Laser-Enhanced Ionization Spectrometry - American Chemical Society

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Laser-Enhanced Ionization S p e c t r o m e t r y The common atom reservoirs for analytical atomic spectrometry are at mospheric pressure flames, furnaces, and electrical plasmas. Such environ ments promote some level of ioniza tion of atomic and molecular species, depending dramatically on the ioniza tion potential of the species. When a tunable laser is used to resonantly ex cite an atomic species, the laser-in duced excited atom population ob viously has a lower effective ionization potential than the ground-state popu lation and is more readily ionized by thermal collisions. This process and several variations constitute the basis of the optogalvanic effect in flames (1), or laser-enhanced ionization (LEI) (2-7). To detect the occurrence of LEI, a voltage is applied across the flame, and the resulting current is moni tored. LEI signals appear as changes in this current, and are thus most easi ly detected by using pulsed or ampli tude-modulated continuous wave la sers with synchronous detection. The periodic table shown in Figure 1 indicates the elements that are cur rently detected by LEI in analytical flames. Limits of detection are given in nanograms of analyte per milliliter of distilled water aspirated into the flame. One ng/mL corresponds ap proximately to an atom density of 108 c m - 3 in the flame (8). LEI offers po tential advantages in both selectivity and sensitivity over conventional forms of atomic spectrometry. Optical Enhancement of Thermal Ionization In an analytical flame, collisions occur at a rate of >10 9 s _ 1 . With each collision, the probability of ionization of a given atom or molecule is gov erned by the Arrhenius factor, exp[-CE; - Ej)/kT] where Et is the ionization potential and Ej the occu

pied energy level of the atom (or mole cule), k is the Boltzmann constant, and Τ the flame temperature. At Τ = 2500 Κ, k T is about 0.2 eV and the ioniza tion probability, which is near unity at Ej » Ei, falls at a rate of about two or ders of magnitude per eV increase in (Ei — Ej). The collisional ionization probability for a low-lying atom may be increased by two orders of magni tude for each eV of optical excitation that moves it closer to its ionization limit. This promotion of atoms to higher energy levels "enhances" the rate of ionization and is the basis for LEI (6). The four photoexcitation schemes employed to date for LEI are illustrated schematically in Figure 2 for four elements in a 2500 Κ flame (kT = 1735 cm" 1 ). The dynamics of the fundamental three-level resonant (R) scheme of Figure 2 are illustrated by a hydraulic analogy shown in Figure 3. The tubs represent the three energy levels, with the fluid levels indicating atom or ion populations. For simplicity, state mul tiplicities are ignored, i.e., statistical weights are assumed to be equal. By definition, the pumps representing the laser and thermal energy have pump ing rates proportional to the pressure head (population) as well as the rota tional velocity of the pump rotor (laser power/Arrhenius factor). Before the laser is turned on, the fluid level in the top two tubs is negli gible compared to the level in the bot tom tub. The condition represented in Figure 3 results at some time t fol lowing initiation of a high-power laser pulse. The populations in the bottom two levels have equilibrated (t > 1 0 - 9 s) under optical saturation, because the excitation and stimulated emis sion rate constants significantly ex ceed those for collisional quenching (~10 9 s _1 ) and fluorescence (~10 8 s _ 1 ). Ionization occurring from the ex

