Anal. Chem. 1987. 59, 073-079
073
Quantitative Discrimination of Gas-Phase Species Based on Single-Wavelength Nonlinear Intensity Dependent Pulsed Infrared Laser Excited Photothermal Deflection Signals Stephen E. Bialkowski* and George R. Long’
Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322-0300
A sensitive photothermal deflection spectroscopy experlment Is described whlch allows for sknultaneous collection of pulsed infrared excltatlon laser energy and slgnal data. Processing of these data for elucldatlon of nonllnear excltatlon laser energy dependent absorption effects Is Illustrated for 1,bbutadiene, chlorodlfluoromethane, and dlchlorodtfluoromethane. Both multiphoton and saturation absorptlon behavior are observed for the halocarbons. Thls nonlinear excltatlon energy dependent absorption behavlor Is emplrlcally modeled by a power serles uslng llnear regression. The resultlng power serles are used to analyze mlxtures of the two halocarbon spedes. I t Is found that slnglewavelength dlscrlminatlon and quantltatlon of these species can be performed with analyte levels below 10 ppm (v/v) In argon.
Photothermal spectroscopy (I) of gas-phase analytes using pulsed, infrared laser excitation has been shown to be a very sensitive technique for chemical analysis (2-12). A commonly used laser excitation source is the C02 laser. This laser is tunable over about 70 discrete wavelength “lines” from 9 to 11 pm. Although limited, selectivity due to spectroscopic discrimination can be performed by use of tunable C 0 2 infrared laser light sources (13). This region of the infrared is known as the “fingerprint” region wherein small changes in molecular structures between analytes result in large differences in the absorption spectrum. Subsequently, even with its limited discrete wavelength coverage, the line-tunable COS laser is able to be used to discriminate among many species of importance that are found in natural or industrial atmospheres (14). The high sensitivity of the t e c h i q u e can be understood in terms of the theoretical enhancement over conventional absorption spectrophotometry. In particular, the pulsed laser photothermal lens (PL-TLS) enhancement factor is calculated from the ratio of the PL-TLS signal to that of absorption spectrophotometry. This ratio, which is an accurate measure if both techniques are shot noise limited in detection (15),is expressed as (5)
where E , is the pulse energy equal to the integrated power, the product of the molar density, p , and the molar heat capacity, C,, is the specific heat of the sample, w o is the excitation laser minimum beam waist electric field radius, ( d n l drip is the temperature-dependent refractive index change, and X is the wavelength of excitation. An interesting facet of the theoretical enhancement factor is the direct dependence on the excitation intensity proporCurrently with the National Bureau of Standards, Chemical Kinetics Division, Gaithersburg, MD 20899.
tional to E,/w:. In principle, the PL-TLS signal can be made arbitrarily large by increasing the intensity of the excitation source either by focusing to a smaller beam radius or by increasing the energy in the pulse. However, there are limitations to this signal increase. These limitations are due to the dynamics of radiation absorption and energy coupling to the matrix. The dynamic limitations are particularly critical in gas-phase samples where excited state relaxation is limited by the rate of molecular collision. At high intensities, nonlinear intensity dependent signal magnitudes will result from multiphoton absorption or optical transition saturation. Both of these effects will be dependent on the rate at which excited species relax relative to the rate at which stimulated emission takes place. We recently reported on the observation of saturation in PL-TLS and pulsed laser excited photothermal deflection spectroscopy (PL-PDS) of gas-phase analytes using high-intensity infrared laser sample excitation (7,8,10). In the latter studies, effects due to apparent saturation were modeled based on homogeneous optical saturation. The theories used to describe the effects of optical saturation on the analytical signals observed in PL-TLS and PL-PDS were straightforward extensions of the usual simple lens or prism optical element determinations based on the refractive index profile ( I , 2,16). Effects due to optical saturation are calculated by first determining the effect of intensity on the spatial distribution of the fraction of species excited and then relating this fraction to the spatial temperature change in the sample. When saturation occurs, the radial temperature profile becomes dependent on the laser intensity. The nature of this dependence is determined by whether the absorption is homogeneously or inhomogeneously broadened. The difference in the saturation behavior results from a difference in the number of molecules available for excitation (17). The zero time deflection angle for a homogeneously broadened, twolevel absorption was shown to be ( 7 ) -2rlE,
