Sample aqueous

Most compounds that dissolve in nonpolar solvents are nonpolar and may have significant vapor pressures. Thus, they can be introduced into an IMSs by the direct injection of the solvent or by deposition of the sample solution on a surface where the solvent is evaporated and the analyte is then thermally desorbed into the IMS. While this approach was acceptable for semivolatile compounds dissolved in nonpolar solvents, it still excludes important biological, environmental, and industrial samples dissolved in water. Water is the ubiquitous solvent on Earth. Thus, our living systems have evolved around the use of water to transport chanicals through our environment and within organisms. Most compounds important to chemistry or life on Earth have polar moieties that reduce or eliminate vapor pressure while increasing water solubility. Thus, application of IMS to aqueous samples significantly expands its utility as an analytical tool. 

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Liquid-liquid extractions using ammonium pyrrolidine dithiocarbamate (APDC) as a metal chelating agent are commonly encountered in the analysis of metal ions in aqueous samples. The sample and APDC are mixed together, and the resulting metal-ligand complexes are extracted into methyl isobutyl ketone before analysis. 

The procedure calls for placing a 100-pL aqueous sample containing the thiourea in a 60-mL separatory funnel along with 10 mL of a pH 7 buffer and 10 mL of a 12 pM solution of... 

Adjusting the Analyte s Concentration Analytes present at concentrations too small to give an adequate signal need to be concentrated before analyzing. A side benefit of many of the extraction methods outlined earlier is that they often concentrate the analytes. Volatile organic materials isolated from aqueous samples by a purge and trap, for example, can be concentrated by as much as 1000-fold. 

Let s assume that the solute to be separated is present in an aqueous phase of 1 M HCl and that the organic phase is benzene. Because benzene has the smaller density, it is the upper phase, and 1 M HCl is the lower phase. To begin the countercurrent extraction the aqueous sample containing the solute is placed in tube 0 along with a portion of benzene. As shown in figure A6.1a, initially all the solute is present in phase Lq. After extracting (figure A6.1b), a fraction p of the solute is present in phase Uq, and a fraction q is in phase Lq. This completes step 0 of the countercurrent extraction. Thus far there is no difference between a simple liquid-liquid extraction and a countercurrent extraction. 

Iron(II) can be analy2ed by a luminol—air reaction in the absence of hydrogen peroxide (276). Iron in the aqueous sample is reduced to iron(II) by sulfite other metals which might interfere are also reduced to valence states that are inactive in the absence of hydrogen peroxide. The detection limit is 10 ° M. 

Silver diethyldithiocarbamate [1470-61-7] is a reagent commonly used for the spectrophotometric measurement of arsenic in aqueous samples (51) and for the analysis of antimony (52). Silver iodate is used in the determination of chloride in biological samples such as blood (53). 

Environment. Detection of environmental degradation products of nerve agents directly from the surface of plant leaves using static secondary ion mass spectrometry (sims) has been demonstrated (97). Pinacolylmethylphosphonic acid (PMPA), isopropylmethylphosphonic acid (IMPA), and ethylmethylphosphonic acid (EMPA) were spiked from aqueous samples onto philodendron leaves prior to analysis by static sims. The minimum detection limits on philodendron leaves were estimated to be between 40 and 0.4 ng/mm for PMPA and IMPA and between 40 and 4 ng/mm for EMPA. Sims analyses of IMPA adsorbed on 10 different crop leaves were also performed in order to investigate general apphcabiflty of static sims for... 

The teehniques of membrane extraetion permit an effieient and modern applieation of elassieal liquid-liquid extraetion (LLE) ehemistry to instmmental and automated operation. Various shorteomings of LLE are overeome by membrane extraetion teehniques as they use none or very little organie solvents, high enriehment faetors ean be obtained and there ai e no problems with emulsions. A three phase SLM system (aq/org/aq), where analytes are extraeted from the aqueous sample into an organie liquid, immobilized in a porous hydrophobie membrane support, and further to a seeond aqueous phase, is suitable for the extraetion of polar eompounds (aeidie or basie, ehai ged, metals, ete.) and it is eompatible with reversed phase HPLC. A two-phase system (aq/org) where analytes ai e extraeted into an organie solvent sepai ated from the aqueous sample by a hydrophobie porous membrane is more suitable for hydrophobie analytes and is eompatible with gas ehromatography. 

Figure 2.12 Schematic representation of an on-line SPE-GC system consisting of three switching valves (VI-V3), two pumps (a solvent-delivery unit (SDU) pump and a syringe pump) and a GC system equipped with a solvent-vapour exit (SVE), an MS instrument detector, a retention gap, a retaining precolumn and an analytical column. Reprinted from Journal of Chromatography, AIIS, A. J. H. Eouter et al, Analysis of microcontaminants in aqueous samples hy fully automated on-line solid-phase extraction-gas chromatography-mass selective detection , pp. 67-83, copyright 1996, with permission from Elsevier Science.

Such a system with an atomic emission detector (AED) for the analysis of nitrogen-chlorine- and Sulfur-containing pesticides in aqueous samples (39), as shown in Figure 2.19. 

Figure 2.20 Schematic representation of the set-up used for on-line exti action-GC VI and V2, valves PI and P2, syringe pumps L, sample loop CC flow, countercunent flow CT, cold ti ap. Reprinted from Journal of High Resolution Chromatography, 16, H. G. J. Mol et ai, Use of open-tubular tapping columns for on-line exti action-capillary gas cluomatography of aqueous samples , pp. 413-418, 1993, with permission from Wiley-VCH.

