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Method to Determine PPCPs in the Environment

Application of U.S. EPA Methods to the Determination of Pharmaceuticals and Personal Care Products in the Environment:

Determination of Clofibric Acid in Sewage Effluent by GC/MS

Dennis B. Patterson2, William C. Brumley1, Virginia Kelliher2, and Patrick L. Ferguson3

U.S. Environmental Protection Agency, Environmental Sciences Division, PO Box 93478, Las Vegas, NV 89193-3478

1Author to whom correspondence should be sent.
2Enrollee in the Senior Environmental Employment Program, assisting the EPA under a cooperative agreement with the National Association of Hispanic Elderly.
3Graduate Student Fellow in the EPA National Network Management for Environmental Management Studies (NNEMS) Program

Original citation:
D. B. Patterson, W. C. Brumley, V. Kelliher, and P. L. Ferguson, "Determination of Clofibric Acid in Sewage Effluent by GC/MS: Conversion to the Methyl Ester with Trimethylsilyldiazomethane," Amer. Lab. 34, 20-28 (2000)

ABSTRACT

Pharmaceuticals, their metabolites, and personal care products constitute an emerging area of research with regard to their environmental presence and distribution. U.S. EPA methodology has traditionally been aimed at a variety of toxic substances and persistent chemicals based on acid and base/neutral separations. This approach can be applied with broad applicability to emerging analytes whose effects are still largely unknown. Among potential analytes is the widely-distributed clofibric acid, a metabolite of various lipid regulators. Relatively long-lived in the environment, it is a common contaminant of municipal sewage systems. As a specific example we present a determination of clofibric acid in the sewage effluent of a major southwestern US city. Clofibric acid was recovered from freshly collected samples of the effluent by liquid-liquid extraction (acid fraction) as part of a broad characterization effort. Recoveries from solid phase extraction using styrene/divinylbenzene adsorbent were also studied. Analysis was by EI GC/MS after conversion of the clofibric acid to the methyl ester by means of trimethylsilyldiazomethane. 3,4-Dichlorophenoxyacetic acid was used as an internal standard with additional use of highly orthosubstituted polychlorinated biphenyl congeners as alternative internal standards. The present paper appears to be the first report of the use of trimethylsilyldiazomethane with clofibric acid (trimethylsilyldiazomethane has been used in environmental work with phenoxyacetic acid herbicides, which are structurally similar to clofibric acid) and further supports the usefulness of this reagent as a potential replacement for diazomethane for multiresidue analysis. The extract containing the base/neutral fraction was further characterized by full scan mass spectrometry.

INTRODUCTION

An emerging area of research concerns pharmaceuticals and personal care products (PPCPs) in the environment and their possible impact on biota and ecosystems. The long term effects of constant perfusion of PPCPs into the aquatic environment are presently unknown. Some compounds are known to have physiological effects on nontarget biota at extremely low concentrations (e.g., estrogens and estrogenic mimics and certain antidepressants) (1).

Among the possible target analytes are several compounds possessing chemical structures that are resistant to microbial degradation and/or capable of being bioaccumulated. Acidic metabolites of pharmaceuticals present one type of analyte that appear in the effluent of many publicly operated treatment facilities. The aim of the present study is to begin to understand and assess the potential exposure of biota and associated ecosystems to these compounds. This study is a first step in an overall strategy to understand the fate and transport of these compounds in the affected environment. Such studies are mission relevant and given high priority since the Environmental Chemistry Branch of the Environmental Sciences Division is charged with the assessment of emerging areas of risk under Strategic Plan 2000 for the Environmental Protection Agency.

Clofibric acid [2-(4-chlorophenoxy)-2-methylpropanoic] acid is the bioactive metabolite of various lipid regulating pro-drugs (1). Its structure is suggestive of chlorophenoxy acid herbicides (it is in fact an isomer of one such herbicide, mecoprop [2-(4-chloro-2-methylphenoxy)propionic acid]). However, clofibric acid appears to persist in the environment much longer than do these herbicides (2). Thus, clofibric acid is a common contaminant of sewage systems (3, 4, 5). It has also been detected in Swiss lakes and in the North Sea in the low nanograms-per-liter range (6).

However, clofibric acid is but one piece of a larger puzzle that includes many other acids as well as base/neutrals as components in sewage effluent. For example, recent monitoring programs supported by USGS now include 92 compounds over 40 of which may be classified as PPCPs (7).

