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Anal. Chem. 1983, 55,2441-2444
Capillary Flow Cell for Photoacoustic Detection of Organic Compounds Edward P. C. Lai,* Becky L. Chan, and Lolita L. Chan Department of Chemistry, Bowling Green State University, Bowling Green, Ohio 43403 The waveguide effect for photoacoustic waves in quartz as a substrate material has been shown to be efficient by Rosencwaig and Hindley (I),Pate1 and Tam (2), and Lai et al. (3). This efficiency has enabled the design of a low-temperature piezoelectric-transducer photoacoustic cell ( 4 ) and a capillary tube flow cell for laser-induced photoacoustic detection of high-performance liquid chromatography effluents (5). In both cases, a piezoelectric ceramic tube transducer w& attached via an aluminum coupling cylinder to the outer surface of a quartz tube or capillary. While the capillary tube flow cell provides limits of detection of tens of a nanogram for polynuclear aromatic hydrocarbons (5), the low-temperature (77 K) photoacoustic cell has been found to lack the necessary sensitivity for use in photoacoustic spectroscopy of polynuclear aromatic hydrocarbons in Shpol’skii matrices (4). Delay time analyses for the two photoacoustic detection systems indicated that acoustic waves generated by the excitation laser beam did actually travel in the liquid flow stream (5) or in the frozen polycrystalline n-alkane matrix (8)instead of transferring into the quartz substrate immediately. Thus it seemed reasonable that attachment of a piezoelectric transducer to the end of a quartz tube would provide for more efficient collection of acoustic waves than a side-mounted transducer as had been previously used. A fused quartz capillary flow cell having piezoelectric ceramic tube transducers located at three different positions was constructed, and the performance of the individual transducers was evaluated by using the previously studied compound pyrene (5) and the clinically administered compound disodium protoporphyrin.
EXPERIMENTAL SECTION Apparatus. The construction of the quartz capillary tube flow cell for photoacoustic detection is shown schematically in Figure 1. The cell consists of a fused quartz capillary tube, with outside diameter 4 mm, inside diameter 1mm, and length 110 mm, and three attached PZTdA piezoelectric ceramic tubes of 13 mm (0.5 in.) length, 13 mm (0.5 in.) outside diameter, and 0.8 mm (0.031 in.) thickness (No. 8-8031, Vernitron Piezoelectric Division, Bedford, OH). The center ceramic tube (transducer B) is attached to the quartz capillary tube via an aluminum coupling cylinder of 6.4 mm length as described elsewhere (5). The other two ceramic tubes (transducers A and C) are attached to the two ends of the capillary tube via aluminum coupling cylinders of 8.9 mm length. A cyanoacrylic adhesive (Superglue) is used at the interfaces t o couple the parts together. A 22-gauge stainless steel syringe needle is inserted into the outer face of the aluminum cylinder of transducer A and connected to the capillary bore by a drilled hole of 0.58 mm diameter. An 18 gauge needle and 0.88 mm diapeter hole are used for transducer C. Aluminum is chosen as the coupling medium because it has a low acoustic impedance (1.7 X 10’ kg m-2 s-l), defined as the product of the density of, and the velocity of sound in, the medium (7). The transmission coefficientfor an acoustic wave at normal incidence across a boundary is given by (4R&)/(R1 + R2)2(8), where R, is the acoustic impedance of medium t’. The low acoustic impedance of aluminum renders relatively high transmission coefficients of 0.29 and 0.20 for contacts with water and ethanol, respectively. For the aluminum to fused quartz interface, the transmission coefficient is as high as 98% of the theoretical maximum. Aluminum has the additional advantages of being relatively inert to solvents and easily machined. A schematic diagram of the experimental setup is shown in Figure 2. The light source is an Oriel Model 3021 pulsed xenon light source operated at a repetition rate of 9 Hz (0.2 J/pulse, 6.5 ks pulse width) (Oriel Corp., Stamford, CT). Two f/1.5 UV grade fused silica focusing lenses (Models 6304 and 6450, Oriel 0003-2700/83/0355-2441$01.50/0
Corp.) are used to focus the source radiation into an image of 4 mm length at the center bore of the quartz capillary tube. The liquid sample can be held within the capillary cell, or pumped through (flow mode) from an external reservoir or exit from a high-performance liquid chromatograph. The photoacoustic voltage signal generated by the PZT transducer is amplified with a laboratory-constructed amplifier (high-pass -3 dB cutoff frequency at 8 kHz) and gate-detected with a PAR Model 162/ 164 averager/integrator (EG&G Princeton Applied Research, Princeton, NJ). A strip chart recorder records the pulsed photoacoustic signal as a function of delay time. The light source, boxcar averager/integrator, and oscilloscope are all triggered by the TTL output of a Noveltronics time voltage calibrator (Noveltronics, Mountain View, CA). A Texas Instruments Model 6606x2 pulse generator (Texas Instruments, Inc., Houston, TX) operated at a repetition rate of 200 Hz, pulse width of 0.2 gs, and pulse height of 14 V is used to generate pulsed electroacoustic waves in transducer A, while a Heathkit Model IG-72 audio generator (Heath Co., Benton Harbor, MI) is used to generate a sinusoidal electroacoustic output from the transducer. Reagents. The pyrene (Chem Service Inc., West Chester, PA) was dissolved in dehydrated U.S.P. ethanol (US. Industrial Chemicals Co., Louisville, KY), and the disodium protoporphyrin (Sigma Chemical Co., St. Louis, MO) dissolved in water purified by a Nanopure Barnstead exchanger (Barnstead, Sybron Corp., Boston, MA).
