Dear Vince,

Maybe I can comment because we have both state of the art IP (Stoe IPDS2) 
and CCD systems (Bruker APEX2) systems for MoKa in the same building (as 
well as a Bruker SMART6000 CCD and a MAR345 IP for protein work with 
CuKa).

Modern CCDs have a very much lower noise level than the one you are 
using and 10 minute exposures are no problem. The IPs have a very much 
longer read-out time (more than an order of magnitude) so one is obliged 
to work in wide-scan mode (say 2 degrees) rather than fine-slicing (say 
0.2 degrees for the CCDs). Except for crystals with a very large mosaic 
spread, this means that the X-ray background (fluorescence, air-scattering 
etc.) is an order of magnitude more for the IP systems and we observe
this in practice, both for the small-molecule and for the protein work.

If you have problems with very weakly diffracting crystals you should
also think about the source; there have been major improvements in
sources and especially X-ray optics in the last few years. It often 
happens that we can collect a good dataset with our CuKa/SMART6000 
system for crystals for which we scarcely see any reflections with the 
Stadi2/MoKa, so when we run out of protein crystals we use this machine 
for small molecules too. The reasons are that the absolute scattering 
power of a crystal is proportional to lambda^3, the SMART6000 has a 
rotating anode and multilayer focussing optics are much more effective 
at longer wavelengths. We recently changed the Osmic blue to Incoatec 
Helios optics on this system and the intensity (total number of photons 
hitting the crystal) went up by a factor of 4.5! So the ratio of the 
number of photons per reflection in unit time relative to the 
MoKa / sealed tube / graphite monochromator is well over two orders of 
magnitude, which can make a difference. 

Finally the new MoKa microsources are also worth a look; my colleague 
Dietmar Stalke has tested a MoKa Incoatec ImuS here more or less 
continuously for the last four months. It is attached to a standard
Bruker APEX2/sealed tube system in such a way that one can switch sources
in a few seconds simply by changing 2theta-zero, so one can collect data 
from the same crystal with the same detector for comparison purposes.
The ImuS has proved extremely stable and has required no reallignment or 
servicing in that time. For large crystals the ImuS still has its nose 
ahead despite its air-cooled 30 Watt tube, but for very small crystals 
the much narrower beam of the ImuS gives a factor of 10 or more photons 
hitting the crystal, which often gives a critical improvement in the 
resolution attainable for very small crystals.

Best wishes, George

Prof. George M. Sheldrick FRS
Dept. Structural Chemistry,
University of Goettingen,
Tammannstr. 4,
D37077 Goettingen, Germany
Tel. +49-551-39-3021 or -3068
Fax. +49-551-39-2582


On Fri, 8 Feb 2008, Vince Lynch wrote:

> I was interested to know what the general thoughts were regarding the
> advantages and disadvantages of CCD based detectors versus image plate
> detectors for small molecule crystallography.  I currently use a Nonius
> Kappa CCD.  While a very nice instrument, one drawback comes with very
> weakly scattering crystals.  With such crystals, if I use very long scan
> times, and I mean scan times of 500 - 600 seconds per frame, often the
> inherent noise of the detector will swamp out whatever scattering there is
> to collect.  I understand that the more modern CCD's have a much lower 'dark
> current'.  However, with image plates there is no dark current so exposure
> time isn't an issue beyond the judicious use of the instrument.  I have no
> experience with image plate detectors and so I am probably overlooking some
> practical drawbacks in using one.
> 
> ______________________________________________
> XRD mailing list
> [log in to unmask]
> http://www.jiscmail.ac.uk/lists/xrd.html
> To leave the list use the webpage or send an email to:
> [log in to unmask] with the text
> SIGNOFF XRD
> 

______________________________________________
XRD mailing list
[log in to unmask]
http://www.jiscmail.ac.uk/lists/xrd.html
To leave the list use the webpage or send an email to:
[log in to unmask] with the text
SIGNOFF XRD