GC/MS analysis of such acids is facilitated by conversion to a volatile ester. Methyl esters are particularly convenient for electron ionization (EI) MS and are readily made by reaction of the substrate acids with diazomethane. However, diazomethane, explosive and allergenic, is a hazardous reagent to work with and is typically used immediately after its generation. The use of diazomethane in this manner is set forth in a U.S. EPA Method (8). Clofibric acid itself has been analyzed in the same manner using diazomethane by Ternes et al. (3, 4, 5). Conversion of clofibric acid to the methyl ester directly in the hot GC injector with trimethylsulfonium hydroxide has recently been reported (9). At higher levels, such as in pharmacokinetic studies, clofibric acid may be analyzed directly by HPLC (10).

The diazomethane derivative, trimethylsilyldiazomethane (TMSDM), is a convenient alternative to diazomethane, although more selective, and exhibits the reaction with carboxylic acids to yield methyl esters. TMSDM is much easier and safer to handle than is diazomethane, and its chemistry has been reviewed (11,12). Recently the use of TMSDM for the conversion of phenoxy acid herbicides to the methyl esters for analysis by GC/MS has been demonstrated (13). Thus, the analysis of clofibric acid in effluent presents a challenging analytical problem that tests the ability of TMSDM to function effectively at trace levels in a complex environmental sample. We also note that TMSDM has also been used in conjunction with two U.S. EPA methods, 552.2 (14) and 515.2 (15) and compared to diazomethane.

In this work we report the analysis of effluent for clofibric acid and demonstrate the applicability of standard U.S. EPA methodology to the determination of PPCPs and in characterizing sewage effluent. We find that many other acidic analytes are also amenable to this approach. In addition, the base/neutral fraction of the extracted sample was studied by full scan GC/MS during initial characterization.

EXPERIMENTAL

Chemicals
(a) Trimethylsilyldiazomethane, 2.0 M solution in hexanes. (Aldrich Chemical Co., Milwaukee, WI).
(b) 2-(4-Chlorophenoxy)-2-methylpropanoic acid (clofibric acid). 97%, (Aldrich Chemical Co.).
(c) 3,4-Dichlorophenoxyacetic acid (3,4-D). 96% (Aldrich Chemical Co.).
(d) Sodium hydroxide. 98.9%, (Fisher Scientific, Pittsburgh, PA).
(e) Hydrochloric acid. ACS grade, 36.5-38% (Fisher Scientific).
(f) Methylene chloride. Capillary GC/GC-MS grade (Fisher Scientific).
(g) Methanol.ACS/HPLC grade (Fisher Scientific).
(h) Acetone AR grade (Fisher Scientific).
Solutions
(a) 6N NaOH. 100 mL batches were freshly prepared from 24.3 g of the solid NaOH made up to 100 mL with deionized water.
(b) Hydrochloric acid. Acidification was accomplished with the concentrated acid with the pH being monitored with Hydrion paper.
Other Materials
(a) Vials Silanized via high-temperature treatment with HMDS, 12 x 32-mm wide mouth, screw cap with PTFE/silicone cap liners. (Alltech Associates, Inc., 2051 Waukegan Rd, Deerfield, IL 60015).
(b) SDVB Disks. 3M Empore disks, SDB-XC, 47 mm (Fisher Scientific).

Sample Collection and Treatment

Three 4-liter samples were collected from the sewage effluent. The contents of each 4-liter bottle were treated in parallel fashion following identical procedures. In order first to remove base/neutrals, each was basified to pH 12-14 with 6 N aqueous NaOH. The liquid-liquid extraction used here follows the protocol of an EPA Method (16). After basification each 4-liter sample was extracted three times with 400 mL dichloromethane. The aqueous phases were then taken to pH 1 with concentrated hydrochloric acid and again extracted three times with dichloromethane. The dichloromethane extracts from these acidified solutions were combined, dried over anhydrous sodium sulfate and concentrated down to a volume of 2.4 mL. Concentration was achieved using a refluxing apparatus consisting of a large round bottom flask, boiling chips (Teflon), and a three-ball Snyder column; this ensured that all surfaces were continuously bathed with condensing liquid during concentration. Of the resulting concentrate, 240 µL (10%) was subjected to GC/MS analysis.

