Gas Chromatography Detectors
Electron capture detector & Flame Photometric detector Facilitator: Dr.B.M.Gurupadayya, Professor, Head of the Department, Pharmaceutical Analysis, J.S.S.C.P, Mysore.
Submitted by: M.Indupriya, 1st M.Pharm, PharmaceuticalA nalysis, J.S.S.C.P,
Mysore.
Electron capture detector:
An electron capture detector (ECD)is a device for detecting atoms and molecules in a gas
through the attachment of electrons via electron capture ionization. The device was invented in 1957 by Dr.James E.Lovelock and is used in gas chromatography to detect trace amounts of chemical compounds in a sample.
Used for the detection of halogen containing compounds like pesticides.
Principle:
In ECD specifically a -emitter serves as a source of radiation to generate the ions that help in ionizing the carrier gas molecules to form positive ions and free electrons as expressed in the following equation: C + radiation c+ + e-
Instrument:
The basic ECD consists of a small chamber one or 2ml in volume enclosing two metal
electrodes. The electrodes may be concentric cylinders or metal discs separated by an insulator.
The cell contains the radioactive source, electrically connected to the entrance conduit and to the negative side of the powder supply.
Operating procedure:
One of the electrons is treated with a radio isotope that emits high energy as it decays.
These emitted electrons produce copious amounts of low energy (thermal) secondary electrons in the GC carrier gas, all of which are collected by the other positively polarized electrode.
Molecules that have an affinity for thermal electrons capture electrons as they pass between the electrodes. Hence the amount of steady state current is reduced that forms the base line signal of the ECD detector, thus providing an electrical reproduction of the GC peak.
This output is amplified and recorded as output signal. The output from the sensor is processed by suitable electronics and the output passed to either a potentiometry recorder of a computer data acquisition system.
The carrier gas used in this type of detector depends upon the electron affinity of the compounds analysed.
For compounds with high electron affinity, argon is used as carrier gas. For compounds of low electron affinity nitrogrn, hydrogen, helium can be used as carrier gas.
Advantages:
Highly sensitive. Even nanogram quantities can be detected.
Halogenated compounds, several pesticides etc can be detected using this type of detector.
Disadvantages:
It can be used only for compounds with electron affinity.
Flame photometric detector:
Flame photometric detector is essentially a flame emission photometer. It is similar to the flame ionization detector. FID measures ions produced by organic compounds during combustion.
Whereas FPD analyzes the spectrum of light emitted by the compounds as the luminesce in the flame.
Theory of operation:
The FPD uses one of the two available band filters over a photomultiplier tube (PMT)to selectively detect compounds containing sulphur or phosphorous as they combust in the hydrogen flame.
When compounds are burned in the FPD flame they emit photons of distinct wavelength. Only those photons that are within the frequency range of the filter specifications can pass through the filter to the photo multiplier tube.
The photo multiplier converts the photons it sees through the band pass filter to an analog signal, which is acquired by the peak simple data system.
The most highly developed FPDs uses one of two available band pass filter to selectively detect compounds containing sulphur (or) phosphorous.
Compounds containing phosphorous are detectable with the 526nm filter, which is yellow on one side.
Compounds containing sulphur are detectable with the 394nm filter which is blue on one side.
Detector response to phosphorous compounds is linear, where as the response to compounds containing a single atom of sulphur is proportional to the square of the compound concentration.
To detect phosphorous and sulphur at the same time, the dual FPD featuring two PMTs may be used.
Quenching of the sulphur emission by other organic compounds present in the flame &extinguishing of the flame by solvent peaks are overcome with a dual-flame FPD.
The lower flame causes partial combustion &decomposition of the solute molecules into relatively simple species.
The upper flame then burns these and produces the emission which is detected with the photo multiplier tube.
Although both flames produce the unique green (or) violet molecular emission, only the upper flame is viewed since this is less affected optically by the incoming sample components.
Solvent peaks may extinguish the lower flame, but the upper one remains lit. After passage of the solvent, the lower flame is reignited by flash back from the upper one. The FPD response to sulphur and phosphorous is about 104 times that elicited from hydrocarbons.
Sulphur containing species may act as serious interferents in the detection of phosphorous compounds .It is necessary therefore to select a column which not only separates the sulphur compounds from other, but also separates them from other sample components.
Applications:
Determination of pesticides and pesticide residue containing sulphur and phosphorous. Used to detect gaseous sulphur compounds such as mercaptans or thiophenes in gases & H2S and SO2 and mercaptans in air pollution.
Detection of a variety of volatile metal salts & chelates, lead, alkyls and silylated compounds.
