Anal. Chem. 1984, 56, 2979-2981 2979

r 5 0 X

1351

267 I I

260 250

M'Z Figure 8. Solution introduction C I mass spectrum of adenoslne (2.0 mM in water: CI: isobutane, 0.4 torr; source, 140 'C; probe, 20-240 'C ballistic).

T

"T" 100 120 140 160 180 200 220

1

248.9 501 I

325.0

240 260 280 300 320 340 360 380 400

M / E

Figure 7. Solution introduction NCI mass spectrum of adenosine (2.0 mM in 0.5 M NHSCN; NCI; isobutane, 0.4 torr; source, 140 'C; probe, 20-240 'C ballistic).

(mlz 325, Figure 7). The ion at m/z 249 corresponds to the (M - HzO) anion. As expected the thiocyanate anion, m/z 58, is the base peak in the spectrum. The other major ions in the spectrum correspond to the proton bound thiocyanate

dimer H(SCN); at m / z 117 and the thiocyanate adduct of adenine at m / z 193.

Preliminary experiments indicate that the solution intro- duction technique that we have described is applicable for use with nonpolar as well as polar solvents and is useful in both positive and negative ion modes.

Evidence reported here suggests that this solution intro- duction technique offers very usable sensitivity for obtaining chemical ionization mass spectra from compounds with rel- atively high lattice energies. The method should be useful with virtually any chemical ionization mass spectrometer and requires no modification of the instrument. It is conceivable that this general technique could be used with electron impact ionization; however, dielectric breakdown could be a problem in this case. More detailed studies will be necessary before the full analytical utility of this method can be established. Studies along these lines are presently in progress.

ACKNOWLEDGMENT It is a pleasure to acknowledge useful discussions with G.

Hieftje and technical assistance from J. E. Thean. Registry No. Erythromycin, 114-07-8; dextrose, 50-99-7;

adenosine, 58-61-7.

LITERATURE CITED (1) Schulten, H. R. Int . J. Mass Spectrum. Ion fhys. 1979, 32, 97. (2) Benninghoven, A.; Sichtermann, W. Anal. Chem. 1978, 50, 1180. (3) Barber, M.; Bordoll, R. S.; Sedgwick, R. D.; Tyler, A. N. Nature (Lon-

don) 1981, 293, 270. (4) Tsuchlya, M.; Kuwabara, H. Anal. Chem. 1984, 56, 14-19. (5) Evans, C. A., Jr.; Hedrlcks, C. D. Rev. Sci. Instrum. 1972, 43,

1517-1530. (6) Hardin, E. D.: Fan, T. P.; Blaklev. C. R.; Vestal, M. L. Anal. Chem.

1984 56, 2-7. Macfarlane, R. D.; Torgerson, D. F. Science (Washington, D.C. ) 1976. 197, 920. Blakley, C. R.; Carmody, J. J.; Vestal, M. D. J. Am. Chem. SOC.

Dedieu, M.; Julin, C.; Arplno, P. J.; Guiochon, G. Anal. Chem. 1982,

Rinehart, K. L., Jr.; Cook, J. C., Jr.; Mauren, K. H.; Rapp, U. J. Antibi-

1980, 102, 5931-5933.

54, 2372-2375.

O f . 1974, 126, 1-13.

Ralph C. Dougherty* John de Kanel

Department of Chemistry Florida State University Tallahassee, Florida 32306

R ~ c ~ l v n , for review April 30,1984. Accepted August 29,1984.

High-Resolution Self-scanning Continuous Wave Dye Laser

Sir: This report demonstrates the potential utility of a novel dye laser system for high-resolution (Doppler limited) spec- troscopy. This new laser consists of a standard untuned continuous wave (CW) dye laser oscillator coupled to an ex- tracavity feedback element. The feedback element is a sin- gle-domain crystal of BaTiO:, which acts as a passive-phase conjugate reflector (PPCR) (I). The feedback from this PPCR narrows the laser line width more than 2 orders of magnitude to -5 GHz (0.006 nm) and passively scans the wavelength over a 25-nm range as previously reported (2). We show here, for the first time, that this self-scanning laser maintains a narrow line width throughout a scan and that scans are suf- ficiently continuous and monotonic to be useful for high- resolution measurements.

