Anal. Chern. 1992, 64, 465-468
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Chromium Detection by Laser Desorption and Resonance Ionization Mass Spectrometry N.S.Nogar* Chemical and Laser Sciences Division, MS J565,Los Alamos National Laboratory, Los Alamos, New Mexico 875!5
R. C. Estler Department of Chemistry, Fort Lewis College, Durango, Colorado 81301
Chromium was measured by resonance ionization mass 8 p e d m W y combined wtth laser desorptkn. The desorption process produced a plume of analyte with a temperature = 1000 K. Ionization was effected by a 2 1 (photons to resonance photons to ionize) process, with the first step n it w w o n exdtatbn of the (365s') 'S, (3848') 'Sa trandtlon. Saturation curves were used to estimate the cross sections for these processes: model parameter fits indicate umr = (5 f 2) x cm4*s,u, = (7 f 4) X cm2. The detection limit for this analysb was = I O fg. Thls iLnn could be improved signlticantly by altering the geometry for the analysis.
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INTRODUCTION Resonance ionization mam spectroscopy (RIMS) has found numerous applications in chemical' and i s ~ t o p i canalysis, ~,~ surface a n a l ~ s i s ,optical ~ s p e c t r o ~ c o p y ,optical ~ ~ ~ damage diagnostics for laser ablation events? and ionization potential measurements.lOJ1 RIMS involves a multistep excitation process, taking advantage of intermediate electronic resonances in the species of interest, in combination with mass spectral sorting and detection of the resultant ions. High spectral brightness lasers are typically used to effect ionization, thus leading to very high ionization probabilities, potentially approaching unity.12 When used in conjunction with high detection efficiency mass spectrometers, RIMS offers the possibility of unparalleled sensitivity and selectivity for gas-phase atoms and small molecules. "his allows for potential improvements in minimum sample size, accuracy of measurement, or speed of analysis. Sample size requirements are reduced because of the effective utilization of the available sample; that is, sample is efficiently transformed into signal. This, in turn, can be used to improve the accuracy of measurement for a given sample size or to reduce the time of analvsis for a given level of accuracy. In- commonwith many optical methods for atomic and isotopic analysis, one of the significant problems in achieving ultimate Sensitivity for RIMS analysis lies in Sample prepsration. Since RIMS is both element and quantum-state specific, optimal detection requires production of the species 0003-2700/92/0364-0465$03.00/0
of interest as atoms in the gas phase and in a specific electronic state. Numerous methods have been pursued to this end. Many early experiments utilized sample preparation techniques borrowed from thermal ionization mam spectrometry, including thermal evaporation from metal filaments,13J4overcoating with carbon or refractory metals, and adsorption onto resin beads.Is These methods have the advantage of being mature technologies and reasonably well-understood. On the other hand, there can be difficulty with refractory elements, or those having a tendancy to form strong molecular bonds, particularly oxides. The former can lead to intercalation and/or reaction of the analyte with the metal support and to the production of thermal ions, which potentially interfere with the analysis. The latter lea& to the loss of anal@ from the chemical and electronic form being sought. Both reduce sensitivity and/or selectivity of the ionization process. To combat these difficulties, various other physical means have been used to generate gas-phase atoms from solid or liquid samplea. Chief among these are ion-beam sputtering4s6 and laser desorption or laser ablati~n.l'-~~ The similarities and differences between laser ablation and ion-beam sputtering have been discussed elsewhere.20 In this work, we describe the use of laser desorption, in combination with RIMS, for the detection of trace quantities of chromium. Chromium is of interest because of its toxicity23,24 and frequent occurrence in groundwater and soils2592e around hazardous waste disposal sites,27*28 where it may be leached from disposal pits and surrounding formations. Previous work on detection of trace chromium has included radiochemical detection,29 laser desorption/laser-induced fluorescence,30graphite furnace atomic absorption, and inductively-coupled plasma/atomic emi~sion.~'The LIF experiments30 are particularly relevant. In this case, the solid sample was ablated and atomized into a buffer gas by pulsed-laser radiation; elemental analysis was by optical emission spectrometry and laser-induced fluorescence (LIF). A detection limit of 300 fg was reported.
