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Chiral e-Times 2009 Volume 2
Are scientists using Twitter? Apparently yes. The new social networking site, Twitter, is a cross between blogging, texting, and instant messaging. Using Twitter, members communicate in real time with short bursts of information. If you aren’t familiar with Twitter and are interested in learning, there are loads of tutorials on the web; just query your favorite search engine.
CHIROBIOTIC chiral stationary phases (CSPs) possess ionic functional groups and, unlike cellulose-based CSPs, they are not limited to normal phase operation. They are therefore very useful for separating ionic/ionizable compounds, like amino acids. They operate in mobile phases that are MS-compatible, and that can be optimized based on sample solubility, an important consideration for all forms of preparative separations, including SMB. Chiral Amino Acids The decision tree below is a general guideline on choosing an HPLC CSP for chiral amino acid separations. Many applications and a bibliography are available on our web site sigma-aldrich.com/chiral-chromatography. Please consult our technical services group for further assistance.
A product information flier and technical presentations on Astec CHIROBIOTIC CSPs for SMB applications are available in our learning center.
Astec CHIRALDEX and Supelco DEX lines comprise the widest range of derivatized cyclodextrin (CD) phases for chiral GC separations available today. The 26 different phases cover three different cyclodextrins and nine different derivatives. The phases are grouped into three general categories based on the derivative and types of interactions they are proposed to undergo with analytes. Group 1 is termed “Surface Interactions/Complex Derivatives.” Because the predominant mechanism of retention for phases in this group is based on surface interaction, the gamma-cyclodextrin, with 8 glucose molecules, has been shown to be the most useful. Compared to alpha- and beta-cyclodextrins, the greater number of glucose molecules in a gamma-cyclodextrin results in the greater number of 2,3,6-position hydroxyl functional groups available for derivatization. High derivative concentration is beneficial for maximizing surface interactions. Astec CHIRALDEX G-TA, G-DP, G-PN and G-BP belong to this group. Although the G-TA is by far the dominant player, there are subtle chiral and achiral selectivity differences between the members. Group 2 is termed “Surface & Inclusion Interactions/Simple Derivatives.” This group includes the popular dimethyl and permethyl derivatives. The beta-cyclodextrin has shown the greatest applicability for phases with these derivatives. Astec CHIRALDEX B-DM, B-PM, and Supelco ß-DEX 110, ß-DEX 120, ß-DEX 325 and ß-DEX 225 belong to this group. Group 3 is termed “Inclusion Interactions.” Obviously, this group relies on inclusion interactions as the dominant retention mechanism. The three different size cyclodextrins (alpha, beta, and gamma) allow for separation of a wide variety of different size analytes. Astec CHIRALDEX B-DA and CHIRALDEX B-PH belong to this group. The Astec CHIRALDEX B-DA demonstrates the strongest size selectivity. This phase requires analytes to minimally contain two ring structures, one of which is unsaturated (aromatic). The mechanism of this phase is strongly dependent on the inclusion mechanism and is able to differentiate changes in the base structure. Because the DA phases most effectively separate multi-ring analytes, analysis temperatures are often higher than 150 °C. This phase is primarily used for fingerprinting raw materials and identifying structural differences. The Astec CHIRALDEX B-PH shows at least some selectivity to a great variety of analytes, but is especially effective for saturated analytes with minimal functionality, saturated cyclics, and saturated bicyclics. Interestingly, B-PH and B-DA often exhibit reversal of elution order (enantioreversal). The table below gives some guidelines on the choice of chiral GC phase based on the analyte functional group. Please consult our technical services group if you need any help in deciding which GC CSP is best for your separation. Please also note the limited-time promotion on our chiral GC columns in the Special Offers section.
Sample solubility in the mobile phase is an important consideration when performing preparative HPLC separations and purifications. Generally, the higher the solubility, the more mass per injection and the higher the throughput. Our Chiral Services Laboratory was founded upon using the Astec CHIROBIOTIC phases for preparative as well as analytical scale chiral separations. Now, we have expanded our capabilities to include normal phase preparative separations using cellulosic-type CSPs. With this addition, our Services Laboratory is unique in having expertise in normal phase, reversed-phase, polar-ionic and polar-organic chiral separations. This expertise can be applied to perform all types of chiral services, from rapid column screening using an automated MS protocol to supplying gram-quantities of purified enantiomers. 
The most popular method of resolving racemic mixtures for process-scale operations is still the crystallization of diastereomeric salts. However a non-rationalized approach to the screening of resolving agents can often result in poor recovery, low chiral purity and inefficient, non-scalable crystallization protocols. Experience of the Pharmorphix team has shown that the solid state characterization of the diastereomeric salts is critical for an understanding of the resolution process as different polymorphic forms and / or solvates can provide varying resolution results. For small molecule compounds that lack an ionizable center, separation of racemic mixtures can be possible and can be sometimes achieved by a process of kinetic entrainment. Key Pharmorphix capabilities for crystallization resolution studies include:
For more information on crystallization development and scale-up or the solid-state services offered by Pharmorphix, please contact Paul.Hirst@sial.com or view our Pharmophix Crystallization Development page.
Chirality 2009 (July 12 - 15, 2009, Breckenridge, CO) Our complimentary vendor seminar on chiral method development received a high response, and we were overbooked – we truly appreciate the interest in our talk! Analytical Open Seminar Schedule The first seminar series will be conducted August 10 – 13 in the NJ-PA-DE region. To find out more, including locations, abstracts, speaker biographies and to register online, please visit sigma-aldrich.com/analytical-seminars, or send an email to ac_seminars@sial.com. To see our complete tradeshow schedule for 2009, visit www.sigmaaldrich.com/analytical-events
Once enantiomeric selectivity is achieved on a chiral stationary phase, resolution of the enantiomers may be improved by changing mobile phase composition, temperature, and/or flow rate. A flow rate of 1.0 mL/min. is often used during method development on 4.6 mm I.D. columns packed with 5 µm particles. For method optimization, if enantiomers are separated but not fully resolved at 1 mL/min., decreasing the flow below 1.0 mL/min. may further increase resolution. Van Deemter studies performed on 4.6 mm I.D., 5 µm particle CHIROBIOTIC T and V2 columns in reversed-phase, polar-ionic, and polar-organic modes have shown that the greatest peak efficiency is achieved at flow rates between 0.2 and 0.15 mL/min. (linear velocities between 0.2 and 0.3 mm/s; reduced linear velocities between 0.61 and 0.81). Therefore, decreasing the flow rate beyond the norm may enhance the efficiency of each peak, and thus, improve the resolution of the enantiomers.
Method development and optimization on Astec CHIROBIOTIC and CYCLOBOND CSPs follows a simple, logical protocol that also permits optimization for MS and evaporative detection methods. To help our customers, we have updated the familiar Astec poster on Chiral Method Development to include new information, strategies and tips. The poster should be a welcome addition to any laboratory where chiral HPLC methods are developed.
For a limited time -- Specify Promotion Code 632 to receive a 25% discount off any Astec CHIRALDEX or Supelco DEX chiral GC column. View our Ordering Information for a list of columns. |
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