1442
Anal. Chem. 1981,
ordinary graphite tubes were used instead of pyrolytically coated ones.
CONCLUSIONS As has been shown in this paper, concomitants can affect the vaporization characteristics of the analyte so that (i) the atomization interval is shifted toward lower or higher temperatures and (ii) the rate of atom formation is changed. Such effects cannot be established when the detector/readout system has a too slow response time. This means that many empirical studies of interference effects reported in the literature, where such instruments have been used,are of limited value. Even if a fast response time is used the relative number of free atoms formed cannot be determined if the atomization interval is changed, whether the peak height or the peak area is measured. The degree of interference effects can therefore only be established accurately if the atomization interval is
53,1442-1446
unchanged and, moreover, if peak areas are evaluated.
LITERATURE CITED McWiiiiam, I. G.; Botton, H. C. And. Cbem. 1969, 41, 1755-1762. Piepmeier, E. H.; de Galan, L. Spectrochim. Acta, Pad 6 1976, 318, 163-177. Siemer, D. D.; Baldwin, J. M. Anal. Cbem. 1980, 52, 295-300. Lundgren, G.; Lundmark, L.; Johansson, G. Anal. Cbem. 1974, 46, 1028-1031. Lundberg, E.; Lundmark, L. Cbem. Biomed. Env. Instrum. 1979, 9 , 91-93. Lundberg, E. Cbem. Instrum. 1970, 8, 197-204. Wold, S. Tecbnometrics 1974, 16, 1-11. van den Broek, W. M. G. T.; de Gabn, L. Anal. Cbem. 1977, 49, 2 176-2 186. L’vOv, 6. V. “Atomic Absorption Spectrochemical Analysis”: Adam Hilger: London, 1970. Maessen, F. J. M. J.; b i k e , J.; Massee, R. Spectrochim. Acts, Part 8 1970, 338, 311-324.
RECEIVED for review February 2, 1981. Accepted April 27, 1981.
Condensed Phase Photoacoustic Spectroscopic Detection of Porphyrins and Dyes Edward Volgtman, Arthur Jurgensen, and James Winefordner Department of Chemistry, University of Florida, Gainesville, Florida 326 1 1
A simple piezoelectric detection system suitable for performing highly sensitive iiquid-phase photoacoustic spectroscopy is presented. Performance of the system with respect to linearity, solvent effects, and excitation pulse characteristics Is experimentally evaluated and compared with theoretical figures of merit. Limits of detection for various porphyrins, laser dyes, and drugs are presented.
Although photoacoustic detection has been previously employed in the study of condensed phase materials, the technique employed has most often been of the type studied by Rosencwaig and Gersho ( I ) : a photothermalacoustic detection. Absorption of modulated light by a condensed medium leads to modulated diffusion of resultant heat into the surrounding confined buffer gas which then undergoes modulated pressure fluctuations. The fluctuations are detected with a sensitive capacitance microphone. The slowness of thermal diffusion, signal saturation effects ( 2 ) ,and possibility of scattered light artifacts (3) partially annul the very high sensitivity of the technique. Consequently, other photoacoustic spectroscopy (PAS) schemes have been investigated in hopes of retaining the sensitivity while eliminating the aforementioned difficulties. In 1969, Callis et al. (4) adapted a capacitance microphone to the study of triplet yields of anthracene and acridine orange in ethanol solutions using pulsed broad band excitation. In 1977, Hordvik and Schlossberg ( 5 ) studied absorption coefficients in fluorite crystals using attached and nearby piezoelectric transducers with differential amplification and lock-in amplifier detection. They measured absorptivities approaching cm-’. Lahmann et al. (6) used a frequen-
cy-shift keyed (FSK) argon ion laser together with piezoelectric transducer and lock-in detection to achieve a detection limit of 54 ppt (80 pg ~ m - 2.2 ~ ,X cm-*) for @-carotenein chloroform. Also in that year, Bonch-Bruevich et al. (7) used laser excitation and a piezoelectric transducer to study single and multiple photon absorption in dye solutions. Sensitivities of 1 pJ were achieved with oscilloscopic detection. In 1978, Farrow et al. (8)exploited the acoustic impedence matching advantage of piezoelectric transducers when used with condensed media and demonstrated the feasibility of obtaining excellent PAS spectra with both chopped CW and pulsed laser excitation of various Nd3+-containingsamples. Also in 1978, Oda et al. (9) achieved a detection limit for cadmium as cadmium dithiozonate of 14 ppt (20 pg cm-9 in chloroform. They employed a chopped 514.5-nm argon ion laser line with piezoelectric