that the signal scale of the 842.65 nm line is expanded by a factor of four which reflects the weak absorption of this line in comparison with the two others. In Fig. 7 can be seen that, luckily, the 842.65 nm line is placed between two groups of strong molecular transitions. However, if the intensity of the titanium line is amplified by a factor of 50 (insert in Fig. 7), a structured background can be observed. The reason for the background was not molecular absorption, but interference effects in the optical arrangement [2] which could not be completely avoided by proper alignment of the laser beam. However, because our optical arrangement was sufficiently stable, the structured background could be measured without aqueous sample in the flame and taken into account in the spectra obtained with analyte solution. We used a special procedure to average the titanium absorption signals. Additionally to a wavelength modulation of about 60 kHz, the diode laser was driven by a low frequency, triangular shaped current function (12.5 Hz) from a wavelength position near the maximum to a position near the minimum in the 2f profile (see Figs 6c and 7). The 12.5 Hz modulation was used as reference frequency for a second lock-in amplifier (Ithaca 3921) which was fed with the 2f output of the first amplifier. The lf signal of the second, averaging amplifier, smoothed by a 1 s time constant, was
1468
H. GROLLet al.
wavelength Fig. 6. 2f spectra of titanium lines at (a) 843.57 nm, (b) 843.49 nm, and (c) 842.65 nm measured with concentrations of 1000 and 100 &ml titanium and a blank in a N&acetylene flame.
-
wavelength
Fig. 7. 2f spectrum around the titanium 842.65 nm line.
stored together with the blank signal in the oscilloscope and processed by the personal computer. The calibration curve measured with the weak titanium line at 842.65 nm is given in Fig. 8. A 3a-detection limit of 360 q/ml was found. This value corresponded to a detectable absorbance of 5 x 10e6. Note that our detection limit obtained with this weak line originating from a thermally populated level is about a factor of five better than the characteristic concentration which can be obtained with the titanium resonance line in conventional flame-AAS (see Table 1). The slope of the calibration curve was slightly smaller than unity, which was probably due to the relatively high analyte concentration. It is well known that the population density of thermally excited species is more sensitive to the change of physical parameters, in particular the temperature, than the ground state population. 3.3. Measurement of chromium with frequency-doubled diode laser radiation The most prominent absorption line for hollow cathode-AAS of chromium is the line at 357.87 nm. Characteristic concentrations of about 40 q/ml can be obtained in
WM-LAAS
in analytical flames
1469
Ti concentration, ,,g/ml Fig. 8. Calibration curve of titanium measured by WM-LAAS at 842.65 nm in a N,O-acetylene flame.
Table 1. Detected absorbance and the detection limits (DL) for cesium, titanium and chromium obtained in the present work by WM-LAAS in analytical flames. The characteristic concentrations (cz,,.) measured by conventional hollow cathode flame AAS on different wavelengths than in this work were taken from [l]. The oscillator strengths f of the lines applied were taken from different publications
Element
Line [nm]
f
852.11 842.65 399.86 425.44
0.71* 0.0096t 0.093$ 0.11s
cs Ti Ti Cr
Detected absorbance 1 5 1.3 1
* * * *
10-S 10-e 10--a 10-d
DL [ng/ml]
c& [@ml1
0.5 360 60 6
100 2000 2000 40
* Ref. [8]. t Ref. 191. $ Ref. [lo].
analytical flames [l]. Although it is possible to produce laser diodes with fundamental wavelengths in the range 710-720 nm which have sufficient power for efficient second harmonic generation for that chromium line, there are no commercial laser diodes with such fundamental wavelengths on the market. Therefore, we were obliged to find another strong chromium absorption line whose wavelength can be generated by commercial laser diodes. Since there are powerful laser diodes available with wavelengths in the range of 850 nm, the chromium line at 425.44 nm is very suitable for WM-LAAS. That line is only slightly weaker than the 357.87 nm line. The characteristic chromium concentration which has been found for that line in conventional flame-AAS is only a factor of 2.5 higher than for the 357.87 nm line [l]. The 2f absorption line measured with 0.5 p&ml chromium is displayed in Fig. 9c. It is compared with the lf profile recorded at the same experimental condition (Fig. 9b) and the direct absorption profile measured with ten times higher chromium concentration. The slope of the intensity in the direct absorption spectrum is due to change of power if the laser is tuned by the current [ll, 121. One can readily see the improved signal to noise ratio of WM-LAAS compared with direct LAAS. Furthermore, the advantage of 2f detection over a lf measurement can also be seen in Fig. 9. In comparison with the lf spectrum, there is no dc background in the 2f spectrum. The background in the lf detection is mainly a result of the variation in the laser intensity. It is interesting to show the dependence of the signal and of the noise on the power of the frequency doubled radiation measured with photomultiplier (Fig. 10). A constant noise given by the photomultiplier and the amplifier was measured below about
