Grains under the Microscope: A WHOI Adventure

About a week and a half ago I had the opportunity to visit Woods Hole Oceanographic Institute (WHOI) on the southern part of Cape Cod in Massachusetts.  I took a day trip away from my usual research at BU in order to gather some test chemical data on a couple of my detrital garnet samples.  I received an invitation from a research scientist at WHOI offering me the opportunity to try out a piece of equipment we do not have in our department at BU.

Before I describe what my day at WHOI looked like, let me give you some general background on the goal of the trip and the method that I was using for those of you who are unfamiliar with the SEM.  I traveled to WHOI to use a tabletop scanning electron microscope (SEM).  A scanning electron microscope uses a beam of electrons that bombard the sample.  When the electrons collide with the material being analyzed, different types of detectors can be used to provide information about the electrons that are scattered or energy that is emitted to learn more about the sample being analyzed.  The instrument I was using is smaller than most standard SEMs and is designed to be user-friendly, to take up less space, and to allow for analysis not just of polished, coated samples (which is the standard procedure) but also of rough, uncoated samples using a lower vacuum setting.  This makes this instrument an excellent candidate for analysis of my samples, since the grains do not have to be coated or polished, which would destroy them.  The tabletop SEM is equipped with two different detectors- a BSE detector and an EDS detector.  BSE stands for backscattered electrons and measures the electrons from the beam that are scattered and reflected by the object being analyzed.  It allows for the differentiation between heavy elements and light elements, since heavy elements scatter electrons more strongly.  From this differentiation, an image of the material being analyzed is generated using grey scale that can show surface detail or different mineral phases.  Below is an example from one of the grains that I analyzed at WHOI, though this particular grain is zircon rather than my usual garnet subject.

BSE image showing a zircon grain, analyzed at WHOI 1/20/2012

Besides BSE, the SEM can also do EDS analysis, which stands for energy-dispersive X-ray spectroscopy.  The electron beam hitting the sample can trigger the release of x-rays and the energy of the x-rays released is characteristic of the major elements that compose the object being analyzed.  This makes EDS useful for preliminary chemical characterization and mineral identification.  EDS results in a spectrum showing major element peaks and can also generate atomic and weight percentages for major elements using spot analysis.  Basically, we can get a general idea of the composition of the material being analyzed.

While I was at WHOI, I analyzed twelve different detrital garnet grains.  I used grains from two different locations in Vermont that we have been using as test samples throughout the method development process.  Three of the grains were from one location and the other nine were from a second location.  The sand collected at the second location contains detrital garnet of different colors, so I was able to analyze three grains of each color (red, orange, and pink) hoping to test the ability of the instrument to differentiate between garnet with different compositions.  When doing detrital work, it is important to be able to identify populations of similar grains that likely originated from the same general area and should be roughly the same age, since a single detrital sediment can contain different populations of garnet grains that originated from different locations and were affected by different conditions.  In addition to the garnet analyses, I also analyzed many grains of any other mineral that could remotely resemble garnet under the microscope from one of my other field areas to verify that garnet is not present in the sample.  Here is a picture showing how the grains were mounted for analysis.  The grains are still attached to this mount- they are just too small to see easily in the picture and blend into the carbon sticky tape.  The grain mount is sitting in a small weigh boat to minimize contamination.

Grain mount for SEM using carbon sticky tape

Just for fun, let me give you a look at what my day of research looked like.  My commute to WHOI started around 6:30am and involved two different subway lines, a Peter Pan bus, getting picked up at the bus station and a short drive to WHOI's Quissett campus.  I arrived around 10:00am and started learning how to run the instrument and by 11:00am I was running the instrument independently.  I then spent the next five to six hours (minus a lunch break) analyzing as many grains as possible and accumulating roughly 90 computer files between BSE images, EDS analyses, etc.  Then I took a shuttle bus to Woods Hole, where I ate dinner in an awesome little cafe while waiting for my bus to Boston.  (If you ever find yourself in Woods Hole, MA..I highly recommend eating at Pie in the Sky. It was delicious!)  After a bus ride and two more subways, I arrived home around 10:30pm.  It was a long and tiring day...but definitely worth it.

Here is one final picture, showing me running the SEM at WHOI.  Not only did I learn a new instrument and gather helpful information on some of my samples, I am learning how to be an independent scientist!

Me running the tapletop SEM at WHOI