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Trace Organic Analysis

Multiresidue Determination of Acidic Pesticides in Water by HPLC/DAD with Confirmation by GC/MS Using Conversion to the Methyl Ester with Trimethylsilyldiazomethane


Published in J. Chromatogr. Sci. 47, 343-349 (2003).
[note: minor content and formatting differences exist between this web version and the published version]

Thomas W. Moy1 and William C. Brumley*

U.S. Environmental Protection Agency, Office of Research and Development, National Exposure Research Laboratory, Environmental Sciences Division, PO Box 93478, Las Vegas, NV 89193-3478

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 non-exclusive, 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.

1Enrollee in the Senior Environmental Employment Program, assisting the EPA under a cooperative agreement with the National Association of Hispanic Elderly.

*Author to whom correspondence should be sent. email: brumley.bill@epa.gov

Abstract

A multiresidue pesticide methodology has been studied and results for acidics are reported here with base/neutral to follow. This work studies a literature procedure as a possible general approach to many pesticides and potentially other analytes that are considered to be liquid chromatographic candidates rather than gas chromatographic ones. The analysis of the sewage effluent of a major southwestern US city serves as an example of the application of the methodology to a real sample. Recovery studies were also conducted to validate the proposed extraction step. A gradient elution program was followed for the high performance liquid chromatography leading to a general approach for acidics. Confirmation of identity was by EI GC/MS after conversion of the acids to the methyl ester (or other appropriate methylation) by means of trimethylsilyldiazomethane. The 3,4-dichlorophenoxyacetic acid was used as an internal standard to monitor the reaction and PCB #19 was used for the quantitation internal standard. Although others have reported similar analyses of acids, conversion to the methyl ester was by means of diazomethane itself rather than by the more convenient and safer trimethylsilyldiazomethane. Thus, the present paper supports the use of trimethylsilyldiazomethane with all of these acids (trimethylsilyldiazomethane has been used in environmental work with some phenoxyacetic acid herbicides) and further supports the usefulness of this reagent as a potential replacement for diazomethane. The HPLC approach here could also serve as the separation basis for an LC/MS solution to confirmation of identity as well as quantitation.

Introduction

Under various legislative acts such as the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA), U.S. EPA has been charged with monitoring the levels of pesticides and other substances and in determining their effects on ecosystems and human health (1). There are now a great variety of methods for various groups of pesticides. In particular U.S. EPA Methods 531.1 (carbamates), 631 (benomyl and carbendazim), 627 (dinitroaniline), 632 (carbamate and urea) target various groups of pesticides for determination (2). Each group of compounds presents special problems and may also require different detection limits.

What is missing in considering all of these methods is a general approach to such analytes from the HPLC point of view. By this we mean an analogous methodology to that found in Methods 625 or 8270 (3) where a large number of target analytes can be screened, and the presence of nontarget compounds can be discovered. Part of the difficulty with establishing a multiresidue analytical methodology is the lack of a universal approach to LC/MS for such a diverse class of compounds

Multiresidue pesticide methods present analytical challenges in the form of great diversity of chemical structures and physical properties of the target analytes. Nevertheless, analysts have found some solutions for this complex task (4-15). Thus, with a sufficiently large subset of compounds, some fractionation of the group is probably inevitable in order to keep the separations/detections practicable.

In particular, DiCorcia and Marchetti have presented an isolation based on Carbopack SPE followed by HPLC/DAD for about 89 compounds (4, 5). Their approach divides the analytes into acidic and base/neutral fractions for determination using a C18-derivatized silica HPLC column with gradient elution. Since theirs is one of the largest subsets of compounds approached from a unified viewpoint, their work presents a possible fruitful approach to a general screening method for pesticides and other analytes by HPLC. An additional application of the overall approach is its potential use as a screening method in newer issues such as analysis for pharmaceuticals and personal care products (PPCPs) in effluent and other matrices (15-18). PPCPs constitute an emerging area of environmental research that encompasses a wide range of chemical structures and functionality at trace levels (19-20). Acidic target substances are obviously an important subset of PPCPs.

