Detectability and the resulting requirements for column-detector systems in capillary gas chromatography

Expressions for the minimum detectable amount Qo and the minimum analyte concentration Co as functions of the chromatographic parameters are derived for both mass and concentration sensitive detectors. The effects of pressure drop, column inner diameter and film thickness are given. The minimum analyte concentration for mass flow sensitive detectors. Com, can be reduced considerably by selecting the carrier gas velocity well above its optimum value (related to Hmin) , however, at the cost of long columns and long analysis times. For Qo the improvements can be neglected, and so the analysis can best be performed at Uopt. When the flow rate in the detector, Fd, is equal to the column flow rate Fc, the maximum permissible detector volume of concentration sensitive detectors is proportional to dc2 up to dc3, and so narrow bore columns require detectors of extremely small volume. Make-up gas has to be added when the actual volume is too large, thus deteriorating the detectability. Another approach, vacuum operation of the detector cel appears to be very attractive. On the other hand, when wide bore colums are used in combination with small volume concentration sensitive detectors, very small values of Qoc and Coc are obtainable when the abundant carrier gas can be removed before entering the detector cell. Digital noise filtering can further reduce the obtainable Qo and Co values, especially for broad peaks and thus for wide bore columns.