Anal. Chem. 1989, 61, 1648-1652
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Near-Infrared Surface-Enhanced Raman Spectroscopy Using a Diode Laser Stanley M. Angel* and Michael L. Myrick Environmental Sciences Division, Lawrence Livermore National Laboratory, Livermore, California 94550
Near-Infrared (near-IR) surface-enhanced Raman spectra (SERS) were measured for a highly luminescent compound, W(2,2'-bWWI)II) ([Ru(bpy)8+), with a 78J-nm diode lw uslng Cu and Ag metal electrodes. No iuminescent interference was encountered and spectra were meagured with high aensltivlty. SER spectra were measured for 8 mM sdutkns of this compound ushgonly 4.3 mW of power from the diode laser. I n addHion, SER spectra were measured for pyridine on Ag and Cu electrodes. Relative SER enhancements for pyrldine were compared for both metals using 785-nm-wavelength and 632nmwavelength excitation.
INTRODUCTION Surface-enhanced Raman spectroscopy (SERS) has been a focus for much study since it was first reported by Fleischmann and co-workers in 1974 (I). Thus far, a considerable amount of this study has aimed at understanding the physical origin of the phenomenon (1-4))but recently there have been many investigations of potential analytical uses for SERS (5-9). Although it has not yet come into general use, SERS is an attractive analytical technique. This is partly because for certain types of molecules it can be very sensitive. Furthermore, because SERS is a vibrational spectroscopy, a SER spectrum contains considerable molecular information. At Lawrence Livermore National Laboratory, we are investigating the possibility of making analytical SER measurements over fiber optics for remote monitoring of groundwater contaminants. For this purpose, small powerful laser sources are needed. With the introduction of Fourier transform (FT) Raman instruments (10, 1I), near-infrared (near-IR) Raman spectroscopy has become an excellent technique for eliminating sample fluorescence and photochemistry in Raman measurements. Recently, the range of near-IR Raman techniques was extended to include near-IR SERS (12-14). Most SER studies to date have been performed by use of visible excitation sources such as Ar-ion lasers, and sample luminescence complicates the measurement in this, as in any, Raman technique. While luminescence from adsorbed species is generally quenched by the metal, luminescence may originate from analyte in solution or from other species in complex samples (e.g. natural waters). Though methods exist for removing small Raman signals from a large fluorescence background (15-21), these do not eliminate fluorescence,but rather take advantage of the different time scales or the different polarization properties of Raman scattering and luminescence. Near-IR SERS reduces the magnitude of the fluorescence problem (12-14) because near-IR excitation eliminates most sources of luminescence (10). Potential applications of near-IR SERS are numerous. A principal interest in this laboratory has been remote monitoring of groundwater contaminants over optical fibers. Near-IR SERS offers many advantages for this application. In addition to eliminating fluorescence problems from the optical fiber and sampling region, near-IR excitation should 0003-2700/S9/0361-1648$01.50/0
significantly reduce the intensity of fiber-optic Raman bands due to the n4 dependency of the Raman intensity, while the SERS intensity does not decrease monotonically with excitation wavelength. Also, the absorption losses of many optical fibers are very low for near-IR radiation. Near-IR SERS is also applicable to monitoring highly luminescent molecules. As an example, resonance Raman studies have been performed on tris( 1,lO-phenanthro1ine)ruthenium(II), [Ru(phen)#+, bound to DNA to determine the stereoselectivity of binding (22). The use of near-IR SERS for the same purpose, while introducing possible surface effects, would avoid problems associated with luminescence of the complex or the biological molecules to which the complex binds, as well as possible changes in binding due to electronic excitation of the molecules. Any spurious results due to photoreaction products could also be eliminated. Near-IR SERS also has physical significance, in that this wavelength range is significantly different from the visible wavelength range used in most SER studies. The calculation of near-IR SER enhancement factors might be useful for verification of models currently used to explain the SERS phenomenon (1-4). For SERS to become a widely accepted analytical technique, a number of obstacles must be surmounted, not the least of which is a convenient laser source. Most SER studies have been performed with visible-excitation sources such as Ar-ion lasers. Ion lasers have the disadvantage of being expensive to purchase and maintain, large and usually water cooled, and not applicable for general laboratory analytical measurements. Semiconductor lasers do not suffer from these problems. These solid-state devices are inexpensive, small, easy to use, and long-lived and require little maintenance. The problem with semiconductor lasers is that powerful single-mode lasers are only available at near-IR wavelengths, though this is likely to change in the near future. Semiconductor lasers have already been demonstrated to be remarkably versatile and useful in a number of spectroscopic applications, as indicated by Ishibashi and co-workers (23,24). To date, however, no report has been made of the use of laser diodes for SERS. The ability to perform SERS with a small, inexpensive, and highly portable excitation source could increase the number of laboratories capable of effectively using this technique by providing a relatively easy entry to SERS research. Also, the development of semiconductor-laser excitation would expand the number of potential applications of SERS for nonlaboratory uses such as in situ environmental monitoring. Thus, the purpose of the present work is to demonstrate that near-IR SERS can be performed with laser-diode excitation sources and to describe the characteristics of this technique. The observation of SERS for pyridine and [ R ~ ( b p y ) ~on ] ~Ag + and Cu electrodes using 785-nm excitation from a laser diode is reported. These two analytes were chosen because their SER spectra are well documented in the literature (25, 26).