1470
H. GROLL et al.
wavelength
-
Fig. 9. Spectra of the chromium 425.44 nm line measured in an air-acetylene flame by direct absorption (a) and WM-LAAS applying lf (b) and 2f (c) detection. The chromium concentrations were 5 &ml for (a) and 0.5 kg/ml for (b) and (c).
SHG laser power, nW Fig. 10. Dependences of the WM-LAAS signal (S), the noise (N) and the signal to noise ratio (SIN) on laser power measured with a photomultiplier at the chromium 425.44 nm line.
1.5 nW. For higher power the noise increased slower than with a photodiode detector (see above). The slope was about 0.5 in the double logarithmic plot. Since the signal increased linearly (unity slope) with power, the dependence of the signal to noise ratio on power was one below 1.5 nW and approximately 0.5 above 1.5 nW. The calibration graph for chromium is shown in Fig. 11. The characteristic concentration was 240 ng/ml which is a factor of six higher than the characteristic concentration found in conventional flame-AAS with the 357.87 nm line (see Table 1). However, the 3a detection limit by WM-LAAS was 6 @ml which is a factor of five to six lower than in routine flame-AAS with detectable absorption of the order of 1%. The absorbance at the detection limit was about 10P4. 3.4. Measurement of titanium with frequency-doubled diode laser radiation It is possible to generate deep blue radiation with sufficient power for the titanium 399.86 nm line with commercially available laser diodes. Since the oscillator strength of this line is a factor of ten larger than that of the 842.65 nm line (see Table 1) and
WM-LAAS
in analytical
1471
flames
10’ -
1% absorptmn
J
30 detectmn ltmlt 102
10’
I 104
10’
Cr concentration. nglml
Fig. 11. Calibration
curve of chromium measured by WM-LAAS air-acetylene flame.
at 425.44 nm in an
the population density of the initial level (Ei = 387 cm-l) is also higher than that of the initial level for the near-ir transition, a better detection limit was obtained with frequency-doubled radiation, although the SHG-power was low. Figure 12 shows the WM-LAAS calibration data for titanium measured with the 399.86 nm line. The characteristic concentration was about 3 cl.g/ml and the detection limit 60 ng/ml which corresponded to about 1.3 x 10e4 absorbance. Note that the detection limit is a factor of six better than in the case of the 842.65 nm line.
4. CONCLUSION The results of our demonstration measurements show that WM-LAAS with diode lasers can significantly reduce the element detection limits in flame-AAS. The concentration detection limits which ultimately can be obtained are of the order of the detection limits known in conventional hollow cathode graphite tube AAS. Due to the very low absorbances which can be measured by WM-LAAS, even the choice of weak absorption lines or lines from excited levels with low population can give detection limits which are comparable or better than in routine hollow cathode flame AAS. However, it has to be stressed again that excited states are more sensitive to the flame parameters than the ground state. It means that systematic error may be larger if, for example, the flame is loaded with samples of high concentrations or varied composition.
105 /
.
1 % absorption
loOO
30 detection limit
lo-?
