478
Anal. Chem. 1982, 5 4 , 478-481
Determination of Polychlorinated Biphenyls in Transformer Oils by Capillary Gas Chromatography Robert J. Gordon," Joseph Szlta, and Edward J. Faeder' Global Geochemistry Corporation, Canoga Park, California 9 1303
A procedure for reliable routine analysis of polychlorinated biphenyl (PCB) mixtures in mineral oils was developed. An acetonltrlle/hexane partition followed by passage In hexane through a prepacked Cartridge of slllca serves to remove bulk mineral oil and most Interferences. Gas chromatography (GC) through a capillary slllca column with electron capture detection allows PCB peaks to be dlstlngulshed from other chlorinated species (prlncipaily pesticides), and patterns for various Aroclors (standard PCB mixtures) are readily recognized. A rigorous glassware cleaning procedure Is also required. Recoveries of PCBs from spiked oils were determined at five concentration levels. Several hundred samples have been analyzed.
Presently, there is no single generally accepted or official analytical method for determining PCB mixtures in insulating oils. Methods have been reported for determining PCBs in air, water, soil or sediment, and biological samples (1-4). A proposed method for determining PCBs in petroleum oils is on trial by the US. Environmental Protection Agency (Contract 68-03-3006, Environmental Monitoring and Support Laboratory, Cincinnati, OH). The methods used for determining PCBs in other media are not readily adaptable to used oil matrices because of potential differences in the interferences and the similarity in many properties for both mineral oils and PCBs. The study described in this paper was undertaken to develop a reliable and simple method for determining PCBs in transformer oils, because widely discrepant results were found when replicate samples were submitted to several independent analytical laboratories. Although packed-column GC is most commonly used for routine PCB determination, two factors led us instead to use capillary columns. First, any specific interfering compound surviving the cleanup procedure (e.g., a chlorinated pesticide) has less chance of overlapping any PCB peak, because of enhanced resolution with the capillary column. Second, more detailed and easily recognized patterns from the component peaks allow better resolution of specific Aroclors, especially in mixtures. The column lengths used were shorter than those typically used to determine individual PCB isomers ( I , 5-8) but several-fold longer than the usual packed columns. Electron capture detection has very high sensitivity to compounds containing chlorine and gives relatively very weak response to most hydrocarbons. It is therefore the most generally used detector. The Hall conductivity detector can be made even more specific for halogens than the electron capture detector, but (presently) it is considerably less sensitive and not so commonly available. GC/MS is likewise very specific, but much more expensive for routine use. Various types of cleanup procedures for dirty oils have been reported, including solvent extraction, acid treatment, and Address: Southern California Edison Co., Rosemead, CA. 0003-2700/82/0354-0478$01.25/0
use of various adsorbents. Evaluation trials of a number of these led to the procedure described here. It is intended to be applied routinely and to remove the bulk of the mineral oil and most other interfering materials. EXPERIMENTAL SECTION Precaution in Handling PCBs. High stability and extreme persistence of PCBs necessitate proper safeguards against exposure by skin contact or ingestion. Low vapor pressure makes the danger of inhalation unlikely except when overheating of the materials occurs. The major precautions required are avoidance of skin contact by use of protective clothing, gloves, and fume hoods and adequate personal hygiene after handling. The extreme stability of PCBs, combined with characteristics of high viscosity and ability to wet glass, makes it necessary to take special precautions in cleaning glassware. The cleaning sequence consisted of: solvent rinse, chromic acid soak, exhaustive water rinses, final solvent rinse, and bakeout at 350-400 "C. All solvents used were pesticide grade distilled in glass. Interference Removal. LiquidlLiquid Partition. Into a 50-mL conical centrifuge tube with a Teflon-lined screw cap, a weighed 1.0-mL portion of the oil and 15-mL portion of a mixture of 10% hexane in acetonitrile was added. The tube was then shaken for 30 s and the phases were allowed to separate; centrifugation was carried out, if necessary, to complete the separation. Most of the oil, with some hexane, remained separate and was