1006 A · ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982

cited state (middle tub) empties both lower levels at the same rate because the laser "mixing" rate greatly exceeds the ionization rate. The total system has not reached a steady-state condition since the top tub is filling faster than emptying (i.e., the ionization rate is much greater than the recombination rate). Indeed, electron-ion recombination takes so long (~ms) as to be negligible on the time scale of laser pulses (10~ 8 -10~ 6 s). Extrapolating from Figure 3, it can be seen that ionization proceeds virtu ally to completion, providing the laser remains on long enough. C.A. van Dijk (9) has noted a useful rule of thumb for pulse-length requirements: Unit ionization efficiency may be ap proached with an optically saturating laser pulse whose duration significant ly exceeds the reciprocal of the effec tive ionization rate of the laser-popu lated excited state. The qualifier "ef fective" in the above statement ac commodates the effect of Boltzmannpopulated states above the state in question, as studied by Hollander (10) and discussed by Smyth et al. (11) and van Dijk and Alkemade (7). In the nonresonant mode of LEI (Figure 2), the lower level from which laser excitation occurs (lower transi tion level) is an excited electronic level of the atom, instead of the ground level as in the R scheme. The term "nonresonant" refers here to a transi tion that is not ground state con nected. Both the nonresonant (N) and nonresonant stepwise (NS) schemes shown in Figure 2 work as long as the first excited level is reasonably close to the ground level. For Τ « 2500 Κ, lower transition levels less than about 11 000 c m - 1 above the ground state are suitable for flashlamp dye laser pulses of 1 0 - 6 s, but for 10~ 8 s (nitro gen-, excimer-, or Nd:YAG-pumped dye laser) pulses, lower transition levThis article not subject to U.S. Copyright Published 1982 American Chemical Society

J. C. Travis G. C. Turk Center for Analytical Chemistry U.S. National Bureau of Standards Washington, D.C. 20234

R. B. Green Department of Chemistry University of Arkansas Fayetteville, Ark. 72701

Figure 1 . LEI p e r i o d i c c h a r t of the elements, i n d i c a t i n g experimental l i m i t s of d e t e c t i o n (in ng/mL) and e x c i t a t i o n mode for ele ments observed to date (italics) Observation modes: R = resonant (ground state connected); Ν = nonresonant; S = stepwise, resonant; and NS = nonresonant, stepwise. Other elements shown are expected to yield LEI signals in flames. Omitted elements are not amenable to flame spectrometry

els should be less than about 3500 c m - 1 above the ground state. The stepwise schemes (S and NS of Figure 2) have been developed to yield high LEI sensitivities for high-ionization-potential elements (12,13). If both laser-excited transition steps are optically saturated, the pulse-length rule for total ionization applies direct ly to the highest-lying excited state. To date, seven elements with ioniza tion potentials up to 9.2 eV (e.g., Au) have been determined in this way, with 7.2 eV). Even though nonlinear methods of generating such wavelengths are being developed, high powers would be required to saturate weak transitions, and the optical train would need to be purged of air. Fur thermore, stepwise excitation furnishes advantages in selectivity as well as sensitivity, as will be discussed later. Although LEI ion yields may ap proach 100% of the atoms exposed to laser light, the quantum efficiency— defined as ion pairs produced per pho ton absorbed—is typically much lower, making laser excitation essen tial. For instance, if Na in a H 2 /air flame absorbs a photon to yield a 3pstate atom, the excited atom can be collisionally deactivated back to

ground ( ~ 1 0 - 9 s), fluoresce ( ~ 1 0 - 8 s), or collisionally ionize ( ~ 1 0 - 5 s). Collisional quenching is obviously the dominant process, yielding a quantum efficiency of ~0.1 for fluorescence and 1 0 - 4 for LEI in this example. For lev els very near the ionization potential, ionization rates approach collision rates, yielding a near-unity LEI quan tum efficiency. However, these states are not amenable to significant popu lation by conventional sources. Thus, the low LEI quantum yield for tradi tional transitions and the low proba bility of populating Rydberg states with conventional sources restrict practical optical ionization enhance ment to laser excitation. LEI is closely related to resonance ionization spectroscopy (RIS), which

ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982 · 1007 A

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involves laser excitation of one or more bound transitions followed by direct photoionization of a laser-popu lated excited state (14). The thermal excitation that characterizes LEI is not required, and low-temperature, low-pressure atom reservoirs may be employed. Although 100% ionization efficiency may be achieved with RIS, the final photoionization step requires considerably higher laser irradiance than do the bound transitions. This characteristic of RIS has led to the de velopment of collisionally assisted RIS (15), which is essentially synonymous with stepwise LEI ("S" in Figure 2). The semantic distinction between LEI and collisionally assisted RIS seems to relate to the reservoir temperature. The latter technique assumes temper atures low enough that background ionization is totally negligible. Thus the final laser-excited state must be much closer to the ionization potential than for LEI in flames. In a flame environment, RIS and LEI may both occur (16). The domi nance of one over the other will be de termined by the excitation scheme, ionization potential, and laser spectral irradiance.

