2,5-Dihydroxybenzoic acid butylamine and other ionic liquid matrixes for enhanced MALDI-MS analysis of biomolecules

Development of nanomaterial enabling highly sensitive surface-assisted laser desorption/ionization mass spectrometry peptide analysis

RATIONALE

Surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) is an approach derived from matrix-assisted laser desorption/ionization (MALDI)-MS which overcomes the drawbacks associated with the use of organic matrices required to co-crystallize with the analytes. Indeed, nanomaterials commonly used in SALDI-MS as inert surfaces to promote desorption/ionization (D/I) ensure straightforward direct deposition of samples while providing mass spectra with ions only related to the compound of interest. The objective of this study was to develop a novel SALDI-MS approach based on steel plates that are surfaces very rapidly and easily tuned to perform the most efficient peptide detection as possible. To compare the SALDI efficacy of such metal substrates, D/I efficiency and deposit homogeneity were evaluated according to steel plate fabrication processes.

METHODS

The studied surfaces were nanostructured steel plates that were chemically modified by perfluorosilane and textured according to different frequencies and laser writing powers. The capacity of each tested 100 surfaces was demonstrated by comparative analyses of a mixture of standard peptides (m/z 600-3000) performed with a MALDI-TOF instrument enabling MALDI, SALDI and imaging experiments.

RESULTS

A peptide mix was used to screen the different surfaces depending on their D/I efficiency and their ability to ensure homogeneous deposit of the samples. For that purpose, deposition homogeneity was visualized owing to reconstructed ionic images from all protonated or sodiated ions of the 10 peptides constituting the standard mix.

CONCLUSIONS

Seven surfaces were then selected satisfying the required D/I efficiency and deposit homogeneity criteria. Results obtained with these optimal surfaces were then compared with those recorded by MALDI-MS analyses used as references.

Analysis of Intact Glycoproteins by Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) can be regarded as a key tool to rapidly obtain molecular mass information of intact glycoproteins in glycoproteomic studies and quality control of recombinant biopharmaceuticals. However, MALDI-TOF MS of these glycosylated compounds is a tricky task due to its low ionization efficiency and fragmentation of labile groups such as sialic acids.Here, we offer the reader a practical overview of the available methodologies for the confident analysis of intact glycoproteins with different glycosylation degree by MALDI-TOF MS. The three proposed methods fulfil the requirements of reproducibility and low extent of glycan fragmentation required to successfully analyze intact glycoproteins.

MALDI Matrices for the Analysis of Low Molecular Weight Compounds: Rational Design, Challenges and Perspectives

The analysis of low molecular weight (LMW) compounds is of great interest to detect small pharmaceutical drugs rapidly and sensitively, or to trace and understand metabolic pathways. Matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) plays a central role in the analysis of high molecular weight (bio)molecules. However, its application for LMW compounds is restricted by spectral interferences in the low m/z region, which are produced by conventional organic matrices. Several strategies regarding sample preparation have been investigated to overcome this problem. A different rationale is centred on developing new matrices which not only meet the fundamental requirements of good absorption and high ionization efficiency, but are also vacuum stable and "MALDI silent", i. e., do not give matrix-related signals in the LMW area. This review gives an overview on the rational design strategies used to develop matrix systems for the analysis of LMW compounds, focusing on (i) the modification of well-known matrices, (ii) the search for high molecular weight matrices, (iii) the development of binary, hybrid and nanomaterial-based matrices, (iv) the advance of reactive matrices and (v) the progress made regarding matrices for negative or dual polarity mode.