carefully withdrawn by syringe and discarded. Ten milliliters each of water and hexane were added to the acetonitrile-rich tube contents. The mixture was shaken and separated as before, whereby PCBs were concentrated into the hexane-rich phase. The acetonitrile/water layer was drawn off and discarded. The hexane layer was evaporated gently in a water bath t o a volume of not less than 1 mL. Adsorbent Column Treatment. Preliminary trials, using spiked oils, were made of the adsorbents Florid, silica, and alumina. Subsequently the use of commercial disposable preloaded silica cartridges ("Sep-Pak", Waters Associates) were found to be satisfactory. The final procedure was to load the hexane sample solution (from acetonitrile partition) into a glass syringe and inject it through the cartridge, followed by a 10 mL additional hexane. The combined hexane washings were then evaporated to a small volume on a water bath and made up to an exact volume in hexane. Determination of Recoveries. Oil samples which were found not to contain detectable PCBs were spiked individually with each of the three Aroclors (1242,1254, and 1260) at three concentrations. These samples were run in triplicate through the final partition and adsorbent treatments above. Gas Chromatography. The column used was a HewlettPackard 12.5-m capillary fused silica column, cut from a 50-m column. The stationary phase was OV-101 (methyl silicone) deactivated with Carbowax 20 M. This was mounted in a Varian 3700 chromatograph equipped with a "Ni electron capture detector and a splitless septum injector. The injector was held isothermal at 290 "C, the detector at 310 "C, and the oven programmed from 150 to 275 "C at 6 "C/min. The electrometer output was fed into a Hewlett-Packard integrator for printout. Verification by Other Methods. Three samples of transformer oil containing various amounts of Aroclors (up to about 120 ppm), analyzed by capillary GC with electron capture detection before and after acetonitrile/silica cleanup,were submitted to an independent laboratory (PJB Laboratories, Pasadena, CA) for direct analysis by packed column gas chromatography with 0 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 3, MARCH 1982
l 5
50 100 Aaoclor 1242 spiked, ppm
10
Flgure 1. Recovery of Aroclor 1242 spiked into transformer oils: (solid
line) new oil; (dashed line) used oil. (Vertlcal bars show ranges of triplicate analysls.)
5oo 100
io 50 100 Aroclor 1260 spiked, ppm
5
500
479
500
Flgure 3. Recovery of Aroclor 1260 spiked with transformer oils: (soli
line) new oil; (dashed line) used oil. (Vertical bars show ranges of triplicate analysis.)
r
50
P N 0
2
4
6
8
10
12
14
16
18
20
t,,min
L
' 5
Figure 4. Gas chromatogram of Aroclor 1260, spiked 500 ppm in used oil, diluted 500 times In hexane after cleanup.
50
100
500
Aroclor 1254 spiked, ppm Flgure 2. Recovery of Aroclor 1254 spiked into transformer oils: (solld line) new oil; (dashed line) used oil. (Vertical bars show ranges of
triplicate analysis.)
Hall conductivity detection. A sample of Aroclor 1254 reference solution was also submitted. All samples were coded. An untreated oil sample containing about 120 ppm Aroclors, determined by capillary GC with electron capture detection was submitted to the mass spectrometer facility at the UCLA Institute of Geophysics and Planetary Physics. GC/MS analysis was performed with a Finnigan 4000 mass spectrometer. Computer reconstruction of mass chromatograms and several specific ion fragment spectra was carried out using an INCOS system. Calculation of PCB Content. The specific Aroclors present were identified by inspection and comparison with reference standards. A rough determination of the amount present was made by calculating the ratios of various single peaks with a p propriate standards-this also served to indicate the presence of more than one Aroclor or an interference. As retention times vary slightly from one chromatogram to the next, the pattern of relative peak heights was used to determine correspondence between peaks in the sample and those in the reference standard. The sum of a set of three peaks was selected for each of the common Aroclors (1242,1254, and 1260),which in most cases minimized the overlap between them. All peaks were ratioed to the peak height of the internal standard (Aldrin) before calculation of concentrations. Calculation of Aroclor content in the oil sample, corrected for recovery in cleanup, was made from the curves shown in Figures 1-3, derived from spiked samples. When mixtures of two Aroclors
occur, some peaks used in calculating each Aroclor may overlap. There were corrected iteratively using ratios from nonoverlapping peaks.
