James R. Allkins 15485 One Oak Lane Monte Sereno, Calif. 95030
It has long been the dream of many spectroscopists to have available a high-intensity source of monochromatic radiation that could be tuned to any desired wavelength a t will. Such a source is the tunable laser. It is not surprising, therefore, that in the nine shGrt years since Sorokin and Lankard first observed stimulated coherent emission from a laserpumped organic dye (1) and Schafer et al. ( 2 ) realized the tunability of such a source, the tunable dye laser has experienced an extraordinary growth in its development (3-7). At present, owing in part to anticipated applications and in part to strong commercial competition, continuously tunable dye lasers are available a t a cost that is well within the budget of most industrial laboratories and with an ease of operation that is comparable to that of fixed frequency gas lasers. Before becoming too engrossed, however, it should be recognized that the tunable dye laser is not the immediate answer to all problems, and despite its tremendous potential, it still has certain drawbacks. The first is that the effective tuning range is restricted to the visible region of the electromagnetic spectrum (typically 750-350 nm). Tunable ultraviolet radiation can be and is obtained by frequency doubling the visible radiation inside the laser cavity with a suitable nonlinear crystal ( 8 ) , but even then the upper limit of the tuning range is usually confined to around 260 nm. Secondly, to cover the entire visible region entails the use of many different dyes; therefore, various dye solutions have to be switched in and out of the pumping area as each region is scanned. Tunable sources of coherent radiation other than dye lasers are available for other regions of the electromagnetic spectrum (9-12), particularly the infrared. Semiconductor diode lasers 752A
and spin-flip Raman lasers emit in the “information-rich” infrared fingerprint region (4000-400 cm-’). Unfortunately, they too have their problems such as requiring cryogenic cooling and high-power magnetic fields to operate efficiently. They also cover a rather limited tuning range a t any one time. However, their potential applications are so far reaching that these restrictions should be overcome in the very near future. Other tunable laser sources such as parametric oscillators and frequency mixing devices have, so far, been confined to the laboratory bench since they require a very stable temperature environment and very fine positioning controls. They also should achieve commercial viability once potential applications have been developed. In light of the above preamble, it is not surprising that most applications of interest to chemists with tunable laser sources have been performed with the tunable dye laser. The major part of this article will, therefore, be concerned with the impact of tunable dye lasers on analytical spectroscopy. In particular, applications which have taken full advantage of the laser’s unique properties of high intensity, narrow spectral bandwidth, coherence, small beam diameter, and small divergence directionality will be discussed. The potential impact of other tunable sources will serve as a conclusion.
Tunable Dye Laser A simplified version of a typical dye laser is shown in Figure 1. Both the etalon for frequency narrowing and the nonlinear crystal for intracavity frequency doubling can be inserted in the laser cavity if and when required. To cover most of the near UV-VIS tuning range requires that a series of dyes in solution flow sequentially through the dye cell, since each dye only emits effectively over a limited range of some 20-50 nm. Alternative-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 8, JULY 1975
ly, different dye cells containing different dyes can be swung in and out of the pumping cavity as required. Typical tuning elements and pumping sources are listed in Table I. Dyes found most effective for stimulated emission are the coumarins, rhodamines, cresyl violet, and fluorescein. Factors that could reduce the efficiency of the lasing process such as nonradiative transfers or dimer formation by the dye are minimized by the addition of quenchers and deaggregating agents to the dye solution. A comprehensive discussion of various factors that affect the efficiency of dye lasers has been published recently by Drexhage (13).
Applications of Tunable Dye Lasers Before becoming too deeply involved in the various applications of tunable dye lasers in analytical specTable I . Typical Tuning Elements and Pumping Sources for Dye Laser Systems (Flashlamp and N, laser pumping is usually by way of P,, whereas other lasers pump axially along P,; see Figure 1 ) Pumping sources
Flashlamp N, laser Freq. dbld. ruby laser Freq. dbld. Nd: YAG Freq. dbld. Nd:,glass
Another dye laser C W argon laser Xenon laser
Tuning elements
Grating Prism/rnirror assy Birefringent filter Tilted etalonu Wedged etalon Ly o t f i Itera Acousto-optic filter I nterferometera
a Also used a s frequency narrowing devices.
