29th November 2019 | Author: Dr Simon Burgess
The question ‘What is Standardless Quantitative Analysis?’ was a subject of discussion at the Microanalysis Society (MAS) 2019 Topical Conference on Quantitative Microanalysis (QMA 2019) held in June, at the University of Minnesota in Minneapolis.
From the most important perspective, that of the user, standardless analysis is any method that provides a quantitative composition from EDS data without the need for the user to standardise. That means not needing to keep and maintain a set of certified standards for different elements that are used to standardise the analysis of each element requiring quantification. It also means a system will be able to provide quant results right from the point of installation, straight out of the box.
All commercially available systems now provide this capability, but standardless analysis often has a bad reputation, with questions about its accuracy. Many people view it as a ‘semi-quantitative’ technique with the ability of determining a general view of the composition of a sample at best. For example, the latest addition of the leading textbook Scanning Electron Microscopy and X-ray Microanalysis, 4th Edition (Goldstein et al. 2017) takes a rather dim view of the technique. They view it as a black box tool, which can only give results normalised to 100%, hides errors, and is subject to much wider range of variation in results compared to analysis where standards are collected. Their view is that users should expect results to fall only within 25% of the true value.
When you consider the wide range of parameters that need to be known and controlled to calculate the composition of material from X-ray intensities, without the help of standards, you can see their point. These parameters include beam current, acquisition time, detector solid angle, detector efficiency for each energy measured, and radiative transition probability including fluorescence yields, Coster-Kronig transitions and line weight. With all the uncertainty associated with some of these parameters, you could conclude that if standardless analysis is using this type of approach, a +/- 25% data spread is not a bad effort.
At QMA2019 I was kindly given a platform to present a different view standardless analysis by EDS, with a talk entitled ‘Standardless Quantitative X-ray Microanalysis with Energy Dispersive Spectrometers’. In this presentation I explained the rather different approach employed by Oxford Instruments. In the AZtec software we provide pre-installed standardization databases for micro- and nano-analysis where the standards have been collected by our experts in our factory.
By doing this, the need to know many of the parameters mentioned above disappears, and we only need to worry about the beam current, solid angle, and detector efficiency. Beam current and solid angle can be ignored by normalizing the result to 100%. However, by collecting a calibration spectrum from a single pure element such as copper, cobalt or silicon (beam measurement function in AZtec), un-normalised results can be calculated and the user can take advantage of having an analytical total. That leaves only the detector efficiency, a very tough challenge to characterise to the accuracy required for all X-ray energies for all detectors. The key to solving this one was to measure a detector on a monochromatic synchrotron X-ray source, measuring the change in intensity with energy directly for the first time. The important thing now is to ensure all detectors shipped to our customers have an efficiency which matches the model, one of the reasons why all the detectors we make are tested on an SEM using several standards, prior to shipment.
I presented some tests of certified materials using our approach, at count rates of 50,000cps and 200,000cps. The tests showed 95% of compositions fall within 4% of certified values, for un-normalised analyses for a wide range of elements from boron to bismuth. When we excluded light elements such as boron, nitrogen and oxygen (matching the protocol used for the standardless evaluation reported in Goldstein et al.) we achieved 95% of compositions within 2.1% of their certified value with a mean analytical total of 99.1% (see graph). This is very close to the ‘rule of thumb’ target of 2% often quoted for describing the analysis of materials by WDS using EPMA.
As part of the session at QMA2019, there was a very interesting discussion about what we should call different methods. There are clearly very different methods of achieving quantitative EDS analysis which require no standardisation by the user, but they achieve potentially very different levels of accuracy, from +/-25% for true standardless approaches to approaching +/-2% for the method used in AZtec. The general consensus of the room was that the term ‘standardless analysis’ should be reserved for methods that truly do not use standards, and by implication are less accurate. For methods that use standards, but not collected on the user system, another term is needed. ‘Single Standard Quant Analysis’ was used by Statham (2009) to describe the method, highlighting the need to measure only single pure element to provide un-normalised analysis. Another term which is achieving some traction is ‘remote standards’, or ‘remote standard method’, as discussed in a recent paper by Newbury and Ritchie (2019) in Microscopy and Microanalysis. Whatever we call it, we must not lose sight of what this name must describe, a method which requires no standardization by the user but can provide a result to good accuracy and include a useful analytical total, not normalized to 100%.
If you are interested in finding out more about the spectrum processing and quantitative analysis approach used in AZtec a paper by Pinard et al. ‘Development and validation of standardless and standards-based X-ray microanalysis’ is due to be published soon as part of the proceedings of the 2019 European Microbeam Analysis Society (EMAS) workshop in Trondheim.
This blog is the third in a series about standardless quantification. Discover more now:
Dr Simon Burgess
Business Manager X-ray Products