The
Nitrogen Phosphorus Detector
(
NPD
The nitrogen phosphorous detector (sometimes called the thermionic detector) is a very sensitive but specific detector that responds almost exclusively to nitrogen and phosphorous compounds. It is based on the flame ionization detector but differs in that it contains a rubidium or cesium silicate (glass) bead situated in a heater coil, a little distance from the hydrogen flame. If the detector is to respond to both nitrogen and phosphorous then the hydrogen flow should be minimal so that the gas does not ignite at the jet. If the detector is to respond to phosphorous only, a large flow of hydrogen is used which is burnt at the jet. The heated bead emits electrons by thermionic emission. These electrons are collected under a potential of a few volts by an appropriately placed anode, and provides a background current. When a solute containing nitrogen or phosphorous is eluted from the column, the partially combusted nitrogen and phosphorous materials are adsorbed on the surface of the bead. The adsorbed material reduces the work function of the surface and, as consequence, the emission of electrons is increased which raises the current collected at the electrode. The sensitivity of the detector to phosphorous is about 10-12 gram per ml and for nitrogen about 10-11 gram per ml at a signal to nose ratio of 2. The alkali bead as a finite life and needs regular replacement.
Author: RPW Scott
Book:Capillary Chromatography
Section:Capillary Detectors The-Nitrogen-Phosphorus-Detector
The Nitrogen Phosphorus Detector (NPD)
The NPD, is a highly sensitive but very specific detector. It gives a strong response to organic compounds containing nitrogen and/or phosphorus. Despite its appearance it operates on an entirely different principle to that of the FID. A diagram of an NPD detector is shown in figure 14. The sensor of the NPD is a small rubidium or cesium bead contained inside a small heater coil. The helium carrier gas is mixed with hydrogen and passes into the detector through a small jet. The bead, which is heated by passing a current through the coil, is situated above the jet, and the helium-hydrogen mixture (produced by mixing the column carrier gas, helium with a separate stream of hydrogen) passes over it. If the detector is to respond to both nitrogen and phosphorus, then a minimum hydrogen flow is employed to ensure that the gas does not ignite at the jet. In contrast, if the detector is to respond to phosphorus only, a large flow of hydrogen
Capillary Detectors The-Nitrogen-Phosphorus-Detector
Author: RPW Scott
Book:Gas Chromatography
Section:YES Detectors Nitrogen-Phosphorus-(NPD)
Mixture
The Nitrogen Phosphorus Detector (NPD)
The nitrogen
phosphorus detector (NPD), is a highly sensitive but specific detector and
evolved directly from the FID. It gives a strong response to organic compounds containing nitrogen and/or phosphorus. Although it appears to
function in a very similar manner to the FID, in fact, it operates on an
entirely different principle. A diagram of an NP detector is shown in figure
24.
Figure 24.
The Nitrogen Phosphorus Detector
 
YES Detectors Nitrogen-Phosphorus-(NPD)
Author: RPW Scott
Book:Gas Chromatography
Section:YES Detectors Nitrogen-Phosphorus-(NPD)
detector is to respond to both nitrogen and phosphorus, then a minimum hydrogen
flow is employed to ensure that the gas does not ignite at the jet. In
contrast, if the detector is to respond to phosphorus only, a large flow of hydrogen
can be used and the mixture burned at the jet. A potential is applied between
the bead and the anode. The heated alkali bead emits electrons by thermionic
emission which are collected at the anode and thus produce an ion current. When
a solute containing nitrogen or phosphorus is
eluted, the partially combusted nitrogen and phosphorus materials are adsorbed on the surface of
the bead. This adsorbed material reduces the work function of the surface and,
as consequence, the emission of electrons is increased which raises the anode
current. The sensitivity of the NPD is about 10-12 g/ml for phosphorus and 10-11 g/ml for nitrogen).
Unfortunately,
the performance deteriorates with time. Reese (10) examined the function of the
NPD in
YES Detectors Nitrogen-Phosphorus-(NPD)
Author: RPW Scott
Book:Gas Chromatography Detectors
Section:GC-Detectors Nitrogen-Phosphorus-Detector-(NPD)
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. Thedetector 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
GC-Detectors Nitrogen-Phosphorus-Detector-(NPD)
Author: RPW Scott
Book:Gas Chromatography Detectors
Section:GC-Detectors Nitrogen-Phosphorus-Detector-(NPD)
The NPD sensor differs from that of the FID
by a rubidium or cesium chloride bead contained inside a heater coil situated
close to the hydrogen jet. The bead is situated above a jet and heated by
a coil, over which the nitrogen carrier
gas mixed with hydrogen passes. If the detector is to respond to both nitrogen
and phosphorus, then the hydrogen
flow should be minimal so that the
gas does not ignite at the jet. If the detector is to respond to phosphorus,
only, however, a large flow of hydrogen can be used and the mixture burnt at
the jet. The heated alkali bead emits electrons by thermionic emission which
are collected at the anode and provides background current through the
electrode system. When a solute that contains nitrogen or phosphorusiseluted, thepartiallycombustednitrogenandphosphorusmaterials are adsorbed on the surface of the bead.
Figure
22. The Nitrogen Phosphorus Detector
GC-Detectors Nitrogen-Phosphorus-Detector-(NPD)
Author: RPW Scott
Book:Gas Chromatography
Section:YES Detectors Flame-Ionization
(e.g., halogenated substances by the electron
capture detector). Conversely, those detectors with a catholic response,
although highly sensitive compared to LC detectors (e.g. the flame
ionization detector) are significantly less sensitive than the specific
detectors. The detectors with a catholic response are the most popular and the
majority of GC separations are monitored by the flame ionization detector
(FID). The most commonly used specific detectors are the nitrogen phosphorus
detector (NPD) and the electron capture detector (ECD) The katharometer
detector, although having relatively poor sensitivity is widely used in gas
analysis.
The Flame Ionization Detector
The FID,
invented by Harley and Pretorious (7), and separately by McWilliams and Dewer
(8), evolved from the Heat of Combustion Detector developed by Scott (9). The FID detector employs hydrogen as
the combustion gas which is mixed with the column eluent (helium, nitrogen or
other
YES Detectors Flame-Ionization