Cyclodextrins are an important family of pharmaceutical excipients that are widely used as complexing agents to increase the aqueous solubility of poorly water-soluble drugs. With their characteristic doughnut-shaped structure, comprising a lipophilic central cavity and hydrophilic outer surface, these versatile drug delivery systems are highly effective at enhancing the bioavailability and stability of active guest molecules. However, to ensure the quality and safety of cyclodextrins used in pharmaceutical products, accurate, reliable and convenient methods for assessing their purity are essential.
HPAE-PAD: A trusted technique for cyclodextrin determination
High-performance anion-exchange chromatography coupled with pulsed amperometric detection (HPAE-PAD) is a powerful and popular technique for cyclodextrin analysis. HPAE chromatography utilizes the weakly acidic nature of carbohydrates to achieve highly selective separations under basic conditions, while PAD enables sensitive, derivatization-free analyte quantification, avoiding time-consuming sample preparation steps. Given the technique’s analytical performance, reliability, and widespread use, HPAE-PAD is described by several monograph methods for cyclodextrin determination, including those of the US Pharmacopeia National Formulary (USP-NF) and European Pharmacopoeia.
Ongoing advances in ion chromatography column design are pushing the capabilities of HPAE-PAD for cyclodextrin analysis even further. The development of advanced non-porous polymeric packing materials, such as those found in Thermo Scientific™ Dionex™ CarboPac™ PA200 series columns, has led to significant improvements in the efficiency of HPAE separations for charged and neutral oligosaccharides. With their small particle-size packing materials (5.5 μm) stable over the full pH 0–14 range, Dionex CarboPac PA200columns offer the highest resolution available for oligosaccharide analysis by HPAE-PAD.
Improving cyclodextrin separation using advanced HPAE column chemistries
One of the most widely used cyclodextrin-based solubilizing and stabilizing agents is betadex sulfobutyl ether sodium. The USP-NF monograph for determining ß-cyclodextrin impurities in betadex sulfobutyl ether sodium describes a HPAE-PAD method employing a Thermo Scientific Dionex IonPac AS11 Analytical and Guard Column.
We tested the new Dionex CarboPac PA200 Analytical and Guard Columns for determination of ß-cyclodextrin, using a Thermo Scientific Dionex ICS-5000+ HPIC system (currently available as the Thermo Scientific Dionex ICS-6000 HPIC system). The HPAE-PAD method described in the USP-NF monograph for betadex sulfobutyl ether sodium uses a sodium hydroxide and sodium acetate mobile phase. In our analyses, sodium nitrate was used as a replacement to sodium acetate, to enhance the resolution of higher oligosaccharide polymers. A column regeneration step was also employed to wash strongly retained analytes from the stationary phase.
In comparison to the Dionex IonPac AS11, ß-cyclodextrin was better retained on the new Dionex CarboPac PA200 column, although peak shapes and symmetries were comparable to those of the Dionex IonPac AS11 (Figure 1). An analysis of six replicate injections of a reference standard found the relative standard deviation of the retention time, peak area, and peak height to be <0.2%, 0.6%, and 0.5%, respectively – well within the 5% limit specified in the monograph. Additionally, limits of detection and quantification were estimated from the average peak height of three injections of the reference standard and were found to be 0.03 and 0.1 mg/L, respectively, highlighting the method’s suitability for sensitive cyclodextrin analysis.
High-resolution, sensitive analysis of commercial cyclodextrin samples by HPAE-PAD
This approach was subsequently used to analyze two commercial betadex sulfobutyl ether samples. Both samples were found to contain betadex sulfobutyl ether sodium as impurities at concentrations below the monograph limit of 0.1%.
To test the reproducibility of this approach, the reference standards and commercial samples were analyzed under identical conditions using a second Dionex CarboPac PA200 column. Response times on the two columns were found to differ by just 3%, meeting the system suitability and acceptance criteria.
Overall, the separation, linearity, reproducibility and sensitivity associated with the method met or exceeded the current USP-NF betadex sulfobutyl ether sodium monograph performance requirements, demonstrating the suitability of the Dionex CarboPac PA200 column for this application.
Pushing the limits of performance with advanced HPAE-PAD techniques
Advances in HPAE column chemistries are delivering enhanced analytical performance for cyclodextrin determinations. By providing robust, reliable and reproducible analyses, these advanced column technologies are helping pharmaceutical manufacturers ensure the quality and safety of pharmaceutical formulations.
Want more details on this HPAE-PAD method for cyclodextrin analysis? Read more in this application note.
Figure 1. Comparison of chromatograms of a 2 mg/L USP ß-cyclodextrin reference standard solution separated by Dionex IonPac AS11 and Dionex CarboPac PA200 columns.