Development of the APLI Source

The development of methods for ionising species of interest under atmospheric-pressure conditions has enormously increased the analytical power of mass spectrometry and has allowed the direct determination of analytes present in various matrices. Quadrupole, ion-trap and time-of-flight mass analyzers with electrospray ionisation (ESI) and atmospheric-pressure chemical ionisation (APCI) are already applied in routine analysis. ESI normally produces little fragmentation while forming both protonated and deprotonated ions of most polar compounds in the positive and negative ionisation mode, respectively. In contrast to ESI, APCI generates ions from less polar compounds in combination with a corona discharge assisted by a heated nebulizer. However, APCI may produce in-source fragmentation of thermally unstable compounds and is therefore not preferred for qualitative assays. In the case of non-polar analytes, other approaches are required, such as electrochemistry with ESI, coordination ionspray (CIS) or dissociative electron-capture ionisation with APCI.
 

Photoionisation under atmospheric pressure has been used in connection with ion-mobility spectrometry (IMS) for a long time, and in 2000 Syage et al. described an atmospheric-pressure photoionisation (APPI) as a new ionisation method in LC-MS of non-polar compounds. The ionisation process in APPI is initiated by 10 eV photons that can ionise species with ionisation potentials lower than the energy of the photons. Typical ionisable species are thus compounds with conjugated double bonds, such as aromatic molecules. The principle mechanism for photoionisation of molecule M is photon absorption and electron ejection to form the molecular ion M⁺. In the presence of water vapour or protic solvents, the molecular ion can extract H to form MH⁺. This tends to occur if M has a high proton affinity. Therefore, drug compounds in protic solvents are usually observed as MH⁺, while non-polar compounds like naphthalene usually form M⁺. Earlier in 2000, Bruins and co-workers introduced dopant-assisted atmospheric-pressure photoionisation (dopant-assisted APPI) as a new ionisation method in which the number of ions produced by a discharge lamp will be greatly increased by the addition of a large quantity of an ionisable compound (dopant) to the liquid eluent or to the vapour generated from the eluent. If the dopant is selected so that its photoions have a relatively high recombination energy or a low proton affinity, then the dopant photoions may react by charge exchange or proton transfer with species present in the ionisation region. Besides acetone and toluene, anisole was very recently described as a dopant for APPI. These reactions can improve the sensitivity, but may complicate the analysis of APPI mass spectra by fragments and solvent adducts.

In 2004 and in cooperation with the Department of Physical Chemistry at the University of Wuppertal we developed a new ionisation method, atmospheric-pressure laser ionisation. APLI is based on resonant or near-resonant two-photon ionisation of aromatic ring systems. In contrast to previous work on numerous analytical applications of REMPI in combination with mass-selective detection, APLI operates at atmospheric pressure (AP) in the ion-source region. A typical APLI set-up consists of an MS equipped with an API source fitted with fused silica windows for laser beam entry and exit, a table- top excimer laser running at 248 or 193 nm with a repetition rate of 200 Hz, 10 mJ pulse energy, and a pulse duration of 10 ns or less, leading to power densities of about 1 MW/cm². For initial experiments, the laser beam remains unfocused.

At the moment we are optimizing the coupling stages of HPLC-, CE- and GC-separation systems with the APLI source.