Chemical reactivity in matrix-assisted laser desorption/ionization mass spectrometry

During the control of a multistep organic synthesis on a soluble polymer (PEG) by matrix-assisted laser desorption/ionization (MALDI) mass spectrometry, a chemical reactivity was encountered when the matrix was acidic, for the samples where the amino moiety of the anchored compounds was protected as a Schiff base. Such imine hydrolysis was proven to be solely mediated by the acidic matrix during analyses since the expected protected structures were detected when the experiments were duplicated with a non-acidic matrix. Even if MALDI mass spectrometry was found to be more convenient than electrospray ionization mass spectrometry for the monitoring of liquid phase organic syntheses, the chemical reactivity imparted by the use of a matrix must be taken into account to avoid erroneous spectra interpretations. Copyright 1999 John Wiley & Sons, Ltd.     

Molecular histology analysis by matrix-assisted laser desorption/ionization imaging mass spectrometry using gold nanoparticles as matrix

Gold nanoparticles (AuNPs) were applied and optimized as matrix for matrix-assisted laser desorption/ionization mass spectrometry analysis of animal tissues, and enabled histological analysis of animal tissues at molecular level by imaging mass spectrometry (IMS). AuNPs were coated on animal tissue in a solvent-free manner via argon ion sputtering. Metabolites, including neurotransmitters, fatty acids and nucleobases, were directly detected from mouse brain tissue. Based on region-specific chemical profiles, fine histological features of mouse brain tissue and heterogeneous regions of tumor tissue were both revealed.     

Molecular distribution of some commercial nonylphenol ethoxylates using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Nonylphenol ethoxylates (NPEs) belong to a group of nonionic surfactants that are collectively referred to as alkylphenol ethoxylates (APEs). APEs find widespread use in heavy-duty commercial and household cleaning formulations, shampoos, and industrial processing, i.e. textile manufacture. Their environmental impact depends on the molecular distribution and the extent of their biodegradation in municipal sewage systems, waterways and rivers. We have established two sample preparation methods that have enabled the determination of the molecular distributions of six commercial nonylphenol ethoxylates using matrix-assisted laser desorption/ionization (MALDI) time-of-flight (TOF) mass spectrometry (MS). In both methods, alpha-cyano-4-hydroxycinnamic acid, dissolved in acetonitrile/tetrahydrofuran, was used as the matrix. In one set of experiments, the NPEs were dissolved in an acetonitrile/tetrahydrofuran solvent system prior to mixing with the matrix solution, and the resulting MALDI-TOF mass spectra produced mostly sodiated molecules [M + Na](+). The NPEs, all having the formula 4-(C(9)H(19))-C(6)H(4)-(OCH(2)CH(2))(n)-OH, are Surfonic (R)N-95, N-100, N-102, N-120, N-150 and N-300. Surfonic N-95 and N-100 gave n values of 5-20; Surfonic N-102, N-120, N-150, and N-300 gave n values of 5-21, 5-22, 8-25 and 15-40, respectively. In order to develop a sample preparation method that could be used with less polar NPEs, we dissolved the NPEs (except N-300) in pentane prior to mixing with the matrix solution, and found that the MALDI spectral quality was unaffected by the solvent systems. Copyright 1999 John Wiley & Sons, Ltd.     

Application of nanodiamonds in human body fluid analysis by matrix-assisted laser desorption/ionization mass spectrometry

Direct mass spectrometric analysis of complex biological samples is very important and challenging. In this paper, nanodiamonds have been successfully used in matrix-assisted laser desorption/ionization mass spectrometric analysis of human serum and urine. As a practical tool and platform, it can be widely used in the field of humoral proteomics, and it plays a very promising role in clinical diagnosis, including identification of novel disease-associated biomarkers.     

