Environ. Sci, Technoi. 1984, 18, 477-482
Field Evaluation of a Cost-Effective Screening Procedure for Polynuclear Aromatic Pollutants in Ambient Air Samples Tuan Vo-Dlnh,*+ Tlmothy J. Bruewer,? George C. Colovos,' Thomas J. Wagner,* and Robert H. Jungersl Health and Safety Research Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, Rockwell International, Environmental and Energy Systems Division, Newbury Park, California 91320, PEDCo Environmental, Inc., Cincinnati, Ohio 45246, and Data Management and Analysis Division, U S . Environmental Monitoring Systems Laboratory, Research Triangle Park, North Carolina 2771 1
w The field evaluation of simple and cost-effective luminescence techniques for screening ambient air particulate samples is described. Two analytical methods, synchronous luminescence and room temperature phosphorescence, are employed to estimate the content of polynuclear aromatic species in air particulate extracts collected at two wood-burning communities. The validity and efficacy of this cost-effective screening approach are demonstrated by the good agreement between the screening data and the results obtained by detailed gas chromatography/mass spectrometry and high-performance liquid chromatography. The field results indicate that the screening process can readily discriminate between samples whose PNA content differs much less than 1 order of magnitude.
Introduction The potential health hazards associated with polynuclear aromatic (PNA) compounds present in the atmosphere are of concern because many PNA species have been found to be carcinogenic in laboratory animal tests (1). A comprehensive assessment of the potential health effects related to air pollution can be costly because it requires a long-term monitoring and analysis program designed to handle large numbers of air samples. The objective of this field study is to evaluate the efficacy of two simple rapid and cost-effective analytical techniques, synchronous luminescence (SL) and room temperature phosphorescence (RTP), for routine screening of sample PNA content. The SL and RTP techniques were used to monitor PNA compounds in outdoor ambient air samples collected in two wood-burning residential areas. This field study is an integrated effort in sampling and analysis by several organizations. Table I lists the various participating organizations and summarizes the role each played in the study. This report deals with the luminescence screening procedures developed and conducted at the Oak Ridge National Laboratory (ORNL) to rank all samples on the basis of their PNA content. The efficacy of the screening procedure was then evaluated by comparing the SL and RTP results with the data obtained independently by PEDCo Environmental, Inc., and Rockwell International using gas chromatography/mass spectrometry (GC/MS) and high-performance liquid chromatography (HPLC), respectively. Analysis of complex environmental samples can be conducted by using techniques such as HPLC ( 2 ) or GC/MS (3). These analytical techniques, however, cannot be employed on a routine and systematic basis to study Health and Safety Research Division, Oak Ridge National Laboratory. Rockwell International. 5 PEDCo Environmental, Inc. Data Management and Analysis Division, US.Environmental Protection Agency.
*
0013-936X/84/0918-0477$01.50/0
all samples due to the high cost involved. To reduce the total cost of monitoring programs and environmental assessment studies, it is often desirable to have a screening procedure to prioritize samples such that a detailed analysis needs only to be conducted on a subset of samples. This paper describes the field evaluation of such a screening procedure. The results demonstrate that the SL and RTP screening procedures constitute an effective tool for ranking outdoor and indoor air sample extracts for their PNA content and could serve to reduce the number of GC/MS or HPLC analyses needed to determine PNA compounds in ambient air.
