Nitrogen Phosphorus Detector
Encyclopedia
The nitrogen phosphorus detector (NPD) is a form of gas chromatography in which thermal energy
is used to ionize an analyte
. With this method
, nitrogen
and phosphorus
can be selectively detected with a sensitivity
that is 104 times greater than that for carbon
.
NP-Mode: A minimum concentration of hydrogen
gas is used such that it is just below the minimum required for ignition. A rubidium
or caesium
bead, which is mounted over the nozzle
, ignites the hydrogen (by acting catalytically), and entails the formation of a cold plasma. Excitation
of the alkali metal results in ejection of electrons, which in turn is detected by current flow between an anode
and cathode
in the chamber. In the presence of nitrogen or phosphorus combustion products exiting the column, the work function
of the metal is reduced, resulting in an increase in the current measured.
Since the alkali metal bead is over time consumed, it must be regularly replaced - a disadvantage compared to the related FID detector
which lacks the selectivity for nitrogen and phosphorus.
Thermal energy
Thermal energy is the part of the total internal energy of a thermodynamic system or sample of matter that results in the system's temperature....
is used to ionize an analyte
Analyte
An analyte, or component , is a substance or chemical constituent that is of interest in an analytical procedure. Grammatically, it is important to note that experiments always seek to measure properties of analytes—and that analytes themselves can never be measured. For instance, one cannot...
. With this method
Methodology
Methodology is generally a guideline for solving a problem, with specificcomponents such as phases, tasks, methods, techniques and tools . It can be defined also as follows:...
, nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
and phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
can be selectively detected with a sensitivity
Sensitivity
Stimulus|Sensitivity may refer to:* Sensitivity , the ability to react to a stimulus* Sensitivity , the strength of physical or emotional reaction in people* Sensitivity , variations in process dynamics and control systems...
that is 104 times greater than that for carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
.
NP-Mode: A minimum concentration of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
gas is used such that it is just below the minimum required for ignition. A rubidium
Rubidium
Rubidium is a chemical element with the symbol Rb and atomic number 37. Rubidium is a soft, silvery-white metallic element of the alkali metal group. Its atomic mass is 85.4678. Elemental rubidium is highly reactive, with properties similar to those of other elements in group 1, such as very rapid...
or caesium
Caesium
Caesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C , which makes it one of only five elemental metals that are liquid at room temperature...
bead, which is mounted over the nozzle
Nozzle
A nozzle is a device designed to control the direction or characteristics of a fluid flow as it exits an enclosed chamber or pipe via an orifice....
, ignites the hydrogen (by acting catalytically), and entails the formation of a cold plasma. Excitation
Excited state
Excitation is an elevation in energy level above an arbitrary baseline energy state. In physics there is a specific technical definition for energy level which is often associated with an atom being excited to an excited state....
of the alkali metal results in ejection of electrons, which in turn is detected by current flow between an anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
and cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
in the chamber. In the presence of nitrogen or phosphorus combustion products exiting the column, the work function
Work function
In solid-state physics, the work function is the minimum energy needed to remove an electron from a solid to a point immediately outside the solid surface...
of the metal is reduced, resulting in an increase in the current measured.
Since the alkali metal bead is over time consumed, it must be regularly replaced - a disadvantage compared to the related FID detector
Flame ionization detector
A flame ionization detector is a type of gas detector used in gas chromatography. The first flame ionization detector was developed in 1957 by scientists working for the CSIRO in Melbourne, Australia....
which lacks the selectivity for nitrogen and phosphorus.