Q_10 in nomenclature

[Wolfgang Engelmann]

I have a nomenclature item with the symbol Q_{10}
and the description
a measure of how a reaction rate changes with increasing temperature. It 
is the ratio of the reaction rate when the temperature is increased by 
10°C to the original rate.

I added in   Sort as   Q10

I marked   Literal

I would have expected in the pdf output that it is sorted under Q, but 
it is at the very end of the nomenclature.

Why? What did I do wrong?

Wolfgang