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Anal. Chem. 1992, 64, 1477-1483
On-Site Detection of Polycyclic Aromatic Hydrocarbons in Contaminated Soils by Thermal Desorption Gas ChromatographyIMass Spectrometry Albert Robbat, Jr.,' TyngYun Liu, and Brian M. Abraham Trace Measurements Analytical Laboratory, Chemistry Department, Tufts University, Medford, Massachusetts 02155
Thermal desorption gas chromatography/mass spectrometry (TDGWMS) was evaluatedfor on-site detection of polycyclic a m & hydrocarbons (PAHs). FlekCpractlcal sample cleanup proceduresand TDGWMS methods were developed for rapid, qualitative and quantitative measurement of PAHs (3-20 mln/ sample) based on selected Ion monitoring or total Ion current mass spectrometry. SIXsol1samples from a hazardous waste site were analyzed by TDGC/MS In the field and by standard EPA laboratory methods. Interlaboratory comparison revealed comparable data between the field and laboratory results.
INTRODUCTION Over the last several years, the U.S. Environmental Protection Agency (EPA) has supported efforts to develop field-practical analytical methods for the identification of priority pollutants.'-3 Toward that end, thermal desorption gas chromatography/mass spectrometry (TDGC/MS) was evaluated for on-site detection of polychlorinated biphenyls.* Methods were developed for rapid screening level PCB identification (*a1 I1 .....................................................................................
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Figure 1. Typical total ion current chromatogram of a standard PAH solution (250 ng/compound). Compound identities are listed In Table I.
for 2 min and the extract filtered using a 13-mm, 0.20-pm pore size Nylon filter. Experiments were performed by varying solvents and extraction times. A known aliquot of the extract ( 5 pL) was placed on an aluminum foil-covered petri dish and thermally desorbed as described above. The percent recovered was calculated on a dry weight basis. The PAHisoil thermal extraction efficiency was calculated by the ratio of the response factors (RF) determined directly from soil and methylene chloride. The percent recovery (solvent extracted or thermal desorption from soil),percent moisture of soil,and RF were used to calculate the final analyte concentration, C , (ngig),as shown: C, = (EA,Cis)/(Ai8RFxW,~) (1) where E is the hexane extraction or thermal desorption efficiency from the soil, A, is the target analyte SIM or TIC/SIE signal (ion current), Cis is the known concentration (nanograms injected) of internal standard, Ai, is the corresponding internal standard SIM or TIC/SIE signal, W, is the weight of soil (grams) analyzed, D is the percent dry weight (%), and RF, is the response factor, A.(s~~Ci$AisC,~a~) [Cx(std)is the standard concentration of target analyte (nanograms injected) and Ar(std)is the SIM or TIC/SIE signal of analyte corresponding to Cstd]. Chemicals were purchased and used as received. The PAH standard contained 2000 ng/pL of each PAH (Supelco,Inc.,Bellefonte, PA). Deuterated naphthalene and pyrene served as internal standards (Cambridge Isotope Laboratories, Woburn, MA). All standard solutions and sample extractions were prepared with HPLC-grade solvent (>96 % , Fisher Scientific, Medford, MA). A standard reference soil containing known quantities of PAHs was purchased from the National Institute of Standards and Testing (NIST, Gaithersburg, MD). In addition, soil samples were collected from the Hocomonco Pond (Westborough, MA) hazardous waste site. The samples were homogenized and split by the EPA Region I contractor (Federal Programs Corp., Boston, MA). The Pond and NIST soil samples were prepared as follows: (1)0.5 g of soil was weighed and placed into a 7-mL sample vial; (2) 2 mL of methylene chloride was added to the vial which was hand shaken for 2 min; (3)the extract was filtered through a 13-mm, 0.2-wm pore size filter; (4) a 5-pL aliquot of the extract was coinjected with 1000 ng of the internal standard onto an aluminum foil-covered petri dish and the mixture was thermally desorbed by using the sample probe.