EXPERIMENTAL SECTION SER spectra were measured on the surface of the working electrode of a standard three-electrode electrochemical cell. @ 1989 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 61, NO. 15, AUGUST 1, 1989
Electrodes were prepared from 1-mm-diameterAg or Cu wire that was sealed in a glass capillary tube with Norland brand optical epoxy (type NOA-61) and polished flat to expose only the front 1-mm-diameter face of the wire to solution. Electrodes were polished with 3M-brand Imperial lapping film, using a 1-pm grade for final polishing. Electrodes were repolished and rinsed after each analysis. Measurements are reported in aqueous solution. Electrode potentials were controlled with a PAR Model 362 scanning potentiostat and are listed relative to the saturated calomel electrode (SCE). The supporting electrolyte was 0.1 M KC1 in all cases. Solutions were prepared in distilled water. Pyridine was obtained from J. T. Baker Chemical Co. and was used without further purification. [Ru(bpy),]C12was obtained from Alfa Chemical Co. and contained 6.31% water. Roughening of the electrode surface was performed by repeated in situ oxidation-reduction cycles (ORCs). ORCs either were performed manually, oxidizing for approximately 20-30 s at +400 mV followed by reduction at -400 mV for [ R u ( b p ~ ) ~and ] ~ +-700 mV for pyridine, or were performed automatically, allowing the potentiostat to cycle between these potentials for approximately 10 min at a scan rate of 50 mV/s. ORCs were performed in situ since the ultimate goal of this work is to develop portable remote in situ environmental monitoring technology. Part of this research involves determining whether in situ SERS is possible for identifying contaminants. The spectrometer used for these studies consisted of an f/4 scanning double monochromator (SPEX Model 1681B)with 1200 grooves/mm holographic gratings and an f / l collection lens. The detection system was a GaAs photomultiplier (RCA Model 31034) operated at -1800 V with a photon counting system (EG&GModel 1121A amplifier discriminator with Model 1112 counting system). All SER spectra were recorded with a 2-5 integration time and a nominal spectral resolution for a 1000 cm-' Raman band of 16 cm-' using the 632-nm laser line and 10 cm-l using the 785-nm laser line. Scans typically required about 10 min. The 514.5-nm line of an Ar-ion laser was used for measuring fluorescence spectra, while SER spectra were measured by using the 632-nm line of a helium-neon (HeNe) laser (PMS Model LSTP-0050) or the 785-nm line from a GaAlAs diode laser (D.O. Industries, Model GALA-078-16-8). A very strong broad-band emission from the diode laser was removed by using an 830-nm band-pass interference filter that was tilted at a large angle to allow maximum transmission of the 785-nm laser line. This filter significantly attenuated the output power of the diode laser and was only used because a 785-nm band-pass filter was not available. Instability of the laser diode was noted. It was found that the exact wavelength of lasing depended upon length of operating time (a thermal effect) and applied power. When the diode was allowed to thermally equilibrate at full power for several minutes prior to use, the output of the diode appeared to be stable. Measurements with a United Detector Technologies powermeter indicated that the 632-nm line of the HeNe laser produced approximately 8.5 mW of power. The laser diode, when unfiitered, produced 10 mW of power; however, use of the 830-nm interference filter reduced the maximum usable intensity to 4.3 mW at 785 nm. SERS was performed with electrodes immersed in the analyte solution. With 632-nm and 785-nm excitation, little sample absorbance of the excitation light by the [ R ~ ( b p y ) ~solution ] ~ + was noted, even though there was about a 10-mm thickness of aqueous solution between the SERS electrode and the cell window that faced the laser-focusing lens and also between the electrode and the cell window that faced the spectrometer-collection lens. However, when 514.5-nm excitation was used, this sample strongly attenuated the incident radiation and resulted in intense [Ru( b p ~ ) ~luminescence ]~+ from unadsorbed molecules in the analyte solution. When focused to a tight spot, the intense 514.5-nm radiation appeared to be totally attenuated within 4 to 5 mm of entering the analyte solution so that the electrode had to be placed very close to the walls of the sample cell. A 90" scattering geometry was used to collect the SER spectra. The electrode normal was at about a 60° angle with respect to the incident laser beam. ORCs were performed while the SERS electrodes were illuminated with the laser at the power used to make subsequent measurements.
