The rule to remember is this:
“When you measure absorption spectra (X-ray, UV, Vis, IR…) in transmission mode then you must not have any feature in the spectrum with an absorbance higher than approximately 1.2.”
The principles from which this follows are nicely explained here:
For an absorbance of 1 90% of the incident light is absorbed. So 10% of the light transmits the sample and is detected. This ensures that the detector for the transmitted light has a signal with reliable intensity.
If you have an absorbance of 2 then 99% of the light is absorbed by your sample – so the transmission detector only sees 1% of the original intensity, which is usually not quite enough to ensure low noise and a linear response of the detector.
For an absorbance of 3 the detector only gets 0.1% of the original light intensity. This intensity is already approaching the noise level in many laboratory spectrometers. So what you are measuring is probably not significant – you are probably just seeing a flat noise line due to … – electronic noise.
So if the concentration of your substance is so that the absorbance is >1.2 then you must dilute your sample or shorten the thickness of the sample.
Most of the time dilution is the better way to do it:*
Just take a defined volume (in practice usually 1 mL or 10 mL) from your solution with a volumetric pipette and fill it up to 100 mL or perhaps even 1000 mL, in a clean volumetric flask. Use clean glassware for all of this – plastics may introduce contaminants.
Accuracy is important here – especially in pipetting. A 10% error in the pipette volume will mean a 10% error in the absorbance. You should aim for 1% accuracy.
*Those of you interested in molecular interactions at high concentrations can’t dilute of course – you will need to get special cuvettes providing a low sample thickness.
Our recent PhD graduate Joanna Stevens has been awarded an EPSRC PhD+ grant to continue, now as a postdoc, her successful soft X-ray spectroscopy work on local structure in complex organic solids. Congratulations!
We are partners in an EPSRC Critical Mass Grant of £2.3M that has been awarded to investigate kinetic pathways in crystal nucleation from solution. Our expertise in measurements by soft X-ray absorption spectroscopy and XPS play an important role in the proposed work, which will be carried out jointly with Roger Davey’s team, the University of Leeds, DIAMOND Light Source and HZB Berlin as project partners.
Follow this link for more information about the part of the grant held in Manchester
Follow this link for information about the sister grant held at Leeds University
The Science Advisory Committee (SAC) of DIAMOND Light Source Ltd has recommended that the Versatile Soft X-ray Spectroscopy Beamline project is invited for full proposal.
We are part of the leadership team for the Versatile Soft X-ray Spectroscopy Beamline Project, which would be the first soft X-ray spectroscopy facility in the UK for the characterisation of liquids, soft matter and organic solids under control of environmental conditions. The Expression of Interest to build such a facility has now been submitted to DIAMOND Light Source Ltd.
It is the time of the year where people come and go. We would like to welcome
- Devon Indar and Amani Musharah, who join our team as new PhD students from September 2010. Both will work on the catalytic upgrading of chemicals from sustainable feedstocks.
- Dr Elizabeth Willneff, who took up her position as Arts and Humanities Research Council (AHRC) Fellow in the School on 1 September. Beth will work on strategies for the analysis and conservation of artist’s acrylic emulsion paints.
- Dr Yvonne Gründer, who will join us from the University of Kiel (Germany) on 1 November; Yvonne will be a joint postdoc between the Dryfe and SLMS research groups, working on in situ XAS studies of the nucleation and growth of metal nanoparticles at liquid-liquid interfaces in collaboration with DIAMOND Light Source and Mark Schlossman at the University of Illinois Chicago.
- Lauren Newton, Michael Foo and Will Johnson, who will work with us in the academic year 2010/11 for their final year MEng research projects.
An NSF/EPSRC proposal with Robert Dryfe and Mark Schlossman to study nanoparticle nucleation and growth at liquid-liquid interfaces with synchrotron radiation techniques (XAS and GIXS) has been successful.
Follow this link for more details
Our team will contribute three talks at the 9th International Conference on Crystal Growth of Organic Materials, (CGOM9), 4-7 August in Singapore.
Hamizah will speak about “Surfaces Analysis of Molecular Crystals: Studies of Paracetamol by X-ray Photoelectron Spectroscopy (XPS)”
Vicky presents “Polymorphism from a Solution Perspective: Rationalisation at the Molecular Level”.
Sven will summarise our soft X-ray spectroscopy work: “Co-Crystals, Coatings, Contamination and Clusters: Core Level Spectroscopies as a Probe for Local Bonding in Molecular Systems”