Collisional Ionization

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LEI Detection

The maximum current density that can be extracted from a flame by an applied electric field is a measure of the volume ionization rate (ions gener ated per cm 3 per s) of the flame (17).

An external field may thus be applied to sense a current change resulting from LEI. Figure 4 is a block diagram of a typical LEI experiment utilizing this principle. A variety of electrode configurations may be employed (2-5, 18,19), as discussed below. The ac component of the flame current (i.e., LEI signal) resulting from the pulsed (or chopped CW) laser excitation is separated from the dc background current by a simple resistor-capacitor network. The LEI signal is first pro cessed in a ~ 1 0 6 V/A preamplifier (20) and then signal-averaged in a gated (boxcar) integrator. Placement of the high-gain preamplifier close to the burner minimizes electrical pickup of radio-frequency interferences (RFI) invariably broadcast by pulsed lasers. The detection of ions and electrons differs radically from the photon de tection common to most spectroscopic methods. Charged species may be di rected by electric fields, and they trav el at finite velocities much less than the velocity of light. Both of these properties are illustrated by ion im ages obtained with the electrode con figuration of Figure 5 (18). Flat plates are used as horizontally opposed

Ground-State Atoms

Figure 3. Hydraulic analogy to LEI for photoexcitation scheme (R) of Figure 2 Tubs represent atom (ion) energy levels; water levels indicate populations; pumps correspond to effects of laser irradiation and thermal collisions. See text for discussion


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Figure 4. Block diagram of a typical LEI spectrometer

anode and cathode. The burner head is insulated from the circuit and "floats" at an intermediate potential with minimal influence on ion collec tion. The electric field direction is per pendicular to the electrodes, impart ing a horizontal velocity to the elec trons in the direction of the anode and pulling positive ions toward the cath ode. Positive or negative high voltage is applied to one plate while the other is grounded through a load resistor. The LEI signal produced by chopped, CW laser excitation of Na in the flame is taken from a 1-mm-diameter hori zontal rod just inside the low-voltage electrode. (This differs from the usual practice, in which the signal is extract ed from the low-voltage electrode.) When the rod is translated vertically, ions in transit to the electrode are in tercepted as a function of position. At high voltage, the images for electrons and ions are centered at the same height above the burner head as the laser beam. At low voltage, however, the images are shifted to greater heights above the burner and are broadened, illustrating the contribu tions of flame velocity and diffusion at lower collection velocities. The large mass difference between electrons and ions has some inter esting consequences for LEI. When a pulsed laser is used for LEI, a tran sient electron signal is observed in « 1 μβ. This is the signal normally used for analytical LEI. With the 1-MHzbandwidth preamplifier (20), the shape and peak position of this pulse is conveniently resistant to 10-100-ns changes in electron transit times due to matrix and geometry effects. A

