If A Tree Falls In Liberty Park.... (Part II)

Okay, so we’ve established what we want to do: We want to check the rings of a fallen tree for lead to see if the groundwater lead levels have changed significantly over time, specifically as a result of the use of lead additives in gasoline in the mid-20th century. And we have an instrument, the ICP-MS (inductively coupled plasma mass spectrometer) that can detect the concentration of trace metals like lead to something like a few parts per trillion. That’s good.

Mostly.

The thing is, at that level of sensitivity, such an instrument can start picking up traces of lead that come from sources other than the sample. For example, the acid used to process the sample might have tiny (but detectable at this level) impurities of lead. The containers used to process the sample may have lead contamination from prior use. Even dust in the air might contain enough lead to be detectable. This is problematic because it can lead to a false result, showing more lead than is actually there.

First, a quick bit of terminology is in order. In instrumental analytical chemistry, the reading from the instrument that shows the true amount of the substance we care about in the sample is called the ‘signal’.  The reading for the amount of that same substance that comes from sources other than the sample--processing, contamination, the environment, etc.--is referred to as ‘noise’. To be confident of our results, we need to demonstrate that the reading we get from the instrument is primarily signal, with little or no noise--in other words, we want a strong signal-to-noise ratio.

What is a good signal-to-noise ratio? Depending on the test, you really don’t want more than 5% noise. 1%-2% would be even better, of course, but in many cases, you can live with 5%. Knowing that 95% of your reading is due to the sample and nothing else gives you a certain level of confidence, statistically speaking, that what you’re looking at is real. If the noise gets much higher than that, you begin to lose confidence in your results.

So, how do we know how much of the result is signal and how much is noise? We use a chemical blank. For example, I recently prepared three samples of what’s called a Standard Reference Material (or SRM, in this case, NIST SRM 1515, Apple Leaves) for analysis by placing each sample in a clean, dry crucible and baking it in a furnace overnight at temperatures up to 550°C (~1025°F). Once it cools down, I will take the residue and dissolve it in a mixture of hydrochloric acid and ammonia, and then analyze this solution for lead content. Now, as I prepared and processed these three samples, I also placed three empty crucibles into the furnace. I will also add the hydrochloric acid/ammonia mixture to them and then analyze those samples for lead content. In other words, the empty crucibles will undergo the same processing as the real samples. The empty crucibles are the chemical blank; they tell us (at least in theory) how much lead comes from the environment and not from the sample. They tell us the level of noise.

So when the preliminary samples were processed and tested months ago, a chemical blank was made and tested at the same time. And the results were very interesting. The bark of the tree showed extremely high levels of lead. Well, that’s not entirely unexpected. The bark can pick up lead from the air and soil around the tree as well as the groundwater. The interior rings in the tree showed lead concentrations ranging from around 11 parts per billion (remember, 1 ppb is roughly a paper clip in a swimming pool) to around 30 ppb. That’s not a lot. However, the chemical blank showed a lead concentration of around 2 ppb.

2 ppb of noise compared to 11 ppb of signal is around 18%. That’s way too high.

Furthermore, it’s not just the concentration of lead that matters, it’s also the actual amount. Once we identify how much lead is in the sample, we also want to try to isolate it for further testing (specifically isotopic testing, which I’ll tell you about another time). To do that, we need a certain minimum mass of lead to work with. Now, going back to the definition: in a water solution, parts per billion effectively means means micrograms of the substance we care about (again, lead in this case) per liter of water, or nanograms (0.000 000 001 g) per milliliter of water. Since these samples are almost always 10 mL in volume (because the containers can only hold that much), that means that even the 30 ppb sample contained only 300 nanograms (0.000 000 300 g) of lead. We can run isotopic testing on samples that small, but only if they’re very pure--and remember, thanks to the chemical blank, we already have an uncertainty of 20 ng/300 ng, or about 6.7%, in our purity. That won’t give us results that mean anything useful.

So, what do we do about that? If the processing adds that much lead (and uncertainty) to our results, we’re going to need a different way to process the samples.  I’ll start telling you about that in Part III.

If a Tree Falls In Liberty Park, Does It Reveal Anything About Lead Levels In the Groundwater? (Part I)

For those who are interested (if any), here’s a summary of my summer research project so far. Watch this space for further updates!

A hundred-year-old tree died in Liberty Park in downtown Salt Lake City a few years ago. As living organisms incorporate trace metals (like lead) from their environment, any change in the level of groundwater contamination by these trace metals should be reflected by the organisms that rely on that water. Since the growth and development of trees can be tracked by the ages of their rings, different rings should have different levels of these metals in different rings.

The specific question my project seeks to address is this: Did the use of leaded gasoline in the mid-20th century result in a significant increase in the level of lead in the groundwater in Salt Lake City?

It’s worth noting that the water here is already rich in minerals--it’s very hard water. All groundwater, especially here, has at least some lead in it anyway. It’s also true that mining in the late 19th and early 20th centuries resulted in increased levels of lead entering the local water system. The strategy, then, is to analyze wood samples from the tree from different time periods and see if there is a rise in the lead concentration that coincides with the use of lead additives in gasoline--and if so, how much.

This project was actually started by another researcher who used microwave digestion to process a small number of wood samples. What is microwave digestion? Well, you take a small sample (half a gram or less) and add it to a mixture of concentrated nitric acid and concentrated hydrogen peroxide. (The hydrogen peroxide used in this process is ten times as strong as what you buy at the drugstore.) Place this sample in a plastic container that releases gases at very high pressure, and cook it in an industrial-strength microwave oven for half an hour or so. This process converts all the organic matter--proteins, cellulose, etc.--into water and carbon dioxide. What remains is a solution with the minerals (like lead) dissolved in it.

Samples of this solution are then analyzed for the element in question. The lab I’m working in uses an instrument called an inductively-coupled plasma mass spectrometer (ICP-MS). It’s a big, complex, $400,000 instrument, but here’s the quick oversimplified version: The sample is injected into a stream of argon gas that travels through an ionized plasma at around 4000°C. This knocks an electron off an atom of the sample, giving it a charge. It then travels through an electromagnetic field that bends its path; only atoms with the right mass:charge ratio can get through. The particles that get through are then counted by a detector of some kind. This instrument can separate the particles we care about--lead atoms, in this case--from everything else in very, very small quantities.

How small? Well, you know what the word ‘percent’ means, right? It means one in a hundred--parts per hundred, you might say. One in a thousand, by the same reasoning, would be called parts per thousand. With this instrument, we routinely measure parts per trillion.

How much is one part per trillion? Consider an Olympic-size swimming pool. It holds approximately a million liters of water, with a mass of a million kilograms or a billion grams. How much is a gram? It’s about the mass of a paper clip. One paper clip in an Olympic swimming pool is one part per billion. So, what is one part per trillion? It’s one paper clip in a thousand Olympic swimming pools.

Yeah, the ICP-MS is that sensitive. That’s why we use it for this kind of research. Unfortunately, that’s also part of the problem.

More on this next time.