Journal of Chromatography B, 715 (1998) 341–347 Determination of selenium stable isotopes by gas chromatography– mass spectrometry with negative chemical ionisation * P. Van Dael , D. Barclay, K. Longet, S. Metairon, L.B. Fay ´ Nestle Research Centre, Nestec Ltd, Vers-chez-les-Blanc, P .O. Box 44, CH-1000 Lausanne 26, Switzerland Received 31 October 1997; received in revised form 28 May 1998; accepted 29 May 1998 Abstract A gas chromatography mass spectrometric method using negative chemical ionisation was developed for the determination of stable isotopes of selenium for evaluation of selenium absorption and retention from foods in humans. The method involves an acid digestion to convert all selenium into selenite, which subsequently reacts with 4-nitro-o-phenylene-diamine to form a volatile piazselenole. The piazselenole, after extraction into an organic solvent, was analysed for its isotopic selenium composition by gas chromatography mass spectrometry. Negative chemical ionisation is reported for the first time for the determination of selenium stable isotopes and its analytical characteristics were compared to those of electron impact mass spectrometric ionisation, classically used for the determination of selenium. The negative chemical ionisation technique allowed accurate determination of total selenium by isotope dilution and of selenium isotope ratios in biological samples. The repeatability for total selenium and for stable isotope ratios was good (R.S.D.#10%) within the range of 50 to 250 ng selenium. The detection limit for the investigated selenium isotopes was approximately 1 pg (signal to noise ratio at 3). The applicability of the developed stable isotope methodology was demonstrated by the determination of the selenium absorption and retention from foods in a pilot study using one human adult.  1998 Elsevier Science B.V. All rights reserved. Keywords: Stable isotopes; Selenium 1. Introduction be generally high (40–90%) [3]. Se bioavailability can be estimated by assessing absorption and re- Selenium (Se) is recognised as an essential trace tention of Se stable isotopes [6]. Stable isotopes offer element for humans and dietary recommendations the possibility to evaluate Se absorption and re- were established [1,2]. Se status is determined by tention in all population groups without major con- dietary Se intake and its bioavailability [3]. Popula- straints [6]. These studies require analytical tech- tion groups on restricted diets, such as infants and niques which allow precise and accurate determi- patients on total parenteral nutrition, and people nation of stable isotope ratios at low levels of total living in low Se areas were reported to be at selenium. increased risk for low dietary Se intake [4,5]. The Stable isotopes of Se were determined by neutron bioavailability of Se, influenced by the chemical activation analysis, inductively coupled plasma mass form in which Se is present in the diet, is reported to spectrometry (ICP-MS), and gas chromatography mass spectrometry (GC–MS) [7–11]. GC–MS, * Corresponding author. probably the most widely used technique, employs 0378-4347 / 98 / $19.00  1998 Elsevier Science B.V. All rights reserved. PII: S0378-4347(98)00263-1