Ionisation methods

In order to be detectable by mass spectrometry, the molecules in a sample must first be converted into charged particles (ions). This process is called‘ionisation’. There are a whole range of different ionisation methods, each with its own distinct characteristics. The choice of an ionisation method suitable for the mass spectrometric analysis of a particular sample is therefore very important for the reliability of the analytical results.

An important distinguishing feature of different ionisation methods is the pressure at which ionisation takes place. In the vast majority of mass spectrometric analyses in the fields of biology, medicine and related disciplines, ionisation takes place at atmospheric pressure (AP). These ionisation methods are often grouped under the term ‘Atmospheric Pressure Ionisation’ (API) and offer specific advantages, such as easy compatibility with liquid chromatography (LC) separation systems. In contrast, there is a whole range of ionisation methods that operate at significantly reduced ambient pressure.

The PTC is researching many different ionisation methods, particularly with regard to the chemical and physical conditions to which neutral parent molecules and the generated ions are exposed during and after ionisation, and which can lead to unwanted transformation processes within and after the ion source. The physicochemical conditions play a decisive role in determining which interferences may arise as a result of the chosen ionisation method. A deeper understanding of the chemical processes opens up ways to understand these interferences and, where necessary, to suppress them.

Electrospray is undoubtedly the most important ionisation method at atmospheric pressure (AP), and is of paramount importance for the applicability of mass spectrometry in the life sciences. A solution of the substance to be detected (the analyte) is atomised in a strong electric field. This produces liquid droplets containing ions of the analyte, which have been generated by electrochemical processes. These charged liquid droplets evaporate and fragment through electrodynamic processes. Ultimately, as a result of this process, ions of the analyte are released from the droplets in various ways and can be detected in the mass spectrometer.

The evaporation and release process is highly complex and remains the subject of ongoing research. It has been shown that this process is generally not completed within the ion source itself; rather, large quantities of charged liquid droplets can enter the transfer systems of modern mass spectrometers. As a result, these aspirated charged droplets are exposed to low pressures and strong electric fields. Furthermore, comparatively large quantities of the liquid sample solution enter the instruments’ vacuum systems. To date, little is known about the exact consequences of the aspiration of charged liquid droplets into mass spectrometers.

For this reason, the PTC is investigating the dynamics of the liquid droplets resulting from ESI in the inlet systems of mass spectrometers. Through experimental observations of droplets using various methods, combined with detailed numerical modelling of the internal dynamics and the motion of liquid droplets, we are seeking to understand their behaviour within the instruments and to gain insights into the consequences of their presence.

One elegant method of ionising organic molecules is through their interaction with electromagnetic radiation in the form of light. The absorption of light quanta in the UV range by such molecules leads to the excitation of electrons in the outer electron shell to excited states and, where applicable, also to the ionisation potential being exceeded, thereby resulting in photoionisation. As this process requires only the interaction of light with individual molecules, photoionisation can occur at both high and very low gas pressures.

Depending on the intensity and wavelength of the light used, there are many different mechanisms of photoionisation in detail. Typical ionisation potentials of organic molecules lie in the range of 10 eV. If this energy is supplied in a single step by just one photon, the required wavelengths are around 120 nm, which lies in the vacuum UV (VUV) range and can be provided by specialised gas-discharge light sources. Alternatively, the use of a very high photon density, such as that provided by a pulsed laser, also enables ionisation via the stepwise absorption of several photons. If this multi-step process occurs via resonant quantum mechanical states within the molecule, it is referred to as Resonance-Enhanced Multi-Photon Ionisation (REMPI).

Compared with other methods for the ionisation of certain classes of substances, photoionisation methods offer significant specific advantages, such as very high selectivity in the case of REMPI. PTC has therefore been investigating the specific properties of various photoionisation methods for many years and seeks to draw conclusions regarding the analytical application of these methods based on a precise understanding of these characteristics. Furthermore, laser-based photoionisation techniques also allow for very precise control over the location and timing of ionisation, which can be utilised in specialised experiments that, for example, help to elucidate the flow dynamics within the ion source.

Chemical ionisation methods are based on the transfer of charge from primary ions of a reactant substance to the analyte molecules via chemical reactions. The primary ions required for this process are generated by other physical interactions, such as gas discharges. Chemical ionisation (CI) can take place at atmospheric pressure (APCI) or at significantly lower background pressures. Unlike photoionisation, however, CI requires a direct reaction between primary ions and analyte molecules, and therefore also a certain minimum pressure in the ion source.

CI exhibits specific characteristics that depend on the exact conditions, such as the choice of primary ionisation method, the choice of reactant substance and the analyte to be ionised. As a CI ion source inevitably constitutes a chemically reactive environment, the chemical mechanisms involved – some of which are complex – are a particularly active area of research.

PTC has been conducting intensive research into the fundamentals of chemical ionisation methods for many years. This fundamental knowledge can then be used to develop chemical ionisation methods tailored to specific analytical applications, which, for example, utilise novel primary ionisation methods or minimise chemical interference.