Rapid quality control of medicine and food dual purpose plant polysaccharides by matrix assisted laser desorption/ionization mass spectrometry

With their multiple biological activities and health benefit effects, polysaccharides from medicine and food dual purpose plants (MFDPPPs) have been extensively applied in many fields, including in medical treatments, stock farming, and cosmetics. However, to date, quality issues of MFDPPPs and technologies for the analysis of polysaccharides have posed challenges to chemists. Reported herein is a rapid and high-throughput quality control method for analyzing MFDPPPs, based on matrix assisted laser desorption/ionization mass spectrometry (MALDI-MS). For the analysis of illegally added and doped substances, ferroferric oxide nanoparticles were employed as the MALDI matrix to avoid small molecule interference. Qualitatively, high sensitivity was obtained for both illegal drugs and glucose. Quantitatively, the best linear response (R2 > 0.99) was attained in the concentration range from 0.005 to 1 mg mL-1 for glucose. For the analysis of polysaccharides, 2,5-dihydroxybenzoic acid/N-methylaniline was employed as the MALDI matrix to increase the detection sensitivity and mass range coverage. Furthermore, the established method was successfully applied to the analysis of supplements from Astragalus polysaccharides and Lentinan real samples, showing its potential in quality control for MFDPPPs.

Analysis of High-Molecular-Weight Polyrotaxanes by MALDI-TOF-MS Using 3-Aminoquinoline-Based Ionic Liquid Matrix

Polyrotaxane (PR) is a necklace-like supramolecule composed of cyclic components, such as cyclodextrin (CD), and a threading polymer capped with bulky end groups. PR exhibits peculiar mechanical properties attributed to the intermolecular cross-links with CD. Various CD molecules threaded on a linear PEG chain are often modified with chemical groups to add specific physicochemical properties. In general, the stoichiometry between CD and the PEG chain is a significant parameter that defines the unique physical properties of CD-based polyrotaxane (CD-PR). To date, mass spectrometry (MS) has been applied to investigate the molecular distribution of CD-PR, modifications of CD, and the threaded ratio of CD. However, only molecular weights (MWs) up to several 10s of kDa can be subjected to such analysis, whereas the MW of CD-PR used as industrial materials is much greater. Herein, we applied two ionic liquid matrices composed of 3-aminoquinoline and a high mass detector to analyze PRs using MALDI-TOF-MS. High to very high MW PRs in the range of 90-700 kDa were successfully analyzed using this method. The threaded ratio of CD was estimated from a single MW of CD, PEG, and PR. The ratios obtained were consistent with that obtained using ¹H NMR. Furthermore, a single-stranded form of PR in γ-cyclodextrin threaded PR (γCD-PR) was clearly distinguished from a double-stranded form, which is only possible in γCD -PR because of its large host cavity.

New Strategy to Preserve Phosphate by Ionic Liquid Matrices in Matrix-Assisted Laser Desorption/Ionization: A Case of Adenosine Nucleotides

Adenosine -5′-triphosphate (ATP) plays a valuable role in metabolic activity to produce adequate energy in a biosystem. A high ATP/AMP ratio has a correlation with diabetes that induces suppression of AMP-activated protein kinase (AMPK). Matrix-assisted laser desorption/ionization (MALDI)–mass spectrometry (MS) has outstanding potential in determining the ratio of several types of adenosine phosphates in a sample to rapidly understand the primary energy transfer in metabolism. Although MALDI is viewed as a soft ionization technique for MS analysis, excess photon energy might crack the phosphate bonds leading to misinterpretation of the ATP level. In this work, ionic liquid matrices (ILMs) were employed to reduce fragmentation and increase the detection efficiency during the MALDI process. This study demonstrated for the first time that 2,5-dihydroxybenzoic acid pyridine (DHBP) is one of the most effective matrices for further quantitative analysis of adenosine nucleotides. This systematic screening of ILMs also enhances the fundamental understanding of MALDI.

Soft Matrix-Assisted Laser Desorption/Ionization for Labile Glycoconjugates

Since its introduction, matrix-assisted laser desorption/ionization (MALDI) has been widely used for the mass analysis of biomolecules. The "soft ionization" of MALDI enables accurate mass determination of intact biomolecules. However, the ionization and desorption processes of MALDI are not adequately soft as many labile biomolecules, such as glycoconjugates containing sialic acid or the sulfate functional group, easily dissociate into fragments and sometimes, no intact molecules are observed. In this study, we compared the conventional matrix of MALDI, namely 2,5-dihydroxybenzoic acid, to various soft matrices of MALDI-specifically, 5-methoxysalicylic acid, diamond nanoparticle trilayers, HgTe nanostructures, ionic liquid, and droplets of frozen solutions-by using three labile glycoconjugates as analytes: gangliosides, heparin, and pullulan. We demonstrated that droplets of frozen solution are the softest matrices for gangliosides and heparin. In particular, droplets of frozen solution do not generate fragments for gangliosides and can be used to determine the relative abundance of various gangliosides, whereas ionic liquid 2,5-dihydroxybenzoic acid butylamine is the most suitable matrix for pullulan mass analysis. Graphical Abstract.

