942 (2001) 249–258 Journal of Chromatography A, www.elsevier.com / locate / chroma Contactless conductivity detection for capillary electrophoresis Hardware improvements and optimization of the input-signal amplitude and frequency a b a, * ´ Jose A. Fracassi da Silva , Norberto Guzman , Claudimir L. do Lago a ˜ ´ ´ Departamento de Quımica Fundamental, Instituto de Quımica, Universidade de Sao Paulo, Av. Prof . Lineu Prestes 748, ˜ CEP 05508-900, Sao Paulo ( SP), Brazil b R. W . Johnson Pharmaceutical Research Institute, 1000 Route 202, Box 300, Raritan, NJ 08543, USA Received 14 May 2001; received in revised form 28 August 2001; accepted 9 October 2001 Abstract A new prototype of contactless conductivity detector, smaller and easier to operate than the former version, is described. For a fused-silica capillary with 142-mm wall thickness and voltages up to 25 kV, it can be placed at the low- or high-voltage end of the column. This feature allowed implementation of an apparatus with sample introduction at the grounded end of the column. The input signal is an important parameter for determining the signal-to-noise ratio ( S / N ) of the detection system. An optimization procedure of its amplitude and frequency is proposed. Although the S / N must be determined by introduction of actual samples, the operating conditions can be optimized merely by changing the signal parameters and by using a mathematical procedure. Thus, an easy and fast optimization routine can be carried out. Mathematical and instrumental backgrounds are discussed, and experimental support of the technique’s effectiveness is presented.  2002 Elsevier Science B.V. All rights reserved. Keywords: Conductivity detection; Detection, electrophoresis; Instrumentation; Metal cations 1. Introduction this was made possible by the introduction of commercial CE equipment provided with CD [3,4]. Since its introduction to capillary zone electro- Anions have been quantified in different matrices, phoresis (CZE), conductivity detection (CD) has such as beverages [5–7], environmental samples [8– demonstrated great potential in the field of ion 13], electrodeposition coatings [14–16], and hydro- analysis [1,2]. CD provides a good alternative to gen peroxide [17]. Transient isotachophoresis (ITP) photometric detection, principally in the low con- [18] and ITP–CZE [19] proved to be suitable centration range, when the lack of sensitivity is the methods for trace inorganic anion analysis. Surfac- major drawback for indirect ultraviolet detection. tants have been separated and detected indirectly A large number of ionic compounds have been [20,21]. Bacterial sulfur degradation has been char- separated and analyzed using CZE with CD. In part, acterized by the analysis of sulfur-containing anions with both ion chromatography and CE [22]. Inor- ganic ions were quantified in rat lung airway surface *Corresponding author. Fax: 155-11-815-5579. E-mail address: claudemi@iq.usp.br (C.L. do Lago). fluid [23]. Williams and Boucher [24] performed the 0021-9673 / 02 / $ – see front matter  2002 Elsevier Science B.V. All rights reserved. PII: S0021-9673(01)01380-2