Flame ionization detector sensor

Some altitude effects on the operation of chromatographic instruments are anticipated. To achieve reproducible retention times for identifying compounds, mobile-phase flows need to be controlled so that they are independent of ambient pressure. Detectors may also respond to changes in pressure. For example, the electron capture detector is a concentration-sensitive sensor and exhibits diminished signal as the pressure decreases. Other detectors, such as the flame ionization detector, respond to the mass of the sample and are insensitive to altitude as long as the mass flow is controlled. 

In this manner, a nearly universal and very nonselective detector is created that is a compromise between widespread response and high selectivity. For example, the photoionization detector (PID) can detect part-per-billion levels of benzene but cannot detect methane. Conversely, the flame ionization detector (FID) can detect part-per-billion levels of methane but does not detect chlorinated compounds like CCl very effectively. By combining the filament and electrochemical sensor, all of these chemicals can be detected but only at part-per-million levels and above. Because most chemical vapors have toxic exposure limits above 1 ppm (a few such as hydrazines have limits below 1 ppm), this sensitivity is adequate for the initial applications. Several cases of electrochemical sensors being used at the sub-part-per-million level have been reported (3, 16). The filament and electrochemical sensor form the basic gas sensor required for detecting a wide variety of chemicals in air, but with little or no selectivity. The next step is to use an array of such sensors in a variety of ways (modes) to obtain the information required to perform the qualitative analysis of an unknown airborne chemical. 

The detectors used in gas chromatography for the detection of individual VOCs can also provide information about a mixture without a separation step. These VOC detectors include, for example, the flame-ionization detector (FID) and the photo-ionization detector (PID). Other direct-reading instruments have also been used for determining VOCs, such as photoacoustic spectroscopy (PAS) (see Chapter 1.6). Other types of sensors (e.g. electronic noses ) may become important in the future. 

It was found that these sensors can detect many gases, such as air. He, Ar, and gas mixtures, without gas separation (Modi et al. 2003). We need to note that ionization gas sensors such as photo-ionization detectors (PID), flame-ionization detectors (FID), or electron-capture detectors (ECD) are not suitable for direct application to gas mixtures. These detectors work in conjunction with a gas chromatography setup that separates the mixture into distinct bands that can then be qnalitatively and quantitatively analyzed (Modi et al. 2003). In addition, FID has poor selectivity and requires bulky and hazardous... 

By adapting the Mass Spectrometer (MS) colunm, we obtain the fragmentation spectrum of each of the eluted compounds. From the Total Ion Chromatogram (TIC), we can trace the representative chromatogram of the eluted compounds. Although this method leads to less sensitivity than conventional sensors (e.g. Flame Ionization Detector (FID)), it has become indispensable in a large number of studies. 

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. 

The flame ionization detector works by measuring the current in a flame (into which the gas to be analyzed is passed) across which a potential difference of 200-400 V is applied by means of platinum electrodes. This sensor is not temperature sensitive and is capable of measuring very low concentrations. However, its primary usefulness is in detecting organic carbon atoms, and it is not suitable for hydrogen/water vapor, carbon monoxide/carbon dioxide, or oxygen/sulfur dioxide mixtures. Calibration is required for each gas mixture. 

Hand-held monitoring techniques for VOC and gases flame ionization detection (FID), photoionization detector (PID). thermal conductivity detector (TCD). infrared sensor (IR). see Figure 3. 

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