10-1
100
10’
102
103
Ti concentration, pg/ml Fig. 12. Calibration
curve of titanium measured by WM-LAAS 399.86 nm in a N,O-acetylene flame.
with SHG radiation
at
1412
H. GROLLet al.
Furthermore, it was demonstrated that second harmonic generation of fundamental laser diode radiation is suitable to increase the number of elements which can be measured by WM-LAAS. The detection limits which were achieved were of the same order of magnitude or better than in conventional routine flame-AAS, even with very low powers of frequency-doubled radiation. work was performed in Dortmund within the frame of a scientific and technological co-operation project (no. X229.11) of the Institute of Spectroscopy of the Russian Academy of Sciences, Troitsk (Moscow Region), and the Institut fiir Spektrochemie und Angewandte Spektroskopie, Dortmund, Germany. Funding by the Russian and German governments, the Bundesministerium fib Forschung und Technologie and the Ministerium fur Wissenschaft und Forschung (Nordrhein-Westfalen) is gratefully acknowledged.
Acknowledgements-The
REFERENCES [l] B. Welz, Atomic Absorption Spectrometry, Verlag Chemie, Weinheim (1985). [2] J. A. Silver, Appl. Opt. 31, 707 (1992). [3] C. Schnurer-Patschan, A. Zybin, H. Groll and K. Niemax, J. Anal. At. Specfrom. 8, 1103 (1993). [4] A. Zybin, C. Schniirer-Patschan and K. Niemax, Spectrochim. Acta 48B, 1713 (1993). (51 K. Niemax, H. Groll and C. Schniirer-Patschan, Spectrochim. Acta Rev. 15, 349 (1993). [6] K. C. Ng, A. H. Ali, T. E. Barber and J. D. Winefordner, Appl. Spectrosc. 44, 849 (1990). [7] H. Groll and K. Niemax, Specrrochim. Acta 48B, 633 (1993). [S] J. K. Link, J. Opt. Sot. Am. 56, 1195 (1966). [9] C. Corliss and W. R. Bozman, Experimental Transition Probabilities of Seventy Elements. U.S. National Bureau of Standards, Washington, DC (1962). [lo] W. L. Wiese and G. A. Martin, Wavelengths and Transition Probabilities for Atoms and Atomic Ions, Part II. NSRDS-National Bureau of Standards 68, Washington, DC (1980). [ll] C. E. Wieman and L. Hollberg, Rev. Sci. Instrum. 62, 1 (1991). [12] J. Franzke, A. Schnell and K. Niemax, Specfrochim. Acta Rev. 15, 378 (1993).
Acra, Vol. 498. Nos 12-14. PP 1463-1472. 1994 Copyright 0 1994 Elsevier Saence Ltd Pnnted in Great Britarn. All rights reserved 0x4-8.547/94 57 00 + .cn
0584-8547(94)ooo80
Wavelength
modulation
diode laser atomic absorption analytical flames*
spectrometry
in
H. GROLL,~ CH. SCHN~~RER-PATSCHAN.? YIJ. KURITSYN$and K. NIEMAX@ Institut fur Spektrochemie und Angewandte Spektroskopie an der Universitlt Dortmund, BunsenKirchhoff-Str. (Received
11, D-44139 Dortmund,
3 May 1994; accepted
Germany
19 July 1994)
Abstract-The potential of wavelength modulation laser atomic absorption spectrometry in analytical flames is demonstrated by the measurement of titanium, cesium and chromium applying fundamental and frequency doubled radiation of commercially available semiconductor diode lasers. In dependence on the radiation power, absorbances of the order of 10-4-10-6 are measured, which reveal very low detection limits even when weak absorption lines are used.
