Precise, fast and flexible determination of protein interactions by affinity capillary electrophoresis. Part 2: cations.

The influence of various cations as metal ions (barium, calcium, copper, magnesia, manganese, and nickel), pharmaceuticals (ephedrine, ethambutol, pilocarpine, and pirenzepine), arginine, and guanidine has been tested on BSA, ß-lactoglobulin, and ovalbumin. Influences on proteins regarding changes in size, charge, or mass were of particular interest. ACE proved to be a suitable method to investigate these effects. ACE is able to observe slight but significant changes on proteins due to the excellent precision of the measurements. Therefore, some unexpected behaviors of protein-ligand interactions were found. The protein charge becomes more negative under metal ion influence and some pharmaceutical cations. Probably metal ions bound to the proteins form additional complexes with anions from the surrounding solution. Furthermore, already bound cations could be displaced at the protein surface. Both effects would change the overall charge of the ligand-protein complexes. In all studied cases, multiple-binding stoichiometries have been observed.

Capillary gel electrophoresis for precise protein quantitation.

CGE, also known as SDS-CGE, is being established in the pharmaceutical industry replacing SDS-PAGE. In most cases, the method is applied for the identity and purity control of proteins, for example monoclonal antibodies. In order to quantify these components with sufficient precision using the same quality control method, a RSD for the quantitative analysis under 2% is required. A reliable and highly precise CGE method could be obtained after thorough optimization. It was crucial to increase the sample concentration and the injection volume in order to achieve sufficiently high S/N ratios (>70). The application of hydrodynamic injection is beneficial for the precision of the method compared to the traditionally used electrokinetic one. Linearity was demonstrated and LOD and LOQ were estimated. Both injection modes were compared in long series runs (n = 48). Furthermore, the use of an internal standard was investigated. Thus, the RSD% of the migration time was reduced from 0.9 to 0.2% and the RSD% of peak areas was greatly improved. However, the normalization to the total area further reduced the influence of the injection error. RSD% for the peak area ratios of typically between 1 and 2% was provided.

Analytical instrument qualification in capillary electrophoresis.

Capillary electrophoresis (CE) is a well-established and frequently used technique in the pharmaceutical industry. Therefore an appropriate analytical instrument qualification (AIQ) is required for quality assurance. AIQ forms the basis of a quality management followed by analytical method validation, system suitability tests (SSTs) and quality control checks. Two parts of the AIQ, namely the operational qualification (OQ) and the performance qualification (PQ) are of particular interest in the daily routine of the laboratory. A new concept for OQ and PQ was developed to assure the correct function of a CE system. The significance of each parameter, possible test methods as well as acceptance criteria will be presented and discussed in detail. Especially temperature adjustment by the cooling system and the voltage supply must be tested for accurate and precise operation. The detector noise, wavelength accuracy and detector linearity have to be checked as well. Finally, the injection linearity, accuracy and precision need to be qualified. The proposed set of qualification procedures is easy to implement and was already tested on five CE instruments from three different manufacturers. A time- and cost-saving continuous PQ was derived, using results from method-specific SSTs and some additional experiments. This holistic concept continuously surveys the most relevant parameters, hence assuring the suitability of the used instruments and decreasing their downtimes.

Minimum required signal-to-noise ratio for optimal precision in HPLC and CE.

In a survey, results of more than 100 assay determinations of various analytes (active pharmaceutical ingredients, methylparabene, chlorthalidone, and proteins) at different concentration levels were collected with signal-to-noise ratio (S/N) levels between 2 and higher than 10 000. It must be concluded that without having a S/N of at least 50, repeatabilities (combination of injection, separation process, and integration) of 2% cannot be achieved, in contrast to the general assumption that a S/N of 10 is sufficient for analytical HPLC. These data now confirm an earlier assumption with a much broader fundamental of measurements. The empirical functions %RSD = 58/(S/N) + 0.30 (for HPLC data) and %RSD = 73/(S/N) + 1.07 (for CE data) could be derived. It was shown that a S/N of greater than 100 is necessary for optimal precision. Before optimizing HPLC or CE methods, for example, for sample pretreatment, S/N > 100 should be the prerequisite, else optimal precision will not be achieved. Only after the detection-related scatter is sufficiently reduced, other sources of variation can be successfully tackled.

Infrared-based temperature measurements in capillary electrophoresis.

In capillary electrophoresis (CE), the temperature inside the capillary is one of the most important parameters. In a concept for Analytical Instrument Qualification (AIQ) of CE systems, the temperature accuracy and stability have to be included. This fact requires an accurate look at the measurement of temperature which is generated by the applied electrical power. The generation of Joule heating is measured on the outside of the capillary using an infrared (IR) thermometer. The thermometer linearity is demonstrated over a wide range of various electrical field strength, buffer systems, and different capillary inner diameters. A slope of 6.3 °C m/W was found for the optimal thermometer capillary distance of 8 mm. Furthermore, the temperature measurements are highly precise, depending almost solely on the current variability. The proposed method is compared with three methods calculating the temperature from the conductance, the electroosmotic velocity, or the current. These indirect methods estimate slopes ranging from 7 to 10 °C m/W. In addition, the maximal suitable electrical power per unit length is estimated. Joule heating can often be tolerated up to 4 W/m. However, sensitive analytes can already be affected by using more than 1 W/m. In conclusion, the consideration of the temperature is essential for not only Analytical Instrument Qualification, but also certainly useful for method optimisation and method transfer.