18June2014

XRF Meeting Report 18^th June 2014

A joint BCA/ RSC Atomic Spectroscopy Group Meeting at University of Leicester.

A PDF version of this report (frep14.pdf) with extra photographs can be downloaded from the foot of this page. Clicking the file name will load the file into your browser, clicking the small blue down arrow to the far right of the file name will download a copy to your computer.

Morning Session.

The morning session started with a Handheld workshop.

The first workshop of the day was given by Mathieu Bauer of Bruker Elemental who provided an overview of hand-held XRFS calibration techniques. Recent improvements made to hand-held devices include increased sensitivity for ‘light’ elements (Mg, Al, Si P, S) and integrated cameras/video. Spectral line calibrations using pure elements and/or empirical calibrations with user-defined standards and fundamental parameter (FP) algorithms are available with most models.

Chris Calam of Thermo Scientific introduced Ken Granger of Niton UK who gave a practical demonstration of their hand held XRF instrument and Positive Materials Identification (PMI) software. A Krugerrand coin, business card with thin coating of Gold (to determine the thickness of the Au layer) and a steel cast in the form of a human jaw bone (!) showed that unusual sample shapes could be analysed. Software options include a ‘Search and Match’ database facility for use in a QC environment.

James Parker of Olympus discussed methods of drift correction in hand-held instruments. A docking station containing a multi-element metal alloy (316 stainless steel) was one method used to correct for spectral line drift, thereby reducing the possibility of false positives during analysis. A practical demonstration followed of the auto-drift software used to correct the electronics for changes in temperature during operation.

A talk on safety was given by Steve Allott of SPECTRO Analytical detailing the legal requirements of IRR99 (The Ionising Regulations, 1999) applicable to all electrical equipment emitting ionising radiation and containing components operating at a potential difference of more than 5kV. As handheld XRFS instruments often fall into this category, users are required to notify the HSE, undertake risk assessments and provide method statements and local rules. Designated areas should be clearly defined where hand-held XRFS analysis is taking place and it is mandatory to consult with an RPA before starting work. Under the regulations, manufacturers are also obliged to make the use of hand-held XRFS devices as safe as reasonably practical, e.g. contact sensors, shutters, back scatter shields, dead-mans’ switches so samples cannot be held in one hand while the device is activated by the other and the use of flashing lights to indicate when the X-ray tube is on.

The final talk of the workshop was given by Vivian Wang of Oxford Instruments on sample preparation. As normal with XRFS techniques in general, the more sample preparation that you are able to perform prior to analysis, the better the results obtained. Hand-held instruments were considered suitable for in-situ analysis of light elements, e.g. on drill cores, either directly or through a Prolene film (plastic bags were not recommended) and the use of a light radiation shield to reduced back scatter was advised. Typical analysis times were in the region of 30-60 seconds for samples with water content of around 10-15%. At >20% water, the drying of samples was recommended (possibly by microwave in the field) but this could potentially affect the determination of volatile elements. Differences in water content between samples would also make comparative data difficult to interpret.

Delegates were then given the opportunity to discuss hand-held instrumentation with the suppliers and speakers before a welcome coffee break.

Heather Harrison

Morning Session speakers, from left to right:

Dave Taylor, (Chair), Kevin Solman, David Beveridge and Ros Schwarz

The first user presentation was given by Kev Solman of Plymouth University. His hand-held XRFS was used to assess the potential risks of sediment and water contamination caused by paints from derelict and abandoned boats. Samples of boat paint flakes were analysed primarily in the laboratory using the hand-held device fixed into a portable sample holder. The window of the X-ray tube, normally focussed as an 8mm spot was reduced to 3mm and used in conjunction with a built-in camera to analyse specific areas of the sample material. Paint fragments were found to be non-homogenous, not only were several different paints used on the same boat within a short distance of each other but paints of varying ages were laminated. The depth of penetration of the X-rays was therefore critical to the project and an attempt to correct for the different thicknesses in the paint layers is at an early stage of development. Results from XRFS were compared to ICP-MS, recognising potential issues with ICP-MS analysis caused by the presence of plasticisers. Overall, results compared well with a few exceptions, most notably Sn. The overall consensus was to have greater confidence in the XRF data as the ICP-MS methods were subject to error from incomplete digestion, the formation of Sn complexes and adherence of these complexes to the ICP-MS sample delivery system.

