A helium ionization detector (HID) is a type of detector used in
gas chromatography
Gas chromatography (GC) is a common type of chromatography used in analytical chemistry for separating and analyzing compounds that can be vaporized without decomposition. Typical uses of GC include testing the purity of a particular substance, ...
.
Principle
HID connected to a GC has the great advantage to use helium as both the carrier gas and the ionization gas. HID is an
ion detector which uses a radioactive source, typically β-emitters, to create metastable helium species. The radioactive source ionizes
helium
Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
atoms by bombarding them with emissions. The metastable helium species have an energy of up to 19.8 eV. These metastable helium species can ionize all compounds with the exception of
neon
Neon is a chemical element with the symbol Ne and atomic number 10. It is a noble gas. Neon is a colorless, odorless, inert monatomic gas under standard conditions, with about two-thirds the density of air. It was discovered (along with krypt ...
which has a bigger ionization potential of 21.56 eV. As components elute from the
GC's column they collide with the metastable helium ions, which then ionize the components. The
ion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...
s produce an
electric current
An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The movin ...
, which is the
signal
In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The '' IEEE Transactions on Signal Processing' ...
output of the detector. The greater the concentration of the component, the more ions are produced, and the greater the current.
Application
HIDs are sensitive to a broad range of components. They must use helium as a carrier gas.
HID is classified as a mass sensitive detector, which means that its signal is proportional to the mass of analyte entering the detector per unit time. The analytes are destroyed during reaction, therefore, it is considered a destructive detector.
The drawback to HIDs are that they contain a radioactive source. In the United States, this means they fall under a number of federal regulations concerning their use in the workplace, shipping, disposal, etc.
Discharge ionization detectors have generally supplanted them.
{{DEFAULTSORT:Helium Ionization Detector
Gas chromatography
Helium