To maintain high data quality, instrument performance is routinely monitored using one or more quality control reference materials (secondary standards), typically at both the beginning and end of each analytical session. When a new analytical routine is developed, it is typically tested against multiple reference materials to assess accuracy over a range of concentrations. To facilitate improved accuracy and precision, techniques such as combined EDS-WDS acquisition, blank corrections, quantitative interference corrections, mean atomic number backgrounds, and time-dependent intensity corrections are used on appropriate materials.
Concord's EPMA lab also participates regularly in interlaboratory exercises. This includes G-probe (one or two test glasses per year) and the 2011 INTAV intercomparison of tephrochronology laboratories (Kuehn et al., 2011). These provide a source of ongoing external quality testing. The lab's results on quality control samples are routinely excellent in terms of both accuracy and precision. Below are examples from the silicate glass trace elements procedure, which forms the basis of a family of analytical routines for silicate minerals and glasses, and the routine tephra glass procedure.
Sample Quality Control Data: Silicate glass with trace elements
All results from a single analytical session run in November, 2017 which also included the G-probe 19 test glass
Analytical conditions 15 kV, 40 nA, 15 to 20 mm beam diameter, 5 minute total analysis time; Si and Al by EDS (8 second acquisition time); All other elements by WDS; Mean Atomic Number X-ray backgrounds used for WDS elements
Time-dependent intensity correction used as needed for Na (Lipari, Old Crow, Sheep Track, Erebus, StHs6-80, ATHO-g, T1-g, NIST 612); Multi-standard concensus blank correction for trace-level accuracy
Quantitative corrections implemented for interference of Ti on Ba, Ba on Ti, Mn on Fe, Fe on Co, Co on Ni, Ca on Mg, Si on Sr, Ca on P, P on Zr, Ti on V, V on Cr, and Si on Rb
FeOT is total iron oxide as FeO; n is number of analyses
INTAV reference concentrations are from Kuehn et al., 2011, The INTAV intercomparison of electron-beam microanalysis of glass by tephrochronology laboratories, results and recommendations: Quaternary International. doi:10.1016/j.quaint.2011.08.022
USGS reference concentrations are from USGS certificates of analysis
GeoREM reference concentrations were downloaded from the GeoREM reference material database in fall 2017
Sample Quality Control Data: Routine Tephra Glass
Analysed in two analytical sessions (Oct & Nov 2017); Analytical conditions 14 kV, 10 nA, 8 mm defocussed beam, 3 minute total analysis time
Si and Al by EDS (8 second analysis time), All other elements by WDS
Time-dependent intensity correction for Na on Lipari, Old Crow, and Sheep Track tephras; Multi-standard blank correction for trace-level accuracy