Chiral detectors elution

The chromatograms of the racemic hydroperoxide which was partly decomposed into the corresponding carbinol were recorded by UV and a chiral detector (Fig. 1). First the carbinol was separated and then a better optical resolution and longer retenton time was observed for the hydroperoxide. A comparison of both chromatograms indicates that the optical rotation of the enantiomer of the carbinol, which was eluted first, was plus. This method was successfully used for the enantiomeric separation of other chiral hydroperoxides on an analytical scale [25]. Thus for the first time a practical method is available for the determination of enantiomers of hydroperoxides and related alcohols in the same solution. 

Stalcup aiid co-workers [14] adapted this method to a continuous elution mini-prep electrophoresis apparatus shown in Fig. 11-3. In this apparatus, the end of the electrophoretic gel is continuously washed with elution buffer. The eluent can then be monitored using an HPLC detector (Fig. 11-4) and sent to a fraction collector where the purified enantiomers, as well as the chiral additive, may be recovered. In this system, the gel configuration was approximately 100 mm x 7 mm, and was aircooled. The number of theoretical plates obtained for 0.5 mg of piperoxan with this gel was approximately 200. A larger, water-cooled gel was able to handle 15 mg of... 

All aldehydes used in the experiment were freshly distilled or washed with aqueous NaHC03 solution to minimize the amount of free acid. Chiral HPLC was performed using a chiral OJ-H column (0.46 cm x 25 cm, Daicel industries) with a water 717 auto sampler and a UV-vis detector (254 nm). The eluting solvent used was different ratios of hexane and 2-propanol. Chiral gas chromatography analysis was performed in a Shimadzu auto sampler with cyclodextrins columns as chiral stationary phase (fused-silica capillary column, 30 m X 0.25 mm x 0.25 gm thickness, /3-Dex-120 and /3-Dex-325 from Supelco, USA) using He as a carrier gas (detector temperature 230 °C and injection temperature 220 °C). 

The design has been well proved in quality assurance and origin control of flavours and fragrances. A double-oven system is shown in the Fig. 17.3, with two independent temperature controls and two detectors (DM 1, DM 2). A live switching coupling piece is used to switch the effluent flow to either the first detector or the chiral column. With optimum pneumatic adjustment of the MDGC system, certain fractions are selectively transferred onto the chiral main column as they are eluted from the precolumn (heart-cutting technique) [15]. 

The article is a brief review of the applications of CD as a detector in both preparative and analytical liquid chromatography. The objectives are to identify elution orders for enantiomers, to measure enantiomeric purities and enantiomeric excesses, to analytically determine diastereoisomers, and to selectively determine chiral analytes when present as components in mixtures with achiral substances. 

In its broadest terms the discussion of HPLC detection for chiral species must include the analysis of mixtures with achiral substances as well as the quality testing of, for example, the enantiomeric purity of a chemically pure drug form. The distinction between the definitions of chemical purity versus optical purity can not be overemphasized. In an efficient chiral HPLC system the latter problem is trivial, and if retention times are significantly different then any conventional detector such as RI, electrochemical, absorption, etc., could be used. Co-elutions are a major experimental concern in separations of mixtures and at this juncture it is not only prudent but absolutely necessary to involve a chiroptical detector to preferentially identify the chiral analyte. 

The idea of two sequential detectors, one conventional the other chiroptical, is the basis of a third strategy for enantiomeric purity determinations using HPLC. It differs from the previous two by not involving a chiral separation. In it the enantiomers co-elute and the total amount is determined from an absorbance measurement. Subsequently a chiroptical... 

In the study of mixtures, differentiation between enantiomers is a two level problem which is somewhat independent of whether the LC system is chiral or conventional. The problems common to both systems are the effects of overlapping bands on the performance of the detectorfs). Overlap can be between chiral-achiral species on the one hand and co-eluted chiral-chiral with achiral on the other. On first thought the chiral-achiral distinction should be relatively easy if a chiroptical detector is used because the achiral compounds will not interfere with the detection measurement. In addition the ability of the chiroptical detector to measure both positive and negative signals makes the confirmation of the enantiomeric structure elementary [3,4], As pointed out earlier, enantiomers co-elute from conventional columns and two detectors in sequence will provide the information to measure the enantiomeric ratio provided the mixture is not racemic. Partial or total overlap of the band for a non-chiral species with the chiral eluate band increases significantly the difficulty in measuring an enantiomeric ratio. In this instance the total absorbance that is measured may include a contribution from the non-chiral species which without correction will lead to an overestimation of the amount of chiral material and an erroneous value for the enantiomeric ratio. Under these circumstances there is no other LC option but to develop a separation that is based upon a chiral system. 