RESULTS AND DISCUSSION The triple-transducer quartz capillary tube flow cell design allowed comparison of the three different attachments of PZT ceramic tubes to the flow cell under identical excitation (photoacoustic wave generation) conditions. A high intensity pulsed xenon light source was employed to irradiate samples of necessarily greater concentration (1.3 parts per thousand and 10.5 pa& per thousand for disodium protoporphyrin and pyrene, respectively) since a pulsed laser was unavailable. A dispersing element was not placed before the sample, allowing for total illumination in the 200-900 nm range by the light source. Disodium protoporphyrin was the first compound studied, the source radiation initially being focused at the midpoint M between transducers A and B. The outputs from the three transducers, shown in Figure 3, were acquired consecutively by scanning the aperture delay of the boxcar integrator/averager over a range of 0.2 ms. The aperture duration employed was 0.5 ps, and the observed time constant and signal processing time constant were 0.22 s and 0.1 s, respectively. The transducer outputs when the source radiation was focused at the midpoint N between transducers B and C are shown in Figure 4. Data for the measurements of relative photoacoustic signals and delay times of the first positive excursions of the transducer outputs (which are indicated by an asterisk in Figures 3 and 4)are given in Table I. Also included in Table I are the results from the irradiation, at point N, of pyrene in ethanol. It should be noted that the delay times of the first positive excursions of the transducer C outputs (scans 6 and 9) are longer than the corresponding delay times of the first positive excursions of the transducer A outputs (scans 4 and 7, respectively), even though the excitation radiation is focused nearer to transducer C. The same ratio of intensities of photoacoustic signals between transducer A and transducer C is observed in experiments 1, 2, and 3. The photoacoustic signal output of the side-mounted transducer B (same design as in ref 5) was significantly lower than the end-on attached transducers A and C. Irradiation of the 0 1983 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 14, DECEMBER 1983
Table I. Photoacoustic Detection of Disodium Protoporphyrin and Pyrene with the Quartz Capillary Tube Flow Cell experiment 1
2
sample disodium protoporphyrin in water disodium protoporphyrin in water
N
3 4 5 6 7 8 9
relative PAS signal
signal ratio
448 70 198 4OOd
C
A B
6.4 2.8 6.4
3.1 6.4 0.6 2.1
98.8 92.1 100.4 116.3
...
193 512 55 170
PS
30.5 40.9 55.6 73.9
1.o
*..
C
delay time,c
...
A B C a Refer to Figure 1. Ratio of relative photoacoustic (PAS) signals of transducers A, B, and C within each experiment. The signal ratio for transducer A is arbitrarily set to 6.4 in all the three experiments so that the ratios A:B:C can be easily compared. Delay time of the first positive excursion (indicated by an asterisk in Figures 3 and 4 ) of the transducer output. Undeterminable because of the weak signal. 3
pyrene in ethanol
excitation pointa scan transducer M 1 A 2 B
N
TO AMPLIFIER
a h i
I
U
U
EXCITATION RADIATION
1 cm Figure 1. Schematic of quartz capillary tube flow cell with three attached piezoelectric ceramlc tube transducers, A, B, and C, for photoacowtk detection and M and N points of photoacoustic excitatlon: (a) stainless steel syringe needie; (b) Teflon sleeve; (c) rubber grommet: (d) metallic box; (e) BNC connector; (f) nickle-silver contact; (9) electrical tape; (h) piezoelectric ceramic tube: (i) fused quartz capillary tube; (i) sample solution: (k) aluminum coupling cylinder: (I)drilled hole; (m) Superglue. TIME CALLIBRATOR
Flgure 3. Photoacoustic detection of disodium protoporphyrin in water in experiment no. 1 with excitation at point M: (a) transducer A output in scan no. 1; (b) transducer 6 output in scan no. 2; (c) transducer C output in scan no. 3; (*) first positive excursions of the pulsed photoacoustic signals. x
I
I
I QUARTZ CAPILLARY PHOTOACOUSTIC CELL
Figure 2. Schematic of experimental setup for photoacoustic detection.