EXPERIMENTAL SECTION The Cr atoms detection measurements described below took place in the source region of a time-of-flight(TOF) mass spectrometer as previously described'se and illustrated in Figure 1. Briefly, the Q-switch synch-out from the NdYAG laser (Quanta 0 1992 American Chemical Soclety
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 5, MARCH 1, 1992 I
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Ray/Spectra Physics Model DCR 1A) was used to master the timing sequence. This laser was used as the desorption source and operated at the third harmonic, 355 nm. This wavelength of operation was chosen to ensure strong absorption of the radiation by the substrate disk holding the Cr sample. The laser was equipped with beam-filling optics and produced a nearGaussian profile. The output of this laser was 10 ns in duration and was smooth on the time scale of the detection electronics (- 2 ns). The laser output was focused to a measured spot size of 1 mm diameter onto to a d i k of polymethyhethacrylate (PMMA) polymer onto which the sample had been placed (see below). The angle of incidence (as measured from the normal of the sample surface) was 60".For the desorption studies reported here, a peak laser fluence of approximately 1 J/cm2 was used; the average flcence was ca. half of the peak fluence. The fluence was determined by measuring the energy transmitted through a 10-pm pinhole centered on the laser spot at the focal point and correcting for widow transmission and the angle of incidence. The Gaussian-like profiles were confirmed by scanning the pinhole about the central spot in orthogonal directions. The samples used for this study were prepared from known dilute solutions of Cr(N0j),.9H2O using ethanol as a solvent. Two 10-pL aliquots of each solution were placed onto the suface of a 2.54-cm diameter PMMA disk (0.5 cm thick) by spin coating. The disks were spun at 60 Hz, while the aliquots were placed at the axis of rotation. The spin coater was constructed in such a manner to ensure a repeatable positioning of the aliquot droplet with respect to the spinning disk. Ethanol was used as the solvent rather than water, since the former wet the polymer surface while the latter did not. Each disk was used only once to avoid any poasible contamination problems. After spin coating, the sample was placed within the mass spectrometer through a vacuum load-lock on a feedthrough (Varian Model 1371) that allowed sample translation and rotation while in the mass spectrometer source region. The focal spot of the ablation laser was displaced from the center of rotation of the target. This displacement permitted interrogation of an annulus of sample/substrate surface through target rotation. The size of the annulus (and therefore the relative amount of the total sample removed) was determined for each sample by examining the disks microscopically. Pulses from an excimer-pumped dye laser (Lambda Physik Modela 101/2002), propagating parallel to the target surface, but displaced 3.2 cm (i.e., in the center of the ionization region),were used to interrogate the desorbed neutral Cr species. Any ions produced directly by the desorption event could be strongly discriminated against by maintaining the isolated feedthrough at a negative potential. However, at the laser fluences used for the present study, no such nascent ions were observed. Ionization of the Cr neutral species from the ground electronic state (3d54s1, ?S3) was effected by a twephoton transition to an upper electronic state (3d55s1,?S3,36896 cm-l) followed by the absorption of a third photon of the same frequency (Figure 2). Dye-laser pulses (-542 nm, 2 mJ, 15 ns) produced at a variable delay relative to the ablation laser were spatially fitered and loosely focused through the ionization region (focal diameter 0.7 mm). The delay between
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m e 2. Excitation spectnrm of the chromium (W5s')'S3 (3d54s1) 'S, transition. A simpllfki energy level diigram is shown as the inset. the lasers was empirically set to correspond to the maximum in the timwf-flight velocity distributionof the desorbed atoms. The extraction field was approximately 110 V/cm, followed by a drift tube of 0.4m length at a potential of -500 V. A pair of deflection plates between the extractor and the flight tube could be used to mnnimize the transmission of ions to the detector and minimize any transmission variation that was due to ion velocity components perpendicular to the flight tube. Detection electronics consisted of a channel electron multiplier, a preamplifier, a boxcar averager, and a gated counter (Stanford Research Systems 400). The counter was operated in a mode to minimize pulse pileup distortions. Operationally, signal counts were accumulated during a 5-min time interval as the sample was slowly rotated (2 rpm) exposing fresh surface. All labile Cr atoms within the footprint of the desorbing laser are removed within this interval (most within the first one or two revolutions),as observed in the signal count decaying to background levels during this time expasure. Sample blanks of the solvent only were prepared by following the above procedures and produced an average of 15-18 counts during the 5-min sampling interval. All reported signals have been corrected for background counts. For the purposes of spectroscopically tuning the dye laser to the appropriate transition, a continuous source of the Cr neutrals was available from a resistively heated tantalum ribbon filament (0.075 cm X 0.0025 cm) placed within the ionization region. Resistively heating a filament that had been spiked with a solution of the Cr salt provided a stable source of the neutral species. This source was necessarily not operated during desorption experiments.
