Preparing Doubly Contained Samples for XAS Measurement
At the APS (and presumably at other synchrotrons) certain kinds of hazardous samples require double containment. A common example would be something containing an actinide or radionuclide. My group, for example, occasionally measures sediment samples with uranium contamination. The APS requires double containment – i.e. two physical barriers constraining the sample from leaking out of its sample holder.
There is no single way of preparing samples in this manner, but anything you come up with will be a variation on what I describe here. The technique shown below is the one used by my group. It meets the requirements of the health physics staff at the APS and it makes a good sample for the XAS measurement. When prepared in a glove box, this technique will also protect an oxygen sensitive sample from atmospheric oxygen for many hours.
This picture shows the supplies we will use to prepare our sample. There are three pieces of kapton, two small ones which will be used with the sample holder itself and one large one which will be used to make a baggie. There is a polycarbonate sample frame. This is simply a 50mm x 20mm x 3 mm piece of polycarbonate with a 6mm tall slot milled out. There is nothing special about this frame – any similar object can be used. For instance, a washer makes a fine sample holder. Finally we have a roll of kapton tape, a few tools, and our sample (which is shown here in a weighing boat). Yes, the sample in this demo is some ground coffee. Apparently, we will be measuring the Cf edge ;-)
Step 0: Preparing your sample
This tutorial does not attempt to explain how to prepare your actual sample for XAS measurement. The point of this tutorial is explain how to package your hazardous sample in a way suitable for safe and effective measurement. You must, of course, take all appropriate care to make the sample itself suitable for measurement. That includes making your sample not too thin and not too thick as well as homogeneous and free of pin holes.
Here are some links to useful sample preparation tutorials written by various XAS practitioners. These are all PDF files:
It is wise to spend a bit of extra effort in making a good sample. Beam time is precious. You do not want to squander it with a poorly made sample.
Step 1: Preparing the kapton tape
Here I have cut two lengths of kapton tape. They are held down to the surface with two more pieces of tape. The sticky side is facing up. The two small piece of kapton film were cut to a size slightly larger than the hole in the polycarbonate frame. I have one small piece of film in the center of each piece of kapton tape. The purpose of the small piece of kapton film is to protect the sample from the adhesive on the tape. Some samples may be reactive when exposed to the organic and sulfur compounds in the adhesive.
Step 2: Cover one side of the sample frame
Lift one piece of kapton tape and apply it to the sample frame such that the small piece of kapton film completely covers the slot but is centered on the frame. It is very important that adhesive from the tape touches the sample frame on all sides of the small piece of film. Thus this piece of film must be bigger than the slot, but smaller than the width of the frame.
Flip the sample frame over. We now have a well in which to load our sample.
Step 3: Load the sample
Using a spatula, spoon some material into the sample well.
Clean any stray bits off the surrounding frame. This is an important step – you do not want flecks of your sample interfering with the adhesive.
Step 4: Finishing off the sample holder
Use the second piece of kapton tape with a small piece of kapton film to cover the second side of the sample. Again, take care to center the small piece of film over the sample such that the adhesive of the tape touches the sample frame on all sides of the small piece of film.
With scissors, trim the excess tape. Tah dah! It’s a sample. If your sample is anaerobic and you have performed all steps up to this point in a glove box, the sample will be protected against atmospheric oxygen for several hours. The kapton is essentially impermeable to oxygen. The only path for oxygen to enter the sample is through the adhesive of the tape. So long as you have applied the adhesive smoothly to the sample frame, the diffusion path for the oxygen will be very long and slow.
Step 5: Preparing the secondary containment.
APS rules require two independent forms of sample containment. For our second form of containment, we will make a small bag out of the large piece of kapton and some kapton tape. While there are many conceivable ways of providing secondary containment (a ziplock bag comes to mind) this is the only method approved by the APS. This is a matter of the health physics staff having standard operating procedures (SOP) that they are obliged to follow. At the APS, the SOP for secondary containment of an XAS sample stipulates making a kapton baggie in the manner described here. Both the film and the tape must be kapton – other kinds of film and other tapes are not consistent with the SOP.
Here is a blurry picture of the prepared sample frame and the large piece of kapton.
Fold the large piece of kapton film in half and seal the sides with kapton tape.
Finish sealing the sides of the bag and slip the sample inside.
Step 6: Finishing the secondary containment
Trim the excess material from the top of the bag.
Using more kapton tape, seal the bag. Trim away any excess material. It is a good idea to leave the tape a bit longer than the bag it is sealing. That way, you can be sure there are no small holes in the corners of the bag through which something might leak.
Each sample must have its own baggie. Never put multiple samples in a baggie.
Remember that the XAS measurement will be made through the baggie. For lower energy edges, you should consider using the thinnest available film and tape.
For anoxic samples, it is a good idea to perform all steps inside of a glove box.
To maintain a safe environment for an anoxic sample during transport, slip the entire baggie into a Falcon tube while still in the glove box. For the highly paranoid or if you expect that the sample will spend extensive time outside of a glove box, the Falcon tub can be placed inside of a Mason jar while still in the glove box.
At some facilities, kapton is the only acknowledged material for preparing these samples. In particular, polyethylene bags with zip-lock tops may not be used as a form of containment.