I agree with Ed's suggestion - the folks on that forum are very helpful and very insightful with regards to small-angle scattering.
Some thoughts to offer: Mark - it's hard to evaluate from the Primus screen shots, simply because Primus is not rendering the experimental noise. I'd suggesting plotting your data out on a log-log plot in Origin or via the Igor Pro macros if this data is from an American beamline (NIST, ORNL) and using those renderings to evaluate. Rendering the experimental noise is important, as in those middle D2O concentrations you're going to have a considerable amount of incoherent scatter contributing to your profiles. In your first 30% data shown, the discrepancy is concerning. Check the reduction parameters and make sure the correct correction files were used for that particular sample to detector distance. The rendering of the experimental noise would be helpful in evaluating. In your 50% and 70% data, the experimental noise would be helpful to evaluate, but on the whole they seem pretty good. The discrepancy seen at middle Q in this profile (http://postimg.org/image/m358pazb7/) is a little concerning though. I'm concerned with the first set of 90% data you show, assuming the incoherent scatter is very low at that concentration of D20 and signal-to-noise is high. There seems to be a discrepancy in the middle Q regime (~0.15 Q) Again, might be worth double-checking the correction files used for reduction. HTH, Kushol Kushol Gupta, Ph.D. Research Associate - Van Duyne Laboratory Perelman School of Medicine University of Pennsylvania <BLOCKED::mailto:kgu...@stwing.upenn.edu> kgu...@mail.med.upenn.edu 215-573-7260 / 267-259-0082 From: CCP4 bulletin board [mailto:CCP4BB@JISCMAIL.AC.UK] On Behalf Of Ed Pozharski Sent: Wednesday, August 07, 2013 11:54 AM To: CCP4BB@JISCMAIL.AC.UK Subject: Re: [ccp4bb] Problems with SANS data analysis This question may be better suited for more small-angle-oriented forum, e.g. http://www.saxier.org/forum/ On 08/07/2013 11:22 AM, Remec, Mark wrote: Dear CCP4bb, I have a few questions concerning SANS data recently collected that I'm having trouble analyzing. The data was collected at 2 different detector distances (4m, 2.5m) to achieve higher q-range, but I worry that the curves don't overlap enough at intermediate q, which might indicate a problem with the data. The links below are pictures of the corresponding datasets, before truncating the 4m high-q data and merging them into one. Is there a problem evident with the data, or am I imagining a problem? http://postimg.org/image/qb00y20qr/ http://postimg.org/image/8trbp7akj/ http://postimg.org/image/hni86axj7/ http://postimg.org/image/3sjxnu343/ http://postimg.org/image/4ysj0dgsj/ http://postimg.org/image/9ypz8bmf7/ http://postimg.org/image/m358pazb7/ http://postimg.org/image/jzuthmzib/ My second question concerns the values obtained in the analysis of the final scattering curves. The second sample in my experiment shows serious deviation in the values obtained for I(0) and Rg by Guinier analysis compared to the values obtained by the P(r) analysis. In other words, either the P(r) values match the Guinier and the P(r) fit is terrible, or else the P(r) fit is good but doesn't match the Guinier at all (5-10 difference in Rg, 2x difference in I(0)). I've checked to make sure the buffer subtraction algorithm was OK, and I'm pretty certain that the buffers were exact matches, so I don't know how to explain this variation. There's no evidence of aggregation or polydispersity to throw off the values, either. Does anyone know how this can happen? -- Oh, suddenly throwing a giraffe into a volcano to make water is crazy? Julian, King of Lemurs
