A SILICON SELECTIVE FLAME IONIZATION DETECTOR FOR GAS CHROMATOGRAPHY

This thesis presents a series of papers which have been published or will soon be submitted for publication in which a hydrogen atmosphere flame ionization detector (HAFID) has been investigated for the selective determination of silicon containing compounds after separation by gas chromatography. Although the practice of silylation of certain compounds has long been used in analytical chemistry to facilitate vapor phase chromatography, an accepted method for the selective detection of these derivatives does not exist.;Initial studies of the detector determined that silicon containing compounds produced asymmetric peaks which could not be attributed to chromatographic adsorption but rather were the results of inefficient sweeping of combustion products from the detector housing. By modifying the detector's configuration to a narrow cylindrical tube, peak asymmetry was eliminated and the detector could be used with high resolution chromatographic columns.;Three modes of operation were possible with this detector: one was a non-doped mode in which pure hydrogen gas was introduced into the housing. In this mode, the easiest of the three to operate, silicon containing compounds produced responses about 2600 times greater than those of hydrocarbons and had a minimum detection level around 4 ng. In the second mode of operation, small quantities, typically 5 ppm or less, of pentacarbonyl-iron vapor was doped into the hydrogen atmosphere to increase both sensitivity and selectivity of the detector. The minimum detectable level reached as low as 50 pg for silicon containing compounds and the selectivity against hydrocarbons as high as 10,000. A third mode of operation in which silicon containing compounds produced negative peaks was observed when the concentration of ferrocene vapor was increased to 30 ppm or more in the hydrogen atmosphere. Under these conditions the minimum detectable level for silicon containing compounds was around 1 ng with absolute selectivity. Potentially interfering compounds in this negative mode were found to be hetero-atom compounds containing phosphorus, iron, and chlorine since they also produced inverted peaks.;The basic mechanism of the response for the silicon selective detector is believed to be a series of charge transfer reactions between hydronium background ions, metal containing species, and silicon containing species to produce stable response ions with reduced mobilities favorable for their collection at the distant electrode. Specific reactions which may be responsible for the formation of these response ions are proposed in Chapter 6. Although the mechanism of this detector remains speculative, the analytical potential for application of the detector in the field of gas chromatography is great.;The silicon selective detector described here was constructed from a standard flame ionization detector by interchanging the oxygen and hydrogen inlets so that the flame burned in a hydrogen atmosphere. Also, the collecting electrode was removed from the combustion region (its standard position in a flame ionization detector) to more than 10 cm above the flame to effectively discriminate against the response of hydrocarbons.;As an example of its practical utility, the detector was employed for the selective determination of salicylic acid in human urine. In a comparison with the flame ionization detector, it showed more selectivity for the silylated acid and similar sensitivity.