Preparing samples by single grain fusion mass spectrometry
By Sander Hoogendoorn
What is single grain fusion?
I will apply many analytical techniques during my PhD, but few are as important as single grain fusion mass spectrometry. This technique applies high-intensity infrared or CO2 lasers – may the force be with you – to melt single grains from crushed rock samples. Melting rapidly releases trapped noble gases and other volatiles from the mineral, which can be measured with a mass spectrometer. However, before geochemists can use these measurements to make funny-looking figures, you will need to go through a rigorous preparation scheme. In this blog, I will take you through some of the preparation steps that you’ll need to follow to carry out your own single grain fusion experiments.
Setting up your project
So you have collected rock samples from your field area and your supervisor hints at the idea to do some noble gas analyses! The first thing you will probably do is scratch yourself behind the head. What were the noble gases again? After a quick Wikipedia search you realize that helium, neon, argon, krypton and xenon are the most important noble gases to be aware of. Moreover, you’ll probably find that noble gases are not great in socializing with the rest of chemical society – they have severe attachment issues with other elements and do not tend to form any bonds. That is about as good a start as you will need!
The next thing you will need to do is decide what noble gases are interesting for your project. In my case, argon is an exciting candidate, since it is the natural decay product of potassium. This means that it is widely present in the continental crust and can be used to date minerals.
Enter the lab stage. Here you will need to make the most important decision – what rock samples are you going to analyse? It is unlikely that can analyse everything you collected because time is precious during a PhD. So instead you will need to become a bit superficial and select a few lucky samples based on their appearance, which is somewhat akin to running a beauty contest.
Charnockite from the Agly Massif, southern France. The dominant minerals are feldspar, quartz, biotite, garnet, and orthopyroxene. The black rectangle is indicative of a thin section site.
After choosing your samples, you will need to mark them so that you can cut them into thin sections. These very thin rock slivers can then be further investigated under an optical microscope. This is an important step, since it will allow you to gauge how ‘happy’ the grains are. Happy may seem like a stretch to describe grains – I have never seen someone looking down at the beach and exclaiming how ‘cheerful’ the sand looked. Nevertheless, geologists use this term all the time. It means that grains are devoid of any intergrowths and generally look clean and robust – thus ideal targets to pick for the laser.
Top left shows an intergrowth of amphibole and biotite. The amphibole is ‘unhappy’ and I bet that the biotite is not thrilled with the situation either. Top right shows a clean biotite with inclusions of zircon/monazite, which can be recognized by black radiation halos. Bottom left shows an ‘unhappy’ biotite that has an encrustation of quartz/felspar on its surface. Bottom right shows a feldspar intergrown with biotite. There also seems to be a bit of alteration on its surface. Hence this grain especially, is rather miserable.
Now that you have looked at the grains, the real fun begins. It is time to crush the rocks and sieve the fragments. Some labs have a wide variety of machines that will help you to do this, but in my case I simply used a mortar and pestle. You will end up with a range of glass bottles containing grain and fragments of different sizes. Surrounded by all these vials you may develop a sensation that you are a wizard, although the real magic still needs to happen. You will need to pour some of the material from the vials into a petri dish and carefully place this under yet another microscope. Now it’s time to individually pick out ‘happy’ grains from the crushed material with a pair of tweezers, preferably by repeatedly switching between reflective to transmissive light. This is by far the most time consuming and frustrating step, mostly because grains tend to develop a personality at this stage. Some grains will be easy to pick and adhere to the tweezers without any complaint, whereas others try to bounce around or play hide and seek. A few grains will probably even attempt to jump out of the petri dish to try their luck on the floor. Just like handling teenagers, you will just need to go with the flow and maintain a healthy amount of patience. Fortunately, some labs can accelerate this process by using powerful mineral separation techniques that single out mineral species based on their physical properties, such as their density and magnetic susceptibility.
Left picture shows sample bottles and their corresponding plastic bags. Correct labelling is an absolute must. So be aware for your handwriting… Right picture shows the typical picking set-up. Crushed material is gently poured in a petri dish from which individual grains are picked with a pair of fine tweezers.
Top left shows several unhappy feldspar grains that are intergrown with biotite. These grains I would typically avoid picking. Top right shows a happy K-spar grain that is relatively unaltered and a happy white-mica grain. These grains I would select for cleaning. Bottom left shows the result of cleaning of a biotite grain. Bottom right shows a happy plagioclase grain that appears slightly stained and has a few tiny biotite inclusions.
Cleaning your samples
Finally, you have selected your grains. You may now reward them by a few hours of relaxation in a bath of acetone and deionized water. Of course, you will also need to put on some ultrasonic music to give the grains the full experience. This may be achieved in an ultrasonic bath. The cleaning stage is very important, since traces of organics or salts – for example from your fingers – can cause interference problems during later analyses in the mass spectrometer.
Top picture shows a private hot tube for the selected grains. Although many will welcome this deep cleansing treatment, some will implode and pulverize to dust. Especially micas tend to be a bit fragile. So always pick more grains than you expect to analyse. Bottom picture shows the end result of the folding process, which will leave you with small, one-by-one-centimetre squares filled with the cleaned grains. The packages with and without grains need to be weighed carefully before sending them off for irradiation. This will give you an idea of how much radioactive material you will get back and how long the material needs to ’cool down’ before you can safely work with it.
Packing your samples
Now that the grains are cleaned, you can enter the last stage of preparation, where you will need to perform origami with a pair of tweezers. It is not as bad as it sounds. You just need to fold small packages from a piece of aluminium foil and transfer the grains into these packages. The transferring of the grains is the difficult part – it requires steady hands and precision. I therefore recommend cutting back on the coffee and watching some House or Grey’s Anatomy in preparation the night before. After filling the packages, you carefully seal them off, prepare some additional standards, and send the whole lot to a nuclear reactor for irradiation. You’re finally done!
Finally, a few last pieces of advice
- Dreaming about grains a few weeks after picking is a normal withdraw symptom.
- Singing while picking is a great way to maintain your sanity and to lure some grains to your tweezers.
- The preparation of other noble gases may not require the origami since they are unlikely to be send off for irradiation and just need to be cleaned prior to lasering.
- All joking aside, the sample preparation is a very important process that will make you quite familiar with your minerals. Take your time to look at the grains so that it will be easier to explain the data when your results come in. You have picked the grains, so you’re the expert of your samples.