Paper AS-TuP18
Improving Relative Quantitation in Imaging Lipidomics using ToF-SIMS

Tuesday, November 8, 2016, 6:30 pm, Room Hall D

Introduction

ToF-SIMS is of increasing value to clinicians and has been used on a number of tissue samples to successfully identify and localise different chemical components to various areas of the tissue and answer disease related questions [1]. Compared to traditional histology or fluorescence staining, the main advantage of ToF-SIMS is the label free detection of a large number of different molecules simultaneously on the same tissue section. New gas cluster ion beams (GCIBs) provides benefits for imaging larger molecular species [2]. These improvements still leave issues such as greatly varying ionisation efficiencies for different molecules and matrix effects. In some biological samples changes in Na or K concentration through the sample can lead to different pseudo-molecular ions being preferentially generated from different parts of the sample.

Methods

A J105 ToF-SIMS instrument (Ionoptika Ltd, UK) was used to analyse salt adduct effects on lipids ionization by exposing different standard lipids and tissue samples to different concentrations of sodium and potassium salts mixtures. Tissue sections from mouse heart, rat brain and cancer biopsy samples were imaged on the same instrument adduct formation assessed.

Results

In positive ion mode the relative intensity of [M+H]⁺, [M+Na]⁺ and [M+K]⁺ ions from phospatidylcholine phospholipids changes dramatically with just a small change in in the Na:K ratio in the sample. The situation is further complicated by changes in relative intensities of fragment ions. The ratio of the [M+Na-59]⁺ to the [M+Na]⁺ peak is significantly smaller than the equivalent ratio from potassium adduct ions. Using these different peaks corrected images of different lipids can be generated from images of tissue sections obtained by ToF-SIMS.

Conclusions

Matrix effects, in this case variation in salt content in biological samples, can lead to difficulties in (relative) quantitation of different biological species. The changes in ionisation and fragmentation observed in ToF-SIMS measurements show similar trends to those observed with other mass spectrometry methods where adduct formation is used as a tool for influencing fragmentation in tandem MS experiments[3].

[1] David Toubel, Sandrine Roy, Dominique P. Germain et al., Int. J. Mass Spectrom. 2007, 260(2-3), 158-165, Sjövall Peter, Björn Johansson et.al., App. Surf. Sci, 2008, 255 (4), 1177–1180, Michael A. Robinson, Daniel J. Graham, Fionnuala Morrish et al., Biotinterphases, 2016, 11, 02A303

[2] T. B. Angerer, P. Blenkinsopp, J. S. Fletcher, Int. J. Mass Spectrom. 2015, 377, 591-598

[3] Rian L. Griffiths and Josephine Bunch, Rapid Commun. Mass Spectrom. 2012, 26, 1557–1566