In addition to quantitation and qualitative identification by HPLC/DAD, a confirmatory approach is often based on a complementary separation in HPLC or is based on either GC/MS or LC/MS for greater specificity. For acidic analytes, diazomethane is often chosen to enable gas chromatographic separation of esters or other methylation products where volatility or chromatography is aided by derivatization. Trimethylsilyldiazomethane has been proposed as a safer and more convenient reagent for the derivatization (21-22).

In this work we offer further study of a published multiresidue method for pesticides in water based on SPE and HPLC/DAD and evaluate its potential to act as a broad screening technique with the inclusion of additional analytes. In the present work only the acidic fraction afforded by the methodology is studied. A confirmatory and quantitative determination by GC/MS is added in this work where the application of trimethylsilyldiazomethane is further investigated for acidic analytes.

EXPERIMENTAL

Chemicals

The following chemicals/reagents were used: (a) Acetonitrile.-- HPLC grade (J. T. Baker, Phillipsburg, NJ); (b) Water.-- Filtered (2 micron pore size) deionized water from a Nanopure system.(Waters, Inc., Milford, MA); (c) Methanol.-- HPLC grade (Burdick and Jackson, Muskegon, MI); (d) Trifluoroacetic acid (TFA).-- 99+%, spectrophotometric grade, (Aldrich Chemical Co., Milwaukee, WI); (e) Sodium hydroxide.-- 97% ACS (Aldrich Chemical Co.); (f) Hydrochloric acid.-- ACS (Mallinckrodt Baker Inc., Paris, KY); (g) Sodium Sulfate.-- 99+%, granular, ACS (Aldrich Chemical Co.) (g) Sodium Sulfite.-- 98+%, ACS (Aldrich Chemical Co.)

Solutions

(a)
Pesticides.-- Standards were obtained from EPA Pesticide Repository. The concentration of individual standard solutions was 1 mg/mL in methanol. Combined working standard was prepared by mixing 100 無 of each stock standard and diluting to 10 mL with methanol. Thirteen acidic pesticides were divided into two groups for calibration purposes. Acidic D contains: dicamba, coumafuryl, MCPA, 2,4,5-trichlorophenoxy acetic acid (2,4,5-T), MCPB, and dinoseb. Acidic E contains: bentazone, bromoxynil, 2,4-dichlorophenoxyacetic acid (2,4-D), mecoprop, 2,4-dichlorophenoxy-2-butyric acid (2,4-DB), 2,4,5-trichlorophenoxypropionic acid (2,4,5-TP), and pentachlorophenol (PCP).

(b) Mobile Phase HPLC.-- methanol/acetonitrile (820 mL methanol combined with 180 mL acetonitrile) and water (1.7 mL of TFA added to 998.3 mL water).

(c) 6N NaOH.-- 25.2 g of NaOH was dissolved in water and diluted to 100 mL.

Sample Collection and Treatment

Three 4-liter samples were collected from the sewage effluent. The contents of each 4-liter bottle were dealt with in parallel fashion following identical procedures. To isolate base/neutral substances, each 4-liter sample was basified to pH 12-14 with 6 N aqueous NaOH. 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 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. A 240-uL (10%) aliquot of the concentrate was subjected to GC/MS analysis and HPLC analysis.

Solid Phase Extraction Procedures

Extraction Materials and Apparatus.1-L separatory funnel, 7-cm short stem funnel, 1-L side arm filtering flask, and vacuum pump (Gast, Ann Arbor, MI),Visiprep Solid Phase Extraction Vacuum Manifold (Supelco, Bellefont, PA).

SPE Cartridge. 250mg (6mL) Envi-Carb ( graphitized nonporous carbon, surface area 100 m2/g, 120/400 mesh, Supelco, Inc.)

Envi-Carb Conditioning Procedure. The cartridge was washed with 5mL of methylene chloride (80:20,v/v) followed by 2mL of methanol, air dry 1 min., three 5mL-volumes of 10 g/L ascorbic acid in HCl acidified water (pH 2). Cartridge was not allowed to dry during the final rinses and 4 mL of final solution remained in tube before starting sample extraction ( restart vacuum pump).

