Analysis of fatty acids by graphite plate laser desorption/ionization time-of-flight mass spectrometry

A new update of MALDI-TOF mass spectrometry in lipid research

Matrix-assisted laser desorption and ionization (MALDI) mass spectrometry (MS) is an indispensable tool in modern lipid research since it is fast, sensitive, tolerates sample impurities and provides spectra without major analyte fragmentation. We will discuss some methodological aspects, the related ion-forming processes and the MALDI MS characteristics of the different lipid classes (with the focus on glycerophospholipids) and the progress, which was achieved during the last ten years. Particular attention will be given to quantitative aspects of MALDI MS since this is widely considered as the most serious drawback of the method. Although the detailed role of the matrix is not yet completely understood, it will be explicitly shown that the careful choice of the matrix is crucial (besides the careful evaluation of the positive and negative ion mass spectra) in order to be able to detect all lipid classes of interest. Two developments will be highlighted: spatially resolved Imaging MS is nowadays well established and the distribution of lipids in tissues merits increasing interest because lipids are readily detectable and represent ubiquitous compounds. It will also be shown that a combination of MALDI MS with thin-layer chromatography (TLC) enables a fast spatially resolved screening of an entire TLC plate which makes the method competitive with LC/MS.

Silicon surface assisted laser desorption ionization mass spectrometry for quantitative analysis

The approach to quantitative analysis by silicon Surface Assisted Laser Desorption Ionization Mass Spectrometry (Si-SALDI) is proposed. The approach is based on the new method for forming an active surface layer on a silicon substrate by exposing to laser radiation directly in the ion source of a mass spectrometer. The method can be used repeatedly on the same substrate, providing high reproducibility of its surface ionization properties and high ionization efficiency of organic compounds. Within the proposed approach, the methods of improvement of signal reproducibility are also considered, including continuous monitoring of the silicon surface ionization properties using a Knudsen effusion cell; scanning the surface of a silicon substrate with a laser beam; selecting the optimal value of laser fluence and using a reproducible sample introduction technique. It is demonstrated that this approach can be successfully applied to quantify clinically relevant concentrations of pharmaceutical drugs in extracts of blood.

Recent Developments of Useful MALDI Matrices for the Mass Spectrometric Characterization of Lipids

Matrix-assisted laser desorption/ionization (MALDI) is one of the most successful “soft” ionization methods in the field of mass spectrometry and enables the analysis of a broad range of molecules, including lipids. Although the details of the ionization process are still unknown, the importance of the matrix is commonly accepted. Both, the development of and the search for useful matrices was, and still is, an empirical process, since properties like vacuum stability, high absorption at the laser wavelength, etc. have to be fulfilled by a compound to become a useful matrix. This review provides a survey of successfully used MALDI matrices for the lipid analyses of complex biological samples. The advantages and drawbacks of the established organic matrix molecules (cinnamic or benzoic acid derivatives), liquid crystalline matrices, and mixtures of common matrices will be discussed. Furthermore, we will deal with nanocrystalline matrices, which are most suitable to analyze small molecules, such as free fatty acids. It will be shown that the analysis of mixtures and the quantitative analysis of small molecules can be easily performed if the matrix is carefully selected. Finally, some basic principles of how useful matrix compounds can be “designed” de novo will be introduced.

A direct assay of carboxyl-containing small molecules by SALDI-MS on a AgNP/rGO-based nanoporous hybrid film

Silver nanoparticles (AgNPs) and reduced graphene oxide (rGO) hybrid nanoporous structures fabricated by the layer-by-layer (LBL) electrostatic self-assembly have been applied as a simple platform for the rapid analysis of carboxyl-containing small molecules by surface-assisted laser desorption/ionization (D/I) mass spectrometry (SALDI-MS). By the simple one-step deposition of analytes onto the (AgNP/rGO)9 multilayer film, the MS measurements of various carboxyl-containing small molecules (including amino acids, fatty acids and organic dicarboxylic acids) can be done. In contrast to other energy transfer materials relative to AgNPs, the signal interferences of a Ag cluster (Agn(+) or Agn(-)) and a C cluster (Cn(+) or Cn(-)) have been effectively reduced or eliminated. The effects of various factors, such as the pore structure and composition of the substrates, on the efficiency of D/I have been investigated by comparing with the (AgNP)9 LBL nanoporous structure, (AgNP/rGO)9/(SiO2NP)6 LBL multilayer film and AgNP/prGO nanocomposites.