Report
D I
troscopy, it appears appropriate to define more fully those qualities of the laser that make it unique from other, more conventional, sources of radiation. The primary property of the laser is that all of the available laser radiation is confined in a physically very narrow, low divergence, directional beam that can be placed precisely a t any point in a required area. Furthermore, the beam possesses spatial coherence which means that it can be focused to a very small spot a t any point along its path with a concomitant increase in power density. Not only is the beam narrow in size (spatially), but it is also narrow in frequency spread (spectrally). This means that most of the radiation intensity is contained in a narrow band of frequencies around a central dominant mode. Furthermore, this bandwidth of frequencies can be made even narrower by the insertion of suitable etalons into the laser cavity. The narrow bandwidth is extremely useful for high-resolution spectroscopy since source broadening limitations are almost completely removed. Also, the loss of energy accompanying frequency narrowing of a standard source by filtering through very narrow slits of a standard monochromator is not encountered with the laser, since almost all of the available energy of the laser is channeled into the narrow band of frequencies selected. Thus, we have a narrow beam of high-intensity radiation concentrated in a narrow band of frequencies that can be directed to almost any small area in a system without any substantial loss of power in getting there. Finally, there are the temporal properties of a laser, that is, the variations of output power with time. Tun-
M
able lasers fall into two broad classes, namely pulsed or CW (continuous wave). With pulsed dye lasers, Le., those pumped by a pulsed excitation source, the available energy emerging from the laser is confined in a stream of short bursts or pulses. Pulse characteristics are generally determined by the temporal characteristics of the pumping source. With pulsed nitrogen laser pumped dye lasers, the pulse lengths tend to be short (nsec), whereas the peak powers of each pulse tend to be low. With flashlamp pumping the pulses tend to be longer (wsec), but the peak powers tend to be high. Flashlamp pumping, therefore, gives more energy per pulse, but nitrogen lasers have faster repetition rates (more pulses per sec). Pulsed lasers are used to great advantage in studies of time-related phenomena such as lifetimes, relaxation rates, reaction kinetics, or in studies involving nonlinear optical phenomena in which high peak powers are required. Pulsed lasers may be operated in either a single pulse mode or repetitively in which a stream of pulses is generated with a regular time interval between each pulse. Continuous wave (CW) lasers are pumped by CW sources such as a CW argon ion laser. Although the power output of a CW laser cannot attain the peak power of a pulsed laser, the CW output is continuous and therefore more akin to the conventional sources of radiation used in most spectroscopic techniques. The average power output of a CW laser is usually higher than that of a pulsed laser, but, more importantly, the power stability of a CW source is usually much greater. The power output from a pulsed laser can vary considerably from pulse to
Figure 1. Schematic of typical dye laser Solutions of dyes in dye cell D are pumped by external sources PI or P2 to produce stimulated emission. Generated emission is wavelength tuned by tuning element T and frequency narrowed by optional etalon E. Nonlinear crystal X is inserted into cavity when tunable radiation is required in ultraviolet. A small fraction of tuned radiation is transmitted by output mirror M, the remainder being reflected for further optical amplification. An iris I is often inserted in cavity to control mode characteristics of intracavity beam
pulse. This makes the CW laser especially useful in applications which demand a steady source of very stable power contained in a very narrow bandwidth of frequencies. The following applications exemplify one or more areas where the properties of a tunable laser have proven superior, or at least equivalent, to those of previously used conventional sources. Advantages found with the laser will include greater sensitivity to improve limits of detection, greater resolution to reveal more information content, and better selectivity to aid in identification.
