2436
Anal. Chern. 1985, 57,2436-2441
Ii \!-I
the analyte from the solvent in which it is injected (20). Double-beam thermal lens spectrometry could readily be adapted to detection in liquid chromatography. The 1.25-s time response is quite adequate for resolving closely spaced peaks, and the base line noise is comparable to previous examples of thermal lens detection in HPLC (8,11,21-24). The use of a single laser beam through the sample, however, eliminates the alignment difficulties of the pump-and-probe configuration, which are particularly severe for detection in microbore (8, 23) or capillary (24) HPLC, and lock-in amplification of the signal overcomes any delay or trade-offs in fitting transient single-beam data. Work is in progress to evaluate the double-beam thermal lens method for this application.
“I
LITERATURE CITED 5 min.
Flgure 3. Double-beam thermal lens detection of flow-Injected peaks: lo-’ M I, in CCI,, 100-mV lock-in scale: (b) 1 X lo-‘ M I, in CCI,, 1 . 0 4 lock-in scale. (a) 1 X
volume requirements of FIA which are matched to the capabilities of the laser, the smaller likelihood of sample contamination during processing, and the reduced carry-over from higher concentration samples due to the rinsing efficiency (18, 19).
The flow injection response of double-beam thermal lens detection is shown for two concentrations of injected iodine solution in Figure 3. A significant blank contribution to the measured response was observed in the amplitude and peak shape for the lower concentration samples. Based on its more asymmetric shape, this blank contribution is probably arising from refractive index gradients in the region of the sample zone due to slight differences in composition or temperature between the injected solvent and the carrier. The blank response was fairly constant, however, producing linear calibration results and detection limits based on the reproducwhich were larger than ibility of the blank, Amin= 1.7 X the detection limits based only on the baseline noise, Amin= 5.5 x To obtain base-line-limited detection and avoid the difficulty of exactly matching the optical properties of the injected solvent and carrier, a short chromatographic column could be added to the FIA manifold to temporally separate
(1) Gordon, J. P.; Leite, R. C. C.; Moore, R. S.; Porto, S. P. S.; Whinnery, J. R. J . Appl. PhyS. 1965, 36, 3-8. (2) Harris, J. M. “Analytical Applications of Lasers”; Piepmeier, E. H., Ed.; Wlley: New York, in press. (3) Whinnery, J. R:Acc. Chem. Res. 1974, 7 , 225-231. (4) Hu, C.; Whinnery, J. R. Appl. Opt. 1973, 72, 72-79. (5) Dovlchi, N. J.; Harris, J. M. Anal. Chem. 1981, 53, 106-109. (6) Grabiner, F. R.; Siebart, D. R.; Fiynn, G. W. Chem. f b y s . Lett. 1972, 77, 189-194. (7) Long, M. E.; Swofford, R. L.; Albrecht, A. C. Science 1976, 197, 183-185. (8) Pang, T. J.; Morris, M. D. Anal. Chem. 1984, 56, 1467-1469. (9) Carter, C. A.; Harris, J. M. Anal. Chem. 1983, 55, 1256-1261. (10) Yang, Y. Anal. Chem. 1964, 56, 2336-2338. (11) Pang, T. J.; Morris, M. D. Appl. Specfrosc. 1985, 3 9 , 90-93. (12) Jansen, K. L.; Harris, J. M. Anal. Chem. 1985, 57, 1698-1703. (13) Carter, C. A.; Harris, J. M. Appl. Opt. 1984, 23, 476-481. (14) Ruzicka, J.; Hansen, E. H.; Ramsing, A. U. Anal. Chim. Acta 1982, 134, 55-71. (15) Ruzicka, J.; Hansen, E. H. “Flow Injection Analysis”; Wiley: New York, 1981; Chapter 2. (16) Currie, L. A. Anal. Chem. 1968, 4 0 , 586-593. (17) Bevlngton, P. R. “Data Reduction and Error Analysis for the Physical Sciences”; McGraw-Hill: New York, 1969; Program 11-5. (18) Harrls, J. M. Anal. Chem. 1982, 5 4 , 2337-2340. (19) Leach, R. A.; Harris, J. M. Anal. Chim. Acta 1984, 764, 91-101. (20) Leach, R. A.; Harris, J. M. Anal. Chem. 1984, 56, 2801-2805. (21) Leach, R. A.; Harris, J. M. J . Chromatogr. 1982, 2 1 8 , 15-19. (22) Buffett, C. E.; Morris, M. D. Anal. Chem. 1982, 5 4 , 1824-1825. (23) Buffett, C. E.; Morris, M. D. Anal. Chem. 1983, 55, 376-378. (24) Sepaniak, M. J.; Vargo, J. D.; Kettler, C. N.; Maskarinec, M. P. Anal. Chem. W84, 56, 1252-1257.
