Report
Semiconductor Diode Lasers
in Atomic Spectrometry
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n recent years, there has been a revolution in laser technology—the transition from gas lasers to semiconductor laser diodes doped with group III-V elements. We believe that the impact of semiconductor laser diodes on laser applications will be as strong as the impact of semiconductor elements in electronics in the early 1960s, and that semiconductor laser diodes will replace not only lowpower gas lasers, such as the HeNe laser, but also high-power systems such as the CO lasers used for cutting and welding in material processing Laser diodes already mass produced for instruments such as compact disc plavers barcode scanners laser printers optical systems and telecommunications equioment Of course laser diodes will also have an impact on analytical chemistrv
Kay Niemax A l e k s a n d r Zybin Universitat Hohenheim Christoph S c h n ü r e r - P a t s c h a n Henning Groll LaserSpec Analytik 0003 - 2700/96/0368 -351 A/$12.00/0 © 1996 American Chemical Society
Semiconductor laser diodes are reliable, small, and easy to operate as tunable radiation sources for analytical spectrometry Diode laser basics Low-power semiconductor laser diodes suitable for analytical spectroscopy, often called etalon-type laser diodes, are very small (300 um x 300 um x 150 um) and convert electrical power into optical radiation with high (typically 10-30%) efficiency. The active laser region is a spatially confined layer in the form of a stripe that is a few micrometers wide, less than 1-um thick, and the length of the laser chip. Because the index of refraction of
the semiconductor material is high, the ends of the laser substrate act as resonator mirrors. The active region can be confined eiiher by the eaplication oo femiconductor materials of different indices of refraction (index-guided laser diodes) or by a spatially confined current through the active layer (gain-guided laser diodes). The first commercial laser diodes that could be operated at room temperature and were suitable for spectroscopy were lowpower (typically 3 mW) AlGaAs sevices mat provided laser radiation in the near-IR spectral range (770-810 nm). In recent years, the output power and wavelength range of AlGaAs laser diodes have been improved. About 10 years ago, InGaAsP laser diodes, which operated at ~ 670 nm with a power of ~ 3 mW, became commercially available. Subsequently, InGaAsP laser diodes with higher power have become available and the wavelength has been extended down to 630 Widespread use of semiconductor laser diodes has decreased the price of laser diodes and niishpH industrial rpQparch forthe Hpvelopment of new laser tvnes with hirtier power and shorter wavelengths For ex-
Analytical Chemistry News & Features, June 1, 1996 351 A
Report ample, single AlGaAs laser diodes that have a wavelength range of 740-900 nm and powers lower than 50 mW cost less than $150. At the moment, laser diodes are commercially available in the range 630-1600 nm. However, for certain wavelength areas within this range, laser diodes are not produced. For certain low wavelength areas (primarily those under 630 nm), laser diodes are not available. In theory, these "wavelength gaps" could be filled if laser diodes at these wavelengths were of commercial interest. On the other hand new II-VI type diodes (ZnSe) with lasing wavelengths in the blue-green spectral range (470-515 nm) at room temperature have been developed in research laboratories; an InGaN laser diode which or> in the deep-blue spectral range at about 420 nm was recently developed by the Japanese company Nichia Although the*
lifetime of blue laser diodes is still verv short (minutes to a few hours') it is likel t i rease in the r future Spectroscopic properties
and operational properties for analyucal spectroscopy \ir z). iviost commercial single-stnpe, index-guided laser diodes produce radiation of single spatial and longitudinal mode at maximum injection current. Side modes, which are strong only at low diode current just above the laser threshold, often have < 1% oo fhe radiation power at maximum current. If the temperature and the diode injection current are kept constant, the spectral linewidth is narrow (typically 40 fm), and the spectral resolving power is ~ 107, much higher than necessary for complete resolution of atomic or molecular spectral lines in the gas phase. For example, the laser linewidths are 10-100 times narrower than the widths of typical Doppler-broadened atomic lines, a precondition for applying laser diodes to high-resolution Dopplerfree spectroscopy used for isotopically selective analysis. Applying to feedback from dispersive optical elements such as optical gratings or interferometers further reduction of the laser linewidth by a factor of 103 is possible (i). l ^ l o d UlOUGo W1L11 a l l i l l l d llctl g r dUllJ^