100-MHz preamplifier is presently being used at NBS to study the details of electron transport (21). A slow (10-100 μβ) pulse corresponding to ion transport velocities is not used for an alytical LEI, but has been used by Mallard and Smyth to study ion mo bilities in flames (22, 23). A further implication of the differ ence in electron and ion masses relates to the static electric field distribution in the flame. Consider the instanta neous application of a potential differ ence across a flame with plane parallel electrodes. Initially (at t = 0) the flame is quasi-neutral with equal den sities of electrons and ions. The uni form field between the plates is simi lar to that when a potential is applied across a vacuum. At first, electrons are removed at a higher rate than ions, due to their greater mobility, leaving a positive charge accumulation near the cathode. Eventually, when a steadystate condition is achieved, a positive ion space charge or "sheath" extends from the cathode. The positive ions ef fectively form a diffuse anode, modi fying the field distribution in the flame and slowing electron removal to equal the ion removal rate. This phe nomenon has been described by Lawton and Weinberg (17). Under some conditions (high rates of ionization or lower applied poten tial), the positive ion space charge will not extend across the flame from cath ode to anode, but will instead be local ized close to the cathode. Localization of the positive ion sheath means that the applied potential will not be sensed between the edge of the sheath and the anode. If an LEI signal is gen


erated outside of the sheath, no field exists to move the charges toward their respective electrodes, and· the upward velocity of the flame will re move the ions from the detection vol ume without a signal being detected. This is the origin of the ionization in terference discussed in the next sec tion. The sheath expands as the applied voltage is increased, so that loss of LEI signal due to space-charge shielding can be counteracted to some degree by higher applied voltages. The ultimate limit, however, is set by breakdown or arcing across the flame, which occurs at lower voltages for higher ionization rates. The voltage at which the sheath just reaches the anode is referred to as the electrical "saturation voltage" (17). At voltages higher than saturation, subtle changes occur in the field distribution curves but a nonzero field exists ev erywhere. When a field exists every where in the flame, every ion and elec tron produced by thermal ionization is collected, contributing to the current in the external circuit. Thus, for volt ages above saturation, the current be comes constant and is referred to as the "saturation current." Current vs. voltage curves may be used to great advantage for electrode design and in terference studies (18, 24). The evolution of electrode design over the five-year history of LEI is il lustrated in Figure 6. The use of two 1-mm tungsten welding rods in the flame (2) (not shown) was quickly fol-

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Figure 5. Imaging rod electrode config uration ( 18) for obtaining images of the type shown 1 = high-voltage electrode; 2 = laser position; 3 = reaction zone; 4 = burner head; 5 = low-volt age electrode; 6 = translating imaging (signal) electrode at the same potential as 5

Figure 6. Several electrode configurations used for LEI (a) Tungsten rod cathode in flame with the burner head used as anode; (b) rods external to the flame used as a split cathode with the burner head as anode; (c) plates external to the flame used as a split cathode with the burner head as anode (an alternate electrical configuration for the same geometry is shown in Figure 5); (d) water-cooled stainless steel cathode immersed in the flame with the burner head as anode. See figure 5 legend for 1-4

lowed at NBS by the use of a singlerod cathode with the burner head as the anode (Figure 6a). Later, it was found that two such rods outside of the flame could be used as a split cath ode with no loss in sensitivity for high er applied voltages (3) (Figure 6b). The nonintrusive nature of these elec trodes was attractive, as was the negli gible electrode deterioration. (The iridium wires used as immersed elec trodes by van Dijk and co-workers (7, 25) seem to be more robust than the tungsten ones.) When analytical feasibility studies advanced to complex sample solu tions, ion space-charge interferences were noted (3), and explored in some detail by Green and co-workers (26). Figures 7a-7c show considerable im provement in the tolerance of an LEI signal to a readily ionized concomitant species as electrode rod diameter is in creased (26). This reduction in electri cal interferences with increasing diam eter electrodes is due to a commensu rate reduction in field strength. The high fields near small-diameter rods

accentuate the sheathing effects dis cussed earlier. Thus, flat electrodes (Figure 7d) are preferable to rods for LEI (19). As normally used, these par allel plates are held at high negative potential, and the burner head is used as the anode (Figure 6c). Flat electrodes external to the flame are still subject to space-charge shielding at very high concentrations of low ionization potential concomi tants (e.g., part per thousand Na). The "cold" space between the flame and the electrode can accumulate space charge, but is not appropriate as an analytical sampling volume. The water-cooled cathode (24), a 0.25-indiameter stainless steel tube slightly flattened to simulate a plate electrode (Figure 6d), solves two problems si multaneously. It may be immersed in the flame for long periods of time without detectable deterioration, and the laser beam may be positioned close to the electrode so that LEI oc curs in a region of nonzero field even when high ion concentrations cause the sheath to shrink toward the elec


