Normal phase soil extracts

The types of nonbonded phases used for normal-phase SPE are silica, alumina, and magnesium silicate (Florisil). The most popular phase is silica. Several bonded phases may also be used for normal-phase SPE, including aminopropyl, cyanopropyl, and propyldiol (Table 1.1, Fig. 2.7). Water is not used in the mobile phase in normal-phase SPE because it will sorb to the active sites of the sorbent and reduce the interaction between analyte and sorbent. Typically, normal-phase SPE is used as a clean-up procedure for organic extracts of water, soil, food, or other materials. Normal-phase SPE is also used for the isolation of analytes from organic liquids, such as oils. 

An understanding of simple methods development is crucial to developing effective environmental applications of solid-phase extraction (SPE). The lour mechanisms outlined in Chapter 2 (reversed phase, normal phase, ion exchange, and mixed mode) are sufficient for the majority of SPE applications in environmental analysis. The molecule s structure and the sample matrix are (he main factors that will determine which mechanism of isolation and separation will be the most appropriate. The fundamental approach to selection of sorbents will be a key topic and many examples are given. This chapter will also discuss applications of SPE to environmental matrices. These include water, soil, and air, for a variety of compounds. 

Normal-Phase Solid-Phase Extraction of Soil Extracts... 

An example is endrin in soil (Fig. 7.7). This highly chlorinated insecticide is extracted by Soxhlet extraction with hexane from a soil that has been mixed one to one with sodium sulfate to remove water. The hexane extract is further dried over sodium sulfate and the hexane is added to a hexane-loaded silica or CN SPE sorbent that has been prepared with hexane. The endrin is sorbed to the silica or CN by normal-phase mechanisms and may be eluted with a solvent that will overcome the interaction, such as methylene chloride or ethyl acetate. The more polar interferences are sorbed to the silica or CN sorbent and are not eluted with endrin. Thus, the SPE column performs both trace enrichment and SPE clean-up. See Chapter 5 for more examples. 

Fifteen to twenty g of dried soil were extracted in a Soxhlet (8h) with 80 mL of methanol for the LAS determination by reversed-phase HPLC, and with 80 mL of n-hexane for the NP, NPIEO and NP2EO determination by normal-phase HPLC. Before the extraction with methanol, NaOH (10% w/w) and the internal standard, 3-C15-LAS (50-100 pL of 1 pg/pL solution), were added. The methanolic extract was rotaiy evaporated to approximately 2.5 mL, diluted to 10 mL with water containing 2 xiO m SDS and acetone in such a way to yield a final solvent mixture of 1 1 2 MeOH H20 Acetone. Finally, this solution was centrifuged. The n-hexane extract was rotary evaporated and made up to 10 mL. 

Fig.2. Normal-phase chromatograms of a standard solution of NP, NPIEO, and NP2EO (A), and of a n-hexane extract of sludge amended soil 61 days after the sludge application (B).

This paper is the only one in the liquid chromatography portion of this symposium which will attempt to deal with chromatography specifically from the viewpoint of the pesticide metabolism chemist. A residue analyst knows what compound he must analyze for, and develops his method with the properties of that substance in mind. On the other hand, the pesticide metabolism chemist has a different problem. At the conclusion of the treatment, exposure, and harvest phases of a radiolabeled metabolism study, he divides his material into appropriate samples, and extracts each sample with selected solvents to obtain the radioactive materials in soluble form. Typically these extracts consist of low levels (ppm) of carbon-14 labeled metabolites in a complicated mixture of normal natural products from the plant, animal, or soil source. The identity of each metabolite is unknown, and each must be isolated from the natural background and from other labeled metabolites in sufficient quantity and in adequate purity for identification studies, usually by mass spectrometry. The situation is rather like looking for the proverbial "needle in the haystack" when one does not know the size, shape,or composition of the needle, or even how many needles there are in the stack. At this point a separation technique must be selected with certain important requirements in mind. 

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