CMP s advantages stem from the fact that it is cheaper, since it uses achiral stationary phases, and the chiral additive can be purchased at a low cost the approach is flexible, because after using a chiral additive, the chromatographic column can be washed out from the chiral additive and a new additive can be employed. On the other hand, the mechanism is difficult to predict due to the constant presence of a secondary chemical equilibrium in the column. Since the enantiomeric analytes are eluted out of the column as diastereomeric complexes, the detector response may be different for each complex. Also, the sample capacity is relatively small. 

The CSP approach also has advantages and disadvantages. The advantages stem from the fact that the mechanism of chiral separation is easier to predict. The enantiomers are eluted out of the column as enantiomeric entities thus, they have the same detector response. The disadvantages consist of the high price of chiral columns and the fact that, as in the case of CMPs, the sample capacity is relatively small. 

The enantiomeric excess of this product was determined to be >99.5X using a chiral stationary phase HPLC (preparative Regis Pirkle Type 1-A, 10 x 250 tm I.D., 7.5 mt/min flow rate, 1000 psi pressure, lOX 2-propanol in hexane, detector at 284 nm). The R-(-)-enant1omer Is eluted first and the peaks are well separated. Another batch of phosphate ([ 3p -507.7 C, THF, e 1.17) was shown to have 96.5% ee using the same conditions. 

The amounts of 1,3-BDO and 4H2B were measured by gas chromatography under the following conditions column, PoraPak PS (Waters Corporation, Milford, MA, USA) column temperature, 165 °C carrier gas, N2 detection, flame ionization detector. The optical purity of 1,3-BDO was measured as 1,3-BDO diacetyl by chiral HPLC with a Chiralcel OB packed column (4.6x250 mm, Daicel Chemical Industries, Ltd., Tokyo, Japan) at 40 °C, eluted with n-hexane 2-propanol (19 1) at a flow-rate of 1 mL/min, and detected at 220 nm. 

A commercial CE system and a micropacked capillary was used to separate N—, O—, and S-containing heterocyclic compounds. Migration time reproducibility, linearity, and detector response was found to be comparable to HPLC. A study of the heterocyclic compound s elution order followed that predicted by the octanol-water partition coefficients (354). While chiral CEC provides improved resolution and higher efficiencies, additional work is needed since chiral CEC capillaries are not available commercially. The separation principles and chiral recognition mechanism for the separation of enantiomers have been reviewed (355). Furthermore, a comprehensive collection of drug applications and other compounds of interest has been reported (356). Direct enantiomeric separations by CEC were studied using a capillary packed with alpha-1-acid glycoprotein chiral stationary phase (357). Chiral resolution was achieved for enantiomers of benzoin, hexobarbital, pentobarbital, fosfamide, disopyramide, methoprolol, oxprenolol, and propanolol. The effects of pH, electrolyte concentration, and con-... 

Each enantiomer carried by the mobile phase interacts in a different manner with the stationary phase which contains an enantiomerically pure chiral element. There again, it is diastereomeric interactions that are translated into different elution rates through the column. In principle, whatever the detector (ultraviolet-visible spectrometer, refractometer, etc.) the response factor is identical for the two enantiomers being analysed. As a result, integration of the peak areas corresponding to each enantiomer leads to a measure of the ee by the simple relationship ee = (Si — S2)/(Si + S2) (Figure 2.66). 

Polymer particles are suspended in water (25% MeCN) and then slurry-packed into stainless steel columns (250 mm x 4.6 mm i.d.) using an air-driven fluid pump (Haskel, Burbank, CA, USA) and water (25% MeCN) as the packing solvent. The packed columns are washed on-line on a Beckman HPLC system (comprising a solvent module 126 and diode array detector 168) using MeCN (20% acetic acid) to remove the print molecule until a stable base line is obtained. The mobile phase is then changed to a citrate buflfer (pH 3.0, 25 mM citrate) containing 10% MeCN (v/v) at a flow rate of 1 ml. min . For a test of chiral resolution, a racemic mixture of (+)- and ( )-isoproterenol (20 pL at 2 mM in the mobile phase) is injected, and the elution monitored at 280 nm. Acetone can be used as a void marker for the calculation of capacitor factor (k ) and separation factor (oc). 

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