An Empirical Retention Model for the Optimization of On-Line Normal-Phase LC-GCfor the Multi-Analyte Determination of Hydrophobic Compounds in FattySamples 2000,52,433-438 G. R. van der Hoffl / R. Hoogerbrugge 1/ R. A. Baumann ~/ U. A. Th. Brinkrnan 2 / P. van Zoonen ~* 1 Laboratory of Organic-Analytical Chemistry National Institute of Public Health and the Environment (RIVM), P.O. Box 1, 3720 BA Bilthoven, The Netherlands 2 Department of Analytical Chemistry and Applied Spectroscopy Free Universi N Department of Analytical Chemistry De Boelelaan 1083, 1081 HVAmsterdam, The Netherlands Key Words Column liquid chromatography Liquid chromatography - gas chromatography Triglyceride matrices Retention modelling Summary Normal-phase LC (NPLC) is a powerful method for the clean-up of fatly samples in the determi- nation of organochlorine pesticides (OCPs). The injected sample deactivates the stationary phase and the triglyceride matrix therefore serves as a polarily modifier in the NPLC separa- tion. Thus, the amount of sample injected is the key to both selectivily and sensitivily in matrix- modified LC coupled to capillary GC. In coupled LC-GC the NPLC separation becomes particularly critical because only a limited amount of the LC eluent can be transferred to the GC and the triglyceride matrix must be pre- vented from entering the GC, because it degrades the performance of the injector and the col- umn. In previous applications method development was seriously hampered by these bound- ary conditions and tedious and lengthy trial-and-error experiments were required to determine suitable experimental conditions. In this study an empirical model was developed that de- scribes the NPLC separation process in terms of column dimensions and fat Ioadabilily. The out- put is given as the probabilily of achieving successful LC-GC analysis of a particular set of ana- lytes, thus furnishing a useful tool for the development of new applications in the field of expo- sure assessment and analysis of residues of apolar compounds in fatly samples. The limitations of current procedures - maximum transfer volumes and minimal separation - are also dis- cussed. Introduction Applications of on-line-coupled normal- phase L~GC have usually dealt with the transfer of a single target analyte, e.g. raspberry ketone in raspberry sauce [1] or dicamba in a wax ester matrix originating from tobacco leaves [2]. Multi-analyte ap- plications are usually restricted to group separations, for example hydrocarbons in foodstuffs [3], aromatic compounds in jute bags [4], sterol esters in olive oil [5], PCBs in fish [6] and p,p'-DDE and PCBs in human fat [7]. Hyvonen et al. [8] used a cyano-bonded silica LC column for clean- up in the determination of PCBs in fish. The same group of analytes was deter- mined in plasma by Grimvall et al. [9], by use of a dinitroanilinopropyl-modified si- lica LC column. In these applications a re- latively small heart-cut from the LC suf- ficed to transfer the analytes to the GC, because the target compounds have simi- lar chromatographic behaviour on the LC column. For pesticide residue analysis in fat-containing matrices two requirements must be met. The target compounds must be separated from the triglyceride matrix, but the fraction volume in which the rather heterogeneous set of target com- pounds elutes must be kept as small as possible. The clean-up technique most com- monly used in pesticide residue analysis is size-exclusion chromatography (SEC). This technique is quite efficient for elimi- nation of macromolecular matrix consti- tuents, but does not add selectivity for compounds of similar molecular mass [10]. For more complex types of sample it is, therefore, often combined with clean- up on a silica mini-column [11, 12]. For on-line coupling of SEC with other chro- matographic techniques miniaturization is necessary to reduce the transfer volume. On-line coupling of SEC to GC has been used by Vreuls et al. for the determination of organophosphorus pesticides in olive oil [13]. An important drawback of SEC in on-line coupling is, however, the order of elution because major components elute before the target fraction, tailing can cause severe problems [14, 15]. Because in- corporation of additional clean-up on sili- ca in a coupled SEC GC system would in- volve two transfer steps and result in very Original 0009-5893/00/02 Chromatographia Vol. 52, No. 7/8, October 2000 433- 06 $ 03.00/0 9 2000 Friedr. Vieweg & Sohn Verlagsgesellschaft mbH 433