Ion/Molecule Reactions

Ion/molecule reactions (IMR) have been shown to have great analytical utility, especially in the case of structural elucidation [23-25]. IMRs in the gas phase offer several benefits compared to their solution counterparts. Very little neutral reagent is required for an IMR and often the headspace vapor is sufficient to generate measurable product. Reaction rates and efficiencies are also inherently high for most IMRs, meaning that analytes in trace quantities will still form a detectable product [24]. This is especially true for IMRs performed at atmospheric pressure in IMS instruments, as the number of collisions per second is dramatically increased in comparison to the same reaction performed in an ion trap under vacuum [26]. However, the lack of straightforward identification of products in IMS generally requires the use of tandem IMS-MS instrumentation [26]. Often, significant

Cutaway view of modified electrode system showing opening for neutral vapor introduction

Fig. 3.3 Cutaway view of modified electrode system showing opening for neutral vapor introduction

modification to MS instruments must be made in order to perform ion/molecule reactions, which can be costly and time-consuming. The coupling of IMS to MS instruments suffers from similar drawbacks.

As a proof-of-concept demonstration of an ion-molecule reaction performed with the plastic electrode system in air. Protonated tert-butylamine and cyclo- hexylamine ions were generated by nanoESI from 10 ppm solutions in methanol. As these ions passed through the electrode system they were exposed to the vapor emanating from cotton swab saturated with a solution of 1000 ^g/mL dimethyl methylphosphonate (DMMP) in the last region (E3) of the electrode system shown in Fig. 3.3. In order to introduce DMMP vapor, the final electrode was replaced with an electrode having a hole through which the cotton swab could be inserted. A depiction of this is shown in Fig. 3.1a, however it should be noted that for IMR experiments the electrodes were arranged such that the final electrode (E3) contained the hole and not E2 as shown in Fig. 3.1a. Accurate positioning of the electrodes was performed by mounting the assembly to a 3 axis moving stage such that the exiting ions were sampled with the API of an LTQ linear ion trap (Thermo).

 
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