Device for the Reversed-Phase Separation and On-Target Deposition of Peptides Incorporating a Hydrophobic Sample Barrier for Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry

The separation of peptide mixtures from proteolytic cleavage is often necessary prior to mass spectrometry (MS) to enhance sensitivity and peptide mapping coverage. When buffers, salts, and other higher abundance peptides/contaminants are present, competition for charge during the electrospray ionization and matrix-assisted laser desorption/ionization (MALDI) processes can lead to ion suppression for the targeted analyte(s). In this note, a simple reversed-phase microcolumn sample separation and deposition device (Sep-Dep) is described. The use of this device improves or renders possible the analysis of complex or contaminated peptide mixtures by MALDI-MS. The method is simple and inexpensive and utilizes single-use low-cost Geloader-type columns packed with reversed-phase material. The device described utilizes an open column, allowing for a gradient or narrow-step gradient to be applied by any solvent delivery system or manually with a pipet. A key feature of the device is a deposition chamber that can be custom-built to hold any MALDI target. The Sep-Dep device is attached directly to an in-house vacuum line and draws solvent from the open-ended LC column. The elution of separated peptides is performed directly onto a target that has been treated with a hydrophobic barrier. This barrier effectively isolates fractions and improves the quality and morphology of the matrix crystals. The method produces efficient separations of proteolytic peptides, significantly reducing signal suppression effects in MALDI.

Design of a sheathless capillary eletrophoresis-mass spectrometry probe for operation with a Z-spray ionization source

The construction of a sheathless interface for capillary electrophoresis-electrospray ionization mass spectrometry (CE-ESI-MS), for operation with a Z-Spray source on a Micromass Quattro-LC triple quadrupole mass spectrometer is described. Designing the interface involved machining a probe compatible with the setup already in place on the mass spectrometer, i.e., MegaFlow-Z ESI. The probe was made of Lexan with the same dimensions as the ESI probe supplied with the instrument. The electrical connection at the electrospray end of the CE capillary was made possible by gold-coating (sheathless CE-ESI-MS). The probe design as well as the electrical and power supply requirements are described in detail. Experiments were performed using this interface, and CE separations of mixtures containing pmole and sub-pmole amounts of peptides were monitored by on-line MS. For a standard peptide mixture (10(-4) M), separation efficiency was typically characterized by N > 10(4) theoretical plates with S/N > 400. Using the same experimental setup, it was also possible to conduct on-line CE-ESI-tandem MS (MS/MS) experiments on the same peptide mixture, and to determine the sequence of the peptides. MS/MS scan functions for different precursor ions were used either alternately or sequentially and the results from both methods were compared. The possibility of peptide mass mapping was explored, and CE-ESI-MS results were obtained for the digestion products of equine myoglobin. Separation efficiencies and S/N values were similar to those obtained for standard peptides. A complete map of the digestion products was obtained.