Qualitative and quantitative analysis of low molecular weight compounds by ultraviolet matrix-assisted laser desorption/ionization mass spectrometry using ionic liquid matrices

Microchip coupled with MALDI-TOF MS for the investigation of bacterial contamination of fish muscle products

Bacterial contamination is a significant concern in food safety. Traditional methods, though being a gold standard for bacterial detection, are time-consuming. In this work, we managed to establish a simple and versatile magnetic-assisted microfluidic method for rapid bacterial detection of fish muscle products, by manipulating anti-human IgG functionalized magnetic beads in a zig-zag shaped microfluidic channel, increasing the probability for bacteria capture. The captured bacteria were characterized by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS). This method is capable of isolating Escherichia coli, Staphylococcus aureus and Klebsiella pneumoniae from 5 μL of sablefish sarcoplasmic protein sample, and detecting Escherichia coli in the range of 6.0 to 6.0×10⁴ CFU/mL with a detection limit of 6 CFU/mL. Bacterial growth on salmon sashimi during its period of storage was successfully monitored. The current protocol holds great potential for pathogen detection and microbial control in the food industry.

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.

Quantification of nitroaromatic explosives in contaminated soil using MALDI-TOF mass spectrometry

Contamination from various sources is a global environmental and health threat, with mining and military activities in particular having spread nitroaromatic compounds, such as 2,4,6-trinitrotoluene and its degradation products and by-products, to the soil. The investigation and monitoring of large contaminated areas requires new detection methods since the established ones are expensive and time-consuming. Hence, we established a matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) method using 1,5-diaminonaphthalene as the matrix substance and an internal standard for quantification. Analyzing standard substances, we found specific signals for radical and fragment ions of different nitrotoluenes and nitrobenzenes with good reproducibility and detection limits down to 0.25 ng/μL. The analysis of soil sample extracts from a former production site showed clear signals for 2,4,6-trinitrotoluene and the primary degradation products aminodinitrotoluenes. Furthermore, quantification gave results comparable to those obtained by conventional liquid chromatography-tandem mass spectrometry analysis. The MALDI-TOF MS method has a comparatively lower reproducibility, with relative standard deviations of 6% to 20% for multiple measurements of standard solutions and soil sample extracts. Nevertheless, a comparison of both methods revealed the advantages of MALDI-TOF MS analysis of explosive-contaminated areas with regard to costs, time, and handling. Finally, our MALDI-TOF MS method fulfills all the needs for high sample throughput and can therefore be a valuable screening tool for explosive-contaminated areas. Graphical abstract.

Dual-Channel Enzymatic Inhibition Measurement (DEIM) Coupling Isotope Substrate via Matrix-Assisted Laser Desorption/Ionization Time of Flight Mass Spectrometry

A novel dual-channel enzymatic inhibition measurement (DEIM) method was developed to improve the repeatability with light/heavy isotope substrates, producing reliable relative standard deviations (< 3%) by employing acetylcholinesterase (AChE) as the model enzyme. The matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS) was adapted for enzyme-inhibited method due to its good salt-tolerance and high throughput; meanwhile, dual-channel enzymatic reactions were performed to improve the repeatability of each well. The acetylcholinesterase inhibition measurement was conducted by mixing the quenched enzyme reaction solution of blank group (with heavy isotope as substrate) and experimental group (with light isotope as substrate), of which the inhibition rate might be affected by isotope effects. Hence, inverse study and Km measurement were implemented to validate the method. The inverse study shows similar inhibition rate (68.9 and 70.3%) and the Km of isotope substrates are analogous (0.139 and 0.135 mM), which demonstrated that the novel method is feasible to AChE inhibition measurement. Finally, the method was applied to herb extracts, half of which exhibit inhibition to AChE. The precise dual-channel enzymatic inhibition measurement (DEIM) method could be regarded as a promising approach to potential enzyme inhibitor screening. 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.

