If A Tree Falls In Liberty Park (Part III)
“You know, Thomas Edison tried and failed nearly 2000 times to develop the carbonized cotton filament for the incandescent lightbulb ...And when asked about it he said, ‘I didn't fail. I found out 2000 ways how not to make a light bulb,’ but he only needed to find one way to make it work.”
-National Treasure
Today, I’m going to start to tell you about some of the many, many ways not to test for lead in organic samples...and how we discovered one way that will work (probably).
To review: The goal here is to analyze samples of an old tree from Liberty Park in downtown Salt Lake City to determine whether there was an increase in groundwater lead contamination due to the use of lead additives in gasoline in the 60s, 70s, and 80s. Since tree rings can be dated pretty easily, all we have to do is take wood samples from different rings corresponding to different time periods and compare the amounts of lead in them. If we see more lead in a ring from, say, 1980 than we do in a ring from, let’s say, 1940, that should tell us something--especially if we get consistent results from subsequent comparisons. The more such samples we can analyze, the better idea we will have of what really happened over time.
When we think about analytical chemistry, we often think about how we’re going to test the sample. We can use a number of different techniques: gravimetric analysis, titration, or in this case, mass spectrometry. What we may not think about is this: How are we going to get the sample into a form that we can test? You can’t just stick a chunk of wood from a tree into a mass spectrometer. You have to process it to get it into a form that the instrument can use--and you have to process it in such a way that minimizes possible contamination of the sample from the environment.
For inductively coupled plasma mass spectrometry (ICP-MS), it’s great if you can process your sample into an acidified solution. Dissolve the sample in acid so that the minerals you’re interested in come out into the solution. One way to do this is called chemical digestion. I talked about this a little in Part I. Put your sample into a solution of concentrated nitric acid and concentrated hydrogen peroxide and microwave it for half an hour or more. This breaks down all the organic stuff into water, carbon dioxide, and other gases, and leaves the minerals you care about dissolved in solution. Sadly, this only works with very small samples, half a gram or less. The preliminary tests showed that samples of a gram or more would be needed to get out enough lead to do anything useful with it. Chemical digestion usually works quite well for isolating metal content (like strontium) in living tissue, but in this specific case, it’s not effective. So chemical digestion is out.
Another technique for eliminating the organic material is called ashing. It involves placing the sample in a crucible and heating it in a muffle furnace to 550°C (~1125°F). This burns off all the organic material and leaves an ash (hence the name) of mineral material. The mineral residue is then recovered and diluted as necessary for analysis in the ICP-MS. Again, there are two steps to the process: ashing, and recovery. Introduction of lead contaminants in either case would be a bad thing.
One hypothesis for the comparatively high levels of lead observed in the chemical digestion tests is that the process was leaching lead out of the digestion apparatus during processing. So the first job was to determine whether the proposed process would leach lead out of the crucibles as well.
We began with ceramic crucibles, because there are dozens of them in the lab. We found three brand new, unopened crucibles and designated them for testing. I cleaned them with MilliQ water (double distilled) and cleaned them using an ultrasound bath. Then, in a positive-pressure hood called a laminar flow hood, I placed a 7-8 mL sample of 5% hydrochloric acid (HCl) and allowed them to sit for five minutes. I transferred these samples into clean, labeled test tubes for analysis in the ICP-MS. I also set up three samples of HCl straight from the bottle for use as blanks (or ‘controls’, as they’re commonly called in high school discussions of the scientific method). I then set thee up for testing and analyzed the results the following morning. It turned out that the blanks had no detectable levels of lead, which was great news! The samples from the crucibles showed lead levels of around 0.3 ppb, which is better than what we saw in the microwave digestion tests, but still not as low as we’d like. Still, it gave us hope that ashing might work.
From here, we actually have to talk some chemistry to explain why we’re going to do (or not do) some things. I think I’ll hold off on that until next time.
-National Treasure
Today, I’m going to start to tell you about some of the many, many ways not to test for lead in organic samples...and how we discovered one way that will work (probably).
To review: The goal here is to analyze samples of an old tree from Liberty Park in downtown Salt Lake City to determine whether there was an increase in groundwater lead contamination due to the use of lead additives in gasoline in the 60s, 70s, and 80s. Since tree rings can be dated pretty easily, all we have to do is take wood samples from different rings corresponding to different time periods and compare the amounts of lead in them. If we see more lead in a ring from, say, 1980 than we do in a ring from, let’s say, 1940, that should tell us something--especially if we get consistent results from subsequent comparisons. The more such samples we can analyze, the better idea we will have of what really happened over time.
When we think about analytical chemistry, we often think about how we’re going to test the sample. We can use a number of different techniques: gravimetric analysis, titration, or in this case, mass spectrometry. What we may not think about is this: How are we going to get the sample into a form that we can test? You can’t just stick a chunk of wood from a tree into a mass spectrometer. You have to process it to get it into a form that the instrument can use--and you have to process it in such a way that minimizes possible contamination of the sample from the environment.
For inductively coupled plasma mass spectrometry (ICP-MS), it’s great if you can process your sample into an acidified solution. Dissolve the sample in acid so that the minerals you’re interested in come out into the solution. One way to do this is called chemical digestion. I talked about this a little in Part I. Put your sample into a solution of concentrated nitric acid and concentrated hydrogen peroxide and microwave it for half an hour or more. This breaks down all the organic stuff into water, carbon dioxide, and other gases, and leaves the minerals you care about dissolved in solution. Sadly, this only works with very small samples, half a gram or less. The preliminary tests showed that samples of a gram or more would be needed to get out enough lead to do anything useful with it. Chemical digestion usually works quite well for isolating metal content (like strontium) in living tissue, but in this specific case, it’s not effective. So chemical digestion is out.
Another technique for eliminating the organic material is called ashing. It involves placing the sample in a crucible and heating it in a muffle furnace to 550°C (~1125°F). This burns off all the organic material and leaves an ash (hence the name) of mineral material. The mineral residue is then recovered and diluted as necessary for analysis in the ICP-MS. Again, there are two steps to the process: ashing, and recovery. Introduction of lead contaminants in either case would be a bad thing.
One hypothesis for the comparatively high levels of lead observed in the chemical digestion tests is that the process was leaching lead out of the digestion apparatus during processing. So the first job was to determine whether the proposed process would leach lead out of the crucibles as well.
We began with ceramic crucibles, because there are dozens of them in the lab. We found three brand new, unopened crucibles and designated them for testing. I cleaned them with MilliQ water (double distilled) and cleaned them using an ultrasound bath. Then, in a positive-pressure hood called a laminar flow hood, I placed a 7-8 mL sample of 5% hydrochloric acid (HCl) and allowed them to sit for five minutes. I transferred these samples into clean, labeled test tubes for analysis in the ICP-MS. I also set up three samples of HCl straight from the bottle for use as blanks (or ‘controls’, as they’re commonly called in high school discussions of the scientific method). I then set thee up for testing and analyzed the results the following morning. It turned out that the blanks had no detectable levels of lead, which was great news! The samples from the crucibles showed lead levels of around 0.3 ppb, which is better than what we saw in the microwave digestion tests, but still not as low as we’d like. Still, it gave us hope that ashing might work.
From here, we actually have to talk some chemistry to explain why we’re going to do (or not do) some things. I think I’ll hold off on that until next time.
posted by Michael at
9:27 AM
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