# Current Progress in Femtosecond Laser Ablation/Ionisation Time-of-Flight Mass Spectrometry
## 4 Performance of fs-LIMS Instruments in the Element and Isotope Analysis
### Measurements of Elements and Isotopes
#### 4.4.2 Measurements of Isotopes  
Studies by the LI-O-TOFMS instrument conducted on metal and natural samples have demonstrated the isotope ratio measurements accuracies with less than 5-10 % deviation from the quoted values [161; 162]. In other studies, the combined Thermal Ionisation Mass Spectrometry (TIMS) and LIMS studies on boron isotopes concluded the LIMS isotope accuracies similar to that derived from the TIMS method [163; 164]. Some new inputs to the capabilities of LIMS to isotope measurements were provided by ns-LMS while investigating Pb isotopes in Galena samples [135; 147]. With optimised laser ablation ion source conditions, the accuracy and precision at per mill level could be achieved as required for accurate dating. Similar results were achieved during measurements by TIMS. In recent studies conducted with IR fs-laser ablation ion source on lunar KREEP/zircon grains containing U, Th, and Pb, elements with concentrations in the range of 10s to 100s ppm, measurement quality was sufficient to determine the age similar to that measured by other techniques [165]. Early studies on NIST steel reference samples showed that the accuracy and precision of the isotope abundance measurements decrease with decreasing atomic abundance [21; 132]. To measure relative isotope abundances with accuracies better than the per cent-level, the isotope abundances have to be larger than about 2 ppm, and at 100 ppb isotope abundances, a relative accuracy of 4% can be measured by optimising the ablation ion source parameters [132] (Figure 9A). Elements with concentrations above 100 ppm can be measured with an accuracy of per mill or better, as it has been shown for \({}^{206}\)Pb/\({}^{207}\)Pb isotopes, whereas for isotopes with lower abundances values were in the per cent range [147]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & & **DIN 1716** & **LMS** & **ICP-MS** & **LA-ICP-MS** \\ \hline \multirow{3}{*}{CuSn10 PB10} & Cu & 78.0–82.0 & 78.61 \(\pm\) 11.87 & 82.26 \(\pm\) 4.13 & 76.99 \(\pm\) 0.75 \\ \cline{2-6}  & Sn & 9.0–11.0 & 9.94 \(\pm\) 2.45 & 6.69 \(\pm\) 0.79 & 10.91 \(\pm\) 0.19 \\ \cline{2-6}  & Pb & 8.0–11.0 & 12.25 \(\pm\) 3.28 & 11.13 \(\pm\) 0.63 & 8.56 \(\pm\) 0.47 \\ \hline \multirow{3}{*}{CuSn7 Pb15} & Cu & 75.0–79.0 & 77.93 \(\pm\) 11.77 & 75.55 \(\pm\) 3.73 & 70.67 \(\pm\) 1.03 \\ \cline{2-6}  & Sn & 7.0–9.0 & 5.65 \(\pm\) 1.39 & 6.56 \(\pm\) 0.40 & 7.30 \(\pm\) 0.06 \\ \cline{2-6}  & Pb & 13.0–17.0 & 14.89 \(\pm\) 3.99 & 19.45 \(\pm\) 0.57 & 18.42 \(\pm\) 1.04 \\ \hline \multirow{3}{*}{CuSn5 Pb20} & Cu & 69.0–76.0 & 76.37 \(\pm\) 11.53 & 75.63 \(\pm\) 3.39 & 74.94 \(\pm\) 0.68 \\ \cline{2-6}  & Sn & 4.0–6.0 & 5.99 \(\pm\) 1.48 & 7.08 \(\pm\) 0.73 & 6.92 \(\pm\) 0.11 \\ \cline{2-6}  & Pb & 18.0–23.0 & 15.63 \(\pm\) 4.19 & 20.59 \(\pm\) 1.25 & 16.59 \(\pm\) 0.62 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Summary of composition measurements for three Cu-Sn-Pb alloy samples obtained by LMS, ICP-CRI-MS, and LA-ICP-MS techniques. Abundances and errors are given in wt% [159]. Figure 9: (**A**) Correlation between relative measurement accuracy of isotope ratios and isotope abundance determined from the mass spectra of the NIST SRM 661 steel sample measured by IR fs-LMS. Black and blue points indicate the measurements with laser ablation optics focusing to spot 40 \(\upmu\)m and 10 \(\upmu\)m, respectively; [27] (**B**) Isotope analysis of NIST SRM 658 Ni isotope standard sample showing the composite mass spectrum of Ni isotopes (**a**); depth profile built on \({}^{58}\)Ni and \({}^{60}\)Ni isotopes (**b**) and the intensity correlation curve (**c**). After filtering of the depth profile data, the accuracy and precision from 1% down to 1 per mill level can be achieved [152]. Reprinted with permission [27,152]. Recent UV fs-LMS studies identified several effects on the accuracy and precision of the isotope determination [34]. Compared to ion intensities produced in ns-laser ablation, the ion intensities produced by fs-ablation are significantly lower.