Absorption and Fluorescence Spectroscopy Two of the more interesting properties of a tunable laser are its high spectral brightness and narrow frequency bandwidth. This implies that both high-sensitivity and high-resolution spectroscopy should be natural candidates for tunable laser applications. The sodium atom has received particular attention in this respect, since two of its primary atomic absorption lines (D1 and D2) lie close in wavelength to the emission peak of one of the more efficient lasing dyes, namely, rhodamine 6G. T o demonstrate the potential detec-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 8, JULY 1975
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TLC SEPARATIONS AREA SNAP
Flgure 2. High-resolution spectrum showing hyperfine structure of sodium D2 line recorded with CW dye laser Lower part shows theoretical structure. F' and Fdenote quantum numbers of total angular momentum of hyperfine levels of 32P3/2 and *S1/2 levels, respectively Figure taken from ref. 15 and reproduced with permission of Springer-Verlag. New York
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tion sensitivity of a system involving a dye laser as its source, Jennings and Keller (14) observed the laser-excited fluorescence emission from a sample of sodium contained as a vapor in a simple heated cell. Using a frequency narrowed (-0.003 nm) CW dye laser with 50-mW output power a t 589.6 nm as the source, they were able to detect gram (4.2 X as little as 0.016 X lo5 atoms) of sodium vapor. Detection was by simple visual observation of the fluorescence emission. Their system has since been modified to increase the sensitivity by a factor of 200 ( 5 ) ,which is an improvement in sensitivity of approximately lo6 over sodium detection by vapor lamp excitation. In the area of high-resolution spectroscopy, considerable interest has been generated in the past two years since this is one area in which the laser can show dramatic improvements over conventional techniques. As the line-
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ANALYTICAL CHEMISTRY, VOL. 47, NO. 8, JULY 1975
width of the laser can be frequency narrowed to a value of approximately 'booth of that normally observed for a single atomic absorption line, hyperfine structure can be easily resolved with a laser source. To obtain highresolution spectra requires a source with small bandwidth, excellent frequency stability, and good stable tunability. The frequency stability must be appreciably better than the desired resolution, and the laser must be able to tune smoothly without mode hopping. Figure 2 illustrates the high-resolution capabilities of a frequency stabilized CW dye laser as recorded by Hartig and Walther ( 1 5 ) for the sodium Dz line. Demonstrated resolution nm (30 was of the order of 3.5 X MHz) [this has since been improved to 10 MHz (6)], and the frequency of the laser was stabilized to f 1 . 5 MHz. Hyperfine structure is clearly resolved. T o observe such hyperfine structure requires not only that the source be
narrow, but also that line broadening of the sample be reduced to a minimum. The two primary causes of spectral line broadening in the visible are collision broadening and Doppler broadening. Collision broadening may be reduced by lowering the pressure in the sample cell. The high spectral brightness of the laser can more than compensate for the corresponding reduction in atom density. Doppler broadening, arising from axial motion of the sample atoms in the sampling beam, can be reduced by such techniques as saturation spectroscopy (16),by confining the sample in an atomic beam ( 1 5 ) ,or by the relatively new technique of two-photon absorption spectroscopy (17). Using a special jet stream dye-flow system and a CW argon laser pump, Wu et al. (18)have recently recorded the hyperfine structure of 12 with a resolution of one part in 109. The iodine sample was contained in a molecular beam, and the laser stabilized to six parts in 10'3. On a more practical note, Gibson and Sandford have taken advantage of the sodium doub1et:rhodamine 6G intensity match to remotely monitor sodium concentrations in the atmosphere. They were particularly interested in variances of the naturally occurring sodium layer found a t an altitude of about 90 km. Results have been obtained for both day (19) and nighttime (20) concentrations. Other gaseous constituents of the atmosphere including NOz, SOz, and 12 have also been detected remotely using tunable dye laser sources (21, 22), as have algae concentrations in the sea (23). On the laboratory scale, one of the most widely reported uses of dye lasers in analytical spectroscopy has been in the field of laser-excited atomic fluorescence flame spectrometry (AFFS), a technique for analyzing trace elements (24-27). To detect small atom concentrations in a flame requires good signal-to-noise ratios for the atomic fluorescence emissions. To obtain strong fluorescence emission requires an excitation source of high spectral brightness that is also monochromatic. The source should also be stable when repetitively pulsed, have a high peak power output, and a small duty cycle (small ratio of on-to-off time). Such a source is the pulsed dye laser. The tunability of the dye laser gives the added advantage that one source may be used to study many different atoms in a sample, and a variety of atoms have been studied by this technique. A typical system is shown in Figure 3, in which gated detection is employed to remove all noise except that generated by scattering of the laser ra-
