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Anal. Chem. 1985, 57, 758-762
choice of phase. Thus an absorber should be distinguishable modulation. from a nonabsorber even if they have the same redox potential, Registry No. TAA, 13050-56-1. but care must be taken when determining an absorbing redox system in the presence of a large concentration of a nonabLITERATURE CITED sorbing electroactive species. (1) Roston, D. A.; Shoup, R. E.; Kissinger, P. T. Anal. Chem. 1982, 5 4 , 1417A. The experiments in Figures 8 and 9 and eq 5 and 6 are very Wlghtman, R. M. Anal. Chem. 1982, 5 4 , (2) Caudili, W. L.; Howell, J. 0.; similar to the corresponding expressions for ac polarography, 2532. and the two methods have the same peak width. The two (3) Vydra, F.; Stulik, K.:Julakova, E. "Electrochemlcal Stripping Analysis"; Halsted Press: New York, 1977. techniques are conceptually similar, in that their potential (4) Flato, J. D. Anal. Chem. 1972, 4 4 , 75A. dependence is derived from the fraction of a redox system in (5) Adams, R. N. Anal. Chem. 1978, 4 8 , 1126A. one redox state at the electrode surface. An important dif(6) Wightman, R. M. Anal. Chem. 1981, 5 3 , 1125A. (7) Heineman, W. R.; Hawkridge, F. M.; Blount, H. N. I n "Electroanalytical ference is that ac voltammetry measures the flux of redox Chemistry"; Bard, A. J., Ed.: Marcel Dekker: New York, 1984; Vol. material reaching or leaving the surface, while the spectroe13, pp 1-113. (8) McCreery, R. L. I n "Physical Methods in Chemistry"; Rossiter, B., Ed.; lectrochemical method measures the concentrationat or near Wiiey: New York, in press. the surface. In addition, the spectroelectrochemical method (9) Robinson, R. S.; McCurdy, C. W.; McCreery, R. L. Anal. Chem. 1982, 5 4 , 2356. possesses the added selectivity of a spectroscopic absorption (10) Bancroft, E. E.; Sidwell, J. S.; Blount, H. N. Anal. Chem. 1981, 5 3 , measurement of redox activity over more common monitoring 1390. of current. (11) Tyson, J. F.; West, T. S. Talanta 1980, 27, 335. (12) Pruiksma, R.; McCreery, R. L. Anal. Chem. 1981, 5 3 , 202. A direct comparison of the present method with other (13) Zak, J.; Porter, M. D.; Kuwana, T. K. Anal. Chem. 1983, 55, 2219. spectroelectrochemical techniques is not straightforward due (14) Porter, M. D.; Kuwana, T. Anal. Chem. 1084, 5 6 , 529. to differences in time scale and instrumental variables. In (15) Brewster, J. D.; Anderson, J. L. Anal. Chem. 1982, 5 4 , 2560. (16) Rossi, P.; McCurdy, C. W.; McCreery, R. L. J . Am. Chem. SOC. 1981, several cases, techniques were designed for examining reaction 103, 2524. mechanisms, and analytical sensitivity was not optimized. (17) Rossi, P.; McCreery, R. L. J . Necfroanal. Chem. 1983, 757, 47. (18) Bard, A. J.; Fauikner, L. R. "Electrochemical Methods"; Wiley: New With these caveats in mind, Table I1 compares several techYork, 1980; p 161. niques with respect to their ability to measure the minimum (19) Hinman, A. S.; McAleer, J. F.; Pons, S. J . Nectroanal. Chem. 1983, product of molar absorptivity and bulk concentration, ( C C ~ ) ~ ~ 154, 45. (20) Bard, A. J.; Faulkner, L. R. "Electrochemical Methods"; Wiley: New This quantity provides the most relevant comparison of deYork, 1980; pp 331-332. tection limits since it is composed of variables related only (21) Nelson, R. F.; Adams, R. N. J . Am. Chem. SOC. 1968, 9 0 , 3925. (22) Winograd, N.; Kuwana, T. J . A m . Chem. SOC. 1971, 9 3 , 4343. to the analyte and not the instrument or technique. The (23) Winograd, N.; Kuwana, T. Anal. Chem. 1971, 4 3 , 252. spectroelectrochemical techniques listed vary significantly in (24) Baumgartner, C. E.; Marks, G. T.; Aikens. D. A,; Richtol, H. H. Anal. Chem. 1980, 52, 267. both their optical path lengths and their minimum measurable (25) Skuily, J. P.; McCreery, R. L. Anal. Chem. 1980, 52, 1885. absorbance, Amin. With comparable Amin,those techniques with long path lengths will have better detection limits. Furthermore, those methods which are amenable to signal RECEIVED for review July 27, 1984. Accepted December 3, averaging will improve detection limits by decreasing Amin. 1984. Major support for this work was provided by the The low detection limits of the modulated diffraction techchemical analysis division of the National Science Foundation, nique presented here result from both relatively long path with additional support from the Technicon Instrument length and signal to noise enhancement provided by potential Corporation.
