Assignment of purity

As discussed earlier, the assignment of purity is ideally done by a relative method, comparing analyte peaks in chromatograms of sample solutions with analyte peaks in chromatograms of a reference standard of assigned purity. The difficulty that arises when no suitable reference standard exists has already been alluded to. Purity then has to be assigned on a 100% detected impurities basis. The nature of the difficulties that arise in assigning purity in this way is discussed in the next section. 

The use of the phrase relevant reference injections is highly relevant Ideally under fully equilibrated LC conditions the response would be identical for all injections of reference solutions. In practice there may be a slow long term change (drift) of the response. To account for this each sample solution or sets of sample solutions are bracketed by standard solutions. At the end of the run the analyst must decide whether it is more appropriate for any sample solution to use the average response over the whole run or the average response from standard solutions immediately before and immediately after the sample solution. To be able to quote the sample purity with any degree of statistical confidence, a minimum of 5-6 injections of sample solutions should be carried out. 

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The critical parameter in the discussion of reference standards is the process of qualification. FDA guidance on this topic states that A non-compendial standard should be thoroughly characterized to assure its identity, strength, quality and purity. 3 It is therefore left to the pharmaceutical firm to establish thorough characterization for the ARS of each NCE. The overall qualification process for an ARS includes characterization of the material, proper form selection, and assignment of purity. Some general considerations in this process include... 

Two distinct sesquiterpenes exist in the essential oil of Santalum album, known as a-santalene and /3-santalene. 8-santalene is probably a bicyclic and a-santalene a tricyclic sesquiterpene. These sesquiterpenes were discovered by Guerbet. From the method of their preparation it is doubtful whether they have been obtained in a state of purity, so that the characters assigned to them must be accepted with some reserve, and as probably being only approximate. 

Structure of Oxy-F Compound F is extremely unstable and is difficult to obtain at a level of purity suitable for NMR studies. However, an oxidation product, Oxy-F, formed when F is left standing at — 20° C, is considerably more stable than F and can be purified to a sufficiently high level of purity. Oxy-F is nonfluorescent and shows absorption maxima at 237 nm and 275 nm (shoulder). The high-resolution FAB mass spectrum indicated the molecular formula of Oxy-F to be C33H3809N4Na2 [m/z 703.2363 (M + Na)+ and 681.2483 (M + H)"1"]. The H and 13C NMR data allowed the assignment of structure 7 to oxy-F (Fig. 3.2.6 Nakamura et al., 1988). 

In general, the observation of opposite senses of nonequivalence for substituents on opposite faces of the plane defined by primary and secondary interactions will be the hallmark of a normal solvation model. Deviations are of no consequence for enantiomeric purity determinations but should raise questions concerning the validity of the usual model for the assignment of absolute configuration based on the observed senses of nonequivalence. Since knowledge of solute structure often allows anticipation of such third interactions, ... 

The addition of electrophilic reagents to chiral a,/3-unsaturated sulfoxides is also accompanied by asymmetric induction. Stirling and Abbott (318,322) found that the addition of bromine to the optically active (.R)-vinyl-p-tolyl sulfoxide 319 yields a mixture of diastereo-meric a,/3-dibromosulfoxides 320. Oxidation of this mixture gives the optically active sulfone 321, with a center of chirality at the a-carbon atom only. The optical purity (32%) of this sulfone was estimated by comparing its specific rotation with that obtained as a result of oxidation of diastereomerically pure sulfoxide (/ )-320. The assignment of configuration at the a-carbon atom was based on the analysis of the polarizabilities of substituents. 

Optically active octyn-3-ol was obtained by first converting the salt 42 to the ester acids 43 and then hydrolysis of the ester to compound 44. Optical purity and assignment of the absolute configuration as S was established by methods known from analogous compounds from the literature. Fried and his co-worker proved that preparation of the ferr.-butyl-ether derivatives and deprotection with trifiuoroacetic acid is possible without a trace of racemisation. This guarantees that the S (normal) or the R (epi) 15-hydroxyprostanoids can be synthesized by using the resolved fert.-butylethers in the alkylation reaction. 

Kinetic resolution of vinylic epoxides by the reduction with terpenyl dialkylboranes of high optical purity is a simple and direct method for the preparation of enantiomerically enriched epoxides. It allows for convenient configurational assignment of epoxides if the configuration of the corresponding allylic alcohol is known. Both enantiomers can be prepared using readily available enantiomeric diisopinocampheylboranes. 

Enantiomeric distinction can also be achieved using chiral solvents without shift reagents. The determination of enantiomeric purity and the assignment of absolute configuration of cyclic and acyclic sulphinate esters has recently been achieved (573) using chiral l-aryl-2,2,2-trifluoroethanols as solvents. Enhanced enantiomeric distinction has been demonstrated using a combination of both a chiral solvent and an achiral LSR. (514)... 

A fully characterised nmr spectrum is always a useful piece of data and can also be used as evidence of purity. A spectrum is particularly useful when a compound has most of its proton resonances in the same region of the spectrum. The level of assignment will depend on how well the spectrum can be interpreted. It may simply be a list of chemical shifts, it may be a list of chemical shifts with signals assigned as CH3, CH2, CH or C, or you may be able to give actual assignments to individual carbon atoms. 

Structural assignments and purity determination of the macrocyclic and/or linear oligo- and polymeric pyridinium compounds represent a peculiar aspect in the study of these compounds, due to the presence of a repetitive alkylpyridinium subunit in their polycationic structure. 

A consistent, scientifically defensible process is followed when purity values are assigned. A clear rationale for the calculation of purity values as well as clear instructions on the use of purity values must also be included. 

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