Measuring Cr in high ferrous material

does anyone have an explanation for the fact, that I have a lower result for Cr (approx. 30%) with material with a high iron content, e.g. Millscale, with the measuring mode 'NoGas'? Measured with He mode, the results are correct. Measured with ICP-MS Agilent 7800, digestion in HNO3, HBF4 and water.
Thank’s and regards

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