Operation Principle

APLI is based on resonance enhanced multiphoton ionization (REMPI) and uses pulsed laser systems as light source. 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 windows for laser beam entry and exit, an appropriate pulsed laser system with wavelengths between 266 and 193 nm and repetition rates up to 300 Hz and pulse energies up to 20 mJ. Typical pulse durations ranges between 5 and 10 ns, leading to power densities of about 1 MW/cm2.

Figure 1 is showing a photograph of a typical APLI MS set-up, Figure 2 a schematic diagram.

Schematic of an APLI setup
Figure 2

The benefits of APLI - besides strongly enhanced sensitivities (cf. Figure 4 below) - are:

  1. The ionization process is selective towards aromatic hydrocarbons; other compounds present are tolerated in large excess without noticeable interference.

  2. In practical terms, matrix compounds do not absorb the laser radiation – which means that matrix induced interferences are not observed.

If we take Figure 3 as an illustration of selected REMPI ionization schemes, we can restrict our attention basically just to case a): Step-wise two-photon excitation of the analyte. Other excitation schemes are hardly useful in APLI; the reasons are discussed in more depth further below. For now we just briefly summarize:

  1. Upon increasing the power density from MW/cm2 to GW/cm2 and larger, significant two-photon absorption occurs in the matrix compounds typically present in overwhelming excess. This means about 10 eV (2 x 248 nm) of electronic energy are coupled into the matrix molecules – an unfavorable situation comparable to APPI, see below.

  2. At such high power densities, extensive ionic fragmentation occurs; at AP the subsequently induced ion-molecule chemistry is generally complex and leads to ambiguous results.

 

Schematic overview of different REMPI processes
Figure 3

The ideal analyte has the following properties:

  1. Strong one-photon cross section at the laser wavelength (248 or 193 nm).
  2. A lifetime around 1 ns or larger in the excited state.
  3. Favorable Franck-Condon factors for the ionization step from the excited state.
  4. Favorable selection rules for both absorption processes.
  5. An ionization potential below the sum of the energy required to drive the overall two-step process.

By fortunate coincidence, one compound class generally features all of the above properties: Aromatic hydrocarbons, particularly larger systems with two or more rings, e.g. PAH. It is thus not surprising that initial APLI experiments were performed with PAH and related compounds. The results were extremely promising; APLI outperformed APCI as well as APPI by orders of magnitude with respect to sensitivity upon analyzing non-polar PAH mixtures, as illustrated in Figure 4.

Example Mass Spectra of APPI and APLI
Figure 4

In the mean time, we have drastically extended the range of compounds accessible with APLI:

  1. Upon adding a dopant, non-aromatic analytes with positive proton affinity yield the quasi molecular ion in large yields, directly comparable to DA APPI, as shown in Figure 5.
  2. Upon tagging non-aromatic compounds with REMPI labels (in analogy to derivatization in fluorescence analysis) selective APLI of tagged analytes is possible. In figure 6 a couple of examples are given.
Comparison of Mass Spectra with APLI and Dopant Assisted APLI
Figure 5
Mass Spectra of APLI Labeled Analytes
Figure 6