Detector operation, best practices

From the practical point of view, dual-channel phase detectors operated in quadrature appear to be the best hardware-detection choice. Unlike other techniques, phase detection is sensitive to RF frequency offset from resonance and to the RF phase which, on the one hand, makes the signals more complex to use (as well as and more sensitive to instrument instabilities) but, on the other hand, leads to a number of important advantages, such as ... 

Canines are a proven operational tool for substance detection, but as a practice, canine detection remains more of a craft than a technology. The quasi-technological state of detector dog technology fosters variabiHty in detection capability and reliabiHty, resulting in varying degrees of skepticism. Nevertheless, canine detection is used as the basehne capability to which instmmental devices are compared. Furthermore, despite aU the unknowns and uncertainties, there are stiU many situations where detection dogs remain the best or, indeed, the only available option. 

Let us consider some of the special problems encountered in the operation of a radioisotope detector and the compromises that must be considered. Like any chromatographic detector, a carbon-14 detector should have a small volume and a short hold-up time in order to minimize band spreading and loss of resolution. Unfortunately radioisotopes are measured with an inherent time factor - disintegrations per minute. Therefore, the smaller the cell and the shorter the hold-up, the lower will be the sensitivity, a circumstance which is totally at odds with the first requirement. In practice, we have found that a U-tube with a cross-section diameter of 2mm is generally satisfactory. This gives a cell with a void volume of 200-300 yl, which is high compared to the 2-10 yl volumes of many UV flow cells, and may introduce some band spreading when used with the best new HPLC columns. 

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