Atmospheric pressure matrix-assisted laser desorption/ionisation ion trap mass spectrometry of synthetic polymers: a comparison with vacuum matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry

Atmospheric pressure matrix-assisted laser desorption/ionisation quadrupole ion trap (AP-MALDI/QIT) mass spectrometry has been investigated for the analysis of polyethylene glycol (PEG 1500) and a hyperbranched polymer (polyglycidol) in the presence of alkali-metal salts. Mass spectra of PEG 1500 obtained at atmospheric pressure showed dimetallated matrix/analyte adducts, in addition to the expected alkali-metal/PEG ions, for all matrix/alkali-metal salt combinations. The relative intensities of the desorbed ions were dependent on the matrix, the alkali-metal salt added to aid cationisation and the ion trap interface conditions [capillary temperature, in-source collisionally-induced dissociation (CID)]. These data indicate that the adducts are rapidly stabilised by collisional cooling enabling them to be transferred into the ion trap. Experiments using identical sample preparation conditions were carried out on a vacuum MALDI time-of-flight (ToF) mass spectrometer. In all cases, vacuum MALDI-ToF spectra showed only alkali-metal/PEG ions and no matrix/analyte adducts. The tandem mass spectrometry (MS/MS) capability of the ion trap has been demonstrated for a lithiated polyglycol yielding a rich fragment-ion spectrum. Analysis of the hyperbranched polymer polyglycidol by AP-MALDI/QIT reveals the characteristic ion series for these polymers as also observed under vacuum MALDI-ToF conditions.     

A new method for analysis of disulfide-containing proteins by matrix-assisted laser desorption ionization (MALDI) mass spectrometry

A simple and high-throughput method for the identification of disulfide-containing peptides utilizing peptide-matrix adducts is described. Some commonly used matrices in MALDI mass spectrometry were found to specifically react with sulfhydryl groups within peptide, thus allowing the observation of the peptide-matrix adduct ion [M+n+n′ matrix+H]⁺ or [M+n+n′ matrix+Na]⁺ (n = the number of cysteine residues, n′=1, 2,…, n) in MALDI mass spectra after chemical reduction of disulfide-linked peptides. Among several matrices tested, α-cyano-4-hydroxycinnamic acid (CHCA, molecular mass 189 Da) and α-cyano-3-hydroxycinnamic acid (3-HCCA) were found to be more effective for MALDI analysis of disulfide-containing peptides/proteins. Two reduced cysteines involved in a disulfide bridge resulted in a mass shift of 189 Da per cysteine, so the number of disulfide bonds could then be determined, while for the other matrices (sinapinic acid, ferulic acid, and caffeic acid), a similar addition reaction could not occur unless the reaction was carried out under alkaline conditions. The underlying mechanism of the reaction of the matrix addition at sulfhydryl groups is proposed, and several factors that might affect the formation of the peptide-matrix adducts were investigated. In general, this method is fast, effective, and robust to identify disulfide bonds in proteins/peptides.     

New MALDI matrices based on lithium salts for the analysis of hydrocarbons and wax esters

Lithium salts of organic aromatic acids (lithium benzoate, lithium salicylate, lithium vanillate, lithium 2,5‐dimethoxybenzoate, lithium 2,5‐dihydroxyterephthalate, lithium α‐cyano‐4‐hydroxycinnamate and lithium sinapate) were synthesized and tested as potential matrices for the matrix‐assisted laser desorption/ionization (MALDI)‐mass spectrometry analysis of hydrocarbons and wax esters. The analytes were desorbed using nitrogen laser (337.1 nm) and ionized via the attachment of a lithium cation, yielding [M + Li]⁺ adducts. The sample preparation and the experimental conditions were optimized for each matrix using stearyl behenate and n‐triacontane standards. The performance of the new matrices in terms of signal intensity and reproducibility, the mass range occupied by matrix ions and the laser power threshold were studied and compared with a previously recommended lithium 2,5‐dihydroxybenzoate matrix (LiDHB) (Cva?ka and Svato?, Rapid Commun. Mass Spectrom. 2003, 17, 2203). Several of the new matrices performed better than LiDHB. Lithium vanillate offered a 2–3 times and 7–9 times higher signal for wax esters and hydrocarbons, respectively. Also, the signal reproducibility improved substantially, making this matrix a suitable candidate for imaging applications. In addition, the diffuse reflectance spectra and solubility of the synthesized compounds were investigated and discussed with respect to the compound's ability to serve as MALDI matrices. The applicability of selected matrices was tested on natural samples of wax esters and hydrocarbons. Copyright © 2014 John Wiley & Sons, Ltd.     