Liquid Native MALDI Mass Spectrometry for the Detection of Protein-Protein Complexes

Native mass spectrometry (MS) encompasses methods to keep noncovalent interactions of biomolecular complexes intact in the gas phase throughout the instrument and to measure the mass-to-charge ratios of supramolecular complexes directly in the mass spectrometer. Electrospray ionization (ESI) in nondenaturing conditions is now an established method to characterize noncovalent systems. Matrix-assisted laser desorption/ionization (MALDI), on the other hand, consumes low quantities of samples and largely tolerates contaminants, making it a priori attractive for native MS. However, so-called native MALDI approaches have so far been based on solid deposits, where the rapid transition of the sample through a solid state can engender the loss of native conformations. Here we present a new method for native MS based on liquid deposits and MALDI ionization, unambiguously detecting intact noncovalent protein complexes by direct desorption from a liquid spot for the first time. To control for aggregation, we worked with HUαβ, a heterodimer that does not spontaneously rearrange into homodimers in solution. Screening through numerous matrix solutions to observe first the monomeric protein, then the dimer complex, we settled on a nondenaturing binary matrix solution composed of acidic and basic organic matrices in glycerol, which is stable in vacuo. The role of temporal and spatial laser irradiation patterns was found to be critical. Both a protein-protein and a protein-ligand complex could be observed free of aggregation. To minimize gas-phase dissociation, source parameters were optimized to achieve a conservation of complexes above 50% for both systems. Graphical Abstract ᅟ.

Ionic Liquid-Assisted Laser Desorption/Ionization-Mass Spectrometry: Matrices, Microextraction, and Separation

Ionic liquids (ILs) have advanced a variety of applications, including matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS). ILs can be used as matrices and solvents for analyte extraction and separation prior to analysis using laser desorption/ionization-mass spectrometry (LDI-MS). Most ILs show high stability with negligible sublimation under vacuum, provide high ionization efficiency, can be used for qualitative and quantitative analyses with and without internal standards, show high reproducibility, form homogenous spots during sampling, and offer high solvation efficiency for a wide range of analytes. Ionic liquids can be used as solvents and pseudo-stationary phases for extraction and separation of a wide range of analytes, including proteins, peptides, lipids, carbohydrates, pathogenic bacteria, and small molecules. This review article summarizes the recent advances of ILs applications using MALDI-MS. The applications of ILs as matrices, solvents, and pseudo-stationary phases, are also reviewed.

The combination of 2,5-dihydroxybenzoic acid and 2,5-dihydroxyacetophenone matrices for unequivocal assignment of phosphatidylethanolamine species in complex mixtures