On-Tissue Derivatization via Electrospray Deposition for Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry Imaging of Endogenous Fatty Acids in Rat Brain Tissues

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry imaging (MSI) is used for the multiplex detection and characterization of diverse analytes over a wide mass range directly from tissues. However, analyte coverage with MALDI MSI is typically limited to the more abundant compounds, which have m/z values that are distinct from MALDI matrix-related ions. On-tissue analyte derivatization addresses these issues by selectively tagging functional groups specific to a class of analytes, while simultaneously changing their molecular masses and improving their desorption and ionization efficiency. We evaluated electrospray deposition of liquid-phase derivatization agents as a means of on-tissue analyte derivatization using 2-picolylamine; we were able to detect a range of endogenous fatty acids with MALDI MSI. When compared with airbrush application, electrospray led to a 3-fold improvement in detection limits and decreased analyte delocalization. Six fatty acids were detected and visualized from rat cerebrum tissue using a MALDI MSI instrument operating in positive mode. MALDI MSI of the hippocampal area allowed targeted fatty acid analysis of the dentate gyrus granule cell layer and the CA1 pyramidal layer with a 20-μm pixel width, without degrading the localization of other lipids during liquid-phase analyte derivatization.

Analysis of free fatty acids by ultraviolet laser desorption ionization mass spectrometry using insect wings as hydrophobic sample substrates

Physiologically relevant free fatty acids (FFAs) were analyzed by UV-laser desorption/ionization orthogonal extracting time-of-flight mass spectrometry (LDI-oTOF-MS). Dissected wings from Drosophila melanogaster fruit flies were used as the hydrophobic, laser energy strongly absorbing sample substrates. Using untreated substrates produces predominantly molecular [M + K](+) ions of the FFAs, whereas other alkali metal adducts can be generated by treating the wings with the corresponding alkali hydroxide before spotting of analyte. Limits of detection for the positive ion mode were determined for mixtures of isolated FFAs to values in the low 10 pmol range. Specific values depend on chain length and degree of unsaturation. R(2) coefficients for the analysis of saturated FFAs were found to be generally close to 0.98 over about 3 orders of magnitude if an internal standard (15:0 FFA) was added. Semiquantitative analyses of mixtures containing unsaturated FFAs are also possible but require more effort on the calibration strategy. Notably, both saturated and (poly-)unsaturated FFAs are detected sensitively in the presence of relatively high concentrations of other physiologically abundant lipids (phospholipids and triacyclglycerols). This simplifies screening of the FFA composition in crude tissue extracts. This feature is demonstrated by the analysis of a crude liver extract and that of fingermarks.

Determination of hidden hazelnut oil proteins in extra virgin olive oil by cold acetone precipitation followed by in-solution tryptic digestion and MALDI-TOF-MS analysis

Adulteration of extra-virgin olive oil (EVOO) with hazelnut oil (HO) is an illegal practice that could have severe health consequences for consumers due to the possible exposure to hidden hazelnut allergens. Here, matrix-assisted laser-desorption/ionization (MALDI) mass spectrometry (MS) was used as a rapid and sensitive technique for the detection of a low concentration of hazelnut proteins in oil samples. Different protocols were tested for protein extraction, and the most efficient (cold acetone) was applied to HO and EVOO adulterated with HO. The subsequent in-solution tryptic digestion of protein extracts and MALDI-MS analysis, using α-cyano-4-chlorocinnamic acid as matrix, allowed the detection of stable hazelnut peptide markers (i.e., the m/z ions 1002.52, 1356.71, 1394.70, 1440.81, 1453.85, 1555.76, 1629.83, 1363.73, and 1528.67) attributable to the main hazelnut proteins Cor a 9, Cor a 11, and Cor a 1. Thus, the approach might allow the direct detection of specific hazelnut allergens in EVOO at low concentration without time-consuming pretreatments.

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.