Procedure. One liter of DI water containing 0.2 g of sodium sulfite and fortified with known standards including an internal standard was mixed in a separatory funnel. Water from the separatory funnel was forced through a 7-cm short stem funnel and through the cartridge at 60 ml/min. Just after the sample was passed through the cartridge, the sides of the funnels were washed down with DI water to remove traces of aqueous sample. The upper frit was pushed against carbon bed. Pressure was reduced with air drying to remove all traces of water. Cartridge was moved to Visiprep SPE Vacuum Manifold. A round bottom test tube ( 1.4 x 12.5 cm) was located below tube; an unwanted fraction was removed by passing through 1 mL of methanol, drop wise, the last drops of methanol removed by reducing the pressure.

The acidic pesticides were collected in a second tube by drawing through the cartridge 3 x 4-mL aliquots of methylene chloride/methanol (80:20, v/v) acidified with TFA (0.2 %, v/v). This solution was stored in the freezer when not being used, and was prepared fresh every other day. Reduce the pressure to remove the last drops of eluant. Before blow down the fraction was neutralized with 50 無 of water/methanol solution of ammonia (2 mL of concentrated ammonia diluting to 10 mL with methanol) mix by vortexing. Sample tube was placed in a water bath for evaporation to dryness at 30 oC under a gentle stream of nitrogen. The sample was reconstituted with 300 無 of water/methanol(60:40,v/v) acidified with TFA (0.05/5, v/v), vortexed, and sonicated for 20 minutes.

Recovery Studies of Acidic Pesticides

Samples of 1 L of DI water were fortified with 0.5 and 1.0 痢 of combined pesticides and then extracted according to above setup.

Derivatization with Trimethylsilyldiazomethane

Derivatizations with trimethylsilyldiazomethane were carried out in commercially silanized (via high-temperature treatment with HMDS) vials (12 x 32 mm wide mouth screw cap with PTFE/Silicone cap liners). These vials were purchased from Alltech Associates, Inc., 2051 Waukegan Rd, Deerfield, IL 60015.

For calibration purposes, the vials were charged with an acetone solution containing varying amounts of a stock solution of 2,4-D and 2,4,5-T in acetone (0.0506 g in 100 mL) so that the herbicide concentrations in a 1 mL total volume systematically varied for calibration purposes. A total of seven levels was employed. 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 無 of the 2.0 M trimethylsilyldiazomethane reagent and 100 無 of methanol. The 1 mL of extract in methanol recovered from the sewage effluent was treated in the same way. After thorough mixing, the homogenous reaction mixtures were allowed to stand at ambient temperature for two hours.


GC/MS ANALYSIS

Agilent

GC/MS analysis was carried out directly on the reaction vial contents on an Agilent Technologies 6890 GC/5973 MSD. A 30 meter x 0.25 mm ID HP 5MS column with a 0.25 micron film was used with the MSD. The temperature program was 46.0 min long and ramped as follows: 60 oC to 150 oC @10.00 oC/ min (9 min); 150 oC to 250 oC @ 4.00 oC/minute (25 min); 250 oC to 300 oC - 10.00 oC/ 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 He at a flow rate of 1.0 mL/min with pressure programming and the instrument was operated in EI mode. The retention times of 2,4-D methyl ester and 2,4,5-T methyl ester were 11.70 and 14.48 minutes, respectively. The ion masses monitored (dwell time 50 msec, resulting EM 2035.3 V) were, respectively: 2,4-D methyl ester and 2,4,5-T methyl ester at m/z 234.0, 236.0, 219.0; and 2,4,6-T at m/z 268.0, 270.0, 253.0,; PCB#104 m/z 325.9.

VG

GC/MS analysis was carried out directly on the reaction vial contents on a VG 70SE. A 30 meter x 0.25 mm ID DB 5MS column (J&W, Folsom, CA) with a 0.25 痠 film was used. The temperature program was the same as for the MSD.

Injections were 2 無 using an on-column injection mode. The carrier gas was He at a flow of 30 cm/sec and the instrument was operated in EI mode. The retention times of the methyl esters of 2,4-D and 2,4,5-T were 13:20 and 16:10 min:sec, respectively. The ion masses monitored (dwell time 50 msec, settling time 30 msec, PM setting 410) were, respectively: 2,4-D methyl ester and 2,4,5-T methyl ester at m/z 233.9850, 235.9821; and 2,4,5-T methyl ester at m/z 267.9461, 269.9431; 257.9584 (PCB#19).