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Figure 1. SERS of 0.1 M pyridine in 0.1 M KCl(aq) at -0.7 V vs SCE. Curve A shows SERS for 785-nm excitation with a diode laser on a Cu electrode. Curves 6 and C are for Ag electrodes. The spectrum for 785-nm excitation is shown by B, and for 632-nm excitation by C.
RESULTS AND DISCUSSION SERS of Pyridine. The SER spectra of pyridine are well documented in the literature, and this molecule has been used as a model in many SERS studies. Recent reports from this laboratory (13, 14) and Chase and Parkinson (12) have indicated that pyridine displays a large SER effect when excited in the near-IR region by a Nd-YAG laser. For this reason, pyridine is used as a model in the present study. Figure 1 shows SER spectra for 0.1 M pyridine in 0.1 M KCl(aq) solution. Curve A gives data for pyridine adsorbed on an ORC-roughened Cu electrode when excited by 785-nm radiation from a GaAlAs diode laser filtered to prevent interference from the broad-band emission of the laser. This spectrum is similar to that obtained on a roughened Ag electrode using 785-nm excitation (curve B), except for the relative intensities of a few bands. Differences between pyridine bands on Cu and Ag electrodes using visible-wavelength excitation have been described in the literature (25) and will not be elaborated on here. Curve C shows the spectrum of pyridine adsorbed on a roughened Ag electrode using 632-nm excitation. This spectrum is very similar to the 785-nm excited spectra, the exceptions being the higher-energy part of the SER spectrum. There is significant attenuation of the intensity of the pyridine bands above about 1500 cm-' when 785-nm excitation is used, and this is due to the reduced sensitivity of the photomultiplier-tube detection system in the near-IR region. Relative SER enhancements were measured for pyridine on the Cu and Ag electrodes by comparing the intensity of the SER band measured a t 1008-cm-' using each excitation wavelength to the intensity of the corresponding band using each excitation wavelength of the regular Raman spectra of neat pyridine. The intensity of the band measured at 1008cm-' for the neat pyridine spectrum was used to normalized differences in the system response and laser power at the two wavelengths. For Ag, the SER enhancement for pyridine at 785 nm is about the same magnitude as the enhancement a t 632 nm within a factor of about 2. However, for Cu, the enhancement at 785 nm is a factor of about 6 larger than a t 632 nm. Both of these values are in agreement with published enhancements at 632 nm and 780 nm (27). It seems apparent that a properly filtered and stabilized semiconductor laser can yield useful near-IR-SER spectra for compounds like pyridine. In succeeding experiments, we have attempted to make use of the properties of near-IR excitation to record the SER spectrum of a more interesting and strongly luminescent metal complex, [ R ~ ( b p y ) ~ in ] ~a+lower , concentration range. SERS of [ R u ( b ~ y ) ~ ] ~[ R + ~. ( b p y ) ~and ] ~ +simiIar com-
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1989
ANALYTICAL CHEMISTRY, VOL. 61, NO. 15, AUGUST 1,
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Table I. Positions and Relative Intensities of SER Bands for [ R ~ ( b p y ) ~on ] ~ Cu + and Ag Electrodes at -0.4 V vs SCE Using 785-nm Excitation from a Laser Diode (All Values in cm-')O
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369 sh 379 s 425 vw 552 vw 646 m 666 w 769 s 809 m 1023 sh 1044 ws 1069 m 1110 w 1174 w 1273 m
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378 s 425 vw 550 w 644 m 665 m
765 s 802 vw 1025 m 1039 ws 1063 m 1107 w 1170 s 1271 s 1315 vs 1486 vs
Relative intensities for high-energy bands are much reduced due to low near-IR PMT response. The spectral resolution is nominally 10 cm". Key: s, strong; m, medium; w, weak; v, very; sh, shoulder. bTaken from ref 26, excitation wavelength is 647.1 nm; some very weak bands are not listed.
