Instrumentation

Instrumental developments of TIMS to make it a powerful tool in solving interesting problems within the areas of health and energy

Background

The main instrumentation in the Bleiholder lab was developed within a joint development agreement with Bruker Daltonics and is a version of their recently commercialized trapped ion mobility spectrometry-mass spectrometry (TIMS-MS) apparatus. TIMS-MS has attracted much attention lately as it has been shown to have mobility resolution of ~200, which is significantly higher than other ion mobility spectrometry (IMS) methods. Because TIMS-MS is a novel method, fundamental studies must be performed to ensure that the method can be used confidently for structural elucidation to discover methods and attributes of the instrument that can be further used to separate and determine the structures of analyte compounds. The mission of the lab is to develop methodology that allows the TIMS-MS system to be utilized to study native-like structures of biomolecules. Further instrumental developments will be paramount in making TIMS a powerful tool in solving interesting problems within the areas of health and energy.

How does TIMS work?

Figure 1: Schematic of the TIMS-MS apparatus in the Bleiholder lab with zoom in of the TIMS cartridge (blue box).

In its current configuration, (see Figure 1) the instrument is set up with a micro-spray ESI source although nano-spray, captive-spray, and APPI sources are available in house. Ions are produced at the source, and traverse a resistive glass capillary to an ion funnel. Ions are then focused to the entrance of the TIMS analyzer by DC and RF fields. Once the analyzer has been filled for a set amount of time, voltages are flipped, preventing additional ions from entering the trap.

Figure 2: Schematics of the analyzer tunnel of a TIMS device. (A) The analyzer tunnel comprises 25 electrodes through which a buffer gas flows. A resistor chain forms a voltage divider and creates electrical resistances between the electrodes. The resistance between the electrodes increases for the first 13 electrodes and then remains constant. (B) Electric potentials applied to the first (Vramp) and last electrodes (Vexit) create a voltage profile within the analyzer tunnel due to the resistor chain. The potential at the first electrode, Vramp, is raised during the course of a TIMS measurement. (C) At a given time, the resulting electric field increases linearly in the first section of the TIMS analyzer tunnel and remains constant for the second section. During the course of the measurement, the electric field is decreased. (D) The electric field in the analyzer tunnel is decreased in a stepwise fashion at rate ˇ during the course of a measurement.

Within the analyzer, (see Figure 2A) ions are separated and trapped according to their mobility. Ions in the analyzer are trapped between two opposing forces. The first is a drag force created by collisions with a flow of buffer gas streaming towards the exit of the analyzer. The buffer gas flow is created by a differential in pressure between the entrance and exit funnel regions. The drag force is counterbalanced by a counteracting electric field gradient. The voltage profile is created by applying electric potentials to the first and last electrodes of the TIMS analyzer (see Figure 2B). One can visualize the counteracting electric field gradient as a rising edge over the first half of the analyzer, giving way to a plateau in the second half of the analyzer (see Figure 2C). Consequently, ions having larger cross sections will move further into the electric field because of increased collisions with the buffer gas and will be eluted first. Elution of separated ions is achieved by very slowly decreasing the electric field gradient (see Figure 2D) to a point where the drag force overcomes the electric force, eluting ions to the plateau and downstream optics. Post mobility separation, ions traverse two ion funnels and a hexapole ion guide. Ions then move from the hexapole into a quadrupole where they can be transferred or mass selected prior to moving into the collision cell. The collision cell is an RF and DC operated collision cell that is designed to both perform collision induced dissociation (CID) and transport ion packets without perturbing the high resolution of the mobility separation. From the collision cell, ions reach a time of flight mass analyzer, where mass analysis of mobility separated ions takes place.

Advantages of TIMS and future work

The TIMS-MS instrument is a novel method that can attain mobility resolutions of R>200, which is much higher than that of conventional IMS methods. This high resolution ability lends the needed confidence to the structural interpretation of the data. In addition, an unique feature in TIMS is the ability to trap mobility selected ions for upwards of 15.6 seconds in our current set up. This innovative feature allows us to monitor gas phase stability or the structural changes of the analyte over time. The trapping ability can also be used to facilitate ion-ion and ion-molecule reactions with dopant gasses or other additives. In order to take full advantage of these innovative features and apply them to structural biology, the mission of the lab and current work is focused into characterizing the TIMS device. This will allow appropriate methodologies to be developed, and will enable us to confidently attain high resolution structural information of native analyte compounds.