Furoic and mefenamic acids as new matrices for matrix assisted laser desorption/ionization-(MALDI)-mass spectrometry

The present study introduces two novel organic matrices for matrix assisted laser desorption/ionization mass spectrometry (MALDI-MS) for the analysis of small molecules. The first matrix is "2-amino-4,5-diphenylfuran-3-carboxylic acid" (also called furoic acid, FA) which was synthesized and then characterized by ultraviolet (UV), infrared (FTIR), nuclear magnetic resonance NMR ((1)H and (13)C) and mass spectrometry. The compound has organic semiconductor properties and exhibits intense UV-absorption which is suitable for the UV-MALDI laser (N2 laser, 337 nm). The second matrix is mefenamic acid (MA). The two matrices can be successfully applied for various classes of compounds including adenosine-5'-triphosphate (ATP, 0.5 µL(10.0 nmol)), spectinomycin (spect, 0.5 µL(14.0 nmol)), glutathione (GSH, 0.5 µL(9.0 nmol)), sulfamethazole (SMT, 0.5 µL(2.0 nmol)) and mixture of peptides gramicidin D (GD, 0.5µL (9.0 nmol)). The two matrices can effectively absorb the laser energy, resulting in excellent desorption/ionization of small molecules. The new matrices offer a significant enhancement of ionization, less fragmentation, few interferences, nice reproducibility, and excellent stability under vacuum. Theoretical calculations of the physical parameters demonstrated increase in polarizability, molar volume and refractivity than the conventional organic matrices which can effectively enhance the proton transfer reactions between the matrices with the analyte molecules. While the reduction in density, surface tension and index of refraction can enhance homogeneity between the two new matrices with the analytes. Due to the sublimation energy of mefenamic acid is (1.2 times) higher than that of the DHB, it is more stable to be used in the vacuum.

Laser desorption/ionization mass spectrometric analysis of surfactants on functionalized carbon nanotubes

RATIONALE

Recently, unmodified and carboxylated carbon nanotubes have been used as assisting surfaces laser desorption/ionization (LDI) in mass spectrometry. The functionalization of carbon nanotubes with organic compounds should lead to a gamut of other promising LDI-assisting surfaces.

METHODS

Carboxylated carbon nanotubes were functionalized with sinapinic acid either covalently or by creating an ionic macro-complex. Polyether-based surfactants were used as analytes to examine the properties of these new matrices. Mass spectrometric analysis was conducted on a LDI-quadrupole time-of-flight (QTOF) mass spectrometer. Carbon nanotube surfaces were deposited from suspension using the dried-droplet method.

RESULTS

The functionalization of the carbon nanotubes was confirmed with Fourier transform infrared (FTIR) spectroscopy. The usefulness of each material was examined with two poly(ethylene glycol) hexadecyl ether amphiphiles (Brij® 52 and Brij® 56) and a poly(ethylene glycol) monomethyl ether as analytes. Generally, the mass spectra obtained with carbon nanotubes covalently functionalized with sinapinic acid as a matrix had peaks with higher intensities than those obtained with carbon nanotubes functionalized by ionic macro-complex formation.

CONCLUSIONS

The presented new materials based on functionalized carbon nanotubes are effective in the LDI mass analysis of polyether amphiphiles and poly(ethylene glycol) monomethyl ether. This type of assisting surfaces can be highly modified by appropriate functionalization procedures.

Graphite supported preparation (GSP) of α-cyano-4-hydroxycinnamic acid (CHCA) for matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) for peptides and proteins

Graphite as MALDI matrix or in combination with other substances has been reported in recent years. Here, we demonstrate that graphite can be used as target coating supporting the crystallization of the α-cyano-4-hydroxycinnamic acid matrix. A conventional dried-droplet preparation of matrix and analyte solution on a graphite-coated metal target leads to a thin, uniform layer of cubic crystals with about 1 μm edge length. Commercially available graphite powder of 1-2 μm particle size is gently wiped over the target using a cotton Q-tip, leading to an ultra-thin, not-visible film. This surface modification considerably improves analysis of peptides and proteins for MALDI MS using conventional dried-droplet preparation. Compared with untreated targets, the signal intensities of standard peptides are up to eight times higher when using the graphite supported crystallization. The relative standard deviation in peak area of angiotensin II for sample amounts between 1 and 50 fmol is reduced to about 15 % compared with 45 % for untreated sample holders. For a quantification of 1 fmol of the peptide using an internal standard the coefficient of variation is reduced to 3.5 % from 8 %. The new graphite supported preparation (GSP) protocol is very simple and does not require any technical nor manual skills. All standard solvents for peptides and proteins can be used.

