Repetitive injections

The relative standard deviations for repetitive injections vary between 2 % and 7 % for the different amino acids. 

Size exclusion chromatography (SEC), also known as gel permeation chromatography (GPC), was used for the separation and fractionation of macromolecules on an analytical and preparative scale [17]. The separation occurs predominantly by the hydrodynamic volume of the macromolecules in solution, however, in some cases the polarity of the molecules can also influence the retention times. Like HPLC, the SEC technique is generally very reproducible with regard to its elution times (typically < 1 h) and hence can be used for automated synthesis. But because the cost for an automated SEC system is high, it must be considered as a serial separation technique. In addition, larger scale separations > 100 mg, usually require repetitive injection of small aliquots. 

Automatic operation of the metering pump allows repetitive injections with unattended operation. Precise control of the carrier-gas flow ensures stable chromatograms and reproducible timings for collection of samples. Independent column oven and vaporizer temperatures are available up to 300°C and these can be operated with temperature programming or isothermally. The latter option is the most common. 

Figure 13.12 Performance of the chip with the sharp inlet interface. Typical electro-pherograms of repetitive injections of sample mixtures containing (a) 20 ppm TNB, (b) TNT, and (c) DNT. (Reproduced by permission of the Royal Society of Chemistry [44].)...

Packed-column SFC also is suitable for preparative-scale enatioseparations. Compared with preparative LC, sub- or supercritical fluid chromatography results in easier product and solvent recovery, reduced solvent waste and cost, and higher output per unit time. Because of its reduced sample capacity, SFC usually allows the separation of 10-100 mg samples per run. Chromatographers can compensate for these sample amounts by using shorter analysis times and repetitive injections (Wolf and Pirkle, 1997). 

System suitability should be based on criteria and parameters collected as a group that will be able to define the performance of the system. Some of the common parameters used include precision of repetitive injections (usually five or six), resolution (R), tailing factor (T), number of theoretical plates (N), and capacity factor ( ). 

Preparation of milligram quantities of substances can readily be performed with an analytical gas chromatographic column by repetitive injection and collection. Larger sample quantities require modifications to an analytical apparatus, but are more easily obtained with the use of a special preparative unit. It has been postulated that the sample size approximately increases with the fourth power of the column diameter (up to 1 g). 

Repetitive injections allowed the collection of 250 mg each of enflurane enantiomer of ee = 99.9 % in one day suitable for chiroptical trials. The specific rotation was determined, i.e., - 4.6 (first eluted enantiomer)... 

However, a contaminated mass spectrometer may cause various response trends toward different compounds. For example, after a triple quadrupole MS was contaminated with ascorbic acid, different response patterns were observed for levothyroxine (ANA), liothy-ronine (ANB), and their internal standard (thyroxine-13C6) during the repetitive injections of pure reference solutions containing these three components (Fig. 11). The responses of ANA were relatively stable while those of ANB and IS decreased significantly over a time period of about 1 and half hours, which resulted in increasing analyte/IS response ratios for levothyroxine and decreasing ratios for liothyronine. 

The reproducibility of injection was demonstrated by repetitive injections (n=6) of a standard solution of corticosteroids at two different days. R.S.D. for retention times on day 1 and day 2 were 0.51 and 0.93, and for peak areas 2.49 and 1.78, respectively. The injection linearity in terms of analyte concentration and injection volume was also good (r > 0.998) [40]. Retention time and peak area reproducibility for corticosteroids extracted from urine was characterized by R.S.D. 2 and 7 % respectively [41]. Due to differences in UV transmission of the mobile phase, the baseline drifted with increasing acetonitrile concentration. 

Sterri, S.H., Lyngas, S., Fonnum, F. (1981). Toxicity of soman after repetitive injection of suhlethal doses in guinea pig and mouse. Acta Pharmacol. Toxicol. 49 8-13. 

Figure 2.1 Apparatus used for repetitive injection gas chromatography analysis of volatile polymer decomposition products. (Reproduced from the Journal of Chromatographic Science by permission of Preston Publications, a Division of Preston Industries, Inc)...

Figure 2.2 Evolved gas chromatograms obtained by repetitive injection GC/MS for (a) PE-HZSM-5, (b) PE-HY, and (c) PE-MCM-41. (Reproduced by permission of John Wiley Sons Ltd)...

Figure 2.7 contains plots of repetitive injection GC/MS chromatographic peak areas (integrated total ion current) as a function of sample temperature for benzene and styrene as well as selected products representing alkyl aromatics (ethyl benzene), indanes (methyl indane) and indenes (indene) evolved from PS-catalyst samples. The plots show that benzene is by far the most abundant volatile product. All of the PS-catalyst samples produce alkyl benzenes and indanes, however samples containing HZSM-5 catalyst generate significantly lower relative yields of these products. 

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