4 = (dn/dT)?,---
now2pCp (1
( I J I , ) exp(-2r2/w2)
+ ( I o / I J exp(-2r2/w2))’
(2)
where r is the radial offset of the probe laser relative to the pump, 1 is the path length of the cell, nois the refractive index, Et is the transition energy, the intensity is defined as Io = 2Ep/aw2,the saturation intensity is I , = E , / ~ u Tu ,is the absorption cross section, and T is the lifetime of the excited state. In general, saturation of inhomogeneously broadened transitions will result in less PL-PDS excitation intensity dependence over that of the homogeneous transition. In conventional absorption spectrophotometry, the measured absorbance decreases with increasing intensity of the light source when optical saturation of a homogeneously broadened transition occurs ( I 7). Optical saturation effects in the signal magnitudes observed in PL-PDS are quite different. The PL-PDS signal magnitude is proportional to the amount of energy absorbed by the sample and subsequently
0003-2700/87/0359-0873$01.50/0 0 1987 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL 59, NO. 6 , MARCH 15, 1987
converted to heat ( I , 2). Although the spectrophotometric absorbance decreases with intensity when saturation occurs, the amount of energy absorbed by the sample increases with increasing source intensity. Also, optical saturation distorts the Gaussian spatial refractive index profile that results from Gaussian shaped, TEM,, laser beam excitation. This profile will approach a “top hat” for beam center intensities that are well beyond the saturation intensity. This distortion results in different signal-intensity behaviors for PL-TLS and PLPDS The initial PL-TLS signal decreases with increasing source intensity due to the decreased curvature of the profile (71. On the other hand, the PL-PDS signal increases with increasing source intensity because the gradient of the profile increases ( 8 ) . The effects of multiphoton absorption on the PL-PDS signal have not been previously examined. However, Twarowski and Kliger derived the temperature change and subsequent refractive index change that occurs in a sample after pulsed laser excitation and in which multiphoton transitions occur ( 2 ) . For a multiphoton absorption in an optically thin sample excited with a Gaussian TEM,,, source. this temperature change is
T(r,f) =
where n is the number of photons absorbed, a,, is the exponential absorption coefficient for the n photon transition, t is time, and t, is the characteristic time constant for temperature equilibration. Assuming that the radial dependence of the temperature profile does not change over the path length limited by the physical boundaries of the cell and that the probe laser beam waist is much less than that of the pump laser, the deflection angle based on this temperature change is obtained from optical path integral over the refractive index gradient (161
4 = (dn/dT),
-4rln2a, n,w2pCp(1 + 2nt/t,)* exp(-2nr2/w2(1
+ 2nt/tc))
(4)
Reflecting the temperature change, the maximum deflection angle will occur at zero time. It is also a function of the pump to probe laser beam offset radius, r, and the number of photons absorbed in the excitation process. One may choose as a reference the optimum pump-probe beam offset for a single photon process a t zero time. However, the exact offset is not important for pump laser energy dependent PL-PDS signal studies where the species are undergoing a single type, Le., n photon, transition. In this case the energy dependence is described solely by the E,” term. In this paper we report on experimental results obtained for intensity-dependent PL-PDS signals of medium to large molecule analyte species in argon. Both multiphoton and saturation type nonlinear absorption behavior are observed. T h e purpose of this study is to demonstrate the application of nonlineig PL-PDS to chemical analysis problems and to provide a phenomenological method of data treatment which allows for the easy interpretation and classification of nonlinear data.
EXPERIMENTAL SECTION The PL-PDS apparatus is similar to that described previously ( I O ) . As illustrated in Figure 1, the apparatus consists of four major components: the pump laser and associated optics, the probe laser optics and detectors, signal collection and processing electronics, and the sample handling apparatus. The pump laser was a pulsed, line-tunable TEA-CO, laser (9-11 gm) constructed
;-PH
Figure 1. Experimental schematic of the setup used in these experiments showing the infrared attenuator (IRA), spatial filter comprised of two lenses (L) and a pinhole (PH),the unlabeled germanium beam splitter, energy monitor (EM), analog to digital converter (A/D), all used for the infrared beam. The spatial filter (SF), beam expansion lens, razor edge (E), intensity detection photodiode (PD2), and signal photodiode (PDI), past the second edge (E), are for the visible probe laser. The amplifiers (A), divider, transient digitizer, and computer are also shown.
