Liquid Waste Analysis Garry Smith, XRF Application Specialist SciMed XRF, a division of Scientific and Medical Products Ltd
About Us SciMed represent Rigaku (RESE and ART) and Seiko (SIINT) XRF ranges Over 20 years history of working with XRF A wide range of UK installations across many types of applications
Summary What are the difficulties with liquid waste samples and how does the sample introduce error into the test result? Examples of errors induced on analysis: Suspended solids and settling Immiscible liquids and separation What are the practical solutions?
What are the difficulties? Liquid waste samples are rarely ideal as they are rarely homogeneous. May contain solids which settle over time. May be mixtures of immiscible liquids which separate over time.
An Example Example photos of multiphase sample before, during, and after separation and settling.
Why is this a Problem? For any analysis, the measured sample must be representative of the bulk material being tested. With XRF we are not always measuring everything in the sample cell. If components separate during analysis they may not be measured accurately.
Critical Depth Fluorescent photons from analytes are reabsorbed by the sample. Absorption follows the Beer-Lambert law: I = I 0 e -μρx where: μ = mass absorption coefficient ρ = matrix density x = path length
Critical Depth Critical Depth is defined as the depth of sample from which 99% of the fluorescent photons are re-absorbed. To calculate we can re-arrange Beer- Lambert to express path length, x, where: Io = 100 I
Critical Depth The equation simplifies to: Critical depth, x(mm) = 46.605 μρ Critical depth increases with increasing photon energy. Critical depth decreases with increasing average atomic number and density of the sample.
Critical Depth Liquid Sample in Cell Zn Cl Fluorescent photon reabsorbed by sample Critical depth of Zn Kα Cl Critical depth of Cl Kα Incident X-rays Fluorescent photon to detector
Critical Depth Typical critical depths of selected XRF lines in oil and water Line Energy (kev) Typical Critical Depth (mm) Oil Water S Kα 2.31 0.36 0.12 Cl Kα 2.62 0.50 0.17 Cr Kα 5.41 4.33 1.44 Zn Kα 8.64 16.3 5.42 Pb Lα 10.55 27.9 9.74
Example 1 Effect on Peak Intensity of Analyte by Settling of Sediment (Analyte in Solution) Overlay view of spectra for S in heavily sedimented oil measured at 2 minute intervals, showing attenuation of peak as sediment settles. The analyte is concentrated in the solution.
Example 1 Effect on Peak Intensity of Analyte by Settling of Sediment (Analyte in Solution)(cont.) Overlay view of corresponding spectra for Zn in in the same sample. However there is virtually no change in intensity with time. The higher energy of Zn Kα has greater critical depth in the sample. Intensity is less variable with distribution throughout sample.
Measured S (ppm) Measured Zn (ppm) Example 1 Effect on Calculated Concentration of Analytes by Settling of Sediment (Analyte in Solution) Measured S and Zn Concentration vs. Time for Waste Oil 6000 250 240 5500 230 5000 220 210 4500 200 4000 190 180 3500 170 160 3000 150 0 2 4 6 8 10 12 14 16 18 Time after mixing (min)
Example 2 Effect on Peak Intensity of Analytes by Partitioning of Liquids (Aqueous Analyte) Overlay view of spectra for Cl measured at 2 minute intervals after mixing, showing enhancement of measured peak as aqueous component containing chloride separates to bottom of liquid cell.
Example 2 Effect on Peak Intensity of Analytes by Partitioning of Liquids (Oil Analyte) Overlay view of corresponding spectra for Cr measurement. Cr is concentrated into the oil component and is attenuated as this layer separates to the top of the liquid cell.
Measured Cl (ppm) Measured Cr (ppm) Example 2 Effect on Calculated Concentration of Analytes by Partitioning of Liquids (Analytes in Both Phases) 90000 85000 80000 Measured Cl (aqueous), and Cr (oil) vs. Time in Oil / Aqueous Mixture 250 230 75000 70000 65000 60000 55000 50000 45000 210 190 170 150 0 2 4 6 8 10 12 14 Time after mixing (min) Measured Cl (ppm) Measured Cr (ppm)
Summary Effect on measured intensity may be attenuation or enhancement depending on circumstances. Rate is dependent on the specific sample, e.g. particle size, viscosity, etc. Amount of error varies with analyte higher energy lines (usually heavier elements) affected less than lower energy lines (usually lighter elements).
In the case of real samples, these effects are in practice impossible to predict.
What are the Solutions? Ensure analysis is completed before phases separate (approximate analysis) Separate phases to remove unwanted components or analyse separately Stop phases from separating Use solid binders to immobilise components
Aqueous Samples Aqueous samples containing solids can be dealt with quite easily: Allow sample to settle or centrifuge sample and pipette supernatant liquid (dissolved components only). Filter sample and analyse separate components as required. Note can also be applied to organic liquids.
Immiscible Liquids Oil / hydrocarbon samples may contain water. While it may be possible to use emulsifier to stabilise, no universal solution for all sample types. More robust solutions: Separation Immobilisation using binder
Separation of Liquids - Centrifuging Centrifuging can provide a simple approach to separating aqueous component from oils and hydrocarbons Also removes solids Component required for analysis can simply be pipetted after centrifuging Reference: ASTM D4294 Sulphur in Petroleum and Petroleum Products by Energy Dispersive X- Ray Fluorescence Spectrometry.
Immobilisation Using Binder Separation of liquids or solids not always a practical solution (especially for waste oils) Both immiscible liquids and solids can be immobilised by mixing the sample with a powder binder material Graphite powder Activated alumina
Graphite Powder Fine graphite powder provides an inert and spectrally pure carrier material for oil / hydrocarbon samples Typically mix 10g sample with 4-6g graphite to produce a slurry Transfer to liquid sample cell and measure Reference: ASTM D5839 Trace Element Analysis of Hazardous Waste Fuel by Energy Dispersive X-Ray Fluorescence Spectrometry.
Activated Alumina Activated alumina provides a stable carrier material for aqueous / organic solvent samples Typically mix 5g sample with 15g activated alumina to produce a slurry Transfer to liquid sample cell and measure Reference: ASTM D6052-97 Preparation and Elemental Analysis of Liquid Hazardous Waste by Energy Dispersive X-Ray Fluorescence.
In Closing Multiphase liquid samples can introduce significant errors into analysis results if not treated appropriately These errors are a result of the potential for phases to separate during measurement and the varying critical depth of different analytes. A number of potential solutions to remove or neutralise sources of error. Standard methodology often incorporates these precautions.