Knowledge base: Types of detectors used in Gas Chromatography.
Gas chromatography is a term used to describe the group of analytical separation techniques used to analyse volatile substances in the gas phase. In gas chromatography, the components of a sample are dissolved in a solvent and vaporized in order to separate the analytes by distributing the sample between two phases: a stationary phase and a mobile phase. The mobile phase is a chemically inert gas that serves to carry the molecules of the analyte through the heated column. Gas chromatography does not utilize the mobile phase for interacting with the analyte. The stationary phase is either a solid adsorbent, termed gas-solid chromatography (GSC), or a liquid on an inert support, termed gas-liquid chromatography (GLC).
Gas chromatography detectors
A chromatography detector is a device that locates in the dimensions of space and time, the positions of the components of a mixture that has been subjected to a chromatographic process and thus permits the senses to appreciate the nature of the separation.
Classification of Detectors
Detectors can be classified into two types, bulk property detectors and solute property detectors. The bulk property detector measures some bulk physical property of the eluent (such as dielectric constant or refractive index) and the solvent property detector, measures some physical or chemical property that is unique to the solute (such as heat of combustion or fluorescence). Detectors can also be classified as concentration sensitive devices such as the katharometer or mass sensitive devices such as the flame ionization detector (FID). Another method of classification is to define detectors as specific or non-specific. An example of a specific detector would be the nitrogen phosphorous detector (NPD), which as its name implies detects only those substances that contain nitrogen or phosphorous. A non-specific detector would be the katharometer detector which senses all vapours that have specific heats or thermal conductivity different from those of the carrier gas. In general, (though not always), non-specific detectors have lower sensitivities than the specific detectors.
Mass Spectrometry Detectors
Mass Spectrometry (MS) detectors are most powerful of all gas chromatography detectors. In a GC/MS system, the mass spectrometer scans the masses continuously throughout the separation. When the sample exits the chromatography column, it is passed through a transfer line into the inlet of the mass spectrometer. The sample is then ionized and fragmented, typically by an electron-impact ion source. During this process, the sample is bombarded by energetic electrons which ionize the molecule by causing them to lose an electron due to electrostatic repulsion. Further bombardment causes the ions to fragment. The ions are then passed into a mass analyser where the ions are sorted according to their m/z value, or mass-to-charge ratio. Most ions are only singly charged. The chromatogram will point out the retention times and the mass spectrometer will use the peaks to determine what kind of molecules are existing in the mixture. (Chou 200)
typical mass spectrum of water with the absorption peaks at the appropriate m/z ratios.
Ions are allowed into the cavity through a grid in the upper end cap. A variable radio-frequency is applied to the ring electrode and ions with an appropriate m/z value orbit around the cavity. As the radio-frequency is increased linearly, ions of a stable m/z value are ejected by mass-selective ejection in order of mass. Ions that are too heavy or too light are destabilized and their charge is neutralized upon collision with the ring electrode wall. Emitted ions then strike an electron multiplier which converts the detected ions into an electrical signal. This electrical signal is then picked up by the computer through various programs. As an end result, a chromatogram is produced representing the m/z ratio versus the abundance of the sample.
GC/MS units are advantageous because they allow for the immediate determination of the mass of the analyte and can be used to identify the components of incomplete separations. They are rugged, easy to use and can analyze the sample almost as quickly as it is eluted. The disadvantages of mass spectrometry detectors are the tendency for samples to thermally degrade before detection and the end result of obliterating all the sample by fragmentation.
GC/MS system.
Flame Ionization Detectors
In a Flame ionization detectors (FID), the sample is directed at an air-hydrogen flame after exiting the column. At the high temperature of the air-hydrogen flame, the sample undergoes pyrolysis, or chemical decomposition through intense heating. Pyrolized hydrocarbons release ions and electrons that carry current. A high-impedance Pico ammeter measures this current to monitor the sample's elution.
It is advantageous to used FID because the detector is unaffected by flow rate, non-combustible gases and water. These properties allow FID high sensitivity and low noise. The unit is both reliable and relatively easy to use. However, this technique requires flammable gas and also destroys the sample.
Thermal Conductivity Detectors
Thermal conductivity detectors (TCD) works by measuring the change in carrier gas thermal conductivity caused by the presence of the sample, which has a different thermal conductivity from that of the carrier gas. Their design is relatively simple, and consists of an electrically heated source that is maintained at constant power. The temperature of the source depends upon the thermal conductivities of the surrounding gases. The source is usually a thin wire made of platinum or gold. The resistance within the wire depends upon temperature, which is dependent upon the thermal conductivity of the gas.
