Matrix-assisted laser desorption/ionization with an external ion source Fourier-transform mass spectrometer

Lasers for matrix-assisted laser desorption ionization

Matrix-assisted laser desorption ionization (MALDI) was introduced 35 years ago and has advanced from a general method for producing intact ions from large biomolecules to wide use in applications ranging from bacteria identification to tissue imaging. MALDI was enabled by the development of high energy pulsed lasers that create ions from solid samples for analysis by mass spectrometry. The original lasers used for MALDI were ultraviolet fixed-wavelength nitrogen and Nd:YAG lasers, and a number of additional laser sources have been subsequently introduced with wavelengths ranging from the infrared to the ultraviolet and pulse widths from nanosecond to femtosecond. Wavelength tunable sources have been employed both in the IR and UV, and repetition rates have increased from tens of Hz to tens of kHz as MALDI has moved into mass spectrometry imaging. Dual-pulse configurations have been implemented with two lasers directed at the target or with a second laser creating ions in the plume of desorbed material. This review provides a brief history of the use of lasers for ionization in mass spectrometry and describes the various types of lasers and configurations used for MALDI.

In-source H/D exchange and ion-molecule reactions using matrix assisted laser desorption/ionization Fourier transform ion cyclotron resonance mass spectrometry with pulsed collision and reaction gases

Controlled in-source ion-molecule reactions are performed for the first time in an external matrix assisted laser desorption ionization (MALDI) source of a Fourier transform ion cyclotron resonance mass spectrometer. The MALDI source with a hexapole ion guide that was originally designed to incorporate pulsed gas to collisionally cool ions (Baykut, G.; Jertz, R.; Witt, M. Rapid Commun. Mass Spectrom. 2000, 14, 1238-1247) has been modified to allow the study of in-source ion-molecule reactions. Upon laser desorption, a reaction gas was introduced through a second inlet and allowed to interact with the MALDI-generated ions trapped in the hexapole ion guide. Performing ion-molecule reactions in the high pressure range of the ion source prior to analysis in the ion cyclotron resonance (ICR) cell allows to maintain the ultra high vacuum in the cell which is crucial for high mass resolution measurements. In addition, due to the reaction gas pressure in the hexapole product ion formation is much faster than would be otherwise possible in the ICR cell. H/D exchange reactions with different peptides are investigated, as are proton-bound complex formations. A typical experimental sequence would be ion accumulation in the hexapole ion guide from multiple laser shots, addition of cooling gas during ion formation, addition of reaction gas, varied time delays for the ion-molecule reactions, and transmission of the product ions into the ICR cell for mass analysis. In this MALDI source H/D exchange reactions for different protonated peptides are investigated, as well as proton-bound complex formations with the reaction gas triethylamine. Amino acid sequence, structural flexibility and folding state of the peptides can be seen to play a part in the reactivity of such ions.

A combined ion source for fast switching between electrospray and matrix-assisted laser desorption/ionization in Fourier transform ion cyclotron resonance mass spectrometry

A new ion source has been developed for Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS) that enables quick changes between matrix-assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI) modes. When operating as an ESI source, the sample solution is sprayed through an angled nebulizer. The generated ions pass through a glass capillary followed by a skimmer and three sequential hexapole ion guides. Ions can be accumulated in the third hexapole (storage hexapole) before they are injected into the ICR trap. The second hexapole is mounted on a movable platform which also carries the MALDI sample plate. During the switch from ESI to MALDI, this platform moves the second hexapole out of the hexapole series and locates a MALDI sample plate with 384 sample positions into the area directly in front of the storage hexapole. The storage hexapole is in a medium pressure chamber (MPC) which has windows both for the incoming laser beam and for the observation optics, as well as a gas tube for pulsing collision gas into the chamber. During the MALDI operation the focused laser beam enters the MPC, passes between the hexapole rods and irradiates a MALDI sample on the target plate. The sample molecules are desorbed/ionized into the storage hexapole and simultaneously cooled by collisions with the pulsed gas. Ions desorbed from multiple laser shots can be accumulated in this hexapole before they are transferred to the ICR trap. With the combined ion source a computer-controlled switch between MALDI and ESI modes is possible in less than a minute, depending on the position of the MALDI target on the 384-spot plate. Immediate acquisition of mass spectra is possible after mode switching without the need for tuning or re-calibration.