Recovery levels from effluent spiking studies

Recoveries were assessed from spiking studies carried out as follows. 1-Liter samples of deionized water were fortified with a level of 1 痢/L of 2,4-D and 2,4,5-T and processed by SPE.

For GC/MS analysis, eluant was evaporated to dryness and then subjected to partitioning between methylene chloride and water (acidified with HCl). The methylene chloride fraction was then dried over sodium sulfate and subjected to the derivatization procedure.

HPLC Separations

Beckman System Gold software (Beckman, Fullerton, CA, USA) with a Beckman Model 126 pump unit HPLC system. LC18, 5 痠 packing in a 150 x 2.00 mm Luna column (Phenomenex, Santa Clara, CA, USA). Acidic pesticides were chromatographed with methanol/acetonitrile (82:18, v/v) and water acidified with TFA (0.17 % v/v). The initial mobile phase consists of organic/acidified water ( 50:50 ) and is linearly increased to 88% organic after 35 min; hold for 5 min; 5 min to 50% organic; equilibration 12mins. Flow rate is 0.2 mL/min and the UV detector set at 230 nm.


RESULTS AND DISCUSSION

Recoveries

Table 1 tabulates results for two spiking levels, 0.5 痢/L and 1.0 痢/L for the 13 acidic analytes. Generally, the recoveries were quantitative ranging from 57.6 % to 126.7% with an overall average recovery of 83.2%. The values in the table represent either 4 or 5 separate determinations. The standard deviations occasionally exceeded 20 %, and for those compounds some care in the interpretation of the quantitations should be taken. The referenced work obtained recoveries above 90% consistently. Although our results were not quite as good, they were comparable to results reported in the original work lending evidence that the methodology is practical for adoption by other laboratories.

Table 1. Recoveries of 13 acidic analytes by SPE
Analyte 0.5 ppb level 1.0 ppb level
bentazon 74.2 14.6 % 57.6 14.5 %
dicamba 110.2 4.5 % 100.1 8.7 %
bromoxynil 86.6 4.4 % 88.6 6.7 %
coumafuryl 85.8 64.7 % 63.3 32.1 %
2,4-D 88.5 6.2 % 94.6 7.6 %
MCPA 93.2 14.7 % 80.6 8.0 %
mecoprop 62.9 11.4 % 71.6 17.0 %
2, 4, 5-T 111.2 5.0 % 91.5 7.1 %
2, 4-DB 60.4 10.5 % 64.6 26.3 %
MCPB 62.6 11.2 % 64.0 3.0 %
2, 4, 5-TP 124.3 5.7 % 126.7 9.6 %
dinoseb 100.1 8.3 % 99.5 10.6 %
PCP 58.6 19.3 % 58.4 30.0 %
Overall average both levels 83.2

Since one of our stated goals was to test the methodology for applicability to a broad range of analytes, these data support the method's ability to recover multiple residues. This, then, opens the possibility of applying the approach to an even broader range of compounds in order to establish its ability to act as a general screening tool consisting of preconcentration by SPE and HPLC/DAD-UV.

To add support to a broader range of compound applicability, we obtained recoveries for salicylic acid (analgesic metabolite), trichloropyridinol (pesticide metabolite), and clofibric acid (lipid regulator metabolite). Average recoveries were 85% 15% at the 1 痢/L fortification level. These data indicate that the solid phase extraction followed by HPLC/DAD performs well for compounds and compound classes other than the original acidic (and base/neutral) pesticides.

HPLC Separation

Our approach differs from the original work in using a 2 mm ID HPLC column. This diameter column presents a compromise between the standard analytical column (4.6 mm ID) in terms of concentration detection limits (approximately a factor of 2 higher with the narrow bore) and reducing solvent usage (reduced to 19% of original) while maintaining full compatibility with potential LC/MS analysis (200 無/min flow rate when using atmospheric pressure chemical ionization interface). According to Figure 1, we did not achieve full baseline separation of all 13 compounds with this column.

