Dielectric barrier discharge ionization-mass spectrometry (DBDI-MS), which is based on the use of a low temperature helium plasma as ionization source, is used for the determination of trace amounts ...of triacetone triperoxide (TATP) and its homologue diacetone diperoxide (DADP) from surfaces. TATP is observed as M+NH4+ adduct, whereas DADP is observed as M+O+NH4+. Measurement of DADP with varying deuteration degrees (DADP, DADP-d 6, and DADP-d 12) indicates that DADP undergoes oxidation when ionized by DBDI. If acetonitrile is used as deposition solvent, TATP tends to show fragmentation and is not only detected as M+NH4+ but as M–CH4+NH4+ and M–C2H4+NH4+ as well. Quantification of TATP solutions from glass surfaces by DBDI-MS, using TATP-3,6,9-13C as internal standard, was done and validated using an LC/APCI-MS method. Achievable limits of detection (LOD) for TATP are equivalent to the deposition of 15 ng TATP and are comparable with other ambient desorption/ionization mass spectrometric techniques like desorption electrospray ionization (DESI).
Hydrophilic interaction chromatography (HILIC) was applied for the separation of small, noncovalent metal species, and free ligands in plants, using two different zwitterionic stationary phases at pH ...7.3. Ligands ranged from amino acids and phytosiderophores to organic acids and synthetic chelators like EDTA. Our results confirmed the suitability of zwitterionic stationary phases for the separation of such ligands and their metal complexes. In particular, the chromatographic behavior of phytosiderophore complexes of copper, nickel, and zinc on a sulfobetaine-type material was compared to that on a phosphorylcholine-type material under otherwise identical conditions. A compression of the usable retention range for phytosiderophore species on the latter phase was found, which can be traced back to the reversed charge orientation of the zwitterionic functionalities. Differences were also found for the integrity of more labile metal chelates during separation on the two columns. In particular, Ni-malate could only be analyzed on the sulfobetaine phase, and Cu-glutathione, Ni-aspartate, and Ni-citrate complexes were only stable on the phosphorylcholine stationary phase at pH 7.3. Ni-histidine species were only found after lowering the pH to 4-5. The identification of separated species is possible by online ESI-MS in the negative ionization mode.
•Chromatographic separation of bis(monoacylglycero)phosphate and phosphatidylglycerol by HILIC.•Baseline separation of eight additional phospholipid classes was achieved.•Lipid identification was ...performed by tandem high-resolution mass spectrometry.•Method application to a cell culture model of oncogene-induced senescence in MCF-7/NeuT breast cancer cells can be used to investigate a senescence-associated remodeling of BMP species.
Changes in lipid composition of cells or tissue are often linked to various diseases. Studies indicate alterations of bis(monoacylglycero)phosphate (BMP) species in diseases such as cancer. Therefore, an extended phospholipid profiling method based on hydrophilic interaction liquid chromatography (HILIC) coupled to high-resolution mass spectrometry (MS) and data-dependent MS/MS acquisition was developed to separate and unambiguously identify BMP species. Lipid species identification was based on retention time, accurate mass and specific MS/MS fragments. The developed method was applied in a proof of concept study to lipid extracts of a cell culture model of conditional oncogene overexpression in MCF-7/NeuT breast cancer cells. Comparison of control and oncogene-induced MCF-7/NeuT breast cancer cells showed changes in BMP species distribution. Thereby, a shift from long-chain to shorter-chain fatty acid composition in BMP species was detected.
An atmospheric pressure microplasma ionization source based on a dielectric barrier discharge with a helium plasma cone outside the electrode region has been developed for liquid chromatography/mass ...spectrometry (LC/MS). For this purpose, the plasma was realized in a commercial atmospheric pressure ionization source. Dielectric barrier discharge ionization (DBDI) was compared to conventional electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), and atmospheric pressure photoionization (APPI) in the positive ionization mode. Therefore, a heterogeneous compound library was investigated that covered polar compounds such as amino acids, water-soluble vitamins, and nonpolar compounds like polycyclic aromatic hydrocarbons and functionalized hydrocarbons. It turned out that DBDI can be regarded as a soft ionization technique characterized by only minor fragmentation similar to APCI. Mainly protonated molecules were detected. Additionally, molecular ions were observed for polycyclic aromatic hydrocarbons and derivatives thereof. During DBDI, adduct formation with acetonitrile occurred. For aromatic compounds, addition of one to four oxygen atoms and to a smaller extend one nitrogen and oxygen was observed which delivered insight into the complexity of the ionization prossesses. In general, compounds covering a wider range of polarities can be ionized by DBDI than by ESI. Furthermore, limits of detection compared to APCI are in most cases equal or even better.