Graphene and graphene oxide: two ideal choices for the enrichment and ionization of long-chain fatty acids free from matrix-assisted laser desorption/ionization matrix interference

In this work, graphene (G) and graphene oxide (GO) were utilized to enrich and ionize long-chain fatty acids. All together five long-chain fatty acids were selected as models here, n-dodecanoic acid (C12), n-tetradecanoic acid (C14), n-hexadecanoic acid (C16), n-octadecanoic acid (C18), and n-eicosanoic acid (C20). Due to the large surface area and strong interaction force of G or GO, all the five long-chain fatty models were effectively enriched by G or GO. On the other hand, the excellent electronic, thermal, and mechanical properties enable G and GO to be prefect energy receptacles for laser radiation, which make the ionization steps more effective. Eventually, the promoted G and GO methodology can sensitively detect the five long-chain fatty acid models from real biological samples even at low concentrations. Meanwhile, by adopting our promoted methodology, the detection of long-chain fatty acids by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) was demonstrated to be simple, sensitive, fast, cost effective and high throughput, which is meaningful as to practical usage.

Analysis of deprotonated acids with silicon nanoparticle-assisted laser desorption/ ionization mass spectrometry

Chemically modified silicon nanoparticles were applied for the laser desorption/negative ionization of small acids. A series of substituted sulfonic acids and fatty acids was studied. Compared to desorption ionization on porous silicon (DIOS) and other matrix-less laser desorption/ionization techniques, silicon nanoparticle-assisted laser desorption/ionization (SPALDI) mass spectrometry allows for the analysis of acids in the negative ion mode without the observation of multimers or cation adducts. Using SPALDI, detection limits of many acids reached levels down to 50 pmol/µl. SPALDI of fatty acids with unmodified silicon nanoparticles was compared to SPALDI using the fluoroalkyl silylated silicon powder, with the unmodified particles showing better sensitivity for fatty acids, but with more low-mass background due to impurities and surfactants in the untreated silicon powder. The fatty acids exhibited a size-dependent response in both SPALDI and unmodified SPALDI, showing a signal intensity increase with the chain length of the fatty acids (C12-C18), leveling off at chain lengths of C18-C22. The size effect may be due to the crystallization of long chain fatty acids on the silicon. This hypothesis was further explored and supported by SPALDI of several, similar sized, unsaturated fatty acids with various crystallinities. Fatty acids in milk lipids and tick nymph samples were directly detected and their concentration ratios were determined by SPALDI mass spectrometry without complicated and time-consuming purification and esterification required in the traditional analysis of fatty acids by gas chromatography (GC). These results suggest that SPALDI mass spectrometry has the potential application in fast screening for small acids in crude samples with minimal sample preparation.

An update of MALDI-TOF mass spectrometry in lipid research

Although matrix-assisted laser desorption and ionization (MALDI) mass spectrometry (MS)--often but not exclusively coupled with a time-of-flight (TOF) mass analyzer--is primarily established in the protein field, there is increasing evidence that MALDI MS is also very useful in lipid research: MALDI MS is fast, sensitive, tolerates sample impurities to a relatively high extent and provides very simple mass spectra without major fragmentation of the analyte. Additionally, MALDI MS devices originally purchased for "proteomics" can be used also for lipids without the need of major system alterations. After a short introduction into the method and the related ion-forming process, the MALDI mass spectrometric characteristics of the individual lipid (ranging from completely apolar hydrocarbons to complex glycolipids with the focus on glycerophospholipids) classes will be discussed and the progress achieved in the last years emphasized. Special attention will be paid to quantitative aspects of MALDI MS because this is normally considered to be the "weak" point of the method, particularly if complex lipid mixtures are to be analyzed. Although the detailed role of the matrix is not yet completely clear, it will be also explicitly shown that the careful choice of the matrix is crucial in order to be able to detect all compounds of interest. Two rather recent developments will be highlighted: "Imaging" MS is nowadays widely established and significant interest is paid in this context to the analysis of lipids because lipids ionize particularly well and are, thus, more sensitively detectable in tissue slices than other biomolecules such as proteins. It will also be shown that MALDI MS can be very easily combined with thin-layer chromatography (TLC) allowing the spatially-resolved screening of the entire TLC plate and the detection of lipids with a higher sensitivity than common staining protocols.