RECEIVED for review May 31, 1985. Accepted July 8, 1985. This research was supported in part by the National Science Foundation under Grants CHE82-06898 add CHE85-06667.
Time-Resolved Thermal Lens Measurements in Flowing Samples Wayne A. Weimer and Norman J. Dovichi” Department of Chemistry, University of Wyoming, Laramie, Wyoming 82071 Thermal lens calorlmetry Is considered for flowing llquld samples. Under conditions of known flow profile, as in lamlnar flow, It is possible to construct an accurate model of the time-resolved thermal lens signal. This model Is useful In regression analysls of data taken In flowing llquld samples. Furthermore, the time-resolved model may be used In the optimization of thermal lens experiments In liquid chromatography detection. The model Is verified for flowlng liquid samples withln a square duct.
Since its development by Whinnery ( 1 4 , thermal lens techniques have been recognized as useful in the analysis of
weakly absorbing species (6-16). Specifically, the technique has been considered as a detector for chromatography and flow injection analysis ( I 7-22). Optimization of thermal lens detection requires the consideration of the effect of flow upon the signal. Further interest in thermal lens behavior within flowing liquid streams in found in convective perturbation of thermal lens signals (2, 6 , I I ) and in the intercavity thermal lens formed within a flowing dye laser stream (23, 24). Unfortunately, there has been little work on the theory of thermal lens calorimetry in flowing samples, although the temperature profile produced by a laser beam moving over a surface has been considered (25). In general, it is recognized that flowing streams produce an additional contribution to
0003-2700/85/0357-2436$.01.50/00 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 13, NOVEMBER 1985
t, =
2437
W2PCp 4k
(3)
where C, is the heat capaciby, p is the density, and w is the pump beam spot size. To account for sample flow, it is assumed that the temperature distribution is displaced in the direction of flow a distance proportional to the flow velocity. We neglect heat flow along the pump beam axis which should be acceptable for small temperature displacements. This assumption will only be useful for low flow rate samples. The displaced temperature distribution is given by Figure 1. Coordinate system for thermal lens calorimetry in flowing samples. The beams propagate along the Z axis. Flow Is along the Y axis. The distance from the probe beam waist to the sample is given by Z,and the distance from the sample to the detector is given by Z,. The spot size of the probe beam at the far field in the x direction is given by w, and in the y direction by w y .
heat loss within the sample; both the relaxation time and steady-state intensity change will decrease for step excitation (12). Flow has no influence on the initial probe laser intensity in either impulse (26)or step (12) excitation but does increase the rate of change in probe laser intensity for pulsed excitation (26). Kinetic methods of analysis are known to produce significant improvements in signal-to-noise and detection limits for thermal lens calorimetry (11). A kinetic model would be advantageous for flowing samples. For example, improved detection limits are likely to result using the kinetic model compared to other signal processing techniques. The time constant for the effect may be used as an additional degree of freedom in analysis of relatively concentrated samples. Furthermore, variations in probe laser intensity due to drift in flow rate could easily be corrected with the regression analysis by allowing the flow rate parameter to vary in the calculation. Lastly, the kinetic model would facilitate the optimization of the signal with respect to flow rate and chopping frequency.