Figure 3. Block diagram of experimental system for laser-ex:cited atomic fluorescence flame spectrometry Figure taken from ref. 25
diation within the flame. Background scatter can be reduced even more by the use of suitable nebulizers or by recording fluorescence emissions from nonresonance transitions. It would appear that with such an ideal source as the pulsed dye laser for AFFS, lower limits of detection and wider range, linear analytical curves would result from improved signalto-noise ratios when compared with conventional trace analytical methods such as atomic absorption, atomic emission, or atomic flame fluorescence excited by conventional sources. Extensive results (28), however, show that at best the detection limits for laser-excited flame fluorescence are only slightly better than those of other atomic flame spectrometric techniques. The reason for this anomaly has been traced in part to a saturation effect (29, 30). In this effect, the intensity of the laser irradiance reaches such a level that the atomic absorption transition becomes essentially saturated. Any further increase in laser irradiance simply results in increased background scattering. A limit is therefore set on the linear relationship between the fluorescence signal and the exciting laser radiation intensity. In more exact terms, the atomic fluorescence radiance assumes a nonlinear dependence on the exciting spectral radiance once the excitation radiance reaches a certain level. By irradiating only certain areas of the flame and by careful focusing of the laser beam to a power level approaching saturation, Omenetto and his coworkers (31)were able to extend the linear range of certain analytical curves by a factor of 1000. One other advantage observed when working near saturation is that the atomic fluorescence emission signal is virtually independent of source fluctuations and is also less affected by collision quenching. Further improvements in detection limits are expected when laser tuning ranges are finally extended further into the UV since many of the more sensitive absorption lines (Le., those with high ab756A
sorption oscillator strengths) are in the mid to far UV region. Initial results obtained in the ultraviolet region for Mg, Ni, and P b have already been reported by Kuhl and Spitschan (32) using a frequency doubled dye laser as the excitation source. Other recent applications involving tunable laser excitation of flame fluorescence have been in the detection and measurement of CH (33)and C2 (34) radicals in flames. Flameless atomic fluorescence spectroscopy with laser excitation has also been demonstrated recently by Neumann and Kriese (35) using a frequency doubled flashlamp pumped dye laser. They were able to detect a low limit of 0.2 pg of P b and extend the linear range of the P b analytical curve by two orders of magnitude over that obtained with more conventional sources. Finally, the first report has recently appeared on the analytical applications of laser excited fluorimetry of molecules in the condensed phase ( 3 6 ) . Another technique that has received considerable attention in the past few years is laser enhanced absorption spectroscopy (37) or selective intracavity laser quenching, as it is some-
times called. In this technique, a sample is introduced into the cavity of a dye laser operating under broadband conditions, as shown in Figure 4. Broadband conditions are obtained by replacing the tuning element in the cavity by a broadband totally reflecting mirror. All wavelengths of the stimulated dye are then emitted simultaneously without spectral narrowing. If the sample has an absorption or absorptions at wavelengths within the broadband emission curve, losses will occur in the laser cavity at these wavelengths. The result is that the usually smooth emission profile from the dye laser now contains absorption bands definitive of the sample. A rather striking example is shown in Figure 5 in which a sample of NO2 has been introduced into the cavity of a flashlamp pumped dye laser (38). This method is particularly sensitive since it involves a selective spoiling of the gain inside a laser cavity. Single pass gain in a cavity is critical for lasing to occur, especially near threshold. A further advantage of the method is that the whole spectrum is obtained in one shot, as it were, since
