Letter: Multiply Charged Ions in Matrix-Assisted Laser Desorption/Ionization Generated from Electrosprayed Sample Layers

Abstract

Ultraviolet matrix-assisted laser desorption/ionization (UV-MALDI) detection of ions with charge states as high as those usually seen with electrospray ionization (ESI), was recently demonstrated by Gologan, Cooks and co-workers in soft-landing experiments. 1 In theses experiments, multiply charged ions of proteins generated by ESI were landed in high vacuum (~10 -8 torr) onto self-assembled monolayer (SAM) surfaces. To analyze the deposited protein material, laser desorption mass spectrometry (LD-MS) was used. By comparing the mass spectra from various samples, it was found that from some SAMs, LD-MS shows multiply charged ions, with the charge states being the same as in the original electrospray used for deposition. The interpretation was that multiply charged ions generated by ESI survive during soft-landing and laser desorption if an optimized SAM surface is used. It was noted that such a sample preparation can be used for the enhancement of multiply charged ions yielded in MALDI and can be useful, for example, for applying mass spectrometers with limited m/z range.From the experiments cited above, it appears that the type and structure of the surface used for soft landing greatly infl uences the yield of multiply charged ions. Based on this, we decided to investigate whether organic compounds used as MALDI matrices could also be used for ESI deposition of multiply charged ions. This is often referred to as the electrospray sample preparation method for MALDI. [2][3][4] Simultaneously, we wanted to investigate whether electrospray deposition at atmospheric pressure would also give a significant yield of multiply charged ions in MALDI.The effi ciency of desorption/ionization in MALDI and, in particular, the formation of multiply charged ions strongly depends on the type of matrix and laser irradiance. It has been shown that using matrices such as α-cyano-4-hydroxycinamic acid (HCCA) in UV-MALDI and caffeic acid in infrared (IR)-MALDI and laser irradiance near the threshold the yield of multiply charged ions can be increased. 5-7 The sample preparation method is also known to shift the charge state distribution in MALDI, 8 although it is generally significantly lower compared with ESI. Electrospray deposition of bovine-insulin onto an α-cyano-4-hydroxycinamic acid (HCCA) matrix layer was investigated in detail. It was found that, using electrospray preparation, certain HCCA matrix layers and low laser irradiance showing a signifi cant shift of the insulin ion distribution to higher charge states (lower mass-to-charge values) can be observed.The experiments were all performed on a commercial MALDI time-of-flight mass spectrometer (Axima CRF, Kratos/Shimadzu, Manchester, UK) equipped with a nitrogen laser (337 nm emission wavelength, 10 ns pulse duration, 100 µm × 100 µm laser spot size). The laser irradiance was varied in the range of 3-5 MW cm -2 (near the MALDI threshold) using a computer-controlled attenuator. The actual values were determined from a calibration curve relating to the attenuator setting and i...

Compelling Advantages of Negative Ion Mode Detection in High-Mass MALDI-MS for Homomeric Protein Complexes

“…Previous research on the charge distribution focused on the opposite, namely the search for parameters to increase the number of multiply charged cations since higher charge states open the way to extend the accessible mass range of instruments with limited m/z range and enhance the fragmentation for structural investigations. Elevated gas pressure in the source region [12,13] laser powers just above the threshold [12], addition of glycerol [14], electrospray-deposition onto a matrix layer [15], low abundance of photoelectrons by thick sample layers or deposition on an "electron-free" surface [16][17][18], and matrices with low proton affinity (such as alphacyano-4-hydroxycinnamic acid (CHCA)) [19], favor the presence of higher charge states for polypeptides.…”

Fragmentation of multiply-charged intact protein ions using MALDI TOF-TOF mass spectrometry

“…Indeed, Demirev et al have demonstrated the utility of this approach in profiling bacterial proteomes [32]. A key factor in expanding the useable mass range will be the ability to generate multiply charged ions from MALDI ion sources: therefore, examination of MALDI sample preparation strategies, such as non-metallic surfaces [28] and electrospray deposition [31] or even the recently reported MALDESI approach [44], is an important area of future investigation. Obviously, the analytical parameter space available for exploration is large, including sample preparation, activation methods, hardware/software improvements; however, the utility of the approach has been demonstrated albeit with a limited number of small proteins.…”

“…When the matrix/ analyte is co-crystallized on polyether ether ketone, an "electron free" surface, an unusually large fraction of multiply charged ions is produced, compared with the normally used stainless steel surface [28]. The charge state, as well as the signal intensity, is also related to the way the sample is spotted as evidenced by electrospray deposition of analyte resulting in multiply charged ions being produced at threshold laser irradiance [31]. There is one report of the use of multiply charged precursor ions in MALDI TOF-TOF experiments for protein identification in bacterial proteomes [32] that demonstrates the advantage of using multiple precursors to enhance database searching results.…”

Liquid AP‐UV‐MALDI Enables Stable Ion Yields of Multiply Charged Peptide and Protein Ions for Sensitive Analysis by Mass Spectrometry

“…One of these employed electrospray deposition of at least 200 pmol of analyte on various preformed MALDI matrix layers, showing that under specific conditions highly charged insulin ions can be detected albeit at a low signal-to-noise ratio. [15] Herein, we report progress in achieving high and prolonged yields of multiply charged peptide and protein ions using liquid UV-MALDI matrices and an AP ion source with an ion transfer tube that can be used at variable elevated temperatures of up to 400 8C. The liquid matrices described herein are based on 2,5-dihydroxybenzoic acid (DHB) or acyano-4-hydroxycinnamic acid (CHCA) with the addition of glycerol and triethylamine in various concentrations.…”