1. INTRODUCTION ATOMIC absorption spectrometry (AAS) in flames is a relatively old technique for element analysis. It is still popular and widely used in spectrochemistry (see, for example, Ref.[l] and references therein). The popularity of flame-AAS would be even higher if lower detection limits could be obtained with this technique. In routine analysis with commercial instruments, the detection limits are typically given by the characteristic concentration (1% absorption which corresponds to 4.4 x lop3 absorbance). The performance can be improved, with some effort, by optimization and stabilization of the instrumental parameters and by measurements with long integration times [l]. However, if low concentrations have to be determined, the more sensitive graphite furnace-AAS rather than flame-AAS is preferred in routine analysis. This paper will show that there is a powerful method which yields detection limits in flame-AAS as low as in conventional graphite furnace-AAS. It is the technique of wavelength modulation-laser atomic absorption spectrometry (WM-LAAS) with semiconductor diode lasers. As discussed extensively in the literature (see, for example, the review by SILVER [2]), wavelength modulation spectrometry can eliminate the l/f noise in absorption measurements and reduce the total noise to the shot noise limit. This means that, with optimum conditions, absorbances of the order of 10-7-10-8 can be detected in wavelength modulation spectrometry. Recently, the analytical potential of WM-LAAS with diode lasers has been shown by our group [3, 41. In the first paper, rubidium, lanthanum and aluminium were measured in commercial graphite tube atomizers [3]. While rubidium and lanthanum were studied by absorption of fundamental diode laser radiation at 780.03 and 670.95 nm, aluminium was measured with deep blue light at 396.15 nm produced by second harmonic generation (SHG) in a non-linear crystal. The aluminium experiment at 396.15 nm in particular demonstrated the possibility of measuring about 50 elements in absorption [5] with blue and near-uv radiation generated by commercial laser diodes. The 3a-detection limits for WM-LAAS in a graphite furnace were 1 pg/ml,
* This paper was published in the Special Honor Issue of Spectrochimica Acfa Part B, dedicated to J. D. Winefordner. t Present address: Institut fiir Physik, Universitlt Hohenheim, Garbenstrasse 30, D-70599 Stuttgart, Germany. $ Institute of Spectroscopy of the Russian Academy of Sciences, 142092 Troitsk, Moscow Region, Russia. B Author to whom correspondence should be addressed. 1463
1464
H. GROLL etal. SHG dde
Fig. 1. Experimental
analytIca photodlodr 01 photomultqd~rr
r----------
arrangement with and without second harmonic generation box). Ll, L2 and L3 are lenses.
(in dashed
7.5 @ml and 0.1 ng/ml for rubidium, lanthanum and aluminium, respectively. The second paper reported on WM-LAAS measurements of metastable chlorine, fluorine and oxygen atoms in a low-pressure dc plasma [4]. It is the credit of the group of Prof. Winefordner to have performed the first absorption measurement with a semiconductor diode laser in analytical flames in 1990 [6]. They used a multimode laser diode for the measurement of rubidium. Multielement measurements in flames by simultaneous operation of six diode lasers and the extension of the dynamic range to high concentrations by wavelength tuning to the wing of the absorption lines have been demonstrated by our group [7]. This present paper will show that the technique of WM-LAAS can also extend the dynamic range of flame absorption spectrometry to lower element concentrations. We will show, as in the paper on graphite furnace WM-LAAS [3], that cesium and titanium can be measured successfully by absorption of fundamental radiation of commercial laser diodes at the resonance line and a weak absorption line, respectively. On the other hand, the use of SHG will be demonstrated in measurements of strong absorption lines in chromium and titanium. Furthermore, we will compare the 3a-detection limits obtained by WM-LAAS with the characteristic concentrations (1% absorption) reported for hollow cathode AAS in flames applying the most sensitive absorption lines, since in routine measurements the typical detection limits of hollow cathode AAS are of the order of the characteristic concentrations. It is known that the detection limits of hollow cathode AAS in analytical flames can be better than the characteristic concentration if long integration times are applied. However, very stable flames and sample introduction are required in such measurements which are difficult to realize in analytical routine laboratories.
2. EXPERIMENT The experimental arrangement for WM-LAAS, presented schematically in Fig. 1, has been already described in our recent paper on WM-LAAS in graphite furnaces [3]. Therefore, it will be only described shortly. However, the modifications necessary for WM-LAAS measurements in analytical flames are given here in more detail. Two different GaAlAs laser diodes were used, a Hitachi HL 8318 (power: 50 mW) and a Sharp LTO 16 (power: 30 mW). A commercial power supply (LDC 400 by Profile) provided stable wavelength tuning of the laser diodes by temperature and current. At constant temperature and current, the stability of the optical laser frequency was better than 50 MHz over several hours. Taking into account the widths of the pressure and Doppler broadened lines in the flame, the experimental wavelength stability was sufficient to avoid systematic errors by wavelength drifts. Cesium and titanium were measured by the D2 resonance line at 852.11 nm (oscillator strength
WM-LAAS
1465
in analytical flames
wavelength
-
Fig. 2. 2f profiles of the cesium DZ-line measured in an air-propane flame with three different modulation amplitudes. The cesium concentration was 1 @ml.