in this laboratory. This C 0 2 laser was operated with an internal cavity iris aperture adjusted to result in primarily TEM, mode output as well as a reduced pulse-to-pulse pointing error. The output of this laser was a pulse of 170-11s duration with a 1 0 - p ~ ‘.tail”. The relative energy in the “tail” portion of the pulse was minimized by using low N2 partial pressures in the CO, laser gas mixture. It is important to maintain the spatial and temporal profile of the pump laser beam while attenuating the energy. The pump laser pulse energy was attenuated by placing a “venetian blind” style infrared attenuator in the beam path prior to a spatial filter. Spatial filtering and recollimation of the pump were accomplished by using a pair of BaF, lenses and a pinhole aperture in Cu substrate placed a t the focus of the first lens. The second lens was used to focus the diverging beam past the pinhole aperture into the sample cell. Because of the regular spatial periodicity of the attenuator, several distinct “spots” were imaged on the spatial filter Cu substrate. The central spot was due to the zero-order diffraction component of the beam and was transmitted through the pinhole of the spatial filter. Spots to either side of this zero-order component correspond to higher diffraction orders due to the periodic structure of the attenuator. The zero-order component did not move as the attenuator mechanism was rotated, but higher order components did move since the spatial frequency changed upon rotation. Past the spatial filter, the beam did not exhibit spatial periodicity. The beam waist of the focused CO, laser in the cell was measured with a razor blade edge on a micrometer driven translation stage. The electric field beam waist calculated from the razor blade excursion resulting in 10% to 90% of the maximum pulse energy was = 0.05 cm. The probe laser was a polarized 5-mW continuous-wave HeNe laser (632.8 nm) Uniphase Model 1305-P. The laser beam was spatial filtered with an NRC Model 910 filter. The filtered bem was collimated and expanded through a CPC f / X 5 35-80 mm macro zoom lens with Pentax K mount. The collimated beam had a beam waist radius of approximately 0.5 cm. A microscope slide placed directly after the zoom lens was used to split off 10% of the beam intensity for monitoring with a photodiode. Photothermal deflection was monitored by using two razor blade edges and 25 focal length fused silica imaging lens. The lens was placed 25 cm in front of the sample cell so that the focus of the probe laser would coincide with that of the pump laser. One edge was placed 34 cm in front of this lens and was adjusted such that half of the probe laser beam was blocked. The second edge was mounted on a micrometer-driven translating base and placed at the image plane of the first edge formed by the lens (99 cm past this lens and 74 cm past the sample cell). The image of the first
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 6, MARCH 15, 1987
edge is inverted at this point and consisted of a shadow of the f i t edge with diffraction l i e s above the edge shadow. Both edges were used in the same orientation. Maximum PL-PDS signal was found to occur when the second edge was adjusted such that only about half of the most intense diffraction line was transmitted. In this case the majority of the probe laser beam was attenuated. Pump and probe beams were spatially mixed with a germanium beam splitter which transmitted 57% of the pump laser energy and reflected nominally 100% of the probe laser. This beam splitter was located past the third focusing lens for the infrared beam. The reflected pump energy was used to monitor the pulse energy. Minor probe to pump laser beam offsets at the focus in the sample cell were adjusted by translation of the beam splitter in a direction perpendicular to the beam propagation direction. Gross adjustments were made with the adjustable mirror and beam splitter mounts. Probe laser beam deflection occurs as a consequence of sample heating by pump laser excitation. This deflection was measured as a linear displacement of the probe laser beam 74 cm past the sample cell. This displacement was monitored as the intensity of the transmitted probe beam with a United Detector Technology Model PIN-1ODP photovoltaic silicon detector. A second PIN-1ODP was used to monitor the HeNe laser intensity past the beam expander. Signals from these photodiodes were buffered with equivalent National Semiconductor Model LF357 amplifiers in transconductance circuits. An Analog Devices Model 436B operational divider was used to divide the PL-PDS signal by the HeNe laser intensity signal. The divider was necessary because of large (-10%) intensity variances of the Uniphase HeNe due to temperature-dependent mode sweeping. Output from the operational divider was independent of the probe laser intensity but proportional the position of the laser beam on the razor edge. Linearity of this detector system over the range of deflection angles in this study was checked with a United Detector Technology Model LSC-5D lateral position sensing detector. The deflection signals were digitized with a Physical Data, Inc., Model 522-A eight-bit transient waveform recorder. Analytical signals were processed by using matched filter smoothing software (11,12). The integrated pulsed laser energies were measured with a Laser Precision Model RjP735 pyroelectric energy monitor and digitized with a 12-bit analog to digital converter (A/D). Since the laser pulse duration does not change from pulse to pulse, and the focus spot size of the pump beam does not change, the integrated pulse energy is directly proportional to the intensity. Data collection and procesing were performed with a DEC LSI 11/23 microprocessor. Upon each pulse of the pump laser the PL-PDS signal magnitude was estimated by using the matched filter and the COz laser energy was monitored with the A/D converter. During the experiment, two separate data files were recorded. One being the pump laser energy and the other the signal estimate per pulse. These files were later analyzed by use of standard linear regression routines or the robust regression routine discussed previously (IO). Robust regression was used in cases where portions of the data were altered by interferences of known origin. The theoretical model for excitation energy dependent PL-PDS signals due to saturation is nonlinear in laser energy. Subsequently, reiterative nonlinear regression procedures would have to be used to fit this model to the data. To avoid the long times required for regression of this model with the data sets, typically 2048 samples per set, these data were fit to a power series in excitation laser energy. Samples were introduced into a stainless steel cell of 10 cm length and fitted with NaCl windows that transmit the infrared pump and visible probe laser radiation. The analytes were introduced into the sample cell through an oil diffusion pumped gas manifold. Pressures in the gas cell were measured with a capacitance manometer. All experiments were performed with the analyte diluted in argon. The compounds used in this study were dichlorodifluoromethane(CFC-22)99.9% purity, 1,3-butadiene 99.86%, and argon 99.999% purity from Matheson.
