Выражение встречается в следующем контексте:

Standard Test Methods for
Water in Crude Oils by Coulometric Karl Fischer Titration

This test method covers the determination of water in
the range from 0.02 to 5 mass or volume % in crude oils.
Mercaptan (RSH) and sulfide (S− or H2S) as sulfur are known
to interfere with this test method, but at levels of less than
500 μg/g (ppm), the interference from these compounds is
insignificant.

и еще

At levels of less than 500 μg/g (ppm)
(as sulfur), the interference from these compounds is insignificant.
Most crude oils, including crude oils classified as “sour
crude,” have mercaptan and sulfide levels of less than 500 μg/g
(ppm) as sulfur.

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