Unequivocal assignment of phospholipid peaks in complex mixtures is difficult if only the m/z values but no tandem mass spectrometry (MS/MS) data are available. This is usually the case for matrix-assisted laser/desorption ionization time-of-flight (MALDI-TOF) MS imaging experiments and the analysis has normally to be performed without prior separation. Another problem might be the often matrix-induced loss of one methyl group in phosphatidylcholine (PC) species, which makes them detectable as negative ions becoming isomers of some phosphatidylethanolamines (PEs). Selected lipid mixtures of known compositions were investigated by negative ion MALDI-TOF MS and various imaging experiments. In addition to common matrices such as 2,5-dihydroxybenzoic acid (DHB) and 9-aminoacridine (9-AA), different binary matrices, including 2,5-dihydroxyacetophenone (2,5-DHAP) as matrix additive to DHB, were tested to probe their performance in both ionization modes. Beside artificial PC and PE mixtures of known compositions, egg yolk and liver extracts as well as cryosections from liver and pancreas tissue were selected as biologically relevant systems. The majority of the binary MALDI matrices used here leads to the loss of a methyl group from PC in the negative ion mode, which makes the clear identification of PE species ambiguous. However, this problem does not apply if a mixture of DHB and 2,5-DHAP is used. Therefore, the application of DHB/2,5-DHAP as matrix is a simple method to unequivocally identify PEs even in complex mixtures and tissue sections as negative ions and without the necessity to separate the individual lipid classes prior to MS detection. Graphical abstract Many common MALDI matrices (such as 9-AA) induce the loss of a methyl group from PC rendering the PC detectable as negative ion. These ions (m/z 744.6 in the upper trace) represent isomers of typical PE species. It will be shown that this problem can be avoided if mixtures between DHB and 2,5-DHAP are applied. At these conditions, POPC is exclusively detectable as a matrix adduct with DHB (at m/z 912.6, lower trace) and does not interfere with PE. This approach can also be used in MALDI MS imaging.

Ionic liquids-coordinated Au catalysts for acetylene hydrochlorination: DFT approach towards reaction mechanism and adsorption energy

A DFT study of Au–IL catalysts for acetylene hydrochlorination was carried out and consistency with experimental data was obtained.

High yield matrix-free ionization of biomolecules by pulse-heating ion source

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry has been widely used for biomolecular analysis. However, with conventional MALDI, it is difficult to analyse low-molecular-weight compounds because of the interference of matrix ion signals. Here, we report a matrix-free on-chip pulse-heating desorption/ionization (PHDI) method for a wide range of biomolecules ranging from low molecular-weight substances such as glycine (75.7 Da) to large species such as α-lactalbumin (14.2 kDa). Compared with the conventional MALDI, the matrix-free PHDI method affords high yields of singly charged ions with very less fragmentation and background using only one-pulse without light (laser). We believe that this new technique for matrix-free biomolecules analysis would overcome the limitations of the conventional MALDI.

Novel ionic liquid matrices for qualitative and quantitative detection of carbohydrates by matrix assisted laser desorption/ionization mass spectrometry

Analysis of carbohydrates based on matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is still challenging and researchers have been devoting themselves to efficient matrices discovery. In the present study, the design, synthesis, qualitative and quantitative performance of non-derivative ionic liquid matrices (ILMs) were reported. DHB/N-methylaniline (N-MA) and DHB/N-ethylaniline (N-EA), performing best for carbohydrate detection, have been screened out. The limit of detection for oligosaccharide provided by DHB/N-MA and DHB/N-EA were as low as 10 fmol. DHB/N-MA and DHB/N-EA showed significantly higher ion generation efficiency than DHB. The comparison of capacity to probe polysaccharide between these two ILMs and DHB also revealed their powerful potential. Their outstanding performance were probably due to lower proton affinities and stronger UV absorption at λ = 355 nm. What is more, taking DHB/N-MA as an example, quantitative analysis of fructo-oligosaccharide mixtures extracted and identified from rice noodles has been accomplished sensitively using an internal standard method. Overall, DHB/N-MA and DHB/N-EA exhibited excellent performance and might be significant sources as the carbohydrate matrices.

Independent assessment of matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) sample preparation quality: Effect of sample preparation on MALDI-MS of synthetic polymers

RATIONALE

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) provides detailed and in-depth information about the molecular characteristics of synthetic polymers. To obtain the most accurate results the sample preparation parameters should be chosen to suit the sample and the aim of the experiment. Because the underlying principles of MALDI are still not fully known, a priori determination of optimal sample preparation protocols is often not possible.

METHODS

Employing an automated sample preparation quality assessment method recently presented by us we quantified the sample preparation quality obtained using various sample preparation protocols. Six conventional matrices with and without added potassium as a cationization agent and six ionic liquid matrices (ILMs) were assessed using poly(ethylene glycol) (PEG), polytetrahydrofuran (PTHF) and poly(methyl methacrylate) (PMMA) as samples. All sample preparation protocols were scored and ranked based on predefined quality parameters and spot-to-spot repeatability.

