June is upon us folks, the days are thoroughly longer and the nights are a hell of a lot warmer… Well maybe in some nice warm parts of the world, unlike Belfast which can have four seasons in the space of an hour! The Queen’s jubilee has passed, Nadal won another masters tennis tournament and the Republic of Ireland have played their first two games of the European Championships in twenty-four years! While many of my fellow countrymen and countrywomen have made the journey to Poznan and Gdansk (hence the photograph with the complimentary Father Ted reference), I have been locked up in the lab labouring over my mineralogical powder x-ray diffraction data (pXRD). To quote Richard Feynman, you can gather from some of the previous blog entries I’m somewhat of an aficionado of Feynman (well who wouldn’t want to be an aficionado of the bongo playing, atom-bomb building, Nobel laureate who used to work out equations in a strip club!):
“The scientist has a lot of experience with ignorance and doubt and uncertainty, and this experience is of very great importance, I think. When a scientist doesn’t know the answer to a problem, he is ignorant. When he has a hunch as to what the result is, he is uncertain. And when he is pretty darn sure of what the result is going to be, he is in some doubt. We have found it of paramount importance that in order to progress we must recognize the ignorance and leave room for doubt. Scientific knowledge is a body of statements of varying degrees of certainty –some most unsure, some nearly sure, none absolutely certain.”
Now, one does have to draw the line between being doubtful and having zero-confidence in your data and in your analysis. Essentially, you have to try and be as precise and accurate as you possibly can but bearing in mind that it could all be completely debunked by a colleague. So, I was a fair bit sceptical of my own data and truth be told it did give me sleepless nights as I contemplated: “How do I know that those minerals are there?” Now, such questions may be a matter of confidence: ‘Ah sure it’s grand, they’re probably there anyway!’ However, that’s not so easy when you are part of a tradition that demands possessing doubt and uncertainty and indeed Feynman is completely right, in order to progress you must recognise the ignorance and leave room for doubt. Although one might think that having such little confidence in data might be damaging, you couldn’t be further from the truth and I will explain this with regards to my earlier labouring over the pXRD data – Rietveld refinement.
Qualitative and quantitative analyses in any geochemical approach are essentially two sides of the same coin as they ask the important questions: “What is there?” and “How much?” However, with Rietveld we start seeing these two questions merging together. The way this method works is that the intensities calculated from the crystallographic data are scaled to match the corresponding observed intensities in the same diffraction pattern via a common scale factor. Now, what this means is that you’re comparing your observed data (i.e. the peak heights and widths) with a calculated profile (i.e. a model) of peak heights and widths. The model is then adjusted to fit your observed data using a least-squares approach. So using this method, based on the crystallographic parameters of; background light scatter, a scale factor, structure symmetry, atomic planes, corrections to the experimental geometry, atomic structure factors and profile functions, you can robustly demonstrate what minerals are present and how abundant they are in relative terms.
Without a doubt it was definitely worth the effort! After working on this data for some time, this is a celebratory moment as this method represents one of the more robust approaches to mineralogical analysis using pXRD. Next to actually seeing the individual minerals, this approach has given me what every scientist craves: a robust dataset. We’re never going to be 100% sure of anything, albeit watching a football match or watching a natural process. However, recognising this and trying to come to some robust theory of what we think is going on is all we’re doing and hence why Feynman’s statement is highly appropriate and worth bearing in mind as it’s that uncertainty and doubt that is the driving force behind better and better approaches to analysis.
Now, I do believe a refreshing pint of stout is in order while I watch Trapattoni’s army take on the might of the Italians… Come on you boys in green!
Till next time folks!
Rory Flood, PhD Student, Queen’s University Belfast, School of Geography, Archaeology and Palaeoecology (GAP), http://www.qub.ac.uk/schools/gap/