The Laser-Abation Time-of-Flight Mass Spectrometer instrument (LMS) at the Physikalisches Institut of the University of Bern is designed for in-situ measurements with high acuracy and sensitivity for elemental and isoptopic composition measurements of regolith material on celestial objects. High accurate measurements on major, minor and especially trace elements down to the ppb range and on isotopic ratios play a crucial role for a depper and better understanding on the evolution of our planetery system and the question of the origin of life.
The experimental system consists of a pulsed laser ablation ion source, a miniaturised reflectron-time-of-flight mass spectrometer and a dedicated experimental optimisation part. The pulsed 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 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. The ions passe the field-free region and are backreflected by the reflectron towards the multichannel plate (MCP) detector. The ions arrive at the MCP detector in sequence, at times proportional to the square root of their mass-to-charge ratio (m/q).
There are images of the experiment available as well as an up to date list of publications that contain data obtained with the LMS instrument.
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 .
The parameters (1) accucracy in elemental measurements, (2) accuracy in isotopic measurements, (3) detection sensitivity, (4) mass resolution and (5) mass calibration are the most importat parameters for spectroscopic instruments. In the following a short overview of these parameters of the LMS are given. Further information can be found in the publication section.
The LMS instrument can be calibrated using measurements on standard reference materials/samples. With this procedure one gets for each element a so called releative sensitivity coefiicient (RSC) factor. Using these factors for each element an unknown sample can be measured straigh forward. The measurement accuracy is hereby in the low per cent level. Below, an overview of these RSC factors is shown using the standard reference materials 664 and 661 from NIST. The ns-laser system in the IR range was hereby used (see Tulej, 2011).
Depending on the abundance of the element in the sample measurements on elemental isotopes can be done in the sub per cent level. High resolution measurements (>600) is hereby the crucial key parameter. Further information can be found in the bachelor thesis written by Stefan Meyer in 2011 (see publication list).
With the Laser-ablation Time-of-Flight Mass Spectrometer designed in our group a detetion sensitivity in the low ppb range is possible. Below, a typical spectrum measured with the LMS instrument from NIST SRM (Standard Reference Material) 665 is shwon. Ti49 with ~300ppb is clear visible.
Using the deticated self-writtem computer-controlled performance optimiser based on a adaptive particel swarm algorithm a mass m/δm resolution exceeding 800 is possible! Below, the Fe54 peak from NIST SRM 664 before and after only one iteration of the optimser is shown. The mass resolution was increased by a factor of about 4.5 from m/δm of 122 to 743!
After the first optimiser iteration the laser fluence and the distance between sample and entrance plate can be shifted slightly to start the next optimiser iteration. Finally a mass resolution m/δm over 800 can be reached. Below, the optimised Fe54 peak of the NIST SRM 664 is shown. The mass resolution of the optimised peak is hereby ~885.
Mass calibration with an accuracy in the per mille and sub permille range is achievable. Bellow, the mass calibration and the resulting residuals of a NIST SRM (Standard Reference Material) 664 is shown. The residuals behave random around a relative error of 0, no systematic trend is visible.
In comparision with other laboratory systems, e.g. Thermal Ionisation Mass Spectrometer (TIMS) and others, the Laser Ablation Time-of-Flight Mass Spectrometer instrumet designed in our group requires no sample preparation. No chemicals are used to disolve samples nor ovens to vaporise them. This circumstance plays a crucial role in the field of space instrumentation. An Oven for instance has a high power consumption but on spacecraft not a lot of power is in fact available. And how to store chemical solvents, other agents and gas bottles over a long time in spacecraft? This requires a lot of additional space and so weight which is also very limited in space missions.
To ensure stable mesurement conditions a temperature, humidity and dust stabilised environment for the laboratory systems, e.g. laser systems, optics, computers, etc., is essential.
The whole laboratory is temperature and humidity computer controlled. The temperature is hereby kept about 21.6°C wheras at day the mean temperature is about 22.0°C, a result of working people inside the lab.
The humidity inside the lab is kept at (41 +/- 2)% rel. humidity. The variations in humidity are on the one hand a result of day/night humidity fluctuations and on the other hand of if poeple are inside the lab or not.
To ensure that no dust riches optics, lasers and sensitive instruments, e.g. vacuum chamber, from outside there is an overpressure inside the lab relative to the outside. Even more a laminar flow module with filter is installed on the top of the laboratory to ensure a purity level of 100. The laminar flow module is divided in three parts, separated by plastic drapes. Two outer, small parts and one center part with all the sensitive instruments. Using this principle a dust free and clean circulation from the outer to the inner part of the lab is guaranteed.