Nitrogen Selective Detectors

To a certain extent these have been covered in the section above but the commonest nitrogen-selective detector is the flame thermionic detector. This is abbreviated to FTD or NPD, since by operating under different conditions the detector can be made selective for nitrogen or phosphorus (but not both simultaneously). Like the FPD this detector has a long history of development originating in the observation that alkali metal compounds introduced into a flame gave a high response for halogens. Indeed, early detectors were made by modifying leak detectors used to monitor the escape of halogenated gases from refrigeration units. By operating under different conditions, selectivity for nitrogen and phosphorus could also be obtained. The next development was to use a small

Same as in an FID to impinge upon a pellet of an alkali metal salt; later versions had an alkali metal salt Same tip. All these versions suffered from the fact that, as the salt was gradually vaporized in the Same, the response of the detector altered and consequently required frequent calibration. The current version looks very similar to a conventional FID in construction but has a bead of rubidium glass that can be electrically heated to 600-800°C between the Same jet and the main collector electrode. This acts in a somewhat analogous manner to the grid in an electronic triode valve. Since the bead does not depend on a combustion Same for heating it is more stable and less susceptible to variations in response. For nitrogen the hydrogen gas supply to the detector is very small (about 4mLmin~1) and the area of reaction is described as a hydrogen plasma rather than a Same. Increasing the hydrogen Sow rate to the Same to about 30 mL min"1 increases the sensitivity for phosphorus and reduces the nitrogen response. Typical performance characteristics for the NPD are shown in Table 1.

There is still considerable controversy about the mechanism of response of the NPD and for an in-depth discussion of this the reader should consult Patterson (see Further Reading).

The NPD is capable of giving excellent results if used by a skilled operator and it has been used in the nitrogen mode for a wide variety of samples. However, the difficulties in its use for routine analysis and the alternative detectors now available seem to indicate a decline in its use in the future.

Figure 2A shows an FID chromatogram of a sample containing 2 ng amounts of cocaine and heroin and Figure 2B shows the equivalent chro-matogram obtained with an NPD. The improved sensitivity and selectivity of the NPD are clearly demonstrated.

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Solar Panel Basics

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