RESULTS AND DISCUSSION Total ion current (TIC)and selected ion monitoring (SIM) mass spectrometry coupled with thermal desorption gas chromatography were developed and evaluated for the analysis of polycyclic aromatic hydrocarbons (PAHs) in contaminated soils. T o determine instrument performance and data quality, the linear dynamic range, minimum amount detected, and response factor stability, as well as measurement precision and accuracy, were studied. Soil samples from NIST and from a creosote-contaminated site were analyzed by several methods including TDGC/MS (on-site), GC/MS, and HPLC with fluorescence detection. TDGCiMS sample introduction and analysis for PAHs was accomplished in the following way. Organic extracts of PAHs
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Flgure 2. SIM signal (amount)vs time chromatogram of fluorene (250 ng/compound). and internal standards (naphthalene-da and pyrene-dlo) were coinjected onto an aluminum foil-covered dish. The sample probe (260 "C) was placed directly onto the foil, resulting in thermal desorption of the organics onto the column. The PAHs eluted within 350 s and were detected by either SIM or TIC. Figure 1illustrates a typical TIC chromatogram for a PAH standard mixture (250 ng/compound). The short column length and rapid temperature program limited the separation of some PAH isomers, e.g., phenanthrene/ anthracene, and chrysene/benzo[alanthracene. The instrument's upper temperature limit of 240 OC was insufficient to elute, with sufficient chromatographic peak height, PAHs whose molecular weights were greater than those shown in the figure. Figure 2 illustrates the SIM signal vs time plot for fluorene from the same standard solution used to obtain the TIC chromatogram. The left-hand y axis indicates ion current in log units and the r axis, the analysis time. The ion current chromatograms are shown in the figure for the four ions selected for monitoring. On the basis of the ion fragment relative abundances normalized to the molecular ion, the instrument records the PAHs as detected and reports the current as a log value when the four ion currents reach the same height for three consecutive scans. In addition to mass spectral compound identification, a retention window of 15 s was established, Le., t,(unknown) 5 t,(standard) h 7.5 s. Comparison of fluorene TIC and SIM retention times depicts typical runto-run variations under the same TDGC conditions. Simultaneous observation of SIM signal response for all targeted PAHs is shown in Figure 3. Cells A-J represent the four channel ion currents for each of the PAHs and internal
ANALYTICAL CHEMISTRY, VOL. 64, NO. 13, JULY 1, 1992
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Flgure 4. SIM linear dynamic range for a PAH standard soiutlon serlaily diluted from 800 to 0.1 ng/compound. Shown below are the hear regresslon parameters, minimum amount detected (MAD), and average % RSD calculated from the average signals obtained over the concentration range studied (n = 3, at each concentration).
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Flguro 3. Typlcal SIM readout of a standard PAH solution (250 ng/ compound). Table 11. Comparison of On-Column and Thermal Desorption Sample Introduction Techniques Followed by SIM MS Detection in Logarithmic Units (n = 3)
compda
on-column injection
thermal desorption
%
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PAH
slope
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5 739 101419 9 232 13 901 51 025 31 517 14 416
D E F G
intercept 44 755 110 221 -43 895 259 706 38 663 102 799 112 606
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% RSD
0.99 0.99 0.99 0.99 0.99 0.99 0.99
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15 20 21 22 14 14 12
107 87 96 120 87 96 87 87 a
See Table I for compound identity.
standards detected. The white bars indicate the background level for each of the selected ions while the black bars represent the amount of PAH ions detected. When the respective PAH ion currents are above background (S/N 2 3/1) and reach the same current levels, the compound is recorded as present in the sample. As described above, the instrument's readout for the detected PAHs is given as the log value of the recorded ion current. To evaluate thermal desorption efficiency from the aluminum foil-covered dish and PAH passage through the stainless steel membrane probe head, standards were introduced onto the GC column by TD and by on-column injection after removing the membrane. In this experiment, the PAHs were detected by SIM. The results of this study are shown in Table 11. The data suggest that greater than 85% of the PAHs enter the column after thermal desorption. Because the SIM signal is reported as a logarithmic value to one decimal point, the percent relative standard deviation (9% RSD) of the measurement is dependent on the mantissa roundoff. Figures 4 and 5 illustrate the SIM and TIC/SIE 1'inear dynamic range, respectively, for serially diluted PAH standards. Shown in the figure legends are the linear regression parameters, minimum amount detected, and the average % RSD for the average signal responses obtained for each PAH over the concentration range studied. Because the initial stock solution contained equal quantities of all PAHs, the detectable quantity for E, F, and G was twice the concentration of the other PAHs. Three repetitive experiments were performed at each concentration. For benzo[blfluoranthene,
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Figure 5. TICISIE linear dynamic range for PAH standard solutions serially dlluted from 1000 to 0.2 ng/compound. Shown below are the linear regression parameters, mlnlmum amount detected (MAD), and average % RSD calculated from the average signals obtalned over the concentration range studied (n = 3, at each concentration), PAH
slope
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92419 249232 2268176 360001 364451 6442949 4408190
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benzo[lt]fluoranthene, and benzo[a]pyrene, the peak was ill-defined. Thus, single-point calibrations were required for quantitation of these PAHs. Evident from the plots waa the 3-order magnitude in linearity for both detection modes. A lower limit of detection was found for SIM (-0.1 ng/PAH) than for TIC/SIE (- 2 ng/PAH); in contrast, the average signal % RSD for SIM was larger than for TIC/SIE. For example, the average signal % RSD over all concentrations and for all PAHs was 16% and 11% for SIM and TIC/SIE, respectively,
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 13, JULY 1, 1992
Table 111. SIM Dynamic Range of PAHe Directly Desorbed from 0.5 e of Soil (Ion Count f 5% RSD) compda
concn (ng) 4000 2000 1600 800 120 80 40 4000 2000 1600 800 80 40b 4000 2000 1600 800 80 40b 4000 2000 1600 800 120 40b 8000 4000 3200 1600 240 80b 8000 4000 3200 1600 240 8000
signal (n = 3) 510 084 f 23 255 648 f 23 210 541 f 31 110 357 f 13 28 371 f 13 5 312 f 23 1995 f 22 255 648 f 23 129 245 f 26 94 858 f 11 51 454 f 26 3 575 f 13 794 f 17 94 858 f 11 23 397 f 13 20 307 f 23 9 936 f 35 740 f 13 251 f 15 52 714 f 11 24 197 f 33 18 585 f 13 8 971 f 13 371 f 13 251 f 15 42 578 f 35 21 245 f 12 16 271 f 26 8 084 f 23 877 f 24 316 f 20 17 498 f 24 8 629 f 14 6 854 f 14 3 221 f 40 371 f 13 325 f 32
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Figure 6. Thermal desorption instrument response measured as a function of RF(soil)/RF(organic)times 100over the concentration range studied. 13%- 2068 r = 0.99
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a See Table I for compound identity. These concentrations were not included in the linear regression analysis.