trode. Figure 7e shows the greatly im proved tolerance of the immersed cathode to easily ionized matrices. The increase in LEI signal recovery that precedes signal loss in all electri cal interference studies (Figure 7) is not understood. Speculations on the origin of this enhancement remain to be tested. The dc background current arising from natural flame ions is the ultimate source of the limiting noise for ideal samples. Although the background current itself is blocked by an ac-coupling capacitor (Figure 4), statistical shot noise fluctuations in the current accompany the signal. For a 10-μΑ (~10 14 electrons/s) current, a l-μβ av erager gate width, and a 10-Hz laser, about 109 electrons and ions arrive at the electrodes during the 10 sampling intervals of a 1-s averaging time. The statistical uncertainty in the Poisson distribution is the square root of the number of collected electrons or ~ 3 X 104 electrons. Calculating back to cur rent gives a shot noise limit of ~0.3 nA. A S/N ratio of three at the limit of detection extrapolates to a 0.9-nA sig nal during a l-μβ gate width. This cor responds to ~10 4 electronic charges. For a typical ~0.1-cm 3 laser irradia tion volume, a limiting detectability of ~10 4 atoms (~10 5 atoms cm - 3 ) would be possible for 100% ionization effi ciency and 100% collection efficiency. Analytical Properties and Applications

Figure 1 gives limits of detection (LODs), i.e., concentrations yielding a S/N ratio of three, for the elements determined to date by LEI. The picogram per mL LOD for Li is approxi mately equal to the ~ 1 0 5 atom/cm 3 theoretical limit discussed above. If 100% efficiency were attained for atomization, ionization, and collection, LODs for all elements would be equiv alent. The variations observed in Fig ure 1 result partially from the varia tion in elemental atom fractions in a given flame (27). Also, the laser exci tation schemes used for many of the single photon LODs did not result in 100% ionization (6). The LODs shown are the results for acidified solutions of an appropriate salt in purified water. For actual sam ples, the LOD increases due to shot noise from increased background cur rent and several types of wavelengthindependent laser-induced ionization, discussed below. Similarly, LODs in C2H2/N2O flames may be expected to be poorer than those for C^IWair or H2/air, because the inherent flame ionization rate and the matrix atom ionization rate will both be higher in a C2H2/N2O flame. The only element determined to date in C2H2/N2O is Al,

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Figure 7. The effect of electrode design on LEI signal recovery in the presence of varying Na concentrations (abcissa) (a) 1-mm-diameter rods; (b) 1.5-mm-diameter rods; (c) 2.35-mm-diameter rods; (d) 5-mm-wide X 125mm-long plates (26); (e) water-cooled immersed cathode

with a ~ 1 ng/mL LOD. Many of the elements not examined yet require this flame for effective atomization. Under high-power (MW/cm 2 ) irradiation, complex samples often yield a laser-related ionization signal that differs from the LEI signal by being nominally wavelength independent. This pulsed background signal originates from several sources, including: 1) nonresonant multiphoton ionization (MPI) of virtually any species; 2) LEI resulting from excitation in the "wing" of a strong absorption line of a major matrix species (e.g., Na); and 3) direct photoionization of a Boltzmann-populated excited state species. Broadband ionization of indigenous flame species does not affect the accuracy of LEI, since the same additive background occurs for both sample and standards and cancels in the calibration process. For background laser-induced ionization from matrix-related species, however, accuracy must be ensured by background correction (normally done by scanning the laser wavelength across the analytical line). Dynamic ranges for LEI have not been studied exhaustively, but ranges around 10 4 -10 5 (5) are typical. The upper limit at tens of micrograms per mL is set by optical preabsorption of laser radiation for strong transitions. Focusing or use of higher power lasers may extend the upper limit by "bleaching" the prefilter via optical saturation. However, one may also increase the broadband laser-induced