Detection of Radical Adducts with Small Molecular Weights by Matrix-Assisted Laser Desorption/Ionization with Fourier Transform Mass Spectrometry

As an alternative method, matrix-assisted laser desorption/ionization with Fourier transform mass spectrometry （MALDI-FTMS） has been successfully used to detect and identify free radical adducts with small molecular weights of hydroxyl and 2-cyano-2-propyl radicals trapped with 5,5-dimethylpyrroline N-oxide （DMPO）. The detection and identification by MS/MS experiments using sustained offresonance irradiation collision-induced dissociation （SORI-CID） of [（DMPO＋·OH-·H）＋H^＋] （m/z 130.0868） and [DMPO＋2 ·CH（CH3）2CN＋H^＋] （m/z 250.1917） have demonstrated that MALDI-FTMS could be an effective method for detection and identification of free radical adducts. Other radical adducts have been also detected and identified. The approach of MALDI-FTMS is simple, fast, and sensitive which has potential for high-throughput analysis.     

Improved spectra for MALDI MSI of peptides using ammonium phosphate monobasic in MALDI matrix

MALDI mass spectrometry imaging (MSI) enables analysis of peptides along with histology. However, there are several critical steps in MALDI MSI of peptides, 1 of which is spectral quality. Suppression of MALDI matrix clusters by the aid of ammonium salts in MALDI experiments is well known. It is asserted that addition of ammonium salts dissociates potential matrix adducts and thereafter decreases matrix cluster formation. Consequently, MALDI MS sensitivity and mass accuracy increase. Up to our knowledge, a limited number of MALDI MSI studies used ammonium salts as matrix additives to suppress matrix clusters and enhance peptide signals. In this work, we investigated the effect of ammonium phosphate monobasic (AmP) as alpha‐cyano‐4‐hydroxycinnamic acid (α‐CHCA) matrix additive in MALDI MSI of peptides. Prior to MALDI MSI, the effect of varying concentrations of AmP in α‐CHCA was assessed in bovine serum albumin tryptic digests and compared with the control (α‐CHCA without AmP). Based on our data, the addition of AmP as matrix additive decreased matrix cluster formation regardless of its concentration, and specifically, 8 mM AmP and 10 mM AmP increased bovine serum albumin peptide signal intensities. In MALDI MSI of peptides, both 8 and 10 mM AmP in α‐CHCA improved peptide signals especially in the mass range of m/z 2000 to 3000. In particular, 9 peptide signals were found to have differential intensities within the tissues deposited with AmP in α‐CHCA (AUC > 0.60). To the best of our knowledge, this is the first MALDI MSI of peptides work investigating different concentrations of AmP as α‐CHCA matrix additive to enhance peptide signals in formalin‐fixed paraffin‐embedded (FFPE) tissues. Further, AmP as part of α‐CHCA matrix could enhance protein identifications and support MALDI MSI‐based proteomic approaches.     

A tandem mass spectrometric investigation of N-(3-ferrocenyl-2-naphthoyl) dipeptide ethyl esters and N-(6-ferrocenyl-2-naphthoyl) dipeptide ethyl esters