transducer and phase sensitive detection. Tam and Patel (IO), in 1979, used a “submersed” lead zirconate titanate (PZT)piezoelectric transducer, gated integrator detection, and pulsed laser excitation to measure cm-I with incident pulse enabsorptivities approaching ergies of approximately 1 mJ thus yielding a thermalized energy sensitivity near 1 nJ. Additional studies undertaken by Patel and co-workers with their system included nonlinear Raman gain spectroscopy ( I I ) , C-H stretch overtones in benzene (12),and evaluation of a cuvette cell and “C-clamp” transducer mounting scheme (13). The origin of the photoacoustic effect underlying the above results has been heuristically derived by Tam and Patel (IO) and rigorously derived by Atalar (14);it is summarized briefly below. Absorption of light produces heat in the illuminated region which then expands, assuming a positive coefficient of expansion. The expansion pulse impinges on the lightly damped transducer producing a ringing damped output. The
0003-2700/81/0353-1442$01.25/0Q 1981 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 9, AUGUST 1981
[
TEFLON CAP
1443
(bypassed)
OUARTZ CUVETTE
PIEZOELECTRIC TRANSOUCER pzT-5A 1 - &22Mn 2 N 5 4 8 ! ! # 4 f f
P
SIGNAL OUT
q-1
OUARTZ
- 9 V (bypassed)
Figure 2. Flgure 1.
Photoacoustic preamplifier circuit schematlc.
Schematic of the liquid-phase photoacoustic cell.
transducer output, S, in V, is proportional to both incident pulse energy Ep,in mJ, and to sample optical absorptivity a, in cm-l, for small absorptivities. The proportionality constant R , in V/(mJ cm-l), is called the responsivity and is shown to be dependent on the detection system geometry. The responsivity is also proportional to the solvent thermal expansion coefficient 0, in K-I, and solvent speed of sound c, in cm d, and inversely proportioiial to the solvent specific heat Cp, in cal/(g-mol K). If the limiting detectable absorptivity, aL,is defined as the absorptivity of the soluite a t the limiting detectable concentration CL, where S I N = 3, then it is apparent that higher pulse energy lasers may allow lower CL and aL values to be achieved without improvement in the PAS cell or signal processing. Therefore, a]> values, though widely used as figures of merit of the sensitivity, are actually misleading. A much better figure of merit for comparison of the sensitivities of two PAS systems is the limiting detectable thermalized energy EL,defined as (YLIE,,where aL is as above, 1 is the illuminated path length, in cm, and E, is the incident laser pulse energy, in J. This figure of merit is implicit in the work of BonchBruevich et al. (7) and is explicitly used by Tam et al. (15) as.ni,€ Physical processes liberating more than EL of thermalized energy per excitation pulse would be expected to be observable with the PAS system. The cell design used by Tam and Patel (IO),though quite sensitive by the above criteria, consists of a stainless steel cylinder with quartz windows, “0”-ring seals, and side ports for the transducer and sample admission. Though well-suited to their studies, it would be useless for rapid analysis of potentially hazardous materials and difficult to clean thoroughly. Consequently, they developed a sensitive “C-clamp” arrangement for holding their PZT transducer assembly in contact with a standard 1-cm cuvette (13). Unfortunately, we found that the various “C-clamp” cell versions we built were quite sensitive, but offered poor signal reproducibility when the cuvette was removed from the clamp assembly and then replaced. Signal amplitudes frequently varied by factors of 2. In addition, stray incident light and scattered light impinging on the “C-clamp” itself generated acoustic transients which could not be temporally resolved from the genuine PAS signal. Therefore, we developed an extremely simple photoacoustic detection system by simplifying the designs used by previous investigators. Our design, which employs a “submersed” PZT transducer and a standard 1-cm Suprasil fluorescence cuvette, does not appreciably suffer from the aforementioned difficulties. This paper details our cuvette PAS system and preamplifier electronics and their use in detecting various porphyrins, laser dyes, and drugs. Calibration data establishing the approximate thermalized energy sensitivity of the system are presented and several pulsed laser systems are compared with respect to pulse energy and pulse width effects in our system. Since the magnitude of the photoacoustic effect utilized here
APERTURE SAMPLE CELL I ~
I
rCL LASER
d
Figure 3.