THEORY The development of the kinetic model for thermal lens calorimetry in flowing samples falls into four parts. First, the heat equation must be solved to determine the temperature distribution within the sample for impulse excitation. Second, the inverse focal length for impulse excitation is determined as a function of position across the sample cell and then integrated along the probe beam path. Third, the far-field probe beam center intensity is calculated. Last, the impulse response is convoluted with an appropriate excitation function to construct the time-resolved thermal lens signal. Temperature Distribution. Twarowski and Kliger have developed an expression for the temperature distribution within a semiinfinite homogeneous sample excited by a pulsed TEMW Gaussian laser beam (27). By use of the coordinate system of Figure 1,the temperature distribution at time t after the laser pulse, AT(x,y,t), is given by
(4) where Vis the flow velocity in the Y direction and which varies along the pump beam path, Figure 1. This velocity could be determined experimentally using time-resolved photothermal refraction (28,29)or computed for laminar flow. In this paper, we consider laminar flow inside a square duct. Analogous formulas may be applied to other flow geometries. Bird gives an expression for the velocity profile inside a square duct under laminar flow (30)
(5)
V = =16B2 [l-$][l-++$]
where W is the volume flow rate, B is half the cell width, and K is a constant which contains the flow terms not dependent upon 2. Note that the origin of the coordinate system is at the center of the flow cell. Focal Length. The inverse focal length of the thermal lens is given by Jackson (31)
1
fi- = -(dn/dT)
lBa2si
Bd2AT(x,y,t;V)
dz
Si = X,Y
where Siis the direction perpendicular to the propagation axis. For flowing samples, the focal lengths in 3c and y will differ and are treated separately. The integration is over the optical path, in our case from -B to +B. The change of refractive index with temperature, dn/dT, is a proportionality constant to convert from a temperature distribution to a refractive index distribution. d n / d T is evaluated at the probe laser wavelength. Combining eq 4-6 yields expression for the inverse focal lengths in the 1c and y directions, evaluated at the pump beam center
_1 -fz
4 A T ( t = O)(dn/dT) w2(1+ 2t/tC)2
B
-2t2P(1 - 22/,2)2
J B
u2(1 + 2 t / t c )
_1 --
fv The steady-state probe laser intensity in the absence of flow is related to the initial temperature rise a t the pump beam center, AT(t = 0)
2.303EtC AT(t = 0) = 2nkt, where E is the energy of the laser pulse, e is the molar absorptivity, C is the sample concentration, and k is the thermal conductivity. The time constant is given by
(6)
4AT(t = O)(dn/dT)
w2(1 + 2 t / t c ) 2
JBB[
1-
1 1
dz (7)
4t21C2(1 - z ' / B ' ) ~ w2(1 + 2 t / t c )
Unfortunately, we have been unable to evaluate these integrals in closed form and instead rely upon numerical integration.
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 13, NOVEMBER 1985
Far-Field Probe Beam Center Intensity. The temperature distribution will act to both deflect and defocus the probe beam. The gradient of the temperature distribution will act to deflect the probe beam and the curvature of the temperature distribution will act to defocus the probe beam. Different detectors are used to isolate the two effects. Either a small area photodetector located a t the inflection point of the Gaussian intensity probe beam or a position-sensitive detector may be used to isolate the deflection signal. A small area detector located in the probe beam center will isolate the defocusing component of the thermooptical effect. The differential change in probe beam intensity with respect to deflection angle is zero at the probe beam center; small deflection angles will produce a negligible change in intensity. In this paper, we are concerned with defocusing of the probe beam caused by the thermal lens. Extension to deflection is straightforward (28, 31). The intensity at the center of a Gaussian laser beam is given by
I=- 2P
(9)
a*x*y
where w, and w y are the probe beam spot size aligned along the x and y axes. Harris and Dovichi have presented the results of an ABCD law calculation which may be used to find the far-field probe beam center intensity (9) 0;=
-[ (:) woz2
2,
); ] 2
+(1
-
Chopper
MI
Pump
Loser
0
FD:
n :
Transient
Sample
11
Digitizer
,,
Flgure 2. Experimental diagram for thermal lens calorimetry in flowing samples. 8s are beam splitters, M1-3 are mirrors, DBS is a dicroic beam splitter, FG is a piece of frosted glass, L is a lens, F is an interference filter which passes the probe beam but not the pump beam, PD1-3 are photodiodes, and I A is an instrumentation amplifier.