RESULTS AND DISCUSSION Data obtained in these experiments were analyzed primarily in terms of the pump laser energy or intensity dependence of the photothermal signal. Three types of behavior are expected: linear behavior for large analyte species or small
.O?
-.
P X S E ENERGY
875
(mJ)
Flgure 2. Photothermal deflection signal vs. excitation laser pulse energy for 133 Pa of 1,3-butadiene in 101 kPa Ar. The P(28)line of
the C02 laser at 1039.36 cm-’ was used. species with low absorption coefficients; multiphoton behavior manifested as concave up nonlinear energy dependent signal, and saturation for small to medium sized molecules with large linear absorption coefficients and without large multiphoton absorption coefficients, apparent by concave down behavior (18). Saturation is loosely defined here as any phenomenon that results in diminishing signal with increased excitation energy. Thus we include photolysis in this definition since this too will result in a diminished signal. Nonlinear optical behavior is a molecule-specific phenomenon. There have been several studies of nonlinear infrared absorption behavior. As a consequence, many of the molecular parameters that effect the nonlinear behavior are, at least qualitatively, known (18-20). The fact that nonlinear behavior is molecule specific implies that information pertinent to the identity of the molecule may be obtained by studying nonlinear behavior, and perhaps species identification can be performed from the elucidation of this behavior. To determine if this is possible, it is necessary to consider what molecular parameters are best described by the nonlinear behavior and how this is manifested in a photothermal spectroscopy experiment. Important molecular parameters in determining nonlinear behavior are the rates at which excited-state species can couple or relax into background or heat-bath rovibrational states relative to the rate of optical excitation and the density of these rovibrational states at the excited state energy (20). If the density of states is high and coupling into these states is rapid, neither saturation nor multiphoton absorption will occur. Instead, multiple photon absorption will be operative until enough photons are absorbed to cause dissociation. In this case, optical excitation is the rate-limiting process. The PL-PDS signal will increase linearly with excitation energy until dissociation occurs. Prior to dissociation, there is no distortion of the spatial profiles, and the signal will be directly proportional to the small signal absorption coefficient. Since single photon absorption is much more probable than multiphoton absorption, the latter are not observed. A high density of rovibrational states occurs in large polyatomic molecules and medium-sized polyatomics with heavy atoms. In smaller molecules the density of rovibrational states is low; consequently, saturation and/or multiphoton absorption occur. The exact nature of this behavior is determined by the spectroscopic structure of the low-lying rovibrational energy levels. Small molecules generally exhibit unique nonlinear absorption behavior. Figure 2 illustrates the excitation laser energy dependent PL-PDS signal of 1,3-butadiene. It can be seen that the
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 6, MARCH 15, 1987
is that of a fifth-order power series in excitation energy: -7.753 X 0.055063 - 0.01707E2- 0.004655E3 + 0.007483E40.001920E5,where E is the energy in millijoules. The coefficients for the power series were determined by linear regression and “goodness of fit” was confirmed by inspection of the residuals plot. These data appear to be linear from the lowest excitation energy up to about 0.4 mJ/pulse. Above this energy, the PL-PDS signal increases in a nonlinear fashion. The concave down curvature of the data suggests saturation. The saturation behavior could be due either to optical saturation, to photolysis, or to both mechanisms. Infrared photolysis of CFC-12 is a well-studied phenomenon (18). The degree to which infrared photolysis occurs is a function of the excitation laser energy and, therefore, intensity as well as buffer gas pressure. Photolysis can be ruled out as a major contribution to the nonlinear behavior at the high buffer gas pressures used in these experiments since the PL-PDS signal magnitude did not change (to within 1 % ) after several thousand pulses of the excitation laser. In contrast, data obtained at an argon buffer gas pressure of 13.3 kPa exhibited an 80% increase in signal magnitude after only 2000 pulses with an average energy of 1 mJ/pulse. This increase could only be due to the formation of photochemical products, presumably halogenated ethylenes (IS), which have significantly different absorption characteristics. The absence of the signal increase at the higher buffer gas pressure indicates an absence of these photolysis products and subsequently infrared photolysis was probably not occurring. In order to provide a simple working description of nonlinear PL-PDS, we define the PL-PDS signal in terms of a “local” nonlinear absorption effect. By “local” we mean at a particular excitation energy. This definition allows for a description of the PL-PDS signal in terms of the particular absorption effect occurring at a particular excitation energy. To formulate this description the log of both laser energy dependent PL-PDS signal and laser energy is taken. The excitation energy dependence of the absorption coefficient which best describes the absorption behavior is determined by examining the log energy vs. log signal of the data smoothed by regression. The slope of these data is the energy dependence of the PL-PDS signal and is equal to n,the number of photons required for the transition. Values of n equal to unity indicate linearity, values greater than unity indicate multiphoton absorptions, and values less than one indicate optical saturation. The intercept of data analyzed in this fashion is a constant that contains information about the PL-PDS signal for a given absorbance. The value of n may also be dependent on excitation energy, making the log plot nonlinear, in this case the intercept will not be related to the concentration. Nevertheless, such a plot makes the identification and classification of nonlinear phenomena facile and provides insights into how the specific nonlinear behavior may best be used in chemical analysis. A log plot of the 1,3-butadiene data of Figure 2 is illustrated in Figure 4. The PL-PDS signal axis intercept from linear regression of the raw data was substracted prior to taking the log. Grouping of data a t the low side of the log energy axis is due to quantization error in the analog to digital converter. Taking the log expands the scale a t low values so that this quantization effect is more pronounced. The linear leastsquares line drawn through these data has a slope of 1.0004, indicating that high-energy infrared absorption is a normal single photon process. The residuals plot of the log data to the regression line did not exhibit apparent correlations, so that linear model seems valid. log data for CFC-12 are shown in Figure 5 . The smooth line drawn through the log data is a quadratic equation with parameters found from linear regression. At first glance, this
+
. =IIL, .
