Analytical procedures label position

The techniques used to demonstrate selectivity will depend on the intended objective of the analytical procedure. The selectivity of a procedure may be confirmed by obtaining positive results from samples containing the analyte, while obtaining negative results from blank samples. For chromatographic procedures, representative chromatograms should be used to demonstrate selectivity and individual components should be appropriately labeled. Similar considerations should be given to other separation techniques. 

Assays requiring an internal calibration depend on the near identical behaviour of the calibration compound and the analyte. This is easily achieved by using an analogue of the analyte which has some protons in its stmcture replaced by deuterons. Provided that the deuterons are in such positions that they are stable to the analytical procedure and not back exchanged for proton, and do not affect the physical properties of the molecule (for instance changing the proton affinity) then the substitution of three or more protons will shift the deuterium labelled response clear from the isotopic peaks of the analyte due to natural and contributions. Such compounds act as excellent internal standards. Suitable labelling also enables the deuterium labelled compound to act as a carrier to improve recovery of the analyte at low levels (the so called stable isotope carrier effect [32]). 

This consists of obtaining the necessaiy information regarding the specific application of the labelled compound. Careful consideration must be given to the different synthesis procedures (laboratory, pilot, industrial) that can be applied to obtain the final product as well as to the availability of the intermediate non-labelled compounds, their specifications and purity. Also, analytical data, such as chromatography results for intermediate and final compounds, must be collected. The labelling position and starting material must then be chosem This preliminary step also establishes the amount of starting material required for the final synthesis. 

Figure 12.3 Sample application procedure using guide strips for PLC. Up to 1 mg of lipid was streaked across the center 10 cm of a 20 x 20-cm analytical plate, which was developed in the dual solvent system of Skipski et al. (Chapter 15). Major bands from the organism contained sterol (II) and phospholipid (I). The band labeled "blank" was lipid negative. Lanes 1 and 4 of the edge strips (5 cm wide) were spotted with neutral lipid standards, lanes 2 and 3 with sample. The edge strips were cut and visualized with 10% phosphomo-lybdic acid in ethanol and were then matched with the center to determine the position of the bands.

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