Miniaturized dispersive liquid-liquid microextraction and MALDI MS using ionic liquid matrices for the detection of bacterial communication molecules and virulence factors

The identification and quantification of molecules involved in bacterial communication are major prerequisites for the understanding of interspecies interactions at the molecular level. We developed a procedure allowing the determination of 2-heptyl-4(1H)-quinolone (HHQ) and 2-heptyl-3-hydroxy-4(1H)-quinolone (PQS) and the virulence factor pyocyanin (PYO) formed by the Gram-negative bacterium Pseudomonas aeruginosa. The method is based on dispersive liquid-liquid microextraction from small supernatant volumes (below 10 μL) followed by quantitative matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS). The use of ionic liquid matrix led to a lowered limit of detection for pyocyanin and, due to suppression of matrix background signals, easy to interpret mass spectra compared to crystalline matrices. Using an isotope-labeled pyocyanin standard synthesized in small-scale synthesis, quantitative analysis spanning approximately one order of magnitude (0.5 to 250 fmol) was feasible. The method was successfully applied to the detection of the signaling molecules PQS and HHQ in cultures of P. aeruginosa strains isolated from sputum of cystic fibrosis patients and allowed a highly sensitive quantification of PYO from these cultures. Hence, the developed method bears the potential to be used for screening purposes in clinical settings and will help to decipher the molecular basis of bacterial communication. Graphical abstract Ionic liquid matrices for the detection and quantification of the toxin pyocyanin and other signaling molecules from P. aeruginosa by MALDI MS.

Identification and Quantification of N-Acyl Homoserine Lactones Involved in Bacterial Communication by Small-Scale Synthesis of Internal Standards and Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry

N-acyl homoserine lactones (AHL) are small signal molecules involved in the quorum sensing of many gram-negative bacteria, and play an important role in biofilm formation and pathogenesis. Present analytical methods for identification and quantification of AHL require time-consuming sample preparation steps and are hampered by the lack of appropriate standards. By aiming at a fast and straightforward method for AHL analytics, we investigated the applicability of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). Suitable MALDI matrices, including crystalline and ionic liquid matrices, were tested and the fragmentation of different AHL in collision-induced dissociation MS/MS was studied, providing information about characteristic marker fragments ions. Employing small-scale synthesis protocols, we established a versatile and cost-efficient procedure for fast generation of isotope-labeled AHL standards, which can be used without extensive purification and yielded accurate standard curves. Quantitative analysis was possible in the low pico-molar range, with lower limits of quantification reaching from 1 to 5 pmol for different AHL. The developed methodology was successfully applied in a quantitative MALDI MS analysis of low-volume culture supernatants of Pseudomonas aeruginosa. Graphical abstract ᅟ.

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.

Critical factors determining the quantification capability of matrix-assisted laser desorption/ionization- time-of-flight mass spectrometry

Quantitative analysis with mass spectrometry (MS) is important but challenging. Matrix-assisted laser desorption/ionization (MALDI) coupled with time-of-flight (TOF) MS offers superior sensitivity, resolution and speed, but such techniques have numerous disadvantages that hinder quantitative analyses. This review summarizes essential obstacles to analyte quantification with MALDI-TOF MS, including the complex ionization mechanism of MALDI, sensitive characteristics of the applied electric fields and the mass-dependent detection efficiency of ion detectors. General quantitative ionization and desorption interpretations of ion production are described. Important instrument parameters and available methods of MALDI-TOF MS used for quantitative analysis are also reviewed.This article is part of the themed issue 'Quantitative mass spectrometry'.

Identification of lipopeptide isoforms by MALDI-TOF-MS/MS based on the simultaneous purification of iturin, fengycin, and surfactin by RP-HPLC

A three-stage linear gradient strategy using reverse-phase high-performance liquid chromatography (HPLC) was optimized for rapid, high-quality, and simultaneous purification of the lipopeptide isoforms of iturin, fengycin, and surfactin, which may differ in composition by only a single amino acid and/or the fatty acid residue. Matrix-assisted laser desorption ionization time-of-flight tandem mass spectrometry (MALDI-TOF-MS/MS) was applied to detect the lipopeptides harvested from each reversed-phase HPLC peak. Amino acid analysis based on phenyl isothiocyanate derivatization was further used for confirmation of the amino acid species and molar ratio in a certain HPLC fraction. By this MALDI-TOF-MS/MS coupled with amino acid analysis, it was revealed that iturin at m/z 1,043 consists of a circular Asn-Tyr-Asn-Gln-Pro-Asn-Ser peptide and C14 β-OH fatty acid. Surfactin homologs from Bacillus subtilis THY-7 at m/z 1,030, 1,044, 1,058, and 1,072 possess a circular Glu-Leu-Leu-Val-Asp-Leu-Leu peptide and the β-OH fatty acid with a different length (C13-C16). Fengycin species at m/z 1,463 and 1,477 are homologs possessing the circular peptide Glu-Orn-Tyr-Thr-Glu-Ala-Pro-Gln-Tyr-Ile linked to a C16 or C17 γ-OH fatty acid, whereas fengycin at m/z 1,505 contains a Glu-Orn-Tyr-Thr-Glu-Val-Pro-Gln-Tyr-Ile sequence with a Val instead of Ala at position 6. The method developed in this work provided an efficient approach for characterization of diverse lipopeptide isoforms from the iturin, fengycin, and surfactin families.