ENERGY METER
L - I
0 1 I ‘ . AMPLIFIER
Nitrogen laser excited photoacoustlc detection scheme.
is dependent on certain solvent physical properties, data are presented comparing the magnitude of the observed signals in the normal alkanes (C6-C9),methanol, ethanol, acetonitrile, carbon tetrachloride, and water.
EXPERIMENTAL SECTION The simple cuvette/transducer arrangement is shown in Figure 1. Standard Suprasil fluorescence cuvettes are employed and the transducer is PZT-5A composition with 0.5 in. diameter, 0.12 in. thickness, and 200 kHz fundamental mechanical resonant frequency (no. 8125, Vernitron Piezoelectric Division, Bedford, OH). The transducer is attached to the bottom of a fused quartz flat with cyanoacrylate adhesive while the cuvette is placed on the upper surface of the flat. A drop of glycerol on the flat greatly improves acoustic coupling efficiency and is much easier to remove than vacuum grease. Prior to using this arrangement, we found that a “C-clamp” arrangement was unsatisfactory for the reasons given above. To eliminate the attendant acoustic artifacts, we replaced the “C-clamp” with a simple arrangement in which a PZT transducer was cemented to one end of an Alnico rod magnet and a polished steel disk was cemented to the bottom of a Suprasil cuvette. Magnetic attraction provided a reproducible coupling force. We found that use of a coupling fluid was much more important in obtaining a sensitive and reproducible signal than the use of a relatively strong coupling force. Opaque flats made of stainless steel and tantalum were then used until it became clear that scattered light artifacts would not be problematic. The transducer is shielded from external electrical fields since the quartz flat is thick enough (0.5 in.) t o breach the preamplifier enclosure wall. A rubber gasket reduces acoustic coupling between the enclosure wall and the quartz flat. No other attempts were made to acoustically or thermally shield the apparatus. Figure 2 shows the preamplifier circuit employed. It is a slight modification of the preamplifier circuit used by Tam and Patel (IO). Frequency response is 28-400 kHz (-3 db), voltage gain is 37.5, and noise is 4 pV rms. Nine-volt alkaline batteries are used to power the preamplifier to avoid power line hum pickup. The transducer is located as close to the preamplifier input as feasible (1 in.) and the leads are carefully soldered to the transducer
1444
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ANALYTICAL CHEMISTRY, VOL.
3 20
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53, NO. 9, AUGUST 1981
/V
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PAS SIGNAL
-IO-
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P
BOXCAR GATE (-5V )
-20
-30
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I
I
I
I
I
I
I
I
2
4
6
8
10
12
14
16
10
1
20
TIME ( p s )
-
Photoacoustic signal produced by 12.85 ppm of tris(2,4pentadlono)chromiurn(III)and excitation pulse energy of 15 pJ at A = 337.1 nrn. Flgure 4.
