Insights from industry

Why it is Important to Measure the Uncertainty of Spark Measurements

Thought LeadersWilhelm SandersThomas Asam 
Dr. Ulrike Corradi
In this interview Wilhelm Sanders, Product Manager for Spark Optical Emission Spectrometry at Thermo Fisher Scientific, Thomas Asam, Managing Director, and Dr. Ulrike Corradi, Quality Control Management at Taz GmbH talk to AZoM about why it is important to understand the uncertainty surrounding spark measurement. 

How does Thermo Fisher Scientific's analytical software support the calculation of measurement uncertainty?

Wilhelm Sanders: According to DIN EN ISO 17025, it is necessary to calculate the measurement uncertainty for each measurement result. The state-of-the-art guidelines for doing so can be found in the Guide to the Expression of Uncertainty in Measurement (GUM). However, it can be challenging to implement the guidelines in GUM, especially for those unfamiliar with the necessary mathematical and statistical concepts.

Testing and calibration personnel may look for readily available tools to assist with the uncertainty analysis. Thermo Fisher Scientific has implemented an off-the-shelf tool in its analytical software to calculate the measurement uncertainty as required by DIN EN ISO 17025.

What is the definition of measurement uncertainty and how is it characterized?

Wilhelm Sanders: According to the Guide to the Expression of Uncertainty in Measurement (GUM) and the International Vocabulary of Metrology (VIM), the uncertainty of measurement is a measure of the dispersion of values that can reasonably be attributed to a measurement result. It is a parameter associated with the result of a measurement.

Another way to define uncertainty is as a range of values usually centered on the measurement value and includes a true value with the stated probability.

Image Credit:Shutterstock/New Africa 

What factors contribute to measurement uncertainty, and how can these be minimized?

Wilhelm Sanders: Various factors can contribute to the uncertainty in measurements. These can be divided into internal and external influences.

External influences include errors before the measurement, such as during sample taking and preparation, and the environment in which the measurement takes place, such as temperature, humidity, vibrations, and dirt.

The operator's behavior and the measurement sequence can also significantly contribute to measurement uncertainty.

Other external factors include the measured material (surface, homogeneity, and size), the measurement strategy and resources used, and the measurement method itself.

Internal influences on measurement uncertainty include the instrument's standardization, the reference material used for calibration, the calibration model, corrections for inter-element effects or line overlaps, and the precision of measurements of unknown samples.

It is possible to directly evaluate and combine the calibration uncertainty and the precision of measurements of unknown samples, and estimate other sources of uncertainty by carrying out repetitive tests with different operators or different sample preparation materials and applying a factor.

What are the different types of reference materials available for spark spectrometry and how do they differ in terms of quality and intended use?

Thomas Asam: Four types of reference materials can be used for spark spectrometry. The first is Setting Up Samples, which are homogeneous but do not have certified exact concentration values. These samples can be used for control cards, instrument recalibration, or homogeneity testing.

Reference materials (RMs) have concentration values and uncertainties certified. Certified reference materials (CRMs) are produced according to relevant standards and have their concentrations calculated through a round-robin test.

The highest quality reference materials are traceable CRMs produced by national institutes such as PTB and BAM in Germany or NIST in the USA. The competence of CRM producers can also be proven by accreditation according to ISO 17034.

What is the structure of reference materials certificates, and how do they differ in terms of the information about concentrations, uncertainties, and traceability?

Thomas Asam: Setting up samples, such as RE12, do not have assigned concentration values for all elements in the certificate and do not provide information about measurement uncertainties. These samples cannot be used for calculating measurement uncertainties.

In contrast, reference materials (RMs) have assigned concentration values and information about the uncertainty of each element. RM certificates also list the specific use of the sample in each laboratory and the names of the participants in the characterization study.

Certified reference materials (CRMs) provide average concentration values and the uncertainty of each element, as well as information about the method of preparation, sampling, chemical analysis, estimation of uncertainties, and traceability. CRM certificates also describe the details of production and certification according to relevant standards (such as ISO Guide 31, 35 and ISO 17034) and the Guide to the Expression of Uncertainty in Measurement (GUM) requirements.

What are the benefits of using calibration certificates to understand measurement uncertainty on a spark spectrometer?

