Anal. Chem. 1984, 56,80-82
80
Infrared Photoacoustic Spectroscopy of Liquids with an F-Center Laser Pao-Yuan Chen and James S. Shirk* Department of Chemistry, Illinois Institute of Technology, Chicago, Illinois 60616
An Infrared photoacoustic llquld cell wlth excitation by a tunable F-center laser Is descrlbed. Modulation near 126 kHz excites low order, modest 0 (eo), radial resonance modes of the sample. The resuitlng photoacoustlc slgnal Is Independent of sample path length over a range 0.3-4.4 cm. An absorbance of 1 X 10" provldes a S / N = 1 wlth a laser power of 3.5 mW. The slgnal scales with laser power but the nolse Is domlnated by detector thermal (Johnson) nolse, so greater sensltlvlty can be achleved wlth more powerful lasers. The effect of sample resonances on photoacoustlc experlments wlth pulsed laser excltatlon Is dlscussed.
Table I. Calculated Frequency Values for Different Resonance Modes and Their Experimental Frequency Values measd value,
calcd value,
mode (p,m,n)
Hz
Hz
1,0,0 2.0.0 31010 0,190 0,2,0 0,390 0,OJ 082
1.16 x 104 2.75 x 104 3.5 x 104 4.1 x 104 7.2 Y 104 9.7 x 104 8.43 x 104 16.05 X l o 4 12.25 X l o 4
OJ,1
Photoacoustic spectroscopy (PAS) is useful for trace analysis, especially in the gas phase. It has been also widely used in the IR region; there are commercially available photoacoustic cells for some IR spectrometers (1). These cells are very sensitive for gaseous samples but are not particularly sensitive when used for the IR detection of trace constituents or weak absorptions in liquid phases. The relatively low sensitivity for IR-PAS detection in liquids arises because usually the acoustic signal is detected with a capacitance microphone in the gas above the sample. Sensitivity suffers from the poor acoustic coupling of the condensed phase with a gas. Photoacoustic detection of condensed phase absorptions with microphones in the gas above the liquid is generally useful only for reasonably strong absorbances. Immersed piezoelectric detectors have better coupling to the condensed phase but suffer from a lower sensitivity (by about 2 orders of magnitude) than gas-phase microphones (2). There has been some recent success at high sensitivity PAS in the visible region with piezoelectric microphones that have good acoustic impedance matching to the solution. Voigtman, Jurgensen, and Winefordner review some of the previous studies (all of these are in the visible or near-IR region) (3). Usually pulsed sources with gated detection (so that a true photoacoustic and not a photothermalacoustic pulse is detected) are used. For example Patel and co-workers (4-6) using a pulsed dye laser and measuring the height of the first acoustic peak, were able to detect molecules with absorbances of lo-' cm-l in liquids in the visible and near-IR regions. They were able to measure the absorption spectrum of water in the visible region. The initial motivation for our work came from the papers of Tam and Patel. Since tunable high power pulsed lasers, analogous to dye lasers, are not available in the fundamental IR and yet trace PAS IR detection seemed a desirable goal, we decided to investigate the possibility of trading high peak power at a low repetition rate for the low peak powers a t a high repetition rate which could be obtained by modulating an F-center laser. We report here on the design, construction, and operation of an F-center laser-driven IR-PAS spectrometer for the detection of weak IR absorptions in liquids, including a comparison of pulsed modulation with sine wave modulation.
1.05 x 2.11 x 3.17 x 4.29 x 7.12 x 9.79 x 8.93 x 16.35 X 12.35 X
104 104 104 104 104 104 104
lo4 lo4
EXPERIMENTAL SECTION The IR radiation source was a computer-controlled Burleigh Model FCL-20 F-center laser, used here without an etalon, pumped by a Spectra Physics Model 164-11Kr+ laser. the details of this laser have been described in connection with other experiments (7). This laser is continuously tunable from 2.3 to 3.3 pm. The Kr+ laser pump beam was modulated either as a sine wave or in a discrete pulsed mode using a photoacoustic modulator (Interaction Corp., Model ADM-40). We also did some experiments in which the modulated Kr+ laser beam excited the sample directly. The photoacoustic transducer and preamplifier were identical in design and construction with that described by Tam and Patel. The piezoelectric element was a lead zirconate-titanate cylinder made by Transducer Products. The transducer is enclosed in a housing so that only a polished stainless steel face is in contact with the sample. A variety of sample containers were used. Most experiments were done in sample cells made of Teflon. In these cells the sample chamber was 1.3 cm in diameter and was lined with a stainless steel cylinder. One cell was 4.4 cm long. A second was 1.85 cm long; in this cell the sample length could be varied from 0.3 to 1.85 cm by adjusting a Teflon plug. The transducer assembly was inserted radially into the cell. The cell had salt or Pyrex windows sealed by an indium gasket. The output of the microphone preamplifier was fed into a Princeton Applied Research Model HR-8 Lock-in Amplifier for modulated CW experiments. For detection of pulsed experiments the preamplifier output was amplified (XlOOO) and fed into a PAR Boxcar Averager Model 165 with a Model 162 plug-in module. All liquid samples were Spectrophotometric grade dried over 4A molecular sieves and filtered through a Millipore filter. Samples were introduced into the cell under an Nzatmosphere. Even with these precautions water estimated from the IR absorbance spectra to be in the high parts-per-billion range remained in the samples. RESULTS AND DISCUSSION In a reasonable sized liquid PAS sample cell with dimensions on the order of a centimeter there are a large number of closely spaced acoustic resonances in the frequency range above lo4 Hz. For example, Table I gives the calculated resonances (8)for one of our sample cells and Figure 1shows the observed acoustic response as a function of frequency for this cell; most of the maxima can be assigned to low order
0003-2700/84/0356-0060$01.50/00 1983 American Chemloai Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 1, JANUARY 1984
I " Time psec)
Modulation
Frequency ( ~ z )
Flgure 1. Acoustic response as a function of signal frequency for our approximately 1.3 cm diameter cell. The indexes of the nearest acoustic resonances are given.