RESULTS

Clearly distinctive preferences were observed in matrix identity and cationization agent for PEG, PTHF and PMMA, as the addition of an excess of potassium cationization agent results in an increased score for PMMA and a contrasting matrix-dependent effect for PTHF and PEG. The addition of excess cationization agent to sample mixtures dissipates any overrepresentation of high molecular weight polymer species. Our results show reduced ionization efficiency and similar sample deposit homogeneity for all tested ILMs, compared with well-performing conventional MALDI matrices.

CONCLUSIONS

The results published here represent a start in the unsupervised quantification of sample preparation quality for MALDI samples. This method can select the best sample preparation parameters for any synthetic polymer sample and the results can be used to formulate hypotheses on MALDI principles. Copyright © 2016 John Wiley & Sons, Ltd.

A facile method for cellular N-glycomic profiling by matrix-assisted laser desorption/ionization mass spectrometry

Rapid and highly sensitive analysis of cellular N-glycans with co-derivatization strategy using matrix-assisted laser/desorption mass spectrometry.

A cool and high salt-tolerant ionic liquid matrix for preferential ionization of phosphopeptides by negative ion MALDI-MS

An optimized ILM G₃THAP/PA matrix significantly improved the detection of phosphopeptides by negative ion MALDI-MS compared with using 3-AQ/CHCA/ADP and DHB/PA matrices.

In-Source Decay and Pseudo-MS3 of Peptide and Protein Ions Using Liquid AP-MALDI

Atmospheric pressure MALDI on a Q-Exactive instrument was optimized for in-source decay and pseudo-MS³. The dependence of AP-MALDI ISD on the MALDI liquid matrix was investigated for peptides and proteins. The liquid matrices enabled long-life ISD signal, and exhibited high fragment ion yield and signal stability. Extensive a-, b-, c-, y-, and z-type fragment series were observed depending on the matrix used but were most extensive with 2,5-DHB. Complete sequence coverage of small peptide and intact protein-terminus sequence tags were obtained and confirmed using HCD as a pseudo-MS³ method. Graphical Abstract ᅟ.

Binding Selectivity of Macrocycle Ionophores in Ionic Liquids versus Aqueous Solution and Solvent-free Conditions

The understanding of supramolecular recognition in room-temperature ionic liquids (RTILs) is key to develop the full potential of these materials. In this work, we provide insights into the selectivity of the binding of alkali metal cations by standard cyclodextrin and calixarene macrocycles in RTILs. A direct laser desorption/ionization mass spectrometry approach is employed to determine the relative abundances of the inclusion complexes formed through competitive binding in RTIL solutions. The results are compared with the binding selectivities measured under solvent-free conditions and in water/methanol solutions. Cyclodextrins and calixarenes in which the peripheral OH groups are substituted by bulkier side groups preferentially bind to Cs(+) . Such specific ionophoric behavior is substantially enhanced by solvation effects in the RTIL. This finding is rationalized with the aid of quantum mechanical calculations, in terms of the conformational features and steric interactions that drive the solvation of the inclusion complexes by the bulky RTIL counterions.

Ionic liquid matrix-based dispersive liquid-liquid microextraction for enhanced MALDI-MS analysis of phospholipids in soybean

Ionic liquid matrix (ILM) is found to be a very versatile substance for analysis of broad range of organic molecules in matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) due to good solubility for a variety of analytes, formation of homogenous crystals and high vacuum stability of the matrix. In the present work, an ILM, cyno-4-hydroxycinnamic acid-butylamine (CHCAB) was employed in dispersive liquid-liquid microextraction (DLLME) as sample probe and matrix for extraction and ionization of phospholipids from food samples (soybean) prior to MALDI-MS analysis. With the employed technique, 8-125 fold improvement in signal intensity and limit of detection were achieved for the analysis of phospholipids. The best extraction efficiency of phospholipids in ILM-DLLME was obtained with 5min extraction time in presence 30mg/mL CHCAB and 1.2% NaCl using chloroform as an extracting solvent and methanol as a dispersing solvent. Further, the developed ILM-DLLME procedure has been successfully applied for the analysis of phospholipids in soybean samples in MALDI-MS.