Experimental Methods and Instrumentation Spectroscopic Techniques. Luminescence often provides one of the most sensitive analytical tools to detect PNA compounds because many PNA species are strongly luminescent ( 4 ) . The choice of two spectral parameters, i.e., emission and excitation wavelength, provides greater selectivity than absorption spectroscopy. However, in spite of the excellent sensitivity and superior selectivity, use of luminescence to analyze complex environmental samples is often limited because of spectral overlap due to the large number of PNA species in most real-life samples. Practical limitations of the applications of conventional fixed-excitation luminescence methods to complex mixtures with superimposed spectra led us to investigate a new approach based on the synchronous scanning technique. (1) Synchronous Luminescence. The theory and rationale behind the SL methodology have been described previously in detail (5-8). Only the major features of this technique are summarized here. Conventional luminescence spectroscopyutilizes either a fixed emission or fixed excitation wavelength. With synchronous excitation spectroscopy, the luminescence signal is recorded while both emission and excitation monochromators are simultaneously scanned; the wavelength interval, Ax, between the two monochromators is kept constant throughout the measurement. Improved selectivity without loss of simplicity in experimental protocol is the main advantage of this method. In general, the spectral structure of the composite mixture is more resolved because the spectra of the individual components are simplified and the bandwidth narrowed (5). The decrease of spectral overlap from various components in the mixture is another attribute of the synchronous technique. The concept of synchronous excitation can be applied both to fluorescence, i.e., synchronous fluorometry (SF), and to phosphorescence,i.e., synchronous phosphorimetry (SP). In the case of SF, the Stokes shift, i.e., the wavelength difference between the 0-0 bands in emission and absorption, determines the optimum value of AA, which is often set at 3 nm. For SP, it is the singlet-triplet energy difference that determines the optimum value of AA, which usually ranges from 150 to 250 nm (9). A noteworthy attribute of the synchronous technique is simplicity of instrumentation. The technique often
0 1984 American Chemical Society
Environ. Sci. Technol., Vol. 18, No. 6, 1984 477
Table I. Participating Organizations and Their Responsibilities organizations
responsibilities
U S . Environmental Protection Agency Tennessee Valley Authority Harvard University PEDCo Environmental, Inc. Rockwell International, Inc. Oak Ridge National Laboratory
sponsor of the study field sampling (outdoor air) at Petersville, AL field sampling (outdoor and indoor air) a t Waterburg, VT field test project coordination; GC/MS analysis HPLC and TLC analysis development and field evaluation of SL/RTP screening protocol
requires no additional equipment other than an inexpensive switch for most available spectrometers currently used in conventional fixed-excitation measurements. Several instruments with interlocking capability of two monochromators as a standard feature are now commercially available (10, 11). (2) Room Temperature Phosphorimetry. The phosphorescence signal of PNA compounds in solution or in the gas phase is extremely weak a t ambient temperatures. Due to its long lifetime s to several seconds), the phosphorescence emission is completely quenched by collisions with surrounding molecules present in solutions or in the air. Intramolecular vibrational and rotational deactivation processes are also possible phosphorescence quenching mechanisms. Therefore, conventional phosphorimetry requires the use of low-temperature matrices to reduce the collisional quenching mechanisms and radiationless deactivation processes. Due to the requirement of cryogenic equipment and refrigerant, conventional phosphorimetry has limited usefulness for routine applications in field measurements. Unlike conventional phosphorimetry, RTP is a technique based on the phosphorescence from organic compounds adsorbed on solid substrates at room temperature. It has been shown that the adsorbed state of the analyte molecule on the solid substrate results in increased molecular rigidity, thus reducing collisional and rotational deactivation. Although a variety of solid substrates such as silica, alumina, paper, and asbestos can be utilized, filter paper is normally used because of its convenience, low cost, and applicability to a large variety of PNA species. The RTP technique has recently been reviewed (12-15). A process that provides an invaluable aid to RTP analyses is the enhancement of the phosphorescence emission by the heavy-atom effect which