that it was obtained in the analyte solution with no evidence of luminescence from or attenuation of the laser beam by analyte in solution, even though there was 1 cm of solution on the excitation and collection sides of the electrode. SER I i spectra could not be obtained by using 514.5-nm excitation 360. 680. 1000 1320 1640 XE 0 under these same conditions because of intense [ R u ( b p ~ ) ~ ] ~ + Raman S h i f t (cm-1) luminescence and attenuation of the laser line. These spectra Flgure 3. SER spectrum of 6 mM [Ru(bpy),]*+ in 0.1 M KCl(aq) on (summarized in Table I) agree with data previously published an Ag electrode at -0.4 V vs SCE. Curve A gives data for 785-nm for this complex (26) within the experimental error. By excitation from a diode laser, whlle B gives data for 632nm excitation. comparison, the spectrum shown in curve B, obtained by excitation a t 632 qm with a more intense HeNe laser, is plexes, such as [ R ~ ( p h e n ) ~ ]may ~ + ,have important biological complicated by background luminescence despite the fact that applications; recent work indicates they may be intercalated into DNA and sensitize the photocatalytic cleavage of the the peak absorbance of the complex is a t 455 nm in roomDNA strands (28). Vibrational studies of this phenomenon temperature aqueous solution and decreases rapidly a t lower energy. This 632-nm-excited luminescence is visible to the would be difficult to perform with resonance-enhanced Raman eye when an appropriate filter is used and interferes with the spectroscopy because kinetic phenomena would be occurring measurement of the SER spectra. as the molecules absorbed light. Normal b a n spectroscopy An additional point to be made concerning the spectra of would be insufficiently sensitive to the small quantities of csmplex involved, and thus near-IR SERS would appear to Figure 3 concerns the relative enhancements obtained at the two wavelengths for [ R u ( b p ~ ) ~ ] Using ~ + . the same electrode have potential applicability to this system. Shown in Figure 2 is the luminescence of 6 mh4 [ R u ( b p ~ ) ~ ] ~ + and solution, we found that the absolute magnitude (in counts in 0.1 M KCl(aq) a t room temperature. This emission was per second) of 785-nm-excited SERS is slightly larger than that of the 632-nm-excited spectrum. Relative enhancements obtained with 514.5-nm excitation from an Ar-ion laser. The are difficult to obtain quantitatively due to differences in unusual band shape of the luminescence shown here is due detector response, optical alignment, and excitation intensity. to instrument response variations of our spectrometer over However, the relative enhancement obtained for the nearthe large energy range covered. As shown, the luminescence IR-excited spectrum is qualitatively larger than for the visicovers a large energy range extending from approximately 540 ble-excited spectrum. Optics were aligned for maximum signal to 900 nm. This very bright luminescence, when measured in both cases. However, detector response falls rapidly in the under the same conditions as the SER spectra, is much more near-IR region, and the excitation power available from the intense than the Raman signal over most of this range when near-IR diode was only 51% that of the 632-nm line. excitation is in the visible region. As will be shown below, even low-energy visible excitation results in significant luminesIn Figure 4 is shown the SERS of 6 mM [ R u ( b p ~ ) ~on ]~+ cence, which interferes with the SER signal. Luminescence a Cu electrode. Enhancements for this compound on the Cu following high-energy excitation is frequently a significant electrodes appeared smaller than on Ag electrodes, though problem even for "nonluminescent" molecules (29),the reof the same general magnitude, which is consistent with residual luminescent impurities contributing to this difficulty. ported wavelength-dependent SER enhancements of pyridine Figure 3 shows the SER spectra of 6 mM [ R u ( b p ~ ) ~in] ~ + on Cu and Ag electrodes (27). 0.1 M KCl(aq) solution on an Ag electrode roughened by Curve A of Figure 4 gives the SER spectrum on Cu for excitation at 785 nm with the semiconductor laser. Again, several ORCs. Curve A of Figure 3 gives the SER spectrum a reasonably clear spectrum is obtained with this source. In of [ R u ( b p ~ ) ~ on ] ~ 'an Ag electrode when the molecule is excited at 785 nm. What is remarkable about this spectrum is contrast, the data of curve B, showing SERS using 632-nm
ANALYTICAL CHEMISTRY, VOL. 61, NO. 15, AUGUST 1, 1989 xE 4 ,900
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To the left are measurements of the laser line for two different applied power levels (the laser line positiins are offset by about 250 cm-' for clarity). To the right is a SEI? spectrum of [Ru(bpy),]*+on an Ag electrode using an interma dlate power of the diode laser. Illustrated are effects of mode hopping: splitting of SER bands due to multiple simultaneous discrete modes of the solid-state excitation source.