structure in me suu&irdic, caiied dioinuuted ieeuD 100 uW (10,11). are modulation spectra of the same line at a Typical absorbances of ~ 10"4 have been Cr concentration that is 10 times lower. obtained in a graphite furnace (10) )nd in a The wavelength modulation frequency was flame (11) with low SHG power. Com5 kHz, and the peak-to-peak amplltudes of pared with conventional hollow-cathode the modulation corresponded to the width AAS, for which the detection limits rouof the Cr line. The spectrum in Figure 3b tinely correspond to absorbances of ~ 4.4 x was obtained by phase-sensitive detection 10~3 (1% absorption)) ,he improvement tacof the first harmonic (If detection), whereas tor is between about 10 and 1000, dependthe spectrum in Figure 3c was measured at inj? on the ltiser po^ve^ For example the second harmonic of the modulation obtained a Cr detection limit of 6 ng/mL in frequency (2f detection). a flame using ordinary pneumatic nebuliThe constant slope of the background zation and only ~ 50 nW SHG power ((11 intensity in the direct absorption specIf one takes into account the average imtrum is caused by the increasing power of provement of 2 orders of magnitude by apthe laser diode as the current is raised plication of modulated laser diodes instead and gives a constant signal background in of hollow cathode lamps in AAS, ,he detecThe fact that the wavelength of laser di- the If spectrum of the absorption line. tion power of wavelength modulation AAS odes can be current modulated allows reThe 2f line detection has the advantage in graphite tube furnaces can compete duction of l//noise and measurements near that there is only a small amount of backwith powerful and expensive techniques the shot noise limit Wavelength modulaground. For small modulation amplitudes, such as ICPMS. Furthermore, the detection atomic and molecular absorption spec- the If and 2f line profiles are given by the tion limits in wavelength modulation laser troscopy with laser diodes that have frefirst and second derivatives, respectively, AAS in analytical flames are comparable to quencies in the MHz range has been sucof the direct absorption line. The actual those provided by conventional hollow cathcessfully applied to detect absorbances of analytical measurement is obtained by tun- ode AAS in graphite furnaces lO^-lO""8 in gas or vapor cells (9) ing the modulated wavelength of the laWavelength modulation AAS in analytiTypical spectra obtained by use of ser diode in the 2f profile periodically becal flames has been used for elementwavelength modulation laser AAS in an an- tween one of the maxima and the mini- selective detection for LC (12), as shown in alytical flame are shown in Figure 3. The mum. Figure 4. A detection limit of ~ 0.5 ng/mL was found for Cr(VT) in deionized water; determination of Cr(III) was limited by Cr contamination in one of the acids used in sample preparation. Comparable detection limits were found for drinking water samples. The Cr detection limits using wavelength modulation laser AAS in the flame were of the same order of magnitude as those determined by ICPMS coupled to the same nebulizer/LC system (13). Theoreticallv the detection limits by wavelength modulation AAS can be lowered further by Flame and graphite furnace AAS. Because of the extended wavelength range produced by SHG, most elements routinely measured by using conventional hollow-cathode AAS in analytical flames or graphite tube atomizers can be measured by AAS wiih laser diodes. Figure 2 shows elements that can be measured by laser AAS from the ground or low-lying states (energy within kT) by using commercially available diode lasers and SHG. Uranium, the actinides, and other radioactive species are not included, although some of them have absorption lines that are in the range of fundamental or SHG diode laser radiation. Some important elements, such as Be Mg, As, and Hg cannot be determined by laser AAS because laser diodes with the appropriate wavelengths are not commercially available
