Why determine counter ions?
With more than 50 % of pharmaceuticals on the market using counter ions, their analysis constitutes an essential part of drug development, QC and lot release processes to ensure patient safety and drug efficacy.
Salt formation is important during pharmaceutical and biopharmaceutical drug development as it enables the selective targeting of physicochemical properties such as solubility, drug stability and dissolution rates to ensure good bioavailability. A broad selection of inorganic and organic ions can be used as counter ions to the active pharmaceutical ingredient (API), depending upon the API being protonated or deprotonated, requiring anion or cation counter ions. In order to detect these diverse counter ions, there are two commonly used analytical techniques:
- Ion Chromatography (IC) with conductivity detection
- Ultra-High Performance Liquid Chromatography (UHPLC) with Charged Aerosol Detection (CAD) and/or UV detection
Both IC and UHPLC are capable of drug salt, or counter ion detection, but the choice of technique can depend on the type of analysis, the information required and sample throughput.
The determination of cations and anions using IC typically uses suppressed conductivity to reduce interference from the mobile phase, while increasing the response of analytes; rapid separations and quantitative information can be obtained when scaling from standard bore columns to capillary scale, as well as the ability to reduce system start-up and equilibration times—all without the need for reagents!
For inorganic counter ion analysis, ion chromatography is typically used due to its sensitivity and selectivity. The examples below shows chloride counteranion determination from a drug commonly used to determine type 2 diabetes, and three counter cations found in a drug which is used to control cholesterol levels.
When it comes to the analysis of organic counter ions, IC can provide the ultimate sensitivity through its selectivity; many organic acids can be hard to analyse due to poor UV sensitivity, or conversely, high UV absorption which can cause too much interference.
The example here shows the rapid separation of benzenesulfonate, an organic counter ion that forms part of amlodipine besylate, used to treat hypertension and angina.
IC forms the basis for many USP monographs, and is also being positioned to modernize numerous monographs which are commonly based on tedious titration-based assays. An example of this would be the modernization of the USP lithium hydroxide monograph, where IC has been shown to easily exceed the required limits, as well as offering the capabilities to determine calcium in the same assay (inset shows enlarged view to highlight calcium peak).
Ultra-high performance liquid chromatography
UHPLC can provide universal separations due to its great flexibility, large choice of eluents and columns as well as detection options. While not traditionally used for counter ion detection, UHPLC is seeing increasing use for counter ion detection due to the addition of charged aerosol detection, more powerful separation methods and the ability to provide more information in less time when compared to IC.
IC is well suited for high-throughput environments where anions or cations are required to be measured independently, and there is sufficient throughput to justify instrument purchase for these analyses.
With increasingly complex drugs coming to market, UHPLC offers the ability to analyse and detect both cationic and anionic species within the same analysis, as well as performing many other types of analyses within the laboratory.
An example would be Hydrophilic Interaction Liquid Chromatography (HILIC) separation of common inorganic counter ions with CAD detection, as illustrated here.
One of the issues with typical inorganic and organic counter ions is the lack of a chromophore, ruling out the use of UV detection. CAD does not require any chromophores, as the principle of measurement is to charge all species, with the relative concentrations being proportional to the concentration, which is why CAD is sometimes referred to as the universal detector due to its ability to detect all non- or semi-volatile components of a formulation.
A further advantage of a UHPLC system utilizing both UV and CAD detection is the detection of not only counter ions, but of the API (and any impurities) as demonstrated within the image shown for the analysis of Adderall®. Unique chemistries which allow both cation and anion exchange chromatography, provide great flexibility for method development and optimization and can be found as a complete solution, streamlining laboratory processes for simultaneous detection of counter ions.
Need more confidence in your counter ion analyses?
If you need absolute confidence for your counter ion analyses, you’ll be glad to know that you can hyphenate not only UHPLC, but IC to your mass spectrometer (MS) too! Being able to utilize MS detection provides you with an additional level of security for the identification of your components, or could even offer lower limits of detection.
Tools to help you with your counter ion analyses
Estimate your cost savings with a Thermo Scientific Dionex IC system. You might just be surprised.
With many thousands of pre-configured methods for all types of analyses, AppsLab allows you to download 1-click workflows so you can run, process and report your samples!
With this virtual column simulation tool, you’ll be able to find the best separation conditions for your analytes before you even set foot in the lab.