Liquid supports for ultraviolet atmospheric pressure matrix-assisted laser desorption/ionization

Nanoparticle-based applications by atmospheric pressure matrix assisted desorption/ionization mass spectrometry

Recently, the development of atmospheric pressure matrix assisted desorption/ionization mass spectrometry (AP MALDI MS) has made contributions not only to biomolecule analysis but also to spatial distribution. This has positioned AP MALDI as a powerful tool in multiple domains, thanks to its comprehensive advantages compared to conventional MALDI MS. These developments have addressed challenges associated with previous AP MALDI analysis systems, such as optimization of apparatus settings, synthesis of novel matrices, preconcentration and isolation strategies before analysis. Herein, applications in different fields using AP MALDI MS were described, including peptide and protein analysis, metabolite analysis, pharmaceutical analysis, and mass spectrometry imaging.

Inline pneumatically assisted atmospheric pressure matrix-assisted laser desorption/ionization ion trap mass spectrometry

Atmospheric pressure (AP) matrix-assisted laser desorption/ionization (MALDI) is known to suffer from poor ion transfer efficiencies as compared to conventional vacuum MALDI (vMALDI). To mitigate these issues, a new AP-MALDI ion source utilizing a coaxial gas flow was developed. Nitrogen, helium, and sulfur hexafluoride were tested for their abilities as ion carriers for a standard peptide and small drug molecules. Nitrogen showed the best ion transport efficiency, with sensitivity gains of up to 1900% and 20% for a peptide standard when the target plate voltage was either continuous or pulsed, respectively. The addition of carrier gas not only entrained the ions efficiently but also deflected background species and declustered analyte-matrix adducts, resulting in higher absolute analyte signal intensities and greater signal-to-noise (S/N) ratios. With the increased sensitivity of pneumatically assisted (PA) AP-MALDI, the limits of detection of angiotensin I were 20 or 3 fmols for continuous or pulsed target plate voltage, respectively. For analyzing low-mass analytes, it was found that very low gas flow rates (0.3-0.6 l min(-1)) were preferable owing to increased fragmentation at higher gas flows. The analyte lability, type of gas, and nature of the extraction field between the target plate and mass spectrometer inlet were observed to be the most important factors affecting the performance of the in-line PA-AP-MALDI ion source.

On-chip solid-phase extraction pre-concentration/focusing substrates coupled to atmospheric pressure matrix-assisted laser desorption/ionization ion trap mass spectrometry for high sensitivity biomolecule analysis

Atmospheric pressure matrix-assisted laser desorption/ionization (AP-MALDI) has proven a convenient and rapid method for ion production in the mass spectrometric (MS) analysis of biomolecules. AP-MALDI and electrospray ionization (ESI) sources are easily interchangeable in most mass spectrometers. However, AP-MALDI suffers from less-than-optimal sensitivity due to ion losses during transport from the atmosphere into the vacuum of the mass spectrometer. Here, we study the signal-to-noise ratio (S/N) gains observed when an on-chip dynamic pre-concentration/focusing approach is coupled to AP-MALDI for the MS analysis of neuropeptides and protein digests. It was found that, in comparison with conventional AP-MALDI targets, focusing targets showed (1) a sensitivity enhancement of approximately two orders of magnitude with S/N gains of 200-900 for hydrophobic substrates, and 150-400 for weak cation-exchange (WCX) substrates; (2) improved detection limits as low as 5 fmol/microL for standard peptides; (3) significantly reduced matrix background; and (4) higher inter-day reproducibility. The improved sensitivity allowed successful tandem mass spectrometric (MS/MS) sequencing of dilute solutions of a derivatized tryptic digest of a protein standard, and enabled the first reported AP-MALDI MS detection of neuropeptides from Aedes aegypti mosquito heads.

