Opps I got the contamination the wrong way round thanks for pointing this out. I realise the 0 order is not photomericaly clean but in your result it certainly showed an increase. I will try various strategies as as long as I get a signal it is ok as I will save past spectra over a period long enough even if the trigger is a bit late. I am planning to use my past experience of control charts to spot the signal.
Thanks again and for offering to seek out the raw data..
Andrew