Quantification of citrulline by parallel fragmentation monitoring--a novel method using graphitized carbon nanoparticles and MALDI-TOF/TOF mass spectrometry

Serum lipidomic profiling for liver cancer screening using surface-assisted laser desorption ionization MS and machine learning

The liver is a major organ in metabolism, and alterations in serum lipids are associated with liver disorders. Here, a rapid, easy, and reliable screening technique based on lipidomic profiling was developed using machine learning and surface-assisted laser desorption ionization mass spectrometry (SALDI MS) for liver cancer diagnosis. A graphitized carbon matrix (GCM) was created for serum lipid profiling in SALDI MS and demonstrated a better performance for neutral lipids analysis than conventional organic matrices. The fingerprint of serum lipids, including triacylglycerols (TGs), diacylglycerols (DGs), cholesteryl esters (CEs), glycerophospholipids (GPs), and other components, could be directly obtained by GCM-assisted LDI MS without extraction. Five machine learning methods were applied to distinguish liver cancer (LC) patients from healthy controls (HC) and chronic hepatitis B (CHB) patients. The best diagnostic performance was attained by linear discriminant analysis (LDA), which has a confusion matrix accuracy of 98.3 %. The receiver operating characteristic (ROC) curve for liver cancer exhibited an area under the curve (AUC) of 0.99, indicating a high degree of prediction accuracy. One-way ANOVA analysis revealed that numerous TGs were down-regulated in LC group. The results demonstrated the viability of GCM-assisted LDI MS as a valuable diagnostic tool for liver cancer.

Using surfactants as matrix for the matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF-MS) of amino acids: Sodium dodecyl sulfate (SDS) and sodium octyl sulfate (SOS)

In the present study, the applicability of two surfactants including sodium dodecyl sulfate (SDS) and sodium octyl sulfate (SOS) as the matrix for the Matrix-Assisted Laser Desorption/Ionization (MALDI) of several amino acids (phenylalanine (Phe), valine (Val), proline (Pro), alanine (Ala), and tyrosine (Tyr)) is investigated. Also, the effect of the material of the repeller plate of the ionization part of the used time of flight (TOF) mass spectrometer on the spectral patterns of the amino acids is studied. Furthermore, the recorded MALDI spectra of amino acids are compared with their corresponding direct laser desorption/ionization (direct-LDI) TOF mass spectra. It is observed that the SDS is an appropriate matrix for the Na⁺ transfer to the Phe and Val amino acids, especially, when the Ag metal is selected as the material of the repeller plate. In this case, the peaks of the [M + Na]⁺ and [M-H + 2Na]⁺ species are considerably more intense compared to when the NaF salt is used as a Na⁺ source in the LDI of these amino acids. Unlike Phe and Val, the SDS is not a good matrix for the other selected amino acids. The decrease of the carbonic chain length of the surfactant on the MALDI spectrum of Phe is investigated and it is seen that the mentioned important peaks disappeared in the presence of SOS as the matrix. The density functional theory (DFT) calculation is employed to characterize the structure of [M + Na]⁺ and [M-H + 2Na]⁺ species and determine the interaction sites of amino acids for the Na⁺ attachment. Also, the change in standard Gibbs free energy (∆G^°) of the M + Na⁺ → [M + Na]⁺ and [M + Na]⁺ + Na⁺ → [M-H + 2Na]⁺ + H⁺ reactions are calculated. Based on the values of ∆G^°, the attachment of the first Na⁺ to the amino acid takes place in the gas phase while the attachment of the second one to [M + Na]⁺ is not a favorable process in the gas phase.

Recent advances in inorganic materials for LDI-MS analysis of small molecules

In this review, various inorganic materials were summarized for the analysis of small molecules by laser desorption/ionization mass spectrometry (LDI-MS).

Study of Isobaric Interference in Quantification of Citrulline by Parallel Fragmentation Monitoring

Parallel Fragmentation Monitoring (PFM), which is an analogue of selected reaction monitoring (SRM), is a recently developed method for quantification of small molecules by MALDI-TOF/TOF mass spectrometry (MS). It is well known that isobaric interference substances can be occasionally present in complex biological samples, and affect the accuracy of measurement by SRM. Unfortunately, by design it is not possible to assess whether isobaric interference happens in a SRM analysis. In contrast, the unique design of PFM should allow quick inspection for isobaric interference and subsequent correction. In this study, using arginine as an example, interference effect of isobaric structural analogs on the quantification of citrulline by PFM was evaluated. Our results showed that the presence of arginine affected the measured concentrations of citrulline standard solutions in a concentration dependent manner. Such interference could be observed readily in the MS/MS spectra, and contributed by [arginine+H-NH3](+) fragment ion. Because of having highly similar mass, (13)C-isotope of [arginine+H-NH3](+) fragment ion overlapped with monoisotope of [citrulline+H-NH3](+) fragment ion, which was used as the report ion for quantification. However, such interference could be mathematically eliminated or minimized through estimation of the signal intensity of the (13)C-isotopic peak of [arginine+H-NH3](+) from the intensity of the corresponding monoisotopic peak according to isotope distribution of elements. Furthermore, the presence of interfering fragment ions could be avoided by optimizing MALDI ionization condition. In conclusion, isobaric interference can happen in PFM, but can be easily identified in the mass spectra and eliminated (minimized) with simple methods.

Recent advances in bacteria identification by matrix-assisted laser desorption/ionization mass spectrometry using nanomaterials as affinity probes

Identifying trace amounts of bacteria rapidly, accurately, selectively, and with high sensitivity is important to ensuring the safety of food and diagnosing infectious bacterial diseases. Microbial diseases constitute the major cause of death in many developing and developed countries of the world. The early detection of pathogenic bacteria is crucial in preventing, treating, and containing the spread of infections, and there is an urgent requirement for sensitive, specific, and accurate diagnostic tests. Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is an extremely selective and sensitive analytical tool that can be used to characterize different species of pathogenic bacteria. Various functionalized or unmodified nanomaterials can be used as affinity probes to capture and concentrate microorganisms. Recent developments in bacterial detection using nanomaterials-assisted MALDI-MS approaches are highlighted in this article. A comprehensive table listing MALDI-MS approaches for identifying pathogenic bacteria, categorized by the nanomaterials used, is provided.

Advances in MALDI mass spectrometry in clinical diagnostic applications

The concept of matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) was first reported in 1985. Since then, MALDI MS technologies have been evolving, and successfully used in genome, proteome, metabolome, and clinical diagnostic research. These technologies are high-throughput and sensitive. Emerging evidence has shown that they are not only useful in qualitative and quantitative analyses of proteins, but also of other types of biomolecules, such as DNA, glycans, and metabolites. Recently, parallel fragmentation monitoring (PFM), which is a method comparable to selected reaction monitoring, has been reported. This highlights the potentials of MALDI-TOF/TOF tandem MS in quantification of metabolites. Here we critically review the applications of the major MALDI MS technologies, including MALDI-TOF MS, MALDI-TOF/TOF MS, SALDI-TOF MS, MALDI-QqQ MS, and SELDI-TOF MS, to the discovery and quantification of disease biomarkers in biological specimens, especially those in plasma/serum specimens. Using SELDI-TOF MS as an example, the presence of systemic bias in biomarker discovery studies employing MALDI-TOF MS and its possible solutions are also discussed in this chapter. The concepts of MALDI, SALDI, SELDI, and PFM are complementary to each other. Theoretically, all these technologies can be combined, leading to the next generation of the MALDI MS technologies. Real applications of MALDI MS technologies in clinical diagnostics should be forthcoming.