Exploring the frontiers of synthetic eumelanin polymers by high-resolution matrix-assisted laser/desorption ionization mass spectrometry

New trends in material science and nanotechnologies have spurred growing interest in eumelanins black insoluble biopolymers derived by tyrosinase-catalysed oxidation of tyrosine via 5,6-dihydroxyindole (DHI) and its 2-carboxylic acid (DHICA). Efficient antioxidant and photoprotective actions, associated with peculiar optoelectronic properties, are recognised as prominent functions of eumelanin macromolecules within the human and mammalian pigmentary system, making them unique candidates for the realisation of innovative bio-inspired functional soft materials, with structure-based physical-chemical properties. An unprecedented breakthrough into the mechanism of synthetic eumelanin buildup has derived from a detailed investigation of the oxidative polymerization of DHI and its N-methyl derivative (NMDHI) by linear and reflectron matrix-assisted laser/desorption ionization mass spectrometry. Regular collections of oligomers of increasing masses, spanning the entire m/z ranges up to 5000?Da (>30-mer) and 8000?Da (> 50-mer) for the two building blocks, respectively, were disclosed. It is the first time that the in vitro polymerisation of dihydroxyindoles to form synthetic eumelanins is explored up to its high mass limits, giving at the same time information on the polymerisation mode, whether it follows a stepwise pattern (being this the conclusion in our case) or a staking sequencing of small-sized entities. It also highlighted the influence of the N-methyl substituent on the polymerization process; this opens the way to the production of N-functionalized, synthetic eumelanin-inspired soft materials, for possible future technological applications.     

Identification and quantification of amyloid beta-related peptides in human plasma using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Proteolytic processing of the amyloid precursor protein (APP) by β-secretase and γ-secretase leads to the generation and deposition of amyloid β (Aβ) in Alzheimer’s disease (AD). N-terminally or C-terminally truncated Aβ variants have been found in human cerebrospinal fluid and cultured cell media using immunoprecipitation and mass spectrometry. Unfortunately, the profile of plasma Aβ variants has not been revealed due to the difficulty of isolating Aβ from plasma. We present here for the first time studies of Aβ and related peptides in human plasma. Twenty-two Aβ-related peptides including novel peptides truncated before the β-secretase site were detected in human plasma and 20 of the peptides were identified by tandem mass spectrometry. Using an internal standard, we developed a quantitative assay for the Aβ-related peptides and demonstrated plasma dilution linearity and the precision required for their quantitation. The present method should enhance the understanding of APP processing and clearance in AD progression.     

Characterization of partially transesterified poly(beta-hydroxyalkanoate)s using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) was used for the characterization of a partially transesterified poly(beta-hydroxyalkanoate), PHA, polymer produced by the bacterial strain Alcaligenes eutrophus using saponified vegetable oils as the sole carbon sources. The transesterification was carried out separately under acidic and basic conditions to obtain PHA oligomers weighing less than 10 kDa. The intact oligomers were detected in their cationized [M + Na](+) and [M + K](+) forms by MALDI-TOFMS. A composition analysis, using the MALDI-TOF spectra, indicate that the oligomers obtained via acid catalysis were terminated with a methyl 3-hydroxybutyrate end group, and those obtained by base catalysis had a methyl crotonate (olefinic) termination. In addition to HB (hydroxy butyrate), the oligomers were found to contain a small percentage of HV (hydroxy valerate). This was independently confirmed using gas chromatography/mass spectrometry (GC/MS). In comparison, the analysis of a commercial PHA polymer, transesterified under identical conditions, only showed the presence of HB, i.e. a pure PHB homopolymer. Copyright 1999 John Wiley & Sons, Ltd.     

Formation of macrocyclic oligoferrocenes: a matrix-assisted laser desorption/ionization mass spectrometric study

Oligomeric ferrocenes were investigated simultaneously by matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOFMS). The oligomers were obtained by the reaction of tetrahydro-4,4,8,8-tetramethyl-4,8-disila-s-indacene-3a,7a-diyldilithium (Li(2)L) with FeCl(2).1.5 THF (THF = tetrahydrofuran). Depending on the reaction conditions up to ten linear-chain and eleven cyclic ferrocene oligomers with masses between 1139 and 5071 Da could be detected unambiguously. The most abundant macrocycles contained ten and seven iron atoms when the reactions were carried out at -20 and 25 degrees C, respectively. The chains had cyclopentadienes as end groups and formally resulted from replacing one iron of a cycle by two hydrogens, which corresponds to a mass difference of 54 Da. Copyright 1999 John Wiley & Sons, Ltd.     

