New and more complex formulations for lubricants, fuels and oils are constantly being developed because of the drive for lower costs and improved quality, as well as the growth of biofuels.
Consequently, industry and scientists need to analyze liquids for regulation compliance and to check for contamination.
X-ray Fluorescence (XRF) allows contaminants and trace quantities to be detected. From 100 % concentrations, all the way down to parts per billion (>0.00001 %). This means XRF is perfect for monitoring gasoline, diesel fuels, naphtha, gas oils, crude oil, biofuels, lubricant additives and waste from oil refineries, in accordance with international ASTM Standards such as:
- D2622: Standard Test Method for sulfur in Petroleum Products
- D7085: Standard Guide for Determination of Chemical Elements in Fluid Catalytic Cracking Catalysts.
- D6334 & D5059: Standard Test Methods for sulfur and lead in Gasoline, respectively
XRF is not just used to test oils, it can be used in diverse industries like sewage, agriculture, solvent, paints and dye production.
Liquid XRF analysis is simple, fast and requires very little sample preparation compared to other techniques. Sample cups are cheap, simple to assemble and require minimal liquid handling. The main trick is to choose the correct support film that provides a balance of strength and transmission capabilities and contamination.
Here is a guide to how to analyze liquids using XRF. You will need some XRF sample cups, some thin-film window material, and of course an XRF Spectrometer.
Preparation for liquid XRF Analysis
- Choose the right liquid cup: There are a variety of different cup sizes and formats. The diameter is mostly determined by the spectrometer you will be using. There are two major formats, single and double open-end. Singles are open at one end only and can be pre-filled before applying the film window covering. Double open-ended cups have two open ends, as the name suggests. You have to apply the window covering first, then pour the sample directly onto the window. Sample cups are designed to be disposable and you can purchase them usually in batches of 100 or more. In this guide, we’ll describe use of a single open-ended cup.
- Put liquid into cup: Carefully pour or pipette your sample into the cup, making sure you have enough volume to cover the surface of the cup and give a depth of a few millimetres. Users can place the cups onto a mass balance if they wish to know the weight of their sample. If there is any large debris or particulates like soot, you may wish to filter them out.
- Choose a film window covering: Now the sample needs to be covered by a thin layer of material that is transparent to X-Rays. Luckily there is a wide range of foil materials and thicknesses that can be used to contain your sample, such as Kapton®, polypropylene and Mylar®. The thickness of the material affects the amount of X-Ray signal that is detectable through the film, but you also need to factor in chemical compatibility and resistance to tearing or puncturing – if the material breaks, the contents of the cup will be emptied into the spectrometer! Thin film supports are available in several formats. The basic format is a roll of film from which sheets can be cut, but the films are also available as pre-cut discs on a paper backing and in cardboard support frames for ease of application.
- Apply the film onto cup: Lay the film over the open end of the sample cup, gently stretch it taut over the opening. This needs to be as smooth as possible because any wrinkles could affect the reading. The sample cup will have a second component which slides over the opening, capturing and retaining the foil in place. Fit this carefully to avoid tearing the film.
- Analyze your sample: After you’ve sealed your sample cup with the right foil, you can place the cell into the XRF spectrometer. Carefully invert the cup so that the liquid is resting on the thin film window and place this window on the spectrometer for analysis.
Once the analysis is complete, you can either discard the cell, or retrieve your sample for further testing or use.
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