The wavelength of the laser scans repetitively across a given range from shorter to longer wavelengths. The short wave- length end of the range, or starting point, is primarily de- termined by the dye gain medium and mirror reflectivity. The red end of the scan range depends on the dye gain profile. The scanning rate is dependent on the laser power and the spacing between the PPCR and dye oscillator, normally ranging from 0.001 to 1 nm s-l. The wavelength region scanned can be varied by using output mirrors with different spectral re- flectivities and changing the dye gain medium. We have worked in the wavelength range of 570-700 nm with R6G and DCM dyes.

This technique should work with any standing wave CW dye laser with no additional optical components necessary.

0003-2700/84/0356-2979$01.50/0 0 1984 American Chemical Society

2980 ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984 aM Art LASER

OG SIG.

Flgure 1. Schematic diagram of self-scanning CW dye laser system and optogalvanlc experiment. The BaTiO, single crystal coupled to the Ar' pumped dye laser comprises the self-scanning dye laser system. Other components are chopper, C, wedge, W, mirrors, M, etalon, E, aperature, A, photodlodes, PD1, PD2, hollow cathode lamp, HC, ballast resistor and coupling capacitor, B, high-voltage power supply, HVPS.

The phase conjugation properties of the BaTi03 PPCR imply that the feedback into the dye laser oscillator is insensitive to angular alignment and that mode matching of the feedback to the dye laser will be automatic irrespective of the cavity design (3, 4).

The self-scanning dye laser has a narrower bandwidth than other CW dye lasers using a single tuning element. The resolution provided by this laser is near the maximum required by conventional spectroscopy because spectral line broadening limits measurements to ~1 GHz (0.0012 nm). Although there are commercially available CW dye lasers with line widths as much as 4 orders of magnitude narrower than what we report here, the increased resolution is not useful for conventional spectroscopy because of this line broadening limit. Their reduced bandwidth is obtained by using as many as five in- tracavity tuning elements that must be moved in a coordinated fashion to tune the wavelength, making them mechanically complex. In addition, most of these lasers have a continuous tuning range of only 30 GHz (0.036 nm), and all are costly.

EXPERIMENTAL SECTION We demonstrate the capabilities of this laser as a spectroscopic

source by measuring the optogalvanic spectrum of the Na D lines. Figure 1 shows a schematic diagram of the experiment. A Spectra-Physics 171-08 argon-ion laser tuned to the 514.5-nm transition is used to pump a Spectra-Physics 375 jet stream dye laser. The gain medium for the dye laser is 2 X lo4 M Rhodamine 6G (R6G) in ethylene glycol. The standard R6G output mirror is replaced with the output coupler normally used for Coumarin 6 dye (Spectra-Physics G3862-008a). This new mirror shifts the untuned output wavelength from 618 to 585 nm. A 0.5 X 0.5 X 0.5 cm cube of single-domain BaTiOB is placed 19 cm in front of the dye laser output mirror and ads as a PPCR. The output beam of the dye laser enters a face parallel to the crystalline c axis. The dye laser radiation is vertically polarized and enters the crystal as an extraordinary ray, thus taking advantage of the large r42 electrooptical coefficient. (The perspective of Figure 1 is a top view for the laser systems and a side view for all items after the output of the dye laser.) The dye laser-BaTi03 crystal combi- nation comprises the self-scanning high-resolution dye laser system. A small portion of the laser radiation is transmitted through the crystal and is analyzed by a solid spaced etalon, E, with a free spectral range of 14.2 GHz (0.016 nm). Fringes pro- duced by the etalon are monitored by a photodiode, PD2, after spatial filtering. The recording of these fringes provides a fre- quency scale as the dye laser wavelength scans. Radiation is coupled out of the dye laser system through reflection from the input face of the crystal ( ~ 4 % ) . This radiation is directed into

589.59

1 (d)

I I I I I I I I 1 I I 589*) 589. I

WAVELENGTH (nm)

