I second Phil's opinion - it is better to scan and be sure - as long as
the scan results are not hideously abnormal. If you cannot scan for
whatever reason but are sure that the X-ray optical system is properly
calibrated - then use Phil's numbers below :)

Artem

> Always take the scan results ahead of the typical values unless they are
> obviously wrong.  Only use the "predicted" values if the scan is broken
> or too weak (e.g. very small crystals) and in that case I'd be tempted
> to add 10-20 eV to the "typical" peak wavelength to make sure you
> weren't actually collecting the inflection point since they are
> typically very close in SeMet.
>
> In my NSLS X29-dominated data collections, I find I end up using
> something like this for non-oxidized SeMet:
>
> Peak: 12664 eV, 0.9790 Angstrom   (usually in range 12662-12664)
> Infl: 12662 eV, 0.9792 Angstrom   (usually in range 12660-12662)
> I also typically use high energy remote: 12860 eV, 0.964 Angstrom
>
> give or take a few eV.  This tends to translate well between the
> relatively small number of beamlines that I personally end up using.
> But I always prefer to take the results from the Chooch analysis of the
> scan from the actual crystal.
>
> Cheers (and good luck)
> Phil Jeffrey
> Princeton
>
>
> Jerry McCully wrote:
>> Dear All:
>>
>>      Next week we are going to try some seleno-Met labeled crystals.
>>
>>      We checked the literature to try to find out the peak wavelength
>> that has been used for SAD or MAD data collection. But they are slightly
>> different ( may be 50 ev) in different papers.
>>
>>      I guess this is due to the discrepancy between the fluorescence
>> scanning and the theoretical vaules of f' and f''.
>>
>>       When we collect the data, which wavelength should we use? Should
>> we trust the scanning results?
>