true excitation function but also is much simpler to evaluate in numerical integration (28,32). Convolution of the impulse response, eq 12, with the trapezoidal excitation function is given by eq 13, where t l is the time to unblock or block the
112
i = x,y
(10) tl
where Z1 is the distance from the sample to the probe beam waist, Zz is the distance from the sample to the detector, wo is the probe beam waist spot size, and the confocal distance is given by 2, = irw,,?-/X, where X is the probe laser wavelength. Placing the detector far from the probe beam waist allows simplification of eq 10. The resulting far-field beam center intensity is given by
(1 -
Lt, (t-t, ),,
t z + t l t l+ - t ) d r +
(11)
Equation 11may be further simplified by using the binomial expansion for small signals written in terms of the relative change in the far-field beam center intensity c
c t < t2
ti
+ tz < t
(13)
pump beam and t2 is one-half of the chopper cycle. The contribution from previous chopper cycles is given by
]dr+
t+(Zn-l)t,
tl
where Iois the unperturbed probe beam center intensity. Note that flow acts to make the thermal lens astigmatic; that is, the probe beam has different spot sizes along the x and y axes. However, the beam profile is still Gaussian, although elliptical instead of circular. Also note that we have not considered the lens formed by the variation of flow in the x direction which should be negligible for low flow velocities. Response Function. Equation 12 may be evaluated numerically to produce the impulse response of the far-field probe beam center intensity. However, we do not have a pulsed laser available a t this time. Instead, we consider the thermal lens signal produced by a chopped pump laser. As pointed out for time-resolved photothermal refraction (the cross-beam thermal lens experiment), the appropriate excitation function for a chopped Gaussian pump laser is based upon error functions (32). However, a trapezoidal-shaped excitation function may be used instead. The trapezoidalshaped excitation function not only closely approximates the
EXPERIMENTAL SECTION A block diagram for the thermal lens experiment is shown in Figure 2. The system is constructed on a 4 ft by 8 f t optical table, NRC Model KST-48. The helium-cadmium pump laser, Liconix Model 4210B, delivers to the sample a linearly polarized 4-mW beam at 441.6 nm. The beam is modulated with a variable speed chopper, Scitec Model 300, at 25.4 Hz.A microscope slide cover slip, BS, is used to split a small portion of the pump beam to a reference photodetector, PD1. The main beam is reflected from a mirror, M1, through a dichroic beam splitter, DBS, Melles Griot Model 03SWPO11, to a 27-mm focal length focusing lens, L, Edmund Scientific. A 5-mW helium-neon probe laser, Melles Griot Model 05LHP151, produces a linearly polarized beam at 632.8 nm. The probe beam is reflected from a mirror, M2, through a microscope slide, BS, and onto the dichroic beam splitter. The microscope slide serves as a beam splitter to reflect a small fraction of the probe laser intensity to a second reference detector, PD2. A piece of frosted glass, FG, is placed before the reference detector.
ANALYTICAL CHEMISTRY, VOL. 57, NO. 13, NOVEMBER 1985
Translation of the frosted glass along the beam path provides a convenient means of adjusting the laser intensity at PD2. The main portion of the probe beam is aligned to be coaxial with the pump beam. After passing through the focusing lens and the sample, the laser beams are reflected from a mirror, M3, and centered on the signal detector, PD3. An interference filter, F, Melles Griot Model 03LWPOO1, is positioned before the signal detector to selectively block the pump beam and pass the probe beam. A 1mm square bore glass tube, Microcells Model 8100, is used as the flow cuvette. Nonpulsing flow is produced by a gravity feed system from a 250-mL spherical reservoir suspended 65 cm above the laser beams. Sample flow rates are controlled with a micrometer needle valve, Whitey Model 22RS2, positioned in the flow system after the cuvette. Tubing mode of Teflon, 1mm i.d., is wed to connect all components of the system. Flow is measured by weighing the sample which passes through the system in a fixed time interval. By using low flow rates and filling the reservoir halfway, the variation in flow rate as the flow head decreases is minimal. The silicon photodiodes light detectors are conditioned with JFET operational amplifiers configured as current-to-voltage converters, The pump beam reference detector,PD1, is configured with a 1-MQfeedback