RESULTS AND DISCUSSION Liquid-liquid partition removes both interfering compounds and the bulk of the oil itself. (Large amounts of oil in the sample may overload the capillary column.) The acetonitrile-hexane partitioning was effective in most cases in removing many of the broad peaks causing interferences in the GC. The remaining cases could almost always be cleaned with a brief adsorbent treatment after partitioning. Adsorbent alone was somewhat less effective in cleanup, and in order to remove the bulk of the oil, the solvent-toadsorbent ratio and the adsorbent activity had to be controlled closely. The combination of partition with adsorption was less sensitive to minor variations in these factors and gave a cleaner product. Further optimization is undoubtedly possible, but a t present the major contributions to variance in the results come from the ability to make precise separation of phases and the ability to reproduce the other handling operations. An example of a chromatogram of Aroclor 1260 from a spiked used oil treated by the cleanup procedure is given in Figure 4. An initial test of repeatability for the acetonitrile/silica method was conducted. Four oils were sampled twice each a t differing times. Each sample was run in triplicate. Two samples had no detectable PCB content; the others contained Aroclor 1260 in the amounts shown in Table I. Used oil A
480
ANALYTICAL CHEMISTRY, VOL. 54, NO. 3, MARCH 1982
Table I. Sampling Repeatability
Table 111. Summary of Triplicate Spiked Oil Analyses
ppm of Aroclor 1260 in oil new used used used analysis replicate sample oil oil A oil B oil C oil
1 2 3
1 2 1 2 1 2
mean std dev a
nda nd nd
nd nd nd nd
26.1 25.3 25.5 28.5 27.9 26.7
nd nd nd nd nd nd
26.7 1.30
nd
8.3 10.9 9.4 9.1 9.6 7.1 9.07 1.28
nd = not detected.
Table 11. Comparison of PCB Analysis of Transformer Oils with Electron Capture and Hall Condwtivity Detection
sample description used oil D, as is used oil D, after cleanup used oil E, as is used oil E, after cleanup used oil F, as is reference Aroclor 1 2 5 4 , 1 2 ppm a
ppm of Aroclor found in oil electron Hall capture conductivity Aro- AroArOclor clor Aroclor clor 1260 1254 1260 1254 nd
24
4
2
91
14
nda nd
23 10
7 (as 1260) nd 126 18
nd
nd
nd = not detected.
and used oil C showed a standard deviation of 1.3 at levels of 26.7 and 9.1 ppm, respectively. (On the basis of color and chromatograms run on the untreated oils, oil A was the cleaner, and oil C was the dirtier of the two.) Cleanup recoveries were determined as follows: Samples of a new transformer oil (previously found not to contain PCBs) were spiked separately with Aroclors 1242, 1254, and 1260, at five concentrations (5, 15, 50, 150, and 500 ppm). Similarly, a PCB-free used oil was spiked at 5, 50, and 500 ppm levels. The results for triplicate runs using the cleanup procedure are plotted on a log-log basis, with confidence limits, in Figures 1, 2, and 3. Recoveries of the less volatile, more heavily chlorinated Aroclors 1254 and 1260 were in the range of 72-101 % for the various concentration levels, with standard deviations around 1-4% for replicates at each concentration. The more volatile Aroclor 1242 showed poorer recovery, 43-75% with standard deviations up to 8%. Verification of the procedure was carried out with two referee methods which are applicable to samples without cleanup: GC with the Hall conductivity detector, and GC/MS. Neither method was as sensitive as GC with electron capture detection. Samples were supplied to outside laboratories for PCB determination. Results for comparative tests with Hall detection are shown in Table 11. There was agreement to within -10 ppm. In the GC/MS comparison, a used oil found by electron capture to contain 100 ppm Aroclor 1254 and 17 ppm Aroclor 1260 was supplied without cleanup, along with reference Aroclor standards. The oil was run, after dilution in hexane, on the GC/MS. Patterns for Aroclors 1242 and 1260 were not apparent in the oil chromatograms. The Aroclor 1254
ppm of Aroclor found, mean, and std dev of triplicates at spike level 5 15 50 150 500 all P P ~P P ~ppm ppm ppm data New Oil Aroclor 1242 2.24 10.23 37.4 113.3 346 std dev 0.18 1.10 1.8 6.1 36 Aroclor 1254 4.00 13.4 39.8 133 391 std dev 0.16 0.14 3.2 1.2 23 Aroclor 1260 3.50 13.8 50.1 136 384 std dev 0.05 0.29 1.9 0.6 16 Aroclor 1242 std dev Aroclor 1254 std dev Aroclor 1260 std dev overallstd dev overall relstd dev, %
2.09 0.14 4.28 0.18 3.78 0.23 0.17 5.5
