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for review May Ig7'. -4ccepted December 1977. This work supported by the Office of Naval Research. ''9
CORRESPONDENCE Analytical Flame Spectrometry with Laser Enhanced Ionization Sir, A recently reported "Opto-galvanic effect" in flames ( I ) , in which an increase in flame conductivity is observed in response to the absorption of tunable laser light, could provide the first radically new approach to analytical flame spectrometry since the advent of atomic flame fluorescence in the early 1960's ( 2 ) . I n light of the relative stagnation of flame atomic spectrometry over t h e past few years, with flame detection limits being surpassed by graphite furnace absorption and plasma emission results, the new method should be welcomed by flame adherents. The new approach is insensitive to many of the limiting noise sources of conventional flame spectrometry (3-5),while maintaining the principal sources of appeal of the analytical flame, such as continuous sampling, low expense, and ease of operation. Experiments conducted throughout the past year, and reported elsewhere ( 6 ) ,are consistent with the hypothesis that t h e mechanism of t h e opto-galvanic effect in flames is photo-assisted ionization. T h a t is, thermal ionization is more probable for a n atom in an excited state than for the same atom in the ground state. Thus, laser irradiation of a flame a t a characteristic transition wavelength of an injected atomic species is capable of significantly perturbing the thermal energy level distribution and the corresponding ionization rate of the species. The change in ion-pair density is readily sensed with simple probes and routine electronic apparatus. A similar interpretation ( 7 , 8 ) may be given for the opto-galvanic effect in glow discharges (9). The laser enhanced ionization (LEI) approach is thus a hybrid, requiring both laser excitation and thermal ionization. Since the method requires no optical detection, except to monitor laser power, the optical noise sources associated with flame spectrometry are irrelevant. Especially important is the insensitivity to scattered laser light, which is a problem in laser induced resonance fluorescence (20). The requirement of laser excitation for the method also carries with it the bonus of high spectral resolution. Dye lasers are readily available commercially with resolutions greater than lo5. T h e intent of the present work is t o demonstrate the sensitivity of the LEI method for a number of elements, using traditionally preferred transitions of flame atomic absorption. In addition, the technique is demonstrated on transitions that would ordinarily not be preferred, but prove to be most suitable here. EXPERIMENTAL Figure 1 illustrates the system used in this work. A Chromatix CMX-4 flashlamp pumped tunable dye laser is used, with frequency doubling when necessary. The laser output consists of 0003-2700/78/0350-0817$01.OO/O
-0.8-ps pulses with repetition raties of 5 to 30 Hz. The bandwidth ranges from 0.003 to 0.1 nm depending on the spectral regime and choice of intra-cavity optics. The laser used is furnished with intra-cavity etalons to give two stages of spectral narrowing beyond that of the coarse-tuning birefringent filter. With the exception of Mg, the detection limits reported here were obtained without etalons, thereby simplifying the operation of the system but limiting the bandwidth to 0.05-0.1 nm Without etalons. tuning of the laser at fundamental frequencies is comparable to the operation of a small monochromator. Major spectral region changes, involving dye or mirror changes or frequency doubling, obviously are less convenient than with a monochromator. Most measurements are made in the wavelength region near 280 nm where Fluoral-7GA ( 1 1 ) is the dye used with frequency doubling. Sodium at 589 nm is done using Rhodamine 6G. Copper at 324.8 nm is done using Rhodamine 640 with frequency doubling. The sample is aspirated into a lean air-acetylene flame using a Perkin-Elmer Model 370 pre-mix burner assembly with a 5-cm single slot burner head. The signal is detected via a pair of I-mm diameter tungsten welding rods 1 cm apart and -1 cm above the burner headparallel to the slot and in close proximity to opposite sides of the flame. The rods are used as the cathode, and are maintained at -500 V with respect to the burner head, uhich is used as the anode. This configuration has been found to be slightly less sensitive than with the electrodes immersed in the flame ( I ) , but far less subject to drift resulting from electrode deterioration and/or contamination. The burner head is electrically insulated from the burner body by a strip of electrical tape, so that the current may be monitored on the low voltage side of the flame. A 10-cm length of miniature shielded cable connects the burner head to the preamplifier. To observe