---
~
-
---
-
/
.
.
-
Figure 3. Photothermal deflection signal vs. excitation pulse energy for 3.1 Pa CFC-12 in 100 kPa Ar excited by the R(28) line at 1083.47
cm-’.
PL-PDS signal is directly proportional to the pump laser pulse energy throughout the energy range of this experiment. Saturation at lower pulse energies is not indicated. We have found that when the saturation intensity is lower than the lowest intensity measured, a finite (to within the uncertainty of the data), positive signal axis intercept will result when linear regression analysis of the data is performed. This nonzero intercept effect is due to the reduced excitation pulse energy or intensity sensitivity of the signal a t excitation intensities in excess of the saturation intensity. The intercept of the butadiene data illustrated in Figure 2 is zero to within the uncertainty indicated by the data. The data of Figure 2 along with the predicted linearity of the signal also serve to validate the linearity of the PL-PDS instrument response. Butadiene has a relatively low absorption coefficient (-0.6 cm-’ atm-’) at the 9.621-~mwavelength used and a high density of states at the energy corresponding to the excited state due to the many low-frequency vibrational modes. It may thus be classified as being in the large molecule limit mentioned above. In this limit, linear pump laser energy signal dependence should be observed for two reasons. First, the low absorption coefficient will result in a high saturation intensity since I, is inversely proportional to this coefficient. Second, the high density of states will result in rapid intramolecular relaxation and so subsequently a short upper state lifetime. This too will result in a high saturation intensity since I , is inversely proportional to the excited state lifetime. Further, the rapid feedback to lower vibrational states will favor single-photon absorption through the higher probability transitions over multiphoton absorption caused by saturation of states coupled by single-photon transitions. The linear signal to excitation pulse energy correlation for butadiene is a limiting case. This species represents one of the largest molecules to be found in gas-phase samples. We have also examined the excitation laser energy dependent PL-PDS signal for toluene and the three xylenes and have found that these too exhibit linear dependence over a wide intensity range corresponding to up to 2 mJ/pulse (21). At some higher energy, these species should dissociate or isomerize due to one of several infrared photolysis mechanisms (18-20). This may result in an apparent saturation behavior since absorbing species will be chemically modified during the excitation process. But this type of behavior was not observed in our experiments with total gas pressures of 100 kPa. Smaller species with high absorption coefficients are more likely to exhibit nonlinear absorption behavior. An example of high energy saturation behavior is illustrated for CFC-12. Figure 3 shows the excitation laser energy dependence of the PL-PDS signal for this species. The smooth regression line
ANALYTICAL CHEMISTRY, VOL. 59, NO. 6, MARCH 15, 1987
877
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Flgure 6. Signal vs. energy plot of 5.2 Pa CFC-22 in 100 kPa Ar excited at 1083.47 cm-'.
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Flgure 5. log plot of the CFC-12 data of Figure 3.
