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

Away from Boston and Back Again: Christmas Break and a Research Update

Two weeks away from Boston for Christmas break, a week and a half back in Boston working on research without the distraction of classes, and the start of classes impending in the coming week.  It's been a busy month since I last wrote!

I went home to the Midwest to celebrate the holidays with family.  We had lots of laughter, family gatherings, time spent relaxing, present exchanges, and good memories.  Highlights included going to the Indianapolis Symphony to see the annual Christmas Yuletide show as well as having lunch with a good friend to see her new engagement ring!  While being home for two weeks was wonderful, all good things come to an end and it was back to Boston on January 4th.

The past week and a half I have been working on my research without the distraction of classes.  I must admit, having such large blocks of time for research tasks was productive but also pretty strange.  I'm used to doing 20 different things at once.  For example, a year ago I was student teaching full time in a rural high school and writing/defending a senior thesis at the same time and the semester before that I was taking 5 classes, including a couple upper level geology classes, while TAing for another 2 courses, working as a planetarium operator, working on research, and doing classroom observation hours.  The chance to focus on just research...well it's been a while since that chance came along.

It's been a fairly eventful week and a half in regards to research.  I have now learned most of the garnet dissolution and lab techniques that we use frequently and my first actual garnet samples should be analyzed within the next couple weeks... well as soon as the TIMS is working again.  For those of you unfamiliar with the TIMS, it is the main analytical instrument that our lab uses to generate chemical data on garnet.  I don't want to get into all of the particulars of the machine's operation or purpose at the moment (I'll save it for a more extensive post with pictures to illustrate) but suffice it to say that the solution for fixing the instrument involved disconnecting all of the electrical connections and probes, rolling a 900+ pound magnet away from the instrument, and using heating pads, internal heating elements, and insulating blankets to literally bake the instrument to high temperatures to burn off any accumulated debris from years of use.  I got to help our lab tech. go through the steps to prep the instrument for baking and learned a lot about the instrument in the process!  I have also been doing major sample preparation in anticipation of a trip to Woods Hole Oceanographic Institute (WHOI) this coming Friday for preliminary chemical analysis.  Keep an eye out for a post next week on my adventure to Woods Hole!