1) It should indeed be "are" rather than "is". (The author was just tired.)
2) "as" = "in the form of"

#1: Yes, I understand what "as" means here, "as" just sounds more "German" than English to me. May I skip it here ?

. . . a small amount of systemic GC is incorporated into hair as [or you could use "in the form of" lipophilic metabolites. However, the results also indicate . . .

. . . small amounts of systemic GC are incorporated into hair as (or in the form of) lipophilic metabolites. However, the results also indicate . . .

As I understand what you wrote, the as or in the form of are appropriate to explain how the GC is incorporated. The sentence with the as missing sounds incomplete to me.

Of course, I've been wrong before.

I agree with svaihingen. Note that BE and AE sometimes differ as to the number of certain collectives (BE: the government are doing something). The singular is required for nouns where the amount is the concept rather than the count: 50 dollars is a lot of money. But I can't find a justification for small amounts of something IS incorporated.

#4 (Jurist) bringt es auf den Punkt ...

Das Englische orientiert sich in solchen Fällen mehr an der inhaltlichen als an der rein grammatikalischen Logik.

Das Beispiel "50 dollars is a lot of money" finde ich hier sehr anschaulich:

Gemeint sind ja nicht die 50 einzelnen Dollars (Scheine oder Münzen), sondern der Geldbetrag als Ganzes.

(Okay, zugegeben - mit "small amounts" klingt das hier auch für mich ein wenig schräg ... bin allerdings kein Native Speaker.)

I would be surprised if an educated native speaker of English - no matter where s/he was from - advocated mixing plural and singular in this case (small amounts ... is incorporated).

Thank you, I think #1 has already clarfied, that the " amounts is..." most likely has been a slip of the "pen".
I was more confused by the term "incorporated "as" in the sense of "in the form of".
My original version, which was revised (cf. OP), has been: "...that only small amounts of GC metabolites, preferentially more lipophilic ones, are incorporated into...".
For me, it is logic that GC may be metabolized before being incorporated into hair. Thus, the metabolites are incorporated, and IMP there was no need for an "as" or "in the form of". But, of course, I am willingly persuaded by someone who knows better.