HIGHLIGHTS
Analytical chemistry
MARIE-LAURE ABEL University of Surrey, UK
Correlative microscopy combining secondary ion MS and electron microscopy The interest in imaging obtained with different techniques is that it permits a correlation between, for example, microstructure and physico-chemical properties. However, the difficulty in correlative microscopy resides particularly in the different resolutions of said techniques. This point was examined by Vollnhals et al, comparing images obtained by secondary ion mass spectrometry (SIMS) and scanning electron microscopy (SEM) (Analytical Chemistry, 2017, 89, 10702−10710) (Figure 1). These techniques have a different sensitivity; do not yield the same type of information; and have very different spatial resolution. Image fusion methods were evaluated with three case studies representative of a range of fields within life science research. Two methods, intensity- hue-saturation (IHS) usually used for electron microscopy and Laplacian pyramid fusion (LPF), were evaluated and their limitations and/or advantages highlighted as well as the potential artefacts.
Assessing the quality of image fusion is not straightforward because relevant metrics as well as the concept are mostly developed for remote satellite imaging. Comparison of the fusion algorithms indicates that LPF is better than IHS and that no procedure provides artefact free fusion but also interestingly that visual assessment is consistent with such assessments. IHS approach has limitations, operating with an RGB representation of the data rather than numerical data as for LPF, is not good at dealing with mismatch in input images and may lead to misrepresentation of the initial data because of the selection of an invalid colour scheme. It is, however, easy to implement.
LPF seems to be a better method which does not or suffers less from the above as well as being more reliable for hybrid fusion.
Analysis of human gliomas by swab touch spray mass spectrometry Swab touch spray mass spectrometry, or swab TS-MS, is an ambient technique that Piro et al have used to study 29 cryopreserved samples of human brain tumours (Analyst, 2017, 142, 4058-4066) (Figure 2). This provides immediate
information, which may be used to help in deciding the tumour margins and maximum tumour excision. This is not possible with the more commonly used intraoperative histopathology, even though it can inform on the type and grade of the tumour. The main advantages of this technique are its speed, the need for a very small sample and no specimen pretreatment. The swabs, analysed in a linear
ion trap mass spectrometer with a custom built ion source, serve as both sampling devices and electrospray emitters for analysis based on the ambient ionisation method of paper spray mass spectrometry. Three types of information are
looked for: tissue type through expression of phospholipids, tumour infiltration checking the amount of
Figure 1 Example of image fusion for SEM and SIMS using hybrid LPF showing 15N/14N enrichment in tissue. Blue indicates natural 15N abundance (0.37%) and magenta enrichment (9.25%). Field of view: 1 micron for hybrid image. Pixel size SEM: 3.5 nm; SIMS: 78nm.
N-acetylaspartate isocitrate (NAA), and dehydrogenase mutation through the presence of the oncometabolite 2-hydroxyglutarate (2HG). Spectra are compared with DESI-MS databases and tissues can be differentiated as glioma (malignant), white matter or grey matter. The lipid profiles are capable of indicating the type (grey or white matter) of tissue when infiltrated. NAA neurometabolites decrease as the TCP (tumour cells relative to normal cells) increase although the minimum and maximum limits of detection for NAA are still to be found. Measurement of 2HG allows the distinction between mutant gliomas and wild type gliomas with 100% accuracy. At this stage, the sampling may still be improved and further testing is required but it could have a great future as help for intraoperative diagnosis tool.
LC-MS/MS method for the quantification of endogenous asthma/COPD potential urinary biomarkers A tandem mass spectrometry method has been developed by Khamis et al using urine biomarkers to help specific and quick diagnosis of afflictions such as asthma and chronic obstructive pulmonary diseases (COPD) of which many symptoms overlap (Analytica Chimica Acta, 2017, 989, 45-58). This is a novel diagnostic tool
Figure 2 Swab TS-MS experiment, left to right. Sampling of tissue with swab tip which is transferred by rotating the swab on its shaft. The swab is positioned in front of the mass spectrometer and mass spectra are recorded once an electrospray is generated after exposure of the swab to solvent and high voltage
approach, which may help tailor an appropriate treatment as assessment is particularly difficult amongst the elderly and children and several diseases may coexist in one patient. Fifty urinary metabolites used as
biomarkers and which have been ob- served before using nuclear magnetic resonance (1
H NMR) were used; these
are divided into groups according to their chemical nature. This process uses specifically amines and phenols (nineteen metabolites). Other groups include organic acids to be described in another publication and those not
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