6 Jul 2017

How to prepare a liquid sample for XRF | Spectroscopy Guides

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.

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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 an XRF sample cup, some transparent foil and of course an XRF Spectrometer.

If you want more information on the Specac range of XRF sample preparation equipment, get in touch for a price and product demonstration.

Preparation for liquid XRF Analysis

  1. Choose the right liquid cup

    There are a variety of different cup sizes to suit your needs and spectrometer. Standard sizes include, X, Y and Z mm diameters. Once you’ve found the right size, you can then fill up the cup with your sample. 

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    These cups are designed to be disposable and you can purchase them usually in batches of 100 or more.

  2. Put liquid into cup

    The next step is to 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 mm.

    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.

  3. Select the right foil cover

    When you want to analyze liquids, it needs to be contained within a medium 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 best material and thickness to choose depends on the sample you’re analyzing and your application. These films are made and stored under environmentally controlled conditions to avoid introducing trace levels of contaminants that might affect the XRF data.

    Film Materials:

    Kapton – a robust polyimide material that is shiny orange/yellow in colour. It is stable across a wide range of temperatures and is typically found with a 7.5 um thickness. It can support almost any sample but is particularly affected by strong basic solutions. 

    Polypropylene – a thin-film plastic that has a large % Transmittance for both light and heavy elements, even in low concentrations. Typical thickness is either 6 or 12 µm, and this slightly thicker film gives added security against accidental tearing of the film. Polypropylene has better transmission than Mylar but has a lower tensile strength.

    ‚ÄčMylar® – a perfect choice for run-of-the-mill analysis and containment of liquid samples and is often used for detecting sulfur in fuels, lubricants and oils. It is very durable with a high tensile strength that resists the sample tearing through the film. However, it is not chemically compatible with esters, aromatics, ketones, or aliphatic alcohols. ® Mylar is a trademark of E.I. DuPont de Nemours, Inc.

  4. Place foil onto cup

    Once you’ve selected the best foil for your application, take a roll of that film and place it gentle on top of the sample cup.

    Next, take the plastic ring counterpart that is just slightly wider than the sample cup. Press down onto the ring, gently but firmly. You should create a taut smooth film on the top of the sample cup. This needs to be as smooth as possible because any wrinkles could affect the reading.

  5. Analyze your sample

    After you’ve sealed your sample cup with the right foil, you can place the cell into the XRF spectrometer.

    Once the analysis is complete, you can either discard the cell, or retrieve your sample for further testing or use.

Check out #SpectroscopyGuides for more analysis tips and advice.