resistor in parallel with a 470-pF capacitor. The circuits for the probe beam reference detector, PD2, and the probe beam signal detector, PD3, are identical with matched 15 kQ resistors in parallel with 100-pF capacitors. The probe beam reference detector output is subtracted from the probe beam signal detector output with an instrumentation amplifier, IA, Analog Devices Model 524, configured with a gain of 1000. This instrumentation amplifier produces about an order of magnitude reduction in common mode probe laser beam noise. Of course, a precision divider could replace the differential amplifier to ratio probe beam noise. Use of a differential amplifier offers several advantages. First, for small intensity fluctuations, less than 1% in our probe laser, subtraction and division are equivalent in reducing noise, as shown by the Binomial theorem. Second, a high-quality differential amplifier is much less expensive than a divider with similar specifications. Translation of the frosted glass is a convenient and precise method of adjusting the probe beam reference detector output in order to null the dc component of the signal. The double beam instrument is analogous to a double beam in space design for conventional spectrophotometers. Data Collection and Manipulation. The instrumentation for time-resolved data collection and reduction has been described elsewhere (28,29, 32). The program controlling data collection proceeds as follows. The pump beam reference signal triggers the transient digitizer to display two 10-bit,1024-point wave forms. Channel 1displays the probe beam signal and channel 2 displays the pump beam reference signal whereby data points contained in both wave forms correspond in time. The transient digitizer time base is adjusted so that more than four chopper periods are displayed per trace. The algorithm to accumulate the data proceeds as follows. Softwarechecks are incorporated into the algorithm to reject traces containing off-scale data or erroneously triggered sweeps. Averages of both the probe beam signal and the pump beam reference signal are calculated across the four chopper periods. The average of the probe beam signal is subtracted point-by-pointfrom the probe beam data. This subtraction procedure acts as a high pass filter to remove any residual dc offset in the data. The average of the pump beam reference signal is used as a threshold for a software trigger to locate the low-to-high transitions in the signal. The probe beam data between successive low-to-high transitions in the pump beam reference signal are averaged point-by-point. The standard deviation of each data point is also accumulated as part of the data collection procedure. Typically, the data from 100 chopper cycles are averaged. Time-resolved data are fit to eq 13 and 14 over six previous pulses using the Marquardt algorithm (33). Simpson’s rule is used over 100 points to perform the integration of these equations. Three parameters are available in the regression analysis. The first parameter is proportional to flow rate, the second is equal to the steady-state amplitude of the signal for static samples, and the third is equal to the time constant. We determine the time constant from a static sample. Only the steady-state amplitude
2439
and flow rate are used as free parameters. The uncertainty of each parameter is determined from the diagonal terms of the variance-covariance matrix. The regression analysis is terminated when the reduced x2 statistic differs by 0.01 or less between iterations. The program is not fast enough for real-time application in chromatographic applications. However, faster algorithms certainly could be developed if required. Reagents. All chemicals are reagent grade or better. A 1:l water-ethanol solvent solution is prepared by adding 500 mL of ethanol to a 1-L volumetric flask and diluting to volume with deionized water. The solvent solution is then filtered by suction through 0.5-pm filter paper. A stock solution of 0.0012 M iron 1,lO-phenanthroline in a 1:l water-ethanol solvent is prepared by adding in sequence to a 100-mL volumetric flask 0.0487 g of ferrous ammonium sulfate hexahydrate, 50 mL of 1:l waterethanol solution, 0.0752 g of hydroxylamine hydrochloride, 0.5366 g of sodium acetate, and 0.1253 g of 1,lO-phenanthroline monohydrochloride monohydrate and diluting to volume with 1:l water-ethanol solution. A 5.0 X M iron lJ0-phenanthroline sample solution is prepared by adding 20 mL of the stock solution to a 500-mL volumetric flask and diluting to volume with the 1:l water-ethanol solution.
