Chemical characterization of mass-selected ions by infrared multiple photon dissociation spectroscopy

Infrared multiple photon dissociation (IRMPD) spectroscopy is a powerful tool in identifying ion structures in mass spectrometry. In recent years, most IRMPD studies have been conducted at free electron laser (FEL) facilities, which offer tunability and high spectral brightness over the mid-IR range (500-2000 cm-1). Here, we implement a tunable benchtop optical parametric oscillator (OPO) laser in combination with trapping mass spectrometers to carry out IRMPD spectroscopy in the hydrogen stretching range (3000-4000 cm-1). In the Penning trap of a Fourier transform ion cyclotron resonance mass spectrometer, more weakly bound metal-chelated amino acid complexes can be successfully photodissociated. For a series of group II metals (Mg 2+ -- Ba2+) complexed with the amino acids tryptophan (Trp) and methionine (Met), the O-H and N-H stretching vibrations readily allow distinction between the zwitterionic (Z) (i.e., NH3 +-CHR-CO2-) and charge solvated (C) (i.e., NH2-CHR-COOH) forms of the amino acid (where R denotes the side chain). It is thus determined that for the M2+(Trp)2 dimers, the smaller cations favor the Z form. Conversely, for the M 2+(Met)2 dimers, the opposite trend is observed. These trends are complemented by quantum-chemical calculations to rationalize the findings. It is proposed that the bulky indole side chain in tryptophan accounts for the unusual trend in favoring Z structures for smaller cations.;The IRMPD yield is shown to be increased by irradiating the ion cloud with a second non-resonant CO2 laser at a fixed frequency of 10.6 mum, following OPO laser irradiation. Moreover, the trapping voltages of the Penning trap can be adjusted to improve the overlap between the ion cloud and the laser beam, and hence enhance the IRMPD yield. These approaches are found to be particularly useful in boosting the IRMPD yield of weaker modes. Nonetheless, photodissociation of more strongly-bound ions is found to be much more challenging.;A custom-built mass spectrometer is presented, where the ions are irradiated in a reduced pressure (10-5 mbar) "Paul-type" quadrupole ion trap (QIT). The compact ion cloud is subjected to focused laser beams. Comparison of IRMPD of protonated tryptophan in the Penning and Paul traps shows that required laser irradiation times are considerably shorter and that weaker modes become visible. Furthermore, the fragmentation pathways of protonated tryptophan in collision-induced dissociation conditions are interrogated by IRMPD spectroscopy, assisted by quantum-chemical calculations. The loss of NH3 is shown to be mediated by a nucleophilic attack from carbon C3 on the indole side chain. The subsequent CH2CO loss product is also structurally characterized.;These results demonstrate that strongly-bound ions and reaction products from collision-induced dissociation in particular, can now be routinely characterized by IRMPD spectroscopy using a benchtop infrared laser. It is expected that these developments will make the technique of IRMPD spectroscopy more accessible to the wider mass spectrometry community, as opposed to being limited to a few user facilities.