N‐(3‐Ferrocenyl‐2‐naphthoyl) dipeptide ethyl esters 1–4 and N‐(6‐ferrocenyl‐2‐naphthoyl) dipeptide ethyl esters 5–8 were prepared by coupling either 3‐ferrocenylnaphthalene‐2‐carboxylic acid or 6‐ferrocenylnaphthalene‐2‐carboxylic acid to the dipeptide ethyl esters GlyGly(OEt) (1, 5), AlaGly(OEt) (2, 6), GlyPhe(OEt) (3, 7) and GlyLeu(OEt) (4, 8), using the standard N‐(3‐dimethylaminopropyl)‐N'‐ethylcarbodiimide hydrochloride, 1‐hydroxybenzotriazole protocol. Electrospray ionization mass spectrometry (ESI‐MS) and laser desorption ionization mass spectrometry (LDI‐MS) were employed in conjunction with tandem mass spectrometry in the analysis of N‐(3‐ferrocenyl‐2‐naphthoyl) dipeptide ethyl esters 1–4 and N‐(6‐ferrocenyl‐2‐naphthoyl) dipeptide ethyl esters 5–8. Radical cations, [M]^+? and [M + H]⁺ species were both observed in the mass spectra. Intense sodium [M + Na]⁺ and potassium [M + K]⁺ adducts were also present. An important diagnostic ion at m/z [M–65]⁺ was observed in both the MS and MS/MS spectra of the N‐(3‐ferrocenyl‐2‐naphthoyl) dipeptide derivatives. Sequence‐specific ions were generally not observed in the MS/MS spectra of the N‐(3‐ferrocenyl‐2‐naphthoyl) series due to formation of the diagnostic [M–65]⁺ ion. Sequence‐specific ions were observed in the MS/MS spectra of the N‐(6‐ferrocenyl‐2‐naphthoyl) dipeptide esters with charge retention on the derivatized N‐terminal of the dipeptide. Both series of compounds could be successfully analyzed by MALDI without the use of a matrix (LDI). Copyright © 2011 John Wiley & Sons, Ltd.     

Disappearance of interfering alkali-metal adducted peaks from matrix-assisted laser desorption/ionization mass spectra of peptides with serine addition to alpha-cyano-4-hydroxycinnamic acid matrix

It has been described that ion yield in both positive- and negative-ion matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) of peptides is often inhibited by trace amounts of alkali metals and that the MALDI mass spectra are contaminated by the interfering peaks originating from traces of alkali metals, even when sample preparation is carefully performed. Addition of serine to the commonly used MALDI matrix alpha-cyano-4-hydroxycinnamic acid (CHCA) significantly improved and enhanced the signals of both protonated and deprotonated peptides, [M+H](+) and [M-H](-). The addition of serine to CHCA matrix eliminated the alkali-metal ion adducts, [M+Na](+) and [M+K](+), and the CHCA cluster ions from the mass spectra. Serine and serinephosphate as additives to CHCA enhanced and improved the formation of molecular-related ions of phosphopeptides in negative-ion MALDI mass spectra.     

Novel method for [M + Cu]+ ion formation by matrix-assisted laser desorption ionization

Direct deposition of a MALDI sample onto a copper sample stage and irradiation with UV light (337 nm) produces copper adduct ions of both the matrix and analyte molecules. This technique for introducing Cu⁺ into the gas-phase avoids suppression of ion signal that accompanies addition of metal salts to the sample solution. We observe good correlation between the number of basic residues in peptides and the number of Cu⁺ ions that add to the peptide. For example, the peptide KRQHPG contains three basic residues and forms ions with up to three Cu⁺ adducts. Postsource decay experiments demonstrate that for arginine containing peptides, arginine anchors the Cu⁺ ion. That is, all metastable ions contain the arginine complexed to Cu⁺ and the only immonium ion observed is that of arginine–Cu⁺. In addition, preliminary calculations indicate that guanidine has the highest Cu⁺ ion affinity followed by histidine.     

Fast atom bombardment mass spectrometry of alkali metal fluorides and chlorides; a comparison of the solid salt and glycerol matrix spectra

The Fast Atom Bombardment (FAB) mass spectra of the alkali metal chlorides (Na, K, Cs) and fluorides (Na, K, Rb, Cs) were obtained from solids and a glycerol matrix, using a fast atom bombardment source. From solids the fluorides exhibited an ion abundance enhancement of the well-known [M(MF)₄]⁺ cluster, which decreased with increasing cation size. A gradual decrease in the n=4 enhancement was observed as the salt was diluted with glycerol. In the chlorides only sodium chloride showed the n=4 relative enhancement. The mass spectra of the salts from a glycerol matrix at molar ratios of 1:1 to 1:10 showed that the spectra of the 1:1 solutions were similar to those from the solids, while glycerol adducts were found to increase with increasing glycerol concentration. A [M(MX)_n(gly)]⁺ species that featured successive losses of HX was observed. It has not been established whether HX losses take place in solution, in the surface/vacuum interface and/or whether gas phase reactions might be responsible for the observation of the [M(MX)_n(gly)–y HX]^? species in the mass spectra of the MX/glycerol system.     