annlyincr hicrher radiatinn nnwer T h e im-
Drovement in the detection limit should be portional nower because SHG is proportional to snuare of the fundamental laser power and the S/N of the measurement increases with i-U
Figure 3. Laser AAS of Cr in a flame. (a) Direct absorption of 5 ug/mL Cr. Spectra measured at the (b) first and (c) second harmonic of the modulation frequency at 0.5 ug/mL Cr. 354 A
Analytical Chemistry News & Features, June 1, 1996
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the square root of the SHG laser power. Diode laser absorption spectrometry ofmetastabee atoms. Nonmetallic elements such as H, O, S, noble gases, and halogens, which cannot be measured by AAS in furnaces or ffames, have long-lived excited states from which strong
wavelength modulation. Therefore, powerful absorption measurements in a plasma can be performed if the wavelength of the laser diode is modulated as well as the plasma, a technique termed double-modulation laser AAS. The absorption signal must be detected by a lock-in amplifier at the sum or difference of laser and plasma modulation frequencies, and absorbances of ~ 10~7 have been measured at the detection limit (14) About 60 ppt of C2C12F4 4b absorption of metastable chlorine can be detected in a helium plasma this way. Element-selective detection in GC. Several laser diodes can be eperated at the same time to obtain information on the elemental composition of molecular species, as shown in Figure 5. A haloform test mixture was injected into a gas chromatograph and analyzed using double-modulation laser AAS oo fhe CI line et 837.60 0m Figure 4. Instrumentation for speciation of Cr(lll) and Cr(VI) by and the Br line at 827.24 nm. The ratio of wavelength modulation laser AAS in an analytical flame and the CI and Br peaks corresponds to the stoicorresponding chromatograms obtained for deionized water. chiometric ratios of these species in the compounds which allows quantification by absorption transitions can be induced by tion by the plasma. Etalon or interference ef- internal standardization Within experithe red and near-IR radiation of laser difects caused by multireflection of the coher- mental error the CI and Br atoms are auantitativelv produced from molecules in the odes. Furthermore, elements such as Se ent laser radiation between surfaces of opand Hg that cannot be determined by laser tical components in the optical path, such as plasma T h e detection limits of the haloform species were ~ 3 nfr/mL iisinir the CI diode absorption from the ground state can windows and lenses, are discriminated by absorption line Given the injected sample be determined from metastable states usthe plasma modulation technique, and the volume the detection limit was about 0 1 ing SHG radiation from laser diodes. nonspecific background absorption of the t w / s or'l ne absolute modulated plasma can be eliminated by Metastable atoms can be produced in low-pressure plasmas such as a dc plasma or a microwave-induced plasma and measured by laser wavelength modulation AAS. Under optimum plasma conditions, the population densities of metastable states can be 0.1-1% of the total number density, but there are very strong optical transitions in the red and IR spectral ranges between these metastable states and higher levels. Sufficiently high power for good S/N in wavelength modulation laser AAS can be delivered at the fundamental wavelength by commercially available laser diodes The elements that can be measured by laser absorption from metastable states in plasmas are shown
in yellow in Figure 2 For example, very strong transitions from metastable levels of CI, F, Br, and 0 exist at 837.60, 739.87, 827.24, and 777.18 nm, respectively. If the plasma is modulated, lock-in detection of the laser radiation gives the sum of specific absorption by the analyte line and nonspecific absorp-
Figure 5. Element-selective detection of CI and Br in a modulated low-pressure He plasma coupled to a gas chromatograph. Analytical Chemistry News & &eatures, June 1, 1,96
