FEATURE ARTICLE Improved terminology for experimental field-flow fractionation Karl-Gustav Wahlund Published online: 29 September 2013 # Springer-Verlag Berlin Heidelberg 2013 Introduction The terminology for sample analyte retention in field-flow frac- tionation (FFF) rests on the same fundamental theoretical con- cepts as for chromatography [1–6]. The most basic phenomenon to start reasoning from is the migration velocity ν a of the sample analyte zones. If retention occurs, which is a prerequisite for separation, these velocities will be lower than the carrier (mobile phase) velocity, the latter taken as the axial velocity averaged over the cross-sectional dimension, i.e. the cross-sectional mean carrier velocity <υ >. It is natural that quantitative treatment of retention be based on the relationship between these two veloc- ities. For this purpose, the symbol R was introduced by Martin and Synge [1] and LeRosen [2, 3], who noted that “the term R is a fundamental quantity” [3]. It was defined as the relative rate of movement of the analyte zone quantified by R ¼ υ a 〈υ〉 ; ð1Þ i.e. the velocity of the analyte zone divided by the velocity of the carrier. R is then a measure of the retardation of the analyte zone [4], i.e. the factor by which the analyte velocity has been reduced relative to the carrier velocity. R was been termed the retardation factor in column chromatography [7] and the retention ratio [5]. “Retardation factor” is a quite useful term, the more so as it does not contain the term “retention”, which occurs in so many other contexts. Giddings [5] preferred “retention ratio” as the term for the symbol R and stated that R is a general and fundamental concept for all kinds of chromatography, includ- ing FFF, and pointed out its simplicity in being a direct analyte migration velocity parameter. Hence, all theoretical treatments of FFF start out from the retention ratio R [8]. For a given separation system, R can be broken down into analyte-specific property parameters . In partition chromatogra- phy, invented by Martin and Synge [1], it was demonstrated that (1) R is determined by a specific analyte property, the partition coefficient, and (2) chromatographic experimental data can be used to determine this property, i.e. the partition coefficient (or distribution constant, or distribution ratio) between the stationary phase and the mobile phase. Later, gas and liquid chromatogra- phers introduced retention parameters and as such used the retention factor k [7]. This can be regarded as an analyte- specific retention parameter. Its connection to R is given by [7] R ¼ 1 1 þ k ð2Þ The experimental determination of the retention factor can be made from the retention time t r and the void time t 0 by [7] k ¼ t r −t 0 t 0 ; ð3Þ which shows that k increases with the retention time. The retention factor quantifies the mass distribution ratio of the analyte between the stationary phase and the mobile phase and can be further expressed as a function of the analyte-specific distribution ratio and distribution constant. Hence, k is a retention parameter that contains certain characteristic infor- mation on the analyte. It is therefore common that analyte retention in column liquid chromatography is reported in terms of k rather than R . Published in the topical collection Field-Flow Fractionation with guest editors S. Kim R. Williams and Karin D. Caldwell. K.<G. Wahlund (*) Unit for Analysis and Synthesis, Department of Chemistry, Faculty of Engineering LTH, Lund University, P.O. Box 124, 221 00 Lund, Sweden e-mail: karl-gustav.wahlund@analykem.lu.se K.-G. Wahlund e-mail: karl-gustav.wahlund@chem.lu.se Anal Bioanal Chem (2014) 406:1579–1583 DOI 10.1007/s00216-013-7347-6