Environ. Sci. Technol. 7988,22, 338-344
(16) Strang, G. Linear Algebra and Its Applications, 2nd Academic: New York, 1980. (17) Belsley, D. A,; Kuh, E.; Welsh, R. E. Regression Diagnostics: Identifying Influential Data and Sources of Collinearity; Wiley: New York, 1980. (18) Hansch, C.; Quinlan,J. E.; Lawrence, G. L. J . O w . Chem. 1968, 33, 347.
(19) Yakowsky, S. H.; Valvani, S. C.; Mackay, D. Residue Rev. 1983,85,43-55. (20) Miller, M. M.; Wasik, S. P.; Huang, G. L.; Shiu,W. Y.; and Mackay, D. Enuiron. Sci. Technol. 1985,19, 522-529.
Received for review November IO, 1986. Revised manuscript received J u l y 7, 1987, Accepted October 14, 1987.
Use of Electrothermal Vaporization-Multiple-Wavelength Absorption Spectrometry To Qualitatively Screen for the Presence of Polycyclic Aromatic Hydrocarbons Joseph M. Shekiro, Jr., and Rodney K. Skogerboe Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523
Howard E. Taylor" U.S. Geological Survey, Box 25046, MS 407, Denver Federal Center, Denver, Colorado 80225
An electrothermal atomizer normally used for atomic absorption measurements has been used with a spectrometer designed to simultaneously record spectral data over a 150-nm wavelength range to characterize the vapor-phase absorption spectra of several polycyclic aromatic hydrocarbons. I t is shown that a combination of temperature and absorption wavelength discrimination can be used to qualitatively identify polycyclic aromatic hydrocarbon compounds present in mixtures. The specificity, sensitivity, convenience, and speed of the method indicate that it should offer an expedient means of screening for the presence of polycyclic aromatic hydrocarbons prior to analysis by other costly methods. Introduction
Polycyclic aromatic hydrocarbons (PAH) are classed as carcinogens (1)and represent 16 of the 114 organic priority pollutants (2). Incomplete combustion of fossil fuel is an important source of PAH in the environment (3). Therefore, the ability to measure PAH concentrations has become increasingly important. Historically, the qualitative and quantitative analysis of individual PAH has been of interest because the carcinogenic activity varies significantly between compounds. This has led, in particular, to the development of gas (1, 4, 5 ) and liquid (6) chromatographic methods for their separation and determination. Vo-Dinh (7,B) has used room temperature phosphorescence detection for the determination of PAH vapors in air with a passive integrating dosimeter. The necessity for measurement of specific PAH has recently been questioned, and it has been suggested that the sophisticated efforts dedicated to this are neither cost nor information effective (5,9, 10). The use of expensive GC-MS approaches, as required by U.S. EPA recommended protocol (1,2),for samples that may not contain measureable concentrations of PAH has been cited as an example of the problem (9, 10). Consequently, the development of sensitive, expeditious, and inexpensive screening methods to determine the presence or absence of PAH in samples can be identified as an objective that may effect considerably economic savings. In 1979, Thompson and Waystaff (11) measured the absorption of vapor evolved from samples as they were 338
Environ. Sci. Technol., Vol. 22, No. 3, 1988
subjected to a temperature gradient in a graphite furnace normally used for atomic absorption measurements. By monitoring the absorbance a t a single wavelength as a function of the furnace temperature, they were able to identify oils, greases, soaps, and detergents in water (11). Additional measurements at other wavelengths were often needed to enhance the qualitative specificity of the method. Tittarelli et al. (12,13) evaluated this approach for the identification of spilled crude oils. Although generally successful, the method was unable to differentiate between crude oils from contiguous fields (12, 13) owing to the broad-band natures of the spectra obtained, the small differences in the vapor pressure-temperature characteristics of such crudes, and the occasional occurrence of pyrolysis prior to the vapor-phase diffusion of the crude oil from the furnace system. However, results of these reports indicated that the electrothermal vaporizationabsorption spectrometry approach offered potential for the qualitative identification and quantitative determination of compounds having well-defined absorption bands, e.g., the PAH group (10). This paper demonstrates the use of electrothermal vaporization-multiple-wavelength spectrometry (EV-MWS) as a fast, low-cost method for determining PAH compounds. The approach involves temperature ramp vaporization of the compounds while monitoring the vapor-phase absorption over the 200-350-nm region, with a photodiode array detector (14-16). Experimental Section
Instrumentation. A modified Perkin-Elmer Model 305 spectrometer was used as the wavelength isolation and readout unit. This unit was fitted with a 1024-pixelle photodiode array (PDA) and associated readout electronics as previously described (15,16). The modification allowed the simultaneous interrogation of a selected 150-nm wavelength range with a diode-to-diode resolution of 0.15 nm and an effective resolution determined by the spectrometer entrance slit width used; typical conditions used herein provided a 2-nm resolution. This unit was equipped with a Xenon arc lamp as the spectral source (15, 16) so that absorbance measurements over the range of 200-350 nm, found to be optimal for the present purpose, could be
Not subject to U.S. Copyright. Published 1988 by the American Chemical Society
Table I. PAH Compounds Studied and Their Properties PAH
structure
molecular boiling point, 'C mass
melting point, "C
phenanthrene
178.24
101
340
fluoranthene (FLU)
202.26
111
375
202.26
149
393
228.29
197
425
228.29
162
437
chrysene (CHR)
228.29
254
450
benzo [ghi]-
276.34
278
500
coronene
300.00
pwne ( P W
triphenylene
(TRU
benz[a]anthracene,
8 A