Figure 4. Dye laser system set up for broadband operation. Laser enhanced absorption spectrum of intracavity sample is recorded by spectrograph
ANALYTICAL CHEMISTRY, VOL. 47, NO. 8, JULY 1975
Figure 5. lntracavity absorption spectra of NOP Trace at right represents NO2 pressure of 6 mm: trace at left. 7 mm. Mercury emission lines Shown to iell of each trace are included lor wavelength calibration Figure taken horn ref. 98
one pulse of the flashlamp will produce one pulse of laser emission. Thus, high-resolution ahsorption spectra of short-lived species, such as transient free radicals, may be recorded (39). Since pulsed excitation is most often used for this technique, the ahsorption-emission spectrum is usually recorded photographically by means of a spectrograph. Photoelectric detection involving photodiode arrays has also heen reported (40) and will probably become the method of choice, especially when high sensitivity is required. Most recent work has been directed toward developing hoth the quantitative and qualitative potential of intracavity ahsorption spectroscoPY. Raman Spectroscopy One hranch of spectroscopy that has benefited considerably from the discovery of the laser has been that of Raman spectroscopy (41,42). It would seem only natural, therefore, that Raman spectroscopy should benefit even further from the development of the tunable dye laser. In a conventional laser-excited Raman system, the excitation source is fixed in frequency (for example, a CW Ar+ or Kr+ gas laser), and the scattered light from the sample analyzed with a scanning spectrometer. With the appearance of the tunable laser, one thought that immediately springs to mind is why not remove the rather costly spectrometer and instead tune the laser source? An
investigation along these lines was performed hy McNice in 1972 ( 4 3 , hut since then no reports appear IO have heen published. In McNire’sexperiment he used a stack of interference filters in plare of the spectrometer and tuned the laser arross the Raman region of interest. HP sucressfully recorded Raman emission from oxygen and COLIbut noted that the system Sensitivity was limited hy scattered light from the laser flashlamp. The old prohlem in Raman spertrosropy of scattered light rejertion thcrefore prevails in tunable laser Raman spectroscopy. Further limitations included thc restricted tuning range of the dye and the variance of laser output power with both wavclength and time. With future developmenu in stahle o u t p u ~CW tunable laser3 and advances in interference filter terhnolow, further research in the area of no-spectrometer spectroscopy should be forthrominy. Despite the tremendous impetus given to Raman spertroscopy by the laser, the most severe limitation is still the fact that the Raman effect is inherently very weak. Two techniques have been devcloped recently IO sperifirally overcome this sensitivity prohlem by resonant enhanrement of the Raman intensities. Both techniques depend heavily on the tunahility of the excitation source. The first terhniquc i j that of coherent anti-Stokes Raman spectroscopy (44) or CARS, as it is now known. Ex-
tensive theoretical descriptions of the process have already been discussed (45) and will not he repeated here. In essence, with reference to Figure 6, in the normal spontaneous vibrational Stokes-Raman effect, a molecule is excited to some virtual state by interaction with a source of excitation radiation of frequency w p The molecule then returns instantaneously to the first excited vibrational energy level of the electronic ground state, emitting a photon of frequency os.The difference w p - usis equal to a fundamental frequency of the molecule involved. For anti-Stokes emission, the initial energy level is the first excited vihrational level, and the transition terminates a t the ground state, the emitted photon then having a frequency of.,0 If a n intense beam of coherent radiation of frequency wQ such as can he produced by a tunahle dye laser is now focused into the sample so that it crosses the original excitation pump heam upa t a small angle in the Sam. ple, then an intense, coherent beam of anti-Stokes Raman emission is generated from the crossing point with a . * The process is nonlinfrequency u ear and depends on the relationship mas = 2 up - us.The anti-Stokes heam is generated a t an angle with respect to the pump beam and the dye laser heam and can therefore he spatially separated without a spectrometer. As w8 is tuned in frequency across the normal Raman region, anti-Stokes emission will appear every time that the value of up - w, coincides with the frequency of a Raman active vibration. Advantages of this process are