Julie M. Gillis1,2, Sandra M. Osburn2, Michael J. van Stipdonk2,3, Theodore A. Corcovilos1,3

1Dept. of Physics, Duquesne University, Pittsburgh, PA; 2Dept. of Chemistry and Biochemistry, Duquesne University;3Pittsburgh Quantum Institute (http://www.pqi.org)

INTRODUCTIONTandem mass spectrometry (MS) is an essential tool for chemical analysis andcharacterization. The combination of MS with laser spectroscopy opens up a new layer ofdetail for structural and energetic studies of gas-phase ions. In particular, infrared multiplephoton dissociation (IRMPD) fragments ions in ways not seen in collisionally induceddissocation (Ref. 1). This work is traditionally done at large user facilities such as the FELIXfree-electron laser in Nijmegen, the Netherlands, but as access to these lasers is limited, it isuseful to develop table-top experiments for those wavelength accessible with commerciallyavailable lasers. Here we incorporate a high-power CO2 laser (wavelength 10.6 μm) into alinear ion trap mass spectrometer to perform IRMPD.

ACKNOWLEDGEMENTSWe thank Duquesne University for financial and material support, particularly the BayerSchool of Natural and Environmental Sciences, the Office of the Provost, the UndergraduateResearch Program, and the Faculty Development Fund. Thank you to Prof. J. Brodbelt (UT-Austin) for technical advice.

REFERENCES

We have modified a linear ion trap tandem mass spectrometer (ThermoFisher LTQ) to acceptlaser light into the ion trap volume by adding an optical viewport coaxial with the ion trap(Fig. 1). The viewport is located in the accessory flange at the rear of the LTQ. A 16.5mmO.D. through-hole is covered by a 25.4-mm O.D. × 5-mm thick interchangable window. Thewindow mates with the flange via a Viton o-ring, forming a high vacuum (10-4 Pa)compression seal. An additional o-ring sits on the outside of the window for strain relief.Additional blind screw holes are added to the flange to support a mask on the vacuum sideand a 60-mm optical cage for the external optics.

Our current system uses a ZnSe window for compatibility with our laser: a Synrad 48-2CO2 laser with wavelength 10.6 μm (943 cm-1) and CW power 20W. This window can bequickly replaced to admit different wavelengths, for example, from an OPO or Ti:Sapphirelaser. Our ultimate goal is to have a suite of various lasers spanning IR to UV which can beswitched in/out as desired for various spectroscopic measurements of gas-phase ions.

1. V.N. Bagratashvili, et al. Multiple Photon Infrared Laser Photophysics and Photochemistry.Harwood: Chur, Switzerland (1985); J. Oomens, et al. Int. J. Mass Spec. 254: 1-19 (2006).doi:10.1016/j.ijms.2006.05.009

2. C.M Leavitt, et al. J. Phys. Chem. A 113, 2350-2358 (2009). doi:10.1021/jp807651c3. G.S. Groenewold, et al. J Am Chem Soc 128(14), 4802-4813 (2006). doi:10.1021/ja058106n

Instrument modifications

Optical Layout

CONTACT INFORMATIONTed Corcovilos, Department of Physics, Duquesne University, 600 Forbes Ave. 317 Fisher Hall,Pittsburgh, PA 15282. Email: corcovilost@duq.edu Ph: (412) 396-5973

EXPERIMENTAL DESIGN

IRMPD OF URANYL-SOLVENT CLUSTERS

100 150 200 250 300 350 400 450 500

Initial scan

Isolate 291

IRMPD

Isolate 252

IRMPD

Isolate 213

IRMPD

(a)

(b)

(c)

(d)

(e)

(f)

(g)

291

[UO2(dmso)4]2+

252

[UO2(dmso)3]2+

213

[UO2(dmso)2]2+

270

UO2+

Counts

(arb

.)

m/z

Figure 1.(a) – Schematic of the modified LTQ accessory flange and cross-sectionshowing the key components. (b) – Exploded view of the window assembly.(c) – Photograph of the installed window.

(a)(a)

(c)

(b)

The optical layout is optimized to match the laser beam shape to the ion cloud inside themass spectrometer. A series of lenses (Fig. 2) brings the beam to a shallow waist at the centerof the linear ion trap with a 1/e2 beam radius of 250 μm. The final focusing is done bymaximizing photodissociation of molecular ions in the trap (see right column).