Negative ion production from peptides and proteins by matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry

Negative ion production from peptides and proteins was investigated by matrix‐assisted laser desorption/ionization time‐of‐flight (MALDI‐TOF) mass spectrometry. Although most research on peptide and protein identification with ionization by MALDI has involved the detection of positive ions, for some acidic peptides protonated molecules are not easily formed because the side chains of acidic residues are more likely to lose a proton and form a deprotonated species. After investigating more than 30 peptides and proteins in both positive and negative ion modes, [M–H]⁻ ions were detected in the negative ion mode for all peptides and proteins although the matrix used was 2,5‐dihydroxybenzoic acid (DHB), which is a good proton donor and favors the positive ion mode production of [M+H]⁺ ions. Even for highly basic peptides without an acidic site, such as myosin kinase inhibiting peptide and substance P, good negative ion signals were observed. Conversely, gastrin I (1‐14), a peptide without a highly basic site, will form positive ions. In addition, spectra obtained in the negative ion mode are usually cleaner due to absence of alkali metal adducts. This can be useful during precursor ion isolation for MS/MS studies. Copyright © 2008 John Wiley & Sons, Ltd.     

Validation of a fast screening method for the detection of cocaine in hair by MALDI-MS

The sensitivity and specificity of a novel method of screening for cocaine in hair, based on matrix-assisted laser desorption/ionisation (MALDI) mass spectrometry (MS), have been evaluated. The method entails a rapid extraction procedure consisting of shaking 2.5 mg pulverised hair at high frequency in the presence of an acidic solution (160 μL of water, 20 μL of acetonitrile and 20 μL of 1 M trifluoroacetic acid) and a stainless-steel bullet. Following centrifugation, the supernatant is dried under a nitrogen stream, and the residue is reconstituted in 10 μL of methanol/trifluoroacetic acid (7:3; v/v). One microlitre of the extract is deposed on a MALDI sample holder previously scrubbed with graphite; an α-cyano-4-hydroxycinnamic acid (matrix) solution is electrosprayed over the dried sample surface to achieve a uniform distribution of matrix crystals. The identification of cocaine is obtained by post-source decay experiments performed on its MH⁺ ion (m/z 304), with a limit of detection of 0.1 ng/mg of cocaine. A total of 304 hair samples were analysed in parallel by MALDI-MS and a reference gas chromatography-MS method. The obtained results demonstrate specificity and sensitivity of 100% for MALDI-MS. Evidence of cocaine presence was easily obtained even when hair samples exhibiting particularly low cocaine levels (<0.5 ng/mg) were analysed.     

Electrospray ionization and atmospheric pressure matrix‐assisted laser desorption/ionization mass spectrometry of antioxidants applied in lubricants

The aim of this study was to investigate the utility of ion trap mass spectrometry (ITMS) in combination with the two desorption/ionization methods, electrospray (ESI) and atmospheric pressure matrix‐assisted laser desorption/ionization (AP‐MALDI), for the detection of antioxidants which are applied in lubricants. These experiments should form the base for future investigations of antioxidants in tribologically formed thin layers on the surface of frictional systems. Seventeen different antioxidants were selected out of the group of hindered phenolic and aromatic aminic compounds. Practically all antioxidants could be characterized by positive ion ESI‐ and AP‐MALDI‐ITMS, forming various types/species of molecular ions (e.g. [M]^{+ .} , [M+H]⁺, [M+Na]⁺ or [M–2H+H]⁺). A few compounds could be analyzed by negative ion ESI‐MS, too, but none by negative ion AP‐MALDI‐MS. The influence of target materials in AP‐MALDI‐MS (gold‐ and titanium nitride (TiN)‐covered stainless steel, micro‐diamond‐covered hard metal, hand‐polished and sand‐blasted stainless steel targets) with respect to the molecular ion intensity and type of molecular ion of two selected antioxidants was evaluated. The surface properties are of particular interest because in friction tests different materials with different surface characteristics are used. However, the MS results indicate that optimal target surfaces have to be found for individual antioxidants in AP‐MALDI‐MS but in general smooth surfaces were superior to rough surfaces. Finally the gold‐covered stainless steel MALDI target provided the best mass spectra and was selected for all the antioxidants investigated. Copyright © 2009 John Wiley & Sons, Ltd.     

