We report the coupling of a hybrid ionization source, matrix-assisted laser desorption electrospray ionization (MALDESI), to a Fourier transform-ion cyclotron resonance mass spectrometer (FT-ICR MS). The details of the source design and initial data are presented. Analysis of peptides and proteins ranging from 1 to 8.6 kDa resulted in high resolving power single-acquisition FT-ICR mass spectra with average charge-states highly correlated to those obtained by nanoESI, thus, providing strong evidence that the ESI process dictates the observed charge-state distribution. Importantly, unlike the recently introduced electrospray assisted laser desorption ionization (ELDI) source reported by Shiea and coworkers [1,2], the data we have obtained to date rely on the use of an organic acid matrix. The results presented herein provide insight into the charging mechanism of this emerging ionization approach, while also expanding the utility of FT-ICR MS for top-down protein and complex mixture analysis. T he field of mass spectrometry has grown tremendously over the past century due in large part to the continued development of more powerful ionization techniques, mass analyzer technologies, and ion dissociation techniques. New ionization techniques inherently increase the breadth of applications amenable to mass spectrometry, thereby driving the discovery, development, implementation, and refinement of mass analyzers and ion dissociation methods. The advent of electrospray ionization (ESI) [3] and matrixassisted laser desorption ionization (MALDI) [4,5] resulted in a paradigm shift in the biological sciences whereby intact biomacromolecules could be ionized and detected by mass spectrometry.The benefits of direct analysis of samples under ambient conditions with minimal or no sample pretreatment has led to the development of a wide range of novel ionization methods with potentially extraordinary impact in numerous fields (e.g., environmental, forensics, material science, biomedical). These include fused-droplet electrospray ionization (FD-ESI) [6], laser desorption atmospheric pressure chemical ionization (LD-APCI) [7], desorption electrospray ionization (DESI) [8], direct analysis in real time (DART) [9], atmospheric-pressure solids analysis probe (ASAP) [10], and electrospray-assisted laser desorption/ionization (ELDI) [1,2]. Interestingly, modern high-performance mass analyzers are largely based on hybrid technology (e.g., coupling of linear ion trap with a Fourier transform mass spectrometer [11]) and, similarly, so are these new ionization techniques.Electrospray ionization is inherently well-suited for FT-ICR MS due to the inverse relationship between cyclotron frequency and m/z [12]. Because cyclotron frequency dictates mass resolving power, mass accuracy, limits of detection, and top-down tandem MS efficiencies, the multiple charging (lower m/z) afforded by ESI provides significant advantages [13]. Matrixassisted laser desorption ionization (MALDI) can also be coupled to FT-ICR MS where it can be a very powerful tec...

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.

We report the characterization of a recently introduced hybrid ionization source, matrix-assisted laser desorption electrospray ionization (MALDESI), coupled to a quadrupole Fourier transform ion cyclotron resonance mass spectrometry (QFT-ICR-MS) system. We first demonstrate the ability of MALDESI-QFT-ICR MS to directly analyze and provide high mass measurement accuracy (approximately 1 part-per-million) of a polypeptide using internal calibration. Second, we show the potential of MALDESI-QFT-ICR MS for the top-down characterization of multiply charged polypeptide cations. Finally, we demonstrate sub-femtomole detection limits in MALDESI-QFT-ICR MS using a combination of naturally occurring peptides and their respective stable isotope labeled forms. The results presented herein demonstrate the feasibility of several potential applications for MALDESI-QFT-ICR MS for the direct analysis of intact biological molecules.

We have developed an atmospheric pressure ionization technique called liquid matrix-assisted laser desorption electrospray ionization (liq-MALDESI) for the generation of multiply-charged ions by laser desorption from liquid samples deposited onto a stainless steel sample target biased at a high potential. This variant of our previously reported MALDESI source does not utilize an ESI emitter to post-ionize neutrals. Conversely, we report desorption and ionization from a macroscopic charged droplet. We demonstrate high mass resolving power single-acquisition FT-ICR-MS analysis of peptides and proteins ranging from 1 to 8.6 kDa at atmospheric pressure. The liquid sample acts as a macroscopic charged droplet similar to those generated by electrospray ionization, whereby laser irradiation desorbs analyte from organic matrix containing charged droplets generating multiply-charged ions. We have observed a singly-charged radical cation of an electrochemically active species indicating oxidation occurs for analytes and therefore water; the latter would play a key role in the mechanism of ionization. Moreover, we demonstrate an increase in ion abundance and a concurrent decrease in surface tension with an increase in the applied potential.

We report the first collision-induced dissociation tandem mass spectrometry (CID MS/MS) of a thiolate-protected Au nanoparticle that has a crystallographically determined structure. CID spectra assert that dissociation pathways for the mixed monolayer Na(x)Au(25)(SC(2)H(4)Ph)(18-y)(S(C(2)H(4)O)(5)CH(3))(y) centrally involve the semi-ring Au(2)L(3) coordination (L = some combination of the two thiolate ligands) that constitutes the nanoparticle's protecting structure. The data additionally confirm charge state assignments in the mass spectra. Prominent among the fragments is [Na(2)AuL(2)](1+), one precursor of which is identified as another nanoparticle fragment in the higher m/z region. Another detected fragment, [Na(2)Au(2)L(3)](1+), represents a mass loss equivalent to an entire semi-ring, whereas others suggest involvement (fragmentation/rearrangement) of multiple semi-rings, e.g., [NaAu(3)L(3)](1+) and [NaAu(4)L(4)](1+). The detailed dissociation/rearrangement mechanisms of these species are not established, but they are observed in other mass spectrometry experiments, including those under non-CID conditions, namely, electrospray ionization mass spectrometry (ESI-MS) with both time-of-flight (TOF) and FT-ICR analyzers. The latter, previously unreported results show that even soft ionization sources can result in Au nanoparticle fragmentation, including that yielding Au(4)L(4) in ESI-TOF of a much larger thiolate-protected Au(144) nanoparticle under non-CID conditions.