Select reference materials (7.4), i

Select reference materials (7.4), in the range of the
samples to be analyzed. Use at least three such reference
materials, for each range of sulfur values to be tested. Select
one reference material containing at least as much sulfur as the
highest level of sulfur expected. Select two additional reference
materials, one approximately at the mid-point of the range
and one below the lowest level of sulfur expected.