RESULTS AND DISCUSSION Figure 3 presents time-resolved thermal lens calorimetry signals in a flowing liquid stream. The data are presented at the 68% confidence interval and the theory is shown as the smooth curve. In general, the agreement between the theory and data is excellent. Figure 3a presents data for a static, nonflowing sample. The thermal lens signal rises toward a steady state amplitude during the on portion of the pump laser cycle. Since the sample is located before the probe beam waist, the negative lens produced by the pump beam acts to collimate the probe beam and increase the intensity a t the detector. During the off portion of the chopper, the signal decays toward the original value. The effect of flow is shown in Figure 3b-e. Figure 3b presents the time-resolved thermal lens signal at a low flow mL/s. The peak-to-peak magnitude is derate, 3.6 X creased slightly from the static sample data. As the flow rate is increased, Figure 3c-e, several phenomena are observed. First, the peak-to-peak magnitude decreases steadily with flow rate; flow acts to reduce the magnitude and to distort the temperature profile. At high flow rates, the contribution to the thermal lens is smallest from the fastest moving portions of the streamline, in the stream center for laminar flow. The effectiue path length of the measurement decreases with flow because only the slow moving stream near the cell walls contributes to the thermal lens. Second, the probe laser intensity approaches a constant value more quickly at higher flow rates. The quick approach to steady-state simply reflects the increased rate of heat removal from the sample due to flow. Since little information is generated during the constant portion of the thermal lens signal, it would be appropriate to utilize higher chopping frequencies for the higher flow rate samples and analysis near the detection limit. Our model does not exactly reproduce the signals shown a t the highest flow rates. Several possibilities exist for the disagreement. One possibility for this difference is error in the numerical integration routine. Another possibility is nonlaminar flow at higher sample flow rates. The transition region from the round sample tube to the square sample cuvette certainly produces nonlaminar flow in the immediate region down stream. A third possibility is due to thermal diffusion along the pump beam path. A fourth possibility is deflection of the probe beam due to the gradient of the thermal distribution (34). The flow rate predicted from the regression analysis may be plotted as a function of measured flow rate. The predicted flow rate is a linear, although not very precise, function of measured flow rate, r = 0.987. Since most of the signal is
ANALYTICAL CHEMISTRY, VOL. 57, NO. 13, NOVEMBER 1985
2440
E..
9.. OD, 0
> ..
0
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(ms)
w na, 0 n .,
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(ne)
a 0 - 0
a
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(9)
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Flgure 3. Time-resolved photothermal refraction signals for flowing samples, 200 data points per trace, average of 100 traces. The rising potion of the signal corresponds to the on portion of the pump beam and the falling portion of the signal corresponds to the off portlon of the pump beam. The data are shown at the 68% confidence interval and the smooth curve is the result of the regression analysis. All curves are scaled to the same, arbitrary, scale. (a) Flow rate = 0.0. (b) Flow rate = 2.2 X lo-' mL/min. (c) Flow rate = 3.1 X lo-' mL/min. (d) Flow rate = 7.1 X lo-' mL/min. (e) Flow rate = 2.9 mL/min.
produced by the slow moving sample near the wall, thermal lens signals are not well suited to estimating flow rate. A related technique, time-resolved photothermal refraction, is much more useful in measuring flow rates (28). On the other hand, the steady-state amplitude of the thermal lens signal predicted by the regression analysis is independent of flow, demonstrating a maximum 10% deviation from a static sample to a flow rate of 2 x mL/s for constant pump laser intensity and sample absorbance. It is interesting to consider the effect of flow upon the precision of the amplitude measurement. Figure 4 shows the relative standard deviation for the steady-state amplitude as a function of flow rate. The steady-state amplitude predicted by the regression analysis is linear with the analyte concentration. The standard deviation is estimated from the diagonal terms in the variance-covariance matrix used in the Marquart regression analysis algorithm (33). The standard deviation mL/s. of the amplitude is constant for flows below 6 X The standard deviation increases steadily at higher flow rates. Increased standard deviation is not due to an increase in the standard deviation of the individual data points; the deviations
. / . *
$0 .
Flgure 4. Relative standard deviation of the amplitude as a function of the flow rate. Relative standard deviation is estimated from the diagonal terms in the variance-covariance matrix of the Marquart algorithm.
remain constant at all measured flow rates. Two other factors could produce poorer precision a t higher flow rates. First, the theory and data are not in complete agreement, perhaps due to nonlaminar flow. Also, a t higher flow rates, the amplitude and flow rate terms become highly correlated in the regression function. A decrease in the peak-to-peak signal level may be either due to a decrease in amplitude or due to an increase in flow rate. Volume flow rate is not particularly useful in characterizing the behavior of thermal optical techniques in flowing samples. Of more importance is the linear flow rate of the sample across the pump laser beam. At a given volume flow rate, the linear velocity scales inversely with the cuvette area. Our 1mm wide square bore cuvette is the appropriate size for low flow rate, low dispersion chromatographic techniques, such as microbore chromatography. On the other hand, a larger cross section detector should prove useful with higher flow rates. Not only would the linear flow velocity be lower, but the path length would be larger, both of which would increase the thermal lens sensitivity. Thermal lens calorimetry produces a signal which, to first order, is linearly rehted to path length (35). Very small path length cuvettes, roughly 100 km wide or smaller, will produce low sensitivity. On the other hand, the related technique of photothermal refraction produces a signal which is independent of path length for samples larger than a few micrometers. Photothermal refraction has recently been applied to the detection of femtomole quantities of amino acids separated by microbore liquid chromatography (36).