121 cm

5 cm 9 cm ~17 cm

+15 cm -10 cm +15 cm

Laser

Ion trap

λ = 10.6 μm

w = 1.75 mm

θ = 2 mrad

P = 20 W

Figure 2 – Simplified optical layout showing only thelenses and the nominal distances between elements.

The remaining optical components are arranged to bring the beam up to the level of the iontrap and provide beam steering degrees of freedom (Fig. 3). Course laser power control isprovided by removable beam splitters that divert energy into beam dumps. Fine powercontrol is done by pulse-width modulation of the laser at 5 kHz. A cage system is used forstability and to simplify construction. Also, protective shielding is attached to the cage afterassembly to contain the beam (not shown).

(a)

(b)

Figure 3.(a) – Schematic of the optical layout. Removable beam splitters areused to attenuate the beam. A removable mirror allows us to switch adifferent laser into the beam path if desired. The beam shaping lenses arelocated in the upper portion. Alignment is done by adjusting the two gold-plated solid copper mirrors in the periscope, axial positions of the lenses, andtransverse displacement of the final lens. (b) – Photograph of the system.The laser is in the bottom left corner of the photo with the LTQ massspectrometer in the upper right, roughly corresponding with the schematic.

The coarse alignment of the system is performed by removing the electrospray module fromthe mass spectrometer (note that the electrospray nozzle is offset from the trap axis, so thebeam will not travel through it), placing an optical power meter there, and maximizing thelight transmission through the ion trap. To avoid damaging the spectrometer, the laser isoperated at the minimum PWM duty cycle (3%) during alignment and the beam is furtherattenuated by installing beam splitters in the path.

The fine tuning of the laser alignment and focusing requires a signal from the massspectrometer itself. For this we use IRMPD of uranyl (UO2)-solvent clusters produced byelectrospray ionization. In particular, the presence of acetone and dimethylsulfoxide (dmso)were previously shown to shift the O=U=O asymmetric stretch mode near to our laserfrequency of 943 cm-1, making these good candidate systems for IRMPD (Ref. 2, 3).

The results of IRMPD in the UO2-dmso system areshown in Fig. 4. Without the laser, an initialmass/charge (m/z) spectrum of the system (Fig. 4a)shows large peaks at m/z = 291 and 323 which weidentify as [UO2(dmso)4]

2+ and [UO2(dmso)4(O2)2]2+.

By isolating the 291 peak (Fig. 4b) and irradiatingthese ions with the laser at low power (~10%), wedissociate one of the dmso molecules from thecluster, creating a new peak at m/z = 252,[UO2(dmso)3]

2+ (Fig. 4c). We can isolate this newpeak (Fig. 4d), irradiate, and strip off an additionaldmso, yielding [UO2(dmso)2]

2+ (Fig. 4e). FurtherIRMPD brings the system to the final state of freeUO2

+ (Fig. 4g). Curiously, we do not observe[UO2(dmso)1]

2+ (m/z = 174) in this sequence.

The sequence can be performed step-by-step asdescribed above. However, we can also progressthrough the cascade with a single IRMPD step bysimply increasing the power of the laser. Higher laserpowers more quickly remove dmso molecules fromthe cluster. This gives us a useful tool for aligningthe laser. We begin by isolating the m/z = 291 peakand then observe in real time the m/z spectrum aswe adjust the laser alignment at low power. As thelaser beam begins to overlap the ion trap, we willfirst see the growth of the m/z = 252 peak. Furthertuning the alignment and focus gives peaks in thesequence 291→252→213→270 as the overlapbetween the laser and the ion trap increases.

Figure 4 – Mass/charge spectra ofUO2-dmso clusters, showing acascade of dissociation caused byIRMPD. At low laser power the stepsoccur singly, but at high power,multiple steps may occur (e.g. frompanel (b) directly to panel (g)).

FUTURE WORK

We have demonstrated IRMPD using a modified commercial tandem mass spectrometer.Some technical issues remain unresolved, particularly synchronizing the pulse-widthmodulation of the laser with the mass spectrometer timing sequence. We have also added apulsed Nd:YAG laser (1064 nm + harmonics) to the system for UV/Vis dissociationexperiments. We have designed our system to be flexible so that we may add other lasers,such as tunable OPO or Ti:Sapphire lasers, in the future.

