Oxidation, during sample preparation

FIGURE 7.5. A by-product formed by air oxidation during sample preparation... 

Figure 4.11 reveals that Pt is present on the surface of the catalyst as an oxide, in combination with hydrocarbon species (a contaminant during sample preparation) and as a chloride (derived from the Pt precursor, chloroplatinic acid). The results show the composition of the washcoat to be Pt and Rh on alumina and ceria. 

Because the instability of the N-oxide metabolite, which was subjected to decomposition during sample preparation (solvent evaporation during offline SPE), online SPE LC/MS became the method of choice for the application. Hsieh et al. (2004) built a system with two TFC cartridges and one analytical column, and another system with two TFC cartridges and two analytical columns for GLP quantitative bioanalysis of drug candidates. A Turbo C18 (50 x 1.0 mm, 5 /.mi, Cohesive Technologies), an Xterra MS C18 (30 x 2.0 mm, 2.5 /mi), and a guard column were used. Protein precipitation preceded injection. The cycle times for the two systems were 0.8 and 0.4 min. 

The most important point during sample preparation is to prevent oxidation of ascorbic acid. Indeed, it is easily oxidized by an alkaline pH, heavy metal ions (Cu and Fe ), the presence of halogens compounds, and hydrogen peroxide. The most suitable solvent for this purpose is metaphosphoric acid, which inhibits L-ascorbic oxidase and metal catalysis, and it causes the precipitation of proteins. However, it can cause serious analytical interactions with silica-based column, e.g., C18 or amino bonded-phases [542] and it could co-elute with AA. 

A number of hydrolysis products of chloroaluminafe salfs have been defecfed using FAB MS. The problem here is that under some circumstances FAB MS can provide only a restricted view on the surface of fhe sample that is more likely to be oxidized or hydrolyzed rather than the average sample [13]. Nevertheless, this problem is not directly related to MS itself, but rather to a problem of proper sample preparation. Inert conditions during sample preparation and construction of special sample inlet chambers for MS can help avoid fhese reacfions. The addition of phosgene was suggested to reduce problems encountered with such side reactions [22], but here the problems encountered in working with an extremely toxic chemical have to be taken into account. 

A batch of Se-labeled and enriched yeast was characterized with regard to isotopic composition and content of selenium species for later use in a human absorption study based on the method of enriched stable isotopes. SeMet constituted 53% of the total Se content in the yeast. Oxidation of SeMet to selenomethionine-Se-oxide (SeOMet) occurred during sample preparation... 

A wet oxidation procedure which utilizes sulfuric, nitric, and perchloric acid was established as a satisfactory method for destroying the organic matrix of petroleum samples with quantitative retention of the arsenic. Table 5.1 compares the amount of arsenic added to three petroleum samples with the amount of arsenic found on analysis by the method subsequently adopted. In each case the amount found by analysis agrees with the amount added within the precision of the method and indicates that the arsenic is quantitatively retained by the sample preparation procedure. Quantitative retention of the arsenic was further substantiated by neutron activation analysis of a sample which was spiked with a known amount of triphenylarsine. The arsenic concentration was determined at each step of the sample preparation procedure (Table 5., II). The results were in general agreement with the amount added and confirmed the earlier conclusion that arsenic is quantitatively retained during sample preparation. 

Quantitative recovery of selenium was verified for a gasoline sample spiked with a known amount of selenium as dilaurylselenide and a mineral oil (white oil) spiked with dilaurylselenide and selenium dioxide. The samples were oxidized under total reflux, and the selenium was measured by the hydride generation-atomic absorption technique. Quantitative recovery of the added selenium indicated that selenium is not lost during sample preparation (Table 15.1). 

The hydroxyl groups formed during crosshnking can be oxidized to carbonyl groups, especially at the elevated temperature during sample preparation (see Scheme 28.1). This process can cause the band at 1738 cm that appears after post-curing. In addition, the oxidation of carbonyl groups is catalyzed by copper in common chemical practice and this would explain why the band is most pronounced on this substrate. 

Analytical method. The most serious errors are those in the method itself. Examples of method errors include (1) incomplete reaction for chemical methods, (2) unexpected interferences from the sample itself or reagents used, (3) having the analyte in the wrong oxidation state for the measurement, (4) loss of analyte during sample preparation by volatilization or precipitation, and (5) an error in calculation based on incorrect assumptions in the procedure (errors can evolve from assignment of an incorrect formula or molecular weight to the sample). Most analytical chemists developing a method check all the compounds likely to be present in the sample to see if they interfere with the determination of the analyte unlikely interferences may not have been checked. Once a vahd method is developed, an SOP for the method should be written so that it is performed the same way every time it is run. 

ATR on Mineral-Bed Electrodes. ATR at a mineral-bed electrode was employed to study the anodic oxidation of ethyl xanthate (EX) on chalcocite, chalcopyrite, pyrite, and galena [513, 514]. The optical scheme of the SEC cell is shown in Fig. 4.51. Prior to the addition of xanthate to the buffer, the electrode was polarized cathodically in order to remove any oxidation products that are formed during sample preparation. After a polarization period of 15 min at the selected potential, the electrode was pressed against a Ge IRE and the spectrum was measured while applying a potential less than or equal to -l-0.1 V. Otherwise, the spectra were recorded at open-circuit potential (OCP) just after the polarization to avoid corrosion of the Ge IRE. 

Bortolomeazzi et al. (1994) used GC/EI/MS with an ion trap to identify the thermal oxidation products of cholesteryl acetate as the 7P-hydroperoxy and 7a-hydroperoxy cholesteryl acetate, 7keto-cholesteryl acetate, the a and P isomers of 7-hydroxycholesteryl acetate, the a- and P-5,6-epoxy isomers and several derivatives arising from the loss of acetate and water. Dzeletovic et al. (1995b) have observed that saponification during sample preparation did not hydrolyse all of the oxysterol esters completely and that separation of oxysterols from cholesterol by HPLC was tedious and incomplete. They developed a stable isotope dilution GC/EI/MS SIM method for the determination of cholesterol oxidation products in human plasma. Nine oxysterols were determined by using deuterium-labelled internal standards. 

The other factor that needs to be taken into consideration during sample processing is the stability of folate vitamers. Stability of the vitamers differs with respect to the susceptibility to oxidative degradation, thermal, pH, and ultra-violet light [51]. Interconversion of folate vitamers occurs during sample preparation and analysis, for example, 5,10-methylenetetrahydrofolate dissociates from tetrahydrofolate and formaldehyde in the presence of mercaptoethanol (antioxidant) at low pH [49]. Furthermore, it has been argued that 5,10-methenyltetrahydrofolate is difficult to analyze on a reversed-phase (RP-18) HPLC column with a low pH mobile phase [13]. This is because interconversion of 10-formyltetrahydrofoIate to 5,10-methenyltetrahydrofolate occurs at low pH [18]. 

In order to avoid the oxidation of the compounds, several antioxidants can be added as diethyldithiocarbamate [52,79,82], tert-butyl-hydroquinone (TBHQ), [80,83,84] or butylated-hydroxy-toluene (BHT) [81]. Furthermore, internal standards are usually used, such as morin [52,80], luteolin [80], isorhanmetin, [79,82] or kaempferol [79] depending on the phenolic compounds analyzed to monitor losses during sample preparation. 

Preliminary extraction of lipids is important prior to vitamin D analysis by TLC. Jones et al. (1992) provided a table of extraction procedures for vitamin D compounds. Care must be taken to avoid oxidizing vitamin D on the TLC plate. Some workers use 0°C and a nitrogen atmosphere during sample preparation and subsequent TLC of vitamin D. 

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