Gas-phase fluorescence excitation and emission spectroscopy of mass-selected trapped molecular ions

Abstract

A flexible interface to perform optical spectroscopic measurements on gaseous ions stored in a modified commercial quadrupole ion trap (QIT) mass spectrometer is described. The modifications made to the mass spectrometer did not adversely affect its operating characteristics. Gas-phase ions are produced using electrospray ionization, mass isolated and stored in the trapping mass spectrometer. The ions are subsequently irradiated with visible light from a tunable laser and dispersed fluorescence spectra are recorded simultaneously. Mass spectra are recorded after the irradiation period. This set-up allows us to track a range of possible outcomes upon photoexcitation of selected ions including fluorescence, photofragmentation and photodetachment of electrons. The experimental set-up is characterized using rhodamine 590, which is a methyl ester variant of rhodamine 6G. Fluorescence excitation and emission spectra of gaseous rhodamine 590 are measured and compared with solution-phase spectra. Excitation and emission maxima for the gaseous ions are found to lie at higher energy than for the solvated rhodamine 590. In addition, the gas-phase Stokes shift is significantly smaller than the solution-phase Stokes shift. The effects of several experimental parameters on the observed fluorescence signal are investigated, including laser power, relative number of ions, q(z) trapping parameter and buffer gas pressure. In addition to its use for the photophysical characterization of the intrinsic properties of ionic chromophores, this set-up may be used to investigate the properties of mass-selected, dye-labeled biomolecules, both alone and in well-defined complexes and clusters.

Infrared multiple photon dissociation (IRMPD) spectroscopy of oxazine dyes

“…In the case of CV, which contains no diethyl amine moiety, the lack of a band at 1075 cm À1 that is clearly visible in the other three spectra turns out to be a useful diagnostic. Thus, the bands close to 1075 cm À1 are assigned as CH 3 rocking modes of the diethyl amine moiety, in agreement with the vibrational assignments suggested by the calculated spectra for these systems. In addition, the spectrum of CV does not exhibit any features in the 1300-1400 cm À1 region, which can also be attributed to the diethyl amine moiety found in the other three structures.…”

Gas-Phase Fluorescence Excitation and Emission Spectroscopy of Three Xanthene Dyes (Rhodamine 575, Rhodamine 590 and Rhodamine 6G) in a Quadrupole Ion Trap Mass Spectrometer

“…In addition to the work from our group using rhodamine 590 [2,4], Blades and coworkers [12] examined rhodamine 6G in a quadrupole ion trap, Zenobi and coworkers reported results from several organic dyes [18][19][20], and Ervin and coworkers [17] presented detailed investigations of the photodissociation and fluorescence of rhodamine 575 in a quadrupole ion trap. In all of these works, investigations were undertaken without the advantage of a continuously tunable light source.…”

“…Elevating the pressure of the buffer gas in the QIT results in higher rates of collisional cooling, thereby suppressing photofragmentation processes and allowing for the use of higher laser powers to increase fluorescence signals. The gas-phase fluorescence and photodissociation of several other rhodamine dyes have subsequently been investigated by our group [1,2,4] and others [17-20, 25, 38].…”

“…Recently, our group [1][2][3][4] and several others [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] have presented works focusing upon the development, characterization and applications of instrumentation for measuring laser-induced fluorescence from molecular ions in the gas phase. The majority of these studies have used highly fluorescent molecules such as the xanthene-based rhodamine dyes.…”

“…The instrumentation developed features laser light sources coupled into trapping mass spectrometers that are used to mass-select the ion population of interest and to store the selected ion population for extended periods of time. Several types of mass spectrometers have been employed, including 3-D Paul-type quadrupole ion traps (QIT) [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17], Fourier transform ion cyclotron resonance mass spectrometers (FT-ICR-MS) [18][19][20][21][22][23], and linear radiofrequency ion traps (LIT) [24]. Detection of integrated gas-phase fluorescence has been accomplished using high-sensitivity photomultiplier tubes [5-7, 9-11, 13-17, 21-24] or avalanche photodiodes [18,19,25], while dispersed fluorescence has been measured using spectrographs with charge-coupled device (CCD) detectors [1-4, 8, 12, 22].…”