although we have also used nickel-silver terminals from a solderless breadboard socket (Continental Specialties Corp., New Haven, CT) to effect a removable electrical connection to the transducers. A block diagram of the primary configuration of our experimental system is shown in Figure 3. The excitation illumination is provided by an unfiltered Molectron UV-14 N2laser producing pulses of approximately 1.3 mJ at a repetition rate of 20 Hz. The laser pulse width of 10 ns (fwhm) is obtained by using a Hamamatsu streak camera system with C979 temporal disperser. Pulse energies are measured with a Molectron Model 53-05 pyroelectric energy meter. The predominant laser output wavelength is 337.1 nm. The laser beam is passed through an adjustable aperture, usually set at 1 in. diameter, is attenuated, if necessary, by high-quality neutral density filters (Corion Corp., Holliston, MA), is focused with an f2 1.5 in. diameter quartz lens, and passes through the cuvette to the energy meter or a beam stop. The beam is focused 1cm in front of the cuvette and 0.5 cm above the bottom of the cuvette. Sample size is approximately 1 cm3in all cases. Prefocusing the beam gives lower (3x) signals but better stability in the preamplifier output. Beam positioning is not especially critical. The preamplifier output is amplified by a Tektronix 26A2 differential amplifier with selected frequency response of 10-300 kHz and adjustable gain from 1 to 1000. the amplifier output is processed by a PAR Model 160 boxcar averager (EG&G Princeton Applied Research, Princeton, NJ) with gate duration of 0.5-1.5 p s and gate delay of 12-14 ps. The second positive excursion of the signal is usually selected for processing as explained below. "he nitrogen laser, boxcar averager, and Tektronix Model 454 oscilloscope wed for setup and monitoring of the signal during the experiment are triggered with a Wavetek Model 802 pulse generator. Instrumental time constant for the boxcar averager is 100 ps giving an observed time constant of 5 s. Stock solution absorptivities were measured with a Varian 634 spectrophotometer (Varian, Palo Alto,CA). Additional laser excitation sources were (i) a Garching nitrogen laser (Garching, West Germany), (ii) a CMX-4 flash-lamp-pumped tunable dye laser (Chromatix, Sunnyvale, CA), and (iii) a Molectron DL-400 nitrogen laser-pumped tunable dye laser (Molectron Corp., Sunnyvale, CA). The signal waveform, shown in Figure 4, is very similar to that exhibited by Tam and Patel (10). It consists of a damped (7 20 p s ) ringing at the mechanical resonant frequency of the transducer and also contains contributions from other resonances and reflections. With the nitrogen laser operating, but with the laser output blocked, a short duration ( 1ps) RFI transient is observed with the fast oscilloscope. With a cuvette and solvent on the quartz flat and illumination unblocked, a small artifact is observed in the first 5-10 ws of the oscilloscope trace. This artifact is found to be due to scattered light; although it was feared initially that scattered light would prove to be a major problem with our "naked" transducer, this was not found to be the case. The introduction of a thin, highly light absorbing layer between the cuvette and quartz flat had no effect on the delay in the solute-generated signal. The addition of various laser dyes to the glycerol coupling fluid showed that very little scattered light
-
N
-vpo/
/O,ug/L
/OO,ug/L
tmg/L
lOmg/L
C O N ENTRATlON Flgure 5.
Photoacoustic signal vs. solute concentration for tris(2,4-
pentadiono)chromlurn(III).
normally impinges on the transducer. In any event, the artifact could always be avoided by simply using the second positive excursion of the signal waveform for processing. Had scattered light been problematic, it would still have been possible to proceed either with a quartz piezoelectric transducer, which is not pyroelectric (16), or with gated damping of the transducer output with a suitable high speed switch, such as the General Electric HllF3 optically isolated bilateral analog FET switch. As mentioned previously, I-cm3liquid samples are measured, and the cuvette is removed and replaced several times to provide an average signal value. Sixteen blank values are measured and blank standard deviations are 2-4%. The % RSD values are approximately 4%. In order to set the boxcar averager gate delay, which varies with solvent due to speed of sound variations, a relatively concentrated sample solution (10-100 ppm) is used in one cuvette and the freshly prepared dilutions are then measured in a matched cuvette. Cuvettes are cleaned by rinsing them well and immersing them in concentrated nitric acid in a beaker in a fume hood. A small (0.3 mL) quantity of absolute ethanol is added to the nitric acid which causes a vigorous boiling. The cuvettes are then copiously rinsed with large quantities of distilled, demineralized water (Barnstead Sybron Corp., Boston, MA) rinsed with absolute ethanol, and dried. The laser dyes wed are supplied by Exciton Chemical Co., Inc., Dayton, OH. The porphyrins are obtained from Sigma Chemical Co., St. Louis, MO. Privine HCI and Nupercaine HCl are supplied by Ciba-Geigy, Summit, NJ.
