Multiple temperature programs

Isopropyl derivatives were introduced by Pettitt and Stouffer [287] and later studied by other workers [288]. They are prepared by reaction with 2-bromopropane in the presence of sodium hydride in dimethyl sulphoxide. The reaction scheme and the preparation procedure were given in Chapter 4 (see p. 64). Except for Arg, all amino acids under study provided the expected derivatives. The hydroxyl group of Hypro was, however, not protected. The derivatives were found to be stable for a reasonable period of time and were analysed on 3% of OV-17. The extension of this promising one-step method to all protein amino acids did not fulfill expectations, however [288]. Some amino acids (Gly, Gin, Asp and Asn) did not provide detectable derivatives and the others led to multiple peaks. Moreover, significant amounts of by-products were produced, which may interfere. Arg provided a single peak, the mass spectrum of which was identical with that of Orn both derivatives resulted from lactam formation. Isoprop derivatives of 23 common amino acids were separated on 5% of-Carbowax 20M on silanized Chromosorb G with temperature programming (50-240°C). 

Our purpose in this text is principally to present temperature scanning methods. These generally involve multiple rampings as one seeks to delineate the kinetics of a system over a wide range of conditions. However, there is a well known and established technique for the semi-quantitative study of desorption phenomena, the Temperature Programmed Desorption (TPD) method. The equations developed below are also applicable to results that can be obtained using some of the versions of the traditional TPD apparata. In such cases they can be used to quantify the TPD results to yield the kinetics of the process and/or to check for extraneous influences that can result in anomalous results, effects such as mass diffusion, heat diffusion, or purely kinetic effects. 

Thiomethylation only gives derivatives of arsenic-(III) whereas pentavalent arsenic compounds are reduced by the thiol with formation of the disulfide (SGM)2. The yields for the TGM-derivatization depend on the number of introduced thiomethyl groups. The determined yields and recovery rates are in accordance with the publication of Schoene (Schoene et al., 1995). The derivatization is subjected to sometimes strong matrix influence which can be corrected by determination of the yield of proper reference compounds. Multiple derivatized compounds show often thermal instability, thus the use of cold on-column injection or temperature programmed cold injection systems is required. 

The simplest and most common temperature program is the single linear ramp illustrated earher. More complex programs, with multiple ramps interrupted by isothermal intervals may be designed to optimize critical separations and minimize overall analysis time. Environmental and biological samples often contain very slowly eluting heavier background contaminants of no interest to the analysis, which must be cleared off the column before the next injection, lest they elute with and interfere with analyte peaks from... 

An LGC typically uses temperature programming to enhance separation. The PGC is more likely to be run at an isothermal oven temperature, often using multiple coliunns, with column-switching valves being employed to enhance the separation. 

In LGC, a single long capillary column is typically used in a temperature-programmed mode to perform the separation. Although this same separation scheme can be used in PGC, the major drawback is the speed of analysis. The solution is to use multiple column trains and column-switching techniques. This allows the chromatograph to be operated in an isothermal mode. 

The surface reaction generally proceeds through multiple complex pathways. These pathways are unraveled by using the temperature-programmed technique. Under the experimental conditions of TPD, the reaction products and other surface species are desorbed from the sample surface by increasing the surface temperature after a sufficient amount of chemical species is adsorbed at a low temperature. This method is called the temperature programmed reaction method. Furthermore, we can observe the desorbed species during their exposure to the source... 

---