Monday, 20 February 2012

Tracer Tales

In today’s installment, Dr. Marie-Jose Messias has kindly written a few words explaining the operation of the space-age looking equipment used in the tracer container to detect the concentration of dye in the seawater:
Marie-Jose analysing tracer results
The inert molecule CF3SF5
The tracer team - Neill, Ben, Dhruv, Steve, Marie_jose and Andy

“Our mission is to better understand how water from the Antarctic Circumpolar Current (ACC) invades the rest of the world’s oceans. To do this, 80 kg of tracer (trifluoromethyl sulphur pentafluoride, CF3SF5) were released into the Pacific side of the ACC in 2009. Since then, we have been tracking its dispersion vertically (diapycnal mixing) and horizontally (isopycnal mixing) by collecting water at different depths along its path towards the Atlantic and measuring its tracer content. Our measuring instrument (which we have named Electric Barbarella 3000) is very sensitive, and able to detect a concentration of 5 atomoles (10-18 moles/liter) or 0.5 micrograms of tracer within a kilometre cube of seawater. It uses a method called ‘purge and trap analysis’ and is coupled to a gas chromatograph.  Everything is carried through the instrument by a flow of the ‘carrier gas’, Nitrogen (N2).  Water samples flow through into a calibrated volume and then are transferred to a ‘sparge tower’ under vacuum. The sparge tower is where the dissolved gasses (including our tracer CF3SF5) present in the water are removed by bubbles of nitrogen gas. Gases are carried from the water into a ‘trap’, which is cooled to -110°C, causing the gases to condense and become trapped. We cool the trap by immersing it in liquid nitrogen vapor. After 3 minutes of sparging, the trap is lifted out of the liquid nitrogen and heated to +110° C. This releases the trapped gases into the gas chromatograph, where the tracer is separated from the other gasses by going through a series of chromatographic columns. The amount of tracer present is quantified by an electron capture detector (the electrophile CF3SF5 ‘captures’ the electrons). Thus we can tell how much tracer we have in each water sample! The running time per sample is 9 minutes, giving us just enough time to compile and check the results from the previous run, and to fetch the new samples, and maybe make a quick cup of tea...  Next sample please!”

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