Environmental analysis acidic herbicides

The PS-DVB sorbents can be more retentive than the bonded silica sorbents. Polymeric sorbents have been shown to be capable of retaining chemicals in their ionized form even at neutral pH. Pichon et al. [88] reported SPE recovery of selected acidic herbicides using a styrene-divinylbenzene sorbent so retentive that no adjustment of the pH of the solution was necessary to achieve retention from water samples at pH 7. At pH 7 the analytes were ionized and thereby retained in their ionic form. To effect retention of acidic compounds in their nonionized form using bonded silica sorbents, it is necessary to lower the pH of the sample to approximately 2. Analysis at neutral pH can be preferable to reduced pH because at lower pHs undesirable matrix contaminants, such as humic substances in environmental samples, can be coextracted and coeluted with the analytes of interest and subsequently may interfere with chromatographic analyses. 

Membrane extraction is especially effective in discriminating toward macromolecules. In environmental analysis, macro-molecular humic acids are ubiquitous and usually very efficiently removed. Megersa et al. [63] compared SLM and SPE of some triazine herbicides spiked in river water. As is seen from Figure 12.5, the difference is dramatic. Also, there are many examples of how various drugs can be determined in blood plasma and also urine without matrix interferences [25,64]. 

This method complements the U.S. Environmental Protection Agency s gas chromatographic (GC) SW-846 Method 8151 for the determination of chlorinated acid herbicides. Method 8151 prescribes the use of hydrolysis to ensure that these compounds arc in the acid form, reaction of the acids with diazomethane to produce methyl esters, and GC or GC/MS analysis. This HPLC method eliminates the need for most sample preparation steps and allows direct analysis of sample extracts. 

Analysis of Chloronhenoxv Herbicides. Many important target environmental pollutants can only be detected via conventional GC methods by first converting them to derivatives that are less polar and more volatile, e.g., the chlorophenoxy herbicides. A standard EPA method (SW-846 8150) specifies soil extraction and alkaline hydrolysis of any esters present followed by (re) esterification via diazomethane and detection and confirmation by GC/MS. The methylation step is required because the free carboxylic acids will not pass through conventional GC analytical columns. Reversed phase chromatography functions equally well to resolve free carboxylic acids or the corresponding esters and thus can eliminate the diazomethylation step. An interlaboratory check sample provided by the EPA of soil spiked with the chlorophenoxy acid herbicides Silvex and 2,4-D was obtained by our laboratory to demonstrate that LC/MS can offer a simpler and effective method for these compounds. 

Li, N. and Lee, Ft. K., Sample preparation based on dynamic ion-exchange SPE for GC/MS analysis of acidic herbicides in environmental waters. Anal. Chem., 72, 3077-3084, 2000. 

It is used in organic synthesis as a methylating agent to methylate acidic compounds such as carboxylic acids and phenols. It is used in trace environmental analysis to methylate chlorophenoxy acid herbicides. 

Some chlorinated hydrocarbon (chlorobenzenes, HCH isomers) were extracted with CO2 and toluene- and cyclohexane-containing CO2 from soil samples with high recovery (41f) (Table 1). A review with 62 references summarizes the recently-published methods for the analysis of PCBs, OC insecticides, s-triazine herbicides, phenoxycarboxylic acid herbicides, and PAHs in environmental samples (41g) (Table 1). 

LPME or single-drop microextraction (SDME) involves extraction of organic contaminants from an aqueous donor solution into a microdrop (typically 1 jjlL) of an organic acceptor solvent suspended from the tip of a microsyringe. After extracting for a preset period of time, the microdrop is retracted back into the microsyringe and transferred to a gas chromatograph for further analysis [114]. Various types of pesticides have been extracted by LPME from water samples for determination by GC [115-121]. Recently, an automated d)mamic liquid-liquid-liquid microextraction (D-LLLME) system was developed for the extraction of phenoxy acid herbicides from environmental waters and their determination by LC [122]. 

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