Randy Holmes-Farley
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I do not know what tube filling procedures are advised by all ICP companies, but a recent test by Christoph at Oceamo points out a potentially serious concern.
Freezing of seawater causes the formation of fresh water ice and hypersaline seawater. As the ice spreads to more and more of the total water, the saline liquid remaining can be very much concentrated relative to the original sample.
If all of the collected sample remains within the tube, there will be no change the in the element concentrations when the sample is thawed (at least not by the mechanism described below).
But if the tube is filled to near or at the top, having little air volume to compress, the pressure can rise quite substantially during freezing, and this rise can push out some of the hypersaline liquid. Since that liquid no longer remains in the tube when the sample is thawed, those ions are lost from the measurement, and the net concentration of the remaining fluid after thawing is reduced. Christoph showed that a tube filled to the top and frozen to -4 deg F showed a drop in calcium from about 420 ppm to about 300 ppm, magnesium from about 1250 ppm to 900 ppm, etc. he also showed very high ion concentrations in the exuded fluid
These effects are far too large to ignore.
Thus, if there is any chance your samples may freeze, be sure to NOT fill them all the way up. If your ICP company advises filling to the top, I'd suggest contacting them to see if it is OK to only half fill the tubes.
Here is Christoph's data:
www.reef2reef.com
Freezing of seawater causes the formation of fresh water ice and hypersaline seawater. As the ice spreads to more and more of the total water, the saline liquid remaining can be very much concentrated relative to the original sample.
If all of the collected sample remains within the tube, there will be no change the in the element concentrations when the sample is thawed (at least not by the mechanism described below).
But if the tube is filled to near or at the top, having little air volume to compress, the pressure can rise quite substantially during freezing, and this rise can push out some of the hypersaline liquid. Since that liquid no longer remains in the tube when the sample is thawed, those ions are lost from the measurement, and the net concentration of the remaining fluid after thawing is reduced. Christoph showed that a tube filled to the top and frozen to -4 deg F showed a drop in calcium from about 420 ppm to about 300 ppm, magnesium from about 1250 ppm to 900 ppm, etc. he also showed very high ion concentrations in the exuded fluid
These effects are far too large to ignore.
Thus, if there is any chance your samples may freeze, be sure to NOT fill them all the way up. If your ICP company advises filling to the top, I'd suggest contacting them to see if it is OK to only half fill the tubes.
Here is Christoph's data:
Oceamo ICP-MS seawater accuracy and precision data
Dear all, @Randy Holmes-Farley, as in related threads before: If this has too much of an advertisment character, feel free to delete this posting. We try to regularly shed light onto the processes that happen within the Oceamo seawater laboratory. Im having the feeling that this openness is...
www.reef2reef.com
