1. The Dispersion Characteristics of Lipids in High-Field Asymmetric
Waveform Ion Mobility Spectrometry
Ben Jenkins, Luke Marney, Zoe Hall, Albert Koulman
Introduction - High field asymmetric waveform ion mobility spectrometry (FAIMS) is a chip based
adaptation of conventional ion mobility (IM). The FAIMS system utilises a differential compensation
field to focus ions through into the detector. The chip-based system allows for higher field strengths
to be applied such that ion separation can be achieved using a smaller instrument design. This
enables compatibility of the FAIMS system with different mass spectrometry setups/instruments.
FAIMS separation is determined by several factors, including ion mass, collisional cross section,
charge state and charge position. We studied the effect of differential compensation fields on the
mobility of lipids from different classes to determine the utility of FAIMS for separation lipids of
different classes.
Methods - Lipid standards including; fatty acids, phospholipids, glycerolipids and cholesteryl esters
(n=8, 18, 10, 1 respectively) were used at different concentrations (100nm to 1000µm). Each of
these standards, individually as well as in combinations, and plasma extracts were directly infused
using chip based nanospray (Advion TriVersa NanoMate) into the chip based FAIMS system
(prototype from Owlstone, Cambridge UK) and spectra were obtained using a bench top Orbitrap
(Thermo Exactive). A two dimensional approach was used with regards to the FAIMS data
acquisition. The differential separation (compensation field, starting on 150Td going to 300Td) was
cycled over several values for the aspiration mechanism (dispersion field, starting on -1Td and
finishing on +4.25Td). We plotted the data in 3 dimensions, which allowed the behaviour of the ions
to be assessed as a result of the two main varying parameters in FAIMS.

2. Results - The FAIMS system was able to separate ions of different lipid classes depending on the
location of the charge. Therefore isobaric lipids (e.g. PCs v PEs) could be dispersed aiding in the
identification of the ions in question. In addition FAIMS assisted in the discrimination of ions that
were either authentic or an isobaric fragmentation ions. The localisation of the charge affected the
behaviour in the asymmetric field, therefore concentration dependent ionisations mechanisms
introduced an extra level of complexity to mixtures of lipids such biological samples. Further works is
necessary to test if the addition of electrolytes (such as Li+
or Na+
) can lead to uniform ionisation and
therefore a more focussed dispersion in FAIMS.
Conclusions - The FAIMS system allows post ion source separation of isobaric lipids. This separation
is dependent on charge localisation and ionisation mechanism.
Novel Aspect - Isobaric lipid separation, with regards to class and authentic / fragmentation ion
mass matching. Ion-adduct proportionality, for the determination of charge position and adduct
formation.