Gratuze approach

If elemental concentration rather than oxide concentrations are required, then the oxide conversion can be removed at this point. For obsidian (rhyolitic volcanic glass), using silicon as an internal standard yields results in very close agreement with the Gratuze approach, as shown in Table 37.1. 

TABLE 37.1 Comparison of Concentrations in Obsidian Based on Assumed Value of 35% for the Internal Standard (Si) and Based on the Gratuze Approach to Calibration, in Which Oxide Concentrations Are Summed to 100% (See Text for Full Explanation)... 

Assumed 35% Si Internal Standard Concentrations Gratuze Approach ... 

Gratuze s approach relies on measuring most major, minor, and trace elements and then accounting for all oxygen atoms, so that very close to 100% of the whole rock is represented in the list of monitored signal intensities. In this case, the concentration of some element, y, is known to be some proportion of the total concentrations measured. Thus, the slope parameter, Ky, relating standardized concentration to standardized signal, can be used to write... 

Gratuze [49] recommended dividing the elements into several groups to reduce analysis time and minimize problems of fractionation that can occur as the laser ablates deeper into the sample. In this approach, major elements are measured in the first group of elements, and then one of these elements is selected to serve as the internal standard for subsequent groups. If line scans rather than spot ablations are used, then all elements can be collected simultaneously. Also, with time-of-flight (TOF) instruments (see below), analysis time does not increase with the number of elements. Therefore, the total time can be as short as a fraction of a second. 

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