TCDs usually employ two detectors, one of which is used as the reference for the carrier gas and the other which monitors the thermal conductivity of the carrier gas and sample mixture. Carrier gases such as helium and hydrogen has very high thermal conductivities so the addition of even a small amount of sample is readily detected.
The advantages of TCDs are the ease and simplicity of use, the devices' broad application to inorganic and organic compounds, and the ability of the analyte to be collected after separation and detection. The greatest drawback of the TCD is the low sensitivity of the instrument in relation to other detection methods, in addition to flow rate and concentration dependency.
Chromatogram of a Mixture of Gases
In a standard chromatogram regardless of the type detector, the x-axis is the time and the y-axis is the abundance or the absorbance. From these chromatograms, retention times and the peak heights are determined and used to further investigate the chemical properties or the abundance of the samples.
Electron-capture Detectors
Electron-capture detectors (ECD) are highly selective detectors commonly used for detecting environmental samples as the device selectively detects organic compounds with moieties such as halogens, peroxides, quinones and nitro groups and gives little to no response for all other compounds. Therefore, this method is best suited in applications where traces quantities of chemicals such as pesticides are to be detected and other chromatographic methods are unfeasible.
The simplest form of ECD involves gaseous electrons from a radioactive emitter in an electric field. As the analyte leaves the GC column, it is passed over this emitter, which typically consists of nickle-63 or tritium. The electrons from the emitter ionize the nitrogen carrier gas and cause it to release a burst of electrons. In the absence of organic compounds, a constant standing current is maintained between two electrodes. With the addition of organic compounds with electronegative functional groups, the current decreases significantly as the functional groups capture the electrons. The advantages of ECDs are the high selectivity and sensitivity towards certain organic species with electronegative functional groups. However, the detector has a limited signal range and is potentially dangerous owing to its radioactivity. In addition, the signal-to-noise ratio is limited by radioactive decay and the presence of O2 within the detector. Electron capture detectors (ECD) are typically used in environmental testing for detecting PCB’s, organochlorine pesticides, herbicides and various halogenated hydrocarbons.
Atomic Emission Detectors
Atomic emission detectors (AED) are element-selective detectors that utilize plasma, which is a partially ionized gas, to atomize all of the elements of a sample and excite their characteristic atomic emission spectra. AED is an extremely powerful alternative that has a wider applicability due to its based on the detection of atomic emissions. There are three ways of generating plasma: microwave-induced plasma (MIP), inductively coupled plasma (ICP) or direct current plasma (DCP). MIP is the most commonly employed form and is used with a position-able diode array to simultaneously monitor the atomic emission spectra of several elements.
Instrumentation
The components of the Atomic emission detectors include 1) an interface for the incoming capillary GC column to induce plasma chamber,2) a microwave chamber, 3) a cooling system, 4) a diffraction grating that associated optics, and 5) a position adjustable photodiode array interfaced to a computer.
Atomic emission detector.
GC Chemiluminescence Detectors
Chemiluminescence spectroscopy (CS) is a process in which both qualitative and quantitative properties can be be determined using the optical emission from excited chemical species. It is very similar to AES, but the difference is that it utilizes the light emitted from the energized molecules rather than just excited molecules. Moreover, chemiluminescence can occur in either the solution or gas phase whereas AES is designed for gaseous phases. The light source for chemiluminescence comes from the reactions of the chemicals such that it produces light energy as a product. This light band is used instead of a separate source of light such as a light beam.
CS also has its limitations and the major limitation to the detection limits of CS concerns with the use of a photomultiplier tube (PMT). A PMT requires a dark current in it to detect the light emitted from the analyte.
References
http://chemwiki.ucdavis.edu/Core/Analytical_Chemistry/Instrumental_Analysis/Chromatography/Gas_Chromatography
http://hiq.linde-gas.com/en/analytical_methods/gas_chromatography/thermal_conductivity_detector.html
http://hiq.linde-gas.com/en/analytical_methods/gas_chromatography/electron_capture_detector.html
http://delloyd.50megs.com/moreinfo/detectors.html
J. Chou (2000). Hazardous Gas Monitors, McGraw-Hill and Science Technology Publishing
