Clean-up step

The process is carried at moderate (slightly above atmospheric) pressures, but at very high temperatures that reach a maximum of 1900°C. Even though the reaction time is short (0.6—0.8 s) the high temperature prevents the occurrence of any condensable hydrocarbons, phenols, and/or tar in the product gas. The absence of Hquid simplifies the subsequent gas clean-up steps. 

The main advantages of the ms/ms systems are related to the sensitivity and selectivity they provide. Two mass analyzers in tandem significantly enhance selectivity. Thus samples in very complex matrices can be characterized quickly with Htde or no sample clean-up. Direct introduction of samples such as coca leaves or urine into an ms or even a gc/lc/ms system requires a clean-up step that is not needed in tandem mass spectrometry (28,29). Adding the sensitivity of the electron multiplier to this type of selectivity makes ms/ms a powerhil analytical tool, indeed. It should be noted that introduction of very complex materials increases the frequency of ion source cleaning compared to single-stage instmments where sample clean-up is done first. 

For these reasons Reaktions-Chromatographie [7] ( Chromatographie fonctio-nelle sur couche mince [1,2]) is steadily gaining in importance. Here the reaction, which also then takes on the role of a clean-up step, is performed at the start or in the concentration zone of the TLC plate. 

In 1994, Nam and King (68) developed a SFE/SFC/GC instrumentation system for the quantitative analysis of organochlorine and organophosphorus pesticide residues in fatty food samples (chicken fat, ground beef and lard). In this way, SFC was used as an on-line clean-up step to remove extracted material. The fraction containing pesticide residues is then diverted and analysed by GC. 

It is often possible to carry out any clean-up step that may be necessary in the concentrating zone of a suitable chromatographic plate, in any event clean-up is less complex than for other forms of chromatography. 

As is well known the difficulty of analysis of a sample increases as its complexity increases. Analysis usually commences with a rather nonspecific clean-up step and requires that the separation step that follows be highly selective and depends on a detection step that is as specific as possible. As the selectivity of detection increases there is also an increase in the reliability of the identification and it is possible to reduce the demands made on the selectivity of the preceding separation method. This is the case for radiometric and enzymatic methods and also explains the popularity of fluorescence measurements. The latter obtain their selectivity from the freedom to choose excitation and measurement wavelengths. 

A single SFE/ESE instrument may perform (i) pressurised C02 (SFE), (ii) pressurised C02/modifier and (iii) pressurised modifier (i.e. ASE /ESE , organic solvent) extractions. The division between SFE and ASE /ESE blurs when high percentages of modifier are used. Each method has its own unique advantages and applications. ESE is a viable method to conduct matrix/analyte extraction provided a solvent with good solvating power for the analyte is selected. Sample clean-up is necessary for certain matrix/analyte combinations. In some circumstances studied [498], SFE may offer a better choice since recoveries are comparable but the clean-up step is not necessary. 

Yields a sample ready for direct further analysis (no concentration or clean-up steps)... 

Clean-up steps frequently required (separation of nonelutable oligomers, heavy waxes)... 

Water samples are acidified and extracted with solvent (Kawamura and Kaplan 1983 Muir et al. 1981). Clean-up steps may be used (Kawamura and Kaplan 1983). Methylene chloride is often used as the extracting solvent, and it may interfere with the nitrogen-phosphorus detector. In this case, a solvent-exchange step is used (Muir et al. 1981). Analysis by GC/NPD or GC/MS provides specificity (Kawamura and Kaplan 1983 Muir et al. 1981). Accuracy is acceptable (>80%), but precision has not been reported. Detection limits were not reported, but are estimated to be 0.05-0.1 pg/L (Muir et al. 1981). Detection limits at the low ppt level (ng/L) were achieved by concentrating organophosphate esters on XAD-2 resin. The analytes were solvent extracted from the resin and analyzed by GC/NPD and GC/MS. Recovery was acceptable (>70%) and precision was good (<10% RSD) (LeBel et al. 1981). 

In order to suppress interferences due to the presence of inorganic species and reliably determine the proteinaceous composition of the sample, a clean-up step has often been introduced in the analytical procedure. This step may include the extraction of the proteinaceous matter by an ammonia solution [8], the use of a cation-exchange resin [8,55], a chelating agent [9,41,44], the use of a Cig resin or the use of barium chloride solution to suppress sulfates [10,81,82]. Table 9.1 reports the methods used to overcome such problems. 

