K. M. Mohibul Kabir

Exhaled breath containing volatile organic compounds (VOCs) can be analysed non-invasively and has potential applications in disease diagnosis. Previous high-field asymmetric waveform ion mobility spectrometry (FAIMS)-based breath studies have primarily used low-resolution FAIMS with ion current detection rather than mass spectrometry (MS). Here, atmospheric pressure chemical ionisation (APCI) was coupled with high-resolution FAIMS-MS for direct breath analysis. APCI-FAIMS-MS increased the number of detected breath features by >200% compared with APCI-MS alone and consistently detected 60–130 breath features per sample. Within-participant heatmap consistency was comparable across the four participants based on structural similarity index analysis. Exploratory principal component analysis showed participant-associated clustering of breath profiles. These results demonstrate that high-resolution FAIMS substantially enhances breath feature detection and provides an orthogonal separation dimension for untargeted breath profiling. Future studies involving larger cohorts and disease-specific populations will determine whether the increased breath feature coverage afforded by high-resolution FAIMS can improve VOC-based classification and diagnostic performance.

Glycans are ubiquitous, structurally diverse molecules that have specific and general roles involving metabolism, structure, and cell-to-cell signalling. Functional specificity depends strongly on the complexity of structures that polysaccharides can adopt based on their subunit composition, length, extent of branching, glycosidic bond connectivity and anomeric configuration. However, a rapid and comprehensive characterization of glycan isomers can be challenging owing to limitations associated with their separation. Here, ten composition, anomeric and connectivity disaccharide isomers were separated and detected using high-resolution differential ion mobility-mass spectrometry (DMS-MS, also known as FAIMS). Focus was primarily directed to compositional isomers corresponding to epimers that differ by the axial or equatorial position of a single hydroxyl group. DMS resolving power was enhanced 14-fold primarily by increasing the fraction of helium in the ion carrier gas and lowering the flow rate. At relatively high disaccharide concentrations, DMS-MS of each disaccharide resulted in complex and unique multi-peak spectra with up to ten fully and partially resolved peaks for β-1,4-mannobiose (Man-1,4β-Man), which can be attributed to the DMS separation and subsequent dissociation of ionic non-covalently bound oligomers into monomer ions. Each DMS spectrum has at least one differentiating peak that is not in the other spectra, indicating that DMS can be used to fully or partially resolve composition, configuration and connectivity isomers. At relatively low disaccharide concentrations, disaccharide epimers can also be readily separated by DMS. The integration of high-resolution, ambient pressure DMS with complementary reduced-pressure ion mobility and MS-based glycomics and glycoproteomics workflows may be useful for improving the characterization of glycans and glycosylated biomolecules.

Differential ion mobility (DMS) is a versatile ion separation method that is often integrated with mass spectrometry (MS). In DMS, extremely high electric fields are used such that ion mobility depends non-linearly on electric field and thus, ion separations can be more orthogonal to MS than lower field ion mobility-based methods. DMS can have sufficiently high resolution to be used for enantiomer analysis of small molecules and to separate protein ions with peak widths comparable to those obtained for peptides. However, the performance of high resolution DMS-MS can be limited owing to the substantial loss of ions (>10-fold) that can occur upon their transfer from atmospheric pressure (where DMS separation typically occurs) to vacuum through a narrow conductance limited inlet (e.g. capillary) to the MS. Here, results from simulated ion trajectory simulations suggest that in high resolution DMS most ions can be lost by ‘crashing’ onto the narrow capillary inlet after exiting the DMS separation channel. To enhance DMS sensitivity and resolving power, an integrated DMS-MS interface concept is reported that consists of a slit electrode and a 12-electrode atmospheric pressure ion funnel (APIF). By using an APIF with slit entrance, the simulated ion transmission efficiencies increase by up to 257% for singly charged ions ([DMMP + H]⁺, [tryptophan + H]⁺, and [(2-dodecanone)₂ + H]⁺) and by 209% for [ubiquitin + 12H]¹²⁺, without compromising resolving power. The use of APIF improves the ion focussing from the DMS exit to the MS capillary to improve sensitivity, and the slit ensures that ion dispersion in the analytically relevant direction perpendicular to the DMS electrodes is restricted to enhance resolution. By narrowing the slit of the DMS-Slit-APIF interface, the DMS resolving power can be increased further by at least 20%. Overall, these results indicate that the integrated DMS-Slit-APIF interface is promising for improving the sensitivity and resolution for many different types of DMS-MS experiments.

Glycans are ubiquitous, structurally diverse molecules that have specific and general roles involving metabolism, structure, and cell-to-cell signalling. Functional specificity depends strongly on the complexity of structures that polysaccharides can adopt based on their subunit composition, length, extent of branching, glycosidic bond connectivity and anomeric configuration. However, a rapid and comprehensive characterization of glycan isomers can be challenging owing to limitations associated with their separation. Here, ten composition, anomeric and connectivity disaccharide isomers were separated and detected using high-resolution differential ion mobility-mass spectrometry (DMS-MS, also known as FAIMS). Focus was primarily directed to compositional isomers corresponding to epimers that differ by the axial or equatorial position of a single hydroxyl group. DMS resolving power was enhanced 15-fold primarily by increasing the fraction of helium in the ion carrier gas and lowering the flow rate. At relatively high disaccharide concentrations, DMS-MS of each disaccharide resulted in complex and unique multi-peak spectra with up to ten fully and partially resolved peaks for β-1,4-mannobiose (Man-1,4β-Man), which can be attributed to the DMS separation and subsequent dissociation of ionic non-covalently bound oligomers into monomer ions. Each DMS spectrum has at least one differentiating peak that is not in the other spectra, indicating that DMS can be used to fully or partially resolve composition, configuration and connectivity isomers. At relatively low disaccharide concentrations, disaccharide epimers can also be readily separated by DMS. The integration of high-resolution, ambient pressure DMS with complementary reduced-pressure ion mobility and MS-based glycomics and glycoproteomics workflows may be useful for improving the characterization of glycans and glycosylated biomolecules.

