Optimization of a quadrupole ion storage trap as a source for time-of-flight mass spectrometry

A cryogenic cylindrical ion trap velocity map imaging spectrometer

A cryogenic cylindrical ion trap velocity map imaging spectrometer has been developed to study photodissociation spectroscopy and dynamics of gaseous molecular ions and ionic complexes. A cylindrical ion trap made of oxygen-free copper is cryogenically cooled down to ∼7 K by using a closed cycle helium refrigerator and is coupled to a velocity map imaging (VMI) spectrometer. The cold trap is used to cool down the internal temperature of mass selected ions and to reduce the velocity spread of ions after extraction from the trap. For CO₂ ⁺ ions, a rotational temperature of ∼12 K is estimated from the recorded [1 + 1] two-photon dissociation spectrum, and populations in spin-orbit excited X²Π_g,1/2 and vibrationally excited states of CO₂ ⁺ are found to be non-detectable, indicating an efficient internal cooling of the trapped ions. Based on the time-of-flight peak profile and the image of N₃ ⁺, the velocity spread of the ions extracted from the trap, both radially and axially, is interpreted as approximately ±25 m/s. An experimental image of fragmented Ar⁺ from 307 nm photodissociation of Ar₂ ⁺ shows that, benefitting from the well-confined velocity spread of the cold Ar₂ ⁺ ions, a VMI resolution of Δv/v ∼ 2.2% has been obtained. The current instrument resolution is mainly limited by the residual radial speed spread of the parent ions after extraction from the trap.

A velocity map imaging mass spectrometer for photofragments of fast ion beams

We present the details of a fast ion velocity map imaging mass spectrometer that is capable of imaging the photofragments of trap-cooled (≥7 K) ions produced in a versatile ion source. The new instrument has been used to study the predissociation of N₂O⁺ produced by electric discharge and the direct dissociation of Al₂⁺ formed by laser ablation. The instrument's resolution is currently limited by the diameter of the collimating iris to a value of Δv/v = 7.6%. Photofragment images of N₂O⁺ show that when the predissociative state is changed from ²Σ⁺(200) to ²Σ⁺(300) the dominant product channel shifts from a spin-forbidden ground state, N (⁴S) + NO⁺(v = 5), to a spin-allowed pathway, N^*(²D) + NO⁺. The first photofragment images of Al₂⁺ confirm the existence of a directly dissociative parallel transition (²Σ⁺_u ← ²Σ⁺_g) that yields products with a large amount of kinetic energy. D₀ of ground state Al₂⁺ (²Σ⁺_g) measured from these images is 138 ± 5 kJ/mol, which is consistent with the published literature.

Interleaved Distance-of-Flight Mass Spectrometry: A Simple Method to Improve the Instrument Duty Factor

Distance-of-flight mass spectrometry (DOFMS) is a velocity-based, spatially dispersive MS technique in which ions are detected simultaneously along the plane of a spatially selective detector. In DOFMS, ions fly though the instrument and mass separate over a set period of time. The single flight time at which all ions are measured defines the specific m/z values that are detectable; the range of m/z values is dictated by the length of the spatially selective detector. However, because each packet of ions is detected at a single flight time, multiple groups of ions can fly through the instrument concurrently and be detected at a single detector. In this way, DOFMS experiments can be interleaved to perform several mass separation experiments within a single DOF repetition period. Interleaved operation allows the orthogonal acceleration region to be operated at a repetition rate higher than the reciprocal of the flight time, which improves the duty factor of the technique. In this paper, we consider the fundamental parameters of interleaved DOFMS and report first results.