How to use an XRF machine!

The X-Ray Fluorescence (XRF) machine is super important to those of us doing research on heavy metals in soils. The XRF has a simple purpose: to measure, in a given sample, the concentrations of the various elements. It gives accurate concentration data on most elements in the periodic table. Plus, it's quick to use, requires little sample preparation, and doesn't modify or consume the sample it tests - so the same sample can be used again for subsequent unrelated tests and procedures.

The basic idea behind XRF is to bounce X-rays off of a sample at varying intensities and measure the fluorescence from the different elements present in the sample. The more fluorescence, the higher the concentration. It looks something like this: (credit to www.cat.ernet.in)

You start, as with any test, by prepping the samples. For XRF, it's easy! XRF can be a dry test, so all you need is some soil ground into a fine powder. It's definitely time-consuming, and a little dull, but it doesn't involve lots of careful solution-mixing or anything like that.

A few of our 70+ painstakingly ground soil samples

Once the samples are ready, you need to assemble the containers to hold the samples. The containers need to have transparent bottoms which will interfere minimally with the X-ray fluorescence. For this, we use a special kind of plastic wrap called prolene, which is extremely thin and pure. It's also very delicate and clingy, and you shouldn't touch the part that will actually hold the sample, so it can be tricky to work with.

The container comes in two parts: a tube and a ring. The ring snaps snugly around the bottom of the tube. The tube is placed upside-down in a hole in a small wood plank, which just makes the top of the tube more flush with the surrounding surface, and therefore the prolene film is easier to work with.

Upside--down tube on the left, and ring on the right.

Removing a section of prolene film (carefully!)

The prolene film is placed over the ring. The wooden plank gives it something to cling to so it's easier to work with and doesn't fly away.

Finally, the ring is snapped over the tube, stretching the film across its bottom. There should be no wrinkles or dust on the film.

Once the containers are assembled, labeled, and ready to go, fill the containers with approximately 5 grams of sample each.

Empty assembled containers 

A perfect measurement :)

Then load the samples into the special rack for the XRF machine. The rack spins during analysis to switch samples. Screw the rack into the XRF machine and close the lid tightly to shield from X-rays!

The rack filled with samples

Loaded into the machine

Finally, enter the names and masses of the samples into the XRF software and press "Start". Make sure the helium gas, which is used instead of air to minimize interference, is at the correct flow rate. Check it periodically, and come back in about two hours to remove the samples and get your data!

Here's what the full XRF setup looks like:

Machine on the left and computer/software window on the right. On the left of the screen, the circles on the rack change color from red to yellow to green, indicating (respectively) untested samples, the current sample, and finished samples. On the right of the screen are the names and masses of the samples and some other information about them.