GC/FID

The flame ionization detector (FID) is a non-selective detector used in conjunction with gas chromatography. Because it is non-selective, there is a potential for many non-target compounds present in samples to interfere with this analysis and for poor resolution especially in complex samples. The FID works by directing the gas phase output from the column into a hydrogen flame. A voltage of 100-200V is applied between the flame and an electrode located away from the flame. The increased current due to electrons emitted by burning carbon particles is then measured. Although the signal current is very small (the ionization efficiency is only 0.0015%) the noise level is also very small (<10-13 amp) and with a well-optimized system, sensitivities of 5 x 10-12 g/ml for n-heptane at a signal/noise ratio of 2 can be easily realized. Except for a very few organic compounds (e.g. carbon monoxide, etc.) the FID detects all carbon containing compounds. The detector also has an extremely wide linear dynamic range that extends over, at least five orders of magnitude with a response index between 0.98-1.02.

Contents

  • A Diagnostic Tool for Smith-Lemli-Opitz Syndrome
    A Diagnostic Tool for Smith-Lemli-Opitz Syndrome, Alltech Application Note #AN104, July 6, 2004. Smith-Lemli-Opitz syndrome is a hereditary biochemical disorder in which a gene responsible for converting 7-dehydrocholesterol to cholesterol is defective. The resulting lack of cholesterol causes severe birth defects. An elevated level of 7-dehydrocholesterol in plasma is a biochemical marker for this syndrome. Thus, a rapid method for determining both compounds in human plasma would be a useful diagnostic tool. Both cholesterol and 7-dehydrocholesterol (33g/mL in chloroform extracted plasma) were separated without derivatization on an Alltech Heliflex AT-1ms, 15m x 0.25mm x 0.25m capillary column (Part No. 15880) isothermally at 305C using helium carrier gas and a split inlet flow. Detection was by FID at a temperature of 325C. The separation of both compounds took about 4 minutes.
  • Decreasing GC Analysis Time with Small Diameter Columns
    Decreasing GC Analysis Time with Small Diameter Columns, Alltech Application Note CA001, 1998. Shorter capillary columns can result in 50% faster GC analysis. Combined with smaller diameters of 100m or less, the peak resolution and efficiency and hence, the overall resolution can be maintained. To demonstrate the reduction in time and acceptable resolution achievable using shorter (10m) and smaller diameter capillary columns (0.05-0.10m), PAHs, phenols and lemon oil components were chromatographed on these type columns and on standard 30m, 0.25m capillary columns A comparison of a standard capillary column (AT-5 Capillary, 30m x 0.25mm x 0.25m, Part No. 13656) with a high efficiency capillary column (AT-5 Hi-Eff Capillary, 10m x 0.05mm x 0.10m, Part No. 14428) operated under similar conditions with FID detection at 340C showed that 16 PAHs: (naphthalene, acenaphthalene, acenapthene, fluorene, phenanthrene, anthracene, fluoranthrene, pyrene, benzo(a)anthracene, chrysene, benzo(b)fluoranthrene, benzo(k)fluoranthrene, benzo(a)pyrene, indeno(1,2,3,-cd)pyrene, dibenzo(a,h)anthracene and benzo(ghi)perylene, eluted in less than half the time on the high efficiency column, around 11 minutes. A similar comparison of 11 phenols (phenol, 2-chlorophenol, 2-nitrophenol, 2,4-dimethylphenol, 2,4-dichlorphenol, 4-chloro-3-methylphenol, 2,4,6-trichlorophenol, 2,4-dinitrophenol, 4-nitrophenol, 2-methyl-4,6-dinitrophenol, and pentachlorophenol) with FID detection at 325C also showed elution in half the time (11 min. vs. 22). A lemon oil fingerprint showed a total run time of eight minutes on the high efficiency column with FID detection at 325C vs. sixteen by the standard column for 27 compounds: a-thulene, a-pinene, camphene, sabinene, b-pinene, 6-methyl-5-hepten-2-one, myrcene, octanal, a-phellandrene, 3-carene, a-terpinene, p-cymene, limonene, g-terpinene, terpinolene, linalool, nonanal, citronellal, terpinen-4-ol, a-terpineol, neral, geranial, neryl acetate, geranyl acetate, b-caryophyllene, trans-a-bergamolene and b-bisabolene.
  • Polymer Degradation Mechanisms by Pyrolysis GC
    Investigating Thermal Degradation Mechanisms of Polymers by Pyrolysis GC, Alltech Application Note PY001, September 21, 1998. Polymers thermally degrade under pyrolysis conditions (inert atmosphere, high temperature) by one of three mechanisms depending on the type of polymer. In depolymerization, the polymer is degraded to one or more monomers. An example of this is polystyrene which, degrades to produce styrene monomer. In random scission, small fragments of varying chain lengths are produced. Polyethylene degrades by this mechanism. Polyvinyl chloride degrades by a third mechanism, side group elimination occurs, resulting in a polyene macromolecule that ultimately forms aromatic compounds like benzene and naphthalene. To study these degradation reactions, a pyrolysis/GC analysis was performed using an Alltech pyrola 2000 Pyrolyzer consisting of a Pyrolysing Inlet for GC (Part No. 56100) and the pyrola 2000 with Internal Valve (Part No. 56100). Samples were heated to 700C isothermally and pyrolyzed for 2 sec and chromatographed onto an AT-1 Capillary column (30m x 0.25mm x 0.25m, Part No. 13638) with FID detection. The column temperature was held at 50C for 2 min and raised to 300C at 10C/min with the detector at 325C. The pyrolysis of polystyrene produced peaks for benzene, toluene, styrene momonre, arylbenzene, a-methylstyrene, styrene dimmer and styrene trimer. The pyrolysis of polyethylene produced a series of triplet peaks attributable to saturated molecules, molecules containing a double bond on one end and molecules containing two double bonds. Pyrolysis of vinyl chloride produced peaks for benzene, toluene, styrene, 1-propynylbenzene, cyclopropyl(a)indene, cyclopentacycloheptene and biphenyl.
  • Separation of Para- and Meta-Xylene on Capillary GC Column
    BTEX Aromatics and the Separation of Para- and Meta-Xylene on a Carbowax Econo-Cap Column, Alltech Application Note ANE006, 1997. The GC capillary column separation of para- and meta-xylene on Carbowax, a polyethylene glycol stationary phase was demonstrated. The column, Carbowax Econco-Cap (Part No. 19659), 30m x 0.53mm x 1.2m was installed on a HP 5890 gas chromatograph equipped with a FID. A mixture of eleven aromatic hydrocarbons was directly injected: benzene, toluene, ethyl benzene, p-xylene, m-xylene, o-xylene, propyl benzene, chlorobenzene, m-dichlorobenzene, p-dichlorobenzene and o-dichlorobenzene. The initial temperature of 55C was held for 7.5 minutes until the m-xylene eluted. The GC oven was then programmed to increase to 150C at 10C/min. Ethyl benzene and the para- and meta-xylene isomers were not completely separated, but the resolution was said to be better than with the column typically used for this determination, an EPA Method 624 capillary column.