J. J. Vreuls, R. T. Gliijsen, G. J. de Jong and U. A. Th Brinkman, Drying step for introduction of water-free desorption solvent into a gas cliromatograph after on-line liquid clir omatograpliic Cace enrichment of aqueous samples , 7. Chromatogr. 625 237 - 245 (1992). 

A. J. H. Louter, C. A. van Beekvelt, P Cid Montanes, J, Slobodnik, J. J. Vreuls and U. A. Th Brinkman, Analysis of microcontaminants in aqueous samples by fully automated on-line solid-phase exti action-gas cliromatography-mass selective detection , 7. Chromatogrl2S 67-83 (1996). 

H. G. J. Mol, H.-G. Janssen, C. A. Cramers and U. A. Th Brinkman, On-line sample enrichment-capillary gas clir omatography of aqueous samples using geometr ically deformed open-tubular extraction columns , 7. Microcolumn Sep. 7 247-257 (1995). 

Figure 5.3 Analysis of 100 ml of (a) surface water and (b) drinking water sample spiked with 0.1 pig/ml of microcystins, using column-switching HPLC 1, microcystin-RR 2, microcystin-YR 3, microcystin-LR. Reprinted from Journal of Chromatography A, 848, H. S. Lee et al, On-line trace enrichment for the simultaneous determination of microcystins in aqueous samples using high performance liquid chromatography with diode-array detection , pp 179-184, copyright 1999, with permission from Elsevier Science.

H. S. Eee, C. K. Jeong, H. M. Eee, S. J. Choi, K. S. Do, K. Kim and Y. H. Kim, On-line trace emicliment foi the simultaneous determination of microcystins in aqueous samples using high peifoi mance liquid cliromatography with diode-aixay detection , /. Chromatogr. 848 179-184 (1999). 

W. Golkiewicz, C. E. Werkhoven-Goewie, U. A. Th Brinkman, R. W. Erei, H. Colin and G. Guiochon, Use of pyrocarbon sorbents for rtace enrichment of polar compounds from aqueous samples with on-line HPEC analysis , /. Chromatogr. Sci. 21 27-33 (1981). 

Presently, the on-line coupling of NPLC and GC via heart-cutting is an established procedure which has been used successfully for several bioanalytical applications. Obviously, dfrect analysis of aqueous samples is not possible by NPLC, and therefore, a solvent switch by a sample pretreatment step (e.g. liquid-liquid extraction or SPE) is always requfred when biological samples are analysed by NPLC-GC. 

Although solid-phase microextraction (SPME) has only been introduced comparatively recently (134), it has already generated much interest and popularity. SPME is based on the equilibrium between an aqueous sample and a stationary phase coated on a fibre that is mounted in a syringe-like protective holder. Eor extraction, the fibre... 

Another example is the determination of bentazone in aqueous samples. Bentazone is a common medium-polar pesticide, and is an acidic compound which co-elutes with humic and/or fulvic acids. In this application, two additional boundary conditions are important. Eirst, the pH of the M-1 mobile phase should be as low as possible for processing large sample volumes, with a pH of 2.3 being about the best that one can achieve when working with alkyl-modified silicas. Secondly, modifier gradients should be avoided in order to prevent interferences caused by the continuous release of humic and/or fulvic acids from the column during the gradient (46). 

Figure 13.6 Direct RPLC analysis of a blank ground water sample spiked with 4.5 (p-g 1 ETU, (a) with and (b) without column-switching. A 60 X 4.6 mm i.d. column and a 150 X 4.6 mm i.d. column were used for C-1 and C-2, respectively, with pure water as M-1 and methanol-0.025 M ammonium acetate (pH, 7.5) (5 95, v/v) as M-2 S-1 and S-2 aie the interfering peaks. Reprinted from Chromatographia, 31, E. A. Hogendoom et at., Columnswitching RPLC for the trace-level determination of ethylenetlaiourea in aqueous samples , pp. 285-292, 1991, with permission from Vieweg Publishing.

Chemically modified polymers have been used to determine polar compounds in water samples (37, 71). Chemical modification involves introducing a polar group into polymeric resins. These give higher recoveries than their unmodified analogues for polar analytes. This is due to an increase in surface polarity which enables the aqueous sample to make better contact with the surface of the resin (35). 

The on-eolumn interfaee is the one whieh is most often used in LC-GC of aqueous samples beeause it ean be applied to a wider range of eompounds.The loop-type interfaee is limited for determining volatile eompounds that are volatilized together with the solvent and not retained in the retention gap. Several attempts at solving this problem have been made. One option is to add a eo-solvent whieh enters the retention gap before the analytes and thus forms a eo-solvent film in front of the eluate. 

Figure 13.20 GC-FID chromatograms of an exuact obtained by (a) SPE and, (b) lASPE of 10 ml of municipal waste water, spiked with 1 p.g 1 of seven s-triazines (c) represents a blank mn from lASPE-GC-NPD of 10 ml of EIPLC water. Peak identification is as follows 1, ati azine 2, terbuthylazine 3, sebuthylazine 4, simetiyn 5, prometiyn 6, terbutiyn 7, dipropetiyn. Reprinted from Journal of Chromatography, A 830, J. Dalliige et al, On-line coupling of immunoaffinity-based solid-phase exUaction and gas chi-omatography for the determination of 5-triazines in aqueous samples , pp. 377-386, copyright 1999, with permission from Elsevier Science.

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