f = 0.71 [8]) and the weak absorption line at 842.65 nm (f = 0.0096 [9]), respectively. The initial level Ei of the titanium absorption line (Ei = 6661 cm-l) is a thermally populated level. The HL 8318 laser diode was used for both transitions. The HL 8318 was also applied in the measurements of chromium. Here, the radiation was frequencydoubled by focussing the laser light by a large aperture lens system with astigmatism compensation into a LiI03 crystal. This simple procedure provided up to about 100 nW at the strong chromium absorption line at 425.44 nm (f = 0.11 [lo]) applying about 50 mW fundamental power. Furthermore, titanium was not only measured by the weak near-ir line, but also by the strong transition at 399.86 nm (f = 0.093 [lo]). Radiation of this wavelength was generated in a shorter LiI03 crystal by the laser diode LTO 16. The SHG-power was about 50 nW. Different laminar burners by Techtron with pneumatic nebulization operated by a Varian flame controller were used. An air-propane burner (length 1 = 10 cm) was applied in the cesium experiment, while chromium and titanium were measured with an air-acetylene flame (I = 10 cm) and a N#-acetylene flame (I = 5 cm), respectively. The aqueous samples were prepared with deionized water from standard solutions (Cs: 1 mg/ml from CsCl in Hz0 by Riedel-de Haen, Cr: 10 mg/ml from CrCl, in dilute HCl by Merck, and Ti: 10 mg/ml from Tic& in 18% HCl). Sinusoidal current modulation generated by a Wavetek power supply (FG-5000) was added to the diode current. The amplitudes of the current modulation determined the maximum wavelength modulation. Modulation frequencies up to about 100 kHz were applied. The absorption of cesium and titanium in the near-ir spectral range was detected by a photodiode (Hamamatsu S 2386-5K), while a photomultiplier (Hamamatsu R 212) was used to measure the frequency-doubled radiation in the chromium and titanium experiment. In the latter case, an appropriate glass filter blocked the fundamental radiation of the diode laser. The detector signals were processed by a lock-in amplifier (Ithaca 3961B) in the 2f mode. The amplified signals were displayed by a storage oscilloscope (Hameg HM 205-3) with 8 bit resolution. For further processing the data were transferred to a personal computer.
3. EXPERIMENTALRESULTS AND DISCUSSION 3.1. Measurement of cesium Figure 2 shows 2f profiles of the cesium D2 line. The spectra different modulation amplitudes at a modulation frequency of concentration was 1 Fg/ml. The spectra were obtained by tuning a second Wavetek function generator continuously over the line.
were measured with 5 kHz. The cesium the diode laser with The splitting of the
1466
H.
GROLL et al.
modulatmn amphmde, GHz
Fig. 3. 2f WM-LAAS
signal intensity for cesium plotted against the modulation amplitude.
D2 line mainly due to hyperfine splitting of the ground state (9.192 GHz) is clearly visible. It is obvious that the spectral resolution is best if small modulation amplitudes are applied. However, to obtain large signals the amplitude has to be increased. The dependence of the signal on the modulation amplitude is plotted in Fig. 3. It can be seen that there is a broad maximum at about 18 GHz. Therefore, a modulation amplitude of about 18 GHz was used throughout the measurement of cesium in the flame. The laser diode was operated near to its maximum power of 50 mW to reduce the side modes and to have the small line widths [ll, 121. The radiation power was attenuated by a neutral density filter to about 100 l.r.W. A power of 100 p,W was just high enough to give optimum signal to noise ratios. This can be seen in Fig, 4, where the signal and the noise are plotted against radiation power. Since there was no saturation of the optical transition and of the photodiode, the analytical signal increased linearly with the radiation power. On the other hand, the noise was entirely due to the noise of photodiode and amplifier system below about 1 ~LW. Above 1 ~.LWthe noise increases monotonically with radiation power. Therefore, the signal to noise ratio increased for low powers and reached a constant value for laser powers larger than about 10 ~,LW(see Fig. 4). The cesium calibration curve, measured in the air-propane flame with a modulation frequency of 20 kHz and a modulation amplitude of 18 GHz, is presented in Fig. 5. The measurements were performed by driving the diode laser wavelength with 10 Hz between the minimum and maxium signal shown in Fig. 2. The time constant of the lock-in amplifier was 3 ms and the measuring time for each concentration 50 s. The
laser power, PW Fig. 4. Dependences of the WM-LAAS signal (S), the noise (N) and the signal to noise ratio (S/N) on laser power measured with a photodiode at the cesium D2 line.
