Flame ionization detector design

Other Detectors Two additional detectors are similar in design to a flame ionization detector. In the flame photometric detector optical emission from phosphorus and sulfur provides a detector selective for compounds containing these elements. The thermionic detector responds to compounds containing nitrogen or phosphorus. 

Here is a challenge McMinn and co-workers investigated the effect of five factors for optimizing an H2-atmosphere flame ionization detector using a 2 factorial design. The factors and their levels were... 

The advantages of the flame emission detector (FED) have been combined with the flame ionization detector. This design features the ability to detect CO, CO2, N2O4, SO2, N2F4, HF and H2S gases which respond poorly in an FID. In addition, the system showed qualitative differences in structure attributable to different FED/FID ratios as a function of wavelength for various compounds. 

Dynamic headspace GC utilizes a constant purge of the sample with an ultra-high purity gas (i.e., helium). The purged volatiles are then adsorbed onto a trap, followed by heat desorption onto the GC for analysis. Either a flame ionization detector or mass selective detector can be used. The protocol presented here is designed to analyze a meat sample. 

Flame ionization detectors (FIDs) and photoionization detectors (PIDs) can be used for the detection of hydrocarbons. Both detectors have been utilized for combustibles monitoring in portable and fixed installation designs. The FID actually burns the sample in an H2 flame. A charged electrical field is positioned across the flame, and utilizing the ions in the flame can conduct a current. When most combustible materials are introduced into the flame, they produce ions in their combustion products, and these are detected by the increased flow of current across the electric field (flame). 

Virtually every conceivable means of detecting gases and vapors has been exploited in designing GC detectors, and over one hundred have been described. The two most popular ones, the thermal conductivity detector (TCD) and the flame ionization detector (FID), will be described in some detail. They are classified (according to the criteria in Chapter 7) and compared in Table 6. 

The NPD is similar in design to the FID (flame ionization detector), except that the hydrogen flow rate is reduced to about 3 mL/min, and an electrically heated thermionic bead (NPD bead) is positioned near the column orifice. Nitrogen or phosphorus containing molecules exiting the column collide with the hot bead and undergo a catalytic surface chemistry reaction. The resulting ions are attracted to a collector electrode, amplified, and output to the data system. The NPD is 10-100 times more sensitive than FID. 

McWilliams and Dewer [12] described the flame ionization detector which was to be the work horse of all future gas chromatographs. Further developments of the flame thermocouple detector were described by Primavesi etal. [13], the design of the katherometer was simplified by Stuve [14] and Grant [15] described the first thermal emissivity detector. 

The samples were collected from GLC columns by two techniques. A system with KBr as an absorbent was used to collect GLC peaks from single-column operation with a flame ionization detector. This was adapted from an original design for dual column operation (27). It consisted of column eflBuent splitter, collection tube, and fraction collector. The splitting ratio of the flow was 1 1.7, detector output. This system was chosen for the following reasons ... 

Detectors may be classified on the basis of selectivity. A universal detector responds to all compounds in the mobile phase except carrier gas. A selective detector responds only to a related group of substances, and a specific detector responds to a single chemical compound. Most common GC detectors fall into the selective designation. Examples include flame ionization detector (FID), ECD, flame photometric detector (FPD), and thermoionic ionization detector. The common GC detector that has a truly universal response is the thermal conductivity detector (TCD). Mass spectrometer is another commercial detector with either universal or quasi-universal response capabilities. 

The purpose of the detector is to determine when and how much of a compound has emerged from the column. Although the goal of all detectors is to be as sensitive as possible, many detectors are designed to be selective for certain classes of compounds. Dozens of different types of detectors have been developed, but only a few are used routinely. Those are thermal conductivity (TC), thermionic (N/P), electron capture (ECD), flame photometric (FPD), Hall electroconductivity detector (Hall or ELCD), hydrogen flame ionization detector (FID), argon ionization (AI), photoionization (PID), gas density balance (GDB), and the mass spectrometer. Chemists usually select a detector by the following criteria, listed in priority ... 

Fig. 3.3A illustrates a typical design of a glass pyrolytic cell [37]. Similar cells were described by Janak [38], Jones and Moyles [39, 40] and Mlejnek [41]. They have also been used by other investigators. To increase the concentration of the resulting products and use a simpler thermal conductivity detector. Franc and Blaha [42] employed platinum mesh as the pyrolytic cell filament. This enabled them to increase the sample size without increasing the weight of the polymer under investigation per unit area of the heated filament surface, so that they could use a thermal conductivity detector instead of a flame-ionization detector. 

Purity of the carrier gas is very important in modern GC equipment designated for trace analysis. Consequently, it is essential that the gas purifiers, such as the traps containing various adsorbents, be inserted in the gas tine before the injection port. The same requirement usually applies for purification of the combustion gases for the flame ionization detector. The role of these adsorbent traps is to remove even the trace quantities of water, oxygen and organic impurities present in commercial gas cylinders, and thus minimize both the system contamination and chemical alteration of an injected sample. 

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