increases the population of the triplet state. This technique improves both the selectivity and the sensitivity of RTP analyses (16). Several heavy-atom salts such as thallium acetate, lead acetate, cesium iodide, and sodium bromide have been found to be efficient in selectively enhancing the phosphorescence quantum yields of PNA compounds (16). With heavyatom perturbers, the limits of detection can often be decreased by several orders of magnitude. The detection limits of the PNA compounds investigated are in the nanogram and subnanogram range. Instrumentation. The instrument used for SL and RTP measurements was a Perkin-Elmer spectrometer (Model MPF-44A) equipped with an R-777 photomultiplier tube. A 150-W xenon lamp was used for excitation. Standard quartz cells were used for SL measurements of liquid samples. Special laboratory-constructed sample holders were used for RTP measurements (17). These interchangeable, finger-type RTP sample holders allow frontal excitation and right-angle detection geometry. Reagents and Materials. All the PNA compounds used in this work were purchased at the highest purity available and used as received. When in doubt, their purity was checked by fluorescence measurements. The solvent 478
Environ. Sci. Technol., Vol. 18, No. 6, 1984
used in the preparation of all samples was absolute ethyl alcohol (Aaper Alcohol and Chemical Co.). The heavyatom salts for RTP analysis were commercially available and used in ethanol-water mixtures (volume ratio (1:l). Commercial filter paper (Schleicher & Schuell, type 2040A) was used as the RTP sample substrate. Over 50 field samples were received from PEDCo for this study. These samples were ambient air particulates collected in two wood-burning communities in Petersville, AL, and in Waterburg, VT, by two sampling teams from the Tennessee Valley Authority and Harvard University, respectively. The SL technique was used to investigate the photochemical stability of the field samples under study. Two randomly picked field samples, labeled CG654 and CG547, were monitored by SL for a period of 43 days. No significant sample decomposition appeared to occur in these two samples. Screening Protocol. The full details of the protocol for screening procedures are described elsewhere (18). Only the major features are given here. The four major steps involved in the protocol are (1)serial dilution of the sample extracts, (2) SF measurement, (3) RTP measurement, and (4) ranking the samples. The experimental conditions are as follows: Serial dilution: samples diluted lo-, 30-, and 100-fold in ethanol. Sample preparation: delivery of liquid samples into fluorescence quartz cell, 0.5 min/sample; spotting and drying RTP sample (10 min for 20 samples), 0.5 min/ sample. SF measurement: wavelength interval A i = 3 nm; spectrometer band-pass = 1.5 nm; spectral range 260-500 nm; scan speed 120 nm/min; total scan time = 2 min/ sample; instrument optimization 1 min. RTP measurement: fixed excitation at 290,343, and 390 nm; spectrometer band-pass = 5 nm; scan speed = 120 nm/min; total scan time = 2.5 min/sample; instrument optimization and sample orientation 2 min. Total time: total time for SL and RTP screening (after serial dilution) -8.5 min/sample. A simple computer program was developed to calculate the ranking index for SL and RTP screening. This program receives as inputs the peak height intensities of a previously determined number of emission bands, the sensitivity factor of the detector, the dilution factor of the sample, and the peak height of a given band of a known standard reference sample. These peak height intensities are then corrected to the reference standard, normalized to the unity sensitivity scale of the spectrometer and original dilution (1:l) of the sample, and summed. The ranking program can also correct for spectral artifacts such as filter effects and energy transfer commonly encountered in luminescence measurements. The summed value of the peak height for each sample is stored in a one-dimensional array and used as a basis for ranking. Because most of the PNA compounds fluoresce and/or phosphoresce, the rationale of this ranking protocol is based upon the idea that the higher the total peak height of the SL and RTP bands,
ROOM TEMPERATURE PHOSPHORESCENCE SPECTRA OF T H E STANDARD REFERENCE MIXTURE
Table 11. Experimental Conditions for Detecting Various PNA Compounds in the Standard Reference Mixture compound anthracene benzo[a]anthracene benzo[a] pyrene benzo[e]pyrene chrysene fluoranthene phenanthrene perylene pyrene triphenylene
SF, nmb AX AX AX AX AX AX AX AX AX
=3 =3 = 15
= 45 = 30 =3
=3 =3 = 35
RTP, nmc ND" h = 287 A = 380 X = 330 X = 318 X = 360 X = 295
ND h = 334
X = 370
" N D = not detected. bAX = wavelength interval between emission and excitation. X = excitation wavelength.