Thomas Asam: Calibration certificates are important for obtaining information about the measurement uncertainty of all elements in a method across all concentration ranges.

The company TAZ Servicetechnik has got an accreditation according to DIN EN ISO 17025. Their calibration certificates are traceable and include a DAkkS and DKD symbol. They also include information about the calibration range, used reference materials, number of measurements, certified and analyzed values, absolute and relative differences, absolute and relative standard deviations, and enhanced measurement uncertainties.

It is important to carefully review the results to check for a good correlation between certified and analyzed values and to identify any potential interference or matrix effects.

One method for calculating measurement uncertainty is the top-down method, which is relatively easy to use and allows for the global recording of all influence factors by measuring one or two reference samples. It also allows for the recording of hidden systematic influences. This method can be implemented using an Excel file.

Some disadvantages of the top-down method for calculating measurement uncertainty include the fact that reference and test objects may not always be comparable, it may not be suitable for very high-quality requirements.

Image Credit:Shutterstock/QualityStockArts

How is the measurement uncertainty of an unknown sample calculated using reference samples and the top-down method in Excel?

Thomas Asam: To calculate uncertainties using the top-down method, you must analyze your unknown sample with at least four or more measurements. Then, select one or two reference samples with similar composition and the same matrices as the unknown sample and ensure that they are certified, traceable reference samples with similar analytical content to the unknown sample. Measure the control sample under identical conditions, with at least four or more measurements.

Next, calculate the four uncertainty contributions: error from the unknown sample's standard deviation, error from the certificate of the reference sample, error from the analysis of the reference sample's standard deviation, and the difference between the measured and certified value of the reference sample.

Insert the control sample's measured concentration and the absolute standard deviation. The average concentration and the extended combined measurement uncertainty will then be calculated for each element.

An Excel file can be used to automate this process. Enter the sample name and number of measurements for the unknown sample, copy the elements from the spectrometer software, enter the method-measured concentrations of the unknown customer sample and the calculated standard deviations for the control sample, and enter the number of measurements, the measured concentrations of the control sample, and the measured absolute standard deviations.

The Excel file will automatically calculate each element's average concentration and the extended combined measurement uncertainty.

What is the bottom-up method for estimating measurement uncertainty, and how does it use the OXSAS spectrometer control software?

Wilhelm Sanders: Thermo Fisher Scientific's OXSAS spectrometer control software includes a feature called the bottom-up method, which estimates most contributors to measurement uncertainty directly from the measurement results.

This method requires a minimum of two measurements to display the uncertainty, using the standard deviation as a parameter.

In the bottom-up method, the calibration model's uncertainty is considered. For instance, a calibration curve for carbon in low alloy steel can be made, with each point on the curve representing a certified reference material (CRM) used to calculate the intensity recorded during the instrument's calibration, plotted against the known certified mass percentage of the material.

The uncertainty from the CRM's certificate and the precision of the intensities recorded during the measurement of the samples are used to create a "cloud" of uncertainty for each sample, which is displayed as a rectangle and used to calculate the confidence interval of the correlation curve. The correlation curve is linear and independent of the calibration model.

Using this approach, the uncertainty can be calculated from the multi-regression model, and the combined uncertainty can be calculated by combining the uncertainty from the calibration model with the precision of the unknown sample, using the standard deviation recorded during the measurement of the unknown sample.

How does the OXSAS analytical software's bottom-up method compare to the top-down methods?

Wilhelm Sanders: The OXSAS analytical software's bottom-up method can calculate measurement uncertainty using an optical emission spectrometry (OES) instrument. This method does not require statistical skills and can be easily plotted with the analysis and average. It also includes information about sample homogeneity, however the display of the measurement uncertainty can be turned on or off in the format settings.

This “partial” uncertainty of measurements directly calculated with the Spectrometer may be completed with a factor coming from other sources of uncertainty, which can be determined  via experiments with different operators or sample preparation methods.

In addition to meeting the requirements of ISO 17025, this approach can be used in product conformance testing. In this context, the probability of conformance (PC) is used to determine if a product's chemical composition falls within a given set of specification limits for a given element.

