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.
