Gas Chromatography detectors


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Gas Chromatography Detectors By S.Satish

GC Detectors: 

GC Detectors The Detectors that are used for GC are devices which continuously monitor some property of the eluate from the chromatographic column 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 .

Detection Systems Characteristics of the Ideal Detector: : 

Detection Systems Characteristics of the Ideal Detector: The ideal detector for gas chromatography has the following characteristics: 1. Adequate sensitivity 2. Good stability and reproducibility. 3. A linear response to solutes that extends over several orders of magnitude. 4. A temperature range from room temperature to at least 400 o C 5.A short response time that is independent of flow rate. 6. High reliability and ease of use. 7. Similarity in response toward all solutes or a highly selective response toward one or more classes of solutes. 8. Nondestructive of sample.

Detectors : 1. Non Destructive 2. Destructive: 

Detectors : 1. Non Destructive 2. Destructive

Thermal Conductivity Detectors(TCD): 

Thermal Conductivity Detectors(TCD) A very early detector for gas chromatography, and one that still finds wide application, is based upon changes in the thermal conductivity of the gas stream brought about by the presence of analyte molecules. The sensing element of TCD is an electrically heated element whose temperature at constant electrical power depends upon the thermal conductivity of the surrounding gas. The heated element may be a fine platinum, gold, or tungsten wire or a semiconducting thermistor (cont)

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The advantage of the thermal conductivity detector is its simplicity, its large linear dynamic range(~10 5 ), its general response to both organic and inorganic species, and its nondestructive character, which permits collection of solutes after detection. A limitation of the katharometer is its relatively low sensitivity (~10 -8 g solute/ mL carrier gas). Other detectors exceed this sensitivity by factors as large as 10 4 to 10 7 .

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The sample components in the carrier gas pass into the measuring channel . A second channel serves as a reference channel where only pure carrier gas flows. Electrically heated resistance wires are located in both channels. The difference in thermal conductivity between the column effluent flow (sample components in carrier gas) and the reference flow of carrier gas alone, produces a voltage signal proportional to this difference. The signal is proportional to the concentration of the sample components.

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For an optimal and proper response of the TCD, there are critical factors: Temperature of the detector block Flow rate of the carrier gas and the reference gas Resistance of the filaments Helium is typically used as the carrier gas for the TCD because of its high thermal conductivity. However, nitrogen, argon or hydrogen are also used as carrier gases with GC-TCD Thermal conductivities of He and H 2 are ~ 6 – 10 times higher than most organic compounds. Necessitates the use of these gases as carrier gas NOTE : Chemically active compounds like acids and halogenated compounds should be avoided when using TCD since they can attack the filament (wires) and thereby change the resistance and permanently reduce the detector sensitivity. Oxidizing substances, such as oxygen, can also damage the filament, and a leak free environment should be maintained.

Flame Ionization Detectors (FID): 

Flame Ionization Detectors (FID) The flame ionization detector is the most widely used and generally applicable detector for gas chromatography. The effluent from the column is mixed with hydrogen and air and then ignited electrically. Most organic compounds, when pyrolyzed at the temperature of a hydrogen/air flame, produce ions and electrons that can conduct electricity through the flame. (cont)

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A potential of a few hundred volts is applied across the burner tip and a collector electrode located above the flame. The resulting current is amplified and proportional to the number of carbon atoms in the flame. The flame ionization detector exhibits a high sensitivity (~10 -13 g/s), large linear response range (~10 7 ), and low noise. A disadvantage of the flame ionization detector is that it is destructive of sample. However, carbonyl, alcohol, halogen and amine groups yield few electrons. Also insensitive to H 2 0, CO 2 , SO 2 , NO The detection of organic compounds is most effectively done with flame ionization. Biochemical compounds such as proteins, nucleotides, and pharmaceuticals can be studied

Electron-Capture Detectors(ECD): 

