Elemental sulfur as a versatile low-mass-range calibration standard for laser desorption ionization mass spectrometry

Hg-Hg bonding and its influence on the stability of (HgS)n clusters

Pulsed laser ablation of a HgS(s) precursor shows the formation of small cluster ions, (HgS)n=2-4+, together with HgSn=1-8± and [(HgS)n + Sm]±. The computed structure, atomization energy, and HOMO-LUMO gap energy values of the lowest energy ring singlet show stable (HgS)n=2-8. However, the computed bond conductance of the Hg-Hg bond in (HgS)n shows a high value for (HgS)n=2-4 (ξ = 1.072-0.122), whereas it is low for (HgS)n=5-8 (ξ = 0.039-0.006) and decreases significantly as the ring expands, indicating that (HgS)n≥5 is unstable. It evidences that the weak chemical bonding between Hg2+-Hg2+ closed shell (5d10-5d10) electrons plays a significant role in the stability of ring (HgS)n=2-4. Thus, it validates the experimental observation of stable cluster ions up to (HgS)4+. In contrast, the low energy chain triplet (HgS)n=2-8 shows a progressive increase in stability and bond conductance with chain length, indicating sustained mercurophilic interactions in long chain clusters like its crystal structure. Furthermore, the lowest/low energy isomers of HgSn=1-8 have been computed for their energetics, HOMO-LUMO gaps, and electron affinity using DFT-B3LYP/PBE0 methods.

Insights into the sulfur metabolism of Chlorobaculum tepidum by label-free quantitative proteomics

Chlorobaculum tepidum is an anaerobic green sulfur bacterium which oxidizes sulfide, elemental sulfur, and thiosulfate for photosynthetic growth. It can also oxidize sulfide to produce extracellular S0 globules, which can be further oxidized to sulfate and used as an electron donor. Here, we performed label-free quantitative proteomics on total cell lysates prepared from different metabolic states, including a sulfur production state (10 h post-incubation [PI]), the beginning of sulfur consumption (20 h PI), and the end of sulfur consumption (40 h PI), respectively. We observed an increased abundance of the sulfide:quinone oxidoreductase (Sqr) proteins in 10 h PI indicating a sulfur production state. The periplasmic thiosulfate-oxidizing Sox enzymes and the dissimilatory sulfite reductase (Dsr) subunits showed an increased abundance in 20 h PI, corresponding to the sulfur-consuming state. In addition, we found that the abundance of the heterodisulfide-reductase and the sulfhydrogenase operons was influenced by electron donor availability and may be associated with sulfur metabolism. Further, we isolated and analyzed the extracellular sulfur globules in the different metabolic states to study their morphology and the sulfur cluster composition, yielding 58 previously uncharacterized proteins in purified globules. Our results show that C. tepidum regulates the cellular levels of enzymes involved in sulfur metabolism in response to the availability of reduced sulfur compounds.

Comparison of Small Biomolecule Ionization and Fragmentation in Pseudomonas aeruginosa Using Common MALDI Matrices

Different bacterial cell surface associated biomolecules can be analyzed by matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry and coupled with collision induced dissociation (CID) for identification. Pseudomonas aeruginosa is an opportunistic, Gram-negative bacterium that causes acute or chronic biofilm infections. Cells of P. aeruginosa communicate through a system of signaling biomolecules known as quorum sensing (QS). The QS system can result in the production of biosurfactant rhamnolipids known to associate and alter the cellular membrane. MALDI-TOF utilizes a variety of matrices that can interact differently with biomolecules for selective ionization. We examined six common matrices to determine the optimal matrix specific to different molecule classes in P. aeruginosa associated with cell surfaces. Three major molecule classes (quinolones, rhamnolipids, and phospholipids) were observed to ionize selectively with the different matrices tested. Sodiated and protonated adducts differed between matrices utilized in our study. Isobaric ions were identified as different molecule classes depending on the matrix used. We highlight the role of matrix selection in MALDI-TOF identification of molecules within a complex biological mixture.

Reaction of 1,3-dimethylimidazolium dimethylphosphate with elemental sulfur

By the methods of MALDI and mass spectroscopy with the detection of positively and negatively charged ions, it was found that the reaction of elemental sulfur and 1,3-dimethylimidazolium dimethylphosphate is accompanied by the opening of the S8 ring. 1H, 13C, 15N and 31P NMR spectroscopy showed that the interaction of S8 and 1,3-dimethylimidazolium dimethylphosphate proceeds exclusively on the oxygen atom of the dimethylphosphate anion carrying a negative charge. Kohn-Sham calculations at B3LYP/STO-3G, B3LYP/6-31G* and B3LYP/6-311G* levels of theory confirmed that the reaction of S8 with dimethylphosphate anion is possible.