&
(BAA) \
/
/
525
(COR)
made. The intensity of the light source and the sensitivity of the PDA were such that a 500-ms integration period was necessary between successive interrogations of the spectra acquired; this provided the best experimental compromise between detector signal-to-noise ratio and temperature resolution. The readout and control system was based on a minicomputer and ancillary facilities as previously described (13,14). The carbon furnace system was a Model CRA-90 (Varian-Techtron) operated with pyrolytic graphite supports and tubes obtained from the manufacturer. Reagents. The PAH compounds studied (Table I) were generally 9&99+% pure and were obtained from Aldrich, Columbia Organic Chemicals, or Chemical Services. Stock solutions of each at 20 g/L were prepared in spectrophotometric-grade toluene. Single compound samples were prepared fresh daily by diluting the stock solutions with toluene to obtain a final concentration of 1 g/L. Six mixtures were prepared fresh from the stock solutions a t the following concentrations: (1)pyrene (1 g/L) + coronene (5 g/L); (2) benz[a]anthracene (1g/L) + chrysene (3 g/L); (3) benz[a]anthracene (1g/L) + triphenylene (1 .g/L) caronene (5 g/L); (4) pyrene (1g/L) triphenylene (1 g/L) + caronene (5 g/L); (5) benz[a]anthracene (1g/L) + triphenylene (1 g/L) + chrysene (3 g/L); and (6) phenanthrene (1g/L) + pyrene (1g/L) + chrysene (3 g/L) + triphenylene (1 g/L) + benz[a]anthracene (1g/L) + coronene (5 g/L). Procedure. A 5-pL sample aliquot was dispensed into the graphite furnace with a syringe (Hamilton) equipped with a Teflon capillary. The samples were dried a t 100
+
+
"C for 60 s to remove the toluene, and the temperature was then ramped a t 25 deg/s to a temperature of 1000 "C where it was held for 2 s. The furnace was then fired to 2500 "C to remove anything not vaporized at the lower temperature. Data acquisition was initiated, at 500-ms intervals, 5 s prior to the completion of the dry cycle, and the average of these 10 spectral scans was used as the reference intensity (Io)for each sample run. The spectral scans were continued at the same interval throughout the temperature ramp cycle, until a total of 60 scans were collected. Although each scan involved interrogation of 1024 diodes over the 200-300-nm range, the band-pass of the spectrometer was approximately 2 nm at the slit width. Thus, the intensities recorded for each sequential set of 10 diodes were averaged to reduce the data storage requirements, without compromising the limiting resolution of the system as a whole. Each sequential scan provided that absorption spectrum integrated over a nominally 12-13 deg temperature increment. Data treatment methods will be discussed below.
Results and Discussion The eight PAH compounds listed in Table I were selected as generally representative of the group; the series selected covered ranges of melting and boiling points, molecular masses, and ring structures and included isomers. Drying temperature experiments demonstrated appreciable losses of phenanthrene and fluoranthene at 125 "C and an inconveniently extended time to remove the toluene at 75 "C. Drying a t 100 "C removed the toluene in 40-45 s without measureable losses of the analytes. Although several temperature ramp rates were explored, it was determined that the slowest available (25 deg/s) provided the best temperature discrimination for the compounds explored. The results also demonstrated that compound vaporization was complete at temperatures less than -700 "C. However, data collection to a temperature level of 850 "C was maintained as a means of checking for any delayed sample vaporization. The vapor-phase spectra of the PAH compounds were obtained with both 40- and 115-pm entrance slits. Comparison of these spectra indicated that the absorption bandwidths were sufficient to allow use of the wider slit width without appreciable degradation of spectral quality. Since use of this slit width permitted shorter integration time, with a proportionate improvement in temperature resolution, the 115-pm slit width was used. Absorbance profiles obtained for the respective compounds are presented in Figures 1 and 2. These profiles indicate the vaporization of the unpyrolyzed compounds in temperature ranges proportional to their respective boiling points, followed by their gas-phase diffusion out of the optical path. Although not readily apparent in Figures 1and 2, examination of the spectra as a function of increasing temperature revealed thermochromic (17) shifts to longer wavelengths accompanied by bandwidth broadening. For example, the main absorption band maximum for chrysene shifted up 3 nm and broadened by 1nm at the half-intensity point when the temperature was increased from 450 to 600 "C. While these thermochromic effects are not particularly large, they must be recognized as factors to be considered in the assignment and use of wavelengths for absorbance measurements made by this method. The profiles presented in Figures 1and 2 were evaluated to determine the temperatures a t which each of the absorbance profiles reached a maximum; a temperature range in which the absorbance was at or >75% of the maximum value was defined as the appearance temperature range Environ. Sci. Technol., Vol. 22, No. 3, 1988
339
0 5
I 7 IBI w
w
0
0
z
z
4
U
m
m
= 0
c
0
m
v)
m
m
a
4
00 850
2 4
0.6
w w
0
z
U m
0
z
a m
a
cc
0
0
Y)
m
v)
m
a
4
200
Figure 1.
0.0
00
850
850
Wavelenglh-temperature profiles for (A) phenanthrene, (B) fluoranthene,(C) pyrene, and (0)triphenylene.
for each. The results plotted in Figure 3 show that the temperatures at which maximum absorption appeared were linearly related to the boiling points. Regression analysis produced appearance temperature = 299 + 1.48(boiling point) (1) with a correlation coefficient of 0.9854. Note that three temperatwe mnes may be used for discrimination between groups of the present compounds. That is, for temperatures
