Photoionization Mass Spectroscopy

Multiplexed Detection

Figure 6

Multiplexed detection has substantial advantages in measuring kinetics, not only because of the time advantages in looking at all of the expected species simultaneously, but because detecting unexpected species can provide new insights into reaction mechanisms. One example is given by the self-reaction of vinyl radicals. Literature values for this reaction’s rate coefficient were surprisingly high; reinvestigation by both laser and mass spectrometric probing showed a substantially lower rate coefficient (see Figure 6).

The source of this discrepancy is made clear by resolving all reaction products. Determining the second-order rate coefficient requires knowing the absolute radical concentration. The literature reports that the vinyl concentration was derived from measured concentrations of butadiene product in conjunction with a branching fraction for butadiene production that assumed that recombination (to form butadiene) and disproportionation (to form ethene and acetylene) were the sole product channels. However, measurement using multiplexed photoionization mass spectrometry shows that the unexpected radical products methyl (visible at m/z = 15 in Figure 7) and propargyl (m/z = 39) are in fact substantial.

Figure 7

In fact, the data show that the branching to methyl + propargyl is ~50% at 300 K. Including this channel in the interpretation of the literature experiments would therefore increase the derived vinyl radical concentration, and hence reduce the rate coefficient, by approximately a factor of two, bringing them into agreement with the new absolute measurements.