# Current Progress in Femtosecond Laser Ablation/Ionisation Time-of-Flight Mass Spectrometry
## 3 LIMS System: fs-Laser Ablation Ion Source and Time-of-Flight Mass Analyser
### Selected Femtosecond-LIMS Systems
#### 3.3.3 High-Resolution fs-LIMS: LMS-GT  
The high mass resolution fs-LIMS instrument called LMS-GT (GT stands for 'Gran Turismo') supports both superior mass resolution compared to other LIMS using TOFMS and ablation studies with the high lateral and vertical resolution (Figure 6) [24]. The instrument consists of an fs-laser ablation ionisation ion source capable of probing with spot sizes down to 1-2 \(\upmu\)m, a long flight path folded twice by two ion mirrors and a channeltron detector. Figure 6: Schematic of optical and ion-optical system of LMS-GT; (**a**) The layout of the optical system applied in LMS-GT combines optics for laser ablation ion source, microscope system and the surface illumination system for the microscope observations. The Schwarzschild objective, supporting tight focusing conditions, is placed near the sample surface; (**b**) Ion optical design of LMS GT: SA: Sample location, SRC: ion source, DR1–DR4: Drift Tubes, L1–L3: Lens 1–3, DEFL: Deflection plates, R1&R2: Reflectron, RL: Reflectron Lens RET: Retarder Section REF: Reflector Section BP: Backplane, DR: Drift Tubes, DET: Detector (for more details see reference [24]). Reprinted with permission from [24]. The optical system plays a central role in the instrument. It supports tight laser focusing on the sample surface and an in-line microscope system with a resolving power of 2 \(\upmu\)m that allows verification of the ablation crater characteristics and location on the sample. This microscope can also be used to target specific sample micro-features during the measurement campaigns. The microscopy line uses the same objective as the laser. The instrument guides the laser radiation from the laser output to a two-mirror system known as Schwarzschild optics, where it is focused on the sample to a spot of 1-2 \(\upmu\)m in diameter for applied IR fs-laser radiation. A Ti: Sapphire laser CPA system from Clark-MXR Inc. is used to produce femtosecond laser radiation (190 fs, \(\lambda\) = 775 nm). The pulse repetition rate is up to 1 kHz and the maximal output pulse energy is 1 mJ. The focusing of laser radiation is arranged by expanding the laser beam with a beam expander (Altechna 775 nm and 387 nm). The optical system design was optimised using ZEMAX tools [118]. To support optimal ion transport from the ablation spot into a mass analyser, the ion-optical design was merged with the light optical system. The ion-optical design of the instrument was entirely carried out using SIMION [138]. Custom-made MATLAB pre- and post-processors were made to create the initial ion populations and to visualise the simulation results. High mass resolution, in addition to high spectral quality, allows the removal of a significant fraction of isobaric interferences significantly improving the quantitative capabilities of the instrument. With the ion optical system, a large temporary spread of ion packet is effectively compensated with two ion mirror systems. Typically, the ion temporal spread at the ion detector in the LIMS system is a few ns close to the transit-time spread of the currently available ion detectors (<1 ns) [139; 140] and the frequency bandwidth (~1 GHz) of the preamplifiers in data acquisition systems [132]. LMS-GT represents the system in which the ion packet temporary spread is in ns range, thus, comparable with these other time spreads. To reproduce the pulse shape of the signal produced by the detector, the sampling rate of the acquisition system should be 10 GSamples/s or larger. The LMS-GT instrument demonstrated mass resolving powers (m/\(\Delta\)m) of more than 10,000. With the mass accuracy in the 10 ppm range, the direct identification doublet or multiplet mass lines of isotopes, molecules, and clusters are possible. This removes isobaric interference effects, which are limiting the quantitative analysis of low mass resolution LIMS instruments. Trace element sensitivity is found to be in the ppm range.