Metastable decay of peptide ions on a Fourier transform mass spectrometer equipped with an external ion source

Metastable decay rates of two peptides, RPPGFSPF and PKPQQFFGLM, were determined from ions produced in an external matrix-assisted laser desorption/ionization source with a Fourier transform mass spectrometer. An isolation and subtraction method that gives difference spectra was employed to monitor the product formation and metastable decays. The dependence of metastable decay rates on laser fluences and matrixes was demonstrated.

Applications of 1.06-micron IR laser desorption on a Fourier transform mass spectrometer

Various sugars, peptides, and lipids were analyzed on a Fourier transform mass spectrometer using laser desorption and ionization with and without the assistance of matrixes. A compact Nd:YAG laser with an output at 1.06 microns corresponding to fundamental frequency was employed. Gram-negative and Gram-positive bacteria were also subjected to laser desorption mass spectrometry. Characteristics ions of conjugated lipid, formed by attachment of alkali metal cations, endogenous to the cells, were observed. Particle/liquid matrixes (e.g., cobalt in glycerol) proved to be useful with the 1.06-micron laser. The particles absorb efficiently laser radiation in a broad wavelength range. The liquid provides the same advantages as in fast atom bombardment: increased signal-to-noise ratios and enhanced sample lifetimes. The effect of laser power on total ion current was shown to differ for samples with and without the particle/liquid matrix. The Fourier transform analyzer provides MS/MS capability for both positive and negative ions from complex mixtures. Ions desorbed externally are introduced into the cell via a quadrupole ion guide with a lower mass cutoff. Such a setup allows matrix ions to be excluded and thus provides excellent signal-to-noise ratios for lower mass range fragment ions formed inside the cell.

High-resolution MALDI Fourier transform mass spectrometry of oligonucleotides

The matrix-assisted laser desorption/ionization (MALDI) method has been used with an external ion source Fourier transform mass spectrometer (FIMS) to analyze single-stranded, mixed-base oligomers of DNA. It is demonstrated that ultrahigh mass resolution (830 000 fwhm) can be achieved for small oligomers, and high resolution (136 000 fwhm) can be achieved for a 25-mer at m/z 7634. MALDI-FTMS can clearly separate the molecular ion peaks from analyte-matrix adduct peaks and alkali metal-containing species that result from replacement of hydrogen ions with sodium or potassium ions at multiple sites along the phosphate backbone. Previous MALDI-FTMS studies of oligonucleotides had two limitations: (1) low sensitivity due to difficulty in trapping the high kinetic energy ions made by the laser and (2) fragmentation of the ions due to the long delay (tens to hundreds of milliseconds) between their formation and detection. Both of these problems are alleviated in the present study. With the external ion source FTMS instrument, ions made by MALDI are injected at low energy into the analyzer cell by a rf-only quadrupole ion guide, captured by gating the voltage on the trapping plates, and cooled by a 0.5-s pulse of argon gas. Under these conditions, fragmentation is minimized, and DNA ions can be trapped in the FTMS analyzer cell for greater than 50 s. Sensitivity is also improved, as demonstrated by detection of 1 pmol of a single-stranded, mixed-base 20-mer of DNA, with a signal-to-noise ratio greater than 20:1.