Electrospray ionisation (ESI) is renowned for its ability to ionise intact proteins for sensitive detection by mass spectrometry (MS). However, the use of a conventional direct current ESI voltage can result in the formation of relatively large initial droplet sizes, which can limit efficient ion desolvation and sensitivity. Here, pulsed nanoESI (nESI) MS using nanoscale emitters with inner diameters of ~250 nm is reported. In this approach, the nESI voltage is rapidly pulsed from 0 to ~1.5 kV with sub-nanosecond rise times, duty cycles from 10 to 90%, and repetition rates of 10 to 350 kHz. Using pulsed nESI, the performance of MS for the detection of intact proteins can be improved in terms of increased ion abundances and decreased noise. The absolute ion abundances and signal-to-noise levels of protonated ubiquitin, cytochrome C, myoglobin, and carbonic anhydrase II formed from standard denaturing solutions can be increased by up to 82% and 154% using an optimal repetition rate of ~200 kHz compared to conventional nESI-MS. Applying pulsed nESI-MS to a mixture of four proteins resulted in the signal for each protein increasing by up to 184% compared to the more conventional nESI-MS. For smaller ions (≤1032 m/z), the signal can also be increased by the use of high repetition rates (200–250 kHz), which is consistent with the enhanced performance depending more on general factors associated with the ESI process (e.g., smaller initial droplet sizes and reduced Coulombic repulsion in the spray plume) rather than analyte-specific effects (e.g., electrophoretic mobility). The enhanced sensitivity of pulsed nESI is anticipated to be beneficial for many different types of tandem mass spectrometry measurements.

Glycans are ubiquitous, structurally diverse molecules that have specific and general roles involving metabolism, structure, energy storage, and cell-to-cell signalling. Functional specificity depends strongly on the complexity of structures that polysaccharides can adopt based on their subunit composition, length, extent of branching, glycosidic bond connectivity and anomeric configuration. However, the rapid, sensitive, and comprehensive characterization of glycan isomers can be challenging owing to limitations associated with their separation. Here, ten composition, anomeric and connectivity disaccharide isomers were separated and detected using high-resolution differential ion mobility-mass spectrometry (DMS-MS, also known as FAIMS). Focus was primarily directed to compositional isomers corresponding to epimers that differ by the axial or equatorial position of a single hydroxyl group. DMS resolving power was enhanced 15-fold primarily by increasing the fraction of helium in the ion carrier gas and lowering the flow rate. DMS-MS of each disaccharide resulted in surprisingly complex and unique multi-peak spectra with up to ten fully and partially resolved peaks for β-1,4-mannobiose (Man-1,4β-Man). Each DMS spectrum has at least one differentiating peak that is not in the other spectra, indicating that DMS can be used to fully or partially resolve composition, configuration and connectivity isomers. Thus, DMS-MS has the potential to be used for identifying polysaccharide isomers in a spectral ‘fingerprinting’ approach. The integration of high-resolution DMS into high-throughput MS-based glycomics and glycoproteomics workflows may be useful in the future for improving the characterization of glycans and glycosylated biomolecules.

Non-invasive medical diagnosis by analysing volatile organic compounds (VOCs) at the point-of-care is becoming feasible owing to recent advances in portable instrumentation. A number of studies have assessed the performance of a state-of-the-art VOC analyser (micro-chip high-field asymmetric waveform ion mobility spectrometry, FAIMS) for medical diagnosis. However, a comprehensive meta-analysis is needed to investigate the overall diagnostic performance of these novel methods across different medical conditions. An electronic search was performed using the CAplus and MEDLINE database through the SciFinder platform. The review identified a total of 23 studies and 2312 individuals. Eighteen studies were used for meta-analysis. A pooled analysis found an overall sensitivity of 80% (95% CI, 74%–85%, I ² = 62%), and specificity of 78% (95% CI, 70%–84%, I ² = 80%), which corresponds to the overall diagnostic performance of micro-chip FAIMS across many different medical conditions. The diagnostic accuracy was particularly high for coeliac and inflammatory bowel disease (sensitivity and specificity from 74% to 97%). The overall diagnostic performance was similar across breath, urine, and faecal matrices with sparse logistic regression and random forests algorithms resulting in higher diagnostic accuracy. Sources of variability likely arise from differences in sample storage, sampling protocol, method of data analysis, type of disease, sample matrix, and potentially to clinical and disease factors. The results of this meta-analysis indicate that micro-chip FAIMS is a promising candidate for disease screening at the point-of-care, particularly for gastroenterology diseases. This review provides recommendations that should improve the techniques relevant to diagnostic accuracy of future VOC and point-of-care studies.

One of the major challenges in the development of soft electronics is to devise scalable and automated strategies for the microfabrication of deformable and flexible electronic components and sensors. Liquid metals have demonstrated unique characteristics suitable for such applications. To date, conductive patterning using liquid metals has mainly focused on writing or injection of liquid metals into prefabricated channels, requiring multiple complicated procedures and reliance on specific inks formulae. Here a new strategy has been introduced for direct patterning of liquid metal particles onto substrates via UV photolithography. A variety of different tracks, patterns and sensors, on both rigid and flexible substrates, are demonstrated using a dispersion of eutectic alloy of gallium and indium in photoresist, to present the extent of capability of the introduced method. The method provides simple, effective, high-resolution and precision patterning of liquid metals for the emerging fields of soft electronics and sensors.