"
"
I
81
'
30 60 90 Time (ysec)
Flgure 3. Pulse response of the piezoelectric detector (a) with excitation about one-third of the way across the cel and (b) with excitation about two-thirds of the way across the cell. Laser had a I-pspulse width at 3712 cm-l and a pulse energy of 3.5 X IO-' J. 1
I
wave Number (Cm')
Figure 4. PAS spectrum of 4 ppm CH OH in CCi, taken at 3.5 mW laser power. The peaks at 3616 cm-'and 3712 cm-' are H,O.
Distance (cm)
Flgure 2. Acoustic signal as a function of excitation position for the l,l,O mode near 126 kHz.
radial and azimuthal acoustic modes. These acoustic resonances affect the acoustic response to either pulsed or high-frequency sine wave modulated excitation. The observed frequency response, i.e., the resonance frequencies, depends upon the size and shape of the cell. In open cells, such as a cuvette of the type used in previous experiments (3-6), it depends on the sample volume as well, since the airjsolution interface determines the boundary of the effective cell. It also depends upon the position of excitation in the cell; for maximum response the acoustic mode must have pressure maxima at both the microphone and the excitation position. Figure 2 shows the magnitude of the detected signal for the l , l , O mode at 126 kHz in our cell as a function of excitation position. In pulsed experiments considerable care is necessary to achieve consistent results. In PAS with pulsed excitation the observed response for short pulses is the Fourier transform of the frequency response of the cell and the microphone. Thus the observed ringing pattern depends upon the same factors which influence the resonance frequencies of the cell-detector combination. It has been pointed out that simply measuring the initial peak is an arbitrary measure of the absorption strength (9). We confirmed this, for example, Figure 3 shows the pulse response of one of our samples as a function of excitation position. The advantages attributed to pulsed excitation, high sensitivity and discrimination against window absorption, can be maintained by modulated CW excitation into one of the low order radial acoustic modes of the a sample with syn-
chronous detection. High frequency modulated CW experiments were preferable to pulsed experiments for a variety of reasons: (1)the effective repetition rate can be higher since it is not necessary to wait for the "ringing" to die away before exciting the sample again and (2) the small signals and substantial thermal (Johnson) noise from the high-impedance detector results in low SIN ratios at the preamplifier output, sometimes as low as 1/1000. Lock-in amplifiers, which can operate at a very narrow effective band-pass, can extract a PAS signal with a better final S I N than can the boxcar integrators necessary for pulsed experiments. For the 1.3 cm diameter Teflon cells we found that the l , l , O mode at 126 kHz excited ca. one-third of the distance across the cell was convenient and gave a strong resonance signal. The measured Q was 60. Such modest values of Q are advantageous in that a small change in the resonance frequency does not cause large changes in the observed signal and yet the resonance enhancement provides a good signal. Figure 4 gives a typical spectrum recorded in the long cell of 4 ppm CH30H in CCh. The bands at 3712 cm-' and 3616 cm-' are H 2 0 present as an impurity i n the CC14. The peak laser power was about 3.5 mW. The measured S I N was 48. Since the absorbance was 8.4 X an absorbance of 1.7 X would be detectable with a S I N = 1. Similar detection limits were obserbed on other absorptions. The PAS signal was linear with concentration and linear with laser power over the range of power available to us (-4 mW). In a separate experiment, the PAS signal from excitation of a dilute CuS04 solution in H 2 0 in the same cell using a modulated Kr+ laser on the 647.1-nm line, the PAS signal was linear with laser intensity up to 700 mW. Additional experiments using the shorter, variable path length cell showed that the photoacoustic signal was approximately independent of path length over the range available to us: 0.3 cm to 4.4 cm. This was
a2
Anal. Chem. 1984, 56,82-85
expected since we were exciting into a radial acoustic resonance mode of the cell. The long cell had the advantage that we could directly compare the PAS spectrum with an FTIR spectrum of the same sample. The PAS spectrum had about two to three times better SIN. An advantage of the PAS spectrum is that there is no interference from atmospheric absorptions such as water. The sensitivity advantage of PAS was more dramatic with shorter path length samples. Over the range 0.3-4.4 cm, the photoacoustic signal was practically independent of the sample length, so that for a 3-mm sample the detection limit ( S I N = 1)was an absorbance of 1 X a factor of about 50 better than a typical FTIR spectrum. The shortest path length feasible with our current cell and microphone design was 3 mm. Presumably the sample could be made considerably thinner. The observed noise level a t the output of our lock-in amplifier was very close to the expected Johnson noise of our 22 Ma preamplifier input resistor over the band-pass of our lock-in amplifier. This noise determines the detection limit. Higher sensitivities can be achieved by increasing the laser power or cooling the input resistor to the preamplifier. An advantage of operating at these high frequencies is that our detector is able to operate in noisy surroundings and even with flowing samples with little interference. Furthermore using radial acoustic modes we can achieve substantial rejection of interference from window absorption.