Flgure 2. Optogalvanic spectra of Na hollow cathode lamp: (a) me- chanical tuning of two-plate birefringent filter with 300-mW dye laser power, (b) same as (a) except 10-mW dye laser power, (c) spectrum taken with self-scanning laser system, (d) fringes of monitor etabn (FSR = 14.2 GHz).

a Perkin-Elmer hollow cathode lamp containing sodium atoms and argon fill gas. A 500-V dc potential is applied to the lamp through a 56 kQ, 2-W ballast resistor, B, giving -7 mA of dc current. The argon-ion laser is chopped at 400 Hz while we synchronously detect the optogdvanic (OG) signal (5) with a PAR 5204 lock-in amplifier. The OG signal is monitored through a 0.1-pF, 1-kV blocking capacitor while the reference signal is derived from the argon-ion laser through a wedge beam splitter, W, and photodiode, PD1. The argon-ion laser power is initially adjusted to produce 10 mW of dye laser power with no phase conjugate feedback and no chopping. The scan rate can be controlled by adjusting the power of the argon-ion pump laser. The scan rate for this work was 0.0033 nm s-l or about one resolution element per second.

For comparison with the self-scanning phase-conjugate feedback laser, the dye laser was also scanned mechanically by using the standard Spectra-Physics two-plate birefringent filter. The micrometer control for the filter was driven by a 1.33 revolutions/h synchronous motor. the mechanical tuning of the dye laser using the synchronous motor results in a scan rate of 0.0025 nm s-l. For spectra taken with the mechanically tuned laser, the BaTiOB crystal was replaced with a mirror to direct the laser radiation into the hollow cathode lamp.

RESULTS AND DISCUSSION The OG spectrum of the hollow cathode lamp in the vicinity

of the sodium D lines, taken with the mechanically tuned laser, is shown in Figure 2a. The laser is operated at 300-mW output power for this spectrum. All three of the transitions observed produce a line width of -75 GHz. The first transition (588.86 nm) is due to the argon fill gas whereas the other two (589.00 and 589.59 nm) are the 2S!/2 - 2P,p,,3/2 sodium D lines. Most of this line width is attributable to the laser source. The expected line width of these transitions due to Doppler and collisional broadening is e 2 GHz. When the output power of the mechanically tuned laser is reduced to 10 mW, the bandwidth can be reduced to =25 GHz as shown in Figure 2b. Although the resolution of the spectrum is enhanced,

Anal. Chem. 1884, 56, 2981-2983 2881

considerable structure is added by the spectral instability of the laser.

Finally we show the spectrum measured by using the phase-conjugate feedback self-scanping dye laser in Figure 2c. Greatly improved resolution is achieved with highly symmetric line shapes. The fwhm of the Na transitions are 6.7 GHz and the Ar transition measures 5.7 GHz. This is very nearly what we expected considering the =5 GHz line width of the laser, Part d of Figure 2 shows the fringes produced by the monitor etalon. The fringes are 14.2 GHz (0.016 nm) ai8 art, being essentially linear in wavelength over this brief interval. Figure 2, through the spectrum and in particular the etalon fringes, shows for the first time that this self- scanning laser can be useful as a high-resolution spectroscopic source. The narrow bandwidth is maintained across a rela- tively broad range, and scans are continuous and monotonic on this high-resolution scale.

The spectral region shown in Figure 2 is only a small portion of the total scan possible with the dye laser as described. A scan from 585 to 610 nm is possible a t 5-GHz resolution corresponding to more than 4000 separate resolution elements. Scans over other regions of the visible spectrum should be possible using different gain media and mirror reflectivities. The wavelength limits of the scan range determined by a given dye/mirror combination can also be modified. A scan can be stopped at any time and restarted at any wavelength, within the tuning range, by inserting a coarse tuning element in the dye laser for a few seconds. In addition, we have been able to reduce and adjust the tuning range by inserting a broad band filter in the dye cavity. The scan rate of this laser can be adjusted over a broad range, from several seconds to 10 ms per resolution element. The slow scan rates are important when using long time constants for signal-to-noise improve- ments. Higher scan rates might be used with signal averaging

where the etalon fringes could provide spectra registration of the signal or a wavemeter could be used rather than an etalon to provide absolute wavelength measurement during a scan, although at greater expense.