Flgure 7. log plot of CFC-22 data shown in Figure 6.
species exhibits linear behavior a t low excitation energies with nonlinear behavior at higher ones due to saturation. However, the initial slope of this curve indicates that a nonlinear absorption process is occurring at the lower energies as well. The slope of the smooth regression line approaches 312 at low pulse energies and decreases to a little over 1 / 2 at the higher energies. Slopes greater than one indicate a multiphoton absorption and those less than one indicate saturation. The slope a t the lowest excitation energy of this particular data is 1.4. I t is interesting to note that these data as plotted in Figure 3 did not appear to be nonlinear a t low excitation energies. We have examined this low-energy slope in many experiments and have found a low-energy limiting value of 3/2. Thus the excitation is not strictly linear or multiphoton but rather is due to a complicated process or combination of both linear and nonlinear processess (21). CFC-22 is a species that exhibits more pronounced nonlinear absorption behavior. PL-PDS signal w. excitation laser energy data for CFC-22 are illustrated in Figure 6. Two distinct nonlinear absorption behaviors are evident. Multiphoton absorption a t the low energies results in concave up curvature and saturation a t high energy results in concave down curvature. The multiphoton behavior is clearly visible a t excitation energies lower than 0.3 mJ/pulse. The multiphoton absorption behavior of this species apparently saturates a t higher excitation energies. Because of the complex shape of the signal vs. energy data, having both concave up and concave down portions, a seventh-order polynomial was required in regression analysis. The regression line shown is + 0.019243 a seventh-order power series, -3.80 X 0.1635E2 - 0.4022E3 + 0.4524E4 - 0.2856E5 + 0.08986E -
0.01134E7,where again, E is the energy in millijoules. The residual plot of this seventh order polynomial showed little if any correlation to the energy. The log plot of these data is illustrated in Figure 7. The smooth line is a fifth-order polynomial. The slope of this line at low energy is 1.8, indicating a limiting two-photon transition. The slope of the data about the 1-mJ point on the axis is very near one. One explanation for this is that the two-photon transition is saturating and underlying one-photon transitions become predominant a t the higher excitation energies. However, interpretation of the interesting photophysics of this species based on these plots is still speculative, but could be a theme for future studies. Single excitation wavelength, multicomponent analysis can be performed by using the nonlinear excitation energy dependence of the PL-PDS signal. To demonstrate this, experiments were performed by using CFC-22 and CFC-12, at the R(28) line of the 9.2-wm transition at 1083.47 cm-'. Energy-dependence data for CFC-12, CFC-22, and a mixture of the two were obtained. Experimental data sets were obtained consecutively to ensure that the alignment was consistent. It was assumed that there would be no change in the signal due to interactions between the two analytes. Figures 3 and 6, discussed above, illustrate the excitation energy dependent data obtained for the pure analytes in argon and Figure 8 shows data obtained from a mixture of these species. The two empirical polynomial equations describing the energy-dependent signal of the pure compounds were used to model the mixture data. Linear regression was used to determine the relative amounts of each polynomial model in the mixture, and thereby the relative amounts of each com-
+
878
ANALYTICAL CHEMISTRY, VOL. 59, NO. 6, MARCH 15, 1987
. .1’
.
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/ PJLSE ENERG?
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‘mJ)
Flgure 8. Signal vs. energy plot for a mixture of 1.7 Pa CFC-12 and 3.8 Pa CFC-22 in 100 kPa Ar. The smooth line is a linear combination of the polynomials used to model the pure component data illustrated in Figures 3 and 6.
Table I. Values for Linear Regression of CFC-12 and CFC-22 Mixture Data partial partial pressure in compd
pure samples. Pa
CFC-12 CFC-22
3.1 5.2
pressure in mixture,
predicted
Pa
ratio
regression ratio
1.7 3.8
0.55 0.73
0.57 0.73
ponent. Only the energy-dependent terms of the polynomial models were used. Energy axis intercept terms were not included since they would only add to the intercept calculated by regression of the mixture. The intercept term of the mixture data was determined in the regression along with the coefficients for the linear combination of the polynomial models. The smooth line drawn through the mixture data in Figure 8 is that of the linear combination of the empirical power series equation used to fit the pure component data. Examination of the residuals plot indicated no correlation between the excitation energy and the absolute error of the fit. Concentrations predicted in this fashion for the data illustrated here are summarized in Table I. The ratios in Table I are the ratio of the partial pressure of a gas in the mixtttre to that in the pure samples used to determine the empirical equations. Correlation coefficients for the amount of both species modeled were 0.998. These particular results are quite good considering that the error in obtaining gas mixtures pressures was on the order of 2% and that the relative precision of the data was -5%. Often, the partial pressures predicted by pure-component modeling of the mixtures were in error by as much as f 5 % @ I ) , but some of this error can be attributed to using model polynomials with fewer terms than is required for accurate modeling of the pure-component data. The relative concentrations over which this procedure was found to work were always between 20% and 80% of one of the two analytes. Since the relative error of the excitation energy data is about 57’0, it is not be possible to perform accurate analysis of mixtures where the relative signals from the analytes are 20 to 1or less. The ratio of component signals that will allow accurate multicomponent analysis will also be a function of the similarity of the excitation energy dependence curve. In the case of CFC-12 and CFC-22 the differences in the excitation energy dependence curves is quite pronounced. Even with this difference, the general trend in the data is similar and consequently there is high covariance between these data sets. For larger mol-