numerous in that emission intensities are very high compared with the normal Raman process; fluorescence which may mask normal Raman emission is eliminated as the observed output is in the fluorescence-free anti-Stokes region; and no spectrometer is required. However, the efficiency of the process is quadratically dependent on the intensity of the pump beam and therefore requires the high peakpowers availahle from pulsed lasers. Limitations of tuning range and cost are also involved, and the experimental conditions are still very much in the research stage. It is not expected that CARS will replace conventional Raman spectroscopy as an analytical technique but rather be used instead for very specific applications. Such an application has already been reported for the measurement of Hz gas concentration, both in a flame and in a supersonic jet (46). I t is felt, however, that the full potential of CARS for practical analytical spectroscopy has yet to be developed. The second technique for Raman intensity enhancement is that of reso-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 8, JULY 1975
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Flgure 6. Energy level diagram for Raman process, adapted to show generation of stimulated anti-Stokes Raman emission Figure taken from ref 45 and reprdvcsd wih prmission of ma American Instbie of PhPiCB and the a u t h a
nance Raman spectroscopy. If the frequency of the Raman excitation source is tuned to coincide with the frequency of an electronic absorption of the molecule being studied, then a resonance process is set up. The result of this resonance is that the Raman intensities of certain fundamental vihrations of the molecule are tremendously enhanced. Considerable research has already heen performed in the investigation of this technique and in utilizing its advantages for determining spectroscopic constants of small molecules (47). It has also been found useful as an aid in assigning vibrational frequencies. A more practical application of resonance Raman spectroscopy using tunable lasers has been in the study of biological molecules (48). If, in a complex molecule such as a protein, a group of atoms gives rise to an isolated electronic absorption band, then by tuning the excitation source into that isolated electronic absorption, the resonance Raman effect will enhance the intensities of only those Raman emissions associated with vibrations of the atoms of that group, Such a group of atoms may he a biological chromophore. Therefore, by the technique of resonance Raman, certain selective sites in a very complex molecule can he studied in isolation, as though they 758.4
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were independent units, without excessive interference from spectral lines arising from other sites in the molecule. Using this technique, Spiro and his cowork& have carried out extensive studies of the heme proteins especially with regard to changes in structure and bonding arising from different spin and oxidation states (49).By tuning the laser excitation frequency into selective electronic absorption hands of the heme molecule, they have been able t o study vibrations of the porphyrin rings in almost complete isolation. This particular application of tunable lasers is expected to promote a whole new approach t o structure elucidation of complex biological systems. I
Other Tunable Laser Sources As mentioned earlier, other tunable laser sources, particularly those emitting in the infrared, have been fully developed. Only parametric oscillators appear to he available commercially at present. Various excellent reviews have been written on the characteristics, operation, and applications of tunable laser sources other than dye lasers (9-12,50,51);this report will, therefore, he restricted to recent applications pertaining to analytical spectroscopy. For this restricted case (i.e., exclud-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 8. JULY 1975
ing such tunable sources as high-pressure gas lasers and frequency mixing generators), tunahle lasers that operate in the infrared can he classified into three major types: Semiconductor diode (SD)lasers Spin-flip Raman (SFR) lasers Parametric oscillators (PO) In each of the above, tuning is accomplished hy varying either an applied magnetic field, the temperature of the device, or its position in an external pumping beam. At present, hoth SD and SFR lasers require cryogenic cooling which severely limits their application outside of the laboratory. SD and SFR lasers have limited continuous tuning ranges (of the order of cm-'). However, they do operate with very narrow spectral bandwidth's (10-6cm-'). They are, therefore, ideal for ultrahigh resolution spectroscopy, especially in the frequency region in which most fundamental molecular vibrations occur (4000400 cm-': 2.5-25 