  The Planetary Instrument for X-ray Lithochemistry (PIXL) is a rasterable
focused-beam X-ray fluorescence (XRF) spectrometer mounted on the arm of
National Aeronautics and Space Administration's (NASA) Mars 2020 Perseverance
rover. To ensure that PIXL would be capable of performing accurate in-flight
compositional analysis of martian targets, in situ, an elemental calibration
was performed pre-flight on the PIXL flight instrument in a simulated martian
environment. The details of this calibration, and implications for measuring
unknown materials on Mars are the subjects of this paper. The major goals of
this calibration were both to align the spectrometer to perform accurate
elemental analysis and, to derive a matrix of uncertainties that are applied to
XRF measurements of all elements in unknown materials. A small set of pure
element and pure compound targets and geologically relevant reference materials
were measured with the flight hardware in a simulated martian environment.
Elemental calibration and quantifications were carried out using PIXL's XRF
quantification software (PIQUANT). Uncertainties generated were implemented
into the PIQUANT software version employed by the PIXL's data visualization
software (PIXLISE). We outline in this work, a list of factors that impact
micro-XRF accuracy, the methodology and steps involved in the calibration,
details on the fabrication of the uncertainty matrix, instructions on the use
and interpretations of the uncertainties applied to unknowns and an assessment
on the limitations and areas open to future improvement as part of subsequent
calibration efforts.