excitation, reveal a large amount of background luminescence from the bulk analyte solution. These data also indicate that enhancements for [Ru(bpy)$+ on the Cu electrode are greater for 785-nm excitation than for 632-nm excitation. determined for each new experiment. The position of the laser Characteristics of Laser Diode Excitation. It is espeline, after equilibration at full power, did not appear to vary cially important for Raman applications that the laser be significantly over extended periods of time. For this reason, "single-mode"; i.e., it possess a narrow, well-defined frequency. and because of the long life of the laser device (on the order For other applications such as photoacoustic spectroscopy, of 10000 h or greater at full power), it is reasonable for some spectrophotometry, or fluorescence, this requirement is relaxed applications to leave the diode laser at fullpower continuously. because molecular absorption bands are usually broad comThe diode laser used for this work was of moderately low pared to the laser line width, even for multimode diode lasers. power compared to other diode lasers that are commercially However, Raman bands are usually narrow, and the Raman available. It seems likely that much greater sensitivity will band positions are measured relative to the laser line. Acbe possible as more powerful single-mode diode lasers become curate calculation of vibrational frequencies therefore relies available. The use of a more powerful diode laser coupled with upon precise knowledge of the laser frequency. A principal the proper detection system might provide a relatively low-cost difficulty with the use of the laser diode for Raman studies alternative to an F T Raman system for measuring SER (or is its tendency to "mode hop", or change frequencies by disnormal Raman) spectra of highly fluorescent compounds. crete amounts, depending upon operational temperature and applied power. However, this difficulty may be easily overLITERATURE CITED come by taking appropriate precautions before use of the Fleischmann. M.; Hendra, P.; Mcquillan, A. Chem. Phys . Lett. 1974, diode. 2 6 , 163. Shown in Figure 5 is a SER spectrum of 2 mM [ R ~ ( b p y ) ~ ] ~ + Furtak, T. In Advances In Laser Spectroscopy; Garetz, B., Lombardi, J., Eds.; Wiley: New Ycfk, 1983; Vol. 2, p 175. complicated by mode hopping. On the left-hand side of the Surface Enhanced Raman Scattering; Chang, R., Furtak, T.. Eds.; PIS figure, two distinct wavelengths of laser activity are shown, num Press: New York. 1982. measured with the diode laser at two different power levels. Jeanmarie, D.; van Duyne, R. J . Electroanal. Chem. Interfacial Electrochem. 1977, 8 4 , 1; The intensities of the narrow laser lines are scaled arbitrarily Vo-Dinh, T.; Hiromoto, M.; Begun, G.; Moody. R. Anal. Chem. 1984, for easier comparison; however, the higher-energy line was 5.. 6 ,. 1667. ... . Eniow, P.; Bunclck. M.; Warmack, R.; Vo-Dinh, T. Anal. Chem. 1986, obtained by reducing power to the laser. 58. 1119. Also shown in Figure 5 is a SER spectrum of 2 mM [RuAiak, A.; Vo-Dinh, T. Anal. Chem. 1987, 59, 2149. (bpy)S]*+on an Ag electrode. Evident in this spectrum is the Torres, E.; Winefordner, J. Anal. Chem. 1987, 56, 1626. Carrabba, M.; Edmonds, R.; Rauh, R. Anal. Chem. 1987, 5 9 , 2559. result of "mode hopping" of the diode laser; the two largest Hirschfeld, T.; Chase, B. Appl. Spectrosc. 1986, 40. 133. SER bands appear as doublets with pronounced side bands. Chase, D. B. J . Am. Chem. Soc. 1986, 108, 7485. Chase, D.; Parkinson, B. Appl. Spectrosc. 