Derivatization with TMSDM

Derivatizations with TMSDM were carried out in the commercially silanized vials. The vials were charged with 1.0 mL of an acetone solution. The 1.0 mL comprised acetone and various amounts of a stock solution of clofibric acid in acetone (0.0506 g in 100 mL) so that the clofibric acid concentration systematically varied for calibration purposes. A total of seven clofibric acid levels was used. On top of the 1.0 mL in each vial was added 50 無 of a stock solution of 3,4-D in acetone (0.0186 g in 100 mL) as an internal standard, 25 µL of the 2.0 M TMSDM reagent, and 100 無 of methanol. The 3,4-D is not a constituent of commercial pesticide formulations. An additional internal standard was used that consisted of polychlorobiphenyl congeners numbered as 19, 54, 104, 155, 184, and 204 with 50 無 added from a solution that was 2 ng/無 in each congener. These congeners do not occur in commercial Aroclors and are the most highly orthosubstituted congeners of each chlorination level. The 240 無 in dichloromethane recovered from the sewage effluent was treated in the same way after dilution to 1.0 mL with acetone. After thorough mixing, the homogenous reaction mixtures were allowed to stand at ambient temperature for two hours.

GC/MS Analysis

GC/MS analysis was carried out directly on the reaction vial contents on an Agilent Technologies 6890 GC/5973 MSD. A 30-m x 0.25-mm ID HP 5MS column with a 0.25 痠 film was used. The temperature program was 46.0 min long and ramped as follows: 60 oC to 150 oC @ 10.00 deg/min (9 min); 150 oC to 250 oC @ 4.00 deg/min (25 min); 250 oC to 300 oC @ 10.00 deg/ min (5 min); maintained at 300 oC until 46.00 min total.

Injections were 2 無 and pulsed-splitless mode was used. The carrier gas was helium at a flow of 1.0 mL/min with pressure programming and the instrument was operated in EI mode. The retention times of the methyl esters of clofibric acid and 3,4-D were 11.70 and 14.48 minutes, respectively. The PCB#104 was at 20.24 min. The m/z values monitored (dwell time 50 milliseconds, resulting EM voltage 2035.3) were, respectively: clofibric acid methyl ester - 228.0; 230.0; 128.0; 130.0; 169.0; 171.0; 3,4-D methyl ester - 234.0; 236.0; 162.0; PCB#104 - 323.90.

Recovery Levels from Spiking Studies

Recoveries were assessed from spiking studies carried out as follows. Samples of deionized water (1-L) were spiked with clofibric acid at 50.6 and 506 ng/L. In one approach the clofibric acid was recovered by means of SDVB disks. The SDVB disks were prepared according to the manufacturer's directions. The disk was first soaked with methanol (10 mL) until liquid began to drip through. The disk was then dried by applying a vacuum. Vacuum was removed and 10 mL of acetone was added. The disk was soaked a minimum of 3 minutes, and then vacuum was applied until all acetone was pulled through. Vacuum was removed and 10 mL of methanol was added. After 3 minutes water was added and the liquid pulled through to near the top of the disk; more water was added and the process repeated three times leaving a layer of water over the disk. After this the sample was added, acidified with concentrated HCl (2 mL) and pulled through. The disk was dried by pulling air through for 30 to 60 min. The disk was eluted with 7 mL acetone by soaking and pulling to dryness and the process repeated two times for a total of 21 mL of eluate. This volume was then concentrated by nitrogen blow down.

In a second approach the recovery behavior of clofibric acid was studied using liquid/liquid extraction according to standard EPA methodology (16). Since the characterization studies of the effluent were carried out using liquid/liquid extraction, recoveries of clofibric acid by this approach are representative of our recoveries when applied to the real samples. Levels of about 1 痢/L and 200 ng/L were subjected to study via spiking of 100 mL and 1 L of DI water, respectively. The spiked water was acidified with concentrated HCl and immediately extracted with three portions of methylene chloride representing 10%, 5%, and 5% of the sample volume. The combined methlylene chloride extract was concentrated under a gentle nitrogen stream to 1 mL.

RESULTS AND DISCUSSION

Recoveries

Spiking studies using either the liquid-liquid or SDVB methods indicated adequate recovery for both methods. Replicate spiking studies using SDVB disks yielded at the 50.6 ng/L spiking level recoveries of 77.9, 75.7, and 63.2% (avg. 72.3 11% RSD); and at the 506 ng/L spiking level 64.0, 94.3, and 103.0% (avg. 87.1 23.5%). Liquid-liquid extraction yielded recoveries of 58.5, 54.6, and 56.0% (avg. 56.4 3.5% RSD) at the 1012 ng/L spiking level; and 52.8, 57.0, 72.9% (avg. 60.9 17.4%) at the 202.4 ng/L spiking level.