A high-resolution scanning microprobe matrix-assisted laser desorption/ionization ion source for imaging analysis on an ion trap/Fourier transform ion cyclotron resonance mass spectrometer

A new scanning microprobe matrix-assisted laser desorption/ionization (SMALDI) ion source for high spatial resolution has been developed for linear ion trap and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). The source is fully compatible with commercial ion trap flanges (such as the LTQ series, Thermo Fisher Scientific). The source is designed for atmospheric pressure (AP) operation but is also suitable for mid-pressure operation. The AP mode is especially useful for investigating volatile compounds. The source can be interchanged with other ion sources within a minute when operated in the AP mode. Combining high-lateral resolution MALDI imaging with high mass resolution and high mass accuracy mass spectrometry, available in the FT-ICR mode, provides a new quality of analytical information, e.g. from biological samples. First results obtained with the new ion source demonstrate a maximum lateral resolution of 0.6 by 0.5 microm. Depending on the limit of detection of the chosen mass analyzer, however, the size of the focus had to be enlarged to a diameter of up to 8 microm in the FT-ICR mode, in order to create enough ions for detection. Mass spectra acquired for analytical imaging were obtained from single laser pulses per pixel in all the experiments. This mode allows us to investigate biological thin sections with desorption focus diameters in the micrometer range, known to cause complete evaporation of material under the laser focus with a very limited number of laser pulses. As a first example, peptide samples deposited in microstructures were investigated with the new setup. A high quality and validity of the acquired images were obtained in the ion trap mode due to the low limit of detection. High mass resolution and accuracy but poorer image quality were obtained in the ICR mode due to the lower detection sensitivity of the ICR detector.

Charge assisted laser desorption/ionization mass spectrometry of droplets

We propose and evaluate a new mechanism to account for analyte ion signal enhancement in ultraviolet-laser desorption mass spectrometry of droplets in the presence of corona ions. Our new insights are based on timing control of corona ion production, laser desorption, and peptide ion extraction achieved by a novel pulsed corona apparatus. We demonstrate that droplet charging rather than gas-phase ion-neutral reactions is the major contributor to analyte ion generation from an electrically isolated droplet. Implications of the new mechanism, termed charge assisted laser desorption/ionization (CALDI), are discussed and contrasted with those of the laser desorption atmospheric pressure chemical ionization method (LD-APCI). It is also demonstrated that analyte ion generation in CALDI occurs with external electric fields about one order of magnitude lower than those needed for atmospheric pressure matrix assisted laser desorption/ionization or electrospray ionization of droplets.

Peptide analysis: Solid phase extraction–elution on diamond combined with atmospheric pressure matrix-assisted laser desorption/ionization–Fourier transform ion cyclotron resonance mass spectrometry

Electrospray ionization (ESI) has been an indispensable ion generation technique for mass spectrometric analysis of biopolymers such as intact proteins and protein digests operated at atmospheric pressure. Since its advent in 1998, atmospheric pressure matrix-assisted laser desorption/ionization (AP-MALDI) quickly became a popular alternative for the analysis of peptides. Although AP-MALDI sources typically share the same vacuum interface and ion transmission hardware with ESI, it is generally found that ESI is superior in detection sensitivity. Here we present a method based on solid phase extraction and elution with surface-functionalized diamond nanocrystals (which we previously referred to as "SPEED") that not only streamlines AP-MALDI mass spectrometric analyses of peptides and other small biomolecules under typical operational conditions but also outruns ESI in ultimate detectable concentration by at least one order of magnitude.

Ionic (liquid) matrices for matrix-assisted laser desorption/ionization mass spectrometry-applications and perspectives