RESULTS AND DISCUSSION In order to verify that the simple phenominological results derived by Tam and Patel (IO) are applicable to our PAS system, we measured the responsivity of our system as a function of solvent. Responsivities were measured by dividing the input-referred amplitude of the first positive signal waveform excursion by the product of the incident laser pulse energy and the extrapolated solution absorptivity. The compound tris(2,4-pentadiono)chromium(III)was chosen as the solute since it is soluble in the disparate solvents used, is stable, does not luminesce, absorbs well at 337.1 nm, and is readily available in high purity (triple recrystallized). Binary dilutions were prepared and measured as indicated above. The results are shown in Figure 5. Although the preamplifier output signal, which is proportional to the responsivity, is seen to be linearly proportional to solute concentration, as expected, the hexane result is anomalously good. Consequently, solutions of tris(2,4-pentadiono)chromium(III)were prepared in 10 solvents at concentrations of approximately 1ppm. Absorptivities were measured on the 10 ppm solutions from which
ANALYTICAL CHEMISTRY, VOL. 53, NO. 9, AUGUST 1981
sulfide was spectacularly unsuited for use as a solvent at 337.1-nm excitation wavelength. Since the laser power employed is well above the threshold for beam self-focusing in carbon disulfide (-2 kW (18))and carbon disulfide is strongly absorbing at 337.1 nm, a number of unusual effects occur: the liquid bubbled vigorously, a loud clicking sound is produced, a bluish white “laser plasma” is produced, and the Suprasil cuvette is conchoidally fractured on the inner surface nearest the beam focus. Examination of the results in Table I shows that our measured responsivity ratios agree moderately well with the predicted ratios. The discrepancies are most likely due to variations in illuminated sample volume, variations in beam prefocusing distance, and use of first peak oscilloscopic detection rather than second peak boxcar averager detection. Though responsivities are useful figures of merit for condensed phase PAS detection systems, the limiting detectable thermalized energy, EL, is preferable since it contains information concerning the system noise level. These EL values are obtained by measuring the corrected PAS signal vs. solute concentration and dividing the least-squares slope of the resulting line into three times the standard deviation of the blank; concentrations are converted to equivalent absorptivities by performing absorbance measurements on the stock solutions of the solutes. The value of EL is then obtained by multiplying the limiting absorptivity value, CYL,by cell length, 1, and by incident laser pulse energy, E,,. Alternatively, the signal may be measured vs. incident pulse energy (using neutral density filters) and the least-squares slope may be divided into three times the electronic noise level, as was done for the smoky quartz sample in Table 11. The results of such calibrations are given in Table 11. Although it is expected that dilute solutions of different absorbing solutes should give the same EL value, assuming the same solvent is used, this is not seen to be verified by our data, It is possible that the discrepancies in our calibration data are due to ambient temperature changes (samples were not run on the same day) or they may be similar in origin to the aforementioned responsivity discrepancies. Accordingly, investigations are continuing in order to determine the cause of the imprecision in the measured values of the figures of merit discussed above. The effects of laser excitation pulse energy and pulse duration were examined and the results are shown in Table 111. Considering first the effect of laser pulse duration, note that the DL-400 dye laser output is 100 times shorter than the CMX-4 dye laser output but of approximately equal pulse energy. The responsivity and EL values are about twice as good for the DL-400 system as they are for the CMX-4 system. Although this seems to contradict the results of Atalar (14) that higher signal to noise ratios may be obtained by using shortm laser pulses, this result assumes that system bandwidth is commensurate with the laser pulse durations used. With the preamplifier we have used, system bandwidth is approx-
Table I. Photoacoustic IResponsivities of Tris(2,4-pentadiono)chromiurn(III)in Several Solvents with 337.1-nm Excitation responsivity, R , mV/ predicted R,/ ratiosat (mJ no. solvent cm-’) R H 2 0 b 20°C 1 2
3 4 5 6 7 8
9 10
ethanol methanol carbon tetrachloride acetonitrile water n-pentane n-hexane n-heptane n-octane n-nonane
7.8
6.0
12. 17.
11.
5.6
30. 19. 1.5
20. 13.
14.4
1 15. 5.8 11.
22. 8.5 17.
14.
7.7
9.3 12.
18.
a R , is the responsivity for solvent X and RH,O is the responsivity of water.
