Evaluation of Thermospray and Cross-Flow Pneumatic Nebulization as Means of Interfacing a Flame Infrared Emission (FIRE) Radiometer to a High-Performance Liquid Chromatograph

Abstract

A cross-flow pneumatic nebulizer and a thermospray nebulizer were each studied as a means of interfacing a flame infrared emission (FIRE) radiometer to a high-performance liquid chromatograph (HPLC). Both nebulizers introduced significant quantities of liquid mobile phase into the FIRE burner, necessitating modifications to the basic burner design and causing temperature changes in the hydrogen/air flame used as the FIRE source. Tailing in the trailing edge of the peak profiles was also observed with the use of pneumatic nebulization and attributed to dead volume present in the nebulizer spray chamber. Flame background caused by the combustion of carbon-containing mobile phases was also a problem, but could be avoided when the FIRE radiometer was operated in the carbon mode by using water as the mobile phase. Flame background from carbon-containing mobile phases could also be electronically subtracted by using a dual-channel, chlorine-specific FIRE radiometer to selectively determine mixtures of chlorinated hydrocarbons in the presence of a chlorine-free mobile phase. Background subtraction had the added advantage of improving the detection limits for organochlorine compounds by a factor of approximately 3. Detection limits for FIRE/HPLC using pneumatic nebulization and the single-channel FIRE radiometer in the carbon mode were estimated as 3.7 μg s⁻¹ (5.5 × 10² μg) for acetic acid and 0.80 μg s⁻¹ (1.2 × 10² μg) for ethanol with water as the mobile phase. FIRE/HPLC detection limits using thermospray nebulization and a dual-channel FIRE radiometer in the chlorine-specific mode were estimated as 6.2 μg s⁻¹ (3.7 × 10² μg) for methylene chloride and 9.7 μg s⁻¹ (5.8 × 10² μg) for carbon tetrachloride with a combustible mixture of 49% acetonitrile, 20% methanol, 30% water, and 1% acetic acid as the mobile phase.