Electron probe microanalysisWhat’s the point?Goal and IssuesPowerPoint PresentationSlide 5Random ErrorsSystematic errorsSlide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Slide 40Slide 41Slide 42Slide 43Slide 44Slide 45Slide 46Electron probe microanalysisAccuracy and Precision in EPMA:Understanding ErrorsModified 10/31/07What’s the point?How much can I trust the compositions that the probe computer spits out? Are two analyses equivalent? Can I compare my numbers with those published by other researchers using EPMA?Goal and Issues Goal: achievement of high accuracy and precision in quantitative analyses, recognizing sources of errors and minimizing themIssues involved with achieving this goal:• Standards• Instrumental stability• Sample and standard physical condition• Beam impact on sample complications• Spectral issues • Counting statistics• Matrix correctionStandards: how “good” are they? well characterized? homogeneous? Instrumental conditions: beam stability; spectrometer reproducibility; thermal stability; detector pulse height stability/adjustment; reflected light optics (stage Z)Matrix correction: any issues (eg MACs for light elements)? wide range in Z for binary (eg PbO)Sample and standard conditions: rough surface? polish? etched? tilt? sensitive to beam? C coat thickness if usedCounting statistics: enough counting time? Spectral issues: peak and background overlaps?Sample size vs interaction volume: homogeneous? small particles? secondary fluorescence?These can be categorized into “random” and “systematic” errors.Random ErrorsRandom errors include• random nature of X-ray generation and emission• instrumental (random) instability• operator inconsistency (e.g. little attention to correct optical focus)• sample surface roughness• interaction volume intersecting two phases• secondary fluorescence from hidden (below surface) phases• stray cosmic raysSystematic errorsSystematic errors include• instrumental instability (temperature effect on crystal 2d, and on gas pressure; stage Z drifts as it heats up)• inappropriate matrix correction• poor electrical ground of either standard or unknown• beam change/damage to unknown (e.g. Na in glass)• difference in peak shape/position (standard vs unknown)• peak or background interference• pulse height depression on standard• fluorescence across observed phase boundaries (e.g. diffusion couple)Precision and Accuracy in Error Analysis Precision refers to the reproducibility of the counts – and thus the ability to be able to compare compositions, whether within a sample, or between samples, or between analytical sessions. It is directly tied to counting statistics. It is a relative description.Accuracy refers the “truth” of the analysis, and is directly tied to the standards used and the matrix correction applied to the raw data, as well most of the other variables listed previously that could affect the X-ray intensities (background and peak interferences, beam damage, etc). It is an absolute description.EPMA quantitative error analysis is a combination of both, the first being very easy to define, the later more difficult. Precision for major elements could easily be <1%, but when combined with accuracy, total EPMA error probably 1-2% in the best cases (for major elements).Precision and Accuracy Low PrecisionHigh PrecisionLow AccuracyHigh AccuracyInstrumental Errors-1• Beam current stability: with Faraday cup measurements made for each analysis, long term drift should not be a problem as the counts for each analysis are normalized to a common reference current value (could be 1, or 20 nA). For long count times (minutes ) for trace element work, it is recommended that the peak and background counting be constantly cycled so that any longer period issues be spread out over the whole time period.• Spectrometer reproducibility: with modern microprobes, this should not be a serious problem, although problems do crop up with age. Where crystals are flipped, in a small fraction of cases there is an error; generally it is not recommended to flip crystals within analyses. When spectrometer reproducibility is a problem, it is seen as backlash of the gears; to minimize errors, the peaks should always be approached from the same direction. This is set up within the software.Instrumental Errors-2• Thermal stability: Spectrometers could drift if there is a change in the room temperature, though this would presumably be noticeable to the operator (air conditioning fails in hot spell). I have not seen problems with PET nor LIF. P10 gas pressure is sensitive to the temperature change. We attempt to keep the room at 68-70°C and the circulating water temperature in the machine is very close to this. Stage height (Z) drifts due to motor heating during long (overnight) runs.• Detector pulse height adjustment/stability: The bias (voltage) of the gold wire in the detector must be set to the proper value; this is a function of the energy of the X-ray and gas pressure. The operator must verify that the bias, gain and baseline are set properly (the last particularly where the Ar-escape peak is partially resolved).Instrumental Errors-3• Dead time:In WDS, counts are dead time corrected. If dead time is not accurately determined, there could be a systematic error here. Cameca probes operate somewhat differently from JEOL and others, in that Cameca introduces a “hard” constant time delay (e.g. 3 secs) automatically into the counting circuitry and then uses that value to correct the counts. Probe labs should verify (at least once) that the manufacturer’s “official” or “default” dead time factors are correct. This is done by counting on a metal standard (e.g. Si or Ge) at varying Faraday currents, with the dead time correction turned off. These data can then be plugged into a spreadsheet that which Paul Carpenter (NASA) has developed to calculate the most accurate dead time actually present on a particular probe. Also, in our Probe for Windows software, there is an option for an alternate, more complex dead time correction equation, for high count rate (>50K cps)Instrumental Errors-4• Specimen focus (stage height): Samples and standards must be
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