LC-MS Interfaces


The analysis of biomolecules by mass spectrometry (MS) put rather severe constraints on the ionization method, as far as its ability to produce intact biomolecular ions (without uncontrollable fragmentation) is concerned. With the advent of the electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI), two ionization techniques of unprecedented softness appeared. Due to the increasing need for powerful analytics of complex mixtures, coupling of separation methods, such as high-performance liquid chromatography (HPLC), with the mass spectrometric detection appeared prerequisite for dealing with complex mixtures of biomolecules, such as protein digests of cell lysates. Concomitantly, the quest for the best LC-MS interface started early in the 1970's.

Aim of the project

Sheathless Electrospray-Interface for Coupling Capillary Separation Techniques to Mass Spectrometry
A simple and versatile sheathless low-flow ESI-interface is developed, facilitating direct sheathless online coupling of separation capillaries to MS. The device, based on a gold-foil produces durable sheathless electrospray emitters which enable to generate highly stable electrospraying for almost unlimited lifetimes. It can be utilized for capillary separation techniques, but also used for offline sample introduction to MS via flow injection ESI. The emitters generate stable electrosprays from low nl/min up to µl/min flow-rates, just dependent on mobile phase, applied voltage, distance resp. alignment between tip and orifice and chosen tip diameter. The emitters can be fashioned from any kind of capillary and their fabrication does not involve any chemical or thermal treatment. The devices are easy to manufacture and allow simple mounting and alignment of the separation capillary at the MS-interface. Therefore, they are ideally suited as "on-column" emitters for online coupling of coated open tubular or packed capillaries, but particularly monolithic capillary columns to mass spectrometry. This "all-in-one" capillary column emitter concept eliminates unwanted void volumes which may lead to band-broadening.

Figure A: Drawing of the experimental arrangement: (1) Nd:YAG pump laser, (2) OPO, (3) telescope, (4) focusing lens, (5) micro-beam nozzle holder, (6) entrance skimmer of the mass spectrometer (sample cone), and (7) the grounded electrode of the mass spectrometer (baffle). (8) HPLC pump, (9) injection valve with sample loop (9'), and (10) He-Ne laser using for adjustment are also displayed. Photographs show the sample cone region of the mass spectrometer with the liquid beam (horizontal) and the baffle (Figure B), and the head of a droplet-on-demand generator (vertical) with the stroboscope LED (Figure C).

Atmospheric Pressure Free Liquid Infrared MALDI Mass Spectrometry
A new atmospheric pressure (AP)-MALDI-type interface based on a free liquid (FL) micro-beam/micro-droplets and a mid-infrared optical parametric oscillator (mid-IR OPO) is developed. The device is integrated into a standard online nanoESI interface. The generation of molecular ions in the gas phase is believed to be the result of a fast (explosive) infrared laser-induced evaporative dispersion (not desorption) of the micro-beam into statistically charged nanodroplets. To improve the duty cycle and to reduce the sample consumption, a droplet-on-demand system was implemented (generating 100pL droplets). This setup can be operated at flow rates down to low nL/min flow-rates and represents a liquid MALDI alternative to nanoESI. Our particular interest was the application of the developed ion source for on-line coupling of liquid chromatography with mass spectrometry. The flow rates (> 100 µL/min), required for stable operation of the ion source in continuous liquid micro-beam mode matches perfectly the flow rate range of microHPLC.



[1] E Rapp, U Tallarek and D Lubda: Vorrichtung und Verfahren zur Kopplung von kapillaren Trennverfahren und Massenspektrometrie. DE 10 2004 005 888.1 (2004).

[2] E Rapp, A Charvát, H Urlaub and B Abel: Method and apparatus for providing a sample for a subsequent analysis. EP 07013904.3 (2007).

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