Flame ionization positive ions

In a hydrocarbon analyzer using flame ionization, the sample gas is conducted along a heated sampling line to the detector, in the hydrogen flame of which the hydrocarbons are ionized into electrons and positive ions,... 

Reaction 2 has been invoked because C3H3 + is apparently formed in a primary ionization step since the ion appears early in the flame front, its concentration maximizes in rich flames (this is true of no other positive ion observed), and it is present in the flame front in large concentrations (9). However, not all the experimental evidence is consistent with this mechanism for producing C3H3+ it might also be produced through an ion molecule reaction, which will be considered below. 

PDMS = polydimethylsiloxane. PA = polyacrylate. CW = Carbowax. DVB = divinylbenzene. FID = flame ionization detection. NPD = nitrogen-phosphorus detection. TSD = thermionic-specific detection. LOQ = limit of quantitation. LOD = limit of detection. TCA = trichloroacetic acid. PICI-MS = positive ion chemical mass spectrometry. SIM = selected ion monitoring. 

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). 

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. 

The Ionization potential of BaOH(g) was deduced by Kelly and Padley ( ) to be 5.25 0.1 eV. These authors quantitatively examined the total positive ion concentrations produced from Ba aqueous salt additives in fuel rich, premixed Hg + Og + N2 flames. Using current JANAF auxiliary data (2), we recalculate the ionization potential to be 5.36 eV. 

The addition of metallic salts to a flame markedly affects the rate of recombination observed. This was demonstrated dramatically by Knewstubb. He showed that lead was virtually unionized in a hydrogen flame, but was apparently as able to provide electrons as sodium in an acetylene flame. His explanation was that the natural ionization in the reaction zone of the acetylene flame was much greater than in the hydrogen flame, and that the lead transferred an electron to the natural positive ion. Since this was polyatomic the three-body recombination rate for electrons with lead atoms would be much lower than for dissociative recombination with the polyatomic natural ion, as that ionization would persist for some distance downstream. 

The positive-ion-molecule reactions considered to play a role in hydrocarbon flame ionization are catalogued in Table III, and the thermochemical values used are summarized in Table IV. Many of the reactions are only moderately exothermic, so that at the high temperatures of flames, the reverse reactions can contribute significantly to ion concentrations, and often the equilibrium is such that large concentrations of both ions are present. Further, because of the large rate coefficients for ion-molecule reactions, the ion concentrations can attain equilibrium distributions with... 

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