Thermal lability

By using a beam of fast atoms or ions incident onto a nonvolatile liquid containing a sample substance, good molecular or quasi-molecular positive and/or negative ion peaks can be observed up to about 4000-5000 Da. Ionization is mild, and, since it is normally carried out at 25-35°C, it can be used for thermally labile substances such as peptides and sugars. 

The main difference between field ionization (FI) and field desorption ionization (FD) lies in the manner in which the sample is examined. For FI, the substance under investigation is heated in a vacuum so as to volatilize it onto an ionization surface. In FD, the substance to be examined is placed directly onto the surface before ionization is implemented. FI is quite satisfactory for volatile, thermally stable compounds, but FD is needed for nonvolatile and/or thermally labile substances. Therefore, most FI sources are arranged to function also as FD sources, and the technique is known as FI/FD mass spectrometry. 

For simple FI, the substance to be mass measured is volatilized by heating it close to the emitter so that its vapor can condense onto the surface of the electrode. In this form, an FI source can be used with gas chromatography, the GC effluent being passed over the emitter. However, for nonvolatile and/or thermally labile substances, a different approach must be used. 

For nonvolatile or thermally labile samples, a solution of the substance to be examined is applied to the emitter electrode by means of a microsyringe outside the ion source. After evaporation of the solvent, the emitter is put into the ion source and the ionizing voltage is applied. By this means, thermally labile substances, such as peptides, sugars, nucleosides, and so on, can be examined easily and provide excellent molecular mass information. Although still FI, this last ionization is referred to specifically as field desorption (FD). A comparison of FI and FD spectra of D-glucose is shown in Figure 5.6. 

FI and FD are mild or soft methods of ionization that produce abundant molecular or quasi-molecular positive or negative ions from a very wide range of substances. In the FD mode, it is particularly useful for high-molecular-mass and/or thermally labile substances such as polymers, peptides, and carbohydrates. 

The mix of ions, formed essentially at or near ambient temperatures, is passed through a nozzle (or skimmer) into the mass spectrometer for mass analysis. Since the ions are formed in the vapor phase without having undergone significant heating, many thermally labile and normally nonvolatile substances can be examined in this way. 

Evaporation from a spray of charged droplets produced from a stream of liquid yields ions that can be analyzed in a mass spectrometer. Thermally labile and normally nonvolatile substances such as sugars, peptides, and proteins can be examined successfully. 

The efficiency of separation of solvent from solute varies with their nature and the rate of flow of liquid from the HPLC into the interface. Volatile solvents like hexane can be evaporated quickly and tend not to form large clusters, and therefore rates of flow of about 1 ml/min can be accepted from the HPLC apparatus. For less-volatile solvents like water, evaporation is slower, clusters are less easily broken down, and maximum flow rates are about 0.1-0.5 ml/min. Because separation of solvent from solute depends on relative volatilities and rates of diffusion, the greater the molecular mass difference between them, the better is the efficiency of separation. Generally, HPLC is used for substances that are nonvolatile or are thermally labile, as they would otherwise be analyzed by the practically simpler GC method the nonvolatile substances usually have molecular masses considerably larger than those of commonly used HPLC solvents, so separation is good. 

The FAB source operates near room temperature, and ions of the substance of interest are lifted out from the matrix by a momentum-transfer process that deposits little excess of vibrational and rotational energy in the resulting quasi-molecular ion. Thus, a further advantage of FAB/LSIMS over many other methods of ionization lies in its gentle or mild treatment of thermally labile substances such as peptides, proteins, nucleosides, sugars, and so on, which can be ionized without degrading their. structures. 

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... 

As a rule of thumb, one can say that the efficiency of separation of mixtures and the simplicity of operating and maintaining apparatus are much greater for GC than for LC. Hence, other things being equal, GC is most often the technique of first choice and can be used with a very wide variety of compound types. However, for nonvolatile or thermally labile substances like peptides, proteins, nucleotides, sugars, carbohydrates, and many organometallics, GC may be ruled out completely... 

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. 

FAB has been used with a wide variety of substances, including thermally labile compounds such as peptides, sugars, carbohydrates, and organometallics. 