background ionization for complex samples. LEI shares the high selectivity of other analytical atomic methods due to the availability of sharp, characteristic atomic line spectra. Since laser bandwidths may be routinely narrowed to well below the inherent Doppler and pressure-broadened line widths of atoms in flames, spectral selectivity is at least as good as for atomic absorption (AAS). In both LEI and line source (hollow cathode lamp) AAS, the experimental resolution is limited by the actual analyte bandwidth, not a monochromator slit function. However, LEI has the advantage of a wavelength-tunable source. There are a number of unavoidable near-coincidences of spectral lines for different elements encountered by analytical spectrometrists (27). Tables of overlap integrals for such coincidences are of qualitative use for LEI. However, since LEI signal strength depends significantly on ionization potential as well as transition strength, LEI "overlap" may be either less or greater than conventional spectroscopic overlap for any given coincidence. Stepwise excitation improves LEI selectivity dramatically (13), since the probability of both transitions simultaneously coinciding with two transitions of another element goes as the product of two small probabilities. Nonresonance fluorescence enjoys a similar two-wavelength selectivity (excitation and emission), but with monochromator-limited reso-


lution at the emission wavelength. The molecular spectral interferences common to flame spectroscopy have not been encountered to date in analtyical LEI. Spectral features of such indigenous flame species as OH and CH are not observed because of their high ionization potentials. However, several oxides of refractory elements have been observed by LEI (28) and could yield spectral interferences for samples containing large amounts of such species. Also, the recently reported resonantly enhanced multiphoton ionization spectrum (29) of NO in a flame (30) could cause spectral interferences in N2Û-based flames. The space-charge-based ionization interference has been a prime concern in developing analytical LEI (19, 26). Although plate electrodes (19) and the water-cooled immersed electrode (24) have improved the tolerance of LEI to highly ionizable matrices, signal recovery still may vary with matrix composition. Problem samples may be recognized by monitoring the dc background current and observing whether aspiration of the sample causes a noticeable increase in current over aspiration of distilled water. If so, the method of standard additions should be used instead of calibration standards to ensure accuracy. Alloy analyses are particularly amenable to LEI due to the absence of an ionizable matrix, and several analyses of NBS standard reference materials have been reported (13). Figure 8 shows the stepwise LEI signal for Sn in SRM 396 (unalloyed copper) obtained by scanning the second excitation wavelength over the excited Sn transition at 597.0 nm with the firststep excitation fixed at 284.0 nm (13). The LEI result of 0.67 ± 0.05 jig/g compares favorably with the less precise certified value of 0.8 ± 0.3 /ag/g (31). The spectrum shows an off-resonance baseline signal due to LEI from the first step alone and broadband laser-induced background ionization. Noise on the baseline results both from laser amplitude fluctuations and from shot noise accompanying the dc background current. The Future of LEI

There is every reason to anticipate that flame LEI can, and will, be extended to include any element amenable to conventional atomic spectrometry, with superior sensitivity and selectivity in most cases. Even so, the eventual impact of LEI and similar techniques is difficult to predict. In some cases (e.g., Na), the potential sensitivity of LEI far exceeds the ability of the scientist to avoid contamination of the sample during storage, preparation, and processing. In other cases, commercially available