Pulsed Infrared Laser Thermal Lens Spectrophotometry of Flowing Gas Samples Scott L. Nickolaisen and Stephen E. Bialkowski* Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322
The behavlor of the pulsed Infrared laser thermal lens slgnal Is studied for flowing gas samples. Posslble sources of decreased TLS slgnal are dlscussed lncludlng flow rate and cell temperature. The TLS slgnal is found to be Independent of the flow rate at all flow rates studled. I t Is calculated that In argon and 1 X In absorbances as low as 2.5 X llquld carbon dlsulflde should be detectable. The posslbllltles of using this technlque as a chromatography detector for ultratrace amounts of analyte are also dlscussed.
The use of laser analytical techniques has produced interesting results both in terms of sensitivity and in the se-
lectivity of the specific technique ( I ) . This progress has for the most part been due to the high spectral brightness and good optical characteristics of the laser source (2). There are effectively two major classes of laser analytical techniques for spectrophotometric analysis. The first class relies on the luminescent relaxation mechanism. Among this class is laser-induced breakdown spectroscopy (3), single and multiphoton laser excited fluorescence ( 4 ) , and light scattering spectroscopies. The second class derives the analytical signal from dark relaxation pathways of the laser excited analyte (5). The latter category includes photoacoustic (6),photothermal lensing (7) and deflection (8),and interferometry (9). Absorption spectrophotometry utilizing laser light sources may also be placed in this category.
0 1985 American Chemical Society 0003-2700/85/0357-0758$01.50/0
ANALYTICAL CHEMISTRY, VOL. 57, NO. 3, MARCH 1985
Perhaps the most well-known of the laser analytical techniques in the latter category, which derives the signal from an optical source, is thermal lens spectrophotometry (TLS). There are two means by which sample excitation can be performed in TLS. The most common method is to use a chopped continuous wave (CW) laser. This method may be used in either the single or dual laser configuration (7,10-14). The second means by which the sample can be excited is through the use of a pulsed laser. In this case, the excitation laser cannot be used as a probe and the use of a pulsed laser necessitates a dual laser configuration. This method of sample excitation results in signals which are in general quite different than those obtained with CW excitation (7, 13). With the exception of the works by Imasaka et al. (15,16)and ourselves (1 7-19), pulsed laser TLS has not been exploited for analytical purposes. It has, however, found widespread use for solutions to problems of a physical chemistry nature (20-24). I t is well-known that there is a need for development of better detectors for chromatography and the use of laser analytical techniques has not been overlooked (25, 26). However, there have been relatively few studies of the use of TLS for flowing sample analysis (27-30). The latter two studies demonstrated the utility of CW laser TLS for flowing sample detection. However, the sensitivity of the technique was found to decrease with increasing flow rate, thus limitations on the applicability of CW laser TLS were discovered. In this paper we report on the use of pulsed infrared laser TLS for flowing sample analysis. Unlike the CW laser technique, there is no degradation of sensitivity at moderate flow rates. The advantages to the use of pulsed infrared laser sample excitation are as follows: first, the rise time of the analytical signal is much shorter than the time scales for probe laser power fluctuation or gas dynamic effects such as turbulence, mass transfer, and thermal diffusion (5, 7,14-21,24, 27, 28,29,31, 32);second the infrared source allows a wide variety of analytes to be excited, not just those with visible transitions; and third, the high pulse energy of the TEA-C02 laser allows for multiphoton excitation of the analyte, thereby increasing the temperature rise in the sample while narrowing the spatial distribution of this profile (33). The latter effects will result in a greater signal and may even overcome the limitation of infrared excitation, over that of visible-ultraviolet, due to the lower transition energy. The present work specifically addresses the utility of the pulsed infrared laser TLS technique for flowing gas analysis, but there is no reason why this technique could not be used for liquid phase analytes as well.