Role of the UV absorber as a matrix in matrix‐assisted laser desorption/ionization mass spectrometric analysis of a mixture of a UV absorber and a stabilizer

A mixture of a UV absorber (Tinuvin 234 or Tinuvin 329) and a UV stabilizer (Tinuvin 770) was analyzed using matrix‐assisted laser desorption/ionization mass spectrometry (MALDI‐MS) without any matrix. Fragmentation patterns of the UV absorbers and stabilizer were also investigated. The mass spectra showed the [M+H]⁺ ions and some fragment ions. Tinuvin 234, Tinuvin 329, and Tinuvin 770 generated three (m/z 119, 370, 432), one (m/z 252), and two (m/z 124 and 140) fragment ions, repectively. These fragment ions can be used to identify the chemical structures of the UV absorbers and stabilizer. Since the UV absorber performed a role as the matrix, the ion abundance of the UV stabilizer was enhanced by mixing with the UV absorber. When organic materials extracted from polypropylene (PP) containing the UV absorber and stabilizer were directly analyzed using MALDI‐MS without any matrix, the protonated molecule of the UV stabilizer was detected in abundance but the product ions of the UV absorber were not observed. When 2,5‐dihydroxybenzoic acid was used as a matrix, the protonated molecule of the UV absorber was observed. Copyright © 2010 John Wiley & Sons, Ltd.     

Fast atom bombardment mass spectra of some alkylammonium 4-toluenesulfonate salts

The fast atom bombardment positive ion mass spectra of low molecular weight alkylammonium 4-toluenesulfonate salts are dominated by cluster ions of the type [(AB)_nA]⁺ or [(AB)_nH]⁺, where A is a cation, B an anion, n an integer from 0 to 4. With one exception the base peak is the cation A⁺.     

Dehydrogenation and dehalogenation of amines in MALDI‐TOF MS investigated by isotopic labeling

Secondary and tertiary amines have been reported to form [M–H]⁺ that correspond to dehydrogenation in matrix‐assisted laser desorption ionization time of flight mass spectrometry (MALDI‐TOF MS). In this investigation, we studied the dehydrogenation of amines in MALDI‐TOF MS by isotopic labeling. Aliphatic amines were labeled with deuterium on the methylene of an N‐benzyl group, which resulted in the formation of [M–D]⁺ and [M–H]⁺ ions by dedeuteration and dehydrogenation, respectively. This method revealed the proton that was removed. The spectra of most tertiary amines with an N‐benzyl group showed high‐intensity [M–D]⁺ and [M–H]⁺ ion peaks, whereas those of secondary amines showed low‐intensity ion peaks. Ratios between the peak intensities of [M–D]⁺ and [M–H]⁺ greater than 1 suggested chemoselective dehydrogenation at the N‐benzyl groups. The presence of an electron donor group on the N‐benzyl groups enhanced the selectivity. The dehalogenation of amines with an N‐(4‐halobenzyl) group was also observed alongside dehydrogenation. The amino ions from dehalogenation can undergo second dehydrogenation. These results provide the first direct evidence about the position at which dehydrogenation of an amine occurs and the first example of dehalogenation of haloaromatic compounds in MALDI‐TOF MS. These results should be helpful in the structural identification and elucidation of synthetic and natural molecules. Copyright © 2013 John Wiley & Sons, Ltd.     

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.     

MALDI‐based intact spore mass spectrometry of downy and powdery mildews

Fast and easy identification of fungal phytopathogens is of great importance in agriculture. In this context, matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry (MALDI‐TOF MS) has emerged as a powerful tool for analyzing microorganisms. This study deals with a methodology for MALDI‐TOF MS‐based identification of downy and powdery mildews representing obligate biotrophic parasites of crop plants. Experimental approaches for the MS analyses were optimized using Bremia lactucae, cause of lettuce downy mildew, and Oidium neolycopersici, cause of tomato powdery mildew. This involved determining a suitable concentration of spores in the sample, selection of a proper MALDI matrix, looking for the optimal solvent composition, and evaluation of different sample preparation methods. Furthermore, using different MALDI target materials and surfaces (stainless steel vs polymer‐based) and applying various conditions for sample exposure to the acidic MALDI matrix system were investigated. The dried droplet method involving solvent evaporation at room temperature was found to be the most suitable for the deposition of spores and MALDI matrix on the target and the subsequent crystallization. The concentration of spore suspension was optimal between 2 and 5 × 10⁹ spores per ml. The best peptide/protein profiles (in terms of signal‐to‐noise ratio and number of peaks) were obtained by combining ferulic and sinapinic acids as a mixed MALDI matrix. A pretreatment of the spore cell wall with hydrolases was successfully introduced prior to MS measurements to obtain more pronounced signals. Finally, a novel procedure was developed for direct mass spectra acquisition from infected plant leaves. Copyright © 2012 John Wiley & Sons, Ltd.     