ecules, there will be little or no difference in the excitation energy data. Finally, we have found that this type of analysis can only be performed if the optical alignment of the apparatus is not changed between obtaining the pure component and the mixture data. Equation 3 predicts that the multiphoton behavior will be the same for any pump to probe laser displacement in the sample cell. Only the magnitude of the zero time signal will change. The same is not true for the saturation behavior predicted from eq 2. PL-PDS signals obtained with excitation energies in excess of that required for saturation are very dependent on the exact pump to probe beam offset. Given that saturation occurs over at least 50% of the energy range of these data, the constant alignment necessity is understandable. Caution must be exercised when interpreting the nonlinear absorption behavior clearly illustrated in the log data plots. ‘Typicalpulsed COPlasers have temporal pulse widths on the order of 200 ns. At room temperature, pressure-dependent excited vibrational state relaxation times are on the order of 1 ms P a for argon collision, which at standard pressure results in a vibrational lifetime on the order of 10 ns. Thus several excitation followed by relaxation events can occur during a single laser pulse. The actual number of excitation/relaxation events will be compounded since the local temperature will increase during this excitation/relaxation process resulting in increased collision rates and subsequently decreased mean lifetime of the excited states. This process of energy level cycling will occur at constant gas density. The density change will occur on the time scale of acoustic relaxation, which is equal to the excitation beam waist radius divided by the sound velocity (21). The sound velocity in argon is about 320 m/s at room temperature, and for a typical 0.05 cm excitation beam radius, the acoustic rise time of the density change will be on the order of 1.5 ps. As a consequence of these arguments, the apparent multiphoton absorption behavior of CFC-12 and CFC-22 should not be interpreted based on these multiphoton and saturation models alone. Clearly, there is much room for error with these interpretations. Although multiphoton behavior is apparent in both cases, the data can only be thought of as a phenomenalogical effect a t this time. However, it is noteworthy that similar nonlinear behavior has been observed for CFC-12 and CFC-22 using other experimental methods (18).
CONCLUSIONS Simple theories describing the PL-PDS signals predict signals proportional to the excitation laser pulse energy. As a result, large analytical signals are predicted when using bright excitation sources. But the energy that a given molecule can absorb with short pulse excitation is limited. In this study we have examined the limitations of pulse energy dependent analytical signals for the three main types of absorption behaviors expected in high energy excitation of gas phase analytes. The fact that linear pulsed laser energy dependent signals were obtained only for larger gas phase molecular species implies that examination of the absorption characteristics for every absorbing species is required. It is also apparent that proportional division by the pulsed laser energy, similar to that of conventional spectrophotometry, will not suffice to correct for the energy dependence in many cases. Linear signal dependence may be achieved at lower energies, but at the cost of signal magnitude. Large signal magnitudes are obtained at the higher pulse energies where nonlinear signal dependencies occur. The costs paid for high signal magnitudes are that the pulsed laser energy dependence of the analytical signal must be known and that nonlinear regression analysis of the data should be performed. In addition, there are advantages resulting from the determination of the
Anal. Chem. 1987, 5 9 , 879-887
energy-dependent behavior of the nonlinear PL-PDS signals. The information to be found by elucidation of the nonlinear behavior can be used for speciation and discrimination of the analyte over interferences.
LITERATURE CITED (1) Fang, H. L.; Swofford, R. L. I n Ultrasensitive Laser Spectroscopy; Kliger, D. S., Ed.; Academic: New York, 1983. (2) Twarowski, A. J.; Kliger, D. S. Chem. Phys. 1977, 2 0 , 253-258. (3) Bailey, R. T.; Cruickshank, F. R.; Pugh, D.; Johnstone, W. J. Chem. Soc ., Faraday Trans. 2 1980, 7 6 , 633-647. (4) Bailey, R. T.; N. Cruckshank, F. R.; Pugh, D. Johnstone, W. J. Chem. Soc., Faraday Trans. 2 1981, 77, 1387-1389. (5) Mori, K.; Imasaka, T.; Ishibashi. N. Anal. Chem. 1983, 55.
1075-1079. (6) Sell, J. A. Appi. Opt. 1984, 23, 1586-1597. (7) Long, G. R.; Bialkowski, S. E. Anal. Chern. 1985, 5 6 , 2806-2811. (8) Long, G. R.; Bialkowski, S. E. Anal. Chem. 1985, 5 7 , 1079-1083. (9) Nickolaisen, S.L.; Bialkowski, S. E. Anal. Chem. 1985, 56, 758-762. (IO) Long G. R.; Bialkowski, S. E. Anal. Chem. 1986, 5 8 , 80-86. (11) Nickolaisen S. L.; Bialkowski S. E. J. Chem. I n f . Sci. Cornput. Sci. 1986, 26, 57-59. (12) Nickolaisen S . L.; Bialkowski S. E. J . Chromatogr. 1986, 366, 127-133.