pyrene solution in a capillary flow cell having one sidemounted transducer (as in ref 5) provided only a negligible photoacoustic signal. This strongly suggests that the former mode of attachment is considerably less efficient than the end-on attachment design. The longer delay times observed in experiment 2 for transducer C compared to transducer A were puzzling at first. A rationalization for these observations was provided by delay time analysis. The separation distance between M and N is 0.0626 m, while the delay times of the transducer A outputs in scans 1and 4 differ by 4.34 X 10" s. Dividing the distance by the time gives 1.44 X lo3 m s-l, which is very close to the literature value of 1497 m s-l for the velocity of sound in distilled water at 25 O C (9). This suggests that the photoacoustic waves traveled through the aqueous disodium protoporphyrin solution in the quartz capillary tube before reaching transducer A.
I
0
50
I 100 IS0 3ELAY TIME (PS)
200
Flgure 4. Photoacoustic detection of disodium protoporphyrin in water in experiment no. 2 with excitatlon at point N: (a)transducer A output in scan no. 4; (b) transducer 6 output in scan no. 5; (c) transducer C output in scan no. 6: (*) first positive excursions of the pulsed pho-
toacoustlc signals.
When the distance MN is divided by the difference in delay time of the transducer C outputs in scans 3 and 6, the quotient is 1.45 X lo3 m s-l, which agrees well with the value of 1.44 x lo3m s d obtained for transducer A above. It is significant
ANALYTICAL CHEMISTRY, VOL. 55, NO. 14, DECEMBER 1983 I
I
I
0
50
100 DELAY TIME
I I50
200
((LS)
Flgure 5. Output signals of (a)transducer C and (b) transducer B when a 14-V, 0.2-ps duration electrical pulse is applled to transducer A.
that the delay time in scan 6 was longer than that in scan 3. It is possible that the signals detected by transducer C did not come directly from the point of excitation (M or N), but were due to the transferal of some of the primary photoacoustic waves reaching transducer A. Comparison of the delay times in scans 1,4,and 7 with those in scans 3,6,and 9 reveals a common time difference of 25 pus. This difference is most likely due to the time taken by the secondary acoustic waves to travel from transducer A to transducer C, via the aluminum coupling cylinders and the quartz tube. The fairly constant intensity ratio of the two transducers (A and C) further confirms the dependency of the signals from transducer C on nonprimary origins. The secondary nature of the signals from transducer C is made to appear more reasonable when one considers the high transmission coefficient (0.98)for an aluminum-fused quartz interface. It seems unlikely that the diffuse appearance of the signals from transducer C (scan 3, Figure 3, and scan 6,Figure 4)could be attributable to a linear, nondeflected path for the detected acoustic waves. That there is a detectable signal from transducer B in this novel arrangement also supports the existence of a secondary photoacoustic wave traveling along the quartz tube. To verify the possibility of acoustic wave transmission from transducer A to transducers B and C, a pulsed acoustic wave was generated in transducer A by the application of a 14-V electrical pulse of 0.2 ps duration. The resultant signals detected by transducers B and C are shown in Figure 5. The ratio of signal intensities is approximately 1:2.5 (B:C), similar to the signal intensity ratios observed in experiments 1 through 3. When an electrical sine wave pattern (10 V peak-to-peak amplitude) of a given frequency was applied to transducer A, signal outputs from transducers B and C were sinusoidal and of the same frequency. The amplitudes of the output signals from transducers B and C changed significantly with varying applied voltage frequencies, as shown in Table I1 (the signal amplitudes have not been corrected for the frequency response of the amplifier). The intensities of the output signals from transducers B and C depend on the acoustic wave generation efficiency of transducer A, the transmission efficiency from transducer A to transducer B or C, and the piezoelectric efficiency of the detecting transducer. The actual cause of the fluctuation in signal amplitude with frequency remains unclear; however, it might be related to the resonance properties of the flow cell system. The insensitivity of transducer C to primary photoacoustic waves is indicated by the absence of a detectable response in the initial time intervals where these primary signals would be expeded. The drilled hole diameter (and the syringe needle