RESULTS AND DISCUSSION Ionization Process, Rates, and Cross Sections. Figure 2 shows the ionization spectrum of Cr near the (3d55s1)7S3 (3d5C1) 7S3twephoton resonance. This curve was obtained from a heated filament with Cr &-dried onto the surface. Also
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displayed is a simplified energy level scheme showing the participating energy levels, the enhancing intermediate state, and the ionization potential. The width of the resonance line is simila~to the laser line width, indicating that the twephoton excitation is not grossly overdriven. Figure 3 shows the Cr ionization signal as a function of laser pulse energy. Also shown is a fit to this data, based upon a model involving two-photon excitation to the resonant (3d55s1) 7S3state, followed by spontaneous emission or photoionization by a third photon?2 This model is based on a rate-equations formalism,33 and produces a closed-form solution for the ionization rate at a particular intensity. Integration over spatial and temporal coordinates yields the total fraction ionized in a laser pulse. The solid line is a nonlinear least-squares fit34of this model to the data, treating the two-photon cross section, b2hv, the ionization cross section, q, and the rate of spontaneous emission, S,, as adjustable variables. The fit yielded values of uahv= (5 f 2) x cm4.s, uI = (7 f 4) X cm2,and S, = 1 x IO7 s-l. The error limits on the two-photon and
ANALYTICAL CHEMISTRY, VOL. 64, NO. 5, MARCH 1, 1992
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photoionization cross sections are two u values, while the fit was insensitive to the value of S , in the range i05--i08 s-l. We include these values not to suggest that we have accurately determined the cross sections, but rather to show that they fall in the range of v,alues reported previously for similar processea.3236 Further, both the raw data and the calculations suggest that the ionization will approach saturation within the focal volume of the laser beam ( l / e intensity points) for pulse energies above 2 mJ, thus reducing signal fluctuations due to shot-to-shot variations in the laser. Lastly, for the twephoton cross section reported above, a transition rate R , = (5 x 10+)(9 x = 4 x 108 s-l is calculated for exposure at 1.6 mJ. This implies that the excitation is near saturation for pulses in this range. Further, the line width correaponding to this rate is 4 . 1 an-', again consistent with the observation that the observed transition is laser-line width limited. Calibration Curve and Sensitivity. For the desorption experiments, the arrival time distributions of Cr ground-state atoms in the ionization region, for a sample size of IO+ g, were found to be roughly thermal with a temperature of 103K. In subsequent experiments, the delay between the absorption and ionization lasers was adjusted to produce the maximum signal for this distribution. This corresponds to a delay time of =18 MS. Independent analyses suggest that relatively small amounts g) of the support material may also be desorbed by the laser. Figure 4 is a calibration curve for this system, showing a log-log plot of the Cr ion signal (in counts) as a function of desorbed sample size. A t least two features should be noted from this figure. First, the signal is nonlinear with sample size. In the range of 10-1L10-6 g, the signal, S, scales approximatley as S a [Cr]0.2, where [Cr] is the mass of chromium exposed to the desorption laser during the analysis. For sample sizes less than g, S 0: [Cr]0.5.In both cases, the error in the exponent is ~ 0 . 1 . There may be a number of reasons for this nonlinear dependence of signal on sample size. Some may result from laser-material interactions: that is, the sample size may effect the interaction of the desorption laser either with the target, by changing the surface absorption coefficient, or with the desorbed plume, by changing its optical properties. Changing the optical properties may directly affect the partition function of the desorbed plume. Both the fraction of chromium desorbed as monatomic species and the fraction of this subset appearing as neutral ground-state atoms may be strongly affected by interactions with the incident laser.% In addition, second-order effects may result from changes in the density of desorbed material. In particular, at higher densities of desorbed material, one expects both redeposition3? and