Glycerophospholipids (GP) are the building blocks of cellular membranes and play essential roles in cell compartmentation, membrane fluidity or apoptosis. In addition, GPs are sources for ...multifunctional second messengers. Whereas the genome and proteome of the most intensively studied eukaryotic model organism, the baker's yeast (Saccharomyces cerevisiae), are well characterized, the analysis of its lipid composition is still at the beginning. Moreover, different yeast species can be distinguished on the DNA, RNA and protein level, but it is currently unknown if they can also be differentiated by determination of their GP pattern. Therefore, the GP compositions of five different yeast strains, grown under identical environmental conditions, were elucidated using high performance liquid chromatography coupled to negative electrospray ionization-hybrid linear ion trap-Fourier transform ion cyclotron resonance mass spectrometry in single and multistage mode. Using this approach, relative quantification of more than 100 molecular species belonging to nine GP classes was achieved. The comparative lipidomic profiling of Saccharomyces cerevisiae, Saccharomyces bayanus, Kluyveromyces thermotolerans, Pichia angusta, and Yarrowia lipolytica revealed characteristic GP profiles for each strain. However, genetically related yeast strains show similarities in their GP compositions, e.g., Saccharomyces cerevisiae and Saccharomyces bayanus.
In this article, some recent trends and developments in ambient desorption/ionization mass spectrometry (ADI-MS) are reviewed, with a special focus on quantitative analyses with direct, open-air ...sampling. Accurate quantification with ADI-MS is still not routinely performed, but this aspect is considered of utmost importance for the advancement of the field. In fact, several research groups are devoted to the development of novel and optimized ADI-MS approaches. Some key trends include novel sample introduction strategies for improved reproducibility, tailored sample preparation protocols for removing the matrix and matrix effects, and multimode ionization sources. In addition, there is significant interest in quantitative mass spectrometry imaging.
Graphical abstract
Conceptual diagram of the ambient desorption/ionization mass spectrometry approach with different desorption/ionization probes
Rationale
Cardiolipins (CL) are a special lipid class which plays a main role in energy metabolism in mitochondria and is involved in apoptosis. In contrast to other glycerophospholipids, they ...contain four fatty acyl residues which results in a high structural diversity. Oxidation, for example by reactive oxygen species, or lyso forms such as monolyso‐CL (MLCL), increases this diversity. Mass spectrometric analysis and computational identification of CL, MLCL and their oxidation products is therefore a challenging task.
Methods
In order to distinguish CL, MLCL and their oxidation products, a liquid chromatography/tandem mass spectrometry (LC/MS/MS) method was developed. A hydrophilic interaction liquid chromatography (HILIC)‐based solid‐phase extraction (SPE) clean‐up approach was developed for CL enrichment. Graphical analysis of CL, MLCL and their oxidation products was carried out by a three‐dimensional Kendrick mass defect (3D‐KMD) plot module, as well as a refined lipid search module of the open‐source metabolomics data mining software MZmine 2.
Results
The HILIC‐based SPE clean‐up enabled complete separation of polar and nonpolar lipid classes. A yeast (Saccharomyces cerevisiae) lipid extract, which was artificially oxidized by means of the Fenton reaction, was analyzed by the developed LC/MS/MS method. CL species with differences in chain length and degree of unsaturation have been separated by high‐performance liquid chromatography (HPLC). In total 66 CL, MLCL and oxidized species have been identified utilizing 3D‐KMD plots in combination with database matching using MZmine 2. For further characterization of annotated species, MS/MS experiments have been utilized.
Conclusions
3D‐KMD plots capturing chromatographic and high‐resolution mass spectrometry data have been successfully used for graphical identification of CL, MLCL as well as their oxidized species. Therefore, we chose multiple KMD bases such as hydrogen and oxygen to visualize the degree of unsaturation and oxidation capturing chromatographic data by means of a color‐coded paint scale as the third dimension. In combination with database matching, the analysis of low concentrated lipid species in complex samples has been significantly improved.
Rationale
Lipid A is a part of the lipopolysaccharide layer, which is a main component of the outer membrane from Gram‐negative bacteria. It can be sensed by mammalians to identify the presence of ...Gram‐negative bacteria in their tissues and plays a key role in the pathogenesis of bacterial infections. Lipid A is also used as an adjuvant in human vaccines, emphasizing the importance of its structural analysis.
Methods
In order to distinguish and characterize various lipid A species, a liquid chromatography coupled to tandem mass spectrometry (LC/MS/MS) method was developed. Isolation of lipid A from different bacteria was carried out using a modified Bligh and Dyer extraction following a mild acid hydrolysis. Chromatography was performed using a bifunctional reversed‐phase‐based stationary phase. High‐resolution MS using negative electrospray ionization was applied and MS/MS experiments utilizing high‐energy collisional dissociation generated diagnostic product ions, which allowed the assignment of the side chains to distinct positions of the lipid A backbone.
Results
The method was applied to lipid A isolations of Escherichia coli (E. coli), Pseudomonas putida (P. putida) and Pseudomonas taiwanensis (P. taiwanensis). Various lipid A species were identified by their accurate masses and their structures were characterized using MS/MS experiments. Previously described lipid A structures from E. coli were identified and their structures confirmed by MS/MS. For the biotechnologically relevant strains P. putida and P. taiwanensis, we confirmed species by MS/MS, which have previously only been analyzed using MS. In addition, several lipid A species were discovered that have not been previously described in the literature.
Conclusions
The combination of LC and MS/MS enabled the selective and sensitive identification and structural characterization of various lipid A species from Gram‐negative bacteria. These species varied in their substituted side chains, speaking of fatty acids and phosphate groups. Characteristic product ions facilitated the assignment of side chains to distinct positions of the lipid A backbone.
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