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Table 11. Frequency Response of Triple Transducer Quartz Capillary Tube Flow Cell transducer transducer sine wave Ba output, C a output, transducer C output/ transducer B output freq, kHz PV PV 0 0 1 0 0 10 1.o 66 66 20 3.7 362 99 30 3.1 115 362 40 6.0 132 789 50 2.3 247 109 60 4.8 82 395 70 3.5 460 132 71 2.7 72 1320 493 2.3 73 1050 2360 2.5 74 954 2360 1.2 75 961 1150 1.0 76 1110 1080 1.o 1480 77 1480 1.5 78 2110 1380 1.2 1810 79 1540 1.7 987 80 1710 3.1 513 81 1590 3.5 303 82 1050 9.0 161 1450 83 1.3 401 303 85 2.8 164 461 90 1.8 237 418 91 1.2 434 92 513 1.6 444 724 93 3.7 280 94 1030 5.7 388 95 2220 6.3 500 96 3160 3.0 740 97 2250 0.7 987 704 98 1.0 99 1320 1250 1450 1.4 100 2040 2.2 101 855 1920 105 1.5 405 608 4.9 559 2760 106 a Peak-to-peak sine wave outputs when a 10-Vpeak-to]peak sine wave electrical voltage is applied to transducer .A. gauge) appears to have a significant effect on the intensity of the signal from the transducer. A smaller drilled hole diameter increases the detection sensitivity, probably because a larger front surface area (of the aluminum cylinder) is available for the impingement by the primary acoustic waves. Consequently, the present design for transducer A offers an enhancement in sensitivity of at least an order of magnitude over the previous side attachment design. The deflection of acoustic waves from transducer A depends on the matching in acoustic impedance of the different construction materials and reduces the signal intensity of transducer A itself. Such consideration should be given in the future design of photoacoustic cells.
CONCLUSIONS Much useful information was derived from the complex pulsed photoacoustic signal as detected by a piezoelectric transducer. The signal amplitude, delay time, and peak shape contribute to the overall characterization of this novel design of a quartz capillary tube photoacoustic flow cell. The present delay time analysis may be regarded as an extended technique of photoacoustic kinetics (first reported in 1982 (IO)),like the criterion of exponential decay for the pulsed photoacoustic signal (11). Further development of these techniques should prove themselves to be beneficial to the analytical science.
ACKNOWLEDGMENT The authors thank Laszlo Kecskes for assistance in construction of the quartz capillary flow cell, Craig Bedra for
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Anal. Chem. 1083, 55,2444-2445
construction of the amplifier, and Edwin M. Bryant for many helpful suggestions in preparing the manuscript. Registry No. Disodium protoporphyrin, 50865-01-5; pyrene, 129-00-0.
LITERATURE CITED (1) Rosencwaig, A.; Hlndley, T. W. Appl. Opt. 1981, 20, 606-609. (2) Patel, C. K. N.; Tam, A. C. Rev. Mod. f h y s . 1981, 53, 517-550. (3) hi,E. P. C.; Voigtman, E.; Wlnefordner, J. D. Appl. Opt. 1982, 21, 3126-3128.
(6) ial; E. P. C. Ph.D. Dissertation, Universlty of Florida, Gainesvllle. FL, 1982, pp 97-99.
(7) Farrow, M. M.;Burnham, R. K.;Auzanneau, M.;Olsen, S. L., Purdie, N.; Eyring, E. M. Appl. Opt. 1978, 17, 1093-1098. (8) Blitz, J. ”Fundamentals of Ultrasonics”; Plenum: New York, 1967; p 25. (9) Weast, R. C., Ed. “CRC Handbook of Chemistry and Physics”, 55th ed.; CRC Press: Cleveland, OH, 1974; p E-47. (IO) Major, R. W.; Hutton, S. L. Appl. Opt. 1982, 21, 1159-1161. (11) Lal, P. C., unpubllshed work, Bowling Green State University, Bowling Green, OH, 1983.
RECEIVED for review July 7,1983. Accepted September 14, 1983. This work was supported by Grant No. 8318 from the Biomedical Research Support Program and in part by a part-time associateship from the Faculty Research Committee, Bowling Green State University.
