Current from flame-ionization detectors

Figure 3.15 A flame ionization detector. Hydrogen and oxygen are introduced into the gas mixture as it emerges from the column to allow it to be burnt in the detector. Some molecules are ionized in the flame and cause a current to flow between the two polarized electrodes. The degree of ionization varies with the composition of the gas mixture and the resulting changes in current can be monitored.

Figure 1.7 GC-MS (total ion current) chromatograms for samples of fluoxetine hydrochloride from two different suppliers (a) India and (b) Costa Rica. Reproduced from [7]. Reproduced by permission from the publisher and authors. (Column 30 m X 0.25 mm i.d. 1 pm DB-1 (J W) carrier gas helium spht ratio 30 1 flame-ionization detector and oven temperature 100°C for 3 min, to 300°C at 10°C/min, maintain at 300°C for 10 min.)...

Many GC detectors exist, but not all are suitable for phytochemicals. The thermal conductivity detector (TCD) is considered a universal detector and is appropriate for most analytes as long as the thermal conductivity of the carrier gas is different from that of the analytes. During the early development phase of GC, TCD was an easy choice because thermal conductivity measuring devices were already in use (Colon and Baird, 2004). Ionization detection arrived with its improved trace determinations and replaced TCD in many applications. While TCD is still used for some food applications (Allegro et ak, 1997 Sun et ak, 2007) and in the past was used for phenolic acids (Blakely, 1966), currently it is not generally used for phytochemicals. Rather, the flame ionization detector (FID) is better-suited due to its selectivity for organic compounds and superior measuring ability for trace measurements. 

The flame ionization detector (FID) is the most widely used and generally applicable detector for gas chromatography. With a detector such as that shown in Figure 31-8, effluent from the column is directed into a small air/hydrogen flame. Most organic compounds produce ions and electrons when pyrolyzed at the temperature of an air/hydrogen flame. Detection involves monitoring the current produced by... 

The results from a chromatographic analysis by means of SIM are usually presented in the form of mass chromatogram , i.e., a plot of the intensity of a particular ion as a function of analysis time (Figure 2). Often this is plotted in conjunction with the total ion current (TIC), the sum of the intensities of all ions monitored over the same time period. The TIC is analogous to a chromatogram obtained from a universal GC detector system, such as a flame ionization detector. Mass chromatograms may also be derived from full scan data. Whilst this technique is commonly employed for both the identification and quantitation of specific compounds, this process does not yield the improvement in sensitivity afforded by SIM. 

The base current ranges from 1 x 10 to 5 X 10 A, the noise level is about 1.2 x 10 A, and the ionization efficiency is about 0.07%. It is claimed to be about 10 times more sensitive than the flame ionization detector and to have a linear dynamic range of 10. The pulsed helium discharge detector appears to be an attractive alternative to the flame ionization detector and would eliminate the need for three different gas supplies. It does, however, require equipment to provide specially purified helium, which diminishes the advantage of using a single gas. 

Detection is based on a change in a physical property of the gas emerging from the column. The signal must be proportional to the amount of material eluted. For routine analyses, a flame ionization detector is used. It consists of a small Hj-air flame burning at a metal jet. The eluant gas mixes with the H2 before the latter emerges from the jet. The combustion of Hj produces free radicals, but no ions however, the combustion of carbon produces positive ions in the flame as well as free radicals. The ions travel to the collector electrode, where they are measured by the current they induce. GC is of enormous value for the separation of small to medium sized molecules and is routinely used in clinical chemistry, e.g. to detect drugs or anesthetics in serum. 

There are many kinds of detectors used for gas chromatography two common ones are thermal conductivity and flame ionization. In a thermal conductivity detector, there are two sets of hot filaments whose temperature depends on the composition of the gas flowing over them. The detector can be set to zero for pure carrier gas passing through the instrument over both filaments. As the components of the mixture elute over one set of filaments, their temperature will change and the current needed to bring them back to the zero point can be measured and plotted. A flame ionization detector measures the current induced by ions created as the effluent is burned. Again, a zero point for pure carrier gas is determined and deviations from zero are detected and plotted as the components of the mixture elute. 

Flame Ionization Detector FID). This detector utilizes the increased current at a collector electrode obtained from the burning of a sample component from the column effluent in a hydrogen and aiijet flame. 

Pyrolysis - gas chromatographic - mass spectrometric equipment, Perkin Elmer filament pyrolysis unit fitted in a Perkin Elmer Fll gas chromatograph (pyrolysis temperature control 250-550°C) or equivalent. The gas chromatographic column used is a 1.7 m X 3 mm o.d. stainless steel column packed with 30% m/m silicone oil (Embaphase) on acid washed Celite, operated at 80 C with a helium flow rate of 30 ml/min . The column effluent is split in the ratio 2 1 between a flame ionization detector and an API MS 12 mass spectrometer equipped with a glass fit type of molecular separator at 150°C. Mass spectra are scanned from m/e 200 to 20 at 8 seconds per decade under standard electron bombardment conditions, electron energy 70 eV, emission current 500 uA, accelerating voltage 8 kV and source temperature 200°C. 

There are many convenient devices for measuring the composition of two-component gas mixtures currently in use in gas chromatography. Foremost among these devices are thermal conductivity cells, flame ionization detectors, gas density balances, and the microcross-section detector. These sensors provide a continuous signal suitable for feeding into a chart recorder. When coupled with an accurate flow meter, such devices enable a reaction to be followed from the gas composition downstream from the pellet. 

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