Concentration and fractionation of hydrophobic organic acid constituents from natural waters by liquid chromatography

Abstract

A scheme is presented which used adsorption chromatography with pH gradient elution and size-exclusion chromatography to concentrate and separate hyd-ophobic organic acids from water. A review of chromatographic processes involved in the flow scheme is also presented. Organic analytes which appear in each aqueous fraction are quantified by dissolved organic carbon analysis. Hydrophobic organic acids in a water sample are concentrated on a porous acrylic resin. These acids usually const ; tute approximately 30-50 percent of the dissolved organic carbon in an unpolluted water sample and are eluted with an aqueous eluent (dilute base). The concentrate is then passed through a column of polyacryloylmorpholine gel, which separates the a°ids into, highand low-molecular-weight fractions. The highand low-molecular-weight eluates are reconcentrated by adsorption chromatography, then are eluted with a pH gradient into strong acids (predominately carboxylic acids) and weak acids (predominately phenolic compounds). For standard compounds and samples of unpolluted waters, the scheme fractionates humic substances into strong and weak acid fractions that are separated from the low molecular weight acids. A new method utilizing conductivity is also presented to estimate the acidic components in the methanol fraction. INTRODUCTION To effectively study most natural and anthropogenic orgaric substances in water, they must be concentrated and separated. F^cause the concentration of organic substances in water by DOC (dissolved organic carbon) analysis is from 1-20-mg/L (Malcolm and Durum, 1976), trace enrichment is necessary. Liquid chromatography is an analytical technique that can be very efficient in both the concentration and isolation of natural and anthropogenic organic analytes. This paper deals with the utilization of two types of liquid chrcmatography: liquid-solid adsorption chromatography with a porous acrylic resin, and size exclusion chromatography with a polyacryloylmorpholine gel, to concentrate and classify hydrophobic organic acid conGl G2 ORGANIC SUBSTANCES IN WATER stituents from water. The use of these two forms of liquid chromatography coupled with carbon analysis for detection allows a meaningful grouping of hydrophobic organic acid solutes into seven fractions. The mass of carbon in each fraction is quantified by DOC analysis. Although specific compound identification has not yet b«en accomplished, it is greatly facilitated by preconcentration and may be done on fractions of interest. The flow scheme combines various chromatographic procedures which includes the adsorption process (Malcolm and others, 1977; Thurman and others, 1977; Thurman and others, 1978; Aiken and others, 1977), size exclusion, and the most recent work with gradient elution and HPLC (high-performance liquid chromatography) with macroporous resins. The purpose of this report is to discuss integration of these various techniques and to develop a flow scheme for evaluation of hydrophobic-organic-acid constituents in a natural water sample. ACKNOWLEDGMENTS Appreciation is expressed to United States Geological Purvey personnel who made this study possible. EXPERIMENTAL PROCEDURES RESIN AND COLUMN PACKINGS Amberlite1 XAD-8, a porous acrylic ester, was obtained from Rohm and Haas Chemical Co. The 40to 60-mesh resin was extensively cleaned with O.I TV NaOH and sequentially extracted with methanol, ether, acetonitrile, and methanol for three days each, then stored in methanol. Medium grade KO Enzacryl 1 gel that has a polyacryloylmorpholine structure was obtained from Aldrich Chemical Co. The cleaning procedure was similar to that for XAD-8, except only NaOH and methanol were used. The gel was stored in methanol to prevent bacterial growth. INSTRUMENTATION A Varian Aerograph 2700 gas chromatograph, Varian £500 liquid chromatograph, Beckman 915 carbon analyzer with 2ISA infrared detector, and a Dohrmann 52D carbon analyzer with flame-ionization detector were used for sample and standard organic analysis. Peristaltic pumps were used for low-pressure systems with Glenco glass 'The use of the brand names in this report is foi identification purposes only and does not imply erdorsement by the U.S. Geological Survey. HYDROPHOBIC ORGANIC ACID CONSTITUENTS G3 columns, 3500 series. A Perkin Elmer Model 305B atomic absorption spectrophotometer was used for sodium analysis.