Just as there is no completely universal detector, so there is no detector which is completely specific for one element or type of compound, and for this reason it is better to talk of “selective” rather than “specific” detectors. Again, as with the universal detectors, every selective detector has advantages and disadvantages which make it more suitable for some types of work than for others. Some selective detectors have a high specificity but a relatively low sensitivity; with others the reverse is true. A number of selective detectors depend on the performance of certain chemical operations on the sample before actual detection; e.g., nitrogen compounds may be reduced to ammonia and the amount of ammonia measured in some way, sulphur compounds may be oxidized to sulphur dioxide, and so on. These techniques usually involve a considerable dead volume between the end of the column and the detector proper, and it is not possible to use detectors of this sort with high resolution open tubular (capillary) columns.