Stability of volatile organic compounds in environmental water


Nov 1, 1990 - Chemical Preservation of Volatile Organic Compounds in Soil. Alan D. Hewitt. Environmental Science & Technology 1997 31 (1), 67-70...
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Environ. Sci. Technol. 1990, 24, 1665-1670

Stability of Volatile Organic Compounds in Environmental Water Samples during Transport and Storage Mlchael P. Maskarlnec,' Lynne H. Johnson, Susan K. Holladay, Ronnie L. Moody, Charles K. Bayne, and Roger A. Jenkins Analytical Chemistry Division, Oak Ridge National Laboratory, P.O. Box The stability of volatile organic compounds in environmental water samples has been studied, particularly with respect to the establishment of preanalytical holding times. Methods have been developed for the preparation of standard samples containing known concentrations of volatile organics. Three water samples were used distilled water, surface water, and groundwater. Samples were stored at both room temperature and under refrigeration. Data were collected over a 365-day period by gas chromatography/mass spectrometry. In water samples containing low chloride content (distilled water), rapid dehydrohalogenation of tetrachloroethane to trichloroethylene occurred. Such degradation was also evident in the surface water and groundwater samples stored at room temperature. A less rapid conversion of trichloroethane to dichloroethylene occurred in distilled water samples stored at 25 "C. Reduced concentrations of aromatic volatiles were observed in both surface and groundwater matrices after 28 days. Loss of carbon tetrachloride was also apparent in surface water samples stored at room temperature. Subsequently, experiments were conducted to determine the value of reduced pH in sample preservation. It was shown that acidification with hydrochloric acid effectively prevented degradation and allowed indefinite storage. However, sampling and analytical considerations make the use of HC1 impractical. Therefore, a study was carried out using sodium bisulfate and ascorbic acid as preservatives. Both substances effectively preserved the samples, but sodium bisulfate proved to have several advantages over ascorbic acid. Samples preserved with either acid were stable over the 112-day experimental period. The implication is that with preservation the maximum holding times for such samples will be limited only by the need for sample turnaround. Introduction

During the past two decades, there has been a dramatic expansion of environmental legislation, including the Comprehensive Environmental Response, Compensation, and Liability Act; the Resource Conservation and Recovery Act; the Toxic Substance Control Act; the Clean Water Act; the Safe Drinking Water Act; the Marine Act; and, most recently, the Superfund Amendment and Reauthorization Act. One result of these regulatory measures has been a tremendous increase in the number of samples collected and distributed for analysis. One estimate is that federal, state, and local governments combined with private industry accounted for 500 000-700 000 samples in 1986. Furthermore, this number is growing at a rate of 25-40'70 per year (1). Obviously, this has put tremendous strain on the capacity of analytical laboratories. In many cases, samples are collected at a particular site, shipped to a central distribution point, and assigned to individual laboratories on the basis of capacity. All of this is done with relatively little knowledge of the stability of the samples. For each analytical method, maximum preanalytical holding times have been established in a rather arbitrary fashion. With a few exceptions ( 2 ) , limited systematic information on the long-term stability of volatiles in water exists. This work focuses on the develop0013-936X/90/0924-1665$02.50/0

2008, Oak

Ridge, Tennessee 37831-6120

ment of a data base that allows documentation of the stability of volatile organics in water, for purposes of increasing the preanalytical holding times and therefore reducing the cost associated with the analysis. The generation of a data base establishing preanalytical holding times presents formidable experimental difficulties, including the need for a large number of identical sample aliquots, the need for a variety of sample matrices, and the desire for a large number of potential analytes to be present. The high vapor pressure of these analytes requires that precautions be taken to minimize losses during sample aliquot preparation. In addition, since most environmental samples contain only a few of the potential analytes, a laboratory method for the preparation of samples containing all target compounds must be developed. Fortunately, an analytical method (GC/MS) exists that is capable of determining all of these analytes in a single run. However, there are analytical problems related to the long-term drift of the instrument, the stability of standard compounds, and the use of a method that was originally designed for screening purposes, not for highly accurate quantitative determinations. In this work these limitations have been largely overcome, and the data base reported here can be used to make an accurate assessment of the stability of volatile organic compounds in environmental water samples. Experimental Section

This study was designed to take into account as many variables as possible within the limitations of budget and sample capacity. For the initial investigation, two concentration levels were used: 50 and 500 pg/L. For the preservation studies, a single spike level, 100 pg/L, was employed. The exception to this was the spike level for the ketones and carbon disulfide, which was 250 pg/L. Higher levels were not considered since it was expected that stability would improve with increasing concentration. Three matrices were chosen in order to assess the effect of varying water quality parameters on stability. The storage conditions were chosen based on the possibility that samples might not be continuously chilled during collection and storage. Time intervals were chosen on the basis of a logarithmic increase, but were also designed to bracket the existing holding time of 10 days (3). The sample storage vials used were 40-mL borosilicate glass vials with Teflon-faced silicone septa and screw caps with holes. These vials were received fully assembled and precleaned according to EPA 40 CFR 136 and EPA 40 CFR 141 regulations. Three water sample matrices were used for this study. The waters were reagent grade water (water l),groundwater (water 2), and surface water (water 3). The groundwater and surface water sample matrices were obtained on site at Oak Ridge National Laboratory (ORNL). The methanol used was distilled-in-glass grade. Water quality parameters for the water matrices used in this study are reported in Table I. All target compound used either were obtained from the United States Environmental Protection Agency (U.S. EPA) Quality Assurance Materials Bank (Research Triangle Park, NC) ( 4 ) or were of equivalent purity and obtained commercially.

0 1990 American Chemical Society

Environ. Sci. Technol., Vol. 24, No. 11, 1990

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Table I. Water Quality Parametere for Groundwater and Surface Waters Used in Preanalytical Holding Time Study characteristic alkalinity, mg of CaCO,/L BOD,*mg/L COD, m d L c chloride; mg/L fluoride, mg/L nitrate, mg/L PH phosphate, mg/L sulfate, mg/L total hardness, mg/L

distilled ground- surface water water water

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