Electron-Capture Detectors(ECD) The electron-capture detector has become one of the most widely used detectors for environmental samples because this detector selectivity detects halogen containing compounds, such as pesticides and polychlorinated biphenyls. The effluent from the column is passed over a  emitter, usually nickel-63. An electron from the emitter causes ionization of the carrier gas and the production of a burst of electrons. In the absence of organic species, a constant standing current between a pair of electrodes results from this ionization process. The current decreases markedly, however, in the presence of those organic molecules that tend to capture electrons. (cont)

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The electron-capture detector is selective in its response being highly sensitive to molecules containing electronegative functional groups such as halogens, peroxides, quinones , and nitro groups. It is insensitive to functional groups such as amines, alcohols, and hydrocarbons. An important application of the electron-capture detector has been for the detection and determination of chlorinated insecticides. An ECD is 10-1000 times more sensitive than a flame ionization detector (FID), and one million times more sensitive than a thermal conductivity detector (TCD), but has a limited dynamic range and finds its greatest application in analysis of halogenated compounds such as pesticides and CFCs, even at levels of only one part per trillion ( ppt ), thus revolutionizing our understanding of the atmosphere and pollutants

Flame Photometric Detector (FPD): 

Flame Photometric Detector (FPD) FPD uses a H 2 – air flame , as dose the FID . Rather than measure the amount of ionization of the sample components in the flame, the FPD uses a photo multiplier tubes to measure the radiation emitted in the flame by the sample components. Used to measure mainly sulfur or phosphorus and some organometallic compounds which are capable of being exited in H 2 -air flame. The emitting species for sulfur compounds is excited S 2 . The lambda max for emission of excited S 2 is approximately 394 nm. The emitter for phosphorus compounds in the flame is excited HPO (lambda max = doublet 510-526 nm).

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Instrumentation A combustion chamber to house the flame Gas lines for hydrogen (fuel) and air (oxidant) An exhaust chimney to remove combustion products A thermal (band pass) filter to isolate only the visible and UV radiation emitted by the flame. Without this the large amounts of infrared radiation emitted by the flame's combustion reaction would heat up the PMT and increase its background signal. The PMT is also physically insulated from the combustion chamber by using poorly (thermally) conducting metals to attach the PMT housing, filters, etc.

Applications : 

Applications Quantitative Applications Gas chromatography is widely used for the analysis of a diverse array of samples in environmental, clinical, pharmaceutical, biochemical, forensic, food science, and petrochemical laboratories. Examples of these applications are discussed in the following sections. Environmental Analysis Gas chromatography is for the analysis of numerous organic pollutants in air, water ,and wastewater. Clinical Analysis Clinical, pharmaceutical, and forensic labs make frequent use of gas chromatography for the analysis of drugs. Especially used in the determination of anti-epileptic drug valproic acid in blood plasm after solid phase extraction by flame ionization

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Consumer Goods Many flavors, spices, and fragrances are readily analyzed by GC, using headspace analysis or thermal desorption. Foods and beverages are analyzed either directly or following a suitable extraction. Volatile materials, such as those found in spices and fragrances, often can be obtained by headspace sampling. Petroleum Industry Gas chromatography is ideally suited for the analysis of petroleum products, including gasoline, diesel fuel, and oil.

Qualitative Applications: 

Qualitative Applications Kovat’s retention index A means for normalizing retention times by comparing a solute’s retention time with those for normal alkanes . Example : In a separation of a mixture of hydrocarbons, the following adjusted retention times were measured. propane= 2.23 min iso butane= 5.71 min butane= 6.67 min Kovat’s retention index for a normal alkane is 100 times the number of carbons; thus Ipropane = 100 *3 = 300 Ibutane = 100 *4 = 400 To find Kovat’s retention index for isobutane , we use equation

Reference : 

Reference Instrumental methods of analysis – willard Merritt Dean Settle Introduction to instrumental analysis – Robert D. Braun Modern analytical chemistry – David Harvey Pharmaceutical analysis –David G.Watson

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