Volatile stability, during analysis

Host stabilizers are relatively nonvolatile so they do not vaporize during a thermal curing process. Unfortunately, their low volatility make GC analysis impossible for many stabilizers. HPLC works well for the UVA, but HALS are not easily detected by conventional UV or fluorescent detectors. High resolution capillary SFC was shown to be an ideal separation method for twenty-one polymer additives (17). We chose SFC to characterize stabilizers contained in automotive coatings. 

Ease of Detection and Stability. Numerous compounds are thermally produced in foods, but not all are suitable as chemical markers of sterility. Some of the compoimds that need to be ruled out include volatiles and unstable intermediates that rapidly undergo subsequent reactions. Preferably, the marker compound should be easily extracted wi an aqueous solvent and easily determined without many additional operations. The marker should also be stable during analysis. In situ analysis would be ideal however, accurate quantitation by simple in situ methods, such as surface fluorescence or near infrared measurements, is questionable. 

Splitless injection is required for very dilute solutions. It offers high resolution but is poor for quantitative analysis because less volatile compounds can be lost during injection. It is better than split injection for compounds of moderate thermal stability because the injection temperature is lower. Splitless injection introduces sample onto the column slowly, so solvent trapping or cold trapping is required. Therefore, splitless injection cannot be used for isothermal chromatography. Samples containing less than 100 ppm of each analyte can be analyzed with a column fdm thickness < 1 p.m with splitless injection. Samples containing 100-1 000 ppm of each analyte require a column film thickness 1 p.m. 

Rancidity measurements are taken by determining the concentration of either the intermediate compounds, or the more stable end products. Peroxide values (PV), thiobarbituric acid (TBA) test, fatty acid analysis, GC volatile analysis, active oxygen method (AOM), and sensory analysis are just some of the methods currently used for this purpose. Peroxide values and TBA tests are two very common rancidity tests however, the actual point of rancidity is discretionary. Determinations based on intermediate compounds (PV) are limited because the same value can represent two different points on the rancidity curve, thus making interpretations difficult. For example, a low PV can represent a sample just starting to become rancid, as well as a sample that has developed an extreme rancid characteristic. The TBA test has similar limitations, in that TBA values are typically quadratic with increasing oxidation. Due to the stability of some of the end-products, headspace GC is a fast and reliable method for oxidation measurement. Headspace techniques include static, dynamic and solid-phase microextraction (SPME) methods. Hexanal, which is the end-product formed from the oxidation of Q-6 unsaturated fatty acids (linoleate), is often found to be a major compound in the volatile profile of food products, and is often chosen as an indicator of oxidation in meals, especially during the early oxidative changes (Shahidi, 1994). 

During the derivatization reaction, proper attention should be paid to its yield, the stability of the derivatives produced and their volatility, which can be the reason for losses and errors in the analysis. The GC of derivatives can be performed on common instruments, major modifications of which are usually not required. Only a few derivatives are sensitive to the activity of the chromatographic support or the material of the column, and some unstable derivatives are affected by contact with metals. 

Lamkin et al. [276] studied in detail the GC analysis of silylated methylthiohydantoins of all protein amino acids. They effected the silylation with BSA-acetonitrile (1 3) at 100°C for 10 min. They separated the products in a simple column packed with 2% of OV-17 on Gas-Chrom Q at 145—230°C, and Fig. 5.20 illustrates the results. The authors used a flame photometric detector, sensitive to sulphur-containing compounds, in order to ensure sensitive and selective detection. Minor incidental peaks that were often noticed during the analysis of the samples obtained by the Edman degradation of proteins with the use of an FID did not appear and the peak of the solvent was not detected. The baseline stability was good and the response was linear over a range of two orders of magnitude of concentration. Asn and Phe were the only unresolved pair Arg, as in previous instances, did not form a volatile derivative. 

Metal oxide semiconductor chemical sensors in combination with MDA have been shown to be useful to estimate the oxidative stability of polypropylene during processing instead of traditional melt flow index analysis (50). An array of sensors was used to receive a detailed analysis of volatiles. At quality measurements of different poly(butylene adipate)s the use of indicator products has been proven better than analyses of the decrease in molecular weight or mass loss for early degradation detection. Adipic acid, quantified using gas chromatography, was then used as the indicator product [51]. 

A useful method of increasing substantially the stability of derivatives during chromatographic separation that has been applied in the analysis of volatile chelates of a number of elements can be mentioned. 

The reasons for derivatizing carbamates, apart from their thermolability during the gas chromatographic analysis, include increased detector sensitivity (particularly with electron capture detection), increased volatility, better chromatography separation, apphcability to multiresidue and confirmatory analysis, and enhanced compound stability. There are two general approaches to the analysis of Af-methylcarbamates by derivatization, namely derivatization of the intact pesticides and derivatization of a hydrolysis product (one of which will always be the volatile methylamine). The reactions typically used to obtain derivatives of both intact and hydrolysis products of A-methylcarbamates are methylation, silylation, halogenation, acylation, and esterification. 

AOCS has a recommended practice (Cg 3-91) for assessing oil quality and stability (AOCS, 2005) for measuring primary and secondary oxidation products either directly or indirectly. For example, peroxide value analysis (AOCS method Cd 8-53) (AOCS, 2005) determines the hydroperoxide content and is a good analysis of primary oxidation products. To determine secondary oxidation products, the procedure recommends p-anisidine value (AOCS Method Cd 18-90, 2005) volatile comlb by gas chromatography (AOCS Method Cg 4-94, 2005) and flavor evaluation. (AOCS Method Cg 2-83, 2005). The anisidine value method determines the amounts of aldehydes, principally 2-alkenals and 2, 4-dienals, in oils. The volatile compound analysis method measures secondary oxidation products formed during the decomposition of fatty acids. These comlb can be primarily responsible for the flavors in oils. The... 

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