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1. Introduction: The Roles of Metals in Biology

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Browse All Figures Return to Figure. Previous Figure Next Figure. Email or Customer ID. Forgot password? The sources of random error which are considered are calibration uncertainty; system stability; peak and background counting statistics; uncertainty in attenuation corrections; and uncertainty in peak overlap corrections, but the dominating source by far is peak and background counting statistics.

At a minimum, laboratories are to determine annual estimates of the DL using the guidance provided in Reference 5. X-ray spectra are subject to distortion if unusually heavy deposits are analyzed. This is the result of internal absorption of both primary and secondary X-rays within the sample; however, this is not an issue for Pb due to the energetic X-rays used to fluoresce Pb and the energetic characteristic X-rays emitted by Pb. Too little deposit material can also be problematic due to low counting statistics and signal noise.

This maximum loading is acceptable for the analysis of Pb and other high-Z elements with very energetic characteristic X-rays.

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A properly collected sample will have a uniform deposit over the entire collection area. Samples with physical deformities including a visually non-uniform deposit area should not be quantitatively analyzed. Tests on the uniformity of particle deposition on PM 10C filters showed that the non-uniformity of the filter deposit represents a small fraction of the overall uncertainty in ambient Pb concentration measurement.

The analysis beam of the XRF analyzer does not cover the entire filter collection area. The minimum allowable beam size is 10 mm. There can be instances when lines partially overlap the Pb spectral lines, but with the energy resolution of most detectors these overlaps are typically de-convoluted using standard spectral de-convolution software provided by the instrument vendor. A de-convolution protocol must be used to separate all the lines which overlap with Pb. X-ray attenuation is dependent on the X-ray energy, mass sample loading, composition, and particle size.

In some cases, the excitation and fluorescent X-rays are attenuated as they pass through the sample. In order to relate the measured intensity of the X-rays to the thin-film calibration standards used, the magnitude of any attenuation present must be corrected for.


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See references 6, 7, and 8 for more discussion on this issue. Essentially no attenuation corrections are necessary for Pb in PM 10 : Both the incoming excitation X-rays used for analyzing lead and the fluoresced Pb X-rays are sufficiently energetic that for particles in this size range and for normal filter loadings, the Pb X-ray yield is not significantly impacted by attenuation. Measurement method precision is assessed from collocated sampling and analysis.

Background

The goal for acceptable measurement uncertainty, as precision, is defined as an upper 90 percent confidence limit for the coefficient of variation CV of 20 percent. The bias is assessed through an audit using spiked filters. The goal for measurement bias is defined as an upper 95 percent confidence limit for the absolute bias of 15 percent. The low-volume PM 10c sampler shall be used for PM 10 sample collection and operated in accordance with the performance specifications described in part 50, appendix L.

The following requirements are similar to those currently specified for the acceptance of PM 2. For large filter lots greater than filters randomly select 20 filters from a given lot.

Atomic spectrometry update. X-Ray fluorescence spectrometry

For small lots less than filters a lesser number of filters may be taken. Ninety percent, or 18 of the 20 filters, must have an average lead concentration that is less than 4.

Field blank filters shall be collected along with routine samples. Field blank filters will be collected that are transported to the sampling site and placed in the sampler for the duration of sampling without sampling.

Elemental and Chemically Specific X-ray Fluorescence Imaging of Biological Systems

Laboratory blank filters from each filter lot used shall be analyzed with each batch of routine sample filters analyzed. Laboratory blank filters are used in background subtraction as discussed below in Section 6. The four main categories of random and systematic error encountered in X-ray fluorescence analysis include errors from sample collection, the X-ray source, the counting process, and inter-element effects.

These errors are addressed through the calibration process and mathematical corrections in the instrument software. Spectral processing methods are well established and most commercial analyzers have software that can implement the most common approaches references to background subtraction, peak overlap correction, counting and deadtime corrections.

An energy-dispersive XRF system is used. Energy-dispersive XRF systems are available from a number of commercial vendors.

Examples include Thermo www. The specific details of the corrections and calibration algorithms are typically included in commercial analytical instrument software routines for automated spectral acquisition and processing and vary by manufacturer. It is important for the analyst to understand the correction procedures and algorithms of the particular system used, to ensure that the necessary corrections are applied. The mention of commercial products does not imply endorsement by the U. Environmental Protection Agency. Thin film standards are used for calibration because they most closely resemble the layer of particles on a filter.

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Geostandards Newsletter 18 special issue. Hampel, J. Huckell, B. Current Research in the Pleistocene 25, 95— Obsidian Studies in the Great Basin.

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American Laboratory 42, 27— Hull, Kathleen L. Ivanenko, V. X-ray spectrometry 32, 52— Jack, R. Plateau 43, — Shackley Jack, R. In Taylor, R. California division of mines and geology special report , 17— Jackson, T. New York: WileyInterscience. New York: Marcel Dekker. Joyce, R. Kahn, J. Killick, D. Lachance, G. New York: Wiley-Interscience.

Analysing archaeological basalt using nondestructive energy-dispersive X-ray fluorescence EDXRF : Effects of post-depositional chemical weathering and sample size on analytical precision. Martynec, R. Kiva, in press. McCarthy, J.


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In Heinrich, K.