Aliasing Artifacts in Multielemental LA-ICP-TOFMS Mapping: An Old Problem Revisited

Recent works on quadrupole LA-ICP-MS have shown that unsynchronized laser pulses and data acquisition routines can produce pronounced signal aliasing. This phenomenon appears in the form of long-range and low- to medium-frequency alterations in time-averaged signals that do not reflect the actual underlying analyte concentration distribution, leading to wrong quantitation and considerably deteriorated image quality. In quadrupole instruments, total cycle times, settling and dwell times per nuclide could be finely tuned to match the pulse time of the laser (and/or SPR), thereby eliminating these artifacts. By contrast, in LA-ICP-TOFMS, exact synchronization between the two instrumental components is impossible even in principle. The reason lies in the fact that the acquisition time in the mass analyzer is a constrained value, and can only be set as an integer multiple of base acquisition time of the instrument. As a result, some level of aliasing is inevitable even if the acquisition time is set close to the SPR. In this work, we discuss the choice of optimal acquisition time in TOF instruments. Theoretical simulations and experimental mappings of standard reference materials strongly suggest that a compromise has to be made to minimize the spread of the signal without producing too much redundant data.