Selected Ion Flow Tube Mass Spectrometry

Details of Operation

1. Ions are generated, at the entrance to Q1, using one of several possible methods: electron impact ionization, chemical ionization, plasma ionization (and soon electrospray ion evaporation). Plasma ionization is depicted in Figure 1.
2. The reagent ion is mass selected in Q1 and injected through a Venturi interface into a helium (or sometimes argon) buffer gas.
3. The reagent ions are swept down the tube by the buffer gas at a pressure of 0.35 Torr and at an average velocity of ca. 50 m/s. Between the Venturi interface and the 'Reagent Inlet' ions experience ca. 100,000 collisions with the buffer gas. These ion-helium collisions relax the ion into its ground state.
4. Neutral reagent is admitted at the 'Reagent Inlet' and ion-molecule reactions occur downstream. The reaction time (essentially the transit time from reagent inlet to down-stream nose cone), which is governed by both flow conditions and tube diameter, is ca. 10 milliseconds.
5. Product and reactant ion intensities are monitored by a second mass spectrometer (Q2/CEM) as a function of neutral flow yielding a so-called 'Reaction Profile'.

Figure 1: The ICP/SIFT instrument.

Photo 1: The flow tube region of the SIFT.

Photo 2: Overview of ICP/SIFT/QqQ instrument (January 2007).

Photo 3: Our periodic table of metal solutions for the ICP ion source.

Figure 2: The ESI/SIFT/QqQ instrument.

Schematic of the details of the ESI/qQ/SIFT/QqQ instrument.
A - the modified floating skimmer,
B - the q0 reaction cell,
C - the extended stubbies,
D - the extended q0 rod set.

Photo 4: Schematic of the details of the ES/qQ/SIFT/QqQ instrument.

Reaction Profiles

Product and reactant ion intensities are plotted versus neutral flow to yield a reaction profile. On a semi-log plot the primary reagent ion decay should lie along a straight line (see Figure 2). The slope of the reagent ion decay is proportional to the reaction rate co-efficient. Rate constants for secondary (and higher) reactions are determined by fitting the ion signal rise (and decay) to the appropriate rate expression.

Figure 3: Kinetics profile for V⁺ reacting with O₂. Low flow experiment (left) and high flow experiment (right).

Multi-collision Induced Dissociation

Course structural and bond energy information can be determined by varying the electric potential applied to the nose cone in front of Q2. As the nose cone potential is made more negative positive ions are accelerated through the buffer gas with increased velocities. Multiple collisions with the buffer gas slowly build the internal vibrational and rotational energy of the ion until dissociation occurs. Plotting dissociation product intensities versus applied nose cone potential yields information regarding relative bond energies within the collided ion. To access higher center-of-mass energy regimes, Argon, Krypton and occasionally Xenon are added to the buffer gas.

Figure 4: Multi-collision induced dissociation of NbO₆⁺ yielding NbO₂⁺ via the intermediate NbO₄⁺.

Equilibrium Kinetics

When clustering reactions proceed to equilibrium the ratio of product ion intensity to reactant ion intensity plotted versus neutral flow (or concentration) can yield a lower limit to the reaction equilibrium constant (K_eq) and thus an upper limit to the reaction Standard Free Energy change ΔG°

–ΔG° = RT ln(K_eq)

If a reaction entropy change is estimated then a reaction enthalpy change can be determined from the well known equation

ΔG° = ΔH° – TΔS°

Figure 5: Ratio plots for the reaction of In⁺ reacting with C₆H₆ to produce In⁺&·C₆H₆ and In⁺·(C₆H₆)₂.

Reaction Profiles

Figure 6: Measured ion profiles for the reactions of common ESI chemical background ions with dimethyl disulfide (DMDS).

Measured ion profiles for the reactions of common ESI chemical background ions with dimethyl disulfide (DMDS), at 295 ± 2 K in helium buffer-gas at a pressure of 0.35 ± 0.01 Torr. The ion signal (y-axis) is given as a function of flow of DMDS (x-axis) in units of molecules s^-1. The solid lines represent kinetic fits to the experimental data. (A) m/z 149 ion, protonated phthalic anhydride, PHANH⁺, formed from phthalates, (B) m/z 327 ion, protonated triphenylphosphate TPPH⁺, (C) m/z 99 ion, protonated phosphoric acid, [H₃PO₄]H⁺, (D) m/z 42 ion, protonated acetonitrile, ACNH⁺.

Quantum Chemical Computations

To supplement the coarse structural and bond energy information provided by multi-collision induced dissociation experiments and the reaction Free Energy determinations provided by equilibrium kinetics experiments, quantum chemical computations can be performed. Past collaborations have involved the research groups headed by Dr. A.C. Hopkinson at York University and by Dr. H. Schwarz at the Technical University of Berlin.