We feel this high-resolution self-scanning dye laser system has many possible applications in molecular and atomic spectroscopy. this source could be particularly useful for solid-state and gas-phase spectroscopy where high-resolution measurements are useful. The repetitive, passive scanning feature of this system might prove useful for low-resolution spectra as well. Future improvements may include further bandwidth reduction, flexible control of scan ranges, and bandwidth reduction without scanning.

Registry No. R6G, 989-38-8; BaTiOB, 12047-27-7; Na, 7440- 23-5.

LITERATURE CITED (1) Feinberg, J. Opt. Lett. 1982 7 , 488-488. (2) Whltten, W. B.; Ramsey, J. M. Opt. Lett. 1984, 9 , 44-48. (3) Yariv, A. I€€€ J . Quantum €/ecfron. 1978, 74, 850-680. (4) Giuiiano, C. R. Phys. Today 1881, 31, April, 27-35. (5) Travis, J. C.; Turk 0. C.; Oreen, R. 6. Anal. Chern. 1982, 54, 1006A-

J. M. Ramsey* W. B. Whitten

1018A.

Analytical Chemistry Division Oak Ridge National Laboratory Oak Ridge, Tennessee 37831

RECEIVED for review March 26,1984. Resubmitted August 21, 1984. Accepted August 21,1984. Research sponsored by the Office of Energy Research, US. Department of Energy, under Contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc.

Segregated Sampling and Excitation with a Dual Inductively Coupled Plasma

Sir: In recent years increasing attention has been placed on the development of atomic discharge systems which seg- regate the sampling from the excitation step. The sampling step includes those processes which create an atomic and/or ionic reservoir from the analyte of interest. The excitation step involves the coupling of additional energy to the resulting reservoir for spectrochemical observation. In practice this segregation may be in time and/or in space. Also, it is unlikely that absolute or complete separation of the two processes is possible.

For example, we used a high-voltage spark as a sampling device to create a long-lived toroidal-shaped reservoir of species sampled from a metallic electrode surface. Once formed, this material was inductively reexcited with an intense pulse of radio frequency energy (1). The inductive field was established with a coupling coil located circumferentially about the spark interelectrode axis. When compared with direct spark emission, spectra resulting from the reexcitation process (1) exhibit narrow line widths, (2) are simplified with pref- erential population of lower level resonant transitions, and (3) are free from background continuum interferences. Recent results (2,3) indicate excellent detection limits for trace el- ements in aluminum matrices with 3-4 decades of linearity in analytical working curves. These properties are a direct result of segregating the sampling and excitation steps in order to allow independent control and optimization of each process.

0003-2700/84/0356-2961$0 1.50/0

In the present communication a new dual inductively coupled plasma device is introduced which embodies several aspects of segregated sampling and excitation spectroscopy. This device is designed to eventually allow direct introduction of powdered (e.g., particulate) samples into an ICP and to facilitate fundamental studies.

Several approaches to independent sample introduction for the ICP have been reported. These include electrothermal vaporization (4), introduction of spark-sampled aerosol (5-7), and transport of laser-ablated material (8,9) to the ICP. For each experiment enhanced detection limits and long linear working curves were reported. In addition, research has been published which describes both the dropping of powders directly into an ICP (10) as well as conventional aerosol in- jection of powders into an ICP for analysis (11,12). The direct introduction of powders and refractory materials avoids ex- pensive and time-consuming dissolution normally associated with aqueous nebulization of these sample types. It was noted (11) that energy transfer was insufficient to completely va- porize large particles before they excited the plasma zone. Reasonable detection limits were observed, but continuum and other background levels were not specified.

C m o et al. (13) recently reported a simple device for direct injection of coal fly ash samples into a conventional high-power ICP. Detection limits of 1-7 ng were reported for several elements in NBS coal fly ash (No. 16333a) samples. Possible

0 1984 American Chemlcai Society