Performance assessment of a portable mass spectrometer using a linear ion trap operated in non-scanning mode.

RATIONALE: The desire for mass spectrometer portability provides the motivation for simpler, lighter electronics to deliver switched potentials applied to the electrodes of the linear ion trap operated in non-scanning mode. Using a novel method of modelling and theoretical analysis, we simulate the mass analyser performance under these unfavourable operating conditions. METHODS: The electrical fields are simulated using the Charge Particle Optics software which employs the boundary element method. The ion trajectories are computed from the ion cage of the EI source to the interior of the trap where the ions are confined. The spatial/temporal ion distributions during injection are calculated from the individual ion trajectories computed with constant time-steps. Due to geometric non-linearities, ßy = 0 lines close to the apex of the stability diagram have been computed for different initial positions with zero initial velocities in order to define the acceptable maximum axial extension. RESULTS: The DC potential well depth has been estimated at about 15 eV from the axial velocity distribution, and the minimum time of ion injection at 120 µs from the temporal ion distribution. To ensure a mass separation of one unit and the confinement of the whole of the injected ions, buffer gas cooling is necessary to reduce the trajectory excursion amplitudes to 0.1 and 15 mm in the radial and axial directions, respectively. CONCLUSIONS: The portable mass spectrometer is predicted to achieve a mass resolution of better than one mass unit providing that helium buffer gas is used. An additional cooling sequence has to be added prior to moving the operating point toward the apex. Copyright © 2016 John Wiley & Sons, Ltd.

Experimental device for dioxin vapour generation: a steady and adjustable source of gaseous 2,3-DCDD.

PCDD/Fs are Persistent Organic Pollutants (POPs), generated by incomplete combustion of carbonaceous and chlorinated compounds. Because of their toxicity, they have to be monitored at emission, from stationary sources like waste incinerator. In the framework of the development of an on-line analyser, the laboratory prototype requires a gaseous dioxin source. We detail the principles of a dynamic generator based on the sublimation of solid sample and on the diffusion method. Such a device has to supply a polluted flow at steady and adjustable concentration of dioxin. The dioxin generation rate is determined by the weight loss of the solid sample. It is constant during time for temperatures ranging between 75 and 100°C and for generation flow rates greater than or equal to 50 mL min(-1). Variation of generation rate with temperature is predictable. With the increasing of temperature, emitted concentration increases while the increase of the inlet pressure decreases the concentration. Helium carrier gas leads to a higher generation rate than with N2 in agreement with mass and molecular volume ratio. Thus, the outlet concentration and flow rate can be settled independently in the range of 3-100 ppm and 50-200 mL min(-1).

Sensitivity and amplitude calibration of a Fourier transform 3D quadrupole ion trap mass spectrometer.

With a three-dimensional (3D) quadrupole ion trap running in a Fourier transform operating mode, the detected signal is an image of the collective motion of the confined ions. Consequently, it is assumed that the image signal is the sum of the axial trajectories of the simultaneously confined ions. The resulting frequency spectrum after Fourier transformation comprises frequency peaks at the axial secular frequencies of the confined species according to their mass/charge ratio. With a singly confined species, the maximal amplitude of the image signal is proportional to the amplitude of the secular axial frequency peak. The matrix method is employed to express the axial trajectory sampled at the confinement field period. In that case, the expression of the image signal, as well as its maximal amplitude, is calculated as a function of the trap operating conditions and initial axial positions and axial velocities of the ions. The initial position and velocity distributions are connected to the injection mode. With the steady ion flow injection mode (SIFIM) and an initial phase of the confinement field equal to kπ, the maximal amplitude of the image signal is proportional to either the sum of the initial axial positions or the number of confined ions and the mean value of the initial axial positions. By simulation, amplitude fluctuation of the frequency peak is then calculated for a number of ions ranging between some tens to some thousands of ions injected by SIFIM. The peak amplitude fluctuations induced by the fluctuations of the number of ions are seven times greater than those induced by the fluctuations of the distribution of the initial axial positions.