Improving the analytical performance of ion mobility spectrometer using a non-radioactive electron source

A Method for the Determination of Rate Coefficients for the Formation of Protonated Monomers and Proton Bound Dimers of Volatile Organic Compounds in Air at Ambient Pressure by Ion Mobility Spectrometry

Rate coefficients (k) for the reactions of hydrated protons and a sample vapor (M) to form protonated monomers (MH+(H2O)n) and proton bound dimers (M2H+) were determined simultaneously by using ion mobility spectrometry with a tandem drift tube at ambient pressure. In this method, sample vapors were introduced to a first drift region with stepwise reactions: M + H+(H2O)n→ k 1 MH+ (H2O)y + (n - y) H2O and M + MH+ (H2O)y→ k 2 M2H+ + yH2O. Ions with a drift time between 9 and 18 ms in the first drift region were subsequently mobility analyzed in purified air with the second drift region using a synchronized second ion gate with boxcar averaging. Slope for plots of ion abundance against drift time was fitted by successive approximation between peaks of H+(H2O)n and MH+(H2O)y to obtain k2 until computed and experimental slopes matched within a maximal deviation of 0.01. Values for k1 were obtained from the experimental baseline slope when adjusted for k2. Rate coefficients for triethyl phosphate and phenylacetate were determined using this method with k1 values of 1.14 × 10-9 ± 1.87 × 10-10 cm3/s and 0.81 × 10-9 ± 9.26 × 10-11 cm3/s and k2 values of 1.05 × 10-9 ± 4.13 × 10-10 cm3/s and 1.07 × 10-9 ± 1.86 × 10-10 cm3/s, respectively. Relative error for this method was determined using mobility spectra generated with a COMSOL model and artificial rate coefficients. Rate coefficients were then extracted from the modeled mobility spectra and compared to the original artificial values. Relative error of this method was 10% and should be generally applicable for individual substances with reactions forming protonated monomers and proton bound dimers.

Parametric Sensitivity in a Generalized Model for Atmospheric Pressure Chemical Ionization Reactions

Gas phase reactions between hydrated protons H⁺(H₂O)_n and a substance M, as seen in atmospheric pressure chemical ionization (APCI) with mass spectrometry (MS) and ion mobility spectrometry (IMS), were modeled computationally using initial amounts of [M] and [H⁺(H₂O)_n], rate constants k₁ to form protonated monomer (MH⁺(H₂O)_x) and k₂ to form proton bound dimer (M₂H⁺(H₂O)_z), and diffusion constants. At 1 × 10¹⁰ cm^-3 (0.4 ppb) for [H⁺(H₂O)_n] and vapor concentrations for M from 10 ppb to 10 ppm, a maximum signal was reached at 4.5 μs to 4.6 ms for MH⁺(H₂O)_x and 7.8 μs to 46 ms for M₂H⁺(H₂O)_z. Maximum yield for protonated monomer for a reaction time of 1 ms was ∼40% for k₁ from 10^-11 to 10^-8 cm³·s^-1, for k₂/k₁ = 0.8, and specific values of [M]. This model demonstrates that ion distributions could be shifted from [M₂H⁺(H₂O)_z] to [MH⁺(H₂O)_x] using excessive levels of [H⁺(H₂O)_n], even for [M] > 10 ppb, as commonly found in APCI MS and IMS measurements. Ion losses by collisions on surfaces were insignificant with losses of <0.5% for protonated monomer and <0.1% for proton bound dimer of dimethyl methylphosphonate (DMMP) at 5 ms. In this model, ion production in an APCI environment is treated over ranges of parameters important in mass spectrometric measurements. The models establish a foundation for detailed computations on response with mixtures of neutral substances.

Non-radioactive electron source with nanosecond pulse modulation for atmospheric pressure chemical ionization

Ion mobility spectrometers (IMSs) are well-known instruments for fast and ultrasensitive trace gas detection. In recent years, we introduced a compact nonradioactive electron source providing a defined current of free electrons with high kinetic energy at atmospheric pressure for initiating a chemical gas phase ionization of the analytes identical to radioactive sources. Besides its nonradioactivity, one major advantage of this electron source is its controlled electron emission current even in pulsed mode. By optimizing the geometric parameters and developing faster control electronics, we now achieve electron pulses with extremely short pulse widths down to 23 ns. This allows us to kinetically control the formation of reactants and analyte ions by chemical gas phase ionization (e.g., reducing discrimination processes caused by competing ionization), enhancing the analytical performance of the IMS. However, this paper concentrates on the pulsed electron source. For its characterization, we developed a measurement setup, which allows the detection of nanosecond electron pulses with amplitudes of only a few nanoamperes. Furthermore, we investigated the spatial ion distribution in the ionization region depending on several operating parameters, such as the kinetic electron energy or the ionization time.