WM-LAAS
in analyticalflames
1467
/
I ‘;/aabsorption
O.oJ
30 detection limit 1
Fig. 5. Calibration
1
I
10 100 1000 Cs concentration, @ml
curve for cesium measured by WM-LAAS 852.11 nm.
10000
in an air-propane
flame at
characteristic concentration of cesium (1% absorption) was found to be 200 @ml which is a factor of two higher than the value found by hollow cathode AAS in an air-acetylene flame [l]. The difference should be due to the different flame and nebulizer. The lowest detectable absorbance was 1 X lop5 in our WM-LAAS experiment. This datum corresponds to a detection limit of 0.5 @ml. Note, that this detection limit is of the same order of magnitude as the cesium detection limit by graphite furnace AAS with hollow cathode lamp (0.3 ng/ml [l]). 3.2. Measurement of titanium with fundamental radiation of diode lasers There are three titanium lines of different strengths in the wavelength range of the laser diode HL 8318 used. These lines are at 843.57 nm, 843.49 nm and 842.65 nm. The initial levels of the absorption lines are different, but they belong to the same multiplet with excitation energies of 6743 cm-’ (5F4 level), 6843 cm-’ (5Fg) and 6661 cm-’ (5F3), respectively. The oscillator strengths of the lines are 0.011, 0.019 and 0.0096, respectively [9]. Unfortunately, the two strongest lines were not suitable for measurement of titanium. Due to the N,O-acetylene flame, severe molecular line interferences were observed. The interferences can be seen in Fig. 6, where the 2f profiles of the three absorption lines are displayed for titanium concentrations of 1000 and 100 &ml and for the blank. Note that the signal scale of the 842.65 nm line is expanded by a factor of four which reflects the weak absorption of this line in comparison with the two others. In Fig. 7 can be seen that, luckily, the 842.65 nm line is placed between two groups of strong molecular transitions. However, if the intensity of the titanium line is amplified by a factor of 50 (insert in Fig. 7), a structured background can be observed. The reason for the background was not molecular absorption, but interference effects in the optical arrangement [2] which could not be completely avoided by proper alignment of the laser beam. However, because our optical arrangement was sufficiently stable, the structured background could be measured without aqueous sample in the flame and taken into account in the spectra obtained with analyte solution. We used a special procedure to average the titanium absorption signals. Additionally to a wavelength modulation of about 60 kHz, the diode laser was driven by a low frequency, triangular shaped current function (12.5 Hz) from a wavelength position near the maximum to a position near the minimum in the 2f profile (see Figs 6c and 7). The 12.5 Hz modulation was used as reference frequency for a second lock-in amplifier (Ithaca 3921) which was fed with the 2f output of the first amplifier. The lf signal of the second, averaging amplifier, smoothed by a 1 s time constant, was
1468
H. GROLLet al.
wavelength Fig. 6. 2f spectra of titanium lines at (a) 843.57 nm, (b) 843.49 nm, and (c) 842.65 nm measured with concentrations of 1000 and 100 &ml titanium and a blank in a N&acetylene flame.
-
wavelength
Fig. 7. 2f spectrum around the titanium 842.65 nm line.
stored together with the blank signal in the oscilloscope and processed by the personal computer. The calibration curve measured with the weak titanium line at 842.65 nm is given in Fig. 8. A 3a-detection limit of 360 q/ml was found. This value corresponded to a detectable absorbance of 5 x 10e6. Note that our detection limit obtained with this weak line originating from a thermally populated level is about a factor of five better than the characteristic concentration which can be obtained with the titanium resonance line in conventional flame-AAS (see Table 1). The slope of the calibration curve was slightly smaller than unity, which was probably due to the relatively high analyte concentration. It is well known that the population density of thermally excited species is more sensitive to the change of physical parameters, in particular the temperature, than the ground state population. 3.3. Measurement of chromium with frequency-doubled diode laser radiation The most prominent absorption line for hollow cathode-AAS of chromium is the line at 357.87 nm. Characteristic concentrations of about 40 q/ml can be obtained in
WM-LAAS
in analytical flames
1469
Ti concentration, ,,g/ml Fig. 8. Calibration curve of titanium measured by WM-LAAS at 842.65 nm in a N,O-acetylene flame.
