Miniaturization and Integration of Plasma Ionization into the Inlet Capillary of a LC-MS Instrument – Towards Kinetic Control of Ionization
- at -
- ICM Saal 4b
- Type: Lecture
Lecture description
Soft ionization by chemical reaction in transfer (SICRIT) is a flow through ionization based on a concentric dielectric barrier discharge. So far, this technique is always applied on top of the vacuum inlet capillaries resp. skimmers of LC-MS systems in a retrofit manner. The technique has been proven to exhibit unique soft ionization capabilities e.g. for perfluoroalkanes or other very nonpolar molecules. This can be mainly attributed to effective transport and short residence time of the highly reactive ions formed into the inlet system of the MS. Here we present a technical advancement of this technology, fully integrating this form of ionization directly into the inlet system of an LCMS QToF and as close to the vacuum orifice as possible. The inlet system of a Shimdazu 9030 QTof was modified by shortening the Inlet capillary (desolvation line) by 20 mm. A miniaturized version of SICRIT was integrated into the following vacuum orifice. The shortened desolvation line serves as first electrode of the concentric discharge configuration. A second electrode and dielectric element were integrated into the vacuum orifice assembly. By application of a sinusoidal HV on the outer electrode a concentric DBDI is formed 10 mm in front of the Vacuum skimmer.
A set of various chemical components was introduced via a GC- coupling and an “external” SICRIT source. The performance of both sources is compared. For the internal source the residence time of an analyte molecules in the primary ionization region is calculated to be < 25 µs. An ion formed here will be introduce into the vacuum and quad region, preventing further reactions, afterwards in < 200 µs. Whereas the external source leaves about 5 ms before the Ions are introduced into the vacuum.
Comparing the spectra for both sources reveals a significant difference. The internal source is exhibiting more odd electron species M+ and less oxidation species than the external one, indicating kinetic control of the ionization process. Even for n-Hexane the M-H+ species is observed which is almost absent for the external source, where only the oxidized ions ([M-H+O]+ and [M-3H+O2]+) are observed.
Although the internal source exhibits a slightly higher degree of fragmentation than the external SICRIT, it still generates mostly MH+ for analytes that undergo protonation (e.g. Phthalates).
We evaluated the influence of this internal source on the normal performance of the QToF operated with ESI by comparing the internal tuning procedure using NaI. The Performance was found to be comparable with and without internal source, with
little to no loss in sensitivity. Since SICRIT is only operated by application of an AC voltage the internal ionization can be turned on and off in ms, enabling a simultaneous use of ESI and SICRIT in LC- or micro-LC-applications to partially improve the coverage and ionization of non-polar analytes, not accessible by ESI alone.
