Environ. Sci. Technol. 1997, 31, 905-910
Matrix-Enhanced Degradation of p,p′-DDT during Gas Chromatographic Analysis: A Consideration WILLIAM T. FOREMAN* AND PAUL M. GATES Methods Research and Development Program, National Water Quality Laboratory, U.S. Geological Survey, 5293 Ward Road, Arvada, Colorado 80002
Analysis of p,p′-DDT in environmental samples requires monitoring the GC-derived breakdown of this insecticide, which produces p,p′-DDD and/or p,p′-DDE, both also primary environmental degradation products. A performance evaluation standard (PES) containing p,p′-DDT but not p,p′DDD or p,p′-DDE can be injected at regular intervals throughout an analytical sequence to monitor GC degradation. Some U.S. EPA methods limit GC breakdown of DDT in the PES to e20%. GC/MS analysis of large-volume natural water samples fortified with deuterium- and 13C-labeled p,p′DDT exhibited up to 65% DDT breakdown by the GC inlet. These matrix-enhanced GC degradation amounts substantially exceeded the 50% breakdown of the surrogate to DDD-d8 occurred (23). This amount of breakdown was well above that observed for DDT in PES injections bracketing the samples and suggested sample matrix-enhanced GC breakdown of DDT-d8. However, the possibility of surrogate degradation by biological or chemical processes in the water sample or extract prior to GC analysis could not be discounted. The objectives of the present study were to (1) confirm sample matrix-enhanced GC degradation of DDT-d8 (and DDT) in Yakima GLSE extracts, (2) compare DDT-d8 with 13 C12-labeled DDT (DDT-13C) to monitor for sample-specific breakdown of unlabeled DDT during GC analysis, (3) establish if matrix-enhanced GC breakdown occurs in other sample matrices, specifically bed-sediment extracts, and (4) evaluate whether matrix-enhanced GC breakdown of DDT could be monitored with DDT isotopes if analyzing by GC with an electron-capture detector (GC/ECD).
Experimental Section GLSE Sample Preparation. Details of the procedure for collecting, isolating, and analyzing water samples from the Yakima River basin, WA, are described elsewhere (21, 22, 24), and the occurrence of DDT in this basin was discussed by Rinella et al. (25). Briefly, filtered water samples of 4-112 L contained in stainless steel cans were fortified with p,p′-DDT2H (DDT-d , Cambridge Isotope Laboratories, Inc. [CIL], 8 8 Andover, MA) surrogate at 11-50 ng/L and equilibrated for at least 30 min prior to extraction with dichloromethane using the GLSE. The undried GLSE extracts were stored up to 5 months at 4 °C. The extracts were prepared for analysis by removal of residual water, followed by solvent exchange to toluene and reduction to 500 µL. Extracts were analyzed for 68 pesticides of various chemical class by GC/EIMS without
906
9
TABLE 2. Ions Monitored for DDT Isotopes and Breakdown Products
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 3, 1997
label
p,p′-DDT
p,p′-DDD
p,p′-DDE
none
235a
235a
237 165 243a 245 173 249a 177 224
237 165 243a 245 173 249a 177 211
318a 316 248 326a 324 254 260a 332 188
d8 13C
a
12
Quantitation ion.
additional cleanup steps and were stored at -15 or 4 °C for nearly 6 years prior to undertaking this DDT breakdown study. Bed-Sediment Sample Preparation. Bed-sediment samples collected from multiple locations in the United States were extracted in a soxhlet apparatus with dichloromethane, initially cleaned up using gel permeation chromatography, and further cleaned up and fractionated using combined alumina/silica gel adsorption chromatography. The resultant 500-µL hexane extracts initially were analyzed by GC/ECD as detailed elsewhere (26) and were stored 1-2 years at 4 °C until reanalysis by GC/MS in this study. DDT Breakdown Study Analyses. All field sample extracts were fortified immediately prior to GC/MS analysis with 2 ng/µL p,p′-DDT-13C12-ring (DDT-13C) internal standard obtained from CIL (CLM-1281, Lot EB-386, 98% chemically pure solid, 99% isotopic purity per ring position). The DDT performance evaluation standard (PES) was a toluene solution containing 1.5 ng/µL unlabeled p,p′-DDT (U.S. EPA Pesticide and Industrial Chemical Repository, Research Triangle Park, NC, Lot X401, 99.0% pure solid), 1.8 ng/µL DDT-13C, and 1.6 ng/µL DDT-d8 from CIL (DLM-3332-Q, Lot P-874, 98% chemically pure solid, 98% isotopic purity per label position). Extracts were analyzed using a Hewlett-Packard (HP) 5890 GC interfaced to a 5970A mass selective detector operated primarily in the selected ion monitoring mode. Separations were carried out on a 25 m × 0.2 mm HP Ultra 2 capillary column with a 0.33-µm film thickness. Injections of 2 µL at 220 °C were accomplished with a HP 7673A autosampler using a 1-min splitless time. The liner was a recessed gooseneck type (No. 20893, Restek Corp., Bellefonte, PA) that was deactivated using a dichlorodimethylsiloxane solution (Sylon CT, Supelco, Inc., Bellefonte, PA) and contained no glass wool unless noted. Column He flow was ca. 1.8 mL/min with a split flow of ca. 38 mL/min. Detector interface temperature was 250 °C. The GC temperature program was 60 °C for 1 min, 30 °C/min to 180 °C, 1 °C/min to 210 °C, and 4 °C/min to 295 °C. Three ions were monitored for each of the isotope analogs of p,p′-DDT, p,p′-DDD, and p,p′-DDE and were selected to minimize ion overlap for closely eluting analogs (Table 2).
Results and Discussion Labeled vs Unlabeled DDT Breakdown in PES. Figure 2 shows the percentage of unlabeled p,p′-DDT, -DDD, and -DDE observed for seven injections of the PES in relation to injection number during a GC/MS analysis of Yakima GLSE samples (Table 3). The percentage of DDT degradation (eq
FIGURE 2. Observed loss of p,p′-DDT and formation of p,p′-DDD and -DDE in injections of a performance evaluation standard (PES) containing p,p′-DDT analyzed intermittently during GC/EIMS analysis of large-volume water samples.
FIGURE 3. Formation of deuterium (d8) and carbon-13 (13C) labeled p,p′-DDD from GC degradation of isotopically labeled p,p′-DDTs in injections of the same PES as shown in Figure 2. The amount of DDE-d8 and DDE-13C found (not shown) was the same as for unlabeled DDE (Figure 2).
TABLE 3. GC Analytical Sequence, Yakima GLSE Samples injection no.
standard or sample (sample volume)a
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
DDT performance evaluation standard, DDT PES mixed pesticide standard DDT PES Yakima River at Kiona (34.5 L) Sulfur Creek (4.4 L) DDT PES well no. 1 (112 L) South Drain (10.8 L) Moxee Drain (10.9 L) DDT PES Yakima River at Union Gap (36.4 L) Yakima River at Cle Elum (108 L) Spring Creek (35.5 L) DDT PES Snipes Creek (36.4 L) Satus Creek at route 97 (109L) Cooper River (72 L) DDT PES DDT PES
a
Additional GLSE sample descriptions are provided in ref 21.
1) for the PES injections remained