1988. 42, 1186. In effect, this gives rise to a superposition of SER spectra with Angel, S.; Katz, L.; Archibald, L.; Lln, L.; Honigs, D. Appl. Spectrosc. an energy offset of about 25 cm-', which corresponds to the 1988, 42, 1327. energy separation of the two modes of the diode laser. This Angel, S.; Katr, L.; Archibald, D.; Honigs, D. Appl. Spectrosc. 1989, 43, 367. spectrum was obtained by operating the diode laser at a power Yaney, P. J . Opt. SOC. Am. 1972, 8 2 , 1297. level intermediate between those required to obtain the laser van Duyne, R.; Jeanmaire, D.; Shriver. D. Anal. Chem. 1974, 46. 213. lines shown on the left of Figure 5. Morhange, J.; Hiriimann, C. App. Opt. 1976, 75, 2969. To overcome this potential problem, we found it necessary Angel, S.; DeArmond, K.; Hanck, K.; Wertz, D. Anal. Chem. 1984, to allow the device to come to thermal equilibrium before 5 6 , 3000. Mann. C.; Vickers, T. Appl. Spectrosc. 1987, 41. 427. beginning an experiment. This required approximately 15-20 Hamaguchi, H.; Tahara, T.; Tasumi, M. Appl. Spectrosc. 1807, 4 1 , min. In addition, best results were obtained with the laser 1265. Bright, F.; Heiftje, G. Appl. Spectrosc. 1988, 40. 583. operating at the full recommended power of the device. Barton, J.; Danishefshy, A.; Goklberg, J. J . Am. Chem. SOC. 1984, Although some spectra were obtained that had the general 706, 2172. appearance of Figure 5, it was more commonly observed that Nakanishi. K.; Imasaka, T.; Ishibashi, N. Anal. Chem. 1985, 5 7 , 1219. the positions of the peaks in the SER spectra appeared to shift Imasaka, T.; Yoshitake, A.; Ishibashi, N. Anal. Chem. 1984, 5 6 , slightly in a random fashion. This appeared not to be due 1077. Pettinger, B.; Wenning, U.; Wetzel, H. Surf. Sci. 1980, 101, 409. to misalignment of the monochromator but rather to variation Virdw, H.; Hester, R. J . Phys. Chem. 1984, 8 8 , 451. of the lasing mode. Thus, for this particular diode laser, it Pettinger, B.; Wetzel, H. Ber. Bunsen-Ges. Phys. Chem. 1981, 8 5 , was determined that the position of the laser line should be 473.
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(28) Kelly, J.; McConnel, D.;Ohlligin, C.; Tossi, A.; Klrsh-De Meemaeker, A.; Nasielshl, J. J . Chem. Soc., Chem. Commun. 1987, 1821. (29) Hlrschfeld, T. Appl. Spectrmc. 1977,3 1 , 328.
was performed under the auspices of the U.S. Department of Energy under Contract W-7405-Eng-48. The authors express thanks to Paul Duhamel of the Office of Health and Envi-
Studies of Sputtering Atomizers for Atomic Absorption Spectroscopy David S. Gough,* Peter Hannaford, and R. Martin Lowe
CSIRO Division of Materials Science and Technology, Locked Bag 33, Clayton, Victoria 3168, Australia
Factors lnfluenclng absorptlon wnrltlvity and reproduciblllty have been lnvestlgated for several sputtering atomltenr, Includlng the Analyte Corporatlon Atomsource and a slmllar system reported prevlousty. The enhancement In wnsitlvlty (factor of 3) of the Atomsource over that of the earl& system for given sputterlng cond#kns Is shown to resutt melnly from the longer absorptlon path in the Atomsource. The reproduciMUty is found to be comparable for the varkwr atomizers studied, e.g. about 05-1 % relathre dendard devlatbn for the case of chromlum In low-alloy steel. The presence of water vapor In the argon sputterlng gas at k v e b greater than abml 10 ppm is found to have a deleterkw, effect on the sputtering efflclency and reproduclblllty.