The next presentation was given by Ros Schwarz, on the results obtained for DOT-1, an alumino-silicate matrix, spiked with two rare earth elements. Ros was keen to emphasise this was a participant study using ‘normal’ XRFS methods and that the material was not destined to become a CRM. Nick Marsh’s team at Leicester had prepared the sample by riffling and milling before distribution to the 16 labs that took part in the study. Detailed reports will be sent to all those who participated and Ros went on to present some interesting findings from her normalised dataset. For major oxides, data from 13 labs using fusion were produced using 8 different flux blends with the addition of a heavy absorber (lanthanum oxide) still being preferred by a few. Trace elements were analysed by pressed powder pellet data with wax, PVA and cellulose acting as binders. Four labs submitted loose powder data and two ICP-MS results were available for comparison to the XRFS data. When comparing powder pellet results to those obtained by fused bead, the variability of Fe data was greater by pressed powder than fusion and interestingly, sulphur was seen to be retained in the beads. For low level traces, XRFS and ICP-MS results were generally comparable with the exception of Sn (approx. 400 ppm by XRFS and 11 ppm by ICP-MS). Poor sample digest and loss of Sn during analysis may account for the low ICP-MS result and XRFS result may have been influenced by a Sn/Ag line overlap as low-level Ag was present in the sample. Tin may also have given rise to a false positive result for I reported in some XRFS results as no iodine was present in the sample. Poor resolution could also account for XRFS Th data being poor in comparison to ICP-MS due to a Th Lα/Bi Lβ line overlap. Results for Zr also varied widely; ICP-MS fusion digest may not dissolve all the Zr, but line overlaps (Zr Kα1/Sr Kβ and Zr Kβ/Mo Kα) may introduce errors in lower resolution XRF spectrometers particularly when the overlapping elements are present at similar concentrations. One observation highlighted by this study was the potential for problems in determining fundamental parameter (FP) totals using data from different sample preparation techniques, e.g. fused bead for the major oxides and pressed powders for traces.

An introduction to the next participant sample, DOT-2, was given by David Beveridge. This material is a halogenated copper phthalocyanine pigment, namely BASF Heliogen Green D9360 (CI Pigment Green 36). This contains approximately 4% Cu, 6% Cl and 61% Br, and has a reputation for being difficult to analyse; the exact colour depends on the proportion of the halogens, which is therefore an important parameter to control. It is considered to be thermally stable up to 300 °C; if it is heated further, brown fumes (probably bromine) are evolved, so caution is necessary if a fusion is attempted. Careless handling of the powder is liable to colour everything green! Further data on this and related pigments can be found at http://www.supmat.com/download/BASF/heliogen_e[1].pdf David wished all participants good luck with the study.

The worthy winner of the conference prize for the most unusual XRFS sample was David Taylor. David’s sample was a precious and irreplaceable 1 g of medieval window glass taken from the Rose Window of York Minster during a restoration project. The sample was prepared by hand-grinding in an agate pestle and mortar followed by fusion in graphite with Ta as a heavy absorber since this element was unlikely to be present in the sample. Under reducing conditions, a 20 g bead was prepared and cast; this was then lapped and polished before analysis. Late Medieval glass, also known as Forest Glass, contains more K and Ca than modern soda glass so no comparable materials were available for use as standards. This problem was remedied by modification of existing glass CRM’s and the creation of a mixed set of calibration standards of float, Forest and synthetic glass materials. This talk was the first ever given by David in 1976 at an XRFS meeting and we hope it won’t be his last.

Heather Harrison

Afternoon Session.

The afternoon session also started with a workshop.

Afternoon Workshop photograph.

The afternoon workshop featured the smallest benchtop/portable XRF instruments. Steve Davies from PANalytical took as his theme realistic expectations about light element analysis outside a well-equipped laboratory. He suggested that the principal role of portable XRF systems was to determine the heavier ‘money’ elements and not light elements which generally comprise the matrix. He emphasized that some of the most important factors that limit good light element analysis are to do with sample preparation.

Next, Jennifer Horner from Spectro gave us an overview of the detection limits that might be achieved by a portable XRF. She gave some figures obtained for a pure silica matrix in the range from <1 ppm (As, Se) up to about 10 ppm (V, Ce), but pointed out that certainly other elements present at significant concentrations will lead to higher LODs. She showed us spectra illustrating these limits and some analysis figures for reference materials.

Garry Smith from SciMed described the types of calibrations available for portable instruments. Garry showed that the calibration packages now available with bench top and other portable systems were every bit as sophisticated as those enjoyed by their larger laboratory based cousins but again emphasised that whilst this was a great benefit the quality of results would still be to a great extent governed by the sample preparation and the availability of suitably prepared reference materials.