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.

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry of small volatile molecules using a parylene-matrix chip

RATIONALE

In matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry (MS), volatile small molecules have been nearly impossible to analyze because (1) such molecules evaporate under drying and vacuum conditions and (2) the organic matrix creates matrix peaks in the low mass-to-charge (m/z) range (m/z <500). In this work, the analysis of volatile small molecules using MALDI-TOFMS was realized using (1) a parylene-matrix chip to eliminate the matrix peaks of the organic matrix and (2) graphene for the effective adsorption of the small volatile molecules.

METHODS

The parylene-matrix chip was produced by deposition of a partially porous parylene-N thin film on a dried organic matrix array. The sample solution of volatile small molecules was mixed with the graphene and then placed on the parylene-matrix chip for MALDI-TOFMS. Analogs of chemical agents called dimethyl methyl phosphonate (DMMP) and 2-chloroethylethylsulfide (CEES) were used as model compounds for the small volatile molecules, and the sensing parameters were estimated, such as the limit of detection (LOD) and the detection range.

RESULTS

MALDI-TOFMS based on the parylene-matrix chip and graphene as the adsorbent could achieve a LOD of approximately 1 ppb in the detection range of 1 ppm-1 ppb for the highly volatile DMMP and CEES.

CONCLUSIONS

The parylene-matrix chip with graphene can be applied for the detection of volatile small molecule analytes in the m/z ratio range of small molecules (m/z <500) using graphene as an effective adsorbent.

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometric imaging of synthetic polymer sample spots prepared using ionic liquid matrices

RATIONALE

Polymer sample spots for matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) prepared by the dried-droplet method often reveal ring formation accompanied by possible segregation of matrix and sample molecules as well as of the polymer homologs itself. Since the majority of sample spots are prepared by this simple and fast method, a matrix or sample preparation method that excludes such segregation has to be found.

METHODS

Three different ionic liquid matrices based on conventionally used aromatic compounds for MALDI-TOF MS were prepared. The formation of ionic liquids was proven by (1) H NMR spectroscopy. MALDI-Imaging mass spectrometry was applied to monitor the homogeneity.

RESULTS

Our results show a superior sample spot homogeneity using ionic liquid matrices. Spots could be sampled several times without visible differences in the mass spectra. A frequently observed loss of matrix in the mass spectrometer vacuum was not observed. The necessary laser irradiance was reduced, which resulted in less polymer fragmentation.

CONCLUSIONS

Ionic liquid matrices can be used to overcome segregation, a typical drawback of conventional MALDI dried-droplet preparations. Homogeneous sample spots are easy to prepare, stable in the MS vacuum and, thereby, improve the reproducibility of MALDI.

Laser desorption/ionization mass spectrometric analysis of folic acid, vancomycin and Triton® X-100 on variously functionalized carbon nanotubes

RATIONALE

Carbon nanotubes (CNTs) have been ascertained to constitute versatile assisting matrices for laser desorption/ionization mass spectrometric analysis of different molecules. The functionalization thereof can lead to obtaining laser desorption/ionization assisting surfaces that would allow the detection of molecules at lower concentration and produce spectra with a better signal-to-noise ratio.

METHODS

Pristine, -OH and -COOH functionalized multi-walled CNTs were obtained from commercial suppliers. Gallic or sinapinic acid was attached covalently to the CNT surfaces by forming an ester bond. Folic acid, vancomycin and Triton(®) X-100 were used as analytes to examine properties of these new assisting surfaces. Mass spectrometry analysis was conducted on a matrix-assisted laser desorption/ionization quadrupole time-of-flight (MALDIQTOF) mass spectrometer.

