Isotope dilution methods esters

Cambell [78] has described an isotope dilution method for the determination of vinyl acetate in vinyl acetate-vinyl chloride copolymers. In this method a known amount of acetic 2- C acid is added to a solution or suspension of the resin in methyl ethyl ketone and the ester is hydrolysed with sodium hydroxide. The major portion of the sodium acetate is isolated and converted to 2-methyl benzimidazole-methyl C by means of the Phillips reaction [79] with o-phenylene diamine. 

The development of procedures for the identification of CW agents in biomedical samples is ongoing and existing procedures are continuously improved. Quantization is also an important factor, and an isotope dilution GC/MS/MS method was developed for the quantitative determination of five organophosphorus acids derived from the nerve agents VX, tabun, sarin, soman, and cyclohexyl sarin in urine samples. The acids were isolated and converted into their methyl esters by diazomethane. Detection limits in the low p,g I. 1 were obtained using CID of the protonated molecular ion peaks obtained with isobutane Cl(58). 

SP-2401" and 3% SP-2250. ° Detectors used by EPA standards procedures, include photoionization (PID)," electron capture (ECD)," Eourier transform infrared spectrometry (PTIR), " and mass spectrometry detectors (MSD)." ° Method 8061 employs an ECD, so identification of the phthalate esters should be supported by al least one additional qualitative technique. This method also describes the use of an additional column (14% cyanopropyl phenyl polysiloxane) and dual ECD analysis, which fulfills the above mentioned requirement. Among MSDs, most of the procedures employ electron impact (El) ionization, but chemical ionization (CI) ° is also employed. In all MSD methods, except 1625, quantitative analysis is performed using internal standard techniques with a single characteristic m/z- Method 1625 is an isotope dilution procedure. The use of a FTIR detector (method 8410) allows the identification of specific isomers that are not differentiated using GC-MSD. 

Phthalate esters listed as EPA priority pollutants may be analyzed in wastewaters by U.S. EPA (1984) Methods 606 (GC), 625 (GC/MS), and 1625 (isotope dilution). Soils, sediments, groundwater, and hazardons wastes may be analyzed by GC using FID or ECD (Method 8060) or by GC/MS (Method 8270 or 8250) (U.S. EPA 1986). The primary and secondary characteristic ions to identify these compounds by GC/MS (electron-impact ionization) are as follows dimethyl phthalate, 149, 177, and 150 din-butyl phthalate, 149, 150, and 104 butyl benzyl phthalate, 149, 167, and 279 and din-octyl phthalate, 149, 167, and 43. Other phthalate esters may be analyzed by GC or GC/MS techniques. A fused-silica capillary column is suitable for the purpose. 

Serum thyroxine has been precisely measured by selected ion monitoring as its methyl ester-N,0-bis(tri-fiuoroacetyl) [119, 120] and methyl ester-KO-bis(hepta-fluorobutyryl) derivatives [121] in connection with the application of isotope dilution GC-MS as a reference method to control the results obtained from less specific immunoassay methods. The fragment ions monitored are in the higher mass range of around 800 amu, with little interference. 

Heptafluorobutyrate derivatives are popular in isotope dilution reference methods for steroids such as testosterone [141], 17 -oestradiol [142-144] and progesterone [145-147]. Even difficult steroids such as aldosterone can be quantified in serum as the 3-enol-HFB ester by stable isotope dilution GC-MS [I4I] or after a new derivatization procedure with HFB anhydride [148). Pentafluoropropionyl derivatives have a)so found application to the quantification of oestriol [141] and urinary progesterone [149]. 

Recently, LC/MS has made significant contributions in the analysis of sterols and steryl esters. Takatsu and Nishi (1993) have employed discharge-assisted LC/TS/MS for the determination of total serum cholesterol. The method incorporates stable isotope dilution using [3,4- C] cholesterol as an internal standard. [MM — H20] ions were monitored by the SIM method. Satisfactory agreement between the analytical result and the certified value of the National Institute of Standards and Technology standard reference material was obtained with a relative standard deviation of 0.6%. The method does not require sterol derivatization. Yang et al. (1992) used FAB/MS to identify cholesteryl sulphate m/z 465) as the [M — H] ion recovered from the appropriate TLC fraction. 

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. 

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