Monazite typical analyses

Typically, analysis protocols for monazite and/or xenotime call for the same current to be used both for calibration of standards and analysis of unknowns (e g. Scherrer et al. 2000), though adopted current values vary widely (e g.. Finger and Broska 1999, Gratz and Heinrich 1997). However, this approach may cause problems if a standard and unknown generate very different count rates. 

Figure 7. (a) Virtual WDS (Reed and Buckley, 1996) plot for monazite in the region of Gd La, LiF crystal. Virtual counter is sealed, with Xe detector gas. Model monazite has typical LREE (La-Gd) content. Minor interference of Nd LP2,is and La Ly2 + La Lys peaks, and major interference of the Ce Lyi peak render the Gd La peak unusable in monazite analysis, (b) Virtual WDS plot for monazite in the region of Gd Lp, LiF crystal. Virtual counter is sealed, with Xe detector gas. Model monazite has typical LREE (La-Gd) content. Gd Lp avoids the interferences of the Gd La peak, and is sufficiently resolved from Nd Ly to use for quantitative analysis of monazite. 

The analysis of monazite requires detailed WDS scans of both standards and typical unknowns for placement of critical background positions, knowledge of desired detection limits and the analytical time and current needed to achieve these limits, and careful selection of analytical peaks to avoid mutual element interference. Changes in count rate between standard and unknown may cause PHA peak shifts and loss of counts when narrow energy windows are used. Placement of background positions on either side of Ar absorption peaks should be avoided. 

Upon removal from the vacuum chamber, samples are dissolved in acid (HNO3 for apatite, HCl for monazite and xenotime) and spiked for U and Th analysis. At Caltech we spike with U and Th and analyze U and Th isotope ratios on a Finnigan Element double-focusing inductively-coupled plasma mass spectrometer. The accuracy and precision of these measurements is typically better than 0.5%. 

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