RESULTS

The functionalization of CNTs was confirmed with Fourier transform infrared (FTIR) spectroscopy. The obtained mass spectra revealed that all the assisting surfaces are capable of transferring energy to the analytes; moreover, the presence of carboxyl groups in the structures of CNTs highly enhances their ionization properties. Nevertheless, the presence of sinapinic acid on CNT surfaces does not increase their properties to absorb pulse laser energy.

CONCLUSIONS

The presented assisting surfaces are effective in LDI mass analysis of folic acid, vancomycin and Triton(®) X-100. The appropriate functionalization of CNTs can lead to the production of assisting surfaces that can become highly effective in the ionization of particular types of analytes.

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.

Surface-assisted laser desorption/ionization mass spectrometric detection of biomolecules by using functional single-walled carbon nanohorns as the matrix

A surface-assisted laser desorption/ionization time-of-flight mass spectrometric (SALDI-TOF MS) method was developed for the analysis of small biomolecules by using functional single-walled carbon nanohorns (SWNHs) as matrix. The functional SWNHs could transfer energy to the analyte under laser irradiation for accelerating its desorption and ionization, which led to low matrix effect, avoided fragmentation of the analyte, and provided high salt tolerance. Biomolecules including amino acids, peptides, and fatty acids could successfully be analyzed with about 3- and 5-fold higher signals than those obtained using conventional matrix. By integrating the advantages of SWNHs and the recognition ability of aptamers, a selective approach was proposed for simultaneous capture, enrichment, ionization, and MS detection of adenosine triphosphate (ATP). This method showed a greatly improved detection limit (1.0 μM) for the analysis of ATP in complex biological samples. This newly designed protocol not only opened a new application of SWNHs, but also offered a new technique for selective MS analysis of biomolecules based on aptamer recognition systems.

Green bioanalysis: some innovative ideas towards green analytical techniques

The key target of green chemistry is to make compounds and materials available to mankind, while causing no harm to the environment. In the 21st century analytical scientists are more concerned about green analytical method development. The number of literatures on green chemistry has undergone a dramatic increase in the new millennium. Green bioanalytical techniques aim to minimize or eliminate the hazardous waste associated with bioanalytical methods. An efficient and sincere approach towards bioanalytical method development has an enormous contribution towards green analysis. The selection of organic constituents of the mobile phase, choice of sample extraction process, adoption of an appropriate separation procedure and a few others, control the green chemistry approach of the bioanalytical method. In routine practice, UHPLC–MS can be the most suitable approach, while supercritical fluid chromatography is one of the best available techniques for green bioanalytical methods. Nevertheless, there always remains great scope of further research on green bioanalytical methods.

Ionic liquids as matrices in microfluidic sample deposition for high-mass matrix- assisted laser desorption/ionization mass spectrometry

Sample preparation for matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) via a microfluidic deposition device using ionic liquid matrices addresses several problems of standard protocols with crystalline matrices, such as the heterogeneity of sample spots due to the co-crystallization of sample and matrix and the limited capability for high-throughput analysis. Since ionic liquid matrices do not solidify during the measurement, the resulting sample spots are homogeneous. The use of these matrices is also beneficial for automated sample preparation, since crystallization of the matrix is avoided and, thus, no clogging of the spotting device can occur. The applicability of ionic liquids to the analysis of biomolecules with high molecular weights, up to ≈ 1 MDa is shown, as well as a good sensitivity (5 fmol) for recombinant human fibronectin, a protein with a molecular weight of 226 kDa. Microfluidic sample deposition of proteins with high molecular weights will, in the future, allow parallel sample preparation for MALDI-MS and for electron microscopy.