Kr is extracted either by stepwise heating or melting of a sample followed by usually several gas clean-up steps and by separation of Kr from other noble gases by selective adsorption on charcoal at cryogenic temperatures [11]. Kr is analysed in ultra-clean static mass spectrometers which allows recycling of... 

For discovery PK samples, rapid method development is required. For HPLC/MS/MS assays, method development can be achieved within 2 hr if no unusual problems are encountered. Xu et al.101 described a process for rapid method development as part of the discovery PK paradigm. As shown in Figure 7.4, the systematic process is based on using protein precipitation as the sample clean-up step and generic HPLC conditions for the HPLC/MS/MS assay. 

Cotterill [222] has developed a procedure, discussed below, in which the herbicides are extracted from soil with saturated calcium hydroxide solution. After clean-up the residues are ethylated using iodoethane and tetrabutylammonium hydrogen sulphate as counter ion. Liquid-liquid partition and the use of a macroreticular resin column were compared as clean-up steps and the reaction conditions for optimum yield of ethyl ester were evaluated. 

A clean-up step may be employed using gel permeation chromatography, Florisil, silica gel or alumina column fractionation, or solid phase extraction (SPE). 

Figure 2.1.1 shows the most common derivatisation methods for anionic surfactants reported in the literature [1]. The first method of LAS determination by GC consisted of a microdesulfonation procedure in which LASs were desulfonated in boiling phosphoric acid at high temperature [2-4] and the corresponding alkylbenzenes analysed. The microdesulfonation method was further improved by introducing additional concentration and clean-up steps [5—11], which allowed the determination of LAS in influent, effluent and river water samples at low qg L-1 levels [7,8] and sediment and sludge samples [8] at pg g-1. In addition to the desulfonation procedure, several derivatisation techniques have been used to make LAS analysis amenable to GC. 

The complex composition of aqueous environmental sample matrices, especially sewage and marine water samples, and the low concentrations in which the surfactants are generally found, have made it necessary to perform an initial stage of concentration and purification of the analytes prior to its analysis. Traditionally, such steps were carried out off-line with procedures based on liquid—liquid extraction (LLE), sublation or steam distillation, followed by chromatographic clean-up steps. 

As Soxhlet extraction is not a very specific technique, interferences will be present in the raw extract, and in most cases, a clean-up step is necessary. Several clean-up types have been reported, including... 

Further steps may include derivatisation, necessary when using GC methods for the final detection [29]. Derivatisation has been performed with pentafluorobenzyl bromide [22,30], acetic anhydride [24], or N,0-bis(trimethylsilyl)trifluoroacetamide [25] (see also Chapter 2.1). A liquid-liquid separation with i-hexane and an alkaline aqueous solution proved to be unsuccessful as a clean-up step for alkylphenols, as the compounds ended up in both phases [23]. 

The extraction time applied was 20 min at 75°C and 150 atm and a flow rate of 1.5-2 mL min-1. The clean-up step consisted of a SAX Empore disk, which was simultaneously eluted and methylated by methyl iodide in acetonitrile [9]. 

Shang et al. [5] used ASE for the extraction of NP and NPEO from estuarine sediments. A sample of 15-25 g was extracted three times using hexane/acetone at 100°C and 103 atm. This was followed by a clean-up step using CN-SPE. A blank sample was extracted between all samples to avoid contamination. Hexane/acetone was also used in the ASE method for alkylphenols and NPEO by Heemken et al. [12]. Extraction conditions for samples of 0.5-1 g were 100°C, 150 atm, with a static extraction step of 15 min, and a rinse step with 20 mL solvent. After a clean-up by HPLC, the analytes were derivatised with heptafluorobutyric acid anhydride for GC analysis. 

Snyder et al. [12] and Keith et al. [13] used steam distillation for the extraction of NP and NPEOi 3 from fish tissues. Normal phase HLPC was used as a subsequent clean-up step prior to final analysis by GC-MSD using selected ion monitoring (SIM). 

Sample preparation for biological matrices involves solvent extraction followed by clean-up steps. Biological samples are often contaminated with other compounds such as polychlorinated biphenyls (PCBs) therefore, additional clean-up steps and/or confirmation techniques are employed to assure reliable results. 

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