Quoting Ethan Merritt <[log in to unmask]>:
> On Wednesday 22 April 2009 09:23:19 Jacob Keller wrote:
>> Hello All,
>>
>> What is the reason that x-ray fluorescence is neglected in our experiments?
>> Obviously it is measureable, as in EXAFS experiments to determine
>> anomalous edges,
>> but should it not play a role in the intensities as well? What am I missing?
>
> Fluorescence is directly proportional to f", so in one sense we do account
> for it in any calculation that includes the anomalous scattering terms.
>
> If you were thinking of direct contribution of the fluorescent X-rays to the
> measured Bragg peak - that is negligible. Those photons do not retain the
> momentum vector of the original incident photon, and are emitted in all
I am not sure whether this is a good explanation. The elastically
scattered photons (which make up the Bragg peaks) also do not not
retain the momentum of the incident photon.
> directions. I.e., they contribute even less to the diffraction image than
> air-scatter from the direct beam or from the diffracted beam.
Well, this clearly depends on the sample content and on the X-ray
wavelength. There are many examples of data collected at an absorption
edge, where fluorescence is the dominating contributor to the
background, i.e. it is much larger than air-scatter from the direct
beam or from the diffracted beams. For an extreme case, see fig. 4 in
Shepard et al.(2000). Acta Cryst. D56, 1288-1303.
Marc
>
> Ethan
>
>>
>> Jacob
>>
>> *******************************************
>> Jacob Pearson Keller
>> Northwestern University
>> Medical Scientist Training Program
>> Dallos Laboratory
>> F. Searle 1-240
>> 2240 Campus Drive
>> Evanston IL 60208
>> lab: 847.491.2438
>> cel: 773.608.9185
>> email: [log in to unmask]
>> *******************************************
>>
>> ----- Original Message -----
>> From: rui
>> To: [log in to unmask]
>> Sent: Wednesday, April 22, 2009 11:06 AM
>> Subject: [ccp4bb] microbatch vs hanging drop
>>
>>
>> Hi,
>>
>>
>> I have a question about the method for crystallization. With
>> traditional hanging drop(24 wells), one slide can also hold for
>> multiple drops but it requires the buffer quite a lot, > 600uL?
>> Microbatch can save buffers,only 100uL is required, and also can
>> hold up to three samples in the sitting well. Other than saving the
>> buffer, what's the advantage of microbatch? Which method will be
>> easier to get crystals or no big difference? Thanks for sharing.
>>
>>
>> R
>
>
>
> --
> Ethan A Merritt
> Biomolecular Structure Center
> University of Washington, Seattle 98195-7742
>
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