Figure 1. HPLC separation of 13 acidic analytes with retention time in min: 2-chlorobenzoic acid (internal standard) (6.5), dicamba and bentazone (11.5), bromoxynil (13.2), coumafuryl (15.2), MCPA (16.4), 2,4-D (17.0), mecoprop (21.5), 2,4,5-T (22.3), 2,4-DB and MCPB (24.4), 2,4,5-TP (26.9), dinoseb (29.5), PCP (36.6).

Incomplete resolution of bentazon and dicamba as well as 2,4-DB and MCPB was obtained in our laboratory as well as in the original work Thus, recovery studies were carried out in two separate groups.

This incomplete separation gives us some measure of the relative selectivity of the method for these kinds of acids, and we can reasonably expect that coelutions are going to occur in real, complex samples.

Naturally, no one environmental sample is expected to contain all 13 compounds, but may of course contain a number of coextractives that could interfere with determinations. Thus, confirmation of identity and comparative quantitation must be provided to the screening procedure for completeness. Generally, this is provided by GC/MS (with derivatization) or LC/MS, although the original work depended on a second column with HPLC/DAD (4).

GC/MS

In the present study, confirmation was obtained by GC/MS under EI conditions for the methyl esters or other methylation product of the analytes obtained from reaction with trimethylsilyldiazomethane. Capillary GC on DB5 failed to separate all 13 components as did HPLC (Fig. 2). The pair 2, 4, 5-T and MCPB were not resolved (about 0.03 min separation) and bromoxynil and 2,4-D were just resolved (0.16 min). In addition, dinoseb is only slightly resolved from 2,4-DB (0.06 min). However, unique monitoring masses for each compound were selected for confirmation and quantitation.

Figures 2 and 3 present the separation of the compounds in the two recovery groups used.

Figure 2. GC/MS full scan total ion chromatograms of acidic D analytes as methyl esters with retention time in min: dicamba (11.95), coumafuryl (30.4), MCPA (12.73), 2,4,5-T (16.93), MCPB (16.84), and dinoseb (18.96).


Figure 3. GC/MS full scan total ion chromatograms of acidic E analytes as methyl esters with retention time in min: bentazone (18.95), bromoxynil (13.86), 2,4-D (14.01), mecoprop (12.49), 2,4-DB (18.35), 2,4,5-TP (10.87), and PCP (15.61).

Dinoseb gave a very weak response within the first group near 10.3 min. The 3,4-D isomer was originally chosen as internal standard to monitor the completeness of the reaction as well as exhibit quantitation. The use of PCB non-Aroclor congeners was eventually selected as the main and general quantitation agent for this as well as other residue determinations. We present the application of GC/MS methodology as a confirmatory tool in this work with quantitative data presented for all analytes as a demonstration that the approach is also appropriate for quantitation. In addition, further study of separations of target analytes on different gas chromatographic phases could be conducted in search of more complete separations. However, we have chosen to present the work on the most commonly used GC column in environmental work. An approach based on LC/MS is also feasible and warrants further study using the separation presented (or modified).

Sample Analysis

Unfortified and fortified drinking water and sewage effluent were examined as practical applications of the HPLC screening methodology. Figure 4 presents a chromatogram for the preconcentrated effluent using the traditional liquid-liquid extraction technique as part of a broad characterization effort.

Figure 4. HPLC of extract of effluent where responses at RT 13.5 and 21.9 represent presumptive amounts of 0.3 and 0.4 痢/L of bromoxynil and mecoprop in the effluent (unconfirmed by GC/MS).

Tentative peaks for pesticides were detected at < 1 痢/L at retention times of 13.5 and 21.9 min. None of these were confirmed. A very weak response at the retention time of clofibric acid was observed, and this analyte has been the subject of a separate paper where clofibric acid was confirmed in the extract of effluent [23]. The chromatogram suggests the robustness of the separation and the possibility of detecting analytes below 1 痢/L in complex matrices such as effluent.