Table 11. Limiting Detectable Energies, EL,and Solution Absorptivities, a,for Several Photoacoustic Calibration Standards in Aqueous Solution with 337.1-nm Excitation (Smoky Quartz Is Included for Comparison) compound
a , cm-’
EL, nJ
tris( 2,4-pentadiono)chromium(111) potassium permanganate potassium chromate potassium dichromate tetraamminoruthenium hydroxychlorochloride rhodium ammonium chloride smoky quartz
0.665 at 15.5 ppm
230
0.095 at 0.072 at 0.077 at 0.022 at
10.0 ppm
31 83 400 97
0.002 at 13.2 ppm
80
13.0 ppm 12.1 ppm 10.8 ppm
250
~-
the 1ppm solutions were prepared by dilution. Table I gives the measured responsivities, their ratios with that of water, and calculated values (17’)of several of these ratios. Since the solutions were concentrated enough to afford high signal to noise ratios, blank correction was dispensed with and oscilloscopic rather than boxcar averager detection was employed. Laser pulse energy was Continually monitored by using a 50% beam splitter modification of our experimental apparatus. Note that the use of ratios of solvent responsivities with respect to that of water is somewhat misleading since the responsivity of water is strongly temperature dependent. This is due to the temperature dependence of the thermal expansion coefficient of water. In addition to the 10 solvents studied above, several others were found to be unsatisfactory for various reasons. Diethyl ether produced severe fluctuations in the preamplifier output, pyridine absorbed too strongly at 337.1 nm, and carbon di-
Table 111. Laser Excitation Source Comparison of Limiting Detectable Energy, EL, Responsivity, R , and Limiting Detectable Absorptivity, a L, for Tris(2,4-pentadiono)chromium(III)in Hexane“ Molectron UV-14
EL,
nJ
R , V/(mJ ern..')
Garching N, laser
N, laser E , = 1.6 m J b
E,=7d 7 =
1ns
1 0 nsc h = 337.1 nmd
h=
337.1 nm
7
=
6. 0.10
Molectron DL-400 dye laser E , = 54 pJ 7 =
h=
10ns
595 nm
5.
3.
0.08
0.12
4. x 10-6 7. x 10-4 6. X lo-’ a Solution absorptivities were 0.460 cm-‘ at 1 0 ppm (hex.= 337.1 nm) and 0.013 cm-’ at 100 ppm Incident pulse energy. Incident pulse duration. Excitation pulse wavelength. a ~ cm-’ ,
1445
Chromatix CMX-4 dye laser E , = 47 pJ 7 =
1ps
h=
595 nm
6. 0.077 1. x 10-4 (hex =
595 nm).
1448
ANALYTICAL CHEMISTRY, VOL.
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Table IV. Nitrogen Laser Excited Photoacoustic Characteristics of Porphyrins and Laser Dyes at 298 K compound
solvent
slopea
bacteriochlorophyll a bilirubinC biliverdin chlorophyll a chlorophyll b chlorophyll water sol chlorophyllin coproporphyrin I11 tetraethyl esterC coumarin 12OC cytochrome c eosin Y fluorescein disodium hematin hematoporphyrin hemoglobin mesoporphyrin IX dimethyl ester nupercaine HCl oxazine perchlorate privine HCl protoporphyrin IX dimethyl ester protoporphyrin disodium rhodamine 6G riboflavine tetraphenylporphined uroporphyrin I ethyl ester vitamin B,,
ethanol ethanol water ethanol ethanol water water ethanol
0.81 0.62 1.10 0.94 1.06 0.98 1.01 0.72
ethanol water water water water ethanol water ethanol
0.74 0.96 0.98 0.99 1.06
1.02 0.99 1.05
LOD,~ ng/& 1. 2 x 10’ 0.9 0.9
0.3 5.
3. 2. 1.
3 x 10’ 4.
3. 1. 0.3 7 x lo1 0.5 1x 10’
water ethanol water ethanol
0.95 0.51 1.01 1.17
9 x lo2 0.09
water
0.94
1.
water water ether ethanol
1.00 0.97
0.53
3.
2.
1x 10’ 6.
2.
