Pulsed discharge ionization detector
Encyclopedia
PDDs utilize a stable, low powered, pulsed DC discharge in helium
as an ionization
source. Eluants from the column, flowing counter to the flow of helium from the discharge zone, are ionized by photons from the helium discharge. Bias electrode(s) focus the resulting electron
s toward the collector electrode, where they cause changes in the standing current which are quantified as the detector output.
mode, the PDD is a selective detector for monitoring high electron affinity compounds such as freons, chlorinated pesticide
s, and other halogen
compounds. For this type of compound, the minimum detectable quantity (MDQ) is at the femtogram (10-15) or picogram (10-12) level. The PDD is similar in sensitivity and response characteristics to a conventional radioactive ECD, and can be operated at temperatures up to 400°C. For operation in this mode, He and CH4 are introduced just upstream from the column exit.
s in petrochemical or refinery environments, where the flame and use of hydrogen
can be problematic. In addition, when the helium discharge gas is doped with a suitable noble gas
, such as argon
, krypton
, or xenon
(depending on the desired cutoff point), the PDD can function as a specific photoionization detector for selective determination of aliphatics
, aromatics, amine
s, as well as other species.
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
as an ionization
Ionization
Ionization is the process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This is often confused with dissociation. A substance may dissociate without necessarily producing ions. As an example, the molecules of table sugar...
source. Eluants from the column, flowing counter to the flow of helium from the discharge zone, are ionized by photons from the helium discharge. Bias electrode(s) focus the resulting electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s toward the collector electrode, where they cause changes in the standing current which are quantified as the detector output.
Electron Capture Mode
In the electron captureElectron capture
Electron capture is a process in which a proton-rich nuclide absorbs an inner atomic electron and simultaneously emits a neutrino...
mode, the PDD is a selective detector for monitoring high electron affinity compounds such as freons, chlorinated pesticide
Pesticide
Pesticides are substances or mixture of substances intended for preventing, destroying, repelling or mitigating any pest.A pesticide may be a chemical unicycle, biological agent , antimicrobial, disinfectant or device used against any pest...
s, and other halogen
Halogen
The halogens or halogen elements are a series of nonmetal elements from Group 17 IUPAC Style of the periodic table, comprising fluorine , chlorine , bromine , iodine , and astatine...
compounds. For this type of compound, the minimum detectable quantity (MDQ) is at the femtogram (10-15) or picogram (10-12) level. The PDD is similar in sensitivity and response characteristics to a conventional radioactive ECD, and can be operated at temperatures up to 400°C. For operation in this mode, He and CH4 are introduced just upstream from the column exit.
Helium Photoionization Mode
In the helium photoionization mode, the PDD is a universal, non-destructive, high sensitivity detector. The response to both inorganic and organic compounds is linear over a wide range. Response to fixed gases is positive (increase in standing current), with an MDQ in the low ppb range. The PDD in helium photoionization mode is an excellent replacement for flame ionization detectorFlame 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....
s in petrochemical or refinery environments, where the flame and use 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...
can be problematic. In addition, when the helium discharge gas is doped with a suitable noble gas
Noble gas
The noble gases are a group of chemical elements with very similar properties: under standard conditions, they are all odorless, colorless, monatomic gases, with very low chemical reactivity...
, such as argon
Argon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...
, krypton
Krypton
Krypton is a chemical element with the symbol Kr and atomic number 36. It is a member of Group 18 and Period 4 elements. A colorless, odorless, tasteless noble gas, krypton occurs in trace amounts in the atmosphere, is isolated by fractionally distilling liquified air, and is often used with other...
, or xenon
Xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. The element name is pronounced or . A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...
(depending on the desired cutoff point), the PDD can function as a specific photoionization detector for selective determination of aliphatics
Aliphatic compound
In organic chemistry, aliphatic compounds are acyclic or cyclic, non-aromatic carbon compounds.Thus, aliphatic compounds are opposite to aromatic compounds.- Structure :...
, aromatics, amine
Amine
Amines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...
s, as well as other species.