A method for improvement of mass resolution and isotope accuracy for laser ablation time‐of‐flight mass spectrometers

Abstract

Accumulation of spectra is a common approach for improvement of the signalto-noise ratio (SNR) in mass spectrometry. However, severe degradation of the overall spectrum can occur if some individual mass spectra, affected by peak broadening, are included in the accumulation process. In this contribution, we discuss potential sources and effects of spectral distortions by using examples from mass spectra acquired by our miniature laser ablation/ionisation time-of-flight mass spectrometer. We show how recent developments in acquisition systems enable to identify individual spectra subjected to peak broadening and present a filtering method capable of systematic and reproducible exclusion of such spectra from the accumulation process. We show that the method can be used on a wide range of materials and present a detailed case study performed on a trevorite mineral sample. Using this method, improvements of the isotope accuracy of Si, Ni, and Cr by factors between 1.6 and 7.7 were achieved. Finally, we discuss the limitations of the method and provide complementary analysis of other materials in the supplementary documents provided with this contribution. KEYWORDSfiltering method, improvement of quantitative performance, isotope accuracy, laser ablation and ionisation time-of-flight mass spectrometry (LIMS-TOF), mass resolution

Isotope abundance ratio measurements using femtosecond laser ablation ionization mass spectrometry

“…Mass calibration of the spectra, mass peak integration, and the spectra filtering procedure were discussed in details in our previous publications. 23,[35][36][37]…”

“…Recently, we developed a spectrum filtering software that allows for the identification and removal of the distorted mass spectra following a well-defined procedure. 37 Using a score function that describes the spectral quality based on selected mass peaks, spectra of bad quality are identified and removed from the analysis. The remaining mass spectra are subsequently used for further analysis.…”

“…Analysis of peak shape and mass resolution, spectrum calibration, and peak integration are fully automatized and performed by a custom software package written in Matlab. 36,37 The isotope abundances are calculated from the integration of the relevant isotope mass peaks and given in the δ-notation:…”

The LMS-GT instrument – a new perspective for quantification with the LIMS-TOF measurement technique

“…Relative sensitivity coefficients close to one, a pre-requisite for standard-less quantitative analysis, became feasible 14,66,67 . It was also demonstrated that detection limits in the sub-ppm range are nowadays possible 14,68 .…”

“…The measured mass resolution gradually reaches a plateau at m/∆m ≈ 8000 for the accumulation of 20 mass spectra and at m/∆m ≈ 9000 for the accumulation of 20'000 mass spectra. This difference can be explained by the effect of the initial crater formation process during which the laser ablation process is less stable 68 . In the second case more spectra are recorded under stable ablation conditions, which results in increased mass resolution.…”