Atmospheric pressure laser desorption/ionization of plant metabolites and plant tissue using colloidal graphite

Colloidal graphite is a promising matrix for atmospheric pressure laser desorption/ionization mass spectrometry. Intact [M+H](+) and [M-H](-) ions are readily produced from a wide range of small molecule plant metabolites, particularly anthocyanins, fatty acids, lipids, glycerides, and ceramides. Compared with a more traditional organic acid matrix, colloidal graphite provides more efficient ionization for small hydrophobic molecules and has a much cleaner background spectrum, especially in negative ion mode. Some important metabolites, e.g., fatty acids and glycosylated flavonoids, can be observed from Arabidopsis thaliana leaf and flower petal tissues in situ.

A laser desorption ionization time-of-flight mass spectrometry investigation into triacylglycerols oxidation during thermal stressing of edible oils

Laser desorption ionization time-of-flight mass spectrometry (LDI-TOF MS) was used to characterize olive and sunflower oils before and after thermally assisted oxidation in order to develop a rapid fingerprinting method for oil that contains unchanged and oxidized components. No matrix was used to assist laser desorption, and simplified mass spectra were obtained in the mass range of interest (m/z 500-1000), where triacyl- and diacylglycerol ions were observed. Sample preparation was reduced to dissolving oil in chloroform saturated with NaCl. Sodiated triacylglycerols (TAGs), their epoxy/hydroxy and hydroperoxy derivatives, as well as TAGs with shortened chain fatty acids (beta-scission products) were clearly observed in the spectra. LDI-TOF MS rapidly provides semiquantitative information about the oxidation level of edible oil, and thus represents a very useful quality control tool.

The use of pencil lead as a matrix and calibrant for matrix-assisted laser desorption/ionisation

Pencil lead is shown to be an effective matrix and calibrant in matrix-assisted laser desorption/ionisation (MALDI) mass spectrometry. Various groups of analytes, including peptides, polymers and actinide metals, can be readily ionised using MALDI when deposited onto a pencil lead matrix. The matrix is seen to have advantages in sample preparation relating to its hydrophobic properties and almost complete suppression of the matrix during analysis. Using pencil lead as a matrix is a quick and convenient method of qualitative analysis and has been shown to be quantitative for the isotope ratio analysis of actinide metals.

The use of desorption/ionization on porous silicon mass spectrometry for the detection of negative ions for fatty acids

The study of low molecular weight compounds by matrix-assisted laser desorption/ionization (MALDI) is difficult because of the presence of ions originating from the matrix in the low-m/z range. In order to resolve these problems, new matrix-free approaches were developed based on laser desorption/ionization from the surface of various materials such as graphite and porous silicon. Our work involves the use of 'desorption ionization on porous silicon mass spectrometry' (DIOS-MS) in the negative ion mode to study fatty acid compounds. The potential of the DIOS-MS technique is shown and an insight into the ionization mechanism provided.

Silicone/graphite coating for on-target desalting and improved peptide mapping performance of matrix-assisted laser desorption/ionization-mass spectrometry targets in proteomic experiments

In two-dimensional gel electrophoresis-based proteomic experiments matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS) peptide mass fingerprinting is often the technique of choice in identifying proteins. Here, we present a novel surface coating technique for MALDI-MS targets that improves manual and automatic sample analysis. A mixture of silicone and graphite is spread in the form of a thin layer over the target. Due to the hydrophobicity of the coating, aqueous solutions can be applied to relatively small spots very precisely using a robotic system. At least four times more liquid can be concentrated on the same area compared to uncoated steel targets. alpha-cyano-4-hydrocinnamic acid crystallizes in form of very small crystals evenly distributed over the surface. The search for "hot spots" during the analysis is not necessary, which supports the automatic acquisition of data. The homogeneous crystal layer can be very effectively washed on-target without encountering major sample losses. This efficient washing and the focused application of aqueous samples replace expensive and time-consuming reversed phase micro column based sample clean-ups. When analyzing peptide mixtures, the signal intensities are up to five times higher than with preparations of the same un-desalted samples on steel targets, since four times more sample can be loaded. The mass resolution remains unaffected by the surface coating. After usage the coating can be removed, followed by a new coating avoiding any carry-over of sample to the next analysis. All these properties make the precoating of MALDI-MS targets with a silicone/graphite layer an ideal technique for routine analysis in large-scale proteomic experiments.