Finally Colin Slater from Bruker gave a very useful explanation of what drift correction will correct and when it should not be relied on. In particular, drift monitors will easily correct for the predictable loss of intensity as an x-ray tube ages and tungsten is deposited on its window but should not be used for fluctuations caused by temperature changes, for example if the instrument has been switched off for some time.

After the talks, the instruments were discussed and demonstrated in an open session before we all returned downstairs for tea and cake.

Afternoon Session speakers, from left to right:

Mathieu Bouchard, AndyScothern, David Beveridge (Chair), Garry Smith

The final session of a very interesting meeting featured three talks on a wide range of subjects. First, Garry Smith from Scimed XRF discussed the “Selection and Application of Internal Standards for Quantitative XRF Analysis”. Using matrix correction coefficients has one disadvantage: all the matrix elements must be measured. In some cases, such as waste analysis or commercial assay, it is not possible to do this easily so a constant concentration of an internal standard element can be added to the bead or solution and its intensity is ratioed to that of the analyte. Matrix effects (and other systematic interferences such as variations in sample:flux ratio, instrument drift or sample surface imperfections) affect both intensities equally and thus are cancelled out. Balancing these advantages are the disadvantages that any internal standard element only provides correction for one or two analytes and that another labour intensive weighing step is introduced. Garry then turned to the criteria for a successful internal standard element. Because its intensity must be representative of that of the analyte element, there must not be any absorption edges from matrix elements between the two, nor should its lines overlap the analyte line or be overlapped by a matrix element line and finally it must be an element never found in the sample. He emphasised that it is still important to understand the sample matrix to apply an internal standard method. In answer to a question from the floor, he recommended that fluxes with added internal standards should be prefused to avoid problems with settling out.

Next, Mathieu Bouchard from Corporation Scientifique Claisse described “Calibration Strategies for XRF Applications using Sample Preparation by Borate Fusion – Practical Example for Chlorine analysis in the Cement Industry”. He reminded us of the challenges in developing XRF applications; including increasing concentration ranges, adding new elements to a calibration, and finding calibration standards. He briefly covered the well-known advantages of fused beads over pressed powder pellets. Turning to the preparation of calibration beads, Mathieu mentioned certified reference materials, synthetic standards and the possibility of adding spikes of “missing” elements to certified reference materials (CRMs). He used the example of adding chlorine to cement CRMs to extend the calibration range from 0.0183% up to 0.567% and showed some calibration lines from different chlorine sources. He pointed out that in some cases pre-ignition of samples before fusion led to loss of chlorine. In answer to questions from the floor, he told us that he had found that adding NaCl in solution was the most satisfactory method and that some chlorine was lost from the bead during fusion, but that the amount was consistent as long as the fusion method remained exactly the same. That is, beads prepared by gas fusion at 1035^oC were not compatible with beads prepared by electric fusion at 1045-1050^oC (temperatures at crucible base).

Lastly, Andy Scothern from British Gypsum, Saint Gobain, gave us some examples of “XRF on a Micro Scale. How EDX, Coupled with SEM, Can Provide Elemental Data to Solve Product and Research Problems”. Andy started his talk with an overview of the international operations of Saint Gobain before describing the focus of British Gypsum on plaster and plasterboard, specifically the forms of calcium sulphate: gypsum (CaSO₄.2H₂O), hemihydrate (CaSO₄.0.5H₂O) and the two forms of anhydrite (CaSO₄); soluble and insoluble. His laboratory uses XRD to track phase transitions between these forms and has XRF calibrations specific to gypsums, cements and flyash/slags for highest accuracy. They also use loose powder and pressed powder pellet methods for trace heavy element analysis and do semi-quantitative analysis. Andy then showed us images of gypsum crystals from their SEM and described the modes of operation to acquire XRF information. They use point and identify for semi-quantitative analysis and line scan to track gradients in concentration, for example in soluble salt migration. Element mapping over a cross section is used to investigate problems, for example with delamination, as it will detect Ti from paint applied incorrectly between plaster layers. He emphasized that correct sample preparation is very important in all XRF techniques, including SEM, by showing an example of shadowing where an uneven surface had blocked x-rays so they did not reach the detector.

The meeting finished at 5pm with our thanks to all the speakers, the staff and organisers from the University of Leicester, and the members of the XRF committee of the Industrial Group of the BCA.

Ros Schwarz