Table 1. Detected absorbance and the detection limits (DL) for cesium, titanium and chromium obtained in the present work by WM-LAAS in analytical flames. The characteristic concentrations (cz,,.) measured by conventional hollow cathode flame AAS on different wavelengths than in this work were taken from [l]. The oscillator strengths f of the lines applied were taken from different publications
Element
Line [nm]
f
852.11 842.65 399.86 425.44
0.71* 0.0096t 0.093$ 0.11s
cs Ti Ti Cr
Detected absorbance 1 5 1.3 1
* * * *
10-S 10-e 10--a 10-d
DL [ng/ml]
c& [@ml1
0.5 360 60 6
100 2000 2000 40
* Ref. [8]. t Ref. 191. $ Ref. [lo].
analytical flames [l]. Although it is possible to produce laser diodes with fundamental wavelengths in the range 710-720 nm which have sufficient power for efficient second harmonic generation for that chromium line, there are no commercial laser diodes with such fundamental wavelengths on the market. Therefore, we were obliged to find another strong chromium absorption line whose wavelength can be generated by commercial laser diodes. Since there are powerful laser diodes available with wavelengths in the range of 850 nm, the chromium line at 425.44 nm is very suitable for WM-LAAS. That line is only slightly weaker than the 357.87 nm line. The characteristic chromium concentration which has been found for that line in conventional flame-AAS is only a factor of 2.5 higher than for the 357.87 nm line [l]. The 2f absorption line measured with 0.5 p&ml chromium is displayed in Fig. 9c. It is compared with the lf profile recorded at the same experimental condition (Fig. 9b) and the direct absorption profile measured with ten times higher chromium concentration. The slope of the intensity in the direct absorption spectrum is due to change of power if the laser is tuned by the current [ll, 121. One can readily see the improved signal to noise ratio of WM-LAAS compared with direct LAAS. Furthermore, the advantage of 2f detection over a lf measurement can also be seen in Fig. 9. In comparison with the lf spectrum, there is no dc background in the 2f spectrum. The background in the lf detection is mainly a result of the variation in the laser intensity. It is interesting to show the dependence of the signal and of the noise on the power of the frequency doubled radiation measured with photomultiplier (Fig. 10). A constant noise given by the photomultiplier and the amplifier was measured below about
1470
H. GROLL et al.
wavelength
-
Fig. 9. Spectra of the chromium 425.44 nm line measured in an air-acetylene flame by direct absorption (a) and WM-LAAS applying lf (b) and 2f (c) detection. The chromium concentrations were 5 &ml for (a) and 0.5 kg/ml for (b) and (c).
SHG laser power, nW Fig. 10. Dependences of the WM-LAAS signal (S), the noise (N) and the signal to noise ratio (SIN) on laser power measured with a photomultiplier at the chromium 425.44 nm line.
1.5 nW. For higher power the noise increased slower than with a photodiode detector (see above). The slope was about 0.5 in the double logarithmic plot. Since the signal increased linearly (unity slope) with power, the dependence of the signal to noise ratio on power was one below 1.5 nW and approximately 0.5 above 1.5 nW. The calibration graph for chromium is shown in Fig. 11. The characteristic concentration was 240 ng/ml which is a factor of six higher than the characteristic concentration found in conventional flame-AAS with the 357.87 nm line (see Table 1). However, the 3a detection limit by WM-LAAS was 6 @ml which is a factor of five to six lower than in routine flame-AAS with detectable absorption of the order of 1%. The absorbance at the detection limit was about 10P4. 3.4. Measurement of titanium with frequency-doubled diode laser radiation It is possible to generate deep blue radiation with sufficient power for the titanium 399.86 nm line with commercially available laser diodes. Since the oscillator strength of this line is a factor of ten larger than that of the 842.65 nm line (see Table 1) and
WM-LAAS
in analytical
1471
flames
10’ -
1% absorptmn
J
30 detectmn ltmlt 102
10’
I 104
10’
Cr concentration. nglml
Fig. 11. Calibration
curve of chromium measured by WM-LAAS air-acetylene flame.