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(13) Higashi, T.; Imasaka, T.; Ishibashi. N. Anal. Chem. 1984, 5 6 , 2010-2013. (14) Mayer, A.; Comera, J.; Charpentier, H.; Jaussaud, C. Appl. Opt. 1978, 17,391-393. (15) Harris, J. M.; Dovichi, N. J. Anal. Chem. 1980, 52,695A. (16) Jackson, W. B.; Arner, N. M.; Boccara, A. C.; Fournier, D. Appl. Opt. 1981, 20, 1333-1343. (17) Yariv, A. Introduction to Optical ,Electronics, 2nd ed.; Holt, Rinehart and Winston: New York, 1976. (18) Grunwald, E.; Dever. D. F.; Keehn, P. M. Megawatt Infrared Laser Chemistry; Wiley: New York. 1978. (19) Steinfeld, J. I.Laser Induced Chemical Processes; Plenum: New
York, 1981. (20) Steinfeld, J. I.Molecules and Radiation, 2nd ed.; MIT Press: Cambridge, MA, 1985. (21) Long, G. R. PhD. Dissertation, Utah State University, 1986. (22) Barker, J. R.;Rothem, T. Chem. Phys. 1982, 6 8 , 331-339
RECEIVED for review September 8,1986. Accepted November 14, 1986. This work was partially supported by a grant from the Vice-president for Research, Utah State University, and by CHE-8520040 awarded by the National Science Foundation.
Elemental Speciation via High-Performance Liquid Chromatography Combined with Inductively Coupled Plasma Atomic Emission Spectroscopic Detection: Application of a Direct Injection Nebulizer Kimberly E. LaFreniere, Velmer A. Fassel,* and David E. Eckels Ames Laboratory-USDOE
and Department of Chemistry, Iowa State Uaiversity, Ames, Iowa 50011
An evaluation Is presented of a direct injection nebulizer (DIN) interfaced to a hlgh-performance liquid chromatograph (HPLC) with inductively coupled plasma atomlc emlssion spectroscopic (ICP-AES) detection for slmultaneous muitielement speciation. The llmlts of detectlon (LODs) obtained wlth the DIN Interface in the HPLC mode were found to be comparable to those obtained by continuous-flow sample Introduction Into the ICP, or lnferlor by up to only a factor of 4. I n addltion, the DIN allowed for the direct Injection Into the ICP of a variety of common HPLC solvents (up to 100% methanol, acetonitrile, methyl isobutyl ketone, pyridine, and water). The HPLC-DIN-ICP-AES system was compared to other HPLC-atomic spectroscopic detectlon technlques and was found to offer substantlal Improvement over the alternative on-line, detection methods in terms of LODs. Representative appllcatlons of the HPLC-DIN-ICP-AES system to the elemental speciation of coal process streams, shale oil, solvent refined coal, and crude oil are presented.
The speciation of organically or inorganically bound metals in solution presents a formidable challenge. Although the technology required to perform high-performance liquid chromatographic (HPLC) separations of metal-containing species is generally available, conventional HPLC detectors, such as the refractive index (RI), ultraviolet (W),fluorescence (FL), electrochemical (EC), flame ionization (FI), and infrared (IR), lack the desired degree of selectivity, sensitivity, and applicability (1,2). Atomic spectroscopic detectors, on the
Table 1. Element-Specific Detectors for GC, LC, and IC: Idealized Characteristics 1 applicable to the direct, on-line detection and determination
of all elements 2 part-per-billion detection limits (based on content of sample injected into chromatograph) 3 capable of determining empirical formulae (no chemical structure or matrix effect) 3 linear response over 3 to 1 orders of magnitude of concentration changes 5 directly adaptable to collecting -nn ehnenT selective chromatograms simultaneously 6 compatible with various mobile phases and associated flou rates 7 minimum dead volumes 8 simplicity of operation and construction 9 low cost
other hand, are highly specific and are applicable to a wide variety of sample matrices. Among the various atomic spectroscopic detectors, flame atomic absorption spectroscopy (FAAS), graphite furnace atomic absorption spectroscopy (GFAAS), and flame or plasma atomic fluorescence spectroscopy (AFS), as well as microwave-induced plasma (MIP), direct-current plasma (DCP),and inductively coupled plasma (ICP) atomic emission spectroscopy (AES) hold the most potential as element-specificdetectors for HPLC. Indeed, over 125 publications on atomic, element-specific detectors have appeared in the literature in the last 15 years (3). Of the atomic spectroscopic detectors, emission sources are the most readily adapted to simultaneous multielement monitoring, and many researchers agree that plasma emission spectroscopy
0003-2700/87/0359-0879$01.50/0 1987 American Chemical Society