Fourier transform mass spectrometry-advancing years (1992-mid. 1996)

This article is one of a series of Fourier transform mass spectrometry (FTMS) reviews that has appeared in this journal at ca. 3-4 year intervals. A comprehensive review of the recent theoretical developments, instrumental developments, electrospray ionization (ESI), and MALDI is given. Ion dissociation techniques are also discussed because of their contributions to gaining insight into chemical structure. Special sections have been devoted to discussing the emerging fields of surface analysis, polymer analysis, Buckminsterfullerenes (buckyballs), and hydrogen/deuterium exchange studies. This review, although not all-inclusive, is intended to be a starting point for those wishing to learn more about the current status of FTMS, and also as a representative cross-section of the literature for those familiar with the technique. © 1997 John Wiley & Sons, Inc.

Endgroup analysis of polyethylene glycol polymers by matrix-assisted laser desorption/ionization Fourier-transform ion cyclotron resonance mass spectrometry

Fourier-transform ion cyclotron resonance mass spectrometry (FTICR-MS) by external injection of matrix-assisted laser desorbed and ionized (MALDI) polymers offers good possibilities for characterization of low molecular weight homopolymers (MW range up to 10 kDa). The molecular masses of the molecular weight distribution (MWD) components of underivatized and derivatized (dimethyl, dipropyl, dibutyl and diacetyl) polyethylene glycol (PEG) 1000 and 4000 were measured by MALDI-FTICR-MS. These measurements have been performed using a commercial FTICR spectrometer with a home-built external ion source. MALDI of the samples with a 2,5-dihydroxybenzoic acid matrix in a 1000:1 matrix-to-analyte molar ratio produces sodiated molecules in a sufficient yield to trap the ions in the ICR cell. The masses of the molecular weight distribution of PEG components were measured in broad-band mode with a mass accuracy of < 5 ppm in the mass range around 1000 u and within 40 ppm accuracy around 4000 u. From these measurements, the endgroup mass of the polymer was determined by correlation of the measured component mass with the degree of polymerization. The masses of the PEG endgroups have been determined within a deviation of 3-10 millimass units for the PEG1000 derivatives and 10-100 millimass units for the PEG4000 derivatives, thus confirming the identity of the distal parts of the model compounds.

Detection limits for matrix-assisted laser desorption of polypeptides with an external ion source Fourier-transform mass spectrometer

Sensitivity in the low-femtomole range with mass resolution greater than 20,000 is demonstrated for several polypeptides analyzed by a mass spectrometer that pairs matrix-assisted laser desorption/ionization (MALDI) and Fourier-transform mass spectrometry (FTMS). The compounds investigated were substance P, renin substrate, melittin, the B-chain of bovine insulin, and bovine insulin. Standard solutions of the polypeptides were prepared with 30% acetonitrile+water, and micropipettes were used to transfer small amounts (1-20 fmol) to a sample probe. The samples were embedded in a large excess of matrix material (2,5-dihydroxybenzoic acid) and ionized by a pulse from an excimer laser. The FTMS instrument used for these experiments has the MALDI source in a separate chamber outside the magnetic field. Ions are extracted from the source and transported by an RF-only quadrupole ion guide to an FTMS analyzer cell mounted in the homogeneous region of a 6.5 T superconducting magnet. The high sensitivity of MALDI-FTMS is due, in part, to the high transfer efficiency of the ion guide, even for ions with a wide range of kinetic energies. The ion guide is easy to use because there are only two adjustments (RF amplitude and DC offset voltage), and unlike electrostatic ion transport means, alignment of it with the axis of the magnetic field is not critical. The mass resolution and sensitivity of MALDI-FTMS is compared with that of MALDI done with time-of-flight, magnetic sector, and quadrupole ion-trap mass spectrometers.

High-resolution mass spectrometer for protein chemistry

Matrix-assisted laser desorption/ionization is an accurate and sensitive method for measuring the molecular weights of peptides and proteins. Usually time-of-flight mass spectrometry is used to detect the laser-produced ions, but a new method called Fourier transform mass spectrometry offers greater sensitivity and much higher mass measurement accuracy.