There are detectors that are small and have fast readout time and others
that have much larger size and slower readout time. Large detector size
reduces background level very effectively by allowing data collection with
larger crystal-to-detector distance. For well ordered crystals, level of
diffuse scattering is so low that there is no benefit to reduce it further
by narrowing the oscillation range.
10 degree rotation I found useful only for inorganic crystals with very
short unit cell parameters, about 5A. For organic small-molecule crystals
and image plate type detectors, 2-5 degree rotation/oscillation was quite
effective. With cylindrical detectors and using moly wavelength, data up
to 0.4A can be efficiently corrected. Bonding electrons are well visible
in such structures.
Also when one wants to collect data very fast, for example in 5 minutes,
background is not an issue and wide and wide angle oscillation mode of
data collection is appropriate.
> Indexing involves storing all the reflection positions in orthogonal
reciprocal space and then finding a reciprocal lattice that fits most of
them.
> For a 10 degree frame all we know is that a reflection lies somewhere on
a sort of 10 degree arc; only two of the three coordinates are
> precisely known, which makes indexing difficult. For 0.1 degree frames
all three coordinates of each reflection are known. For the
> purpose of indexing, the number of reflections per frame is irrelevant.
Unless one is using a detector like an image plate with a long
> read-out time, there is no advantage in using frames significantly wider
than the reflections, they also have a higher background noise level. This
applies equally for small molecules and macromolecules, though for a
strongly diffracting small molecule a higher background can be
tolerated.
> George
> On 03/25/2014 09:55 AM, Tim Gruene wrote:
>> Dear Felix,
>> as far as I understand we are talking about the frame width, not the
total data range for indexing ("10 degree rotations to get enough spots
per frame"). I have used 180degrees of data for indexing. At least XDS
places the reflection at the centre of the frame so that with a 10deg
frame width the position is know within +/-5 degree - it is not
surprising indexing fails here.
>> Regards,
>> Tim
>> On 03/25/2014 09:41 AM, Felix Frolow wrote:
>>> Dear Tim,
>>> I am sure your statement is too general and is not very precise. 10
deg oscillation range is as precise as 0.1 deg. Positions of
diffraction spots on the area detector have
>>> defined position on rotation axis within the precision/accuracy of the
stepping motor and the spacial resolution of the area detector and NOT
defined by oscillation range. If it does, change your setup. We need
sometimes 10 deg to have enough reflections for indexing. Obviously we
>>> need some reflections to define the orientational matrix and cell
dimensions.
>>> FF
>>> Dr Felix Frolow
>>> Professor of Structural Biology and Biotechnology, Department of
Molecular Microbiology and Biotechnology
>>> Tel Aviv University 69978, Israel
>>> Acta Crystallographica F, co-editor
>>> e-mail: mbfro...@post.tau.ac.il
>>> Tel: ++972-3640-8723
>>> Fax: ++972-3640-9407
>>> Cellular: 0547 459 608
>>> On Mar 25, 2014, at 10:26 , Tim Gruene<t...@shelx.uni-ac.gwdg.de>
wrote:
>>>> Dear David,
>>>> I dare claim that rather you do not know how to use the listed
programs
>>>> in the case for small molecule data rather than none of them were
optimised. E.g. 10degree frames loose all the possible accuracy in the
>>>> phi-direction so I am not surprised you had trouble indexing. There
is
>>>> no reason for that, if you know which parameters to modify.
>>>> In addition to that, concluding from one single data set that
programs
>>>> are not optimised may be a little overinterpreted. In my experience,
this interpretation does not hold.
>>>> Best,
>>>> Tim
>>>> On 03/24/2014 10:03 PM, David Schuller wrote:
>>>>> Coincidentally, I just spent my day trying to index a lattice of ~
10
>>>>> x
>>>>> 10 x 11 A.
>>>>> Mounting samples: if the compound is stable, just glue it to the end
of
>>>>> a steel pin. No muss, no fuss.
>>>>> We had to attenuate our synchrotron beam heavily to make it work;
motors
>>>>> can only turn so fast.
>>>>> We did 10 degree rotations to get enough spots per frame per
imaging.
>>>>> Detector setup allowed for ~ 1 A resolution.
>>>>> Indexing was a challenge for many of the samples, heavily overloaded
spots and streaks seemed to be causing the most problems.
>>>>> We tried various of the usual macromolecular programs for indexing;
HKL2000, iMosFlm, XDS, DPS. None of them seem to be optimised for
this,
>>>>> but some of them actually worked in some instances.
>>>>> On 03/24/14 14:04, Andreas Förster wrote:
>>>>>> Dear all,
>>>>>> I've been approached by a materials student with a petri dish full
of
>>>>>> big, sturdy, salty, yellow crystals. He claims I have the best kit
for single-crystal diffraction on campus.
>>>>>> I would very much appreciate advice on how to deal with this,
anything
>>>>>> in the range from "won't work" to "use software X to analyze data
in
>>>>>> space group P-43N" would be welcome.
>>>>>> Thanks.
>>>>>> Andreas
>>>> --
>>>> Dr Tim Gruene
>>>> Institut fuer anorganische Chemie
>>>> Tammannstr. 4
>>>> D-37077 Goettingen
>>>> GPG Key ID = A46BEE1A
> --
> Prof. George M. Sheldrick FRS
> Dept. Structural Chemistry,
> University of Goettingen,
> Tammannstr. 4,
> D37077 Goettingen, Germany
> Tel. +49-551-39-33021 or -33068
> Fax. +49-551-39-22582
Zbyszek Otwinowski
UT Southwestern Medical Center at Dallas
5323 Harry Hines Blvd.
Dallas, TX 75390-8816
Tel. 214-645-6385
Fax. 214-645-6353