Ionic liquids in bioanalysis

Ionic liquids (ILs) are entirely composed of ions and they possess fascinating properties, including low volatility, tunable viscosity, miscibility and electrolytic conductivity, which make them promising alternatives to traditional organic solvents used in sample preparation. The recent surge in the number of publications clearly indicates an increasing interest of the analytical and bioanalytical community toward these exciting and unique solvents. This article highlights the recent advances in the use of ILs as extraction solvents, as materials for separation and preconcentration in chromatographic techniques, and as matrices in mass spectrometric techniques for bioassays in biocomplex samples. We also briefly discuss the potential applications of ILs in biocatalysis.

MALDI Matrix Research for Biopolymers

Matrices are necessary materials for ionizing analytes in matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS). The choice of a matrix appropriate for each analyte controls the analyses. Thus, in some cases, development or improvement of matrices can become a tool for solving problems. This paper reviews MALDI matrix research that the author has conducted in the recent decade. It describes glycopeptide, carbohydrate, or phosphopeptide analyses using 2,5-dihydroxybenzoic acid (2,5-DHB), 1,1,3,3-tetramethylguanidinium (TMG) salts of p-coumaric acid (CA) (G3CA), 3-aminoquinoline (3-AQ)/α-cyano-4-hydroxycinnamic acid (CHCA) (3-AQ/CHCA) or 3-AQ/CA and gengeral peptide, peptide containing disulfide bonds or hydrophobic peptide analyses using butylamine salt of CHCA (CHCAB), 1,5-diaminonaphthalene (1,5-DAN), octyl 2,5-dihydroxybenzoate (alkylated dihydroxybenzoate, ADHB), or 1-(2,4,6-trihydroxyphenyl)octan-1-one (alkylated trihydroxyacetophenone, ATHAP).

N-Glycosylation with synthetic undecaprenyl pyrophosphate-linked oligosaccharide to oligopeptides by PglB oligosaccharyltransferase from Campylobacter jejuni

The oligosaccharyltransferase PglB from Campylobacter jejuni catalyses the N-glycosylation reaction with undecaprenyl-pyrophosphate-linked Glc1 GalNAc5 Bac1 (Und-PP-Glc1 GalNAc5 Bac1 ). Experiments using chemically synthesized donors coupled to fluorescently tagged peptides confirmed that biosynthetic intermediate Und-PP-Bac1 and Und-PP-GalNAc2 Bac1 are transferred efficiently to the Asn residue in the consensus sequence (D/E-X'-N-X-T/S, X',X≠P). The products were analyzed in detail by tandem MS to confirm their chemical structures.

Quantitative analysis of polyhexamethylene guanidine (PHMG) oligomers via matrix-assisted laser desorption/ionization time-of-flight mass spectrometry with an ionic-liquid matrix

RATIONALE

Quantifying polymers by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) with a conventional crystalline matrix generally suffers from poor sample-to-sample or shot-to-shot reproducibility. An ionic-liquid matrix has been demonstrated to mitigate these reproducibility issues by providing a homogeneous sample surface, which is useful for quantifying polymers. In the present study, we evaluated the use of an ionic liquid matrix, i.e., 1-methylimidazolium α-cyano-4-hydroxycinnamate (1-MeIm-CHCA), to quantify polyhexamethylene guanidine (PHMG) samples that impose a critical health hazard when inhaled in the form of droplets.

METHODS

MALDI-TOF mass spectra were acquired for PHMG oligomers using a variety of ionic-liquid matrices including 1-MeIm-CHCA. Calibration curves were constructed by plotting the sum of the PHMG oligomer peak areas versus PHMG sample concentration with a variety of peptide internal standards.

RESULTS

Compared with the conventional crystalline matrix, the 1-MeIm-CHCA ionic-liquid matrix had much better reproducibility (lower standard deviations). Furthermore, by using an internal peptide standard, good linear calibration plots could be obtained over a range of PMHG concentrations of at least 4 orders of magnitude.

