The LMS experimental setup consists of a laser ablation ion source, a miniaturised reflectron-time-of-flight mass spectrometer and a dedicated experimental optimisation part. The dimensions of the spectrometer are 120 mm x Ø 60 mm. The mass spectrometer and the solid/powder samples are placed within an UHV chamber pumped by a turbomolecular and a ion getter pump. The typical base pressure is in the low 10-8mbar range.
At the moment a Q-switched Nd:YAG laser system (266 nm, τ ≈ 4ns, repetition rate of 20 Hz) is used as ablation ion source. The laser beam is focused to a spot of about 20 µm in diameter on the sample surface. The laser beam is guided via mirrors to be co-linear with the LMS system. The laser beam enters the mass analyser through a window at the top of the vacuum chamber and a focusing lens. The focused beam passes the mass analyser, the detection assembly with a central hole of Ø 6.4 mm and finally the ion optical system before reaching the sample surface .
Positive ions generated in the plasma plume enter the mass analyser through the conical nose-piece, and are accelerated, confined, and collimated by an electrostatic immersion lens. Four electric potentials accelerate and focus the ions into the field-free and reflectron regions from which ions are reflected and detected by a pair of multichannel plates (MCP) used in a chevron configuration.
The applied voltages on the ion optical and reflectron system were predicted by ion optical simulations and are taken as an initial set for the further voltage optimisation procedure controlled by a dedicated and self-written code based on an adaptive swarm algorithm. The signals generated in the MCP are collected by four concentric anode rings. The ions arrive at the MCP detector in sequence, at times proportional to the square root of their mass-to-charge ratio (m/q).
Samples are placed on a sample holder mounted on a XYZ micro-translational stage and are positioned at a fixed distance (~1 mm) from the entrance (grounded) plate.
The laser fluence iscontrolled by use of a polarisation-sensitive attenuator, and laser irradiance in the range 0.1–1 GW/cm2 is usually used (ablation mode).
The laser system initiates the experimental cycle and triggers the data acquisition. The spectra are recorded with two dual–channel, 8-bit PCIe/PCI high-speed digitizer with on-board signal processing ADC card. In single-channel mode acqusiition a sampling rate of 4 GS/s with an analogue bandwidth of 1.5 GHz is possible. Typically, the spectra are recorded in the mass range 0–250 amu/q,which corresponds to flight times up to ~14 μs .