INTRODUCTION The use of cathodic sputtering as a means of atomizing samples for atomic absorption spectroscopy was proposed by Russell and Walsh (1) in 1959, soon after the introduction of the atomic absorption technique. With the sputtering method the sample to be analyzed is made the cathode of a lowpressure rare-gas discharge and subjected to bombardment by energetic rare-gas ions formed in the discharge. Under the action of the ion bombardment, atoms are ejected from the cathode surface, thereby creating an atomic vapor of the cathode material. The f i t reported sputtering atomizer, that of Gatehouse and Walsh (2),required the samples to be in the form of a hollow cathode and to be mounted inside the sputtering chamber. An improved sputtering atomizer described by Gough et al. (3) allowed solid samples with a flat face to be mounted onto the outside of a glass chamber against an insulating disk (called the discharge arrester) and O-ring. The disk had a central hole to confine the discharge to a constant area and a narrow recessed step adjacent to the sample to prevent sputtered material from establishing electrical contact between the sample and inner walls of the chamber. The system utilized a flowing stream of argon t o help remove gaseous impurities from within the chamber,but was not satisfactory for the analysis of readily oxidized metals such as aluminum or zinc. An important advance in the development of sputtering atomizers was to introduce the flowing argon gas as closely as possible to the sample surface. In the system described by Gough (4), which is shown in Figure 1, the gas is admitted into the chamber through a narrow (0.1 mm) annular gap located just below the cathode surface (Figure lb). The pressure developed behind the narrow gap forces the gas to enter the sputtering chamber at 0003-2700/89/0361-1652$01.50/0
high speed. This arrangement has two distinct advantages: (i) the fast gas flow entrains sputtered atoms, greatly reducing lateral and back diffusion, and sweeps them into the light path, resulting in an increased absorption sensitivity of typically an order of magnitude. (ii) The rapid flow of gas at the cathode surface sweeps away gaseous impurities and thus allows metals such as aluminum to be analyzed without difficulty. A newly developed commercial sputtering atomizer, called the Atomsource (Analyte Corp., Grants Pass, OR), also incorporates the principle of high-speed flow of argon at the cathode surface. In this device six jets of argon are directed at the cathode surface to produce a balanced flow of gas that sweeps the sputtered atoms orthogonally away from the surface and into the center of the chamber (see Figure 2). An advantage of the Atomsource is the use of a T-shaped absorption chamber in which the gas flow, with entrained sample atoms, is directed along the light path to increase the absorption sensitivity. The robust design of the Atomsource arrester allows it to be operated at higher powers (factor of about 4) than those of the earlier atomizers, thus increasing the rate of sputtering from the surface of a sample. Applications of the Atomsource have been discussed in a number of recent publications. Ohls (5) has reported a preliminary study of the use of the Atomsource in the analysis of metals and of solutions deposited on metallic cathodes. Kim and Piepmeier (6)have recently reported detailed studies of sample loss rates and discharge conditions in the Atomsource and also in a simple, single-jet atomizer, These authors also carried out scanning electron microscopy studies of the surface profiles produced under various discharge and flow conditions. Chakrabarti et al. (7) have recently reported studies of the transient atomization of solutions deposited on metallic cathodes, and Winchester and Marcus (8) have used the Atomsource to atomize nonconducting powders. In this paper we report investigations into the physical principles underlying differences in absorption sensitivity and reproducibility of various sputtering atomizers that utilize a high-speed flow of gas at the cathode surface.
EXPERIMENTAL SECTION Sputtering Atomizers. A number of glass sputtering chambers were constructed with the three basic configurations shown in Figure 3. The sample mounting arrangement and gas inlet system for these cells is the same as in Figure lb. It consists of a hollow ceramic disk that is sealed by an O-ring between the sample and the main body of the chamber. The argon sputtering gas enters the chamber through the 0.1-mm gap in the arrester and, because of the pressure differential across this gap, is forced into the cell at high speed (- lo4cm/s at the orifice). The rapid @ 1989 American Chemical Society
