Lasers and mass spectrometry

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Goals 1 Develop an optimized AFM-RLMS instrument for routine use, based on investigation and understanding of the physics and chemistry of the underlying sampling processes; 2 Build an extended spectroscopic catalogue for identification of target materials by RLMS; 3 Promote the use of the technique by making the instrument available to researchers in the field to facilitate further development of applications of the technique 4 Foster the next generation of scientists working in field of cultural heritage research and encourage the interdisciplinary integration of research and practical utility for the conservation field.

Articles Shawn C. Owens, Jacob A. Dillon and M. For the HOMO, o-xylene not only has the strongest tunnel ionization, but it also has strong overlap between ionization and recombination, while the other two isomers have weaker ionization as well as weaker overlap since their tunnelling ionization peaks move further away from the photo-recombination peak position. Therefore, in p- and m-xylenes, the dominant contributions are expected to be from the HOMO The above theoretical results can be summed up in Fig.

To mimic the effect of macroscopic propagation, the HHG results were obtained with laser intensity averaging 36 , No depletion effect was taken into account in our calculations. Also, HHG yield is stronger in p-xylene as compared to m-xylene, mostly due to a stronger overlap between tunnel ionization and photo-recombination in p-xylene, although ionization is slightly stronger in m-xylene. We found that the two contributions are mostly in phase for all isomers, see Fig. Calculated high harmonic spectra of xylenes. We also found that these ratios remain nearly unchanged at the lower intensity of 0.

A possible reason is that the ionization calculation based on the SFA does not fully account for the laser wavelength dependence, especially since it neglects all excited states of the targets. HHG spectroscopy is well known for its inherent spatial and temporal resolution that enabled to image orbitals and probe dynamics with Angstrom and attosecond precision 43 , 44 , 45 , Our results show that it is also highly sensitive to small changes in the molecular environment The subtle differences exhibited by xylene isomers both in the high harmonic yield and their dependence on the laser polarization lays the foundation for future experiments on time-resolved dynamics of ring opening and ring permutation processes exhibited by these molecules.

For example, m-xylene is known to undergo ring permutation upon photoexcitation by UV light to form o-xylene through an intermediate state The inherent spatial and temporal resolution offered by HHG spectroscopy combined with its ability to distinguish the isomers can enable to track such interesting isomerization dynamics.

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Both of our approaches to identify molecular isomers probing double ionization and high harmonic spectroscopy do not, in general, require polarization shaping. Linearly polarized transform limited light pulses are sufficient to differentiate xylene isomers without the need for any pulse shaping either in the frequency- or time-domain.

However, the sensitivity can be further enhanced in the spectrometric technique by switching laser polarization from linear to circular. Therefore, it is a direct, simpler and faster method than the existing femtosecond laser mass spectrometry and other conventional tandem mass spectrometric techniques.

In the interaction region, the molecules were randomly oriented. The ions were detected by a microchannel plate MCP. The mass spectrum was calibrated using Xe ions.

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Laser intensity was calibrated by measuring the saturation intensity of Ar. The ion yield as a function of laser pulse energy was plotted in a semi-log plot. Extrapolation of the linear portion of the ion signal curve defined the saturation energy, and comparison with the calculated ion yield using the ADK model provided the saturation intensity.

A finite gas cell was used to generate harmonic emission. The pressure in the gas cell was monitored with a Baratron gauge. The outer cell was connected to a high throughput scroll pump. Different harmonic orders are detected using a grazing-incidence concave grating which disperses and focuses them onto a MCP coupled to a phosphor screen. The screen was imaged with a charge coupled device CCD camera. The MCP voltage was adjusted to keep the detection in the linear regime.

The harmonic spectra were obtained under optimal phase-matching conditions by adjusting the position of the laser focus with respect to the pinholes and the absolute pressure inside the gas cell. The spectrometer was calibrated by using the Cooper minimum in the HHG spectrum of argon as a reference. This technique was validated in the past by measuring the harmonic spectrum with an aluminium filter inserted in the beam path and comparing it with the calculated transmission spectrum of a known thickness of aluminium. Laser intensity was calibrated by monitoring cut-off harmonics in Ar and validated in the past by measuring its saturation intensity using a fast ionization gauge by monitoring the ion signal.

A combination of half wavelplate HWP and a polarizer were used to vary the laser intensity.

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The ellipticity of the incident light defined as the ratio of the two orthogonal electric fields of the laser light was controlled by a combination of another adjustable HWP and a fixed quarter-wave plate. This arrangement ensured that the main axis of the ellipse is fixed in space. The harmonics were generated in a continuous flow jet formed from a nozzle of diameter 0.

The HHG detection system was composed of a flat-field concave grating and a microchannel plate detector coupled to a phosphor screen and charge-coupled device CCD camera. Reference measurements at linear polarization were interlaced to correct for laser drift. Error bars were obtained by bootstrapping with multiple angle scans typically 3. Xylene isomers are substituted benzene derivatives by two methyl groups.

The molecular orbitals and ionization energies for the three xylene isomers were obtained by Gaussian quantum chemistry code The ionization energies of m-, p- and o-xylene were 8. This is in close agreement with experiment values of 8. Angle dependent ionization rates were carried out using the strong-field approximation SFA 50 , 51 within the single-active electron approximation.

We used the wavefunctions generated from the Gaussian quantum chemistry code 49 at the Hartree-Fock level with the augmented correlation-consistent polarized valence triple-zeta aug-cc-pVTZ basis set. Since the HOMO and HOMO-1 have quite close ionization energies, well separated from lower molecular orbitals, we only account for these two orbitals in our simulations. The calculation were performed with a 3-cycle pulse with the laser intensity of 0.

Here we have included the Stark shift correction for the polar molecules o-xylene and m-xylene , following refs 38 , The ratios are virtually unchanged with a half-cycle laser calculation. They only slightly change without the Stark shift correction. High harmonic spectra of xylene molecules were calculated within a single-molecule response approximation using the quantitative rescattering QRS theory 36 , 37 , Within the QRS, HHG yield for a fixed photon energy and fixed laser direction or molecular alignment is proportional to the product of tunneling ionization rate and differential photo-recombination cross sections.

To compare with experiments, averaging over isotropic molecular alignment distribution was carried out. The actual QRS calculations are done at the level of complex amplitudes, although it was found for xylenes that qualitative understanding of the relative HHG intensities can be gained even without the phase consideration. The calculation of tunneling ionization was described above. For HHG simulations, in order to account for a definite sub-cycle electric field direction, only a half-cycle at the peak of the laser pulse is allowed to ionize the molecules.

Furthermore, only electron emission along the laser direction is taken into account. The photo-recombination dipoles were calculated using ePolyScat package 40 , The error has been fixed in the paper.

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Chhabra, N. A review of drug isomerism and its significance. Basic Med.

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Laser microprobe mass spectrometry: principle and applications in biology and medicine.

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Wu, H. Rapid communications in mass spectrometry 17 , Bjarnason, A. Tembreull, R. As a result, this method is much better at detecting qualitatively rather than quantitatively. LMMS is relatively simple to operate compared to other methods. Furthermore, its strength lies in its ability to analyze biological materials to detect certain compounds such as metals or organic materials. LAMMA is particular about the sample which is used.

The sample must be small and thin. Ionization of too much material results in a large microplasma whose time spread and ion energy distribution entering the mass spectrometer can result in undesired peak deformation. From Wikipedia, the free encyclopedia.