Gold Nanoparticles (AuNPs) as Reactive Matrix for Detection of Trace Levels of HCN in Air by Laser Desorption/Ionization Mass Spectrometry (LDI-MS)

Under direct laser desorption/ionization mass spectrometric conditions, the irradiation of target spots made of gold nanoparticle residues generates a series of peaks at m/z 197, 394, 591… representing Au_n^- ions (n = 1-3). In contrast, spectra recorded from gold nanoparticles directly mixed with an alkali cyanide exhibited an additional peak at m/z 249, indicating an abundant generation of gaseous [Au(CN)₂]^- ions upon irradiation. The relative intensity of the m/z 249 peak surged when the amount of cyanide in the mixture was increased. Most remarkably, a peak at m/z 249 was observed even from neat AuNPs upon irradiation, if a nearby spot, which was not irradiated, happened to bear a cyanide sample. We postulated that traces of HCN emanating from the headspace of aqueous cyanide solution during the sample-plate preparation is sufficient to convert gold to AuCN, which is subsequently detected as [Au(CN)₂]^-. Further experiments demonstrated that the relative intensity of the m/z 249 peak diminishes exponentially as the AuNP spot becomes more distant from the putative HCN source. Eventually, the method was developed as an efficient procedure to detect HCN or alkali cyanides. Using KCN, the detection limits were determined to be below 10 pg of CN^- per spot. The method also demonstrated that, upon crushing, the seeds or roots of certain fruits and vegetables such as apple, peach, radish, and cassava, but not carrot, release HCN in amounts detectable by this method. Graphical Abstract.

Brimstone chemistry under laser light assists mass spectrometric detection and imaging the distribution of arsenic in minerals

Singly charged As2n+1 ion clusters (n = 2-11) were generated from elemental arsenic by negative-ion laser-ablation mass spectrometry. The overall abundance of the gaseous As ions generated upon laser irradiation was enhanced nearly a hundred times when As-bearing samples were admixed with sulfur. However, sulfur does not act purely as an inert matrix: irradiating arsenic-sulfur mixtures revealed a novel pathway to generate and detect a series of [AsSn]- clusters (n = 2-6). Intriguingly, the spectra recorded from As2O3, NaAsO2, Na3AsO4, cacodylic acid and 3-amino-4-hydroxyphenylarsonic acid together with sulfur as the matrix were remarkably similar to that acquired from an elemental arsenic and sulfur mixture. This result indicated that arsenic sulfide cluster-ions are generated directly from arsenic compounds by a hitherto unknown pathway. The mechanism of elemental sulfur extracting chemically bound arsenic from compounds and forming [AsSn]- clusters is enigmatic; however, this discovery has a practical value as a general detection method for arsenic compounds. For example, the method was employed for the detection of As in its minerals, and for the imaging of arsenic distribution in minerals such as domeykite. LDI-MS data recorded from a latent image imprinted on a piece of paper from a flat mineral surface, and wetting the paper with a solution of sulfur, enabled the localization of arsenic in the mineral. The distribution of As was visualized as false-color images by extracting from acquired data the relative intensities of m/z 139 (AsS2-) and m/z 171 (AsS3-) ions.

Laser Desorption Ionization of As2Ch3 (Ch = S, Se, and Te) Chalcogenides Using Quadrupole Ion Trap Time-of-Flight Mass Spectrometry: A Comparative Study

Laser desorption ionization using time-of-flight mass spectrometer afforded with quadrupole ion trap was used to study As₂Ch₃ (Ch = S, Se, and Te) bulk chalcogenide materials. The main goal of the study is the identification of species present in the plasma originating from the interaction of laser pulses with solid state material. The generated clusters in both positive and negative ion mode are identified as 10 unary (S _p^+/- and As _m^+/- ) and 34 binary (As _m S _p^+/- ) species for As₂S₃ glass, 2 unary (Se _q^+/- ) and 26 binary (As _m Se _q^+/- ) species for As₂Se₃ glass, 7 unary (Te _r^+/- ) and 23 binary (As _m Te _r^+/- ) species for As₂Te₃ material. The fragmentation of chalcogenide materials was diminished using some polymers and in this way 45 new, higher mass clusters have been detected. This novel approach opens a new possibility for laser desorption ionization mass spectrometry analysis of chalcogenides as well as other materials. Graphical abstract ᅟ.

