Imaging of small molecules in tissue sections with a new intermediate-pressure MALDI linear ion trap mass spectrometer

We describe a new intermediate-pressure MALDI linear ion trap mass spectrometer and its capabilities for imaging mass spectrometry. The instrument design is described and is characterized in terms of four performance issues (1) MALDI performance at intermediate pressure; (2) analysis of samples on non-conductive and conductive glass slides; (3) critical importance of tandem mass spectrometry (both MS² and MS³) for identification of analyte species and imaging of isobaric species; (4) capability for repeated analysis of the same tissue section. Application of the new instrument to imaging phospholipids in rat brain sections is described in detail.     

MALDI produced ions inspected with a linear ion trap-orbitrap hybrid mass analyzer

A MALDI source is interfaced to a modified LTQ Orbitrap XL instrument. This work gives insight into the MALDI source design and shows results obtained with the MALDI source coupled to an accurate mass, high-resolution hybrid mass spectrometer. MALDI-produced ions and fragment ions thereof produced in the mass spectrometer may be analyzed and detected by the Orbitrap analyzer at a maximum mass resolution of 100,000 (FWHM) at m/z 400 with high mass accuracy. An accuracy of ≤2 ppm is achieved by internal mass calibration using lock mass functionality; using external mass calibration, an accuracy of ≤3 ppm is routinely obtained. External mass calibration of the hybrid mass spectrometer is performed using a standard calibration mixture of different peptides and matrix components. The instrumental capabilities are demonstrated for analytical methodologies such as Protein ID using Peptide Mass Fingerprint (PMF) and MS/MS analyses of small molecule samples. Stability of mass accuracy and signal-to-noise ratio for low samples loads (on plates) are demonstrated as well as the experimental dynamic range using α-cyano-4-hydroxy cinnamic acid (CHCA) matrix.     

Mass correlated acceleration in a reflectron MALDI TOF mass spectrometer: an approach for enhanced resolution over a broad mass range

Compared to continuous extraction, pulsed extraction (PE) of ions formed by matrix-assisted laser desorption/ionization (MALDI) in time-of-flight (TOF) mass spectrometers significantly improves mass resolution. Parameters such as extraction voltage, delay time, and correction pulse must be varied, however, to achieve optimum mass resolution over a broad mass range because the PE method is mass dependent. We previously reported a novel method, mass correlated acceleration (MCA), which we have now combined with a reflectron MALDI TOF mass spectrometer to further enhance mass resolution over a broader mass range. Unlike the PE method, MCA is not mass dependent and high resolution mass spectra can be achieved with a single tuning of instrument parameters. The ions may be brought into focus simultaneously, i.e., the multi-channel recording advantage can be more fully realized. The MCA dual-stage ion source design includes an extraction pulse region and an acceleration region that contains a time-dependent waveform correlated with mass. We demonstrate the validity of this novel technique with applications in peptide mixture analysis and protein digests of lysozyme and bovine serum albumin.     

Mass-correlated pulsed extraction: Theoretical analysis and implementation with a linear matrix-assisted laser desorption/ionization time of flight mass spectrometer

The pulsed extraction (PE) of ions produced by matrix-assisted laser desorption/ionization in time-of-flight mass spectrometers greatly improves mass resolution but, unfortunately, this method is mass dependent. Here we report an approach to expand the capabilities of the PE method so as to provide uniform focusing conditions over a wide mass range. Along with an extraction pulse, an additional pulse is applied to correct the mass dependency of the standard PE method. We describe the algorithm for derivation of this correction pulse waveform, where the first-order focusing conditions are valid all along the mass region of interest. Experimental verification of this method for correction of ion velocities demonstrated better mass resolution than standard PE over a wide mass range.     

Influence of disorder on superconductivity in non-magnetic rare-earth nickel borocarbides

The effect of substitutional disorder on the superconducting properties of YNi₂B₂C was studied by partially replacing yttrium and nickel by Lu and Pt, respectively. For the two series of (Y, Lu)Ni₂B₂C and Y(Ni, Pt)₂B₂C compounds, the upper critical field H _c2(T) and the specific heat c _p(T, H) in the superconducting mixed state have been investigated. Disorder is found to reduce several relevant quantities such as T _c, the upper critical field H _c2(0) at T=0 and a characteristic positive curvature of H _c2(T) observed for these compounds near T _c. The H _c2(T) data point to the clean limit for (Y, Lu) substitutions and to a transition to the quasi-dirty limit for (Ni, Pt) substitutions. The electronic specific heat contribution γ(H) exhibits significant deviations from the usual linear γ(H) law. These deviations reduce with growing substitutional disorder but remain even in the quasidirty limit which is reached in the Y(Ni_1−x , Pt _x )₂B₂C samples for x=0.1.     

Non-resonant microwave absorption studies of superconducting MgB2 and MgB2 + MgO

Non-resonant microwave absorption (NRMA) studies of superconducting MgB₂ and a sample containing ∼10% by weight of MgO in MgB₂ are reported. The NRMA results indicate near absence of intergranular weak links in the pure MgB₂ sample. A linear temperature dependence of the lower critical field H _c1 is observed indicating a non-s wave superconductivity. However, the phase reversal of the NRMA signal which could suggest d wave symmetry is also not observed. In the MgB₂ + MgO sample, much larger low field dependent absorption is observed indicating the presence of intergranular weak links. The hysteretic behavior of NRMA is compared and contrasted in the two samples. In the pure MgB₂ sample, a large hysteresis is observed between the forward and the reverse scans of the magnetic field indicating strong pinning of flux lines. This hysteresis saturates a few degrees below T _c while in the MgB₂ + MgO sample, a much slower increase of hysteresis with decreasing temperature is observed, a signature of weaker pinning.     

Miniaturized linear time-of-flight mass spectrometer with pulsed extraction

Improved resolution for a miniaturized instrument is demonstrated at high masses using a pulsed extraction, 3(") linear time-of-flight (TOF) mass analyzer. This illustrates the utility of a small and simple mass spectrometer for biological/medical analyses. Current and future applications suggested by this instrument include rapid mass spectral reading of oligonucleotides that differ in one base (single nucleotide polymorphisms), distinction of biomarker signatures from different species of bacterial spores (biological weapons detection) and point-of-care instruments for proteomics-based diagnostics. We have incorporated a two-stage, pulsed-extraction design that places the focal plane of the ions at the detector channel plate surface. The ions are accelerated to a total energy of 12 keV to enable detection of high-mass proteins in a design that incorporates a floatable flight tube set at the voltage of the front channel plate of the detector. The resultant elimination of post-acceleration at the detector is intended to improve mass resolution by reducing the difference in arrival times between ions and their neutral products. Resolutions of one part in 1200 at m/z 4500 and one part in 600 at m/z 12 000 have been achieved. Proteins with molecular masses up to 66 000 Da, mixtures of oligonucleotides, and biological spores have all been successfully measured, results that increase the potential use of this TOF analyzer.