Liquid-liquid extraction followed the traditional EPA methodology and was used in the present study because we were interested in a broad range of compounds in the effluent. We also included SPE for clofibric as a specific target as a convenience to those who would like to use a fast and solvent-reducing approach for this specific compound. Since the SPE behavior for all of the potential analytes and for other compounds is not completely known, we reasoned that the liquid/liquid approach more likely gave us the larger picture in terms of a characterization effort.

Derivatization Artifacts

Figure 1 shows the GC/MS chromatogram of a standard of clofibric acid (5.5 pg/µL) and the internal standard derivatized by TMSDM while monitoring the suite of ions given in the experimental section.

Image of chromatograph - contact brumley.william@epa.gov for further information

Figure 1. Total ion chromatogram (nine ions) for a 5.5 ng/L standard of clofibric acid (as methyl ester) with retention time 11.66 min; internal standard retention time 14.62 min.

The target compound occurs as an easily discernible peak at 11.66 min while the internal standard occurs at 14.57 min in this chromatogram. The scale of the drawing has been magnified to reveal the many small peaks appearing along with the expected responses. The number of peaks generated is a consequence of the derivatization approach at trace levels. Thus, it is likely that artifacts will be the main limitation to lower detection limits, with additional complications arising from derivatized matrix components.

Derivatization always presents complications to environmental analysis and places extra demands on the separation as well as the detection technique. The artifact peaks from TMSDM consisted largely of silicon-containing organic moieties. Therefore, we are currently investigating the application of a rapid cleanup that takes advantage of the more polar nature of the target compounds to remove artifactual components. The second internal standard (PCB #104) provided an independent assessment of the completeness of the reaction as regards the analyte and the 3,4-D internal standard. In addition, the 3,4-D internal standard showed more sensitivity to column degradation than did clofibric acid itself. Therefore, the PCB internal standard can be used as an effective substitute for 3,4-D. The presence of 3,4-D establishes that the reaction is definitely taking place within a sample. We point out that TMSDM is more selective in its derivatization chemistry than is diazomethane itself and weakly acidic phenols and alcohols do not react. In some contexts this may provide structural inferences for unknowns.

Determination of Clofibric Acid in Effluent

Clofibric acid in a sewage effluent was recovered by liquid-liquid extraction in accordance with EPA methods. After recovery, the clofibric acid and the internal standard, 3,4-D, were converted to the methyl esters using TMSDM for EI GC/MS analysis.

Figure 2 presents a GC/MS total ion chromatogram from the suite of nine ions being monitored.

Image of chromatograph - contact brumley.william@epa.gov for further information

Figure 2. Total ion chromatogram (nine ions) for the effluent sample with retention time 11.66 min for clofibric acid (as methyl ester); internal standard retention time 14.62 min.; second internal standard near 20.0 min (PCB#104).

Clofibric acid methyl ester is a small peak at about 11.66 min in this renormalized view of the response from the sample. A significant background of response is generated from the derivatized sample, and we show this in the summed view. The individual ion chromatograms for m/z 128, 169, and 228 are shown in Figure 3.

Image of chromatograph - contact brumley.william@epa.gov for further information

 

Image of chromatograph - contact brumley.william@epa.gov for further information

 

Image of chromatograph - contact brumley.william@epa.gov for further information

Figure 3. Ion chromatograms of m/z 128, 169, and 228 for the effluent sample extract with clofibric acid methyl ester response at retention time label of 11.7 min.

The response at the expected retention time is clear, but the complexity of the sample is also apparent. The background subtracted mass spectrum of a standard and of the sample are shown in Figure 4, and they agree well in relative abundance confirming the identification with additional support from identical retention times.

Image of chromatograph - contact brumley.william@epa.gov for further information

 

Image of chromatograph - contact brumley.william@epa.gov for further information

Figure 4. Background subtracted mass spectrum (9 ions) based on of a standard of clofibric acid methyl ester (upper) and the background subtracted mass spectrum (9 ions) obtained from the effluent sample extract at the retention time of clofibric acid methyl ester.

