Nitrogen-Phosphorous Detector NPD

The alkali beads are heated to red heat both electrically and in the flame and are excited to alkali emission. They are always at a negative potential compared with the collector electrode. The heated alkali bead emits electrons by thermionic emissions which are collected at the anode providing the background current of the detector. 

Phosphorus-containing substances are first converted in the flame into phosphorus oxides with an uneven number of electrons. Anions formed in the alkali reaction by the addition of an electron are oxidized by OH radicals. The electrons added are released and produce a signal current. 

Like phosphorus, nitrogen has an uneven number of electrons. Under the reducing conditions of the flame cyanide and cyanate radicals are formed, which can undergo the alkali reaction (Table 2.35). For this, the input of hydrogen and air are reduced. Instead of the flame the hydrogen burns in the form of cold plasma around the electrically heated alkali beads. 

In order to form the required cyanide and cyanate radicals, the C-N structure must already be present in the molecule. Nitro compounds are detected, but not nitrate esters, ammonia or nitrogen oxides. By taking part in the alkali reaction the cyanide radical receives an electron. Cyanide ions are formed, which react with other radicals to give neutral species. The electron released provides the detector signal. 

NPD Specific detector Mass-flow-dependent detector  

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Traditional detectors (i.e., FID electron capture detector, BCD nitrogen-phosphorous detector, NPD) supply only retention data. However, in many cases this is not enough for proper identification of analytes. Application of GC coupled with an MS detector gives much more information (i.e., the mass spectmm of each compound). GC-MS is a well known and frequently used technique that combines the highly effective separation of GC with the high sensitivity and selectivity of MS. Moreover, improvements in analytical instruments based on different types of mass analyzers (ion trap, quadrupole, and TOF) and the development of hybrid Q-TOF has enhanced the analytical capabilities of modem hardware. Different kinds of mass spectrometers are presented in Table 14.2 [119]. 

Whenever possible, avoid using phases containing the specific element which interferes with specific detectors [e.g., do not use cyanopropyl phases with a nitrogen-phosphorous detector (NPDs) or trifluoropropyl phases with an electron-capture detector (BCD)]. 

It is a common opinion of the majority of analytical chemists that GC-based separation procedures should be preferred to LC-based separation procedures, when derivatization or conversion of the analytes of interest is not necessary. The success of the GC technique is also due to the possibility of using selective and robust detectors, such as the electron capture detector (ECD), nitrogen-phosphorous detector (NPD), and, chiefly, the nowadays relatively economic MS detector. 

The thermionic detector (TID), also known as the nitrogen-phosphorous detector (NPD), is based on the phenomenon that a metal anode emits positive ions when heated in a gas. It is a commonly used gas chromatographic detector for the selective determination of organic compounds containing nitrogen (N) and phosphorus (P) atoms. These include the detection of pharmaceuticals, pesticides, and environmental pollutants. The detector would appear to function as the FID however, its operation is based on a completely different principle. 

A selective thermionic specific detector (TSD) was developed by Albert (107) but it is more commonly referred to as a nitrogen-phosphorous detector (NPD). It is basically an alkali FID. Figure 13.36 compares the TSD and the microcoulometric detector. More resolution is obtained through the TSD with elimination of the mixing in the transfer line, reactor tube, and titration cell of the... 

Nitrogen-phosphorous detector (NPD, thermionic detector, alkali flame ionization detector) 4 X 10- g to 1 X 10"gof nitrogen compounds 1 X 10" g to 1 X 10" gof phosphorous compounds 1 X 10 10 to 10 by mass selectivity of N or P over carbon Does not respond to inorganic nitrogen such as Nj or NH Jet gas flow rates are critical to optimization Response is temperature dependent Used for trace analysis only, and is very sensitive to contamination Avoid use of phosphate detergents or leak detectors Avoid tobacco use nearby Solvent-quenching is often a problem... 

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