Obtaining structural information for lipids such as phosphatidylcholines, in particular the location of double bonds in their fatty acid constituents, is an ongoing challenge for mass spectrometry (MS) analysis. Here, we present a novel method utilizing the doping of liquid matrix-assisted laser desorption/ionization (MALDI) samples with divalent metal chloride salts, producing ions with the formula [L+M]2+ (L = lipid, M = divalent metal cation). Multiply charged lipid ions were not detected with the investigated trivalent metal cations. Collision-induced dissociation (CID) product ions from these doubly-charged metal-cationized lipids include the singly charged intact fatty acids [snx+M-H]+, where ‘x’ represents the position of the fatty acid on the glycerol backbone. The preference of the divalent metal cation to locate on the sn2 fatty acid during CID was found, enabling stereochemical assignment. Pseudo-MS3 experiments such as in-source decay (ISD)-CID and ion mobility-enabled time-aligned parallel (TAP) MS of [snx+M-H]+ provided diagnostic product ion spectra for determining the location of double bonds on the acyl chain and were applied to identify and characterize lipids extracted from soya milk. This novel method is applicable to lipid profiling in the positive mode, where structural information of lipids is often difficult to obtain

Cramer, Rainer

Hale, Oliver J.

Analytical and Bioanalytical Chemistry

Oliver J. Hale

Rainer Cramer

Crossref

Collision-induced dissociation of doubly-charged barium-cationized lipids generated from liquid samples by atmospheric pressure matrix-assisted laser desorption/ionization provides structurally diagnostic product ions

Matrix-assisted laser desorption/ionisation (MALDI) is a soft ionisation technique suitable for

ionising biological molecules for analysis by mass spectrometry (MS). MALDI MS is typically

performed on vacuum-compatible solid samples and on instruments with intermediate to high

vacuum (10-2-10-7 mBar) ion sources.

Less commonly, MALDI can be performed at atmospheric pressure (AP). However, AP-MALDI

presents useful advantages. For example, compatibility is expanded to include samples with

components usually volatile under vacuum conditions. Liquid samples may be analysed with ease.

Recent developments in the fieldusing liquid support matrices (LSMs), which incorporate a viscous

liquid into a solution of a MALDI matrix compound, have demonstrated the ability to reliably

generate stable ion signal over long periods of time.

Interestingly, the combination of AP-MALDI, LSMs and a heated ion inlet on the mass

spectrometer enables the predominant generation of multiply charged ions. MALDI MS spectra

are usually dominated by signals for singly charged ions, with high charge states generally the

domain of electrospray ionisation (ESI). The m/z of highly charged proteins enables AP-MALDI

analysis on narrow m/z range mass spectrometers. Additionally, tandem mass spectrometry

(MS/MS) experiments also benefit: collision induced dissociation (CID) results in more fragment

ion signals from multiply charged precursor ions and electron transfer dissociation (ETD) is

possible, since it requires a minimum 2+ charge state to be performed at all.

Liquid AP-MALDI has potential for mass profiling and sample screening, due to the reliability of

the signal. In this thesis, the ability to rapidly analyse liquid samples using a prototype AP-MALDI

ion source was demonstrated. Applications included the identification of doubly charged

phospholipid ions by MS/MS; profiling of lipids, peptides and proteins for sample classification;

and the assessment of milk samples for -lactamase activity. Barium adduct ions of

phosphatidylcholines were successfully analysed by liquid AP-MALDI CID and ETD MS/MS to

reveal detailed structural information. Profile analysis of milk extracts was used to classify milk

samples. A major section focuses on the rapid and confident detection of bovine mastitis, a

disease with a considerable economic impact

Hale, Oliver John

Central Archive at the University of Reading

Investigation of liquid atmospheric pressure

matrix-assisted laser desorption/ionisation mass

spectrometry (AP-MALDI MS) for the large-scale

analysis of liquid samples

http://centaur.reading.ac.uk/85138/1/18012325_Hale_thesis.pdf

Collision-induced dissociation of doubly-charged barium-cationized lipids generated from liquid samples by atmospheric pressure matrix-assisted laser desorption/ionization provides structurally diagnostic product ions

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Central Archive at the University of Reading

Central Archive at the University of Reading