Tandem mass spectrometry of poly(ethylene imine)s by electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI)

In this contribution, linear poly(ethylene imine) (PEI) polymers, which are of importance in gene delivery, are investigated in detail by using electrospray ionization-quadrupole-time of flight (ESI-Q-TOF) and matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectrometry (MS). The analyzed PEIs with different end groups were synthesized using the polymerization of substituted 2-oxazoline via a living cationic ring-opening polymerization (CROP) and a subsequent hydrolysis under acidic conditions. The main goal of this study was to identify linear PEI polymers in a detailed way to gain information about their fragmentation pathways. For this purpose, a detailed characterization of three different linear PEIs was performed by using ESI-Q-TOF and MALDI-TOF MS in combination with collision-induced dissociation (CID) experiments. In ESI-MS as well as MALDI-MS analysis, the obtained spectra of PEIs resulted in fitting mass distributions for the investigated PEIs. In the tandem MS analysis, a 1,2-hydride shift with a charge-remote rearrangement via a four-membered cyclic transition state, as well as charge-induced fragmentation reactions, was proposed as the main fragmentation mechanisms according to the obtained fragmentation products from the protonated parent peaks. In addition, heterolytic and homolytic cleavages were proposed as alternative fragmentation pathways. Moreover, a 1,4-hydrogen elimination was proposed to explain different fragmentation products obtained from the sodiated parent peaks.     

Analysis of cancer cell lipids using matrix-assisted laser desorption/ionization 15-T Fourier transform ion cyclotron resonance mass spectrometry

A combination of methodologies using the extremely high mass accuracy and resolution of 15-T Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry (MS) was introduced for the identification of intact cancer cell phospholipids. Lipids from a malignant glioma cell line were initially analyzed at a resolution of >200,000 and identified by setting the mass tolerance to ±1 mDa using matrix-assisted laser desorption/ionization (MALDI) 15-T FT-ICR MS in positive ion mode. In most cases, a database search of potential lipid candidates using the exact masses of the lipids yielded only one possible chemical composition. Extremely high mass accuracy (<0.1?ppm) was then attained by using previously identified lipids as internal standards. This, combined with an extremely high resolution (>800,000), yielded well-resolved isotopic fine structures allowing for the identification of lipids by MALDI 15-T FT-ICR MS without using tandem mass spectrometric (MS/MS) analysis. Using this method, a total of 38 unique lipids were successfully identified.     

Silver nanoparticles as matrix for laser desorption/ionization mass spectrometry of peptides

Silver nanoparticle synthesized from chemical reduction has been successfully utilized as a matrix in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) of peptides. Acting as a substrate to adsorb analytes, as well as a transmission medium for UV laser, silver nanoparticle was found to assist in the desorption/ionization of peptides with little or no induced fragmentation. The size of the nanoparticle was typically in the range of 160 ± 20 nm. One of the key advantages of silver nanoparticle for peptides analysis is its simple step for on-probe sample preparation. In addition, it also minimizes the interferences of sodium dodecyl sulfate (SDS) surfactant background signal, resulting in cleaner mass spectra and more sensitive signal, when compared to α–cyano-4-hydroxycinnamic acid (CCA) matrix.     

Detection of melamine in infant formula and grain powder by surface-assisted laser desorption/ionization mass spectrometry

We have developed a method for the determination of melamine (MEL), ammeline (AMN), and ammelide (AMD) by surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) using gold nanoparticles (Au NPs). The major peaks for MEL, AMN, and AMD at m/z 127.07, 128.05, and 129.04 are assigned to the [MEL + H](+), [AMN + H](+), and [AMD + H](+) ions. Because the three tested compounds adsorb weakly onto the surfaces of the Au NPs through Au-N bonding, they can be easily concentrated from complex samples by applying a simple trapping/centrifugation process. The SALDI-MS method provides limits of detection of 5, 10, and 300 nM for MEL, AMN, and AMD, respectively, at a signal-to-noise ratio of 3. The signal variation for 150-shot average spectra of the three analytes within the same spot was 15%, and the batch-to-batch variation was 20%. We have validated the practicality of this approach by the analysis of these three analytes in infant formula and grain powder. This simple and rapid SALDI-MS approach holds great potential for screening of MEL in foods.