Used Oil 31.1 1.9 40.4 0.7 46.6 0.8 0.67 1.9 6.4 4.9
298 34 390 23 385 7 3.6 25 7.4 5.8 3.2
content in the oil was estimated at -200 ppm. These runs were made in the scanning mode. Use of selected characteristic peaks greatly improves the sensitivity in this method. Analytical Uncertainties. Figures 1-3 show that the recoveries with spiked samples vary with both the Aroclor present and the level of the spike. They are generally in the neighborhood of 70-80%. Recoveries in triplicate runs are repeatable to the degree shown in Table I11 by the standard deviations, which vary with the concentration level from 0.17 ppm at a concentration of 5 ppm, to 25 ppm at a concentration of 500 ppm. The relative standard deviations for each concentration level and for the entire set are shown as percentages of means. The relative standard deviation for the entire set is 5.8%,and those for the various concentration levels are in reasonable agreement with this. The uncertainty of single analytical results, taken as three times the standard deviations, would vary from f0.51 to f75 ppm across the range studied; in relative terms this is f17% overall. At the 50 and 500 ppm regulatory levels, this is f9 and f87 ppm, respectively. In Table I by comparison, the variation in results for used oil A is well below f17%, whereas for used oil C, it slightly exceeds f17 % . No significant differences have been observed by us among Aroclor standard solutions purchased from three different suppliers to date. Both patterns of peak ratios and labeled concentrations appear to be in agreement among suppliers, based upon limited side-by-side comparisons. When closely spaced interference peaks are present, spiking with the Aroclor believed to be present helps to confirm and quantify the amount. It is essential to establish detailed standard methods for cleaning the apparatus and for sample processing (cleanup and dilutions) and to adhere closely to these procedures. Surface active properties and sometimes high viscosity of PCBs may make it difficult to remove films from glass and difficult to make clean phase separations. The method has been applied successfully to the analysis of several hundred transformer oil samples (9). ACKNOWLEDGMENT Steven A. Jones and Shu-Teh Pan assisted in this project. LITERATURE CITED (1) Hutzinger, 0.; Safe, S.; Zitko, V. "The Chemistry of PCB's"; CRC Press: Boca Raton, FL, 1974 Chapters J 2 and 13G. (2) US. Environmental Protection Agency. A Method for the Sampling and Analysis of Polychlorinated Biphenyls (PCB's) in Ambient Air,
48 II
Anal. Chem. 1982, 5 4 , 481-485
(3) (4) (5) (6)
EPA/600/4-78/048; PB-288410 from Natlonal Technical Information Service, US. Department of Commerce: ,,Springfield, VA, 1978. US. Environmental Protection Agency. Proposed Rules: Organochlorine Pestlcldes and PCB's - Method 608" Fed. Regist. 4874, 44 (No. 233,Dec 3) 69501. Sawyer, Leon D. J. Assoc. Off. Anal. Chem. 1978, 6 7 , 272-281, 282-241. Krupcik, J.; Leclercq, P. A.; Slmova, A.; Suchanek, P.; Collak, M.; Hrlonak, J. J. Chrurnatogr. '1978, 119, 271-382. Krupclk, J.; Leclercq, P. A.; Garaj, J.; Simova, A. J. Chrornatogr. 1980, 191, 207-220.
(7) Neu, H. J.; Zell, M.; Ballschmlter, K. Fresenlus' 2.Anal. Chem. 1978, 293, 193-200. ( 8 ) Ballschmiter, K.; Zell, M. Fresenlus' 2. Anal. Chem. M80, 302, 20-3 1. (9) Saperstein, M. D.; Gordon, R. J.; Faeder, E. J. J. Environ. Sci. Health, Part A , Environ. Scl. Eng., in press.
for review IMarch
25, lg8'*Accepted
12, 1981.
Attogram-Level Detection and Relative Resplonse of Strong Electrophores by Gas Chromatography with Electron Capture Detection Jeffrey A. Corkill, Marhus Joppich, Simon H. Kuttab, and Roger W. Giese" Department of Mediclnal Chomlstry In the College of Pharmacy and Allied Health Professions and Institute of Chemical Analysis, Northeastern Un;vers&, Boston, Massachusetts 02 1 15
The responses as peak areas of some divergent compounds, most of which are strong electron absorbers, are measured by gas chromatogrirphywith electron capture detection (GCECD). The most sensitivo compounds aro derlvatlred iodothyronines, which are esslentially 20-fold more sensitive than lindane. N,N-Dlpentafluorobenroylpentafluoroanlllne, a somewhat less scnsltive but more volatlie substance, was selected for determinationof a detection limit. The value was 90 ag (1.6 X lo-'' mol), largely due to an anomalous increase In Its response at the trace level. This Increases the reported sensitivity of GC-ECD by 100-fold.
