Ionisation in the absence of high voltage using supercritical fluid chromatography: a possible route to increased signal

Abstract

Supercritical fluid chromatography (SFC) is fast becoming a technique of choice for the analysis of a wide range of compounds and has been found to be complementary to high-performance liquid chromatography (HPLC). The combination of SFC and mass spectrometry (MS) affords a very useful tool in the separation and analysis of compounds. In this study the ionisation of samples in the absence of an applied electrospray voltage has been observed when using SFC/MS, with some compounds showing increased sensitivity when all ionisation source high voltage (HV) is removed. In an attempt to understand the mechanism of ionisation, a series of test compounds were analysed using standard electrospray ionisation (ESI) and atmospheric pressure chemical ionisation (APCI) source configurations and also different API source designs. In both cases, data were acquired with the applied high voltage on (normal conditions) or with the high voltage off, i.e. no voltage spray (novo-spray). The standards were analysed with a range of pressure and modifier percentage conditions. To understand the nature of the ionisation process observed, this was compared with three established liquid-to-gas ionisation mechanisms. These were thermospray (TSP), charge residue model (CRM) of ESI and sonic spray ionisation (SSI). Experiments were undertaken in an attempt to explain this ionisation phenomenon and quantify any observed change in sensitivity. The most important point to note is that enhanced ionisation was observed under novo-spray conditions in a SFC/MS configuration, which in certain cases provides a lowering in the overall limit of detection (LOD).

Systematic evaluation of acetone and acetonitrile for use in hydrophilic interaction liquid chromatography coupled with electrospray ionization mass spectrometry of basic small molecules

“…We observed previously low optimal capillary voltages (0.8 kV) for the UPLC/MS/MS analysis of the basic solutes quinine and its major metabolite 3( S )‐3‐hydroxyquinine, using an instrument with a similar source design to the one used in the present study. This observation may also be related to the work of Langley and co‐workers noting increased signal in the absence of high voltage using SFC/MS, termed novo‐spray. However, in their study the definitive ionisation mechanism responsible for affording increased signal was not elucidated.…”

Novel formation of [2M-H]+species in positive electrospray mass spectra of indoles

“…As such, further analytical runs were conducted at a constant nebuliser voltage of 4 kV where significant dimer formation was still produced and more intense ion signals allowed more reliable mass measurement and MS/MS scans to be performed. Ionisation in the absence of high voltage has been noted previously, but no dimeric species were reported.…”

Advances in High Throughput Supercritical Fluid Chromatography

“…[80,81] In one study, the absence of high voltage during ES ionization in SFC=MS has been shown to unexpectedly increase the signal intensity for a group of standard samples. Thite et al [82] tested the effect of different post-column makeup fluid compositions, as well as the effects of pressure, SFC flow rate, and modifier composition on the signal sensitivity under high voltage and zero voltage MS conditions for both ESI and APCI sources. Reference standard signal intensity in both ESI and APCI increased under zero voltage conditions when methanol was used as the makeup fluid and signal intensity was lower when elevated voltages were applied.…”