LITERATURE CITED (1) Gordon, J. P.; Leite, R. C. C.; Moore, R. S.; Porto, S. P. S.; Whinnery, J. R. J . Appl. Phys. 1985, 38,3-8. (2) Whinnery, J. R.; Miiier, D. T.; Dabby, F. I€€€ J . Quantum flectfon 1987, QE-3, 382-383. Dabby, F. W.; Boyko, R. W.; Shank, C. V.; Whinnery, J. R. I€€€ J . Quantum Electron. 1969, QE-5,516-520 Hu, C.; Whinnery, J. R. Appl. Opt. 1973, 72. 72-79. Whinnery, J. R. Acc. Chem. Res. 1974, 7 , 225-231. Akhmanov, S. A.; Krindach, D. P.; Miguiin, A. V.; Sukhorukov, A. P.; Khokhlov, R. V. IEEE J . Quantum Electron 1988, Q E - 4 , 568-575. Smith, D. C. I€€€ J . Quantum Nectron. 1989, O f - 5 , 600-607. Dovichi, N. J.; Harris, J. M. Anal. Chem. 1979, 57,728-731. Harris, J. M.; Dovichi, N. J. Anal. Chem. 1980, 52, 695-706A. Dovichi. N. J.; Harris, J. M. Anal. Chem. 1980, 52, 2338-2342. Dovichi, N. J.; Harris, J. M. Anal. Chem. 1981, 5 3 , 106-109. Dovichi, N. J.; Harris, J. M. Anal. Chem. 1981, 53,689-692. Carter, C. A,; Harris, J. M. Anal. Chem. 1984, 5 6 , 922-925. Mori. K.; Imasaka, T.; Ishibashi, N. Anal. Chem. 1982, 5 4 , 2034-2038. Higashi, 1907- 19 10. T.; Imasaka, T.; Ishibashi, N. Anal. Chem. 1983, 55,
Miyaishi, K.; Imasaka, T.; Ishibashi, N. Anal. Chem. 1982, 54, 2039-2044. Leach, R. A,; Harris, J. M. J . Chromatogf. 1981, 218, 15-19. Buffet, C. E.; Morris, M. D. Anal. Chem. 1982, 5 4 , 1824-1825.
Anal. Chem. 1985, 57, 2441-2444 (19) Buffet, C. E.; Morris, M. D. Anal Chem. 1983, 55,376-378. (20) Sepaniak, M. J.; Vargo, J. D.; Kettler, C . N.; Maskarinec, M. P. Anal. Chem. 1984, 56, 1252-1257. (21) Pang, T. J.; Morris, M. D. Anal. Chem. 1984, 56, 1467-1469. (22) Leach, R. A.; Harris, J. M. Anal. Chlm. Acta 1984, 764, 91-101. (23) Wellegehause, B.; Laepple, L.; Welling, H. Appl. Phys. 1975, 6 , 335-340. (24) Teschke, 0.; Whinnery, J. R.; Dienes, A. I€€€,/. Quantum. Nectron. 1976, QE-12, 513-515 (25) Sanders, D. A. Appl. Opt. 1984, 23, 30-35. (26) Nickolaisen, S.L.; Bialkowski, S. E. Anal. Chem. 1985, 57,758-762. (27) Twarowski, A. J.; Kliger, D. S. Chem. Phys. 1977, 20,253-258. (28) Weimer, W. A.; Dovichl, N. J. Appl. Opt., in press. (29) Weimer, W. A,; Dovichi, N. J. Appl. Spectrosc., in press. (30) Bird, R. B.; Stewart, W. E.; Lightfoot, E. N.; Chapman, T. W. "Lectures in Transport Phenomena";American Instltute of Chemical Engineers: New York, 1969.