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Figure 8. Wavelength scan over the 597.028-nm second-step transition of Sn in unalloyed copper (SRM 396), with the first-step wavelength fixed at 283.999 nm Sn concentration in solution (1 g SRM 3 9 6 / 1 0 0 mL) is 6.7 ng/mL ( 13)

techniques already exceed the current demands for sensitivity. Still, with the most pessimistic view of ultratrace techniques, there is undoubtedly a fu ture for extremely sensitive, selective, laser-based methods, including LEI, RIS, and laser-induced fluorescence (32-34). Analytical demand has al ways risen to meet the availability of more sensitive methods. Even now, techniques are needed that are capa ble of providing valid environmental quality "baseline" data for sites at which development (e.g., power plant construction) is anticipated. Sample handling and contamination control are also subject to continued improve ment. Furthermore, the extreme sen sitivity of LEI can be used to greatly improve precision and accuracy at moderate concentration levels and to permit greater-than-normal dilution of solid samples, viscous samples, or microsamples. Improving the present sensitivity of LEI will require abandoning the ana lytical flame as an atom reservoir, since background current is a critical limiting noise source. Reducing the background current requires reducing the temperature of the LEI reservoir, which in turn necessitates further op tical excitation to produce the re quired thermal ionization rates. How ever, the two-laser system required to extend LEI to high ionization poten tial elements is already capable of meeting this requirement. The need for high-temperature atomization followed by moderate-tem


perature LEI suggests a segregated sampling and excitation approach, similar to that of Coleman et al. (35). Plasmas, furnaces, cathodic sputter ing, sparks, etc., could conceivably be used with appropriate coupling to a laser-ionization chamber. In addition to improving sensitivity, the lower temperature ionization envi ronment would virtually eliminate space-charge ionization interferences. Also, with little or no background cur rent, the gated-detection time window could be widened to integrate the total charge under a variety of conditions. Under circumstances of total ioniza tion and collection, the accuracy of LEI could become its most important asset. Acknowledgment

One of the authors (R.B.G.) ac knowledges the support of the Nation al Science Foundation under Grant No. CHE 79-18626. Helpful discus sions with J.R. DeVoe, G.J. Havrilla, P.K. Schenck, and C A . van Dijk are gratefully acknowledged. References (1) Green, R. B.; Keller, R. Α.; Schenck, P. K.; Travis, J. C; Luther, G. G. J. Am. Chem. Soc. 1976, 98,1517-18. (2) Turk, G. C; Travis, J. C; DeVoe, J. R.; O'Haver, T. C. Anal. Chem. 1978,50, 817-20. (3) Travis, J. C; Turk, G. C; Green, R. B. In "New Applications of Lasers to Chem istry"; Hieftje, G. M., Ed.; ACS Sympo sium Series 85, American Chemical Soci ety: Washington, D.C., 1978; pp 91-101. (4) Turk, G. C; Travis, J. C; DeVoe, J. R.; (continued on page 1018 A)

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O'Haver, T. C. Anal. Chem. 1979,51, 1980-96. (5) Travis, J. C ; DeVoe, J. R. In "Lasers in Chemical Analysis"; Hieftje, G. M.; Tra vis, J. C ; Lytle, F. E., Eds.; Humana Press, 1981; pp 93-124. (6) Travis, J. C ; Schenck, P. K.; Turk, G. C ; Mallard, W. G. Anal. them. 1979,51, 1516-20. (7) van Dijk, C. Α.; Alkemade, C. Th. J. Combus. Flame 1980,38, 37-49. (8) Winefordner, J. D.; Vickers, T. J. Anal. Chem. 1964, 36,1939-46. (9) C. A. van Dijk, private communication. (10) Hollander, T. J. AIAA J. 1968, 6, 385-93. (11) Smyth, K. C ; Schenck, P. K.; Mal lard, W. G. In "Laser Probes for Com bustion Chemistry"; Crosley, D. R., Ed.; ACS Symposium Series 134, American Chemical Society: Washington, D.C., 1980; pp 175-81. (12) Turk, G. C ; Mallard, W. G.; Schenck, P. K.; Smyth, K. C. Anal. Chem. 1979, 51, 2408-10. (13) Turk, G. C ; DeVoe, J. R.; Travis, J. C. Anal. Chem. 1982,54, 643-45. (14) Hurst, G. S. Anal. Chem. 1981,53, 1448-56 A. (15) Whitaker, T. J.; Bushaw, B. A. Chem. Phys. Lett. 1981, 79, 506-8. (16) van Dijk, C. Α.; Curran, F. M.; Lin, K. C ; Crouch, S. R. Anal. Chem. 1981, 53,1275-79. (17) Lawton, J.; Weinberg, F. J. "Electrical Aspects of Combustion"; Clarendon Press: Oxford, 1969; pp 5, 315-23. (18) Schenck, P. K.; Travis, J. C ; Turk, G. C ; O'Haver, T. C. J. Phys. Chem. 1981,85,2547-57.