THEORY The theories for both CW laser and pulsed laser excited TLS have been described by a number of authors and have been summarized by Fang and Swofford (7). Of importance to this study are the temporal characteristics of the pulsed laser TLS signal and the differences between the pulsed and CW excitation source. One of the ways by which thermooptical methods of analysis can be compared is by the theoretical enhancement factors (7, 10). The ratio of the pulsed laser enhancement factor to that of the CW laser is (15)
33/2E, K PCW~O2
where E , is the pulsed laser energy, K is the thermal diffusivity, P,, is the CW laser power, and wo is the l / e electric field radius of the excitation laser. This equation is for typical optics with matched probe excitation beam radii in the sample. Typical values for the thermal diffusivity are 1.0 X lo-' m2/s for liquid benzene and 1.9 x mz/s for argon gas at standard conditions. In these experiments coo2 is about 1.5
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X lo-' m2, although smaller beam waist radii may be obtained with visible lasers ( 2 ) . For equivalent average energies, the enhancement factor ratio is close to unity for liquid-phase samples but very much greater than one for gas-phase samples. Thus, the pulsed laser technique is clearly preferable for gas-phase analysis. Another point to consider is that of the rise time of the signal. When a CW laser is used as an excitation source, the temperature change resulting in the TLS signal increases slowly in time as energy is put into the sample. Carter and Harris (34) have shown that for the aberrant lens model, the temporal response of the signal is
where I ( t ) is the transmitted intensity at time t , I ( 0 ) is the transmitted intensity prior to turning the excitation source on, the parameter 8, is a dimensionless collection of variables which is related to the strength of the thermal lens, and t, is the thermal response time constant, t , = w 2 / 4 K . The signal, defined above is not given in the form usually used for CW excited TLS, but rather in that form used for pulsed laser TLS. After a time equal to the thermal time constant, the signal has reached 50% of the maximum value; to reach 90% of this value requires that t = 9t,. In contrast, the temporal behavior of the pulsed laser TLS signal, assuming neglectable contributions due to mass diffusion (31) and acoustic effects (32),is for t, < t , (17)
(3) where, again, 8, is a parameter proportional to the strength of the thermal lens and t, is the first-order time constant for excited state relaxation. This expression was derived assuming a parabolic lens (33,34),a laser pulse which is much shorter than the excited state relaxation time, and a pseudo-first-order relaxation mechanism. The signal reaches 90% of its maximum value in 2.303t,. Values o f t , range from about 1 ps for gas-phase analytes a t standard conditions down to picoseconds or even femtoseconds in solution phase. For example, the relaxation time constant for dichlorodifluoromethane in argon at standard conditions and excited with a COS laser is t, = 0.15 ps (35). This is actually slow relative to the other vibrational relaxation times compared in the latter reference. The thermal response time constant for the same system with wo2 = 1.5 X m2 is t, = 2.0 ms. Thus the time required for the CW laser excited TLS signal to attain 90% of the maximum value is over 5 orders of magnitude longer than that for pulsed laser excitation. The rise time of the pulsed laser TLS signal will be ultimately limited by the acousic transit time (24, 32). This is the time required for the pressure disturbance created by the increased temperature to dissipate from the excited region, t , = w 0 / c , where c is the sound velocity. The sound velocity for argon is c = 321 m/s (36) and the acoustic transit time is t, = 1.2 ps. The sound velocity of benzene is 1395 m/s and the acoustic transition time will be correspondingly shorter in liquid phase.
