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要旨:
Einleitung
In this presentation a new concept of an ion source, working at ambient conditions is depicted - the atmospheric pressure free liquid infrared matrix assisted laser desorption ionization (AP-FL-IR-MALDI) interface. This work is dealing with our efforts towards a unified ESI/liquid MALDI-type interface working at ambient conditions. Besides the operation of this new LC-MS interface in continuous micro-beam mode, developed for online coupling of micro- and narrowbore HPLC with mass spectrometry, a droplet-on-demand system which improves the duty cycle and reduces sample consumption is presented. This liquid MALDI type ion source represents an atmospheric pressure ionization alternative to ESI, which can be operated over/at the entire flow rate range required for LC-MS online coupling.
Experimentelles
This new atmospheric pressure MALDI-type interface is based on a free liquid micro-beam resp. micro-droplets and a mid-infrared optical parametric oscillator. The device is integrated with a standard online nano-ESI interface. It is developed on the basis of our FL-IR-MALDI technique, but operates at ambient pressure. It is more flexible, easier to handle and therefore more user-friendly compared to the previous vacuum version. The generation of molecular ions in gas phase is believed to be the result of a fast (explosive) laser-induced evaporative dispersion (not desorption) of a continuous micro-beam (resp. triggered micro-droplets) into statistically charged nano-droplets. Only low charge states appear in significant abundance in this type of experiment.
Vorläufige Daten
This interface enables us to operate a mass spectrometer in different ionization modes using the same hardware. Switching between (nano-)ESI (high voltage on, laser off) and AP-FL-IR-MALDI (high voltage off, laser on) can be readily done on a minute time scale. Mixed experiments, such as laser spray (both, high voltage and laser on) can be carried out easily. In this work the performance of the prototype of an AP-FL-IR-MALDI mass spectrometer is investigated. Mass spectra of common peptides have been acquired in positive ion mode and limit-of-detection was evaluated to be 17fmol/sec for the continuous micro-beam mode and to be 20 amol/sec for the droplet-on-demand mode. Our particular interest was directed towards the feasibility of this novel interface for online coupling of LC-MS. The first stage of application was the use of the developed ion source for online coupling of micro- and narrowbore HPLC with mass spectrometry. The flow rate range, well suited for this columns, perfectly matches the flow rates, required for stable operation of the ion source (>100 µL/min) in continuous micro-beam mode. Its feasibility is demonstrated by performing online LC-MS experiments in single MS mode, separating artificial peptide mixtures and generating PMFs of tryptic digested BSA. In the latter case, a sequence coverage of more than 80 % has been achieved. On a second stage and in spite of desired miniaturization (viz.: handling of limited sample amounts), a droplet-on-demand version was developed, operating at flow rates down to 100 nL/min or even lower, generating 100 pL droplets at 20 Hz. Synchronized with the laser pulses the duty cycle improves dramatically and sample consumption is decreased by several orders of magnitude. First results regarding its performance and applicability are presented, proving that the interface also fulfills the need of sub-femtomole sample consumption.
Grundlegende neue Aspekte
This liquid MALDI-type ion source is a step towards a unified liquid chromatography ESI/MALDI MS interface and represents an API alternative to ESI, which can be operated from nL to mL flow-rate-range.
Stichworte:
Liquid Chromatography (LC), Ion Source, Ionization, Laser, Mass Spectrometry (MS)