700 350
350
700 400
750 480
740
WAVELENGTH ( n m )
the more concentrated the samples are in PNA species. The SL ranking index is a relative number proportional to the total peak height of the major SL emission bands. The RTP ranking index is obtained as a relative number proportional to the average peak height of RTP emission by using three excitation wavelengths to excite most PNA compounds of interest. The relative standard deviations of the data obtained were typically i 5 % and i 1 5 % for SL and RTP, respectively. Results and Discussion SL and RTP Spectra of Multicomponent Mixtures. Measurements were conducted with individual PNA compounds and also with a standard reference mixture prepared to investigate the performance of the SL and RTP techniques. The 10 PNA compounds in this reference mixture were anthracene, benz[a]anthracene, benzo[a]pyrene, benzo[e]pyrene, chrysene, fluoranthene, phenanthrene, perylene, pyrene, and triphenylene. These compounds were selected because of their frequent occurrence in field samples. Initial measurements were performed on the individual compounds to check their purity and to determine the optimal conditions for their detection. Table I1 shows the optimum conditions under which these 10 reference compounds can be detected by synchronous fluorescence and RTP measurements. As shown in Table 11, a single scan by synchronous fluorescence using AX = 3 nm can preferentially detect anthracene, benz[a]anthracene, fluoranthene, phenanthrene, and perylene. Other AX values were employed to monitor benzo[a]pyrene (AX = 15 or 7 nm), benzo[e]pyrene (Ai = 45 nm), chrysene (AX = 30 nm), and pyrene (AX = 45 nm) with higher sensitivity. Except for anthracene and perylene, the selected PNA compounds could be detected by RTP using various excitation wavelengths between 280 and 370 nm. Fixed-excitation RTP measurements were conducted by using broad-band-pass (15 nm) excitation at 290,343, and 390 nm. Since the RTP excitation (absorption) bands of PNA compounds are generally broad (- 20-40 nm), these three excitation wavelengths excite a large variety of PNA compounds, including most of the 10 reference compounds. Figure 1 illustrates some examples of RTP spectra that show the characteristic peaks for triphenylene (TRIP), phenanthrene (PHEN), pyrene (PYR), chrysene (CHRY), fluoranthene (FLUOR), benz[a]anthracene (BAA), and benzo[e]pyrene (BEP) that were present in the reference mixture. Anthracene and perylene, which have no or extremely weak phosphorescence emission, were easily detected by their intense SF bands at 380 and 440 nm, respectively. These preliminary measurements of the
Figure 1. Room temperature phosphorescence spectra of the standard reference mixture using various excitation wavelengths. TRIP = triphenylene; PHEN = phenanthrene; FLUOR = fluoranthene; BEP = benzo[e]pyrene; BAA = benz[a]anthracene; PYR = pyrene; CHRY = chrysene. i
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300
400 WAVELENGTH
500
600
1 nm)
Figure 2. Synchronous luminescence (fluorescence)profiles of several outdoor air samples of the first series.
standard mixture underscored the complementary nature of fluorescence and phosphorescence in the detection of PNA species. Principle of Qualitative Screening for Outdoor Air Sample Extracts. Figures 2 and 3 illustrate the principle of SL screening ( A i = 3 nm) for six samples of outdoor air samples collected by TVA. The background signal of the solvent blank is given in Figure 3d for comparison purposes. Rapid examination of Figures 2 and 3 led to the qualitative ranking of CG654 > CG543 > CG626 > CG634 > CG573 > CG656. Sample CG654 had the highest PNA content since a 50-fold diluted sample exhibited an SL emission intensity equivalent to that of a 10-fold diluted solution in other samples. It is interesting to note that samples CG543 showed three emission bands between 400 and 500 nm similar to those in sample CG626 with some additional bands at the spectral region .
L
CG 5 7 3
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(0
z
2 W
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a
0
400
500
600
700
WAVELENGTH ( n m l
Figure 4. Room temperature phosphorescence spectra of the first air particulate samples series. 300
400 WAVELENGTH (
500 nml
600
Figure 3. Synchronous luminescence profiles of several outdoor air samples of the first series.
than sample CG543. As shown in Figure 3, the ranking for samples CG634 > CG573 > CG656 is also straightforward on the basis of the total intensity of the SL spectra. Figure 4 shows the RTP spectra of the six samples collected by TVA. According to their RTP intensities, these samples were ranked as follows: CG654 > CG543 >, CG634 > CG626 > CG573,> CG656. Table I11 compares the SL and RTP screening results with data obtained by PEDCo using GC/MS measurements. The SL/RTP ranking results were in good agreement with GC/MS data for the six samples from this first field sample series. Ranking Index for PNA Content in Outdoor Air Particulate Samples. A series of seven samples collected at the Petersville community was screened and ranked according to their PNA content by using the same SF and RTP procedures described previously. Table IV gives the results of ranking for these seven field samples by SF and RTP. The SL ranking index is a relative number proportional to the total peak height of the six SL emission bands monitored at 316,336,360,404, 434, and 472 nm. The RTP ranking index is obtained as 480
Environ. Sci. Technol., Vol. 18, No. 6, 1984
a relative number proportional to the average peak height of RTP emissions by using the separate excitations at 290, 343, and 390 nm. These three wavelengths were found to excite most of the PNA compounds of interest to this study. These samples were also independently analyzed by PEDCo and Rockwell International using GC/MS and HPLC, respectively. The results obtained by SF/RTP are compared with those obtained by GC/MS and HPLC in Tables V and VI. The comparative results are plotted in Figures 5 and 6. These data further underscore the close correlation between the screening results and the data obtained by detailed GC/MS and HPLC analysis. This demonstrates the efficacy of SF/ RTP screening procedures to provide a ranking based solely on the total intensity of the luminescence profile and without requiring any physical sample separation. Screening Indoor Air Sample Extracts. One series of samples investigated in this field study dealt with extracts from four indoor air samples. These samples were collected by the Harvard University sampling team at Waterbury. The SL and RTP screenings were the same as those previously described and provided the ranking results given in Table VII. The PNA content in these samples could not be determined either by GC/MS or by
3
Table IV. Calculation of the SL and RTP Ranking Index for Outdoor Air Sample Extracts
sample no.
