Mass spectrometric sampling of flames: how ionic equilibria in flames produce sampling falsifications and “fake” ions, but provide kinetic and thermodynamic data on the reaction occurring

• Published in: Progress in energy and combustion science 2022-01, Vol.88, p.100927, Article 100927
• Main Author: Hayhurst, A.N.
• Format: Article
• Language: English
• Subjects: Electric fields in flames; Flame Sampling; Free electrons in flames; Ions in flames; Mass spectrometry; Negative ions in flames; Positive ions; Sampling nozzles
• ISSN: 0360-1285; 1873-216X
• DOI: 10.1016/j.pecs.2021.100927

Continuously sampling a flame, burning at 1 atm., for mass spectrometry at ≈ 10–8 atm. seriously disturbs the flame. Not only are a flame's temperature and velocity altered, often the composition of a sample is falsified. Thus, “fake” ions appear, even when sampling as quickly as possible, i.e. supersonically, to quench chemical reactions. However, studying these spurious ions is fruitful. They arise, because a sample is unavoidably cooled; the drop in temperature causes a rapid chemical equilibrium to shift position and change the sample's composition. That ions react faster than neutrals (to perturb a sample) magnifies the problem for ions. When continuously sampling a flame, burning at 1 atm., through an inlet at the tip of a hollow, metallic nozzle, cooling can occur in three ways during the formation of a beam for mass spectrometry. Firstly, before a sample passes through the inlet hole to enter the supersonic expansion into the first vacuum chamber of the mass spectrometer, it loses heat to the cooler, sampling nozzle, usually conical in shape. By detecting spurious ions from a flame, this drop in temperature has been measured to be greatest (≈ 400 K) for the smallest orifices. This cooling becomes smaller for larger holes and is trivial for diameters above 150 µm. Secondly, a sample cools (by maybe ≈ 300 K), whatever the orifice's size, on being accelerated to the local speed of sound in the narrowest part, i.e. the throat of the inlet orifice. Thirdly, the drop in temperature in the subsequent, near-adiabatic expansion inside the nozzle is greatest (≈ 1000 K) and most prolonged for the largest inlet holes (diam. > 150 µm). The upshot is that with a small hole (diam. < 100 µm), a sample is cooled by both the sampling nozzle and the acceleration to sonic velocity in the throat of the inlet. However, with a large orifice (diam. > 150 µm), cooling happens in the acceleration to a Mach number of unity and the following supersonic expansion. Analysis shows that, if a positive ion reacts exothermally in a reversible reaction with a time constant briefer than ≈ 0.5 µs, that reaction will be equilibrated early in the flame. In addition, if the orifice is small, the equilibrium will be just fast enough to shift position to that for a temperature reduced in both the thermal boundary layer around the inlet, and in accelerating to the speed of sound. Consequently, the sample begins the expansion with new species. When using a big orifice, the reaction's time co