Read National Academies of Science Press, Project Openbook, Preventing the Forward Contamination of Mars, p. 148

(I don't get what this source is -- perhaps a machine OCR'd catalog entry for ordering the material. They say it's for search engines and not to cut and paste, so I'll summarize.)

In Viking testing they found a contamination problem, so they redesigned the nozzles to lessen contamination, and that reduced erosion too. Tests showed traces of HCN and that was attributed to a fuel impurity which was eliminated, using purified hydrazine -- used hence, though that still gives small contamination of ammonia, nitrogen, and a bit of water, which must be taken into account.

Seems like there's always some contamination and you test your equipment beforehand to know what to expect.

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