tp
SL ranking indexb
CG612 CG532 CG620 CG615 CG623 CG638 CG637
28.9 18.1 9.2 40.0 22.3 7.2 11.2
30 20 10 40 20 5 10
SL intensi-
RTP intensity""
RTP ranking indexd
59.31 8.03 8.84 28.35 24.80 16.07 17.12
e 10 10 30 25 20 15
SL/RTP rankingb
0.02 0.01 0.09 0.10 0.10 0.29
0.02 0 0.06 0.07 0.05 0.20
0.03 0 0.08 0.07 0 0.11
0.05 0 0.23 0.22 0.27 0.19
0.93 0
0.89 0.08
0.15 0.10
0.08 0
0 0
0 0
70
40
30
30
0.29 25
I
1
I
1
I
I
PEDCo data. ORNL data (see sum of ranking indexes on Table IV).
HPLC because of sensitivity limitations of these analytical methods. The RTP signals of these samples were at the background level for nonconcentrated (1:l dilution) samples. The fluorescence technique, however, was sensitive enough to detect SL emission above the solvent background even with a 4-fold dilution of the samples collected indoors. Conclusion The data obtained by SL/RTP screening and their good
o4
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Figure 5. Correlation between the SLIRTP screening results and GCIMS data. n
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AIR SAMPLE NO 1 CG612 2 CG615 3 CG623 4 CG532 5 CG637 6 CG638 7 CG620
07
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03 05
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20 30 40 50 60 70 80 SF/RTP RANKING INDEX ( o r b i t r a r y u n i t s )
10
0
0
0.04 0.03 0.21 0.23 0.12 0.28
0.48
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0
W
0.09 0.05 0.16 0.15 0.44 0.36
1.88 0.86
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02
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GC/MS analyses, pg/mLa CG- CG- CG- CG- CG- CG615 623 532 637 638 620
2.18
total content
I
AIR SAMPLE NO.
Table V. Correlation between GC/MS Data and SL/RTP Ranking for Outdoor Air Sample Extracts
phenanthrene anthracene fluoranthene pyrene benzo[a] anthracene chrysene and/or triphenylene benzo[a]pyrene perylene
I
03
"Peak intensity normalized to a sample dilution of 1:l and to a detector sensitivity of 1. bThe SL ranking index is the rounded value of total intensity of the peaks a t 316, 336, 360, 404, 434, and 472 nm. The average R T P value is taken at one-third of the sum of the intensities of the three spectra using excitation at 290,343, and 390 nm. dThe R T P ranking index corresponds to the rounded value of the average intensity (one-third of total intensity). e Results not quantifiable because of blank contamination.
compounds
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40
l
l
j
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20 30 40 50 60 70 80 SF/RTP RANKING INDEX ( o r b i t r a r y unlts)
j
l
90
100
Figure 6. Correlation between the SLIRTP screening results and HPLC
data.
correlation with GC/MS and HPLC results indicate that the combination of these two luminescence techniques can provide a good estimate of the content of the major PNA species in air samples. Field evaluation conducted with other air samples from a coal conversion facility further underscores the efficacy of the SL/RTP screening approach (19). This study also shows that the sensitivity of the luminescence screening techniques is quite suitable for ambient outdoor and indoor air samples. Most RTP and SL measurements used 1-10-fold and 5-50-fold diluted samples, respectively, whereas GC/MS generally required
Table VI. Comparison between SL/RTP Ranking Results and HPLC Data (ng/mL) HPLCa compound benzo[a]pyrene benzo[k] fluoranthene dibenz[a,h]anthracene diindeno[g,h,i] perylene fluoranthene pyrene total SL/RTPb ranking index
CG532
CG612
CG615
CG620
CG623
247 280 100 320 200 100
