Labile compounds, analysis

Problems may be encountered in the analysis of thermally labile compounds, as heat is required for mobile-phase removal and for the transfer of analyte from the belt into the source of the mass spectrometer, and highly involatile compounds which cannot be desorbed from the belt, unless FAB is used for ionization. 

The APCl ionization regime is much more harsh that ESI and this precludes its use for the study of large biomolecules, with the mass limit for APCl being generally considered as below 2000 Da. Having said this, as will be shown later, the technique may still be used for the analysis of many thermally labile compounds without their decomposition, and small peptides have been studied. 

The recommended technique for the determination of oxime carbamates and their metabolites by HPLC/MS and HPLC/MS/MS is positive ESI. Electrospray is a soft ionization technique and is suitable for thermally labile compounds. Ions are produced in the liquid phase at quasi-ambient temperature and atmospheric pressure, thus leaving the fragile pesticides intact. For oxime carbamates, the molecular adducts that can be monitored during HPLC/MS analysis with electrospray in positive mode are [M- -H]+, [M- -Na]+, or [M- -NH4]+, depending on the nature of mobile phase used. ... 

IonPac KC-811 column separated the labile compounds N-acetylneuraminic acid and N-glycolylneuraminic acid released by mild acid hydrolysis of bovine vitronectin.245 Sialic acid is extremely labile to conditions of handling and must be released by mild acid hydrolysis.246 Derivatization with phe-nylisothiocyanate and separation by reversed phase chromatography was found useful in analysis of hexosamines from gastric mucosa.247 A review on separation of sugars and other carbohydrates which covers many important aspects is available.248... 

Thermal desorption of solid traps by microwave energy is unsuitable for thermally labile compounds. In microwave thermal analysis [431] the (solid) sample is heated directly via interactions of the microwaves with the sample, providing more even heating and reduction of temperature gradients in comparison to heating with electrical furnaces. By passing air over a microwave-heated volatile sample evolved gases may be collected [432]. 

GC injection port) may well be. This was clearly demonstrated by a comparison between cryotrapping/direct injection, cryotrapping/SPME, and solid-phase (Tenax-GC) extraction for sampling of odorous sulfur compounds [71], Thermally labile compounds are likely to break down in the GC injection port/column/transfer line. Instead of SPME-GC, the recently developed SPME-HPLC [72] might be more applicable to analysis of such thermally unstable compounds. 

SFE-GC-MS is particularly useful for (semi)volatile analysis of thermo-labile compounds, which degrade at the higher temperatures used for HS-GC-MS. Vreuls et al. [303] have reported in-vial liquid-liquid extraction with subsequent large-volume on-column injection into GC-MS for the determination of organics in water samples. Automated in-vial LLE-GC-MS requires no sample preparation steps such as filtration or solvent evaporation. On-line SPE-GC-MS has been reported [304], Smart et al. [305] used thermal extraction-gas chromatography-ion trap mass spectrometry (TE-GC-MS) for direct analysis of TLC spots. Scraped-off material was gradually heated, and the analytes were thermally extracted. This thermal desorption method is milder than laser desorption, and allows analysis without extensive decomposition. 

Supercritical fluid chromatography is the name for all chromatographic methods in which the mobile phase is supercritical under the conditions of analysis and the solvating properties of the fluid have a measurable effect on the separation. SFC has some advantages over GC and HPLC it extends the molecular weight range of GC, thermally labile compounds can be separated at lower temperatures, compounds without chromophores can be sensitively detected, and the use of open-tubular and packed columns is feasible. SFC can be employed in both the analysis of natural pigments and synthetic dyes, however it has not been frequently applied in up-to-date analytical practice. 

Because little has been said concerning difficulties arising from derivati-zation of samples to render them suitable for GC analysis, replacement of GC by HPLC for non-volatile or thermally labile compounds is a possibility. However, the demands of reproducible solvent removal for a reliable LC-C-IRMS approach are formidable. Caimi and Brenna [685,686] have developed an instrument based on a moving wire transport system. The analytes are deposited on the wire as they elute from the HPLC column and, after solvent drying at 200 °C, are transported into an 800 °C combustion furnace loaded with CuO, where the resulting C02 is picked up by an He carrier stream and swept via a drying trap into the IRMS. 

The methodologies for the analysis of explosives for both forensic and environmental applications are very similar, using mainly GC/MS and LC/MS. As explosives are thermally labile compounds, LC/MS has an obvious advantage over GC/MS, as the chromatography is carried out at room temperature. 

Vitamins are generally labile compounds and many of them are susceptible to oxidation and breakdown. Since the mid-1970s, the most applied method for vitamins analysis has been HPLC, because this technique does not need hard derivatization and its nondestructive nature allows the use of HPLC both as a preparative purification method as well as for quantification. 

Most ILMs are less acidic than the commonly used acidic matrices alone. This leads to the possibility to synthesize matrices with only weakly acidic or even neutral or basic pH values [48]. These matrices may be beneficial for the analysis of acid-labile compounds [40]. For example, these matrices were successfully used for the measurement of acid-labile compounds like sulphated oligosaccharides, which are a class of compounds with high biological relevance [49]. Using classical preparations, the detection of these challenging analytes was only possible after derivatization or in the form of noncovalent complexes formed with basic peptides. Upon use of the ILM,... 

These are relatively simple and robust procedures. Some of the factors to keep in mind when working with anthocyanins are excessive heat, light, oxidation, and sample handling, as these factors can alter or destroy these labile compounds. The preparation and HPLC separation of anthocyanins on silica C]8 columns (see Basic Protocol 1) is the easiest and most robust of the procedures. Typical care in filtering samples and solvents prior to HPLC analysis is about all that is necessary. 

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