Dipayan Paul wanted to introduce a new way of measuring the radioactive isotope carbon-14 to Groningen. Instead of an expensive and cumbersome accelerator, two lasers would suffice to measure carbon-14, which is widely used to determine the age of biological samples and the origin of greenhouse gasses. But Paul failed to reproduce the results published by an American lab, and could only conclude that the method was wrong. The results can be found his PhD thesis, which he defended on 11 April.
It is not what you want in a PhD project: to find out that it is built on quicksand. ‘It was a very frustrating experience’, Dipayan Paul recalls. In 2008, an American lab published a paper that seemed to revolutionize carbon-14 measurements. This radioactive form of carbon is used to measure the age of organic samples, up to around 40,000 years.
A tool known as an
Accelerated Mass Spectrometer
(AMS) is used to measure carbon-14. Carbon from a sample is turned into graphite, which is mounted on a sample holder in the AMS. Then the sample is bombarded with cesium, which knocks carbon atoms out of the surface of the sample. These atoms are accelerated and sorted by mass. Carbon-14 is a little heavier than the regular carbon-12.
The AMS is an expensive piece of equipment, costing some 2 million euros, so when Daniel Murnick and his colleagues from Rutgers University
a simpler and much cheaper laser-based method in 2008, they attracted quite some attention. Their method used Intracavity Opto-Galvanic Spectroscopy (ICOGS), which uses two lasers, tuned to carbon-14 and carbon-12 resonances. The lasers emit light that excites one of these isotopes in a glow discharge and the excited carbon atoms in turn change the conductivity of the discharge medium.
Paul shows us the setup he built in the Nijenborgh 4 building. The sample and reference cells are housed inside the laser tuned for carbon dioxide containing carbon-14. ‘Electrodes measure the conductivity of the glow discharge in each cell’, Paul explains. Murnick described how by exciting the samples he could measure the amount of carbon-14 in a carbon dioxide sample.
‘I spent eleven months in his lab, learning the ropes’, says Paul. His supervisor Professor Harro Meijer spent three months there. The method did not turn out quite as expected. ‘Reproducibility was a major issue. And we couldn’t observe a conclusive signal from the carbon-14.’ Paul and Meijer did notice that there was some room for improvement to the method, so back in Groningen Paul assembled his own ICOGS system.
However, after months of tinkering, he still couldn’t get a decent signal. ‘We measure carbon-14 at levels present in the atmosphere, and below. But as it turned out, ICOGS didn’t produce reliable signals even if we enriched samples to one billion times the natural level of carbon-14.’ The results from the 2008 paper could not be reproduced in Groningen, nor in the other carbon-14 labs in Uppsala and New York.
‘Some laser-based techniques can be used to measure carbon-14, but not at the low levels we study’, says Paul. ‘They are mainly used for medical purposes, at some 100 times the natural concentration.’ But ICOGS proved to be even less sensitive.
Two years into his PhD project, Paul was left with a setup that didn’t work. ‘Looking back, it seems like a waste of time and money. I estimate that in all, some five million dollars have been invested in the ICOGS technique worldwide.’ Of course, if it had worked, the carbon-14 labs would have been able to do away with expensive AMS machines. The laser setup comes at about five percent of the cost of an AMS. ‘Our AMS in Groningen will need to be replaced in the near future. We had high hopes that we could do this with the simpler laser-based setup, but that was not to be.’
Meanwhile, Paul had to find new research questions for his PhD project. ‘Eventually, I picked up a project that had been started a while ago in our department that involved studying contamination of carbon samples.’ Carbon from samples is turned into graphite, which is pressed into a sample holder. ‘If you leave those samples in the open air for a few hours, the carbon-14 signal is always higher during the initial phase of measurement.’
Paul discovered that the material science community was already aware of a similar phenomenon, which explained the observations: ‘They describe how surfaces become coated with adventitious carbon, even in a vacuum.’ Metals seem to catalyze the formation of this carbonaceous layer. ‘The bottom line is that the contamination of samples appears to be inevitable, but I have been able to recommend some changes.’
Storing graphite in powdered form, rather than pressed into the sample holder, reduces the extent of contamination. ‘Furthermore, all samples are measured multiple times. The initial measurements represent the contaminated surface and the later ones represent the cleaner uncontaminated sample, so if you discard the initial measurements, as is currently the case, the measurements will be more reliable.’
A third project in Paul’s thesis was a proof-of-principle study on recovering carbon dioxide from stratospheric air samples and then determining the radiocarbon in the carbon dioxide samples. These studies could foster a better understanding of greenhouse gas dynamics in the atmosphere. ‘The main challenge is that the samples of 50 millilitres of air only produce about 10 micrograms of carbon. The minimum carbon samples we measure are more than twice that size, and a normal sample would be much larger.’ Paul managed to tweak the technology for extracting carbon dioxide from stratospheric air samples and turning it into graphite in such a way that the AMS could reliably measure 10 micrograms of carbon.
Meanwhile, the debate about ICOGS is still raging. ‘Murnick hasn’t accepted our conclusion that the method doesn’t work. He still believes in ICOGS.’ On 5 April,
a letter by Murnick
challenging the findings of Paul and Meijer and the Uppsala lab appeared in the journal Analytical Chemistry, with a rebuttal by both the
. For Paul, the matter is closed, after his PhD thesis defence on 11 April!
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