Organic acids sample pretreatment

Liquid-liquid extraction (LLE), solid phase extraction (SPE), and solid phase micro-extraction (SPME) are frequently applied for sample pretreatment. The type of extraction is highly related to the type of metabolites selected for determination. In previous metabolomics studies, extraction was focused on compounds of adequate stability that could be extracted together (carbohydrates, esters, amino acids, or organic acids). 

Compared to HPLC, GC, TLC, GLC, and IC, CE offers (1) high resolution efficiency, (2) rapid separation speed, (3) simple sample pretreatment, (4) small consuming sample, and (5) short analysis time. No derivatiza-tion is needed as in GC or a sample extraction as in HPLC or IC. Addition of SDS (anionic surfactant) causes an increase in migration times of all organic acids. (Tang and Wu, 2007). 

Most AEC methods use direct sample injections vial a sample loop, thus achieving adequate detection limits." " Aqueous sample matrices usually do not require any sample pretreatment other than possibly filtration prior to injection. For matrices such as airborne particles, soil, or filters from air collection devices, organic acids are commonly transferred into an aqueous solution which can then be injected directly into the ion chromatograph. ° ° ... 

Most environmental samples require some pretreatment, usually in the form of filtration and centrifugation, before these are injected via a sample loop. lEC separation of organic acids is achieved on cation exchange columns which are characterized by their particle diameter, substrate crosslinking, ion exchange capacity, type of functional group, and hydrophobicity. The most commonly used columns are comprised of fully sulfonated, crosslinked divinylbenzene/polystyrene copolymers (e.g., HPICE-AS6 or -ASl, Dionex). However, a comparison smdy found other columns to be equally as effective for organic acid separation. Unmodified silica gel columns... 

More recently lEC has also been employed for the analysis of landfill leachates.Sample pretreatment steps may include centrifugation, filtration, removal of interfering organic compounds, and carbonate removal. An advantage is that these lEC methods analyze for a wider spectrum of organic acids than the GC methods, hence giving a more complete picture of the organic acid composition in landfill leachates. 

Four types of analyses can be performed in metal determinations, dissolved, suspended, total, and acid extractable metals, based on different sample pretreatment steps. The term total metals includes all inorganically and organically bound metals and requires a vigorous digestion step of the unfiltered sample before analysis. Dissolved metals determination requires filtration of unacidified sample that passes through a 0.45 pm membrane filter. Suspended metals refer to the metals that are retained by a 0.45 pm filter. Acid-extractable metals are determined after the unfiltered sample is treated with hot dilute mineral acid before analysis. 

Filtration using membrane filters with a pore size of 0.45 xm is the most common sample pretreatment procedure for ion chromatography. For biologically active samples, the use of sterile filters with a pore size of 0.2 j,m is strongly advisable. Filters are made either from PTFE, polyvinyldienedifluoride (PVDF), or similar material. Cellulose filters are generally unsuitable. Filter materials differ in pore size, porosity, filtration speed, compatibility with acids, abuse, and organic solvents, as well as in their adsorption properties and blank values. 

In general, any strong acid can be used for the extraction. The choice of acid depends on the sample matrix. Acceptable results have been obtained with hydrochloric and perchloric acids. Nitric acid may oxidize organic matrices making certain constituents in the matrix soluble. Sulfuric acid is another acceptable acid for pretreatment extraction. However, sulfuric acid will cause disturbances if electrothermal AAS is used as the method of magnesium determination. The addition of a surfactant such as Triton X-100 can assist in the digestion of hydrophobic material. 

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