John Fellowes, University of Manchester
Part of the show Chemistry at the Synchrotron
Meera – It seems some museum samples could be harbouring volatile compounds of mercury. But no need to fear, as it is only the people handling the samples themselves that need to watch out. To help them, John Fellowes from the University of Manchester is on the case at the University’s Herbarium to stop these vapours in their tracks.
John – Mercury salts were widely used to preserve museum specimens against damage from bacteria, insects, fungi and so on and so forth, and you can tell by the state of the preservation that these mercury salts have worked, but over time, and what’s been noted, is the initial mercury solutions, you’re talking pretty concentrated mercury solutions, one recipe out there has it a 30g per litre of mercury chloride dissolved into methylated spirits, which were then either sprayed on or paint brushed on or the samples themselves were dipped into this solution and what’s been happening over time is that this mercury chloride has been converted into different forms of mercury, one of which is volatile mercury which is then being released from the samples.
Meera – so is the problem then that when the people go to work with these samples and open them up, they’re being exposed to these volatile levels of mercury?
John – Well, that was one of the things we were investigating, yeah. The first thing we had to test for was making sure there were no dangerous effects in the area. The samples themselves were located in the herbarium and the first thing we tested was the mercury content of the air in the herbarium and we found it was well below the guidelines. So we had 1.7 micrograms per cubic metre of mercury when the guidelines are up at over 25.
Meera – What were the concerns with this to mean you’re are analysing these samples to see the levels of mercury within?
John – The samples themselves may or may not have been contaminated with mercury because the problem is, is that over the course of several hundred years, ‘cause these collections date back hundreds of years, they have been homogenised and sort of rearranged and resorted and reset out, so they don’t know which samples have actually been treated. And the boxes that these samples are kept in are these little airtight containers or little cardboard boxes which are essentially air tight and what we were mostly concerned with was that this build-up of mercury within these boxes, if someone could open this case, then you would have this kind of waft of mercury, which overall across the entire area is not a significant concentration, but it’s that initial opening of the box that we wanted to look at.
Meera – So how have you set about doing that?
John – Ok, so the other aspect we were looking at is, part of my work as a biogeochemistry really is to analyse how efficient our biogenic selenium nanoparticles are, that’s selenium nanoparticles that which been precipitated by bacteria, and one thing that’s been published out in the literature is that selenium is very, very good at capturing this elemental mercury phase, so it was kind of a two-fold research in that one, we wanted to identify, quantify any mercury present within the samples, and secondly to find a way if we could test our selenium nanoparticles and how good they were at capturing this mercury.
Meera – How have you gone about doing that?
John – What we made is little filter membranes. Our selenium starts off as a suspension and you can filter them out onto these filter membranes which are these 0.22 micron filters, and then allow them to air dry and we placed them in some of the boxes where we knew samples had been contaminated with mercury and see if we could capture the mercury as it was coming off the samples.
Meera – So I guess essentially what, you would be making these membranes to soak up the mercury just prior to opening up samples? Or how would this process work if these are proven to be effective at soaking it up?
John – The samples themselves we would be leaving inside the boxes permanently, so hopefully it will decrease any chances of a build up of mercury within the boxes.
Meera – And so how have you been using Diamond to try and visualise this?
John – Diamond was crucial really because we needed to investigate the form of the mercury present which will help us to understand the fate of what happens to the mercury, but also, because the samples are so varied, some have been contaminated with mercury, some haven’t, some of the samples which have been contaminated are only contaminated in certain areas of the sample, so Diamond to start with, with the samples, was helping us to pinpoint where the mercury was and also tell us the kind of concentration we can expect and the form that it’s in. Secondly, it also helped us to characterise the phase the mercury takes once it’s been reacted with the selenium and ensure that it’s not about to come back off again.
Meera – And what were some of your key findings, how badly contaminated were the samples?
John – So within the herbarium there are numerous collections and the one we focussed on was the European Collection and once we’d identified a sample that had got the mercury, we needed to find out whereabouts it was. What we found is that the mercury itself tended to be as little crystalline mercury sulphide phases that happens which was Diamond helped us prove this. There was one particular sample which was really quite good and that was a collection of dust from one of the bottoms of these boxes and just to put this in context; the samples were stuck down onto A3 sheets, and then these A3 sheets were then piled one on top of the other within these cardboard boxes and what we found when we were looking through was, we’re trying to identify where this mercury is and none of the sheets really came very high on the scale of mercury. But when we analysed one of them we found the dust which had collected on the bottom of the box, so none was on the actual sheets it was in the dust at the bottom. When we analysed that we found quite a high mercury concentration which we picked out by XPF and then we sent off to Diamond for XAS. What we found on this is maybe a micron long particle contained on one of the dust particles, which was almost pure mercury sulphide. Once we analysed these samples in Diamond, they were able to scan across a sample set and pick out and say there’s a bright bit there, let’s go in and get our XAS spectrum from this little sample here.
Meera – Now that you’ve been able to see this spread and localisation of mercury within this variety of samples, what are you able to do with the information, what do these findings really mean?
John – So what we can do with this data now is help build up Health and Safety guidelines as to what people are to do with these samples. This will help to say what you want to do when you find one of these boxes is open the box and just leave for 5 minutes and then come back and go through the samples. Because it’s the elemental mercury phase, the volatile phase, which is the dangerous one, you want to watch out for. The mercury sulphide that’s going to be in those, not really too much concerned. We’d like to get to the stage where these selenium membrane filters are used within the boxes to help prevent this build up of mercury within the boxes, so that when someone comes along to look at one of these samples, they can lift off the lid and not have to worry about a waft of mercury coming out from the box.
Meera – So before these membranes are in use, mental note, leave the room when opening a museum sample, well, some of them anyway! That was John Fellowes from the University of Manchester.