Thermolabile compounds

ESI-MS has emerged as a powerful technique for the characterization of biomolecules, and is the most versatile ionization technique in existence today. This highly sensitive and soft ionization technique allows mass spectrometric analysis of thermolabile, non-volatile, and polar compounds and produces intact ions from large and complex species in solution. In addition, it has the ability to introduce liquid samples to a mass detector with minimum manipulation. Volatile acids (such as formic acid and acetic acid) are often added to the mobile phase as well to protonate anthocyanins. A chromatogram with only the base peak for every mass spectrum provides more readily interpretable data because of fewer interference peaks. Cleaner mass spectra are achieved if anthocyanins are isolated from other phenolics by the use of C18 solid phase purification. - ... 

The conditions in PHWE are typically harsh and, therefore, the method is not suitable for thermolabile compounds. Analytes may also react with each other or with the water molecules during the extraction. From an analytical point of view the most salient negative factors of SWE in the continuous mode are co-extraction of undesirable components of the matrix (usually polar components) and dilution of the analyte in the extract. This calls for a clean-up and concentration step prior to individual separation and detection of the target compounds. 

Table 3.45 lists the main characteristics of SPME. The technique is sensitive, reduces analyte loss and can successfully be applied to the analysis of both polar and nonpolar volatile and nonvolatile analytes from solid or liquid and in the gas phase [535]. Room temperature operation of SPME favours thermolabile compounds (only heating during injection into GC). Method... 

LC-TSP-MS without tandem mass capabilities has only met with limited success for additive analysis in most laboratories. Thermospray ionisation was especially applied between 1987 and 1992 in combination with LC-MS for a wide variety of compound classes, e.g. dyes (Fig. 7.31). Thermospray, particle-beam and electrospray LC-MS were used for the analysis of 14 commercial azo and diazo dyes [594]. No significant problems were met in the LC-TSP-MS analysis of neutral and basic azo dyes [594,595], at variance with that of thermolabile sulfonated azo dyes [596,597], LC-TSP-MS has been used to elucidate the structure of Basic Red 14 [598]. The applications of LC-TSP-MS and LC-TSP-MS in dye analysis have been reviewed [599]. 

The synthetic utility of the ring expansion reaction was demonstrated by its application to the synthesis of thermolabile thiepins. When the diazo compound (66) obtained from benzo[c]thiopyrylium salt 65 was treated with palladium catalyst under the same conditions as in the case of 63, the product isolated was ethyl 2-naphthoate (68)48). The plausible reaction pathway is one comprising i) decomposition of 66 to the corresponding carbene intermediate, ii) ring expansion to the... 

Another important drawback involves the necessity to evaporate samples. Although at pressures of 10-5 to 10-6 torr and temperatures of 200 to 300°C many organic compounds evaporate, the method is unacceptable for thermolabile, polar, and high molecular weight compounds. 

LC-APCI-MS has been shown to very useful for the characterization of both neutral ginsenosides as well as thermolabile malonyl-ginsenosides in ginseng extracts (Ma et al., 2005). However, LC-MS with ESI interface is a highly sensitive and soft ionization technique for the LC-MS analysis of thermolabile compounds and is considered to be the best method for the analysis of ginsenosides as it can overcome most problems associated with the thermolabile malonyl-ginsenosides and low molecular ion abim-dance levels. LC-ESI-MS is characterized by abundant adduct formation... 

Diaryl ditellurides are relatively thermolabile compounds and eliminate one tellurium atom by heating at 300°C. ° In the presence of copper metal, however, the extrusion of tellurium is achieved during reflux with toluene or dioxane. " " ... 

Methyl-5-phenylimino-l,2,3,4-thiatriazoline (70) reacts with acetone to give a thermolabile crystalline compound identified as a 1,2,4-oxathiazole derivative (93) (Scheme 16). The reaction is... 

By the irradiation of optically active 58 the helical compound 59 is formed stereo-specifically. However, this product (with the tert. butylgroups at a short distance) is thermolabile. It converts spontaneously into 60, already at 0 °C, thus showing mutarotation. The process was followed by ORD, which showed a complete reversal of the spectrum. 

As in the oxepin-arene oxide system, the resonance effect will also influence the position of equilibrium in the analogous organosulfur series. Compounds (46) and (53) thus appear to exist exclusively in the thiepin form. Since the resonance factor would favor the 1,2-episulfide of naphthalene over the thiepin tautomer (54) it is highly improbable that this thiepin will be detectable at ambient temperature. Both thiepins (46) and (55) have been isolated as thermolabile compounds (78JOC3379, 81MI51700). 

The TSP interface is typically combined with quadrupole MS, but coupling with ion-trap (91) or magnetic sector MS (92) has been also reported. Drawbacks of LC-TSP-MS are the requirements for volatile modifiers and the control of temperature, particularly for thermolabile compounds. Lack of structural information from LC-TSP-MS applications can be overcome by the use of LC-TSP-MS-MS. Use of this tandem MS approach provides enhanced selectivity, generally at the cost of a loss of sensitivity as a consequence of a decreased ion transmission. 

Cold on-column injection. In this approach, the sample is injected directly on-column and its vaporisation occurs after the injection. This requires a very special microsyringe. The needle (steel or silicone) has a diameter of approximately 0.15 mm and it penetrates the column, or pre-column, kept at 4 °C before raising it to its normal operating temperature. This approach, which is very difficult to master without the aid of an autosampler, is useful for thermolabile compounds (applications in... 

The total number of drugs eluted from KOH-treated phases is less than the number eluted from similar phase nontreated coluims. The KOH treatment is done to reduce the adsorption of compounds by the support phase, and although it does this well, it also prevents the elution of phenolic base drugs such as morphine because of conversion to nonvolatile potassium phenolates. Many higher molecular weight alkaloids become thermolabile under such alkaline conditions and do not elute. Non-KOH treated phases have less tendency to cause such decomposition. 

Dehydrofluorinations with alkyllithium reagents offer a facile method to produce alkenes, especially in case of thermolabile compounds, e.g. formation of 17,18 18,19 and 19.28... 

Introduction of the light-scattering detector has made the development of more efficient methods for the separation of natural complex mixtures of TG possible, including those of gradient elution. The evaporator temperature in the mass detector oven may influence the response of thermolabile compounds or compounds with low boiling points. Consequently, low tempera-... 

Determination of urea pesticides has been performed by gas chromatography (37,56,160, 161), a technique that has highly sensitive and selective detectors. Nevertheless, some substituted ureas are thermolabile and decompose during the analysis by GC (102). Therefore, direct determination of unchanged compounds by GC is possible only for some urea pesticides under determined chromatographic conditions (161). In other cases, a degradation product is quantified instead of the parent compound (29,162-164), or these substituted ureas must be derivatized before GC determination (37,102). 

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