EXPERIMENTAL SECTION The TLS optical apparatus used in this study has been previously described (17) and will only be outlined here. A pulsed C 0 2 laser was employed as the excitation source, and a CW H e N e laser was used as the probe laser. The pump laser was a TEA-C02 laser constructed in this laboratory. The rate of pulsing was 3.75 Hz, and the laser was operated on the P32 line of the 10.6-pm transition at 933 cm-l. Transverse mode control was accomplished with the use of an intercavity iris. Each pulse lasted about 150
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 3, MARCH 1985
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-
0.50
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cn
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-I W
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TIME (MSEC) Figure 1. Time-resolved TLS signal for 0.1 % chlorodifluoromethane in argon at flow rates of (A) no flow, (e) 60 mL/min, and (C) 150
FLOW
RATE
(ML/MIN)
Relative TLS signal as a function of flow rate for 9.9 ppm dichlorodifluoromethane in argon at atmospheric pressure. Figure 2.
mL/min.
ns with a 3-ps “tail”. The laser energy per pulse was less than 100 mJ, thus each pulsed delivered approximately 600 kW of power. The probe laser was a 5-mW Uniphase Model 1105P He-Ne laser which operated at the wavelength of 632.8 nm. The COz laser was passed through a germanium flat where approximately half of the beam was transmitted. The transmitted portion of the beam was mixed at the Ge flat with the He-Ne laser so that the two beams were collinear. That portion of the COz beam reflected at the Ge flat was used to monitor the energy of the C02 laser by means of a Laser Precision Model RjP-735 energy monitor. The flowing cell was constructed in this laboratory from stainless steel. It had the dimensions of 0.7 cm i.d. by 5.5 cm length. It was mounted with NaCl windows which were sealed to the cell using O-rings. The inlet and outlet ports were placed as close to the ends of the cell as possible to reduce any dead volume within the cell. The COz and He-Ne lasers were focused through the center of the cell using a 30-cm focal length BaFz lens and a 25-cm focal length fused silica lens, respectively. The positioning of these lenses was such that the He-Ne beam focus was about 15 cm past the cell. The detector used was an EG&G SCD-040-A PIN photodiode with a pinhole size of 0.06 mm radius. The detector was placed about 1 m from the sample cell. The detector signal was amplified by an ac coupled LM741C differentiating amplifier. The diode signal was recorded with a Physical Data Model 522A, 20 MHz, 8 bit transient digitizer. The C02 monitor was connected to an ADAC Corp. Model 1012,16channel, 1 2 bit, programmable gain A/D converter. Both the A/D converter and transient digitizer were interfaced with a DEC LSI 11/2 microprocessor. Data acquisition was accomplished by means of a computer program which corrected the photodiode signal for variations in the energy of the COz laser. For each data point taken, the signal from 100 pulses was averaged and recorded. The gas sample used for the study of flow rate and temperature dependence was a Matheson gas cylinder analyzed to 9.9 ppm dichlorodifluoromethane (FC-12) in argon. The gas cylinder was connected to the cell input port, and a bubble meter by which the flow rate was measured was connected to the output port. The gas flow rate was controlled with a needle valve connected to the cylinder regulator. The temperature of the cell was adjusted by use of a heating cord wrapped around the flow cell. For the GC runs a 1.22 m X 5 mm i.d. 20% Carbowax 20 M on Chromosorb-P 80/100 mesh column in a Gow-Mac Series 550 gas chromatograph was used with argon as the carrier gas. Signals for the GC runs were collected with the A/D converter and COz laser energy fluctuation were not accounted for. RESULTS AND DISCUSSION Typical pulsed laser TLS signals detected by the PIN diode-pinhole detector without preamplification are illustrated in Figure 1. These signals were obtained for 100 averages of a 0.1% by volume mixture of chlorodifluoromethane in the flow cell, The high concentration of chlorodifluoromethane was required to obtain a signal of 50 mV, which is the min-
