Evaporator response analysis

Preparation of soil—sediment of water samples for herbicide analysis generally has consisted of solvent extraction of the sample, followed by cleanup of the extract through Uquid—Uquid or column chromatography, and finally, concentration through evaporation (285). This complex but necessary series of procedures is time-consuming and is responsible for the high cost of herbicide analyses. The advent of soUd-phase extraction techniques in which the sample is simultaneously cleaned up and concentrated has condensed these steps and thus gready simplified sample preparation (286). 

Detection is also frequently a key issue in polymer analysis, so much so that a section below is devoted to detectors. Only two detectors, the ultra-violet-visible spectrophotometer (UV-VIS) and the differential refractive index (DRI), are commonly in use as concentration-sensitive detectors in GPC. Many of the common polymer solvents absorb in the UV, so UV detection is the exception rather than the rule. Refractive index detectors have improved markedly in the last decade, but the limit of detection remains a common problem. Also, it is quite common that one component may have a positive RI response, while a second has a zero or negative response. This can be particularly problematic in co-polymer analysis. Although such problems can often be solved by changing or blending solvents, a third detector, the evaporative light-scattering detector, has found some favor. 

FTIR in multiply hyphenated systems may be either off-line (with on-line collection of peaks) [666,667] or directly on-line [668,669]. Off-line techniques may be essential for minor components in a mixture, where long analysis times are required for FT-based techniques (NMR, IR), or where careful optimisation of the response is needed. In an early study a prototype configuration comprised SEC, a triple quadrupole mass spectrometer, off-line evaporative FTIR with splitting after UV detection see Scheme 7.12c [667]. Off-line IR spectroscopy (LC Transform ) provides good-quality spectra with no interferences from the mobile phase and the potential for very high sensitivity. Advanced approaches consist of an HPLC system incorporating a UV diode array, FTIR (using an ATR flow-cell to obtain on-flow IR spectra), NMR and ToF-MS. 

Adapting the evaporative light scattering device (ELSD) to pHPLC was investigated by Gaudin et al. Quantitative analysis by ELSD is often hindered by nonlinearity however, reduction of the flow rate, resulting in better homogeneity of droplet size distribution, has increased the linearity of the response with ELSD. Despite the predictable effect on droplet size in relation to the reduction of the inner diameter of the capillary inside the nebulizer, ELSD is relatively simple to adapt to micro/ capillary EC. ... 

Evaporative light-scattering detectors have become by far the most widely used detectors in the analysis of phospholipid classes. Generally, this is due not only to their compatibility with a very wide range of eluents but also to the fact that the responses obtained are much more uniform (20). 

Determinative and confirmatory methods of analysis for PIR residue in bovine milk and liver have been developed, based on HPLC-TS-MS (209). Milk sample preparation consisted of precipitating the milk proteins with acidified MeCN followed by partitioning with a mixture of -butylchloride and hexane, LLE of PIR from aqueous phase into methylene chloride, and SPE cleanup. The dry residue after methylene chloride extraction was dissolved in ammonium hydroxide, and this basic solution was transferred to the top of Cl8 SPE column. The PIR elution was accomplished with TEA in MeOH. For liver, the samples were extracted with trifluoroacetic acid (TFA) in MeCN. The aqueous component was released from the organic solvent with n-butyl chloride. The aqueous solution was reduced in volume by evaporation, basified with ammonium hydroxide, and then extracted with methylene chloride. The organic solvent was evaporated to dryness, and the residue was dissolved in ammonium acetate. The overall recovery of PIR in milk was 94.5%, RSD of 8.7%, for liver 97.6%, RSD of 5.1 %. A chromatographically resolved stereoisomer of PIR with TS-MS response characteristics identical to PIR was used as an internal standard for the quantitative analysis of the ratio of peak areas of PIR and internal standard in the pro-tonated molecular-ion chromatogram at m/z 411.2. The mass spectrometer was set for an 8 min SIM-MS acquisition. Six samples can be processed and analyzed in approximately 3 hours. 

Trifluoroacetates of 18 steroids were analysed by Voelter et al. [353] on OV-17, OV-1 and XE-60 stationary phases and compared with the results for TMS derivatives. On the first two stationary phases TFA derivatives have shorter retention times, whereas on XE-60 the reverse applies. With the use of the FED, TFA derivatives gave 30—50% higher responses. These derivatives were also applied to the analysis of the bile acids [354]. In order to eliminate the treatment with diazomethane, the carboxyl group of bile acids was blocked by the reaction with hexafluoroisopropanol, as follows. A 100-pl volume of hexafluoroisopropanol and 200 pi of trifluoroacetic anhydride were added to the dried extract of bile acids and the mixture was heated at 37° C for 30 min. The mixture was evaporated under reduced pressure at room temperature and the residue dissolved in 100 pi of acetonitrile 5 pi were analysed on a 2 m X3 mm I.D. column packed with 1% QF-1 on Chromorosb W (80 100 mesh). As the FID was applied, a high ECD response was not used to advantage. 

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