Thermally labile materials

Liquid chromatography is a separation method that is often applied to nonvolatile, thermally labile materials such as peptides, and, if their mass spectra are required after the separation step, then a mild method of ionization is needed. Since FAB/LSIMS is mild and works with a liquid matrix, it is not surprising that attempts were made to utilize this ionization source as both an inlet... 

Some solid materials are very intractable to analysis by standard methods and cannot be easily vaporized or dissolved in common solvents. Glass, bone, dried paint, and archaeological samples are common examples. These materials would now be examined by laser ablation, a technique that produces an aerosol of particulate matter. The laser can be used in its defocused mode for surface profiling or in its focused mode for depth profiling. Interestingly, lasers can be used to vaporize even thermally labile materials through use of the matrix-assisted laser desorption ionization (MALDI) method variant. 

Unless there are solutes that are very strongly retained, and the maximum operating temperature of the system is reached without all the solutes being eluted, there is usually no need for a final isothermal period of any significant length. There is, however, one exception where a final isothermal period is helpful, and that is for mixtures that contain thermally labile materials. 

As mentioned previously, the formation of droplets requires the use of extra heat to effect complete solvent evaporation and with this comes the potential for decomposition of thermally labile materials. 

From a practical point of view, the DLI, unlike the moving-belt interface, contains no moving parts and is therefore more reliable in operation if adequate precautions are taken to minimize the frequency of the pinhole blocking. In addition, it does not require heat either to remove the mobile phase or to vaporize the analyte into the source of the mass spectrometer. The DLI is, consequently, better for the analysis of thermally labile materials. 

No heat is applied to the interface and it is therefore able to deal with thermally labile materials better than the moving-belt interface. 

It is possible to study thermally labile materials using this type of interface since the only heat applied to the probe tip is that required to prevent freezing of the mobile phase as it evaporates. 

In APCI, droplets are generated by a combination of heat and a nebulizing gas. While the analytes are embedded in a droplet, and thus protected to some extent from the heat, many thermally labile materials are decomposed. In addition, ionization occurs mainly by ion-molecule reactions and yields predominantly singly charged ions. If, therefore, compounds do not undergo thermal degradation, a mass spectrometer with extended mass range would be required to detect any ions formed. 

Fast-atom bombardment An ionization method used for involatile and thermally labile materials. In this technique, the sample is dissolved in a matrix material and bombarded with a high-energy atom or ion beam. 

Drying chamber shape predominantly is either conical or flat-bottomed. The flat-bottomed dryers remove the powder as it falls to the floor of the dryer by use of a rotating pneumatic powder discharger that functions as a vacuum cleaner. These dryers subject the product to significantly more heat than do the cone-bottomed dryers. While for many types of dry flavorings this additional heat is insignificant, thermally labile materials (e.g., natural flavorings - tomato, cheese, and numerous fruit juice based products) may suffer from the additional heat. 

This information would be determined by measuring the heat and airflow at various points of the chamber and then calculating the variability of these conditions in it. Since this kind of information on heat distribution provides assurance that the process equipment is properly designed for the required process, it will be the focus of future QA audits. Furthermore, this knowledge is also essential when a very specific drying temperature is needed for thermally labile materials. The qualification thus not only becomes an integral part of the validation program, but also demonstrates how the information may be used. 

The high energy and shear that result from the movement of the milling media is imparted to the particles as the material is circulated through the milling chamber. The result is the ability to create submicron particles. Thermally labile material is easily handled as the milling chamber is jacketed. By utilizing smaller media (less than 100 p,m) nano-sized (20 nm) particles are achievable. 

Pinkston, J.D., Bowling, D.J., and Delaney, T.E. 1989. Industrial applications of supercritical-fluid chromatography-mass spectrometry involving ohgometric materials of low volatility and thermally labile materials. Journal of Chromatography, 474 97-111. 

Flow programming is not as effective in reducing the elution time of well-retained components and tends to cause increased band dispersion it is, however, more gentle than temperature programming and would be chosen when separating thermally labile materials. The complexity of the theoretical treatment depends on whether the mobile phase is compressible or not. In gas chromatography, the mobile phase is compressible, and this must be taken into account in the first theoretical treatment. 

The body of the injector is heated to ensure the sample is volatilized and inside is an inert glass liner. This glass liner helps minimize any sample decomposition that might occur when thermally labile materials come in contact with hot-metal surfaces. The carrier gas enters behind the glass liner and is thus preheated. The sample is injected into the stream of carrier gas that passes down the center of the tube, a portion passes down the capillary column. 

Supercritical fluids have a number of distinct advantages over conventional liquid solvents. The adjustable solvent strength and favorable transport properties have already been mentioned and it is these features which really differentiate SCFs tom liquid solvents. Most SCFs are low-molecular-weight gases which have relatively low critical temperatures. Operations may therefore be carried out at moderate temperatures which is desirable in the recovery of thermally labile materials. Perhaps the most important advantage offered by SCFs is that after the release of pressure, components are left virtually free of residual supercritical solvent. 

Routine operation of the mass spectrometer interface tip at high temperature might cause the decomposition of thermally-labile materials. Historically, cholesterol has been used to test the thermal activity of GC-MS interfaces and jet separators. It is very sensitive to dehydration across the 3-4 bond upon contact with "active sites" in the system, yielding a species with molecular weight 368 Da (25). 

Decomposition is probably occurring in the source or in the interface. Additional experiments, in which both the source and SFC-MS interface temperatures are varied, will be required to establish the site of decomposition. It is clear, however, that the application of SFC-MS to thermally-labile materials will ultimately be limited, not by the SFC, but by the interface-detector combination. 

Solvent extraction is a very gentle way of isolating the essential oils from a botanical matrix. Consequently, they are employed with thermally labile materials that cannot accommodate elevated temperatures such as jasmine, hyacinth, narcissus, and tuberose. An added advantage is that in many cases solvent extraction also reduces enzymatic... 

Since GC is limited to the analysis of volatile substances, liquid chromatography has been coupled to an IRMS allowing the analysis of high molecular weight, polar, and thermally labile materials. Because of its technical complexity this technique has found only limited use to date. 

Supercritical fluid chromatography (SFC)/ FT-IR spectroscopy, generally with carbon dioxide as the mobile phase, bridges the gap between GC/FT-IR and LC/FT-IR, and is particularly useful for separating nonvolatile or thermally labile materials not amenable to gas chromatographic separation [109J. Flow cells, mobile phase elimination and matrix-isolation techniques are used as SFC/FT-IR interfaces. 

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