pm). Parametric oscillators have a much wider tuning range, but the range is restricted mainly to the midinfrared. Also, the spectral linewidths of Po's are broader than those of SD or SFR lasers, being typically 1-2 cm-l, although this has been narrowed to 0.001 cm-' in special cases. Work is progressing rapidly at present to extend the tuning range of PO'S further into the infrared (52). Mdst of the practical applications developed for SD, SFR lasers, and parametric oscillators have been in the area of atmospheric pollution studies (50).In the case of parametric oscillators, Henningsen e t al. (53) have reported the remote detection of CO using a parametric tunable laser. By the technique of resonance absorption and by tuning the parametric laser over the frequency region of the rotational lines of the first vibrational overtone of CO a t 2.3 pm, they have heen able to measure CO concentrations over a distance of 107 meters. Applications of tunable semiconductor diode lasers to the detection of air pollutants have heen.documented hy Hinkley and Kelley (54). The unique property of the SD laser of condensing approximately 0.25 mW of power into a liuewidth of around em-' makes i t a formidable source for trace pollution studies, hoth for point sampling and remote detection. The superior resolution enables many more infrared absorption lines to be detected than can normally he obsewed with a conventional spectrome ter, and the superior power means that samples can he studied a t such low pressures that collision broadening may be totally neglected. The ability to observe very narrow lines at very high resolution implies that the information content of any spectral region
THE TUNABLE DYE LASER
m e a
more power to Analytical Spectroscopy Finally! the tunable dye laser becomes a routine laboratory tool. Spectroscan 10 combines the power, bandpass and coherence of the laser in a package designed to take the "laser" out of laser spectroscopy. TO START, turn a key, depress a switch. Period. TO SCAN (linear in wavelength) select any of the four built-in optional scan speeds. TO CHANGE DYES, remove one cuvette, replace with another. OPERATING COSTS are counted in pennies a day. One nitrogen cylinder lasts about one year. One charge of dye ( 1 cc) costs maximum 15d and can last for days. PURCHASE PRICE, also, is remarkably low. Spectroscan 10 offers about lo5 more power than a conventional source with a 0.3 nm bandpass from 360-740 nm. Readout (on a digital display) is linear in wavelength. The beam diameter of 0.5 mm can be focussed t o a spot size of 20 pm with a power density approximately 1O9 greater than available from a monochromator source.
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u p t o 20 X los molecules ~ 3 1 (3.8 1 ~ ~ pph). Measurements of HzO ahsorptions showed the stratosphere to he very dry. Future experiments are planned to measure the variance in NO concentrations as the sun sets, as well as measuring O3 Concentrations during the day-night cycle. Conclusion
Wave number. c m ~ l
Figure 7. Infrared absorption spectrum of YJ band of SFS ~ o w e irate r taken with conventional grating spectrometer: sample pressure. 0.1 torr: cell lenglh. 25 cm: resolution. 0.07 cm-'. Upper trace taken Wilh -dicde laser: sample pree55ure. 0.1 torr: cell length, IO Cm: resolution. 3 X 10-e cm-'. P ( laser Spechum Figure taken from "ancement Of sci,
containing infrared absorptions can be vastly increased. It then follows that many different pollutants may he specifically identified by diode laser spectroscopy, both qualitatively and w a n titatively, even when all present toeether...Drovidine a t least some 0f their major absorption frequencies do not ahilioverIan A recent examde of the .. ty of &ode laser spectroscopy to identify organic molecules by high-resolution analysis of the C-H stretching region has been given - hv Nil1 et al. (52). TILe suDerior resolution capahilities of s1D lasers over conventional iufrar?id spectrometers (being somewheire of the order of 2000) is so strikinglji illustrated in Figure 7 that i t J..-A:-I.--- -..-_ &I. h K ~ ~ K U U U C ~ L U X , UC.IT, r s r L~ U V U ~ hears-----this figure has appeared in many recent reviews. With resolution capahilities such as those shown for just one small region'of one baud of SFs hy laser excitation, it is fair t o say that the whole field of infrared sDectroscopy may well undergo a minor revolution in the near future once SD lasers become commercially available. It is always very satisfying to he able to end any review with a stateof-the-art application that is also releI
I
