Anal. Chem. 1983, 55, 1996-1999
1996
fluorescence was sensitized by anthracene (8) and by benz[alanthracene (9),and a good linear relation between analyte concentration and fluorescence signal was obtained. Hornyak also reported the sensitized emission of pentacene (10). A qualitative spot test procedure for detecting polycyclic aromatic hydrocarbons in the field has been described by Smith and Levins (11). Kreps et al. (12) reported the use of colloidal dispersions to detect tetracene in anthracene via energy transfer. Surface-sensitized luminescence offers a relatively simple means for amplifying luminescence and detecting trace amounts of substances. A more detailed examination of this technique is in preparation. Registry No. Naphthalene, 91-20-3; anthracene, 120-12-7; phenanthrene, 85-01-8; azulene, 275-51-4.
LITERATURE CITED
320
340
360
380
400
420
Wavelength (nm) +Figure 4. Fluorescence of naphthalene and azulene on filter paper:
.
- naphthalene (9 pmol) alone; (- - .) azulene (5 nmol) alone; (-) azulene and naphthalene on surface. (- -)
the naphthalene signal by about 30%. Naphthalene-sensitized fluorescences from phenanthrene and azulene on filter paper are illustrated in Figures 3 and 4, respectively. The azulene example is of interest since it represents emission from a second excited singlet state, the well-known anomalous fluorescence of this compound (5,6). The strong quenching of the naphthalene emission in this example indicates that energy transfer is exceptionally efficient, as might be expected from the similarities of the molecular structures. Our attempts to sensitize fluorescence from the lowest excited singlet state of azulene have thus far not been successful. This latter state is known to be only very (7). weakly fluorescent (& = Several earlier studies employing similar systems have been reported but have received little attention. Hornyak (8, 9) adsorbed compounds from solution onto fiiter paper: tetracene
(1) Schulman, S. G. "Fluorescence and Phosphorescence Spectroscopy: Physlochemical Princlples and Practice"; Pergamon Press: New York, 1977. (2) Wehry, E. L. "Modern Fluorescence Spectroscopy"; Plenum Press: New York, 1976; Vols. 1 and 2. (3) Meyers, M. L.; Seybold, P. G. Anal. Chem. 1979, 51, 1609-1612. (4) McHale, J. L.; Seybold, P. G. J. Chem. Educ. 1976, 53, 654-656. (5) Beer, M.; Louguet-Higglns, H. C. J. Chem. Phys. 1955, 2 3 , 1390-139 1. (6) Viswanath, G.; Kasha, M. J. Chem. Phys. 1956, 2 4 , 574-577. (7) Rentzepls, P. M., Chem. Phys. Lett. 1969, 3 , 717-720. (8) Hornyak. I.; Lendvay, E. J. Lumin. 1971, 3 , 369-377. (9) Hornyak, I. Microchim. Acta 1976, 11, 23-29. (IO) Hornyak, J. Lumines. 1975/76, 1 1 , 241-248. (11) Smith E. M.; Levins, P. L. I n "Polynuclear Aromatic Hydrocarbons: Chemlstry and Biological Effects"; BJorseth,A., Dennis, A. J., Eds.; 4th Int. Symp.; Battelle Press: Columbus, OH, 1980; pp 973-982. (12) Kreps, S. I.; Druin, M.; Czorny, B. Anal. Chem. 1965, 3 7 , 586-588.
Paul G. Seybold* Daniel A. Hinckley Timothy A. Heinrichs Department of Chemistry Wright State University Dayton, Ohio 45435 RECEIVED for review March 14, 1983. Accepted June 24,1983. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. D.A.H. was an ACS-PRF scholar. Presented at the 185th American Chemical Society National Meeting, Seattle, WA, March 23, 1983.
Comparison of Reflectance and Photoacoustic Photometry for Determination of Elemental Carbon in Aerosols Sir: Several methods have been developed for the determination of elemental carbon (EC) in atmospheric aerosol samples collected on filters. They include such methods as y-ray analysis for the light elements ( I ) , proton elastic scattering ( 2 ) , Raman spectrometry ( 3 ) , combustion ( 4 , 5 ) , transmittance (6),and reflectance (7). The first three techniques are relatively expensive and involve instruments not common in most laboratories; the combustion method is destructive; transmittance requires optically thin filters and samples; and reflectance can vary with surface characteristics of the filter, requiring measurement of the reflectance prior to loading. The latter two techniques are, however, inexpensive and can be assembled from readily available components. We have investigated the use of photoacoustic
spectroscopy (PAS) for the analysis of the elemental carbon content of atmospheric fine aerosols. For this study a low-cost system was constructed from available components. Details of the construction of the system and preliminary results of the comparison of PAS with reflectance are the subjects of this paper. Photoacoustic spectroscopy has been reviewed previously (8-10) and will therefore be discussed here only briefly. PAS involves two discrete steps-absorption of energy from a modulated light source and detection of the thermal modulations from the sample as pressure waves in the gas surrounding the sample (9). A photoacoustic signal is generated only when the wavelength of the incident light corresponds to an absorption band of the sample; thus PAS spectra are
0003-2700/83/0355-1996$01.50/00 1983 American Chemical Society
ANALYTICAL CHEMISTRY, VOL.