Liquid-liquid extraction and subsequent GC/MS determination yielded a clofibric level in the original effluent of 234 ng/L. This constitutes a challenging demonstration of the applicability of this reagent and EPA methodology for acidic analytes at trace levels in a complex sample.

We have applied this methodology to a number of other pesticide and pharmaceutical analytes including phenoxy acid herbicides, acidic phenols, and salicylic acid (14 compounds) [18,19]. The methodology is capable of quantitating analytes at sub 痢/L levels.

Base/Neutral Fraction

Figure 5 illustrates the complexity of the base/neutral components as revealed by this total ion chromatogram.

Image of chromatograph - contact brumley.william@epa.gov for further information

Figure 5. Total ion chromatogram (50-500 u) for base/neutral fraction of extracted effluent with selective identifications given in Table 1.

Table 1. Representative Tentatively Identified Compounds in the Base/Neutral Fraction of Extracted Sewage Effluent

Retention Time (min)
Compound (MW)
Description/use
10.47
surfynol 104; 2,4,7,9-tetramethyl-5-dicyne-4,7-diol (226)
surfactant
11.24
2,6-di(t-butyl)-4-hydroxy-4-methyl-2,5-hexadien-1-one (236)
BHT breakdown product
11.80
2,6-DI(t-butyl)-5,6-epoxy- 4-methyl-4-hydroxy-2-cyclohexanone (252)
BHT breakdown product
16.5
tri(2-chlorethyl)phosphate Fyrol (284)(249)
flame retardant
16.8
triazine (229)
herbicide
16.9
N-butylbenzenesulfonamide ()
plasticizer
17.2
Fyrol (277)
flame retardant
18.1
caffeine (194)
stimulant, food component
18.3
galaxolide (258)
synthetic musk
18.5
tonalide (258)
synthetic musk
19.3
xycaine (234)
anesthetic
21.0
musk ketone (294)
synthetic musk
28.3
tri(1,3-dichlorisopropyl)phosphate Fyrol (379)
flame retardant
28.7
carbamazepine (236)
antidepressant

There are more than 100 individual peaks discernible with several hundred components making up the content of this fraction. We have selected some representative compounds that have been tentatively identified (based on mass spectra and library fit) and have given them in Table 1. Some of these compounds have been further studied and confirmed by use of standards. The synthetic musks identified have been the subject of a more detailed study where their levels were reported: musk ketone (21.3 to 27.5 ng/L); galaxolide (35.0 to 152 ng/L); tonalide (26.6 to 92.2 ng/L) [20].

The occurrence of breakdown products of BHT [21] is curious and may result from widespread use of BHT as an antioxidant in various products. Persistent chemicals such as fire retardants appear to survive the treatment process as do a number of PPCPs including carbamazepine.

CONCLUSIONS

This work demonstrates the facile determination of clofibric acid as a sewage effluent contaminant down to the 100 parts per trillion level. The clofibric acid may be recovered from the effluent either by liquid-liquid extraction in accordance with EPA methods or through the use of commercial SDVB disks. After concentration by either manner, the clofibric acid and an internal standard, 3,4-dichlorophenoxyacetic acid (3,4-D) are conveniently converted to their respective methyl esters by means of TMSDM, followed by EI GC/MS analysis. The determination represents part of a broader effort to characterize the effluent and test the applicability of standard EPA methods for the determination of PPCPs. Additional acidic analytes and targeting of base/neutral analytes are in progress.

NOTICE

The U.S. Environmental Protection Agency (EPA), through its Office of Research and Development, funded and performed the research described here. This work has been subjected to the Agency's peer review and has been approved as an EPA publication. The U.S. Government has the right to retain a nonexclusive, royalty-free license in and to any copyright covering this article. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.

