Trace enrichment of polar compounds on chemically bonded and carbonaceous sorbents and application to chlorophenol

Abstract

On-ilne trace enrichment of moderately polar compounds on short precolumns prior to their separatlon by means of hlghperformance liquid chromatography (HPLC) has been studled In order to correlate chromatographlc retentlon data wlth sorptlon behavlor from aqueous solutions In preconcentration techniques. Several octadecylalkyl-modified slilcas and different types of carbon materials were selected as sorbents, and a series of chiorophenois were used as model compounds. Special attentlon was devoted to the partlcle characterlstlcs of the sorbent materlals. From the experlmental data a method has been derived for the prediction of the sorptlon of a compound from a purely aqueous solution onto Cis modified-siilca sorbent on the basts of the retention observed on that materlal In a single reversed-phase system using an lsocratlc organic/aqueous mobile phase. As an application, the trace enrichment and HPLC separation of hlghly chlorinated phenols from spiked water samples is descrlbed.Trace enrichment on solid surfaces by means of sorption of sample constituents from aqueous solution has repeatedly been shown to be an attractive method in both off-line and on-line gas and liquid chromatographic methodology. Since off-line techniques require further handling of the sample between the trace enrichment and the separation step, they are not highly suitable for fully automated techniques. In addition, they suffer from the disadvantages of possible sample loss and/or decomposition and are sften complex and timeconsuming.On-line techniques have, for these reasons, become quite in vogue in the recent past, especially in HPLC. For an on-line transfer of a preconcentrated sample constituent from the precolumn to the analytical column, HPLC-quality sorbents-i.e., materials having 5-10 pm particle diameter-in good packing geometry should be used. The diameter of the precolumn should be similar to or smaller than that of the analytical column, and the precolumn should be as short as possible for a particular problem. The retention of the compounds of interest on the precolumn should ideally be similar to, or lower than, that on the analytical column in order to prevent band-broadening problems during the desorption and actual separation step.As regards the small size of the precolumn, this aspect is of importance because of suppression of band broadening due 0003-2700/81/0353-2072$01.25/0 to the precolumn, a low pressure drop-and, thus, a high flow rate-during the sorption step, ease of packing and replacement, and material saving. The minimum size of the precolumn required for the trace enrichment of a sufficient amount of a solute is a function of the absolute detection limit of the solute, ita concentration in the original sample solution, and its retention-Le., its capacity ratio, k ' i n the stationary phase/aqueous sample system under investigation. These aspect$ should be borne in mind when conditions are selected so that the solute of interest is efficiently preconcentrated without occurrence of breakthrough phenomena.In p...

Extraction de composés hydrophiles non ionisables des eaux par entraînement à la vapeur suivi d'une rétention sur résine XAD-4

“…Cette méthodologie, couramment employée pour déterminer les facteurs de capacité sur différents adsorbants (SCHOENMA-KERS P.J. ef al., 1981 ;WERKHOVEN-GOEWIE CE. et al, 1981), permet d'accélérer Pélution des solutés.…”

Use of an open-tubular trapping column as phase-switching interface in on-line coupled reversed-phase liquid chromatography—capillary gas chromatography

“…The breakthrough volume of a component in a trapping column is given by the equation [29] Vb=VO(l+k) l-3--_ ( > fi where V0 is the void volume of the trapping column, k the capacity factor of the solute in the trap and N the plate number of the trapping column (with N>9).…”

The use of solid sorbents for direct accumulation of organic compounds from water matrices–a review of solid‐phase extraction techniques

“…[36,44,45]. As a n explanation, the decrease of the number of theoretical plates 1361, or the occurrence ofnon-equilibrium processes (441 was proposed.…”

“…[47] demonstrated that the fraction recovery, given by the equation can be independent of the volume flow rate, since a large value of V implies a small 6 value, and a short residence time, t, as well Conversely, lower V values increase 6 and slow down diffusion However, a n increasing residence time, t, can compensate for this effect, thereby welding a constantR This conclusion has not been fully confirmed by experiments wth other sorbents [36,44,45,54,861 Summarizing, it is possible to conclude that, for every system, there is a critical value of the flow rate The increase in flow rate over this value leads to the occurrence of non-equilibrium processes and, hence, to a change in retention properties…”