A large number of matrix substances have been used for various applications in matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS). The majority of matrices applied in ultraviolet-MALDI MS are crystalline, low molecular weight compounds. A problem encountered with many of these matrices is the formation of hot spots, which lead to inhomogeneous samples, thus leading to increased measurement times and hampering the application of MALDI MS for quantitative purposes. Recently, ionic (liquid) matrices (ILM or IM) have been introduced as a potential alternative to the classical crystalline matrices. ILM are equimolar mixtures of conventional MALDI matrix compounds such as 2,5-dihydroxybenzoic acid (DHB), alpha-cyano-4-hydroxycinnamic acid (CCA) or sinapinic acid (SA) together with organic bases [e.g., pyridine (Py), tributylamine (TBA) or N,N-dimethylethylenediamine (DMED)]. The present article presents a first overview of this new class of matrices. Characteristic properties of ILM, their influence on mass spectrometric parameters such as sensitivity, resolution and adduct formation and their application in the fields of proteome analysis, the measurement of low molecular weight compounds, the use of MALDI MS for quantitative purposes and in MALDI imaging will be presented. Scopes and limitations for the application of ILM are discussed.

Interfacing liquid chromatography with atmospheric pressure MALDI-MS

Two different strategies for coupling liquid chromatography with atmospheric pressure matrix assisted laser desorption/ionization (AP MALDI) are presented. The first method is flow-injection liquid AP UV-MALDI. Compared with previous similar research, the detection limit was improved 10 times to 8.3 fmol using a solution of 50 nM peptide with 25 mM alpha-cyano-4-hydroxycinnamic acid. The applicability of this method to measure oligosaccharides, actinomycin antibiotics, antibiotics, phosphopeptides, and proteins is demonstrated. The upper mass limit achieved with the current instrumentation is 6,500 Da (doubly charged cytochrome c). The feasibility of a second strategy based on single-droplet IR AP MALDI is demonstrated here. Aqueous peptide solutions were successfully measured by this method.

Liquid ultraviolet matrix-assisted laser desorption/ionization - mass spectrometry for automated proteomic analysis

We have combined several key sample preparation steps for the use of a liquid matrix system to provide high analytical sensitivity in automated ultraviolet -- matrix-assisted laser desorption/ionisation -- mass spectrometry (UV-MALDI-MS). This new sample preparation protocol employs a matrix-mixture which is based on the glycerol matrix-mixture described by Sze et al. The low-femtomole sensitivity that is achievable with this new preparation protocol enables proteomic analysis of protein digests comparable to solid-state matrix systems. For automated data acquisition and analysis, the MALDI performance of this liquid matrix surpasses the conventional solid-state MALDI matrices. Besides the inherent general advantages of liquid samples for automated sample preparation and data acquisition the use of the presented liquid matrix significantly reduces the extent of unspecific ion signals in peptide mass fingerprints compared to typically used solid matrices, such as 2,5-dihydroxybenzoic acid (DHB) or alpha-cyano-hydroxycinnamic acid (CHCA). In particular, matrix and low-mass ion signals and ion signals resulting from cation adduct formation are dramatically reduced. Consequently, the confidence level of protein identification by peptide mass mapping of in-solution and in-gel digests is generally higher.

Functionalized nanoparticles for liquid atmospheric pressure matrix-assisted laser desorption/ionization peptide analysis

Nanoparticles for the extraction of peptides and subsequent analysis using atmospheric pressure matrix-assisted laser desorption/ionization (APMALDI) have been evaluated. The atmospheric pressure source allows for particles to be directly introduced in the liquid matrix, minimizing sample loss and analysis time. Described in this work are two sample preparation procedures for liquid APMALDI analysis: a C18 functionalized silica nanoparticle for hydrophobic extractions, and an aptamer functionalized magnetite core nanoparticle for rapid, affinity extractions. The C18 particles provide a non-selective support for rapid profiling applications, while the aptamer particles are directed towards reducing the complexity in biological samples. The aptamer functionalized particles provide a more selective analyte-nanoparticle interaction whereby the tertiary structure of the analyte becomes more critical to the extraction. In both cases, the liquid APMALDI matrix provides a support for ionization, and acts as the releasing agent for the analyte-particle interaction. Additionally, analyte enrichment was possible due to the large surface-to-volume ratio of the particles. The experiments conducted with functionalized nanoparticles, in an atmospheric pressure liquid matrix, present a basis for further methodologies and utilities of silica nanoparticles to be developed.