Characterization of natural wax esters by MALDI-TOF mass spectrometry

The applicability of matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS) to the analysis of wax esters (WEs) was investigated. A series of metal salts of 2,5-dihydroxybenzoic acid (DHB) was synthesized and tested as possible matrices. Alkali metal (Li, Na, K, Rb, Cs) and transition metal (Cu, Ag) salts were studied. The matrix properties were evaluated, including solubility in organic solvents, threshold laser power that should be applied for successful desorption/ionization of WEs, the nature of the matrix ions and the mass range occupied by them, and the complexity of the isotope clusters for individual metals. Lithium salt of dihydroxybenzoic acid (LiDHB) performed the best and matrices with purified lithium isotopes ((6)LiDHB or (7)LiDHB) were recommended for WEs. Three sample preparation procedures were compared: (1) mixing the sample and matrix in a glass vial and deposition of the mixture on a MALDI plate (Mix), (2) deposition of sample followed by deposition of matrix (Sa/Ma), and (3) deposition of matrix followed by deposition of sample (Ma/Sa). Morphology of the samples was studied by scanning electron microscopy. The best sample preparation technique was Ma/Sa with the optimum sample to matrix molar ratio 1 : 100. Detection limit was in the low picomolar range. The relative response of WEs decreased with their molecular weight, and minor differences between signals of saturated and monounsaturated WEs were observed. MALDI spectra of WEs showed molecular adducts with lithium [M + Li](+). Fragments observed in postsource decay (PSD) spectra were related to the acidic part of WEs [RCOOH + Li](+) and they were used for structure assignment. MALDI with LiDHB was used for several samples of natural origin, including insect and plant WEs. A good agreement with GC/MS data was achieved. Moreover, MALDI allowed higher WEs to be analyzed, up to 64 carbon atoms in Ginkgo biloba leaves extract.     

Comparison of ionization behaviors of ring and linear carbohydrates in MALDI-TOFMS

Ionization efficiencies of cyclodextrins and their linear compounds in matrix-assisted laser desorption and ionisation (MALDI) analysis were compared, and differences in the ionization efficiencies of α- and β-cyclodextrins were also studied. The mass spectra showed a series of the [M+cation]⁺ ions but not the [M+H]⁺ ions. Alkali metal salts of Li⁺, Na⁺, K⁺, and Cs⁺ were used as the cationizing agents to enhance the ionization efficiency. Relative ion intensities of the ring compounds (α- and β-cyclodextrins) were much larger than those of the linear ones (maltohexaose and maltoheptaose), and the difference showed an increasing trend with the size of the alkali metal cation. β-Cyclodextrin had higher ionization efficiency than α-cyclodextrin and the difference increased by increasing the size of the alkali metal cation. It was also found that the ionization efficiency was affected by the counter anion of the salt. The higher ionization efficiencies of cyclodextrins were explained with the number of coordination sites and the binding energies.     

Generation of CsI cluster ions for mass calibration in matrix-assisted laser desorption/ionization mass spectrometry

A simple method was developed for the generation of cesium iodide (CsI) cluster ions up to m/z over 20,000 in matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS). Calibration ions in both positive and negative ion modes can readily be generated from a single MALDI spot of CsI₃ with 2-[(2E)-3-(4-tert-butylphenyl)-2-methylprop-2-enylidene] malononitrile (DCTB) matrix. The major cluster ion series observed in the positive ion mode is [(CsI)_nCs]⁺, and in the negative ion mode is [(CsI)_nI]⁻. In both cluster series, ions spread evenly every 259.81 units. The easy method described here for the production of CsI cluster ions should be useful for MALDI MS calibrations.