at 425.44 nm in an
the population density of the initial level (Ei = 387 cm-l) is also higher than that of the initial level for the near-ir transition, a better detection limit was obtained with frequency-doubled radiation, although the SHG-power was low. Figure 12 shows the WM-LAAS calibration data for titanium measured with the 399.86 nm line. The characteristic concentration was about 3 cl.g/ml and the detection limit 60 ng/ml which corresponded to about 1.3 x 10e4 absorbance. Note that the detection limit is a factor of six better than in the case of the 842.65 nm line.
4. CONCLUSION The results of our demonstration measurements show that WM-LAAS with diode lasers can significantly reduce the element detection limits in flame-AAS. The concentration detection limits which ultimately can be obtained are of the order of the detection limits known in conventional hollow cathode graphite tube AAS. Due to the very low absorbances which can be measured by WM-LAAS, even the choice of weak absorption lines or lines from excited levels with low population can give detection limits which are comparable or better than in routine hollow cathode flame AAS. However, it has to be stressed again that excited states are more sensitive to the flame parameters than the ground state. It means that systematic error may be larger if, for example, the flame is loaded with samples of high concentrations or varied composition.
105 /
.
1 % absorption
loOO
30 detection limit
lo-?
10-1
100
10’
102
103
Ti concentration, pg/ml Fig. 12. Calibration
curve of titanium measured by WM-LAAS 399.86 nm in a N,O-acetylene flame.
with SHG radiation
at
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H. GROLLet al.
Furthermore, it was demonstrated that second harmonic generation of fundamental laser diode radiation is suitable to increase the number of elements which can be measured by WM-LAAS. The detection limits which were achieved were of the same order of magnitude or better than in conventional routine flame-AAS, even with very low powers of frequency-doubled radiation. work was performed in Dortmund within the frame of a scientific and technological co-operation project (no. X229.11) of the Institute of Spectroscopy of the Russian Academy of Sciences, Troitsk (Moscow Region), and the Institut fiir Spektrochemie und Angewandte Spektroskopie, Dortmund, Germany. Funding by the Russian and German governments, the Bundesministerium fib Forschung und Technologie and the Ministerium fur Wissenschaft und Forschung (Nordrhein-Westfalen) is gratefully acknowledged.
Acknowledgements-The
REFERENCES [l] B. Welz, Atomic Absorption Spectrometry, Verlag Chemie, Weinheim (1985). [2] J. A. Silver, Appl. Opt. 31, 707 (1992). [3] C. Schnurer-Patschan, A. Zybin, H. Groll and K. Niemax, J. Anal. At. Specfrom. 8, 1103 (1993). [4] A. Zybin, C. Schniirer-Patschan and K. Niemax, Spectrochim. Acta 48B, 1713 (1993). (51 K. Niemax, H. Groll and C. Schniirer-Patschan, Spectrochim. Acta Rev. 15, 349 (1993). [6] K. C. Ng, A. H. Ali, T. E. Barber and J. D. Winefordner, Appl. Spectrosc. 44, 849 (1990). [7] H. Groll and K. Niemax, Specrrochim. Acta 48B, 633 (1993). [S] J. K. Link, J. Opt. Sot. Am. 56, 1195 (1966). [9] C. Corliss and W. R. Bozman, Experimental Transition Probabilities of Seventy Elements. U.S. National Bureau of Standards, Washington, DC (1962). [lo] W. L. Wiese and G. A. Martin, Wavelengths and Transition Probabilities for Atoms and Atomic Ions, Part II. NSRDS-National Bureau of Standards 68, Washington, DC (1980). [ll] C. E. Wieman and L. Hollberg, Rev. Sci. Instrum. 62, 1 (1991). [12] J. Franzke, A. Schnell and K. Niemax, Specfrochim. Acta Rev. 15, 378 (1993).