Impurities achiral

Let us assume that a given compound has a purity of 98 % ee, and that this compound is reacted with a derivatizing agent which has also a purity of 98 % ee. The two major compounds plus the minor impurities in the compound to be analyzed and the derivatizing agent will create a set of four diastereomers. Two pairs of diastereomers (-i-)-A(-i-)B and (-)-A(-)-B as well as (- )-A(-i-)-B and (-i-)-A(-)-B are enantiomeric pairs, and thus elute together on an achiral column. Therefore, a peak area of 98.011 % will be detected for (-i-)-A(-i-)-B, which leads to a purity of 96.03 % ee for (-i-)-A. This is a quite significant deviation from the true value for (-i-)-A. 

Determination of the drug substance is expected to be enantioselective, and this may be achieved by including a chiral assay in the specification or an achiral assay together with appropriate methods of controlling the enantiomeric impurity. For a drug product where racemization does not occur during manufacture or storage, an achiral assay may suffice. If racemization does happen, then a chiral assay should be used or an achiral method combined with a validated procedure to control the presence of the other enantiomer. 

Achiral-chiral LC-LC is most often used to separate the desired analyte from interfering components, such as matrix components, metabolites, excess derivatiza-tion reagent, or other impurities. Separating such interferents from the analyte allows for better analyte quantification or enantiomeric ratio determination. Also, achiral columns are seen as a way to protect the more expensive chiral columns from matrix components that might become irreversibly retained and deteriorate column performance. Short achiral columns (trap columns) are sometimes used to reconcentrate the chiral analyte after a previous separation (either chiral or achiral) as a type of online enrichment. Configurations that combine an achiral column for increased selectivity and trap column(s) for online enrichment are relatively common, though this type of configuration requires more columns and increases complexity. 

The following applications highlight the vastly different ways achiral-chiral LC-LC can be used. Many of these applications involve analytes that are structurally related. These structurally related compounds may be impurities from the synthesis of the analyte of interest using either traditional laboratory or biological methods. Achiral-chiral separations can be useful in separating these related analytes. 

Ferretti et al. (1988) used an amino column coupled to a derivatized amylose column (Chiralpak AS) operated in the reverse-phase mode to separate the enantiomers of the antifungal agent voriconazole from several chiral impurities and one achiral impurity. Three of the chiral impurities are the other enantiomer and corresponding diastereomers of voriconazole. More chiral impurities result from a chlorinated voriconazole. Additionally, this multidimensional method could baseline separate all but two of the chiral impurities into their respective enantiomers. These separations are shown in Figure 14.5. 

FIGURE 14.5 Separations involving voriconazole (1), its mirror image (2), related diaster-eomers (3), chlorinated impurities (4), and an achiral impurity 5. (a) Achiral separation of compounds 1-5 on an amino column with hexane/ethanol mobile phase (b) Chiral separation of compounds 1-5 on Chiralpak As column with hexane/ethanol mobile phase (c) Achiral-chiral multidimensional separation with the amino and chiral column coupled in series. Reprinted from Ferretti et al. (1998) with permission from Vieweg Verlag. 

D acquarica, I., Gasparrini, F., Giannoli, B., Badaloni, E., Galletti, B., Giorgi, F., Tinti, M.O., Vigevani, A. (2004). Enantio- and chemo-selective HPLC separations hy chiral-achiral tandem-columns approach the combination of CHIROBIOTIC TAG and SCX for the analysis of propionyl carnitine and related impurities. J. Chromatogr. A 1061, 167-173. 

Ferretti, R., Gallinella, B., La, T.F., Zanitti, L. (1988). Direct resolution of a new antifungal agent, voriconazole(UK-109,496) and its potential impurities, by use of coupled achiral-chiral high-performance liquid chromatography. Chromatographia 47, 649-654. 

A new chiral-achiral tandem-columns arrangement based on a commercially available TAG CSP connected in series with a Spherisorb S5 SCX column was developed for the enantio- and chemoselective dosage of propionyl L-carnitine and relative impurities in pharmaceutical batches [166]. Some chromatograms relative to the separation of carnitine and carnitine derivatives on a teicoplanin CSP are collected in Figure 2.18. 

Enlarged specimen electropherogram showing the selectivity towards other achiral impurities present in the sample... 

PLATE I Determination of the enantiomeric purity of active pharmaceutical ingredient (main compound = MC, peak I is the enantiomeric impurity). Conditions lOOmM sodium phosphate buffer pH = 3.0, lOmM trimethyl -cyclodextrin, 60 cm fused silica capillary (effective length 50 cm) X 75 pm I.D., injection 10 s at 35 mbar, 25°C, 20 kV (positive polarity) resulting in a current of approximately lOOpA, detection UV 230 nm. The sample solution is dissolved in a mixture of 55% (v/v) ethanol in water. (A) Typical electropherogram of an API batch spiked with all chiral impurities, (B) overlay electropherograms showing the selectivity of method toward chiral and achiral impurities, a = blank, b = selectivity solution mixture containing all known chiral and achiral compounds, c = API batch, d = racemic mixture of the main compound and the enantiomeric impurity. 

The binding constants of salbutamol enantiomers and also of several chiral and achiral impurities of this compound with CDs were determined by Rogan et al. (23). The results of these calculations, together with some mobility characteristics of the transient diastereomeric complexes, were used... 

FIGURE 17.5 Separation of (5)-timolol and its conceivable chiral and achiral impurities. (A) The mixture solution containing the conceivable chiral and achiral impurities of (5)-timolol, (B) the dissolution solution, and (C) the standard solution at 0.2% enantiomeric impurity. Column Chiralcel OD-H, mobile phase hexane 2-PrOH DEA (965 35 1 v/v/v). Peaks (1) timolol dimer, (2) (i )-timolol, (3) isotimolol, (4) (5)-timolol, (5) dimorpholinothiadiazole, and (6) solvent front. Concentration of analytes 5-10 Xg/mL in (A) and 3 Xg/mL (i )-timolol in (C). (Reprinted from Marini, R.D. et al., Talanta, 68, 1166, 2006. Copyright Elsevier, 2006. With permission.)... 

The precision and accuracy of polarimetry critically depends on temperature, solvent, and traces of achiral and chiral impurities. 

In addition to requiring large sample sizes, high purity of the compound is an indispensible requirement in polarimetry. Chiral and achiral impurities may change the value of the optical... 

While the achiral double anti-Bredt rule compound 56 50) of Cs symmetry was isolated in an impure state and was found to be very labile toward 02 and heat, the chiral 57 of C2 symmetry was assumed to exist very briefly in the pyrolysis of 3,6-dimethylidene-l,7-octadiene51). 

The guidance recommends treating chiral impurities as though they were achiral impurities with the caveat that the ICH identification and... 

For example, the sergeants and soldiers experiment features a preferential screw-sense helical amplification in optical active copolymers with racemic helical structures named by Green et al. [17,18], This effect means that a small portion of enantiopure chiral side groups determines the overall screw sense (P or M) of the helical main chain bearing a majority of achiral side groups, and a population of helicity with one preferential screw-sense is nonlinearly amplified as a function of the chiral impurity. Since the first report of this phenomenon in poly-a-olefin copolymers by Pino et al. [21],... 

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