The Instrument

Schematic of the custom-designed and home-built instrument. The top schematic shows the chambers, while the bottom schematic shows the most important internal components.  Constituents in the source are highlighted in red, the trap/TOF in green, and the detector in yellow.

The Johnson lab research interests include probing the mechanisms of new particle formation, the role of acidity/basicity in CH activation on metal oxide clusters, and energy transfer in tethered gold nanoparticles. A list of funding agencies supporting this work can be found at the bottom of the page.  We utilize a custom-designed and home-built photofragment mass spectrometer, similar to the one developed by Prof. Mark Johnson, to gain insight into these processes. 

Schematics of the instrument can be found above. The instrument is comprised of three main regions: the source (in red), trap/TOF (in green), and detector (in yellow).  Several different experiments can be performed on this instrument, including cryogenic ion vibrational dissociation (CIVP) spectroscopy, mass spectrometry, and molecular uptake. We are currently modifying the instrument  with additional ion traps that will allow us to make thermodynamic and kinetic measurements. 

Each of these experiments begins by generating ions in an atmospheric pressure source, followed by trapping in a variable-temperature octopole, and finally mass separating and detecting the ion clusters. 

Source Region:

Ions are generated at atmospheric pressure via electrospray ionization (ESI) and guided to a cryogenic ion trap (cryo trap).  The ion source was designed to be flexible, and other ionization techniques, such as chemical ionization or laser ablation, can be easily implemented and employed.

We have also modified the ESI source to include environmental controls. By controlling the spray environment, we can manipulate which ions are produced. We either spray in CO2 and H2O scrubbed air or we spike the scrubbed air with molecules of interest (ex. amines, H2O/D2O, organic acids) to produce clusters containing those molecules.  A simple schematic of the ESI source can                                                                                              be found to the left.

Once electrosprayed, ions pass through an inlet, are skimmed, and enter a radio frequency (RF) octopole that can either trap or guide ions.  The ions then enter a quadruple, which can be operated as an ion guide or a mass filter. After exiting the quadrupole, ions are directed though another RF octopoles, a turning quadrupole, and two more RF guides. Finally, the ions enter the cryo trap.

Trap/TOF Region:

Once guided inot the cryogenic octopole ion trap, ions are cooled by collisions with a He buffer gas, and, if desired, tagged with messenger atoms or molecules, such as He,D2, or N2. The temperature range of our trap is 3−310 K. Pictures and a 3D CAD schematic of the cryo trap can be found to the right. The top picture shows the trap without the trapping endcaps, the middle picture includes the endcaps, and the bottom image is a 3D rendered trap a quarter section cut out of it.If uptake studies are being conducted, molecules of interest are introduced into the cryo trap along with the buffer gas to determine the affinity of a cluster towards the molecule. 

Once the ions are cooled and/or tagged, they are released into the extraction region of a Wiley-McLaren time-of-flight (TOF) mass spectrometer where ions are then orthogonally accelerated into a TOF tube. 

At the entrance and exit of the flight tube are a stack of ion optics, each comprised of an einzel lens and a horizontal and linear deflector.  The ion optics focus and guide the ions through the TOF tube and into the detector region.

Detector Region:

Upon exiting the final einzel lens, ions pass through a mass gate and enter the laser interaction region. Next, the ions fly through a reflectron-style TOF mass spectrometer where clusters are analyzed by energy and reflected onto a microchannel plate (MCP) detector.

For mass spectrometric experiments, the signal from the MCP detector is amplified, sent to an oscilloscope, and recorded by a home-written LabVIEW code which saves the ion intensity as a function of mass. The typical resolution of our tandem mass spectrometer is ~3000 m/Δm. An example mass spectrum can be found in the figure below. The mass spec was collected by spraying a 1mM diglycine solution in 50/50 H2O/MeOH. By holding the cryo trap at ~4 K, we are able to adsorb 88 He atoms onto the protonated dipeptide.