Travis

273—80

(21) Havrilla, G. J.; Turk, G. C ; Travis, J. C ; Schenck, P. K., unpublished data. (22) Mallard, W. G.; Smyth, K. C. Combus. Flame 1982, 44, 61-70. (23) Smyth, K. C ; Mallard, W. G. Comb. Sci. Tech. 1981,26, 35-41. (24) Turk, G. C. Anal. Chem. 1981,53, 1187-90. (25) van Dijk, C. Α.; Curran, F. M.; Lin, K. C ; Crouch, S. R. Anal. Chem. 1981, 53 1275—79. (26) 'Green, R. B.; Havrilla, G. J.; Trask, T. 0 . Appl. Spectrosc. 1980, 34, 561-69. (27) Parsons, M. L.; Smith, B. W.; Bentley, G. E. "Handbook of Flame Spectrosco py"; Plenum Press: New York, 1975. (28) Schenck, P. K.; Mallard, W. G.; Tra vis, J. C ; Smyth, K. C. J. Chem. Phys. 1978, 69, 5147-50. (29) Johnson, P. M. Appl. Opt. 1980,19, 3920—23 (30) Mallard, W. G.; Miller, J. H.; Smyth, K. C. J. Chem. Phys. 1982, 76, 3483-92. (31) Office of Standard Reference Materi als, National Bureau of Standards: Washington, D.C. 20234. (32) Fraser, L. M.; Winefordner, J. C. Anal. Chem. 1971, 43,1693-96. (33) Fraser, L. M.; Winefordner, J. C. Anal. Chem. 1972,44,1444-51. (34) Weeks, S. J.; Haraguchi, H.; Wine fordner, J. D. Anal. Chem. 1978,50, 360-68. (35) Coleman, D. M.; Sainz, Μ. Α.; Butler, Η. Τ. Anal. Chem. 1980, 52, 746-53.

Turk

John C. Travis has been a research physicist with the Center for Analyti cal Chemistry of the National Bureau of Standards since 1967. He received' his BA in physics from Austin College and MA and PhD degrees in chemical physics from the University of Texas at Austin. His interests include the use of lasers for trace analysis, with a current emphasis on combined lasermass spectrometric methods. Gregory Turk, a research chemist at the National Bureau of Standards Center for Analytical Chemistry, earned his BA in chemistry from Rut gers College and his PhD in analyti cal chemistry from the University of Maryland in 1978. His research inter ests involve the application of lasers to chemical analysis, in particular


(19) Havrilla, G. J.; Green, R. B. Anal. Chem. 1980,52, 2376-83. (20) Havrilla, G. J.; Green, R. B. Chem. Biomed. Environ. Instrum. 1981,11,

Green

the development of ionization spectrometry metal analysis.

laser-enhanced for trace

Robert Green is an associate profes sor of chemistry at the University of Arkansas, Fayetteville. He received a BS in chemistry from Oklahoma State University in 1966 and a PhD in chemistry from Ohio University in 1974. His major research interest is the application of lasers to the solu tion of problems in analytical chemis try, including some of the initial work on intracavity absorption, and the development and characterization of the optogalvanic effect and laser-en hanced ionization spectrometry. Other areas of interest are atomic and molecular fluorescence and laser probing of flames.

Laser-Enhanced Ionization Spectrometry - American Chemical Society

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