CONCLUSIONS

This study successfully demonstrated that PHMG samples can be quantitatively characterized by MALDI-TOFMS with an ionic-liquid matrix and an internal standard.

High repetition rate atmospheric pressure matrix-assisted laser desorption/ionization in combination with liquid matrices

One major drawback of matrix-assisted laser desorption/ionization (MALDI) is still the relatively poor pulse-to-pulse reproducibility of the signal intensity. This problem, caused by insufficient homogeneity in the matrix/analyte co-crystallization, is usually circumvented by averaging the detected ion intensity over several shots. However, during the consecutive laser pulses, the applied matrix gets depleted and only a number of subsequent experiments can be done on the same sample spot. In order to achieve the desired long-term stability in combination with a sufficient pulse-to-pulse reproducibility, recently liquid MALDI matrices have been introduced. This contribution demonstrates the promising combination of liquid matrices with high repetition rate lasers for atmospheric pressure MALDI (AP-MALDI). To demonstrate the robustness of the new approach, two different kinds of liquid matrices were used in combination with both a typical flashlamp pumped 15 Hz laser and a diode pumped solid state laser operated at 5 kHz. The latter showed a stable ion signal over more than 3,500,000 consecutive laser pulses.

Periodic mesoporous organosilica with ionic liquid framework as a novel fiber coating for headspace solid-phase microextraction of polycyclic aromatic hydrocarbons

Periodic mesoporous organosilica based on alkylimidazolium ionic liquid (PMO-IL) was prepared and used as a highly porous fiber coating material for solid-phase microextraction (SPME). The prepared nanomaterial was immobilized onto a stainless steel wire for fabrication of the SPME fiber. The fiber was evaluated for the extraction of some polycyclic aromatic hydrocarbons (PAHs) from aqueous sample solutions in combination with gas chromatography-mass spectrometry (GC-MS). A one at-the-time optimization strategy was applied for optimizing the important extraction parameters such as extraction temperature, extraction time, ionic strength, stirring rate, and desorption temperature and time. In optimum conditions, the repeatability for one fiber (n=3), expressed as relative standard deviation (R.S.D.%), was between 4.3% and 9.7% for the test compounds. The detection limits for the studied compounds were between 4 and 9 pg mL(-1). The developed method offers the advantage of being simple to use, with shorter analysis time, lower cost of equipment, thermal stability of fiber and high relative recovery in comparison to conventional methods of analysis.

Towards the rational design of ionic liquid matrices for secondary ion mass spectrometry: role of the anion

The role of the ionic liquid (IL) anion structure on analyte signal enhancements has been systematically investigated in secondary ion mass spectrometry (SIMS) using a variety of samples, including lipids, sterols, polymers, and peptides. Twenty-four ILs were synthesized. The 12 matrix acids were cinnamic acid derivatives. Two bases were employed: 1-methylimidazole and tripropylamine. Three matrices, methylimmidazolium o-coumarate, tripropylammonium o-coumarate, and tripropylammonium 3,4,5-trimethoxycinnamate, were "universal" matrices enhancing all analytes tested. The pKa of the matrix acid does not appear to have a strong effect on analyte ion intensities. Rather, it is observed that a single hydroxyl group on the anion aromatic ring leads to significantly increased molecular ion intensities. No analyte signal enhancements were observed for -CH3, -CF3 and -OCH3 groups present on the aromatic ring. The position of the -OH group on the aromatic ring also alters molecular ion intensity enhancements. As well as the chemical identity and position of substituents, the number of moieties on the aromatic ring may affect the analyte signal enhancements observed. These observations suggest that the activation of the IL anion aromatic ring is important for optimizing analyte signal intensities. The implications for SIMS imaging of complex structures, such as biological samples, are discussed.

