GAS CHROMATOGRAPHY DETECTORS

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Gas Chromatography Detectors :

Gas Chromatography Detectors Prepared by:Rajgor Naresh B. Assistant Professor, M.P. Patel College of Pharmacy, Kapadwanj, Gujarat

DETECTORS :

DETECTORS Physical properties of detectors: Thermal conductivity velocity of sound Gas density measurement Magnetic susceptibility Ionisation of α and ß rays from a radio active source. Combustion properties of gases.

Characteristic Of Ideal Detector::

Characteristic Of Ideal Detector: Adequate sensitivity: Range of 10 -8 to-15 g solute/s. Good stability and reproducibility. A temperature range from room temperature to at least 400 ˙c. High reliability and easy of use. A short response time that is independent of flow rate.

PowerPoint Presentation:

Non destructive of sample. High predictable & selective response toward one or more classes of solute.

Thermal Conductivity Detector (T.C.D) or Kathorometer: :

Thermal Conductivity Detector (T.C.D) or Kathorometer: Principle: The rate of heat loss from a heated wire placed in a gas stream depend on thermal conductivity of the gas thus if the composition of gas stream changes,the rate of heat loss from the wire will change and so both the temperature &electrical resistance of the wire will be different.

Detector component& working::

Detector component& working: Filaments are made of platinum ,rhenium,tungsten having large temp. co efficient and corrosion resistant. Thermistor are made of Oxides of manganese ,cobalt or nickel to which some elements are added. The draw back of thermistor is that it should not be used above 100 c. Thermistor Vs Filament:

Factor affecting detector performance::

Factor affecting detector performance: A)Nature and flow rate of carrier gas: 1) The thermal conductivity of a selected carrier gas which differ significantly from that of the analyte. 2) Introduction of small amount of air or oxygen into the cell will destroyed the filaments. 3) At higher flow rate gases spend less time in contact with filament. 4) Flow rate fluctuations can cause detector noise.

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B)Effect of filament current and detector temperature: 1) Sensitivity is proportional to the third power of the filament current. 2) Temp. up to 600 to 700 ˙c is the upper limit for filament . 3) The filament current should be adjusted to satisfy three factor; a . Sensitivity b. Filament temperature. c. Noise. 4)detector temperature should be maintain at or above that of the column to avoid condensation of analyte.

Sensitivity and Linear Dynamic range::

Sensitivity and Linear Dynamic range: Sensitivity is in Sensitivity of TCD can be calculated as follows; S = A x C 1 x C 2 x C 3 / W S = Sensitivity in mV/mg A = Peak area cm2 C 1 = recorder sensitivity,mV/cm of chart C 2 = chart speed,min /cm C 3 = flow rate column exit corrected to column temp. & atmospheric pressure,ml/min W = Weight of sample injected.mg

Precaution ::

Precaution : Never turn on the heating current unless the carrier gas is flowing. If there is no movement of the gas through the cell ,the wires will get very hot & melt or burn out.

Advantages::

Advantages: It is very simple to use. It has large linear dynamic range. It responds to both organic and inorganic compound. It is non-selective & non-destructive . It is stable at higher temperature & has low cost.

Disadvantages::

Disadvantages: Relatively low sensitivity . Fluctuation in carrier gas cause detector noise.

Application ::

Application : It is used to measurement of gas mixtures. It has unique capability for the detection of water. It has applied to detect other solvent residues in samples which are presumed to be contaminated only with water. Determination of CO 2 in flue gases.

Electron Capture Detector::

Electron Capture Detector: Construction : 1) Radio active material metal foil. 2) Anode and cathode electrode. 3)Potential difference of 20V to100V.

Principle &working::

Principle &working: The electron capture detector operates on the basis of differential electron absorption by molecules or functional groups within the molecules. One of the electrodes has on its surface a radioisotope that emits high energy electrons.(beta particles)

PowerPoint Presentation:

These electrons bambard the carrier gas ,resulting in the formation of a plasma of positive ions,radicals &thermal electrons. N 2 ---------  N 2 + + e ¯ Electron absorbing compounds in the carrier gas stream react with the thermal electrons produce negative ions of larger mass. H + e ¯ ---------  H ¯ + Energy

PowerPoint Presentation:

The rate of recombination between negative & positive ion is very faster than between thermal electrons & positive ion . N 2 + + H ¯ ---------  N 2 + H + Energy Thus the decrease in the number of electron & ion will lead to fall in current & thus is related to component entering the detector.

Factor affecting detector performance::

Factor affecting detector performance: Pulsed Vs non –pulsed operation: Effect of carrier gas . Effect of detector temperature. Effect of molecular structure on response.

Advantages::

Advantages: Highly sensitive for halogenated compounds,nitrogen compounds. Not altering sample significance. Detected compounds at 50 FG –1 PG. Level.

Disadvantages::

Disadvantages: More contaminated detector than the other detectors It has high cost. Its linearity is poor.

Application ::

Application : It is applied in non-pharmaceutical disciplines has been its use as a halogen &phosphorus detector in quantitative & ultra micro quantities determination of insecticides in environment & biological system. Determination of organometallic compounds. The major pharmaceutical applications of the ECD are in selective determination of sub micro drug levels in biological specimens such as blood ,urine & tissue. Determination of Gresiofulvin & Hydrochlorthiazide in human plasma.