Procedural errors
- Peak interferences - We have already discussed the effects of interfering peaks (see âFactors to considerâŠâ, above) and discovered that peak interferences result in falsely high X-ray intensities. The âfalse intensityâ caused by the overlapping X-ray peak must be removed by using an interference standard that has the interfering element in it, without the interfered element, and an overlap coefficient may be determined by collecting X-ray signals from the interference standard at the peak position for the interfered element. The intensity of the interfering element is then subtracted from the measured intensity at the interfered peak position.
Kerrick et al. (1973) American Mineralogist 58:920-925
- Background interferences â As previously stated, background overlaps give rise to inflated background intensity counts, and given that background counts are subtracted from peak intensity counts to yield a net intensity count for a given element, an inflated background will lead to an erroneously reduced net intensity.
- Peak shifts/shape differences â Although we have previously discussed peak shifts/drifts over time within an instrument, there is a separate issue of peak shifts caused by the bonding environment of the element in question. It is well documented that the peak positions for Si and Al, for example, will be different between the pure metal, oxide and alumino-silicate phases. These shifts may have significant effects on both accuracy and precision, especially when using standards that are dis-similar in bonding environment compared to unknowns.
- PHA settings â We previously discussed the readjustment of peak-height analyzer parameters owing to variation with time/aging of the electronics, but there is also a potential for systematic errors to creep in when using structurally or chemically disparate standards and unknowns. âBias, gain, and baselines should be checked. Gross errors in them could produce significant errors in the analytical results. Pulse height depression occurs mainly where there is a large discrepancy in count rate between standard and unknown.â (www.geology.wisc.edu/~johnf/g777/ppt/70_Errors.ppt)
- Resetting goniometer â Occasionally, the goniometer may jam as a result of trying to drive the spectrometer to an unreachable position, in which case the position of the goniometer may have to be re-initialized by doing a peak search on a given standard. This can introduce a difference in peak positions if the re-initialization procedure is done incorrectly, or if the error is not cleared properly and the goniometer is at one physical address/angle while the software value is different from the physical address/angle. This can result in a serious reduction of the accuracy of the analysis.
- Resetting operational conditions â On occasion, there may be a need to reset or reboot the microprobe hardware, firmware or software systems to clear an error, or because a catastrophic operational error has occurred, such as a power failure of the system. When the hardware reboots, it is possible that residual charges within the electrical system cause certain mechanical components (e.g., crystal-flipper or stage motors) to âtwitchâ slightly and introduce positional errors in the spectrometers or stage positions, for example. Such small twitches are enough to introduce errors in X-ray intensities.