The process of field ionization presupposes that the substance under investigation has been volatilized by heat, so some molecules of vapor settle onto the tips held at high potential. In such circumstances, thermally labile substances still cannot be examined, even though the ionization process itself is mild. To get around this difficulty, a solution of the substance under investigation can be placed on the wire and the solvent allowed to evaporate. When an electric potential is applied, positive or negative ions are produced, but no heating is necessary to volatilize the substance. This technique is called field desorption (FD) ionization. 

Both FI and FD provide good molecular mass information, but few if any fragment ions, and allow thermally labile substances such as peptides, nucleosides, and glycerides to be examined, as well as inorganic salts. 

Applications. The primary advantage of an Ic/ms is in combining the separation of large, potentially thermally labile compounds with the... 

When the compounds of interest are fragile and thermally labile, thermospray Ic/ms is a good choice. Figure 5, shows the thermospray spectmm for leucine enkephalin [58822-25-6] a pentapeptide of molecular weight 555. The Ic/ms approach has been very helpful in unraveling the stmcture of large biological molecules (21). 

More specific methods involve chromatographic separation of the retinoids and carotenoids followed by an appropriate detection method. This subject has been reviewed (57). Typically, hplc techniques are used and are coupled with detection by uv. For the retinoids, fluorescent detection is possible and picogram quantities of retinol in plasma have been measured (58—62). These techniques are particularly powerful for the separation of isomers. Owing to the thermal lability of these compounds, gc methods have also been used but to a lesser extent. Recently, the utiUty of cool-on-column injection methods for these materials has been demonstrated (63). 

The analysis of penicillins by mass spectrometry (qv) has developed with the advent of novel techniques such as fast atom bombardment. The use of soft ionization techniques has enabled the analysis of thermally labile nonvolatile compounds. These techniques have proven extremely valuable in providing abundant molecular weight information from underivatized penicillins, both as free acids and as metal salts (15). 

Unusual heterocyclic systems can be obtained by photodimerizations and for five-membered heterocycles with two or more heteroatoms such dimerizations need be effected on their ring-fused derivatives. Cyclobutanes are usually obtained as in the photodimerization of the s-triazolo[4,3-a]pyridine (540) to the head-to-head dimer (541). These thermally labile photodimers were formed by dimerization of the 5,6-double bond in one molecule with the 7,8-double bond in another (77T1247). Irradiation of the bis( 1,2,4-triazolo[4,3-a]pyridyl)ethane (542) at 300 nm gave the CK0ifused cyclobutane dimer (543). At 254 nm the cage-like structure (544) was formed (77T1253). 

Oxiranes with lone-pair atoms like nitrogen (67JA3084) or halogens (B-71MI50500) on the ring are thermally labile (e.g. Scheme 21). 

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. 

Another convenient approach to the azapentatriafulvalene system is given by the in situ formation of cyclopropenylium salts from cyclo-propenones and dry HCl gas followed by their electrophilic attack on various indoles. Tire corresponding heterofulvalenium salts of type 35 were isolated as chlorides, which were somewhat photosensitive and thermally labile (68TL5537). 

In this chapter techniques of block copolymer preparation involving thermally labile azo compounds are reviewed. Upon heating, aliphatic azo compounds evolve nitrogen thus forming two carbon centered free radicals. 

The incorporation of thermally labile azo groups into polymer backbones was first reported in the early 1950s [2]. Since then, numerous techniques for synthesizing azo-containing polymers have been developed. The effort to create new azo-containing polymeric materials has been reviewed by several authors [3-8]. 

Concerning the reaction of ACPC with diols, the frequent use of poly(ethylene glycol) has to be mentioned [20-24]. Ueda et al. ([22-24]) reacted preformed poly(ethylene glycol) (Mn between 6 x 10 to 2 x 10 ) with ACPC. In this case, unlike the reaction of ACPA with diols vide ante), no additional condensation agent was needed. The ethylene glycol-based thermally labile polymers were used to produce blocks with poly(vinyl chloride) [22], poly(styrene) [23], poly(methyl acrylate), poly(vinyl acetate), and poly(acrylonitrile) [24]. 

In a later work, Tunca and Yagci [40,41] used two other acid chlorides (adipoyl and terephthaloyl chloride) along with ACPC. By changing the ratio of the different acid chlorides the number of thermally labile azo bonds in the polymer backbone could be regulated. 

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