water 1.02 4. a Slope of the linear portion of the blank-corrected loglog plot of relative photoacoustic signal vs. concentration. The limit of detection is equal to three times the standard deviation of the blank divided by the sensitivity. Data obtained by using oscilloscopic eak detection instead of boxcar averager detection. tl LOD estimated from a single data point with oscilloscopic peak detection. Thermal instabilities in the solvent were severe. imately 400 kHz. Hence, the expected improvement in going from a 1-ps pulse to a 10-ns pulse is approximately loo(1 - e-I(l0 n ~ ) ( X2 400 kHz)l) /( 1 - e-[(’~ d ( X2 4M) kHdl) = 1.45 Our results are in reasonable agreement with the above estimate. The results in Table I11 also indicate that the responsivity and EL figures of merit are relatively unaffected by incident laser pulse energy while the limiting absorptivity values are strongly dependent on pulse energy. The CYLvalue shown for the UV-14 nitrogen laser system having Ep = 1.6 mJ is commensurate with the lowest detectable absorptivity (3.9 X lo4 cm-l at S I N = 3 and Ep = 1 mJ) given by Atalar (14). Having characterized our condensed phase PAS system, we determined a number of limiting detectable concentrations
for various porphyrins, laser dyes, drugs, and vitamins. The results are shown in Table IV. The materials selected were chosen for their general importance and because they exhibit widely varying luminescence yields. Since sample fluorescence could possibly cause an artifact similar to the small observed scattered light artifact, we did not expect spectacular results from such substances as Rhodamine 6G. Nevertheless, the best limit off detection concentration achieved, i.e., -90 ppt for protoporphyrin IX dimethyl ester, is quite good despite the rather high slope of the analytical curve. Our detection limit for vitamin B12was 4 ppb (3 X M) which is commensurate with the reported value of 2 X M by chemiluminescence (19). Our value of 0.5 ppb (9 X M) for mesoporphyrin IX dimethyl ester is commensurate with the value of 1.2 X M (SIN = 3) reported by Hershberger et al. (20) using a windowless flow cell arrangement. Although we do not expect to surpass fluorescence detection techniques, we have demonstrated that nonluminescing substances may be conveniently detected at the parts-per-billion level or below, depending on laser pulse energy, in a simple, easily cleaned PAS cell.
ACKNOWLEDGMENT The authors would like to acknowledge the kind assistance of Andrew Tam, who supplied the preamplifier schematic from which our circuit was adapted. We also which to thank C. K. N. Patel for bringing the “C-clamp” mounting arrangement for a cuvette cell to our attention at the 1979 FACSS meeting in Philadelphia.
LITERATURE CITED Rosencwalg, A.; Gersho, A. J. Appl. Phys. 1978, 47, 64-69. Lln, J. W.-p.; Dudek, L. P. Anal. Chern. 1979, 5 1 , 1627-1632. McClelland, J. F.; Knlseley, R. N. Appl. Opt. 1976, 15, 2967-2968. Callls, J . B.; Gouterman, M.; Danlelson, J. D. S. Rev. Scl. Insfrum. 1989, 40, 1599-1605. Hordvlk, A.; Schlossberg, H. Appl. Opf. 1977, 16, 101-107. Lahmann, W.; Ludewlg, H. J.; Welling, H. Anal. Chem. 1977, 49, 549-55 1. Bonch-Bruevich, A. M.; Razumova, T. K.; Starobogatov, I. 0. Opf. Specffosc. 1977, 42, 45-48. Farrow, M. M.; Burnham, R. K.; Auzanneau, M.; Olsen, S. L.; Purdle, N.; Eyring, E. M. Appl. Opt. 1978, 17, 1093-1098. Oda, S.; Sawada, T.; Kamada, H. Anal. Chem. 1978, 50, 865-867. Tam, A. C.; Patel, C. K. N. Appl. Opt. 1979, 18, 3348-3358. Patel, C. K. N.; Tam, A. C. Appl. Phys. Lett. 1979, 3 4 , 467-470. Patel, C. K. N.; Tam, A. C.; Kerl, R. J. J. Chem. Phys. 1979, 71, 1470- 1474. Tam, A. C.; Patel, C. K. N. Opf. Lett. 1980, 5 , 27-29. Atalar, A. Appl. Opt. 1980, 19, 3204-3210. Tam, A. C.; Patel, C. K. N.; Kerl, R. J. Opf. Lett. 1979, 4 , 81-83. Neubert, H. K. P. “Instrument Transducers”; Oxford at the Clarendon Press, Oxford, 1963. “CRC Handbook of Chemistry and Physlcs”, 49th ed.; The Chemical Rubber Co.: Cleveland, OH, 1968. Yarlv, A. “Quantum Electronics”, 2nd ed.; Wlley: New York, 1975. Sheehan, T. L.; Hercules, D. M. Anal. Chern. 1977, 49, 446-450. Hershberger, L. W.; Callls, J. B.; Christian, G. D. Anal. Chem. 1979, 51. 1444-1446.
RECEIVED for review January 2,1981. Accepted May 6,1981. This work was supported by NIH-GM11373-17 and by DOE-AS05-78EV06022 MOD AOOE.
