OPO Signal (Visible) OPO Idler (Near IR) OPA Idler (Mid IR)
and from this it is possible to obtain coefficients for their motions as function OPO wavelength. This allows for the scanning process to be completely automated.
Figure 6.12. Plot of OPA motor positions vs. OPO Wavelength.
The last step in generating IR light for the desired experiment is to remove the undesired component from the output. This is accomplished by passing the beam through several silicon waveplates that are mounted at Brewster’s angle, which depending on their orientation will select one polarization over the other.
By selecting for only the (e) polarization, the desired Mid-IR is obtained. Figure 6.13 shows a plot of the power corrected to a per-pulse energy as the wavelength range of interest is scanned.
Figure 6.13. Plot of Idler Energy vs. Wavelength (cm-1).
6.4 IRMPD Setup
IRMPD action spectroscopy requires the collection of individual IRMPD mass spectra for a series of wavelengths. These data are then plotted as the percent of
0 5 10 15 20 25 30 35 40 45
2500 2600 2700 2800 2900 3000 3100 3200 3300 3400 3500
laser energy (mj)
WN (cm-1)
Plot of Idler energy vs Frequncy
essentially the same process as used for other FT-MS experiments with the exception that the ions are exposed to IR radiation for fixed periods of time before detection. The irradiation of the ion cloud within our FT-MS requires that the light produced be brought in from the laser by a series steering mirrors and focused (with a 1 m focusing mirror) into the vacuum chamber through CaF2 window. (Figure 6.14) The timing and duration of the irradiation is control by the addition of a mechanical shutter (EOPC SH10, 15 ms response time). This shutter blocks the lights path to the FTMS cell and only allows it to pass when opened with a 10 V pulse produced at the appropriate time by the mass spectrometer.
Figure 6.14. Schematic of the IRMPD Setup.
6.5 Automated Data Acquisition
Automating the acquisition of mass spectra by interfacing the FT-MS and Laser Vision control system is necessary due to the amount of data generated during a typical experiment. This process dramatically decreases the amount of time needed to acquire
the composite spectrum, which not only speeds the experiment up but also improves it considerably. That is, the ion signal fluctuates less, the laser power is more constant, and there is less opportunity for the alignment to change all of which tends to lead to to better data. Automation also allows this composite spectrum to be acquired as a
chromatogram in the Omega data system, which allows for more detailed, and dramatically faster, data analysis.
To automate the collection of spectra, a software tool was developed to interface the Omega scripting language to the Laser Vision system through an RS-232
connection. Within the Omega software, a script is activated and the desired parameters for spectrum collection are utilized. This script signals the laser to move to a desired wavelength and holds it there until a response from the laser is received. This response then triggers the Omega software to acquire the MS signal data for the aforementioned wavelength. This process is repeated, stepping through the spectral range of the laser system and collecting individual mass spectra at each wavelength. The data are then saved to a chromatogram file for subsequent processing.
The Omega software runs within the Visual Basic environment and can modified via the use the Visual Basic scripting language (VBScript). Though this allows for a variety of modificantion, the modern conveniences of Wscript are not available. This is relevant because direct method to allow for a time delay to listen for the response of the laser is containted in Wscript and is not accessible via VBScript. To overcome this problem, an executable was written as a non-memory intensive way of counting. This executable is called via the external application property of Omega
MSComm object, which Microsoft does not allow non-developer access, so the NETComm.ocx wrapper is used to call the MSComm object.9
6.6 Proton Bound Dimer of Glycine
In one of the seminal works for this technique using FT-ICR, McLafferty et. al.
explored the IRMDP spectrum of the proton bound dimer of glycine.10 Since the spectrum of this ion has been well established, it was chosen as our initial substrate to study. Figure 6.15 shows our results plotted in the same form as McLafferty et al. (-ln(depletion) vs. Wavelength). Here the exposure times are adjusted to
15s(experimentally they were determined at 5 s for 3550 to 3580 cm-1 and 60 s for the 3350 cm-1 ) and the depletion is adjusted for laser power as seen in figure Figure numer needed. These results generally match those of McLafferty et al., but we see a much
higher relative intensity for the dominant peak.
Figure 6.15. IRMPD spectrum of Gly2H+.
To examine the effect of relative exposure times, Figure 6.16 shows the
depletion of the starting ion as a function of time versus laser pulse at frequency of 3584 cm-1. These plots show a linear response for both 8 and 10 Hz over several half-lives.
The fragmentation efficiency was observed to vary greatly depending on the laser cross section with the ion cloud and half-lives as small as 2 s were observed with linearity maintained.
Figure 6.16 Time Dependence of fragmentation of Gly2H+ at 3584 cm-1
Future Projects
Initial attempts to look at a variety of covalently bonded substrates were carried out. Though this technique offers the potential to investigate structures of gas-phase ions, there are for a number of potential pitfalls. To date, the majority of studies have been performed on solvated species, or other species that are held together by non-covalent interactions. Since fragmentation is due to multiphoton processes, a small increase in the barrier to fragmentation makes observing fragmentation exponentially harder without a dramatic increase in laser flux. This is especially true at lower frequencies because of the drop in photon energy and laser power. To examine ions
y = 0.0072x -‐ 0.0318