GAS CHROMATOGRAPHY

1. GAS CHROMATOGRAPHY

2. DETECTORS • There are many detectors which can be used in gas chromatography. Different detectors will give different types of selectivity. A non-selective detector responds to all compounds except the carrier gas, a selective detector responds to a range of compounds with a common physical or chemical property and a specific detector responds to a single chemical compound. Detectors can also be grouped into concentration dependant detectors and mass flow dependant detectors.   

3. DETECTORS • The signal from a concentration dependant detector is related to the concentration of solute in the detector, and does not usually destroy the sample Dilution of with make-up gas will lower the detectors response. Mass flow dependant detectors usually destroy the sample, and the signal is related to the rate at which solute molecules enter the detector. The response of a mass flow dependant detector is unaffected by make-up gas. Have a look at this tabular summary of common GC detectors:

4. DETECTORS General characteristic in choosing detectors: • Sensitivity • Measure of the response of the detector per unit of change of sample • Able to detect low concentration doesn’t mean good sensitivity, its about the response towards change • But limit of detection (LOD) commonly been used is 3×S/N ratio • Response and response time • Must show rapid response towards presence of analyte

5. DETECTORS General characteristic in choosing detectors: • Selectivity and specificity • Selectivity: the difference of response of detector towards different compounds or group of compounds • Specific: Discrimination towards analyte based on presence of a specific element or functional group • Linear response range • An ideal detector respose must be linear with the quantity of solute to be determined  graph of response vs [ ] must be a straight line • range of sample amount been measured from the point of LOD to the point where the correlation reaches 5% deviation from the straight line

6. DETECTORS Response 5% deviation Linear response range LOD LOQ Amount of sample

7. Commercially available GC detectors and their applications

8. FLAME IONISATION DETECTORS

9. WORKING OF FLAME IONISATION DETECTORS • The effluent from the column is mixed with hydrogen and air, and ignited. Organic compounds burning in the flame produce ions and electrons which can conduct electricity through the flame. A large electrical potential is applied at the burner tip, and a collector electrode is located above the flame. The current resulting from the pyrolysis of any organic compounds is measured. FIDs are mass sensitive rather than concentration sensitive; this gives the advantage that changes in mobile phase flow rate do not affect the detector's response. The FID is a useful general detector for the analysis of organic compounds; it has high sensitivity, a large linear response range, and low noise. It is also robust and easy to use, but unfortunately, it destroys the sample

10. OUTSTANDING FEATURES OF FID • HIGHLY SENSITIVE TO ORGANIC COMPOUND • LITTILE OR NO RESPONSE TO WATER,CARBONDIOXIDE,CARRIER GAS IMPURITIES & HENCE ZERO SIGNAL WHEN NO SAMPLE IS PRESENT • STABLE BASELINE IS NOT SIGNIFICANTLY AFFECTED BY FLUCTUATION OF CARRIER GAS,FLOW RATE & PRESSURE

11.  Thermal Conductivity Detector (TCD) • Not as sensitive as others, but it is non-specific and non-destructive. • electrically-heated wire or thermistor. • The temperature of the sensing element depends on the thermal conductivity of the gas flowing around it. • Changes in thermal conductivity, such as when organic molecules displace some of the carrier gas, cause a temperature rise in the element which is sensed as a change in resistance.

12. Photoionization Detector (PID) •  selective determination of aromatic hydrocarbons or organo-heteroatom species • Uses ultraviolet light as a means of ionizing an analyte exiting from a GC column. • The ions produced by this process are collected by electrodes. • The current generated is therefore a measure of the analyte concentration.

13. Electron Capture Detector (ECD) •  uses a radioactive Beta emitter (electrons) to ionize some of the carrier gas and produce a current between a biased pair of electrodes. • When organic molecules that contain electronegative functional groups (e.g.: halogens, phosphorous, and nitro groups) pass by the detector, they capture some of the electrons and reduce the current measured between the electrodes. • as sensitive as the FID but has a limited dynamic range and finds its greatest application in analysis of halogenated compounds.

14. Mass Spectrometry Detector • use the difference in mass-to-charge ratio (m/e) of ionized atoms or molecules to separate them from each other. • useful for quantitation of atoms or molecules and also for determining chemical and structural information about molecules.

15. Peak Area Concentration of Standard Qualitative and Quantitative Analysis • Compare retention times between reference sample and unknown • Use multiple columns with different stationary phases • Co-elute the known and unknown and measure changes in peak area • The area of a peak is proportional to the quantity of that compound Peak area increases proportional to concentration of standard if unknown/standard have the identical retention time  same compound