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RECEIVED for review April 29, 1985. Accepted June 27, 1985. This work was funded by the donors of the Petroleum Research Fund, administered by the American Chemical Society, and the National Science Foundation, Grant CHE-8415089.
Pulsed Laser Desorption for Resonance Ionization Mass Spectrometry N. S. Nogar* and R. C. Estler' Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 C. M. Miller Isotope and Nuclear Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
A pulsed Nd:YAG laser (1.06 pm) was used to desorb tantalum atoms from a sample fllament malntalned at a base temperature of 1200 OC; these atoms were subsequently detected by pulsed resonance loniratlon and tlme of flight mass spectrometry. Arrival tlme dlstrlbutlons were obtained by varying the tlme delay between the desorption laser and a probe (lonizatlon) laser. Ground-state tantalum atoms were found to have a most probable hydrodynamlc velocity of -1.0 X I O 5 cm/s for desorptlon pulse lntensltles of I O * W/ cm2. The thermal velocity dlstrlbution, however, was characterized by a width of =2.3 X I O 4 cm/s, and the Internal (electronic) excnation temperature was T < 2100 K. Overlap of the atom pulse and the probe laser pulse was excellent, with 510% effectlve duty cycle. Thls should Increase substantially the sensitivity posslble with resonance Ionization mass spectrometry.
Resonance ionization mass spectrometry (RIMS) is rapidly becoming an established field, with well-defined strengths, weaknesses, and areas of application. A significant fraction of published work has dealt with the use of conventional thermal (hot filament) sources for RIMS (1-18). These sources have a number of significant attributes, including reproducibility, stability, and a substantial literature of work in surface ionization upon which to draw (19). A significant drawback stems from the common use of pulsed lasers for RIMS analyses. The short pulses and relatively low repetition rates of most pulsed lasers, coupled with the constant evaporation of sample from thermal sources, results in a low (typically 110-4) effective duty cycle, and a substantial loss (nonuse) of analyte. This inefficiency can be a substantial burden when the sample is difficult to obtain or when sample size must be Permanent address: Department of Chemistry, Fort Lewis College, Durango, CO 81301.
minimized, as for radioactive materials (18). A number of solutions to this problem have been explored. One possibility is the use of continuous wave (CW) lasers for the resonance ionization process (14, 20). This is a viable alternative, exhibiting average detected currents as great as lo4 those of pulsed lasers, though it may be restricted to elements of rather low ionization potential (IP 5 8.8 eV) due to the limited spectral range available to CW lasers. A second alternative is a pulsed sample evaporation process. This has been demonstrated for laser ablation (21-23), for pulsed sputtering (24-28), and most recently for pulsed thermal sources (11). In the latter case, a thermal pulse is generated by supplying a current pulse to the resistively heated filament. When this pulse is supplied in addition to a continuous current which holds the sample at a temperature slightly below that needed for efficient evaporation, sample vapor pulse widths as narrow as 1 ms may be achieved (11). In this paper, we report on the use of an infrared laser to induce pulsed desorption (29-35) from a metal filament. Atom pulse durations as short as -3.5 ps duration are observed. Preliminary measurements suggest that the desorption process produces a nonthermal distribution of desorbed species with an internal (electronic) temperature somewhat lower than the hydrodynamic translational energy.
EXPERIMENTAL SECTION Pulsed evaporation and ionization took place in the source region of a time-of-flight mass spectrometer described previously (7). Figure l a depicts a schematic of this region, while Figure l b describes the timing sequence used. Briefly, the Q-switch synch-out from the Nd3+:YAGdesorption laser (Quanta Ray/ Spectra Physics, DCR lA, Mountain View, CA) was used to master the timing sequence. The laser output, initially frequency doubled to provide a visible beam, was focused onto the sample filament with the aid of beam-steering prisms and a 250 mm focal length lens. Once alignment was completed, the frequency doubler was detuned, and residual 532-nm light removed with a dichroic mirror, leaving only (299%) the fundamental 1.06-pm beam. The 10-ns pulses from this laser typically arrived at the filament 100 ns
0003-2700/85/0357-2441$01.50/00 1985 American Chemical Society
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