Gas Chromatography Detectors - The Nitrogen Phosphorus Detector (NPD) > Page 47


The adsorbed materialreducestheworkfunction ofthesurfaceand, thus, electron emission is increased and the current collected at the anode rises. The NPD has a very high sensitivity, i.e., about an order of magnitude less than that of the electron capture detector (ca.10-12 g/ml for phosphorus and 10-11 g/ml for nitrogen).

The main disadvantage of this detector is that its performance deteriorates with time. Reese (19) examined the performance of the NPD in considerable detail. The alkali salt employed as the bead is usually a silicate and Reese showed that the loss in response was due to water vapor from the burning hydrogen converting the alkali silicate to the hydroxide and free silica. At the normal operating temperature of the bead, the alkali hydroxide has a significant vapor pressure and consequently, the rubidium or cesium is continually lost during the operation of the detector. Eventually all the alkali is evaporated, leaving a bead of inactive silica. This is an inherent problem with all NP detectors and as a result the bead needs to be replaced regularly if the detector is in continuous use. The detector can be made "linear" over three orders of magnitude although no values for the response index appear to have been reported. Like the FID it is relatively insensitive to pressure, flow rate and temperature changes but is usually thermostatted at 260oC or above. The specific response of the NPD to nitrogen and phosphorus, coupled with its relatively high sensitivity, makes it especially useful for the analysis of many pharmaceuticals and in environmental samples containing herbicides. Employingappropriate column systems traces of herbicides at the 500 pg level can easily be determined.


1/ Eptam® 2/ Sutan® 3/ Vernam® 4/ Tillam®
5/ Odram® 6/ Treflan® 7/ Balan® 8/ Ro-Neet®
9/ Propachlor 10/ Tolban® 11/ Propazine 12/ Atrazine
13/ Simazine 14/ Terbacil 15/ Sencor® 16/ Dual®
17/ Paarlan® 18/ Prowl® 19/ Bromacil 30/ Oxadiazon
21/ Goal® 22/ Hexazinone


Courtesy of Supelco Inc.


Figure 23 The Separation and Specific Detection of Some Herbicides Using the Nitrogen Phosphorus Detector