Thermo-labile

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. 

Isolable pyrazolines (183) are obtained from the (1,3-butadiene)phosphonic acid esters (182 X=S02Me, COOalkyl R "=H or Me R2=Me or Ph) (products from (182 X=CN) are thermo-labile) and diazomethane. Pyrolysis of the phosphorylated pyrazolines affords phosphonopentadienes rather than phosphono-cyclopropanes (contrast (184)) and with NaH give pyrazoles or pyrazolephbsphonic acid esters. 

Further reactions of 5/6 with the sterically less bulky silanes HSiCl3, Ph2SiHCl and Ph2SiH2 give the thermo-labile /x-biscarbyne complex 14, which has been spectroscopically characterized (Table 1). In these cases, formation of the silyl hydrides 11, 13, 15 and 16 as products of an oxidative addition at the 16e -complex generated from 6 (attack at Mn) becomes important. 

Modification of the conditions used for thermal desorption, including a change of adsorbent, were unable to fully resolve the problems of artifact formation and either adsorptive or catalytic loss of thermo-labile species at trace levels. 

General rules can be given to choose between ESI and APCl ESI is preferred for compounds which are ionic or very polar or thermo labile, or with masses higher than 1000, whilst APCl is preferred for compounds of lower molecular mass that are not very polar [264], The use of non-volatile buffers is usually avoided when performing ESI or APCl sources [265] and the analyst must make sure he uses eluents compatible with the stainless steel parts of the mass spectrometer and avoids inorganic salt buildup. 

Furthermore, Fischer rendered this chemistry more practical by generating vinylidene complexes of pentacarbonylchromium and tungsten directly in situ from terminal alkynes [9]. For example, treatment ofterminal alkynes with WCO)5(CH2Cl2), generated by photolysis of W(CO)6 in CH2CI2, gave thermo-labile tt-alkyne W(CO)5... 

Meltser, V. I., Luboshitz, A. I. and Smagina, T. V. (1986). Drying of thermo labile material in reversed jets. Intensification of Thermal Drying Processes, Collection of the Science Transactions, Minsk, ITMO AN BSSR, 62-70 (in Russ). 

The reaction mixture in a total volume of 250 yL contained 50 /xL of 10 mM sodium phosphate buffer (pH 7.2), 50 /xL of enzyme preparation, and 100 /xL of 50 fiM 3 -phosphoadenosine 5 -phosphosulfate. The reaction was started by adding 50 /xL of 12.5 mM p-nitrophenol (when assaying the thermo-labile form) or 50 /xL of 50 yM p-NP (when determining the thermostable form). After incubation for 30 minutes at 37°C, the reaction was stopped by the addition of 25 /xL of 4 M perchloric acid. After centrifugation, a 20 /xL aliquot was injected into the HPLC system. Formation of product was linear with time for both forms of enzyme for up to 45 minutes, and with protein amount up to 0.5 mg. 

Monosubstituted tetrazene (Mc3Si)HN— N=N— NH2 is very thermo-labile (41, 42). In methylene chloride it decomposes above ca. -40 C with the formation of trimethylsilyl azide and bis(trimethylsilyl)amine, as well as ammonium azide. Thus, the (catalyzed ) decomposition of the compound follows Thermolysis Pathway IV. 

The MIP is usually prepared as a highly cross-linked, rigid bulk polymer and the polymerisation reaction is initiated by photo- or thermo-labile free radical initiators such as 2,2 -azobis(isobutyronitrile). For molecular imprint-based CEC systems, the introduction of the imprinted polymer into the capillary column has been focused on and several approaches have been developed (see below). The polymerisation process can be performed in between 1 and 24 h. It has been shown that the temperature during the polymerisation process is important. A lower temperature leads to imprinted polymers with higher selectivity [47] or better chromatographic performance [39]. 

Notwithstanding the foregoing, using a high temperature does not always result in increased extraction efficiency. Adverse effects are especially outstanding with thermo-labile species however, increased temperatures also reduce the density of the extractant, which can lead to a reduction of its extracting power. Thus, Schafer et al. found the yield of extracted saturated and unsaturated fatty acids (FAs) from cereals to increase as the temperature was raised from 100 to 120°C however, no further increase in efficiency was obtained by further raising the temperature. Rather, as the temperature increased to... 

Phytoplasmas can be eliminated from their plant hosts, as they are generally thermo-labile and are not present in the shoot meristem (Lee and Davis, 1992). Furthermore they are sensitive to some antibiotics such as tetracycline (Ishiie et al., 1967 Heintz, 1989). Several methods have been applied to clean plant material for phytoplasmas these include in vitro tissue culture such as shoot tip (Dale and Cheyne, 1993) or micropropagation (Davies and Clark, 1994) sometimes in combination with heat or antibiotic treatment. 