a 30 7% difference in measurement precision. Nevertheless, for rapid, on-site semiquantitative analyses, SIM provides a simpler means for obtaining data to calculate PAH concentration in a sample. TIC/SIE requires software manipulation of the chromatogram, e.g., ion extraction and integration, before calculating the analyte concentration in the sample. Experiments were performed to determine the SIM linear dynamic range of PAHs directly desorbed from 0.5 g of backyard soil (i.e., very poorly sorted clay to coarse sand), minimum amount detected, and PAH thermal extraction efficiency from the soil. Samples were prepared by coinjecting PAHs and the internal standards such that PAH concentrations were between 4000 ng/PAH (8 ppm) and 40 ng/PAH (80 ppb). Table I11 demonstrates the SIM linearity as a function of initial PAH concentration injected into the soil. Note that the linear regression did not include the minimum amount detected with the exception of naphthalene and pyrene/fluoranthene. The minimum detectable amounts for PAHs A-D were 40 ng (80 ppb) and, for E and F, 80 ng (160 ppb) and 240 ng (480 ppb), respectively. Figure 6 illustrates the TDGC/SIM response as a function of (RF(soil)/RF(organic))100 over the concentration range studied. The bar graph shows high thermal desorption efficiency from soil for naphthalene and relatively low efficiency for all other PAHs. For example, the minimum amount of chrysene/ben~ ~ [ a l a n t h r a c e nrequired e for TDGC/SIM detection was greater than 4000 ng in 0.5g of soil. Moreover, it was found that TD extraction efficiency was highly dependent on soil type. Thus, thermal extraction recovery experiments should
be performed to determine PAH content in the different soil types that might be present on site. Despite the poor PAH thermal desorption characteristics, direct soil measurements by SIM provide rapid “qualitative” identification of PAHs at concentrations pertinent to the site assessment. For more quantiative determination of PAHs, soil/solvent extraction was required (see eq 1). Experiments were performed with a variety of solvents and extraction times to determine an optimum, field-practical PAH extraction procedure. It was found that 0.5 g of backyard soil and 2 mL of methylene chloride placed in a 7-mL sample vial and hand shaken for 2 min yielded PAH recoveries between 60 7% and 80 7%. PAH extraction efficiencies will also vary with soil type and soil moisture. PAH response factors were used to calculate PAH content in the sample as well as to monitor instrument performance. A t the beginning of a study, RF values were calculated for each PAH over the concentration range found linear for either SIM or TIC/SIE. From this data, an average response factor, RF,,, and 7% RSD were calculated. The instrument and/or PAH standards were considered to be in good working order if the RF,, % RSD was 130%. At the beginning and end of each day, RF values at one concentration were calculated and compared with RF,,. If the percent difference between RF, and the daily RF value was 130%, the data acquired between RF measurements were considered acceptable; i.e., the instrument yielded little drift during these time periods (Note. RF comparisons are a data quality control requirement for data acceptance employing EPA standardized methods). Figure 7 shows typical, TIC/SIE, RF percent differences obtained over a 2-week period. The line heights indicate the range of percent difference for all PAHs during each day. Days 9 and 13 required regeneration of a new dynamic range RF,,. Four other days had percent differences >30% for one PAH (see Table I for PAH identity). (Note: single-point calibrations were required for PAH group H and, thus, were not included in this study). To validate the sample preparation and TDGC/MS methods, a NIST certified marine sediment sample (SRM) was analyzed as received (4% moisture). NIST PAH concentrations were determined by high-performance reversed-phase liquid chromatography with fluorescence detection, GC with flame ionization detection (FID), and GUMS. Although the TIC/SIE and SIM measurements yielded % RSD’s less than NIST for repetitive measurements (n= 3),the latter provided excellent intermethod data precision (see Table IV). In fact, inclusion of the TDGCiMS data yielded a worst-case interlaboratory % RSD of 36 % for acenaphthylene. The percent
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