Extending the Golay Equation for Coupling a Gas Chromatograph to a Drift Tube IMS

Abstract

When it comes to analyzing complex mixtures with highly sensitive ion mobility spectrometers (IMS), a pre-separation of these mixtures is often required due to chemical cross sensitivities and limitations in IMS resolving power. In most cases, gas chromatographic (GC) pre-separation is used. In contrast to typical GC detectors, such as flame ionization detectors or photo ionization detectors, an IMS can add significant dead volume to the system due to its ionization chamber. This dead volume causes memory effects and peak broadening affecting the performance of the chromatographic pre-separation. Therefore, a new but simple model has been developed to estimate the effect of additional detector dead volume and to obtain the optimal operating parameters. This model considers both geometric parameters, such as column length and column diameter, and operating parameters, such as flow rate and temperature of the gas chromatograph and IMS. In addition, the effects induced by the compressibility of the mobile phase were taken into account. In comparison to the commonly used Golay equation our model predicts increased plate heights at low and medium linear velocities. Our model has been experimentally verified using an ultra-high sensitive IMS detector in combination with capillary columns of different lengths and diameters. A UV lamp has been used for ionization due to its good linearity. The columns have been held in isotherm conditions and, additionally, they have been operated with purified air as mobile phase. For each capillary under investigation, a series of measurements with different linear drift velocities has been performed. From these experimental data, the vast difference in plate heights at low linear velocities compared to the expected values from the Golay equation can be confirmed. Despite the simplicity of our model, the predictions are in good accordance with the measurements. Thus, the GC-IMS operating parameters can be easily optimized with respect to maximum gas chromatic pre-separation.