REFERENCES

  1. Daughton, C. D., Ternes, T. A. Pharmaceuticals and personal care products in the environment: agents of subtle change? Environ. Health Perspect 1999; 107:suppl 6, 907-938. (available at http://www.epa.gov/ppcp/)
  2. Garrison, A. W., Schmitt, P., Martens, D., Kettrup, A. Environ Sci Technol 1996; 30:2449-2455.
  3. Ternes, T. A. Occurrence of drugs in German sewage treatment plants and rivers. Water Research 1998; 32:3245-3260.
  4. Stumpf, M., Ternes, T. A., Haberer, K., Seel, P., Baumann, P. W.Vom Wasser 1996; 86:291-303.
  5. Stumpf, M., Ternes, T. A., Wilken, R. D., Rodrigues, S. V., Baumann, W. Polar drug residues in sewage and natural waters in the state of Rio de Janeiro, Brazil. Sci. Total Environ 1999; 225:135-141.
  6. Buser, H-R., Miller, M.D., Theobald, N. Occurrence of the pharmaceutical drug clofibric acid and the herbicide mecoprop in various swiss lakes and in the north sea. Environ Sci Technol 1998; 32:188-192.
  7. USGS current status at http://www.usgs.gov/ Exiting EPA Disclaimerand http://toxics.usgs.gov/regional/emc.html. Exiting EPA Disclaimer
  8. US EPA Method 8151A (Revision 1- December, 1996). (This and other EPA Methods may be accessed in pdf format at the site http://www.epa.gov/epaoswer/hazwaste/test/main.htm)
  9. Zwiener, C., Glauner, T., Frimmel, F. H.J. High Resolut. Chromatogr. 2000; 23:474-478.
  10. Bachman, W. J., Stewart, J. T. HPLC-photolysis-electrochemical detection in pharmaceutical analysis: application to the determination of clofibric acid in human plasma. J. Liq. Chromatogr. 1989; 12:2947-2959.
  11. Shiori, T., Aoyama, T. Trimethylsilyldiazomethane-A Versatile Synthon for Organic Synthesis in A. Dondoni, ed. Adv. In the Use of Synthons in Organic Chemistry. 1993; 1:51-101.
  12. Hashimoto, N., Aoyma, T., Shiori, T. A simple efficient preparation of methyl esters with trimethylsilyldiazomethane (TMSCHN2) and its application to gas chromatographic analysis of fatty acids. Chem Pharm Bull 1981; 29:1475-1478; Aoyama T., unpublished results.
  13. Johnson, P. D., Rimmer, D.A., Brown, R. H. Adaptation and application of a multi-residue method for the determination of a range of pesticides, including phenoxy acid herbicides in vegetation, based on high-resolution gel permeation chromatographic clean-up and gas chromatographic analysis with mass-selective detection. J. Chromatogr. A 1997; 765:245-250 and Johnson, P. D., Rimmer, D. A., Brown, R. H. Determination of phenoxy acid herbicides in vegetation, utilizing high-resolution gel permeation chromatographic clean-up and methylation with trimethylsilyldiazomethane prior to gas chromatographic analysis with mass-selective detection. J. Chromatogr. A 1996; 755:3-11.
  14. Pawlecki-Vonderheide, A.M., Munch, D. J., Munch, J. W. Research associated with the development of EPA method 552.2. J. Chromatographic Sci. 1997; 35:293-301.
  15. US EPA Method 515.2 (Revision 1.1-1995) listed in 40 CFR 141.24(e) which references EPA Report 600/R-95-131 that contains the detailed method.
  16. US EPA Method 3510C (Revision 3-December, 1996), SW846 CH 4.2.1, available at: http://www.epa.gov/epaoswer/hazwaste/test/main.htm
  17. Daughton, C. G. Emerging pollutants, and communicating the science of environmental chemistry and mass spectrometry: pharmaceuticals in the environment. J Am Soc Mass Spectrom 2001; 12:1067-1076.
  18. Moy, T. W., Brumley, W. C. Multiresidue determination of acidic pesticides in water by hplc/dad with confirmation by GC/MS using conversion to the methyl ester with trimethylsilyl-diazomethane, to be submitted.
  19. Flaherty, S., Wark, S., Street, G., Farley, J. W., Brumley, W. C. Investigation of CE/LIF as a tool in the characterization of sewage effluent for fluorescent acids: determination of salicylic acid. Electrophoresis, submitted.
  20. Osemwengie, L. I., Steinberg, S. On-site solid-phase extraction and laboratory analysis of ultra-trace synthetic musks in municipal sewage effluent using gas chromatography-mass spectrometry in the full-scan mode. J Chrom 2001; 932:107-118.
  21. Brumley, W. C., Warner, C. R., Daniels, D. H., Varner, S., Sphon, J. A. EI mass spectrometry of bht and its alteration products. Biomed Mass Spectrom 1989; 18:207-217.

Trace Organic Analysis Home Page
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Author: William C. Brumley
email: William C. Brumley


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