Incorporation of a Venturi Device in Electrospray Ionization

We report the first quantitative assessment of electrosprayed droplet/ion focusing enabled by the use of a voltage-assisted air amplifier between an electrospray ionization emitter and a hybrid linear ion trap Fourier transform ion cyclotron resonance mass spectrometer (ESI-LTQ-FT-ICR-MS). A solution of fluorescent dye was electrosprayed with a stainless steel mesh screen placed in front of the MS inlet capillary acting as a gas-permeable imaging plate for fluorescence spectroscopy. Without use of the air amplifier, no detectable FT-ICR signal was observed, as well as no detectable fluorescence on the screen upon imaging using a fluorescence scanner. When the air amplifier was turned ON while electrospraying the fluorescent dye, FT-ICR mass spectra with high signal to noise ratio were obtained with an average ion injection time of 21 ms for an AGC target value of 5 ϫ 10 5 . Imaging of the screen using a fluorescence scanner produced a distinct spot of cross-sectional area ϳ33.5 mm 2 in front of the MS inlet capillary. These experimental results provide direct evidence of aerodynamic focusing of electrosprayed droplets/ions enabled by an air amplifier, resulting in improved electrospray droplet/ion capture efficiency and reduced ion injection time. A second set of experiments was carried out to explore whether the air amplifier assists in desolvation. By electrospraying a mix of quaternary amines, ratios of increasingly hydrophobic molecules were obtained. Observation of the solvophobic effect associated with electrospray ionization resulted in a higher abundance of the hydrophobic molecule. This bias was eliminated when the air amplifier was turned ON and a response indicative of the respective component concentrations of the molecules in the bulk solution was observed. (J Am Soc Mass Spectrom 2007, 18, 1909 -1913) © 2007 American Society for Mass Spectrometry I t is well known that electrospray ionization is a soft ionization technique that enables the study of biological samples by mass spectrometry [1]. However, it is estimated that 99.9% or more of the electrosprayed ions are lost in transport to the detector [2-6]. A recent publication indicates that significant sampling efficiency is obtained with a short emitter to capillary distance, at the expense of lower ionization efficiency [7]. At longer distances, less sample is captured, but there is improved ionization due to more efficient sample desolvation. Based on the results presented herein, along with previous reports, the air amplifier has the capabilities to improve capture efficiency and sample desolvation. Devices such as the ion funnel [8 -10], high-field asymmetric ion mobility spectrometry (FAIMS) [11][12][13], the air amplifier [14 -16], and an interface plate [17] have been reported to improve ion abundance. To translate current technology into future advances in mass spectrometry, systematic investigations of devices such as the air amplifier are necessary to improve instrument performance and subsequently the study of biological systems. The ...

Section: Resultssupporting

confidence: 70%

Probing the mechanisms of an air amplifier using a LTQ-FT-ICR-MS and fluorescence spectroscopy

Dixon

Muddiman

Hawkridge

et al. 2007

“…An intersecting gas flow device developed by Covey [23] can provide an increase in sensitivity of more than a factor of 10, and can significantly lower the background in the resulting mass spectra. Recent studies by Lee et al [24] and Muddiman et al [13] have shown large increases in ion abundance resultant from the implementation of a commercial air amplifier or a modified venturi device in the highpressure region. They also reported that these devices generated a concentric high-velocity converging gas flow around the electrospray tip to reduce spreading of the electrospray plume and improve the conductance of ions to the sampling orifice of the MS.…”

mentioning

confidence: 99%

Incorporation of a flared inlet capillary tube on a Fourier transform ion cyclotron resonance mass spectrometer

Zhang

Kaiser

et al. 2006

Flared inlet capillary tubes have been coupled with a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer to help the ion transmission from the atmospheric pressure to the first vacuum region. We investigated different types of atmospheric pressure ionization methods using flared inlet tubes. For most of the ionization methods, such as ESI and DESI, increased ion current transmitted from the atmospheric pressure ion source to the first stage vacuum system was observed with the use of our enhanced ion inlet designs. The corresponding ion intensity detected on a FT-ICR mass spectrometer was also observed to increase two-to fivefold using ESI or DESI with the flared tube inlet. Moreover, increased spray tip positional tolerance was observed with implementation of the flared inlet tube. We also include our preliminary results obtained by coupling AP-MALDI with flared inlet tube in this paper. For AP-MALDI, the measured ion current transferred through the flared inlet tube was about 2 to 3 times larger than the ion current through the control non- and atmospheric pressure matrix-assisted laser desorption/ionization (AP-MALDI), have been widely used with mass spectrometry for proteomics studies [1][2][3][4][5]. Desorption electrospray ionization (DESI) is an exciting new atmospheric pressure desorption ionization method introduced by Cooks et al. [6,7]. Charged solvent droplets are sprayed directly onto an analyte surface through a pneumatically-assisted electrospray ion source. This new method is very attractive due to its utility for direct analysis of different compounds on a variety of surface types [6 -8], and will enable application of mass spectrometry to more diverse, more challenging analytical problems. For all these ionization methods, the ions generated under ambient conditions are transferred through an atmospheric pressure interface such as an inlet tube or an orifice followed by several stages of differential pumping and then ions are sent to the MS detector [1][2][3].A common hurdle encountered in the implementation of any atmospheric ion source is related to the transfer of ions produced at atmospheric pressure to the low vacuum mass analyzer. For all these atmospheric pressure ion sources, the gas-phase ions produced at atmospheric pressure are transferred to the lower pressure regions through conductance limits such as a metal capillary or an orifice [9,10]. For electrospray ionization, it has been reported that gas-phase collisions and charge-charge repulsion after ion formation lead to an expansion of the ion cloud [3,11]. In AP-MALDI and DESI, ions are also dispersed once created at atmospheric pressure. Since ions must enter the MS through a small aperture or capillary with limited cross section, the ion cloud expansion and dispersion can significantly decrease the ion transmission to the lower pressure region, and thus decrease the sensitivity of the analysis. It has also been reported that the ion transmission efficiency between the ESI emitter and the inlet of the mass spectr...

“…This device showed improvement in ion transmission into the mass analyzer, but was not functional at atmospheric pressure. Lee and coworkers have recently shown transmission improvements using a Venturi device, in an approach based on gas dynamic focusing [29]. Perhaps the method of increasing ion transmission that has received the most coverage in the literature is the use of electrostatic lenses, but signal improvements and exact function or benefit of these methods has not been well established [14, 15, 30 -33].…”

mentioning

confidence: 99%

Investigation of electrospray ionization and electrostatic focusing devices using a three-dimensional electrospray current density profiler

Thompson

Eschelbach

Wilburn

et al. 2005

A novel instrument for profiling the current density of nanoelectrospray ionization plumes in three dimensions has been developed. A hemispherically-shaped electrostatic lens at atmospheric pressure is found to be able to compress the space-charge in nano-ESI and increase the average current density in the plume to three times the nominal value. Ion transmission into a single-quadrupole mass spectrometer is found to roughly double using the electrostatic lens. Data also suggest that ion transmission into the first vacuum region for a skimmer-type mass spectrometer interface using nano-ESI may be typically 40% or better with no special focusing device used. (J Am Soc Mass Spectrom 2005, 16, 312-323)