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vant to a concern of the day. I t is fortuitous, therefore, that Pate1 e t al. have recently published their initial findings on the spectroscopic measurement of NO and water vaDor in the stratosphere (56).Their final results should prove conclusive in determining which of the many theoretical models for predicting the effect of SST's on the stratosphere i s mrrwt Spectroscopic measuirements were taken using a spin-flip Raman laser as the source* and a comhination multipass cell an d optoacouirtic cell as the detector. The unit was carried heneath a hrIUOOUto a he ight of 28 km, and measiirements wer'e taken in situ both in t h e presence arid ahsence of ultraviolet radiation fr,om the sun ke., I . 2 uay a d a t night). The detection limit of the system for Nil x z m a an. proximately 1.5 X los molecules cmW3 corresponding t o a volumetric mixing ratio of approximately 0.2 parts per billion a t an altitude of 28 km. No ah- -U- were sorptions attributable to N found at night, implyirLg that the maximum possible concent ration a t night was 1.5 X los molecule s cm-3. After the ultraviolet sunrise, distinct ah., sorptions were ohservea wnicn couia he correlated with NO concentration
As expected, most applications of tunable lasers in analytical spectroscopy to date have been in areas where the unique properties of the laser can replace those of conventional sources with advantage. Extended tuning ranges a t both ends of the spectrum (UV and IR) and stable power outputs should extend these advantages to other areas of spectroscopy. Certain recent trends possibly point the way to the future impact of tunable lasers on spectroscopy. The first is the establishment of laser spectroscopy as a field in its own right as witnessed hy the First International Conference (57) devoted solely to the subject. The second is the recent appearance of systems designed specifically to take advantage of the unique properties of tunable laser sources. These include enhanced sensitivity [58),ultrarapid tuning (59), and improved instrumentation (60). Finally, 3ther novel techniques, similar perhaps to coherent anti-Stokes Raman spectroscopy, which are not possible with conventional sources may he discovered, thereby opening completely new avenues for exploration. As a postscript, Fairhank et al. (61) have recently published their results on the resonance fluorescence detection of sodium using a CW laser and Iphotoelectric detection. The minimum 2sodium vapor density detected was lo2 1toms/cm3 which translates into obLiervation of K 5 resonant atoms, on the average, in the sampling region. They claimthat i t should he possible to measure many other atoms with the same sensitivity. I t is applications such as these that point to the almost inevitable future interdependency of analytical spectroscopy and tunable laser sources. ~
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Acknowledgment
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760A * ANALYTICAL CHEMISTRY, VOL. 47, NO. 8, JULY 1975
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It is a pleasure to acknowledge the assistance of Neil Sandow and Jack Wiley in bringing certain relevant literature to my attention. n, -3ferences
(1,I P. P. Sorokin and J. R. Lankard, IBM J. Res. Deu., 10,162 (1966). (2:1 F.P.Sehiifer, W.Schmidt, and J. Volze, Agpl. Phys. Lett., 9,306 (1966). (31I P. P. orokm, Sm.Am., 220,30 (1969) (4) J. P. Webb, Anal. Chem., 44 (I6),30A (1972).
James R. Allkins was born and educated in London, England. He obtained BS and PhD degrees from London University and then came to the United States as a postdoctoral fellow to work under Ellis Lippincott a t the University of Maryland. After leaving Maryland he spent two years as manager of applications at Spex Industries and then joined Interactive Technology in California as director of applications research. His original interest in lasers was revived by a year at Chromatix where he became particularly interested in tunable lasers as spectroscopic sources.
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Standards To Burn CONOSTAN standards are for
your spectrometer. Burn them in emission or atomic absorption spectrometry or use them in X-ray fluorescence to obtain the highest quality standardization. Where metals in oil are your analytical problem use CONOSTAN standards to help solve it.
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There are 29 elements as individual standards at 5000 ppm in light oil for use in atomic absorption analysis. Mercury and arsenic are available at 100 ppm in light oil. CONOSTAN blends of 12 or 20 metals in heavy oil are the valid reference standards for wear metal analysis. A new 20 metal blend with calcium at 5 times the level of the other metals has been added to the family of standards.
k GONOSTAN CONOSTAN DIVISION. Continental Oil Company P.O. Box 1267 0 Ponca City. Oklahoma 74601 CIRCLE 34 ON READER SERVICE CARD
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ANALYTICAL CHEMISTRY, VOL. 47, NO. 8, JULY 1975
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