Method development for the analysis of resinous materials with MALDI-FT-ICR-MS: novel internal standards and a new matrix material for negative ion mode

Matrix-assisted laser desorption/ionization (MALDI) is a mass spectrometry (MS) ionization technique suitable for a wide variety of sample types including highly complex ones such as natural resinous materials. Coupled with Fourier transform ion cyclotron resonance (FT-ICR) mass analyser, which provides mass spectra with high resolution and accuracy, the method gives a wealth of information about the composition of the sample. One of the key aspects in MALDI-MS is the right choice of matrix compound. We have previously demonstrated that 2,5-dihydroxybenzoic acid is suitable for the positive ion mode analysis of resinous samples. However, 2,5-dihydroxybenzoic acid was found to be unsuitable for the analysis of these samples in the negative ion mode. The second problem addressed was the limited choice of calibration standards offering a flexible selection of m/z values under m/z 1000. This study presents a modified MALDI-FT-ICR-MS method for the analysis of resinous materials, which incorporates a novel matrix compound, 2-aminoacridine for the negative ion mode analysis and extends the selection of internal standards with m/z <1000 for both positive (15 different phosphazenium cations) and negative (anions of four fluorine-rich sulpho-compounds) ion mode. The novel internal calibration compounds and matrix material were tested for the analysis of various natural resins and real-life varnish samples taken from cultural heritage objects. Copyright © 2017 John Wiley & Sons, Ltd.

Detection and imaging of chrome yellow (lead chromate) in latent prints, solid residues, and minerals by laser-desorption/ionization mass spectrometry (LDI-MS)

In the past, chrome yellow (lead chromate, PbCrO₄ ), a bright orange-red substance, has been widely used as an inorganic pigment in the production of paints, coatings, and plastics. Herein, we demonstrate that laser desorption/ionization mass spectrometry (LDI-MS) is a powerful tool for the detection of lead chromate in solid residues. In fact, lead chromate in trace amounts is easily detectable by LDI-MS even from residues left as latent prints. For example, a latent print obtained by stamping the exposed laterally cut surface of a pencil over 50 years old on an acetonitrile-moistened paper, was successfully imaged for both lead and chromate using a Synapt G2 HDMS mass spectrometer. After rastering the print with a 355 nm laser beam and recording positive- and negative-ion mass spectra over the range m/z 50-1200, we generated false-color 'heat maps' (single-ion images) for ²⁰⁸ Pb^+• (m/z 207.98) and Cr₂ O₆^-• (m/z 199.85). The heat maps matched closely with the faint visual image of the pencil imprint. Moreover, our results confirmed that lead chromate was used in the pigment coatings of old pencils. Evidently, LDI-MS imaging is an efficient procedure to survey for the presence of lead and chromate in minerals and other materials. Copyright © 2017 John Wiley & Sons, Ltd.

Enhancement of laser desorption ionization mass spectrometric signals of cesium iodide by elemental sulfur

RATIONALE

The utility of elemental sulfur as a matrix for inorganic salts such as CsI, AgI, and KI was investigated because the conventional matrices deployed to generate gaseous ions from organic compounds, upon irradiation with a laser beam, are not suitable for inorganic salts.

METHODS

Sulfur and inorganic salts were admixed and irradiated with a 337-nm UV laser. Laser desorption ionization (LDI) mass spectra were recorded in both positive and negative ion mode on a time-of-flight mass spectrometer.

RESULTS

The positive ion laser desorption ionization mass spectrum of CsI showed peaks at m/z 133, 393, etc. for [(CsI)(n)Cs](+) ions. Similarly, negative ion spectra showed peaks at m/z 387, 647, etc. for [(CsI)(n)I](-) ions. However, for n >2 ion clusters, the intensities of peaks were negligibly small in both ionization modes. In contrast, spectra recorded from CsI admixed with elemental sulfur showed peaks up to n = 13 for (CsI)(n)Cs(+), and n = 9 for (CsI)(n)I(-). A similar enhancement of ion abundances by sulfur was observed for the cluster ions generated from KI and AgI.

CONCLUSIONS

The dramatic increase in intensities of the higher-mass CsI cluster peaks suggests that sulfur acts as a laser-absorbing matrix for inorganic salts far superior to conventional matrices such as 2,5-dihydroxybenzoic acid and α-cyano-4-hydroxycinnamic acid.