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Isotope-selective analysis. Because the linewidths of laser diodes are considerably narrower than those of atoms in a thermal atomizer, spectra can be recorded with very high resolution. Furthermore, if atomization takes place at reduced pressure (i.e., at about 1-5 hPa), the contribution of pressure broadening (homogeneous broadening) to the width of an atomic spectral line is small compared with that of Doppler broadening (inhomogeneous broadening). The isotope shifts in spectral lines of light and heavy elements are often larger than the Doppler widths of the lines a fact that makes isotopically selective measurements possible For example 238IJ and 235y j-o^jng in enriched and deDieted samples have been measured by Donnler-limited optoffalvanic spectrosconv in hollow-cathode discharges using laser diodes (15,16). Laser diodes can also be used to eliminate the Doppler effect and to perform highresolution spectrometry. Eight years ago, we used laser diodes to resolve isotopic components of spectral lines that are normally hidden in the Doppler profile and to measure isotope ratios (17). Furthermore, researchers at Oak Ridge National Laboratory have used resonance ionization MS with laser diodes for high-resolution measurements of lanthanum (18,19). Doppler-free methods, such as doubleresonance spectrometry with two laser beams in the co- or counter-propagating direction or saturation spectrometry, combine the excellent detection power of laser spectrometry with the possibility of applying the isotope dilution technique from MS for quantitation. Future trends We expect laser diode applications in both elemental and molecular analysis to be an area of active research for years to come as these powerful techniques are transferred from the research laboratory to use as routine methods. We are convinced that laser diode-based instruments can compete with established techniques because comparable or superior analytical power can be offered in economically priced and compact instruments. The first commercial instruments based on analytical diode laser absorption spectrometry will be released soon and further progress in the development of laser di-
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Analytical Chemistry News & Features, June 1, 1996
odes with higher power and shorter wavelengths should result in commercial instruments for spectroscopic techniques that need high laser power, such as LIF or laser ionization spectrometry. We believe that classical spectroscopic instruments such as grating spectrographs and monochromators will be partially replaced by diode laser spectrometers. References (1) Wieman, C; Hollberg, L. Rev. Sci. lustrum. 1991,62,1-20. (2) Franzke, J.; Schnell, A.; Niemax, K. Spectrochim. Acta Rev. .993,15,379-95. (3) Telle, H. R. Spectrochim. Acta Rev. 1993, 15,301-27. (4) Niemax, K.; Groll, H;; Schniirer-Patschan, C. Spectrochim. Acta Rev. 1993,15,34977. (5) Imasaka, T. Spectrochim. Acta Rev. 1993, 15,329-48. (6) Yamamoto, K; Mizuuchi, K. IEEE Photon. Technol. Lett. 1992,2,435-37. (7) Eger, D;; Oron, M;; Katz, M;; Zussman, A. Appl. Rhys. Lett. 1994, 64,3208-09. (8) Zybin, A.; Schniirer-Patschan, C; Niemax, K. Spectrochim. Acta 1992,47B, 151924. (9) Silver, J. A. Appl. Opt. 1992,31,707-17. (10) Schniirer-Patschan, C; Zybin, A;; Groll, H.; Niemax, K./. Anal. At. Spectrom. 1993,8,1103-07. (11) Groll, H.; Schniirer-Patschan, C; Kuritsyn, Yu.; Niemax, K. Spectrochim. Acta 1994, 49B, 1463-72. (12) Groll, H.; Schaldach, G.; Berndt, H.; Niemax, K. Spectrochim. Acta 1995,50B, 1293-98. (13) Zybin, A; Schniirer-Patschan, C; Niemax, K.J. Anal. At. Spectrom. 1995,10, 56367. (14) Jakubowski, N;; Jepkens, B.; Stiiwer, D.; Berndt, H./. Anal. At. Spectrom. 1994, 4, 193-98. (15) Barshick, C. M;; Shaw, R. W.; Young, J. P.; Ramsey, J. M. Anal. Chem. 1994, 66, 4154-58. (16) Barshick, C. M;; Shaw, R. W.. Young, J. P.; Ramsey, J. M. Anal. Chem. 1995, 67, 3814-18. (17) Lawrenz, J.; Obrebski, A;; Niemax, K. Anal. Chem. 1987759,1236-38. (18) Shaw, R. W.; Young, J. P.; Smith, D. H. Anal. Chem. .989, 61, 695-97. (19) Shaw, R. W;; Young, J. P.; Smith, D. H.; Bonanno, A S.; Dale, J. M. Phys. Rev. A 1990,41,2566-73. Kay Niemax is professor ofphysics at Universitdt Hohenheim (Germany). AleksandrZybin is a senior scientist on leave from the Institute of Spectroscopy of fhe Russian Academy of Sciences. Christoph Schniirer-Patschan and Kenning Groll are researcc scientists at LaserSpec Analytik (Germany)) Address correspondence about this article to Niemax at the Institute of Physics, Universitat Hohenheim 70593 Stuttgart Germany (e-mail
[email protected]).