imum input range of the transient digitizer. The rapid rise time of the signal is apparent. In fact, the rise time of the data illustrated in Figure 1is limited by the sampling interval of 5 11s. Thus, the probe laser intensity change occurred on a time scale less than 5 ps. This figure also illustrates the effect of sample flow on the TLS signal. While the signal maxima are the same, the signal decay rate increases with flow rate. The gas flow rate dependent TLS signal for the dichlorodifluoromethane/argon mixture a t 100 kPa is illustrated in Figure 2. The flow rate ranges from about 3 mL/min to 100 mL/min corresponding to a maximum linear flow of 4.3 cm/s through the 3.8 mm cross section of the flow cell. As can be seen, the signal is found to be essentially constant over the range studied. This observation is in sharp contrast to previous studies of the flow rate dependent CW laser TLS signal. Dovichi and Harris (27,28) found that with a sample residence time in a liquid flow cell of 3.04 s, the CW laser TLS signal was diminished by a factor of 43%. The calculated minimum residence time in these experiments is 26 ms. No signal decrease was observed. This significant difference is due to the nature of the TLS signals. In these experiments, the data are obtained over a short (100 p s ) time. The short signal rise times, and thus the data acquisition time, have several important implications in chromatographic and flowing sample applications. The first implication is that the signal does not decrease as rapidly with flow rate as does the CW TLS signal. We would not expect to observe a signal decrease unless the gas dynamic mass transfer time scale was on the order of that of the rise time of this signal. This will occur a t about t , = wo/V,where V is the linear flow rate of the sample. Since the rise time of the signal in these experiments is limited by the acoustic transit time, as discussed above, the linear velocity may have to be on the order of the sound velocity for signal degradation to occur. The second implication of the rapid signal acquisition time is that the sample rate can be higher than that of the CW laser TLS case. In this case however, it is not the rise time of the signal that limits the sample rate, but rather the decay time. The decay time constant for pulsed laser TLS signals is shorter than that for the CW case since the signal decreases proportional to (1 + 2t/tJ2. So in the time required for the CW laser signal to decay to 1 / 2 the maximum value, the pulsed laser signal has decayed to 1/9, 1/4 to 1/49, etc. The time required for the signal to decay to 1% of the maximum in these experiments is 44 ms. In essence, the time required to obtain a CW laser signal is that of the characteristic rise time. If, for example, the analyte composition was to change abruptly, it would take 9t, for the CW laser signal to respond. For analytes that elute off of a chromatography column in time less than t,, the signal would be that of the convolution of the
ANALYTICAL CHEMISTRY, VOL. 57, NO. 3, MARCH 1985
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4
TIME
I
8
1
e
1
10
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Figure 3. Demonstration of the use of pulsed laser TLS for packed column GC. The three peaks are due to chlorotrifluoromethane,dichlorodifluoromethane, and ethanol, respectively. Response due to benzene solvent was not observed at the excitation wavelength used.