Laser desorption/ionization time-of-flight mass spectrometry of triacylglycerols in oils

Selected triacylglycerols (TAGs) were directly analyzed on a standard stainless steel target plate by laser desorption/ionization time-of-flight mass spectrometry (LDI-TOFMS). Sodium and potassium ion adducts of TAGs were produced, and the thermal desorption/ionization mechanism was invoked to rationalize the experimental observations. The method permits a simple and fast qualitative analysis of TAGs. Advantages of this approach relative to matrix-assisted laser desorption/ionization (MALDI) are simpler sample preparation, lack of need to use a matrix with consequent absence of matrix interference peaks in the spectra, and potential improvements in shot-to-shot reproducibility due to the absence of the crystallization step resulting in a more homogenously deposited sample. The procedure was successfully applied to the determination of TAGs in whole oils, yielding very fast TAG fingerprints.

Oxidized carbon nanotubes as matrix for matrix-assisted laser desorption/ionization time-of-flight mass spectrometric analysis of biomolecules

Oxidized carbon nanotubes (CNTs), which can form a stable homogeneous suspension in water close to a solution phase, were synthesized and used for matrix-assisted desorption/ionization mass spectrometric (MALDI-MS) analysis of biomolecules. Infrared (IR) spectra, transmission electron microscopy (TEM) and particle size analysis were used for the characterization of the oxidized CNTs. The results indicate that the physical structure of the CNTs was not damaged, but carboxylate groups were introduced onto the surface of the CNTs. In addition, impurities including amorphous carbon, which is one of the main reasons for ion source contamination, were destroyed by the oxidization. The carboxyl groups on the oxidized surface of the CNTs can not only provide an additional proton source, but can also increase the surface polarity and solubility of the CNTs, making it easier to manipulate which is important for MALDI analysis of some biomolecules, especially larger peptides and proteins. The oxidized CNTs were successfully applied to the analysis of neutral oligosaccharides, peptides, and insulin, and thus promise to be an efficient matrix for MALDI-MS analysis of biomolecules.

Carbon nanotubes as affinity probes for peptides and proteins in MALDI MS analysis

Recently, carbon nanotubes (CNTs) have been reported to be an effective MALDI matrix for small molecules (Anal. Chem.2003, 75, 6191). In a somewhat related study, we have employed CNTs produced by using NaH-treated anodic aluminum oxide (Na@AAO) as a reactive template as the assisting matrix for MALDI analysis upon the addition of high concentrations of citrate buffer. Our results indicate that the mass range can be extended to ca. 12,000 Da and that alkali metal adducts of analytes are effectively reduced. Furthermore, we have employed citric acid-treated CNTs as affinity probes to selectively concentrate traces of analytes from aqueous solutions. High concentrations of salts and surfactants in the sample solutions are also tolerated. This approach is very suitable for the MALDI analysis of small proteins, peptides, and protein enzymatic digest products.

Matrix-assisted laser desorption and ionization time-of-flight (MALDI-TOF) mass spectrometry in lipid and phospholipid research

The interest in the analysis of lipids and phospholipids is continuously increasing due to the importance of these molecules in biochemistry (e.g. in the context of biomembranes and lipid second messengers) as well as in industry. Unfortunately, commonly used methods of lipid analysis are often time-consuming and tedious because they include previous separation and/or derivatization steps. With the development of "soft-ionization techniques" like electrospray ionization (ESI) or matrix-assisted laser desorption and ionization time-of-flight (MALDI-TOF), mass spectrometry became also applicable to lipid analysis. The aim of this review is to summarize so far available experiences in MALDI-TOF mass spectrometric analysis of lipids. It will be shown that MALDI-TOF MS can be applied to all known lipid classes and the characteristics of individual lipids will be discussed. Additionally, some selected applications in medicine and biology, e.g. mixture analysis, cell and tissue analysis and the determination of enzyme activities will be described. Advantages and disadvantages of MALDI-TOF MS in comparison to other established lipid analysis methods will be also discussed.