Ionic liquid matrix-enhanced secondary ion mass spectrometry: the role of proton transfer

Room temperature ionic liquids (ILs) are effective matrices in secondary ion mass spectrometry (SIMS) and matrix assisted laser desorption ionization (MALDI). In this paper, we examine the role of proton transfer in the mechanism of secondary ion enhancement using IL matrices in SIMS. We employ hydrogenated and deuterated 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) as analytes to investigate the origin of proton transfer. The data indicate that protons from the IL anion transfer to the analyte in solution leading to an increase in the secondary ion intensity of the protonated molecular ion. The chemical identity of the matrix cation also affects analyte signal intensities. Using deuterated DPPC we observe that protons (deuterium) from the DPPC tail group react with the cation of the IL liquid leading to an increase in (cation + D)(+) ion intensities. Further, the data suggest that the transfer kinetics of deuterium (hydrogen) is correlated with the secondary ion enhancements observed. The highest secondary ion enhancements are observed for the least sterically hindered cation. Neither the proton affinity nor the pKa of the IL cation have a large effect on the analyte ion intensities, suggesting that steric factors are important in determining the efficacy of IL matrices for a given analyte.

An optimized matrix-assisted laser desorption/ionization sample preparation using a liquid matrix, 3-aminoquinoline/α-cyano-4-hydroxycinnamic acid, for phosphopeptides

RATIONALE

A liquid matrix, 3-aminoquinoline (3-AQ)/α-cyano-4-hydroxycinnamic acid (CHCA), introduced by Kolli et al. in 1996 for matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), has been reported for peptides and proteins, oligonucleotides, oligosaccharides, and glycopeptides. However, it has not been validated for phosphopeptides.

METHODS

We optimized sample preparation using 3-AQ/CHCA for phosphopeptides. The sensitivity of six phosphopeptide species as isolated or in digests was systematically evaluated by using MALDI-quadropole ion trap (QIT)-time of flight (TOF) MS in positive and negative ion modes, and compared with the conventional methods using a solid matrix, 2,5-dihydroxybenzoic acid (2,5-DHB).

RESULTS

The sensitivity of mono- and tetraphosphopeptides was improved 10- to 10 000-fold with the optimized preparation method using 3-AQ/CHCA compared with the conventional methods using 2,5-DHB. Improvement by 3-AQ/CHCA itself was 10-fold. Adding ammonium dihydrogen phosphate or an analyte solvent composition was also effectively improved the sensitivity. Phosphopeptides in isolated form or in digests were detected at femto- or subfemtomole levels.

CONCLUSIONS

Sensitivity of phosphopeptides was improved by the optimized sample preparation method using 3-AQ/CHCA compared with the conventional method using 2,5-DHB. The validation of 3-AQ/CHCA for phosphopeptides was systematically confirmed, expanding the potential of this matrix to phosphoproteomics.

Matrix assisted ionization: new aromatic and nonaromatic matrix compounds producing multiply charged lipid, peptide, and protein ions in the positive and negative mode observed directly from surfaces

Matrix assisted inlet ionization (MAII) is a method in which a matrix:analyte mixture produces mass spectra nearly identical to electrospray ionization without the application of a voltage or the use of a laser as is required in laserspray ionization (LSI), a subset of MAII. In MAII, the sample is introduced by, for example, tapping particles of dried matrix:analyte into the inlet of the mass spectrometer and, therefore, permits the study of conditions pertinent to the formation of multiply charged ions without the need of absorption at a laser wavelength. Crucial for the production of highly charged ions are desolvation conditions to remove matrix molecules from charged matrix:analyte clusters. Important factors affecting desolvation include heat, vacuum, collisions with gases and surfaces, and even radio frequency fields. Other parameters affecting multiply charged ion production is sample preparation, including pH and solvent composition. Here, findings from over 100 compounds found to produce multiply charged analyte ions using MAII with the inlet tube set at 450 °C are presented. Of the compounds tested, many have -OH or -NH(2) functionality, but several have neither (e.g., anthracene), nor aromaticity or conjugation. Binary matrices are shown to be applicable for LSI and solvent-free sample preparation can be applied to solubility restricted compounds, and matrix compounds too volatile to allow drying from common solvents. Our findings suggest that the physical properties of the matrix such as its morphology after evaporation of the solvent, its propensity to evaporate/sublime, and its acidity are more important than its structure and functional groups.