Electrophores iin the vapor state are molecules or substituents which absorb thermal electrons (1,2). This type of electron uptake takes place in the electron capture detector in both gas chromatograplhy (GC-ECD, e.g., ref 3) and liquid chromatography (4),and also negative chemical ionization mass spectrometry (5,6).Relatively few molecules are both volatile and intensely electrophoric. The relative response ad different electrophores has been studied previously, as has been reviewed (3,7,8),but this work can be extended in several respects. Only a limited variety of electrophoric structures were often examined in any particular study. Certain interesting molecules were not included, such as the derivatized iodothyronines (9),or were not compared with other electrophores, such as flophemesyl (7). Various types of EC detectors and GC-ECD conditions were involved, which cain influence the response (8). Thus, it is useful to conduct (a sensitivity study of strong electrophores under a set of conshnt and typical conditions. The conditions employed here are capillary GC with a fused silica column and a constant current, variable frequency, small volume ECD. The purpose of this study is to defiie the types of structures which are most sensitive as well as volatile by GC-ECD, in order to guide the future development of improved derivatizing agents for GC-ECD and related techniques. The main conclusions reached here are that (1)the derivatized iodothyronines are the most 3ensitive but least volatile of the compounds examined, (2) the flophemesyl group, which has 0003-2700/82/0354-0481$01.25/0
been employed as a derivatizing group for analyses by GCECD (7), is not highly sensitive, at least for the derivatization of alcohols, (3) the lpentafluorobenzamide group (penta fluorobenzoylamide) offers a good combination of sensitivity, volatility, and chemical accessibility for incorporation into potential derivatizing agents, and (4)the response for N , N dipentafluorobenzoylpentafluoroanilineincreases at the tract! level, yielding a detection limit for this compound of 90 ag. EXPERIMENTAL SECTION Materials and Met hods. Methyl pentafluorobenzene, p e n tafluorostyrene, pentafluoroiodobenzene, pentafluoroacetophenone, decafluorobenlzophenone,pentafluorobenzoyl chloride, and pentafluoroaniline were obtained from PCR Research Chemicals, Inc. 1,4-Diibromobenzene, l-octanol, ethylamine, pentachlorobenzene, 1-butanol,2,4,6-triicdophenol,glycine methyl1 ester hydrochloride, @alanine, 2,5-diiodobenzoic acid, 3,5-dibromotyrosine, 3-iodotyrosine, 3,5-diiodotyrosine, 3,5-diiodothyronine, 3,3',5-triiodothyronine, thyroxine, trifluoroacetic an. hydride, pentafluoropropionic anhydride, and heptafluorobutyric anhydride were obtained from Aldrich Chemical Co. Flophemesyl chloride was purchased from Lancaster Synthesis Ltd., Lancaster England, and used to derivatize l-octanol according to the procedure of Poole et al. (10). A standard solution of lindane and aldrin in isooctane was obtained from Varian Associates. N-Acetylpentafluoroaniline.Pentafluoroaniline (0.4 g, 0.002 mol) and freshly distilled acetic anhydride (2 mL, 0.02 mol) were heated under reflux at 90 OC for 1h. After the mixture was cooled to 4 "C, water (5 mL) was added followed by methylene chloride (10 mL) and shaking. The organic layer was separated, washed with 10 mL of water, and. dried over anhydrous magnesium sulfate overnight. Nitrogen was used to evaporate the solvent, yielding white crystals. The product was recrystallized from toluene, and then acetonitrite, mp 144-145 OC. The structure was confirmed by mass spectrometry. Perfluoroacyl Alkyl Ester Derivatives of the Amino Acids. Typically the dried amino acid (0.0003 mol) was heated at 60 OC with a 25% (w/w) methanolic solution of HCl(15 mL) for 1.5 h. Excess reagents were removed at 50 "C on a rotary evaporator under reduced pressure (water aspirator) and the product was dried in vacuo over sodium hydroxide in a desiccator overnight. The methyl ester amino acid hydrochloride was dissolved in dry acetonitrile (10 mL) and freshly distilled per. fluoroacyl anhydride (0.01 mol) was added. The mixture was heated under reflux for 1.5 h. Excess reagents were removed at 0 1982 American Chemical Society
