Laser desorption chemical ionization mass spectrometry/mass spectrometry

Tabletop extreme ultraviolet time-of-flight spectrometry for trace analysis of high ionization energy samples

RATIONALE

Species with ionization energies beyond what is accessible using state-of-the-art lab sources are affected by poor detection limits in ordinary mass spectrometry setups, whose throughput is also often limited. Extreme ultraviolet (XUV) photoionization mass spectrometry, in combination with linear time-of-flight (TOF), is necessary for the sensitive detection of high ionization energy compounds at trace level. XUV photoionization is available at beamlines, although with limited access. A tabletop setup may fill such a gap.

METHODS

A self-developed tabletop system, based on a plasma discharge with extreme ultraviolet emission (λ = 5-50 nm) coupled to a TOF mass spectrometer, was used in this study. Simultaneous validation measurements with a reference electron ionization quadrupole mass filter were carried out. An in-house developed hollow toroidal coil (HTC) induction detector was used for concomitant photoelectron detection.

RESULTS

Straightforward XUV mass spectra without fragmentation, thanks to the single-photon ionization, were acquired. The measurements with the reference quadrupole were in agreement with the spectra acquired by XUV-TOF. The resolution obtained for N₂ was at least factor of 2 higher than that measured with the reference quadrupole. Initial energy distributions of photoelectrons were retrieved by cross-correlation that gave access to the photoionization distribution.

CONCLUSIONS

The system allows XUV single-photon ionization of elements and molecules with IE >10 eV that are of fundamental interest e.g. for water splitting and catalysis research. The demonstrated performance is now suitable for a prototype platform.

Characterization of the chemical components on the surface of different solids with electrospray-assisted laser desorption ionization mass spectrometry

In this study we demonstrate that electrospray-assisted laser desorption ionization (ELDI) mass spectrometry (MS) can be used to rapidly characterize major chemical components on the surfaces of different solids under ambient conditions. The major chemical components in (a) dried milks with different fat contents, (b) different color-regions of a painting, (c) the thin coating on a compact disc, (d) drug tablets, and (e) porcine brain tissue were rapidly characterized as protonated molecules [M+H](+) or sodiated molecules [M+Na](+) by ELDI-MS with minimum sample pretreatment. The ionized ions of synthetic polymer and dye standards were detected directly from dried sample solutions using either positive or negative ion mode. Further structural information for the FD&C Red dye was obtained through tandem mass spectrometric (MS/MS) analysis using an ion trap mass analyzer attached to the ELDI source.

Electrospray-assisted laser desorption/ionization mass spectrometry for direct ambient analysis of solids

A new method of electrospray-assisted laser desorption/ionization (ELDI) mass spectrometry, which combines laser desorption with post-ionization by electrospray, was applied to rapid analysis of solid materials under ambient conditions. Analytes were desorbed from solid metallic and insulating substrata using a pulsed nitrogen laser. Post-ionization produced high-quality mass spectra characteristic of electrospray, including protein multiple charging. For the first time, mass spectra of intact proteins were obtained using laser desorption without adding a matrix. Bovine cytochrome c and an illicit drug containing methaqualone were chosen in this study to demonstrate the applicability of ELDI to the analysis of proteins and synthetic organic compounds.

Laser desorption-atmospheric pressure chemical ionization mass spectrometry for the analysis of peptides from aqueous solutions

A recently reported ionization method, comprising an infrared (IR) laser pulse to desorb (LD) analyte species, followed by atmospheric pressure chemical ionization (APCI) with a corona discharge (LD-APCI) to effect ionization of the desorbed neutral analyte molecules, is described for the direct analysis of aqueous peptide solutions. The source employs a heated capillary atmospheric pressure (AP) inlet coupled to a quadrupole ion trap mass spectrometer and allows sampling under normal ambient air conditions. By use of the corona discharge, signals of the atmospheric pressure infrared matrix-assisted laser desorption/ionization (AP-IR-MALDI)-generated analyte protonated molecule were enhanced by factors as large as 1400. In addition, the acid modifier trifluoroacetic acid (TFA) was found to improve the AP-IR-MALDI-generated signal by a factor of approximately 10, whereas the LD-APCI generated signal yielded a 100-fold increase. In this study, the use of the corona discharge is described to enhance the analyte signal generated via AP-IR-MALDI and, as a tool, to probe the gas-phase neutral molecule population generated by the MALDI process. Finally, through the decoupling of desorption from ionization, implications regarding the application of LD-APCI for the direct analysis of numerous new analyte containing matrixes (e.g., polyacrylamide gel electrophoresis (PAGE), tissue, etc.) are discussed.

Atmospheric pressure laser desorption/chemical ionization mass spectrometry: a new ionization method based on existing themes

We have designed and constructed an atmospheric pressure laser desorption/chemical ionization (AP-LD/CI) source that utilizes a laser pulse to desorb intact neutral molecules, followed by chemical ionization via reagent ions produced by a corona discharge. This source employs a heated capillary atmospheric pressure inlet coupled to a quadrupole ion trap mass spectrometer and allows sampling under normal ambient air conditions. Preliminary results demonstrate that this technique provides approximately 150-fold increase in analyte ions compared to the ion population generated by atmospheric pressure infrared matrix-assisted laser desorption/ionization (AP-IR-MALDI).