High‐resolution peak analysis in TOF SIMS data

Abstract

High mass resolution time‐of‐flight secondary ion mass spectrometry (TOF SIMS) can provide a wealth of chemical information about a sample, but the analysis of such data is complicated by detector dead‐time effects that lead to systematic shifts in peak shapes, positions, and intensities. We introduce a new maximum‐likelihood analysis that incorporates the detector behavior in the likelihood function, such that a parametric spectrum model can be fit directly to as‐measured data. In numerical testing, this approach is shown to be the most precise and lowest‐bias option when compared with both weighted and unweighted least‐squares fitting of data corrected for dead‐time effects. Unweighted least‐squares analysis is the next best, while weighted least‐squares suffers from significant bias when the number of pulses used is small. We also provide best‐case estimates of the achievable precision in fitting TOF SIMS peak positions and intensities and investigate the biases introduced by ignoring background intensity and by fitting to just the intense part of a peak. We apply the maximum‐likelihood method to fit two experimental data sets: a positive‐ion spectrum from a multilayer MoS2 sample and a positive‐ion spectrum from a TiZrNi bulk metallic glass sample. The precision of extracted isotope masses and relative abundances obtained is close to the best‐case predictions from the numerical simulations despite the use of inexact peak shape functions and other approximations. Implications for instrument calibration, incorporation of prior information about the sample, and extension of this approach to the analysis of imaging data are also discussed.

Identification of Ubiquitously Present Polymeric Adlayers on 2D Transition Metal Dichalcogenides

“…For the transferred films, r is 0.220 for PS, 0.309 for PVA/PMMA, and 0.276 for PMMA. All of these ratio values by far exceed reported literature values of 0.145 . Though this previously reported value, from a bulk powder sample, may differ from a CVD-grown material, the marked increase in r values in the present materials still indicates a trend of increasing contaminant concentration going from PS to PMMA to PVA/PMMA, noting that the ideal ratio is zero for the theoretical Mo isotope ratio with no hydride species present.…”

“…As mentioned previously the overlapping of Mo + and MoH + species leads to an intermixing of peaks and their intensities. In a report from Gelb et al, the resulting intensities for the intermixed peaks at m / z 95, 96, 97, and 98 were described as italicI ( x ) = italicI ( Mo x ) + italicr · italicI ( Mo x − 1 ) where I ( x Mo) represents the theoretical intensity of an isotope of certain mass x , I ( x ) is the experimentally measured intensity, and r is the ratio factor between them (hydride to metal). From the ratio of the intensities at m / z 92 and 93, where the pure 92 Mo and the pure 92 Mo + H isotopes are located, the ratio factor r can be calculated.…”

“…The seven Mo + isotope peaks with high abundance, namely, 92 Mo, 94 Mo, 95 Mo, 96 Mo, 97 Mo, 98 Mo, and 100 Mo, are located between m / z 92 and 100 in the positive ion spectrum. The peak analysis of MoH + is more complex as the metal and metal hydride peaks may be very close (for example, 94 MoH and 95 Mo differ by only 7.07 × 10 –3 amu), which is not resolvable even with best mass resolution settings . Overlapping occurs at nominal m / z 95, 96, 97, and 98, while peaks at m / z 92, 94, and 100 are purely from Mo + and m / z 93, 99, and 101 are from MoH + isotopes only.…”

“…The peak analysis of MoH + is more complex as the metal and metal hydride peaks may be very close (for example, 94 MoH and 95 Mo differ by only 7.07 × 10 −3 amu), which is not resolvable even with best mass resolution settings. 39 Overlapping occurs at nominal m/z 95, 96, 97, and 98, while peaks at m/z 92, 94, and 100 are purely from Mo + and m/z 93, 99, and 101 are from MoH + isotopes only.…”

“…As mentioned previously the overlapping of Mo + and MoH + species leads to an intermixing of peaks and their intensities. In a report from Gelb et al, 39 the resulting intensities for the intermixed peaks at m/z 95, 96, 97, and 98 were described as…”