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I have an issue with possible sulfur interference in connection with analytical Method 1664 A (SGT-HEM), determining non-polar material (NPM). Yellow crystals form in some of our samples. It is not clear if the crystals are a result of acidification (as required by the Method) of samples containing certain sulfur compounds (like ammonium thiosulfate), or from some other source. I know that the final residue (which is weighed as NPM) is a yellow powder that contains elemental sulfur. Does anyone have information about sulfur and NPM analysis, or a modification to Method 1664 A when there may be sulfur interference? I'd appreciate any help you can give me. Thank you, Don Wanamaker American Chemical Society Member #01158318 Environmental Management, Ltd. 845-429-1141