Helium ionization detectors

The helium ionization detector (HID) is probably the least used of the ionization detectors. It is a universal and ultra sensitive detector with a reputation for unreliability and... 

The helium ionization detector (HID) is a sensitive universal detector. In the detector, Ti3H2 or Sc3H3 is used as an ionization source of helium. Helium is ionized to the metastable state and possesses an ionization potential of 19.8 eV. As metastable helium has a higher ionization potential than most species except for neon, it will be able to transfer its excitation energy to all other atoms. As other species enter the ionization field the metastable helium will transfer its excitation energy to other species of lower ionization potential, and an increase in ionization will be measured over the standing current. 

F. Andrawes and S. Greenhouse, Applications of the helium ionization detector in trace analysis, J. Chromatogr. Sci., 26 153-159 (1988). 

Detectors Flame-ionization detectors are used for most analyses, with lesser use made of thermal conductivity, electron-capture, nitrogen-phosphoms, and mass spectrometric detectors. For quantitative analyses, detectors must have a wide linear dynamic range the response must be directly proportional to the amount of compound present in the detector over a wide range of concentrations. Flame-ionization detectors have a wide linear range ( 106) and are sensitive to organic compounds. Unless otherwise specified in individual monographs, flame-ionization detectors with either helium or nitrogen carrier gas are to be used for packed columns, and helium is used for capillary columns. 

The pulsed discharge electron capture detector is an extension of the previously discussed pulsed discharge helium ionization detector, a... 

The sensor consists of two sections the upper section where the discharge takes place has a small diameter and the lower section where the column eluent is sensed and the electron capturing occurs has a wider diameter. As with the pulsed discharge helium ionization detector, the potential across the discharge electrodes is pulsed at about 3 kHz with a discharge pulse width of about 45 psec for... 

Detectors commonly used in GC and specified in the USPP include FID, alkali FID (NPD, TD), BCD, and TCD. A description of these detectors, including their operational principles and relative performance, was presented in a previous volume of this encyclopedia. Various other useful detectors for GC include photoionization (PID), flame photometric (FPD), electrolytic conductivity (BLCD), redox (RCD) and sulfur chemiluminescence (SCD), and helium ionization (HID).[4 1 Table 1 summarizes some of the features of detectors used in GC. 

The type of detector to be employed determines the nature of the carrier gas which may be used. Argon is used with the argon ionization detector. Helium is used with flame-ionization, thermal conductivity, thermionic emission, and cross-section detectors. Hydrogen may be used in thermal conductivity detectors to give maximum sensitivity. Probably the commonest and cheapest carrier gas is nitrogen, which can be used with flame-ionization, electron capture, thermal conductivity, and cross-section detectors. Argon-methane mixtures may be used with electron capture detectors. 

All ionization detectors have the same base body. They are all miniaturized. They are not universal with the exception of the helium ionization detector. 

Abbreviations GC, Gas Chromatography IRMS, Isotope Ratio Mass Spectrometry ECD, Electron Capture Detector FID, Flame Ionization Detector TCD, Thermal Conductivity Detector MS, Mass Spectrometry PDHID, Pulsed-Discharge Helium Ionization Detector mol. sieve, Molecular Sieve HP, Hewlett-Packard PE, Perkin-Elmer. 

At the temperatures and pressures generally used in gas chromatography the common carrier gases employed behave as perfect insulators. In the absence of conduction by the gas molecules themselves, the increased conductivity due to the presence of very few charged species is easily measured, providing the low sample detection limits characteristic of ionization based detectors [259]. Examples of ionization detectors in current use include the flame ionization detector (FID), thermionic ionization detector (TID), photoionization detector (PID), the electron-capture detector (ECD), and the helium ionization detector (HID). Each detector employs a different method of ion production, but in all cases the quantitative basis of detector operation corresponds to the fluctuations of an ion current in the presence of organic vapors. 

Figure 12.18 Separation and detection of fixed gases and low MW hydrocarbons on high surface area molecular sieve using a helium ionization detector (HID). (Adapted with permission of Restek Inc.)...

Low resolution mass spectrometry was performed by the Penn State Chemistry Department on a Kratos MS 9/50. Capillary gas chromatography was performed on a Hewlett-Packard 5880A Series GC. The capillary column used was a 12 m crosslinked methylsilicone fast analysis column utilizing a flame ionization detector. Helium was used as the carrier gas at a flow rate of 2.2 mL/min under various temperature programming conditions. 

For infinite dilution operation the carrier gas flows directly to the column which is inserted into a thermostated oil bath (to get a more precise temperature control than in a conventional GLC oven). The output of the column is measured with a flame ionization detector or alternately with a thermal conductivity detector. Helium is used today as carrier gas (nitrogen in earlier work). From the difference between the retention time of the injected solvent sample and the retention time of a non-interacting gas (marker gas), the thermodynamic equilibrium behavior can be obtained (equations see below). Most experiments were made up to now with packed columns, but capillary columns were used, too. The experimental conditions must be chosen so that real thermodynamic data can be obtained, i.e., equilibrium bulk absorption conditions. Errors caused by unsuitable gas flow rates, unsuitable polymer loading percentages on the solid support material and support surface effects as well as any interactions between the injected sample and the solid support in packed columns, unsuitable sample size of the injected probes, carrier gas effects, and imprecise knowledge of the real amount of polymer in the column, can be sources of problems, whether data are nominally measured under real thermodynamic equilibrium conditions or not, and have to be eliminated. The sizeable pressure drop through the column must be measured and accounted for. 

In GC, the mobile phase or carrier phase is an inert gas such as helium and the stationary phase is a very thin layer of liquid or polymer on an inert solid support inside a column. The volatile analytes interact with the walls of the column, and are eluted based on the temperature of the column at specific retention times (Grob Barry, 2004). The eluted compoimds are identified with detectors. Flame ionization and mass spectrometry are the most commonly used detectors for flavour analysis (Vas Vekey, 2004). 

Figure 2 Gas chromatographic separation of hydrocarbons found in an urban air sample. Open capillary, 0.32 mm i.d. x 60 m length stationary phase, DB-1 (dimethyl polysiloxane) film thickness, 0.25 pm carrier gas, helium temperature programme, 5°C isothermal for 3 min, 5-50°C at a rate of 3°C min 50-220°C at a rate of 5°C min detector, flame ionization. With this method, a total of 142 hydrocarbons could be separated and identified 128 of them were found in the urban air sample. (After Ciccioli P, Cecinato A, Brancaleoni E, Frattoni M, and Liberti A (1992) Use of carbon adsorption traps combined with high resolution GC-MS for the analysis of polar and nonpolar C4-C14 hydrocarbons involved in photochemical smog formation. Journal of High Resolution Chromatography 15 75.)...

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