Sunday, October 14, 2012

The Princess – she keeps me up all night!

by Hussein Sayani (@hsayani)

Scientists will often name instruments and equipment in the lab. I like to think it’s out of pride and love, kind of like how people name their cars, and not because we talk about them all the time and just want to blend in with normal people waiting in line for coffee. For example, our Delta V plus mass spectrometer and Kiel carbonate device are called Matilda and Damien (named after certain movie characters). My friends know this, so I’ll occasionally get calls that essentially go: “I haven’t heard from you in a while. How’s Damien doing?” See, it makes us sound normal! Anyway …

The NENIMF houses two ion microprobes, the Cameca IMS 3F and the Cameca IMS 1280. For this project, we’ve been using the IMS 1280, a very large beastie affectionately known as The Princess.

The obligatory glamour shots

A quick rundown of how this ion microprobe works:
Schematic of the IMS 1280, borrowed from the NENIMF site.
A duoplasmatron (a type of ion source) is used to generate a primary beam of oxygen ions.  This primary beam then digs into the sample, ejecting ions from its surface (this is called sputtering).  Various lenses and deflectors are used to channel the sputtered ions towards a large magnet which separates the ions by tiny differences in mass (every element has a different mass).  After being separated, our charged particles are sent to the ion counters which tell us how much of each ion is present in the sample.  Pretty nifty, right?  For my samples, the measurement process takes about 15 minutes per spot.  We measure about 60-65 spots a day and the sampling is only partially automated. Hooray for 17 hour work da … wait, what?

The Princess and her bling:
Preparing samples for The Princess is a little different. With the other analytic techniques we employ, we usually mill some coral powder and then dissolve it in acid. The Princess on the other hand, prefers smooth, flat, and shiny gold discs. SIMS samples are prepared by cutting of a small section from the slab. The coral skeleton is very porous, so the side of the coral section that we want to measure is embedded in epoxy to create an even surface. After some polishing to completely flatten and smooth this side, a small slice is taken and attached to a glass slide with more epoxy. Then come the hours and hours of polishing the slide to get it perfectly smooth and flat. Finally, the slide is coated in gold (ooh shiny!). 

The thin sections (slightly larger than a quarter) are locked into a steel mount and loaded into the SIMS (red arrow).

Since the slice of coral on the slide is very thin (hence the name thin section), we can use a microscope shine a light through it and see the various different crystals that make up the skeleton. Besides being very cool, this lets us know what features within the skeleton to avoid (as different features have different chemical compositions).

The left side is what we see on the SIMS. The right side is what the slide actually looks like.
You need a map of both to know what to avoid (e.g the black line running down our column - see the white arrow).

I'm sorry, Earth!
Loading the sample into the SIMS and waiting for the instrument to reach high vacuum can take a couple hours, so it’s best to know beforehand where you want to make your measurements. After spending some time familiarizing myself with each slide (and series of slides as we have 2-3 per coral), I use the microscope to build a map to help me navigate each slide. Back in January I did this using paper, scissors, and tape (and you thought science was complicated). Out of kindness to trees and the planet, this time I made the maps on my laptop instead.

Now that we’re all familiar with the SIMS technique, let’s take a look at what this beastie can do. We’re interested in figuring out past ocean temperature, and so we’ll be measuring the ratio of strontium to calcium (Sr/Ca) at each spot on the coral as this is a commonly used  proxy for such reconstructions. Remember, our goal is to determine how useful, reliable, and reproducible SIMS measurements are. So the first thing we want to check is whether or not our SIMS measurements look like temperature. We measured Sr/Ca at almost weekly resolution (one measurements every 6-8 days). As you can see on the graph (light red line), the data looks very noisy and it’s hard to see any long-term trends. What we want to see is an annual cycle (a warm summer period and a cool winter period), and to get that we’ll have to do some averaging.  Since we’ll be comparing these Sr/Ca measurements to monthly ocean temperatures, a monthly running average is appropriate.  Applying this to the data set, we get a Sr/Ca record that happens to look a lot like temperature (thick red and black lines)! This is great because it tells us that we’re doing something right.

We still need to test how reliable this technique is – i.e. can we measure similar samples and get the same numbers? To test this, we measured samples from different corals that grew at the same time.  The idea is to get the same Sr/Ca record for each of them.  Surprisingly, the records from two of these overlapping corals were an almost 75% match. That is much better than what we were expecting.

There is much more data left to analyze, but it looks very promising. We’re hoping that the 800 measurements we’ve made will provide us with a roadmap to the most efficient and effective way to use this technique to test the integrity of our fossil corals.

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