1485
Anal. Chem. 1982, 54, 1485-1489
LITERATURE CITED (1) Smith, A. L. "Applied Infrared Spectroscopy': Fundamentals, Techniques and Analytical Problem Solving"; Wiley-Interscience: New York, 1979; Chapter 6. (2) "Abstracts of Papers", 31st Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Atlantic City, NJ, March 1980; Infrared Spectroscopy---Quantitative Analysis, 359-367. (3) "Abstracts of Papers", 32nd Plttsburgh Conference on Analytical Chemlstry and Applied Spectroscopy, Atlantic City, NJ, March 1981; Infrared Quantitative Analysls, 910-919. (4) Frye, H.; Kletsch, R.; hdlllle, G. J. J. Lab. C l h . Med. 1977, 90 (l), 109-113. (5) Lehmann, C.; Rosenfeld. M.; Parker, M. Anal. Lett. 1980, 73 (81% 1303-13 15. (6) Freeman, N. K. J. Lbia'Res. 1984, 5 , 2364241. (7) Freeman, N. K. I n "Blotxl Lipids and Lipoproteins: Quantitation, Com-
position, and Metabolism"; Nelson, G. J., Ed.; Wiley-Interscience:
.-.. Vnrk . -. .., 1F173. .- . -, n
,Maw
r
1136
(8) Tietz, N. W. "Fundamentals of Clinical Chemistry", 2nd ed.; W. B. Saunders Co.: Philadelphla, PA, 1976. (9) Brown, C. W.; Lynch, P. F.; Obremski, R. J. Anal. Cbem., In press. (10) Gendreau. R. M.; Griffiths, P. R.; Ellis, L. E.; Anfinsen, J. R. Anal. Cbem. 1978, 48 (13), 1907. (11) Diaz-Rueda, J.; Sloane, H. J.; Obremski, R. J. Appl. Spectrosc. 1977, 31, 298. (12) Freeman, N. K. I n "Blood Lipids and Lipoproteins: Quantitation, Composition, and Metabolism"; Nelson, G. J., Ed.; Wiley-Interscience: New York, 1972; p 155.
RECEIVED for review July 13,1981. Resubmitted and accepted April 15, 1982.
Determination of Nitrogen Dioxide by Visible Photoacoustic Spectroscopy 0. Poizat' and G. H. .Atkinson* Department of Chemistty, Syracuse University, Syracuse, New York 132 10
Vlslble, photoacoustic spectroscopy, performed wlth a CW krypton laser, Is applied1 to the quantitative detectlon of NO2 In alr samples at total pressures of 1 atm. The optlmlratlon of the photoacoustlc slgrial In accord wth a theoretical model Is discussed and a dual beam (sample slgnal mlnus reference slgnal) method of reduclng the background noise level Is described. Data were obitalned by using two cell deslgns and two types of mlcrophonles. NO2 concentrations as small as 2 ppb were detected, rind the photoacoustic slgnal was linearlly dependent on NO2 pressure over ,approximately 6 orders of magnltude.
We report in this paper, the photoacoustic detection of NOz at concentrations as low as 2 ppb in samples of air at total pressures of an atmosphere. The photoacoustic signal was driven by fixed frequency excitation between 406.7 and 530.9 nm. The photoacoustic signal was linear over approximately 6 orders of magnitude in NOz pressure. The instrumentation was optimized with respect to the generation and propagation of photoacoustical signals under conditions which simulate those found in the environmental monitoring of real, captured air samples, In this regard, the simplicity of the instrumentation as well as its performance capabilities were considered in the design of the experiment.
THEORY The resurgent intererit in photoacoustiic spectroscopy fostered by the development of versatile lasers was rapidly directed toward practical applications such as the quantitative detection of atmospheric pollutants (1-14))particularly NO2 (1,6-10). The increased efficiency of photoacoustic detection at higher pressures makes it well-suited for applications such as atmospheric pollution monitoring. Moot of the initial work in the area was performed by using infrared laser excitation (1-5). For many atmospheric pollutants, however, photoacoustic spectroscopy initiated by excitation in the visible and ultraviolet regions provides significant aidvantages over excitation in the infrared region. First, the energy deposited in the molecule by visible excitation, and thus that available for nonradiative, acoustically detected decay, is much larger than that for infrared excitation (assuming the probability of nonradiative decay remains approximately the same). Second, the selectivity of the technique for monitoring gas mixtures is higher since it is less likely that different gases will have interference bekween their absorption spectra in the visible. Third, laser systems operating in the visible and ultraviolet regions have become extremely versatile with respect to tunability, bandwidth, and power. These characteristics may provide the enhanced sensitivity and specificity required in practical atmospheric monitoring.
The fundamental relationships describing the photoacoustic signal induced in a gas sample have been derived for a twolevel system, in the cases of a chopped modulation (15))a low amplitude modulation (13,14),or a single pulse radiation (16), and for a three-level system in the general case (17,18). Each derivation, of course, has incorporated specific assumptions and simplifications. Let us consider the case of a gas mixture enclosed in a constant volume and containing only one absorbing gas species. Assuming an ideal gas behavior, the pressure P and the variation of kinetic energy dEkinare given by P = NkT
d E k i , = (dEki,/dT)v d T = Cv d T Hence, the time derivative of pressure is d P=-p. = N k. -Ekin
dt
where N is the total number of molecules (absorbing molecules and buffer molecules) per unit volume, k is the Boltzmann constant, and Cv is the specific heat of the gas at constant volume. In the case of a two-level system, the variation of kinetic energy in the gas can be expressed by Ekin
Permanent address: Centre N a t i o n a l de Recherche Scientifique
2, R u e Henri-Dunant, 94320 Thiais, France. 0003-2700/82/0354-1485$01.25/0
CV
=
k1oCN1(E1 -
EO)
(2)
in terms of Eo and E,, the energies of the ground and excited 0 1982 American Chemical Society
1486
ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982
-
states, respectively, kl,C, the El Eo collisional rate constant, and Nl, the number of excited molecules (NT = N1+ No = total number of absorbing molecules). Then, by combining eq 1 and 2
P
Nk
- Eo)
= -k",N(,E1
CV
q 7,
(3)
The conservation of the number of molecules is written as
fi1 = kO1'No - (klo'
+ kloc)N1
I
A-B
sh
(4)
hR
or
fi1 = kolaNT - (kola + k1or + kloc))N1
(4a)
where kola and k,& are the radiative rate constants. The kola part consists of only the absorption term ( p a o l ) while the kl& part has two separate terms describing stimulated (p,,Bl0) and spontaneous emission (klo?. The radiation density is expressed as pv = I(El - Eo)/c where I is the intensity of the radiation incident on the sample. For exciting radiation modulated at a frequency w and with amplitude 6, I = Io(l+ 6 cos ut). With these definitions and the fact that Bol = Blo, expression 4 can now be written
A-8 -
C
Flgure 1. Schematic representation of the experimental instrumentation. Symbols are PD, photodiode; M, microphones; A, B, and C, voltage signals from the sample cell microphone, the reference cell microphone, and the photodiode, respectlvely; S and R, the signal and reference inputs of the lock-in amplifiers; RM, ratiometer.
resonant frequency, wj, according to the quality factor, Qj, of the chamber for the resonant mode, j . This factor is defined (13, 14, 19) as j'
2
(El - E0)61301 C
1
cos ut N1 ( 5 )
In the case where 6