When proper performance of the spray and the repeller electrode is established, fine tuning of the vaporizer temperature for the analyte(s) of interest can be performed. Within a temperature range of ca. 30 °C, depending on the flow rate and the mobile phase composition and to some extent to the age and history of the vaporizer, the vaporizer temperature has to be optimized, which sometimes results in a tenfold gain in analyte signal. Especially for thermo-labile compounds the vaporizer temperature is an important parameter adjustment of the vaporizer temperature can have a considerable influence on the mass spectrum of the analyte, because for thermo-labile compounds mixed spectra of analyte and its decomposition products are obtained. When set at a potential between 0 and 100 V the repeller electrode in most cases has not much influence on the analyte signal. Therefore the repeller potential only needs systematic optimization when minor gains in sensitivity are needed. 

Because of the destabilizing effect of the mutations on the structure of the mutant proteins, the NA mutant and some of the HA mutants display a marked thermo-lability. The NA mutant loses both enzyme activity as well as reactivity to the NC-10 mAb, which only recognizes the native NA (7). The HA mutants lose the ability to bind to the receptor on red blood cells, resulting in a decrease in the hemagglutination titer, or to the sialic acids on fetuin in a fetuin-based ELISA assay. 

The utilization of the previous techniques is limited by the necessity of evaporating compounds with a very low vapor pressure which are also often thermo-labile. Nevertheless, the vaporization at high temperature is often more rapid than the decomposition reactions of the ions produced after passage into the vapor phase (reverse of the situation at lower temperatures). Thus, Anderson et al. [68] introduced the flash desorption technique , in which the sample is brought from 25 to 1000°C in less than 0.5 second and is ionized by electron impact. This technique appears to be much more efficient than the conventional technique. It is sufficiently powerful to obtain the vaporization of a polypeptide so as to obtain intense protonated molecules, MH and identifiable fragmentations, leading to the peptide sequence. 

We have summarized in the previous pages the recent developments (prior to 1980 ) of new soft ionization techniques and of various peripherals. These aspects often responded to the need to obtain molecular ions, in order to determine the molecular masses of these often thermo-labile biological molecules, leading to the development of soft ionization methods and to obtain sufficient vaporization (or desorption) of the sample studied. Among other methods, field desorption (and DCI), the use of lasers and radioactive decomposition of Cf have been introduced. 

Polysaccharides are often thermo-labile, even when soft ionization methods such as field desorption are utilized. Pyrolysis effects may occur, leading to a low abundant molecular ion, in spite of the slight energy excess. This generates a spectrum which occasionally is complex. 

Considerable experimental difficulties often arise when non-volatile and thermo-labile molecules are analyzed. The exploitation of MIKE (and MIKE/CAD) spectra of these high-molecular-mass ions is rendered delicate by poor energy resolution. 

Thebaine can be converted into a photo- and thermo-labile quaternary salt, thebaine cyclomethine perchlorate, containing the ion (94) the quaternary salt of the true methine base (95) has been isolated. 

For most syntheses sec- or /er/-butyllithium can be replaced by the very reactive combination of -butyllithium and potassium r-butoxide in THF [236, 239], This is a clear solution which can be prepared by mixing solutions of /-BuOK in THF and rz-BuLi in hexane at < — 90 °C. The specific advantage of the BuLi /-BuOK-THF combination is the possibility of carrying out deprotonations that give thermo-labile metallic intermediates. Effectively, the reagent can be considered as an... 

Elevated temperatures may degrade thermo-labile analytes... 

Production processes described by Ciba-Geigy for fenpiclonil [17] and fludioxonil [18] both use TosMIC (5) as a key reagent. Crystalline TosMIC is thermo labile and has a potential for deflagration and was therefore not available in bulk quantities. EP 378 046 [17] described a production process for TosMIC and its safe handling as a solution in organic solvents. 

Short contact time and parallel flow makes it possible to dry thermo-labile materials. 

The problems related to the nature of the analytes appeared most difficult in the early years of LC-MS, but meanwhile a variety of so-called soft-ionization techniques have been developed, sometimes as a spin-off or in conjunction with LC-MS developments. Electrospray ionization (ESI) is by far the most successful of these techniques. They allow the mass spectrometric analysis of highly polar, ionic, and (thermo)labile compounds. As a result, the problem can be considered to be solved. In the majority of the liquid-based soft-ionization methods, the analytes, either as neutrals or as preformed ions in solutions, are transferred to the gas phase by nebulization of the liquid and subsequent desolvation of the droplets. In this way, the amount of heat required in the transfer has been minimized, which is favorable for the type of compounds to be analyzed. 

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