-
CONCLUSIONS IR absorption spectra of weak absorbers and trace contaminants in liquids can be observed by using a photoacoustic detection and a modest power F-center laser. The moderate Q natural acoustic resonances of typically sized liquid cells are used to enhance the signal. Even with a modest power laser (3.6 mW) absorbances in the range of 1 x; corre-
sponding to a sample concentration for a typical absorber in the OH region of a few parts per million and a sample path of 3 mm are detectable. The current apparatus is about 50 times more sensitive than a typical FTIR. The limiting noise is thermal (Johnson) noise in the input resistor, which is of course independent of signal. The signal scales linearly with the laser power, so the sensitivity should scale as the laser power. Thus higher sensitivities will be possible as more powerful IR lasers become available. The high frequency used makes the technique useful in high ambient noise applications and for flowing samples. It also provides discrimination against contributions to a background signal from window absorption.
ACKNOWLEDGMENT We thank James Mitchell and Timothy Harris for their interest and aid in obtaining a duplicate of the Tam and Patel microphone. The initial experiments which led to this research were done in collaboration with T. Harris. LITERATURE CITED (1) McClelland, J. F. Anal. Chem. 1983, 55, 89A. (2) Rosencwaig, A. "Photoacoustics and Photoacoustic Spectroscopy"; Why-Interscience: New York, 1980. (3) Voigtman, E.; Jurgensen, A.; Winefordner, J. Anal. Chem. 1983, 53, 1442. (4) Patel, C. K. N.; Tam, A. C. Rev. Mod. Phys. 1981, 53,517. (5) Patel, C. K. N.; Tam, A. C. Appl. Phys. Lett. 1979, 34, 760. (6) Nelson, E. T.; Patel, C. K. N. Appl. Phys. Lett. 1981, 3 9 , 537. (7) McDonald, P. A.; Shirk, J. S. J. Chem. Phys. 1982, 7 7 , 2355. (8) Morse, P. "Vibration and Sound"; McGraw-Hill: New York, 1948; p 398f. (9) Fisher, M. R.; Fasano, D. M.; Nogar, N. S.Appl. Spectrosc. 1982, 3 6 , 125.
RECEIVED for review August 11, 1983. Accepted October 7 , 1983. This work was initially supported by the NSF under Grant No. CHE 79-09380.
Phase Plane Method for Deconvolution of Luminescence Decay Data with a Scattered-Light Component J. C. Love and J. N. Demas* Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901
A three-parameter version of the phase-plane method for deconvolutlon of lumlnescence decay data Is presented that corrects for contrlbutlons of scattered exclted llght In the observed decay. Through computer slmulatlons, we tested the method with numerous comblnatlons of Ilfetlmes, noise levels, and scatter coefflclents. The modlfled equatlon was found to be computatlonally rapld and yielded excellent preclslon and accuracy In the decay parameters.
Excited state lifetime measurements are pervasive and provide crucial information in analytical chemistry, photochemistry, photophysics, and photobiology (1,2). The most common approaches to lifetime measurements involve exciting the sample with a short optical pulse and monitoring the sample decay. If the excitation source is short enough and the detection system responds quickly enough, the observed
sample decay is the desired sample impulse response. Frequently, however, the sample decays on a time scale comparable to the excitation pulse width and the response time of the detection system. The observed decay is then a complex function given by the convolution of the system response and the sample impulse (2). The process of extracting decay parameters from the observed excitation and decay profiles is called deconvolution. The most important deconvolution problem involves Samples with impulse responses that are single exponential decays. The observed sample decay, D ( t ) , is then given by
D ( t ) = K exp(-t/r)JfE(x) 0
exp(x/r) dx
E ( t ) = 0; t I O
+ aE(t)
(1) where D ( t ) is the observed decay signal vs. time, t , K is the proportionality constant, E ( t ) is the observed excitation profile, and 7 is the sample lifetime (1-4). The first term on
0003-2700/84/0356-0082$01.50/00 1983 American Chemical Society