Effluent was spiked at 1 痢/L in 2,4-D and 2,4,5-T and the sample was subjected to SPE. The effluent sample afforded by SPE directly caused immediate vigorous bubbling with addition of derivatizing agent. To overcome coextractive effects and competition from TFA, the sample eluant was evaporated to dryness and then subjected to partitioning between methylene chloride and water (acidified with HCl). The methylene chloride fraction was then dried over sodium sulfate and subjected to the derivatization procedure. Figure 5 shows the result of confirmation of 2,4,5-T as the methyl ester which was quantitated at 85% recovery (850 ppt). Ions at m/z 234, 268, and 270 are shown. The 2,4-D spike was recovered at 114% as the methyl ester. Thus, the feasibility of using GC/MS with derivatization for confirmation of identity and quantitation is shown.



Figure 5. GC/MS selected ion recording results for 2,4,5-T as the methyl ester confirmed in effluent spiked at 1.0 痢/L and quantitated at 0.85 痢/L.

An additional test and application of the GC/MS methodology was undertaken. The attenuation of herbicides determined in runoff water was studied using an experimental test plot. After spraying the plot with herbicides, the area was watered and runoff water analyzed over a period of 14 days. These determinations were performed using the GC/MS methodology as a test of its robustness. The results are tabulated in Table 2.

Table 2. Amounts of herbicides and other acidics in runoff water (20 痢 total application each compound) reported in ng isolated from 500 mL runoff water each sampling day. (Levels below 300 ng are not confirmed).
Compound
Day 1
Day 4
Day 7
Day 12
Day 14
Dicamba ME
65.1
272.
109.
105.
122.
Bentazone ME
1297.
657.
106.
200.
162.
2,4,5-TME
3966.
2577.
830.
757.
795.
DinosebME
2230.
1661.
284.
286.
294.
MCPAME
3087.
3016.
1158.
1030.
991.
2,4,5TPME
4050.
3120.
963.
781.
647.
MCPBME
2916.
967.
173.
82.4
362.
BromoxynilME
5714.
3152.
722.
466.
422.
2,4-DME
4920.
4212.
1309.
61.0
68.
MecopropME
5025.
3315.
1098.
844.
911.
2,4-DBME
4072.
1752.
295.
122.
123.
PCPME
6124.
976.
312.
266.
239.
Methyl Salicylate
17.9
11.0
24.0
8.06
9.47
Methyl Clofibrate
66.7
59.3
9.91
38.5
27.9
TCP methyl ester
570.
439.
400.
95.3
93.6
CoumofurylME
226.
156.
146.
148.6
175.6

Some of the compounds were almost immediately dissipated, presumably either through decomposition or irreversible adsorption. Others could be followed in a descending presence in runoff water. In some cases, partial interferences were observed on some ions. These issues point up the problems with environmental monitoring of herbicides. The herbicides frequently dissipate rapidly, the application rates may be relatively low, and the sampling events must be closely tied to the application events. Interferences are a major issue with low level detection, and the development of efficient class separations as cleanup for complex extracts containing these analytes remains a future research goal.

Conclusions

The present work further explored a multiresidue method for pesticides (acid fraction) with the determination of target compounds to less than the 1 痢/L level. This methodology is proposed as a general approach to the analysis of pesticides by HPLC following acid and base-neutral partitioning. At the same time we presented a confirmatory/quantitation technique for acids based on derivatization with trimethylsilyldiazomethane followed by GC/MS and demonstrated its effectiveness at levels below 痢/L. The SPE-HPLC/DAD approach appears to be broadly applicable and was demonstrated to work with both pesticide and pharmaceutical metabolites. This suggests that the methodology applies to a broad spectrum of analytes with the ability to detect nontargeted compounds. Future work will address the separations and confirmation of the base-neutral fraction as well as effective cleanup approaches.

References

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22. N. Hashimoto, T. Aoyama, and T. Shiori. A simple efficient preparation of methyl esters with trimethylsilyldiazomethane [TMSCHN2] and its application to gas chromatographic analysis of fatty acids. Chem. Pharm. Bull. 29: 1475-1478 (1981).

Trace Organic Analysis Home Page
Analytical Environmental Chemistry
[ Environmental Sciences ]
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[ National Exposure Research Laboratory]
Author: William C. Brumley


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