I
1
Flgure 1. Block diagram of reflectance/photoacoustic system. LIGHT 2 5 om
+I IC
DDW
7
ICROPH
'1I
P F
+iDom+
Figure 2. Dlagram of the sample cell. The gasket seallng the two halves of the cell has been omitted for clarity.
very similar to conventional optical spectra. If the source is modulated at a frequency corresponding to the audible range (20-20000 Hz),the acoustic signal can be detected with a conventional microphone. Since only the thermally generated signal is monitored in PAS, the amount of scattered light in the system can have a minimal effect, in contrast to its effect on the reflectance signal especially for optically thin samples (11,12). In the present system, a white-light source was used to increase the sensitivity of the method (carbon is a broadband absorber) and to reduce any potential spectral interferences.
Ik
--- -
4.01
4
- I SVDC
. 2 2 TANTALUM
IN914
-MuACin
1997
EXPERIMENTAL SECTION A block diagram of the system is shown in Figure 1. The instrument consists of a tungsten filament bulb (GE no. 1158)powered by a 12-V power supply, a mechanical chopper with variable speed (variac) control, a sample cell (discussed below), microphone with associated preamp and output device, and a photodiode (PD) to measure the reflectance. The cell (Figure 2) was constructed from two 2.5 X 7.5 X 12.5 cm aluminum blocks, machined to hold a filter mounted in a 5 X 5 cm photographic slide mount. A 1.5 cm diameter chamber machined in the front block and sealed (RTV) with a quartz window provided the photoacoustic cavity. The photodiode (FP 106,Radio Shack, $0.89) was mounted in the chamber approximately 180" to the incident beam to measure back-reflected light intensity. Perpendicular to the light beam, a 0.3 cm hole was drilled, connecting the chamber to a microphone (Electret, Model 270-092A, Radio Shack $2.98), which was sealed with epoxy resin into a recess drilled into the cell wall. The two blocks were held together with screws and sealed with cork gasketing material. Samples were introduced through a slot machined in the top of the cell. A gasketed aluminum lid was used to seal the cell after the sample was introduced. The signal from the microphone was amplified with the preamp shown in Figure 3. The ac signal was filtered, passed through an ac/dc converter (see Figure 3), and fed t o a recorder for display. In the present system, a lock-in amplifier Photoacoustic System.
LID
BATTERY
OCTOBER 1983
Due to their low carbon background, glass or quartz fiber filters are used for collection of aerosol samples for subsequent carbon analysis (13,14). Such filters scatter transmitted light and have high surface roughness, which affects their reflectance. Carbon black has been used as a calibrant for some PAS systems, due to its high molar extinction coefficient (15). PAS has been employed in a number of studies of surfaces (16,17), and has been used to measure aerosol soot content in the gas phase (18) and of samples collected on filters (19). Studies have been performed on laboratory generated aerosols a t high loadings (up to several hundred Kg/cm2) that indicate matrix corrections due to the presence of ammonium sulfate in ambient fine aerosols, while significant for transmittance methods, have only a minimal effect on the photoacoustic signal due to EC (20).
I
PHOTODIODE
55, NO. 12,
20k
DC wt
20 k
Flgure 3. Circuit diagrams of preamp (upper) and ac/dc converter (lower).
1998
ANALYTICAL CHEMISTRY, VOL. 55,
NO. 12, OCTOBER 1983
Table I. Reflectance and PAS Signals for Butane-Generated EC Standards
sample blank blank blank blank blank 51 52 53 54 55 56 57 58 59 510 511 51 2 51 3 514 51 5 a
N.A.
=
1 2 3 4 5
pg of C/cmZ 0.0 0.0 0.0
0.0 0.0
N.A.a 5.0
N.A. N.A. 7.8
N.A. N.A. 8.4 10.6 12.4 13.3 16.4 17.7 33.0 68.0
PAS signal, arbitrary units
reflectance, arbitrary units
0.20 0.20 0.20 0.25 0.23 1.70 2.30 2.35 2.40 2.70 3.00 3.30 3.35 3.90 3.90 3.95 4.60 5.20 6.20 7.30
0.997 0.997 0.997 1.003 1.003 2.41 3.81 3.89 4.21 4.46 5.74 5.80 6.40 7.17 7.01 7.58 8.37 8.87 9.59 9.86
Not analyzed via combustion.
Flgure 4. Plot of reciprocal of reflectance and PAS signals vs. carbon loading on filters. 10,
3 c 21
5 6 L
was not available, requiring special care in acoustic isolation of the cell. The cell was mounted on a slate slab which rested on an inflated inner tube sitting on a marble balance stand. Preparation and Verification of Carbon Standards. All samples and standards were collected on 37-mm glass fiber filters (Gelman GF/F, Gelman Instrument Co., Ann Arbor, MI). The reflectance of each filter was measured prior to use. Soot was generated by directing a butane flame at a cold porcelain surface. The soot was aspirated through a funnel into a vessel t o permit settling of larger particles, and onto the fiiter mounted in an in-line filter holder. Since the carbon loading, in units of pg/cm2, was measured by combustion to COz, the standards were placed in an oven for 30 min at 350 "C to remove any organic (nonelemental) carbon. The filters were then mounted in slide mounts and the reflectance and photoacoustic signals measured. As discussed previously (7), the soot produced was found to be optically similar to atmospheric fine aerosols (