Thermal Gradient Lamps for Dispersive Flame Atomic Fluorescence Spectrometry M. D. Seltzer and R. G . Michel* Department of Chemistry, University of Connecticut, Storrs, Connecticut 06268 Many of the characteristics of thermal gradient lamps (TGLs) have been described by Gough and Sullivan (1-3). Their work demonstrated that for a variety of volatile metals (arsenic, cadmium, selenium, zinc) TGLs are useful as sources for nondispersive atomic fluorescence spectrometry (AFS)and for atomic absorption spectrometry (AAS). This is because of their narrow line widths and high intensities. No results have yet been reported for the use of TGLs as the excitation source for AFS with conventional dispersive detection. Our experience with rigorously optimized microwave excited electrodeless discharge lamps (EDLs) (4-7)for dispersive AFS allows a meaningful quantitative comparison of the intensity of TGLs when compared to microwave excited EDLs, in order to confirm the potential of TGLs for use in atomic spectrometry.
EXPERIMENTAL SECTION The instrumentation used was that described in ref 7 and involved the use of a nitrogen separated air-acetylene flame, an f/4.2 double monochromator with a 0.5-nm spectral bandwidth, a UV-sensitive photomultiplier tube, and photon counting detection. For selenium and arsenic lamps, detection limits were measured with the monochromator flushed with nitrogen to reduce absorption in the UV below 200 nm. Three thermal gradient lamps, selenium, cadmium,and arsenic, and their associated power supply were supplied by Scientific Glass Engineering, Inc. (SGE), of Austin, TX. Atomic fluorescence signals were measured and detection limits were calculated for the TGLs in the same way as for EDLs in all previous papers (4-7). Cadmium metal, selenium oxide, and arsenious oxide were used to prepare 2000 Fg/mL aqueous stock solutions,0.04 M in hydrochloric acid. The arsenious oxide was first dissolved in a minimum volume of 20% NaOH. Chemicals were analytical reagent grade.
RESULTS AND DISCUSSION The detection limits obtained here for cadmium and selenium TGLs are compared in Table I with EDL detection limits obtained by us. Cadmium and selenium TGL detection limits were measured 5 OC above the recommended operating temperatures which were 160 ‘C and 170 OC, respectively. The cadmium TGL detection limit was a factor of 7 worse than the EDL, and the selenium TGL detection limit was a factor of 3 worse. The difference in detection limits obtained with TGLs and EDLs is a result of the difference in the relative intensities of the two types of lamps because background signals and noise magnitudes were the same for both TGL and EDL measurements. 0003-2700/83/0355-2444$01.50/0
Table I. Comparison of Atomic Fluorescence Detection Limitsa (rg/L) EDL wavelength, (microwave metal nm TGL excited) 0.07 228.8 0.5 cadmium selenium 100 30 196.0 Cadmium detection a Signal-to-noise ratio of two, limits were measured by using a 5 Mg/L solution and selenium limits with a 1 0 Wg/mL solution. The cadmium EDL detection limit was reported in ref 5 and has been reproduced consistently since then on the instrumentation used here. The selenium EDL detection limit was reported in ref 6 using the same instrumentation as ref 5 . Hence the EDL detection limits are probably very consistent although selenium EDLs were not directly compared in this work. A stability check of the change in cadmium TGL atomic fluorescence signal over an hour revealed a drift of about 1% per hour. The lamp was operated at a temperature of 160 ‘C which is the manufacturer’s recommended operating temperature. This stability is somewhat better than that observed for microwave excited EDLs which have typically a downward drift of 3-5% per hour. This c o n f i i s the TGL drift observed by Gough and Sullivan by direct measurement of TGL output light intensity. Short-term fluctuations in TGL intensity were insignificant since detection limits in the work reported here were limited by flame background noise. At higher concentrations, short term lamp fluctuations did not increase the noise on the fluorescence signal. The power supply provided for the TGLs allows control of the operating temperature of the lamps with much the same idea in mind as the thermostating of EDLs suggested by Browner and Winefordner (8) and used by us ( 7 ) . Temperature control of the TGLs is by heated filament rather than heated air. However, the effect is much the same as shown in Figure 1, where a rapid increase in atomic fluorescence signal with temperature is observed. At operating temperatures higher than those recommended by the manufacturer, lamp stability was the same as a t the recommended temperature but warmup periods of more than half an hour were required in order to attain stable operation. We did not attempt to go beyond the highest temperatures shown in Figure 1because the manufacturer claims reduced lifetime of the TGLs at high temperatures. Hence, it was not 0 1983 American Chemical Society