Performance evaluation on a wide set of matrix-assisted laser desorption ionization matrices for the detection of oligosaccharides in a high-throughput mass spectrometric screening of carbohydrate depolymerizing enzymes

Compared to other analytical methods, matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) presents several unique advantages for the structural characterization of degradation products of carbohydrates. Our final goal is to implement this technique as a high-throughput platform, with the aim of exploring natural bio-diversity to discover new carbohydrate depolymerizing enzymes. In this approach, a variety of carbohydrates will be used as enzymes substrates and MALDI-MS will be employed to monitor the oligosaccharides produced. One drawback of MALDI, however, is that the choice of the matrix is largely dependent on the chemical properties of the analyte. In this context, our objective in the present work was to find the smallest set of MALDI matrices able to detect chemically heterogeneous oligosaccharides. This was done through the performance evaluation of more than 40 MALDI matrices preparations. Homogeneity of analyte-matrix deposits was considered as a critical feature, especially since the final objective is to fully automate the analyses. Evaluation of the matrices was done by means of a rigorous statistical approach. Amongst all tested compounds, our work proposes the use of the DHB/DMA ionic matrix as the most generic matrix, for rapid detection of a variety of polysaccharides including neutral, anionic, methylated, sulfated, and acetylated compounds. The selected matrices were then used to screen crude bacterial incubation media for the detection of enzymatic degradation products.

Matrix interference-free method for the analysis of small molecules by using negative ion laser desorption/ionization on graphene flakes

This work presents a new approach for the analysis of small molecules with direct negative ion laser desorption/ionization (LDI) on graphene flakes. A series of matrix interference-free mass spectra were obtained for the analysis of a wide range of small molecules including peptides, amino acids, fatty acids, as well as nucleosides and nucleotides. The mixture of analytes and graphene flakes suspension were directly pipetted onto a sample plate for LDI-time-of-flight mass spectrometry (TOFMS) analysis. Deprotonated monomeric species [M-H](-) ions were homogeneously obtained on uniform graphene flakes film when negative ion mode was applied. In positive ion mode, the analytes were detected in form of multiple adduct ions such as sodium adduct [M+Na](+), potassium adduct [M+K](+), double sodium adduct [M+2Na-H](+), double potassium adduct [M+2K-H](+), as well as sodium and potassium mixed adduct [M+Na+K-H](+). Better sensitivity and reproducibility were achieved in negative ion mode compared to positive ion mode. It is believed that the new method of matrix interference-free negative ion LDI on graphene flakes may be expanded for LDI-MS analysis of various small molecules.

Biomedical application of MALDI mass spectrometry for small-molecule analysis

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) is an emerging analytical tool for the analysis of molecules with molar masses below 1,000 Da; that is, small molecules. This technique offers rapid analysis, high sensitivity, low sample consumption, a relative high tolerance towards salts and buffers, and the possibility to store sample on the target plate. The successful application of the technique is, however, hampered by low molecular weight (LMW) matrix-derived interference signals and by poor reproducibility of signal intensities during quantitative analyses. In this review, we focus on the biomedical application of MALDI-MS for the analysis of small molecules and discuss its favorable properties and its challenges as well as strategies to improve the performance of the technique. Furthermore, practical aspects and applications are presented.

MALDI immunoscreening (MiSCREEN): a method for selection of anti-peptide monoclonal antibodies for use in immunoproteomics

A scalable method for screening and selection of peptide-specific monoclonal antibodies (mAbs) is described. To identify high affinity anti-peptide mAbs in hybridoma supernatants, antibodies were captured by magnetic affinity beads followed by binding of specific peptides from solution. After timed washing steps, the remaining bound peptides were eluted from the beads and detected by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry (MS). This allowed measurement of monovalent interactions of peptides with single antigen binding sites on the antibodies, thus reflecting antibody affinity rather than avidity. Antibodies that were able to bind target peptides from solution phase and retain them during washing for a minimum of 10 min were identified by the strength of the appropriate m/z peptide MS signals obtained. This wash time reflects the minimum peptide dissociation time required for use of these antibodies in several current immuno-mass spectrometry assays. Kinetic analysis of antibody-peptide binding by surface plasmon resonance (SPR) showed that the selected antibodies were of high affinity and, most importantly, had low dissociation constants. This method, called MALDI immunoscreening (MiSCREEN), thus enables rapid screening and selection of high affinity anti-peptide antibodies that are useful for a variety of immunoproteomics applications. To demonstrate their functional utility in immuno-mass spectrometry assays, we used the selected, purified RabMAbs to enrich natural (tryptic) peptides from digested human plasma.