The dynamic modification of gas chromatographic stationary phases by vapours

Abstract

A general method for the modification of stationary phase selectivity by vapours in capillary gas chromatography is presented. The modification is temporary and reversible. There are two approaches. The first procedure involves injection of a modifier of low volatility, as a large vapour plug, onto a capillary column at an elevated temperature. At subsequent lower analysis temperatures, the elution of the modifier is so slow as to enable the normal use of the flame ionization detector(FID). A silicone phase was modified to achieve chiral separation and the polarity of a polyethylene glycol column was significantly increased by this dynamic procedure. The reversible nature of the process presents a convenient way to study intermolecular interactions and potential selectors for novel GC stationary phases. In the second approach, a volatile modifier is continuously added to the carrier gas. Water vapour, to which the Fl D shows no response, was used to alter the retention properties of a polyethylene glycol coated capillary GC column. A dramatic increase in hydrogen bonding interactions was observed towards alcohols and carboxylic acids. The Kovats index of methanol was found to increase by 353 units. The application of the dynamically modified stationary phase to the analysis of alcohol additives in petrol was investigated. Dynamic modification of stationary phases can be performed with minimum alteration to existing equipment. Special selectivity can be obtained without the need for expensive custom-made columns.