Intact and top-down characterization of biomolecules and direct analysis using infrared matrix-assisted laser desorption electrospray ionization coupled to FT-ICR mass spectrometry

Abstract

We report the implementation of an infrared laser onto our previously reported matrix-assisted laser desorption electrospray ionization (MALDESI) source with ESI post-ionization yielding multiply charged peptides and proteins. Infrared (IR)-MALDESI is demonstrated for atmospheric pressure desorption and ionization of biological molecules ranging in molecular weight from 1.2 to 17 kDa. High resolving power, high mass accuracy single-acquisition Fourier transform ion cyclotron resonance (FT-ICR) mass spectra were generated from liquid-and solid-state peptide and protein samples by desorption with an infrared laser (2.94 µm) followed by ESI post-ionization. Intact and top-down analysis of equine myoglobin (17 kDa) desorbed from the solid state with ESI post-ionization demonstrates the sequencing capabilities using IR-MALDESI coupled to FT-ICR mass spectrometry. Carbohydrates and lipids were detected through direct analysis of milk and egg yolk using both UV- and IR-MALDESI with minimal sample preparation. Three of the four classes of biological macromolecules (proteins, carbohydrates, and lipids) have been ionized and detected using MALDESI with minimal sample preparation. Sequencing of O-linked glycans, cleaved from mucin using reductive β-elimination chemistry, is also demonstrated.

What can we learn from ambient ionization techniques?

“…At around the same time IR laser-assisted desorption electrospray ionization (IR LADESI) was introduced with the use of a nanoESI source for ionization [128]. Sampson et al proposed LAESI and IR LADESI "may be better described as IR-MALDESI" (MALDESI ϭ matrix-assisted laser desorption electrospray ionization) because of the use of water as a matrix [25]. More recently, ESI was coupled to laser-induced acoustic desorption (LIAD) where a laser beam is directed at the back of a substrate surface containing the sample and desorption occurs due to the action of the propagating acoustic wave [26].…”

Infrared Laser Ablation Sample Transfer for MALDI and Electrospray

“…The additional ion at m/z 533 (indicated with an asterisk) corresponds to a singly charged lipid ion or other component of the milk. The mass spectra of chicken egg yolk and egg white from a fresh egg are shown in Figure 6c [19,39,[47][48][49]. A mass spectrum obtained from egg white is shown in Figure 6d.…”

“…In contrast, the greater material removal of IR lasers is an advantage for ambient ionization techniques such as MALDESI and the related LAESI technique [19][20][21]. The volumetric energy deposition with a mid-IR laser is comparable to that of UV lasers, but the depth of laser penetration is an order of magnitude larger [22].…”

Ambient Molecular Analysis of Biological Tissue Using Low-Energy, Femtosecond Laser Vaporization and Nanospray Postionization Mass Spectrometry

“…Several such techniques, electrospray-assisted laser desorption ionization (ELDI) [27], laser ablation electrospray ionization (LAESI) [13], infrared matrix-assisted laser desorption electrospray ionization (IR-MALDESI) [28], and laser electrospray mass spectrometry (LEMS) [29], can provide a lateral resolution of 20−200 μm without the necessity of adding external matrix to the sample. Different mechanisms have been proposed for the laser desorption step of these laser/electrospray hybrid techniques depending on the laser wavelength and pulse duration while they share similar ionization mechanism.…”

“…Different mechanisms have been proposed for the laser desorption step of these laser/electrospray hybrid techniques depending on the laser wavelength and pulse duration while they share similar ionization mechanism. LAESI and IR-MALDESI employ a nanosecond IR laser at a wavelength of 2.94 μm and take advantage of the inherent water present in many biological systems to absorb the energy required to induce desorption because water has a strong absorption band at the laser radiation wavelength [13,28]. In LEMS, laser pulses as short as 60 fs, usually at 800 nm, interact with the analytes via a nonlinear, multiphoton excitation mechanism [29].…”