Michal Jaros ˇ 1 Tomoyoshi Soga 2 Tom van de Goor 3 Bohuslav Gas ˇ 1 1 Faculty of Science, Charles University, Prague, Czech Republic 2 Institute for Advanced Biosciences, Keio University, Yamagata, Japan 3 Agilent Laboratories, Palo Alto, CA, USA Conductivity detection in capillary zone electrophoresis: Inspection by PeakMaster A simple rule stating that the signal in conductivity detection in capillary zone electro- phoresis is proportional to the difference between the analyte mobility and mobility of the background electrolyte (BGE) co-ion is valid only for systems with fully ionized electrolytes. In zone electrophoresis systems with weak electrolytes both conductivity signaland electromigration dispersion of analyte peaks depend on the conductivity and pH effects. This allows optimization of the composition of BGEs to give a good conductivity signal of analytes while still keeping electromigration dispersion near zero, regardless of the injected amount of sample. The demands to achieve minimum elec- tromigration dispersion and high sensitivity in conductivity detection can be accom- plished at the same time. PeakMaster software is used for inspection of BGEs com- monly used for separation of sugars (carbohydrates, saccharides) at highly alkaline pH. It is shown that the terms direct and indirect conductivity detection are misleading and should not be used. Keywords: Background electrolyte / Capillary electrophoresis / Conductivity detection / Electro- migration dispersion / PeakMaster software DOI 10.1002/elps.200410366 1 Introduction Conductivity detection has been successfully used in capillary electromigration separation methods for a long time.Especially in capillary isotachophoresis it is used almost from its inception. Conductivity detection in capil- lary electromigration methods is mostly performed as an on-column detection technique, which means thatthe detection cellis part of the column.In contact con- ductivity detectors [1–5] the sensing electrodes of the conductivity cell are directly in contact with the solution. Unfortunately, due to a high superimposed driving elec- trical potential there is a significant danger of polarization or deterioration of the electrodes by electrolysis products. Gasˇ et al. [6, 7]used for the first time the “contactless” capacitive coupling of the sensing electrodes with the capillary solution using a high-frequency measuring cur- rent. The detectorwhich they developed, was termed contactless conductivity detector (CCD) and consisted of four electrodes placed radially around the separation column.Later,Zemann etal. [8] proposed a different construction ofthe CCD consisting of two cylindrical electrodes placed axially around the column. Independ- ently, da Silva and do Lago [9] published a similarly con- structed CCD. The contactless detector enabled the re- producible use ofconductivity detection for many ana- lyticaltasks in electromigration methods, as there are no phenomena that lead to the deterioration of the detector signal. In spite of the increasing use of conductivity detection in capillary zone electrophoresis (CZE) there is still a lack of understanding what is, for various analytes, the amplitude of the conductivity signal in different background electro- lytes (BGEs). It is often incorrectly stated that the ampli- tude of the conductivity signal of a particular analyte is proportional to the difference between the analyte mobil- ity and mobility of the co-ion of the BGE. In addition, there is confusion concerning electromigration dispersion of the peak. It is often assumed that matching the mobility of the co-ion to thatof the analyte will eliminate electro- migration dispersion (and also cancel the conductivity signal). It was, however,already shown [10]thatthe quantity responsible for the amplitude of the conductivity signal is not the difference between the analyte mobility and mo- bility of the BGE co-ion, but the molar conductivity detection response b X defined as: b X ¼ lim c X !0 dk dc X (1) Correspondence: Dr. Bohuslav Gas ˇ, Faculty of Science, Charles University, Albertov 2030, CZ-128 40 Prague 2, Czech Republic E-mail: gas@natur.cuni.cz Fax: 1420-2-2491-9752 Abbreviations: CCD, contactless conductivity detector; HIBA, hydroxyisobutyric acid; NANA,N-acetylneuraminic acid; PDC, 2,6-pyridine dicarboxylic acid; TTAOH,tetradecyltrimethylam- monium hydroxide 1948 Electrophoresis 2005, 26, 1948–1953  2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim