Infrared laser desorption and ionization of polypeptides from a polyacrylamide gel

Atmospheric pressure laser desorption/ionization using a 6-7 µm-band mid-infrared tunable laser and liquid water matrix

Due to the characteristic absorption peaks in the IR region, various molecules can be used as a matrix for infrared matrix-assisted laser desorption/ionization (IR-MALDI). Especially in the 6-7 µm-band IR region, solvents used as the mobile phase for liquid chromatography have absorption peaks that correspond to their functional groups, such as O-H, C=O, and CH3. Additionally, atmospheric pressure (AP) IR-MALDI, which is applicable to liquid-state samples, is a promising technique to directly analyze untreated samples. Herein we perform AP-IR-MALDI mass spectrometry of a peptide, angiotensin II, using a mid-IR tunable laser with a tunable wavelength range of 5.50-10.00 µm and several different matrices. The wavelength dependences of the ion signal intensity of [M + H](+) of the peptide are measured using a conventional solid matrix, α-cyano-4-hydroxycinnamic acid (CHCA) and a liquid matrix composed of CHCA and 3-aminoquinoline. Other than the O-H stretching and bending vibration modes, the characteristic absorption peaks are useful for AP-IR-MALDI. Peptide ions are also observed from an aqueous solution of the peptide without an additional matrix, and the highest peak intensity of [M + H](+) is at 6.00 µm, which is somewhat shorter than the absorption peak wavelength of liquid water corresponding to the O-H bending vibration mode. Moreover, long-lasting and stable ion signals are obtained from the aqueous solution. AP-IR-MALDI using a 6-7 µm-band IR tunable laser and solvents as the matrix may provide a novel on-line interface between liquid chromatography and mass spectrometry.

Analysis of gangliosides directly from thin-layer chromatography plates by infrared matrix-assisted laser desorption/ionization orthogonal time-of-flight mass spectrometry with a glycerol matrix

A novel method is presented for direct coupling of high-performance thin-layer chromatography (HPTLC) with matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) for the analysis of biomolecules. A first key feature is the use of a liquid matrix (glycerol), which provides a homogeneous wetting of the silica gel and a simple and fast MALDI preparation protocol. A second is the use of an Er:YAG infrared laser, which ablates layers of approximately 10-microm thickness of analyte-loaded silica gel and provides a soft desorption/ionization of even very labile analyte molecules. The orthogonal time-of-flight mass spectrometer employed in this study, finally provides a high accuracy of the mass determination, which is independent of any irregularity of the silica gel surface. The analytical potential of the method is demonstrated by the compositional mapping of a native GM3 (II(3)-alpha-Neu5Ac-LacCer) ganglioside mixture from cultured Chinese hamster ovary cells. The analysis is characterized by a high relative sensitivity, allowing the simultaneous detection of various major and minor GM3 species directly from individual HPTLC analyte bands. The lateral resolution of the direct HPTLC-MALDI-MS analysis is defined by the laser focus diameter of currently approximately 200 microm. This allows one to determine mobility profiles of individual species with a higher resolution than by reading off the chromatogram by optical absorption. The fluorescent dye primuline was, furthermore, successfully tested as a nondestructive, MALDI-compatible staining agent.

Interfacing capillary gel microfluidic chips with infrared laser desorption mass spectrometry

We report on the fabrication and performance of a gel microfluidic chip interfaced to laser desorption/ionization (LDI) mass spectrometry with a time-of-flight mass analyzer. The chip was fabricated from poly(methylmethacrylate) with a poly(dimethyl siloxane) cover. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis was performed in the channel of the microfluidic chip. After electrophoresis, the cover was removed and either the PDMS chip or the PMMA cover was mounted in a modified MALDI ion source for analysis. Ions were formed by irradiating the channel with 2.95 microm radiation from a pulsed optical parametric oscillator (OPO), which is coincident with IR absorption by N-H and O-H stretch of the gel components. No matrix was added. The microfluidic chip design allowed a decrease in the volume of material required for analysis over conventional gel slabs, thus enabling improvement in the detection limit to a pmol level, a three orders of magnitude improvement over previous studies in which desorption was achieved from an excised section of a conventional gel.

Particle formation by infrared laser ablation of glycerol: implications for ion formation

The quantity and size distribution of micrometer-sized particles ejected from thin films of glycerol were measured using light scattering particle sizing. Thin glycerol films were irradiated at atmospheric pressure with an infrared optical parametric oscillator at wavelengths between 2.95 and 3.1 microm. Particulate material resulting from the ablation was sampled directly into a particle-sizing instrument and particles with diameters greater than 500 nm were detected and sized by light scattering. The fluence threshold for particle formation was between 2000 and 3000 J/m2 for all laser wavelengths. At threshold, fewer than 100 particles/cm3 were detected and this value increased to several thousand particles/cm3 at twice the threshold fluence. The average size of the coarse particles ranged from 900 nm to 1.6 microm at threshold and decreased by 10-20% at twice the threshold fluence. The coarse particle formation observations were compared with ion formation behavior in matrix-assisted laser desorption ionization and interpreted in terms of a photomechanical mechanism for material ablation and ion formation.

Characterization of infrared matrix-assisted laser desorption ionization samples by Fourier transform infrared attenuated total reflection spectroscopy

Fourier transform infrared attenuated total reflection (FT-IR ATR) spectroscopy was used to characterize thin films of succinic acid, a matrix compound commonly used with infrared matrix-assisted laser desorption ionization (IR-MALDI) mass spectrometry. IR spectra of succinic acid thin films deposited alone and in combination with the analyte biomolecules insulin and cytochrome c were obtained by FT-IR ATR spectroscopy. Spectra of analyte and matrix alone were similar to those obtained previously from KBr pellets, Nujol mull, or thin-film absorption, although the ATR spectra have significantly lower background interferences. Thin films deposited from mixtures of water and methanol have additional peaks compared to films deposited from a methanol solution. These additional peaks are attributed to carboxylate groups stabilized by residual water molecules. No evidence was found to suggest that residual water absorption contributes to absorption at wavelengths typically used for IR-MALDI. Absorption of energy by analyte vibrational modes with rapid energy transfer to the matrix is suggested as a contributor to desorption and ionization consistent with the FT-IR ATR results.

Direct from Polyacrylamide Gel Infrared Laser Desorption/Ionization

The direct combination of gel electrophoresis and infrared laser desorption/ionization time-of-flight mass spectrometry has been demonstrated. We present results for infrared laser desorption and ionization mass spectrometry of peptides and proteins directly from a polyacrylamide gel without the addition of a matrix. Analyte molecules up to 6 kDa were ionized directly from a vacuum-dried sodium dodecyl sulfate-polyacrylamide gel after electrophoretic separation. Mass spectra were obtained at the wavelength of 2.94 microm, which is consistent with IR absorption by N-H and O-H stretch vibrations of water and other constituents of the gel. A 5-nmol quantity of peptide or protein was loaded per gel slot, although it was possible to obtain mass spectra from a small fraction of the gel spot. This technique shows promise for the direct identification of both parent and fragment masses of proteins contained in polyacrylamide gels.

Direct Atmospheric Pressure Coupling of Polyacrylamide Gel Electrophoresis to Mass Spectrometry for Rapid Protein Sequence Analysis

Using laser desorption-atmospheric pressure chemical ionization we describe a novel approach for coupling mass spectrometry to polyacrylamide gel electrophoresis. In contrast to other approaches, the method allows for the direct sampling of a polyacrylamide gel-embedded protein without the addition of any exogenous matrixes and is performed at atmospheric pressure. After electrophoresis and enzymatic digestion, the gel is analyzed at AP by photons that desorb neutral peptide molecules, followed by corona discharge ionization in the gas-phase, and subsequent mass analysis. Our experimental results demonstrate the method to (1) rapidly identify electrophoresed proteins via "peptide fingerprinting" using protein databases, (2) detect single-amino acid polymorphisms, and (3) has potential for sub-picomole sensitivity while still maintaining in situ gel desorption-ionization at ambient conditions.

Laser desorption-atmospheric pressure chemical ionization: a novel ion source for the direct coupling of polyacrylamide gel electrophoresis to mass spectrometry

Laser desorption-atmospheric pressure chemical ionization-mass spectrometry (LD-APCI-MS) is presented for the atmospheric pressure (AP) sampling of tryptic peptides directly from a polyacrylamide gel. In contrast to other gel sampling mass spectrometric approaches, this technique does not require the addition of any exogenous matrices to the gel to assist with ionization. In this arrangement, a CO(2) laser at 10.6 micro m is used to desorb intact neutral peptide molecules from the gel, followed by ionization in the gas-phase with APCI. The ions are then sampled via a heated capillary inlet and transferred to a quadrupole ion trap mass spectrometer for mass analysis. Preliminary results suggest the polyacrylamide gel electrophoresis-LD-APCI-MS technique provides several advantages that could translate into a more convenient, robust methodology for the rapid identification and characterization of proteins. Finally, strategies regarding the further development of the method are presented.