Our scientific goals are to further study uranyl-solvent clusters for comparison withstructure and energy calculations. We have also begun studies of peptide sequences with thegoal of identifying the sequences through their IRMPD fragmentation patterns.

Precise laser alignment

121 cm

5 cm 9 cm ~17 cm

+15 cm -10 cm +15 cm

Laser

Ion trap

λ = 10.6 μm

w = 1.75 mm

θ = 2 mrad

P = 20 W

Figure 2 – Simplified optical layout showing only thelenses and the nominal distances between elements.

The remaining optical components are arranged to bring the beam up to the level of the iontrap and provide beam steering degrees of freedom (Fig. 3). Course laser power control isprovided by removable beam splitters that divert energy into beam dumps. Fine powercontrol is done by pulse-width modulation of the laser at 5 kHz. A cage system is used forstability and to simplify construction. Also, protective shielding is attached to the cage afterassembly to contain the beam (not shown).

(a)

(b)

The coarse alignment of the system is performed by removing the electrospray module fromthe mass spectrometer (note that the electrospray nozzle is offset from the trap axis, so thebeam will not travel through it), placing an optical power meter there, and maximizing thelight transmission through the ion trap. To avoid damaging the spectrometer, the laser isoperated at the minimum PWM duty cycle (3%) during alignment and the beam is furtherattenuated by installing beam splitters in the path.

The fine tuning of the laser alignment and focusing requires a signal from the massspectrometer itself. For this we use IRMPD of uranyl (UO2)-solvent clusters produced byelectrospray ionization. In particular, the presence of acetone and dimethylsulfoxide (dmso)were previously shown to shift the O=U=O asymmetric stretch mode near to our laserfrequency of 943 cm-1, making these good candidate systems for IRMPD (Ref. 2, 3).

The results of IRMPD in the UO2-dmso system areshown in Fig. 4. Without the laser, an initialmass/charge (m/z) spectrum of the system (Fig. 4a)shows large peaks at m/z = 291 and 323 which weidentify as [UO2(dmso)4]

2+ and [UO2(dmso)4(O2)2]2+.

By isolating the 291 peak (Fig. 4b) and irradiatingthese ions with the laser at low power (~10%), wedissociate one of the dmso molecules from thecluster, creating a new peak at m/z = 252,[UO2(dmso)3]

2+ (Fig. 4c). We can isolate this newpeak (Fig. 4d), irradiate, and strip off an additionaldmso, yielding [UO2(dmso)2]

2+ (Fig. 4e). FurtherIRMPD brings the system to the final state of freeUO2

+ (Fig. 4g). Curiously, we do not observe[UO2(dmso)1]

2+ (m/z = 174) in this sequence.

The sequence can be performed step-by-step asdescribed above. However, we can also progressthrough the cascade with a single IRMPD step bysimply increasing the power of the laser. Higher laserpowers more quickly remove dmso molecules fromthe cluster. This gives us a useful tool for aligningthe laser. We begin by isolating the m/z = 291 peakand then observe in real time the m/z spectrum aswe adjust the laser alignment at low power. As thelaser beam begins to overlap the ion trap, we willfirst see the growth of the m/z = 252 peak. Furthertuning the alignment and focus gives peaks in thesequence 291→252→213→270 as the overlapbetween the laser and the ion trap increases.

Figure 4 – Mass/charge spectra ofUO2-dmso clusters, showing acascade of dissociation caused byIRMPD. At low laser power the stepsoccur singly, but at high power,multiple steps may occur (e.g. frompanel (b) directly to panel (g)).

FUTURE WORK

We have demonstrated IRMPD using a modified commercial tandem mass spectrometer.Some technical issues remain unresolved, particularly synchronizing the pulse-widthmodulation of the laser with the mass spectrometer timing sequence. We have also added apulsed Nd:YAG laser (1064 nm + harmonics) to the system for UV/Vis dissociationexperiments. We have designed our system to be flexible so that we may add other lasers,such as tunable OPO or Ti:Sapphire lasers, in the future.

Our scientific goals are to further study uranyl-solvent clusters for comparison withstructure and energy calculations. We have also begun studies of peptide sequences with thegoal of identifying the sequences through their IRMPD fragmentation patterns.

Precise laser alignment