thermal Green's function (7) with that of the temperature source due to the transient analyte. This would serve to diminish the maximum signal as well as lower the effective resolution of the separation process. For the pulse laser case, if the analyte elutes in a time less than the repetition rate of the laser, then there is a chance that no signal will result. The signal decay time limits this rate to about 22 Hz, and any analyte which elutes in less than 44 ms could be missed. Figure 3 illustrates the response of the pulsed TLS apparatus to a GC separated mixture of halocarbons and alcohols in benzene solvent. In this case a packed column was used so that the resolution is not as high as could be obtained with modern capillary columns. The data were collected as a function of time in such a manner as to sum each consecutive 10 data into one datum. The effective time constant is thus ten times the repetition rate. However, due to the large pulse to pulse energy variations of the COz laser used in these experiments, this was necessary to obtain acceptable SIN. It is apparent from this figure that there is no problem with using the low repetition rates for packed column GC analyte detection. The data in Figure 2 indicate that the TLS signal obtained at the lower flow rates might be less than that for a high one. In investigating this we found that the signal obtained just after the COzlaser was turned on was in fact greater than that obtained after the sample had been irradiated for several minutes. Further, the recovery time was found to be about 5 min. With even the slowest flow rate of 3 mL/min, the time required to replace the contents of the flow cell is less than 1min. We subsequently rule out the possibility of a chemical transformation taking place as a result of laser irradiation which diminishes the signal. It is more likely that the temperature of the cell is increasing due to the buildup of heat from the repetitively pulsed sample. With the faster flow rates, the rate of cooling of the flow cell is sufficient to overcome the heating rate and thus the signal does not diminish in time. The effect of temperature of the TLS signal for gas samples can be determined rather easily. The parameter, e,, in eq 3 is (17)
(4) Here a is the exponential absorption coefficient, 1 is the length of the sample cell, no is the refractive index of the gas, p is the molar density and C, is the heat capacity. Since C, and a do not vary significantly over a small temperature range,
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the main temperature-dependent parameters are p and (dn/dT),. Bailey et al. (20) have shown that the temperature dependence of (dn/dT), a l/p,and that of p is 1/T. Overall, the TLS signal should decrease as 1/T. We have confirmed this effect by measuring the signal at a flow rate of 67 mL/min while varying the flow cell temperature from 30 OC to 110 OC. The implication of this effect in the use of pulsed laser TLS in GC effluent detection is that since the sample cell will generally be operated a t elevated temperature, the signal can be expected to decrease over that a t room temperature. The third implication of the fast rise time of the pulsed laser TLS signal is that the effects of turbulence and other mass transport effects will be minimized. If the rise time of the signal is faster than the characteristic mass transport effect time, then these effects will not be manifested in the signal. One effect that may be important in gas-phase analytes is that of mass diffusion. The characteristic time constant for mass diffusion is t, = wz/4D, D being the mass diffusion coefficient. This is not an important consideration at pressures above 5 kPa. The turbulence time scales are much more difficult to estimate but are prsbably about that of the flow rate limited mass transport through the laser beam diameter in the cell. Thus, if the flow rate is fast enough that the signal diminishes over that of static conditions, turbulence could be expected to decrease the precision of the measurement (28,29). The standard deviations of the TLS signals obtained over the range of flow rates demonstrated no significant variation with flow rate. In these experiments, the major contribution to fluctuations in the TLS signal was not that of the turbulence in the flow cell, as found by Dovichi and Harris (28, 29), but rather was due to the pulse to pulse laser energy and mode changes. The relative precieion due to these effects can be seen in Figure 2 at the lower flow rates. The relative precision of the data in Figure 2 is 4% and that of each point, an average of 100, ranged from 1.4% to 5%. The precision of the measurements obtained in these experiments may be expressed as the sum of those relative deviations due to pump laser mode variations, intensity changes in both the pump and probe lasers, alignment errors from vibrations and laser pointing noise, intrinsic gas dynamic variances, and instrumental errors as a result of digitization of the signals, instrumental noise, and interferences. Signal variations as a function of pump laser energy fluctuations were compensated for in the data reduction. Instrumental noise and interference were less than the least significant bit of the transient digitizer (