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molecule (including dimer) formation to increase, thus leading to less efficient production of ground-state chromium atoms. Minor changes in the velocity and angular distributions of the desorbed plume may also result from increases in the mass or density of desorbed material. Thus, dramatic changes in the mass of material removed from the surface may significantly alter the fraction of ground-state chromium atoms present in the volume defined by the ionization laser and the ion optics during the ionization pulse. This, in turn,will affect ionization and may lead to the relatively low-order dependence of signal on sample size. Second, it is also useful to consider the efficiency with which the sample is used. For the desorption of g of Cr, we detect approximately 160 ions. This corresponds to a detedion efficiency, t = 160/(1.2 X lolo) = lo+. This degree of sample utilization is not atypical38and is the result of inefficiencies in both sample utilization and analysis. In the former case, sample loss may be due to nonevaporation (intercalation) of the analyte from the sample platform, as well as postdesorption losses such as redeposition. In the latter case,spatial and temporal overlap, partition function (analyte desorbed in other than atomic ground state), incomplete ionization, and imperfect transmission and detection of the ions produced all lead to a reduction in potential signal levels. A rough numerical estimate can be obtained for these numbers. For an isotropic (cos 0) distribution of desorbed this overlap may be as small as In addition, at this relatively large distance from sample to interrogation region, the velocity spread of the desorbed analyte dictates that the temporal overlap of the laser pulse and the desorbed plume may be as small as We expect the combined transmission/detection efficiency of this time-of-flight instrument to be -10-l. The fraction of chromium desorbed as ground-state atoms may vary over a wide range, from as large as 10-' to as small as perhaps lo4. Thus, we expect the fraction detected in fall in the range f d I A possible third source of the low-order dependence of signal on sample size, and in particular the variation of the dependence on sample size,is pulse pileup errors in the m e n t experimental configuration. Presently, the p u b counting rate is frequency limited by a 150-MHz preamp; this means that no more than one ion can be detected for each laser pulse. In the laser ablation process, where there exists a laser energy fluence threshold for removal of analyte, instantaneous counting rates during the pulse ionization detection process may exceed this limit. In looking at a typical data point, the majority of the ions are collected over a period of =l min (two revolutions of the sample). At 10 Hz, this corresponds to =lo00 shots. At higher sample doses, we observed 11o00 ions,
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 5, MARCH 1, 1992
suggesting that pulse pileup may be a significant concern. Detection Limit and Comparison with Other Methods. The nonlinear dependence of signal on sample size can be both an advantage and a detriment. On one hand, a nonlinear dependence requires the generation of extensive calibration curves. On the other hand the observed low-orderdependence leads directly to a dramatic increase in the dynamic range of the measurement. It may be possible to correct for some of the nonlinearity by ratioing the observed RIMS signal to a signal obtained either from the sample matrix or from the sample platform itself.40 The detection limit reported here, 10 fg, compares favorably with previous reports for laser-based analysis.3o In addition, the use of mass spectral interrogation allows the potential for isotopic analysis. It should be noted also that the RIMS system used for these experiments must serve multiple purposes8.9J0and is not optimized for best sensitivity. In particular, the relatively long distance between the sample surface and the ionization laser, 3.2 cm, dictates that the spatial overlap of the laser-desorbed plume with the ionization laser is small. Alteration of our source geometry to reduce this distance could result in a sensitivity increase of perhaps 4 orders of magnitude.
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RECEIV~, for review September 24,1991. Accepted November 27, 1991.