The uncertainty of measurement plays a role in this determination. Overall, the OXSAS analytical software's bottom-up method is a fast and easy way to determine measurement uncertainty.

Image Credit:Shutterstock/InWay

What are the key features of Thermo Fisher's benchtop optical emission spectrometer and the iSpark 8820 and 8860 high-resolution spectrometers?

Wilhelm Sanders: Thermo Fisher's optical emission spectrometer instrument portfolio includes several options ranging from value to high performance.

The benchtop instrument is suitable for process control and quality assurance in small and mid-sized foundries and metal processing industries. It is equipped with high-performance CCD optics and can analyze various elements, including nitrogen, steel, and sodium lithium in aluminum matrixes. It is easy to use and maintain and has a low cost of ownership.

The iSpark family includes the 8820, a pre-configured instrument with a range of predefined elemental sets, and the 8860, a fully configurable single or multi-base metals analyzer that can support up to 80 element channels. The iSpark 8860 also offers the option for ultra-fast inclusion analysis to determine non-metallic inclusions.

Several Thermo Fisher's X-ray fluorescence (XRF) instruments, including the ARL OPTIM'X, the PERFORM'X, and the 9900 series, as well as automation systems, can also use the measurement uncertainty feature provided by OXSAS.

How can uncertainty for measured values be indicated in practice?

Thomas Asam: There are two ways to specify measurement uncertainties in Thermo software. The first is for the software to provide the measurement uncertainties automatically. The second is for the user to specify the maximum possible matrix-independent process uncertainties using traceable calibration certificates. In this case, the results of the calibration certificates are used to indicate the maximum possible uncertainties.

Where can the calibration certificates with traceability for Ti-Base be obtained?

Thomas Asam: The TAZ Servicetechnik is an accredited calibration laboratory for Spark Spectrometry. The company offers traceable calibration certificates for Spark spectrometers for all important matrices.

What details about Spark Spectroscopy must be considered in the audit, apart from the control cards?

Thomas Asam: Several considerations need to be taken into account when using a Spark spectrometer. It is necessary to have reference samples with a range of concentrations for each element being measured and knowledge of the uncertainty of measurement for all elements in all concentration ranges.

Training employees on the use of the instrument is also important. It is also necessary to have calibration certificates and to validate the Spark spectrometer to meet audit requirements.

What is reproducibility and why is it important?

Dr. Ulrike Corradi: Reproducibility is the part of precision in measurements under conditions that may involve different locations, operators, measuring systems and replicate measurements on the same or similar objects. To know the reproducibility of a measurement system is a key parameter in the validation of the measurement capabilities of this system.

Can you compare the resolution with Spark OES and ICP for OES?

Thomas Asam: It is possible to compare the results of inductively coupled plasma (ICP) and optical emission spectrometry (OES) if both instruments are of good quality and the operator is skilled.

In reference materials tests, comparable results have been obtained using both ICP and OES instruments when the instruments and operator are of high quality.

About Wilhelm Sanders

In 1989, Wilhelm started his professional career at SPECTRO Analytical Instruments after studying Physical engineering at the Rheinische Akademie in Cologne, where he gained his first experience with optical emission spectrometers, which also ignited his passion for this technology. Since then, Wilhelm has worked in this field for various companies and roles. First in applications, then in automation, and later in service and sales. Since 2011, he worked for Oxford Instruments, whose industrial analysis division was acquired by Hitachi-High-Tech in 2017. There he was working as a product manager responsible for the complete OES product portfolio which included bench top and mobile spectrometer types only. Since 2021, Wilhelm is working in this role for Thermo Fisher Scientific, as the task of developing high-resolution "high-end" spark spectrometers really appealed to him. What he likes about this role is that it enables close contact with our customers and interaction with research and development.

About Thomas Asam

Dipl. Ing. Thomas Asam ventured into self-employment in 1995 and founded TAZ GmbH.

Fracture assessments and damage analyses, GDOES depth profile analyses, XRF mappings, CSNOH determinations, and a whole range of other metallographic examinations in the field of accreditation according to DIN EN ISO 17025 are offered here on behalf of customers.

TAZ also offers the production and certification of customer-specific reference samples according to DIN EN ISO 17034 as well as traceable calibration certificates with DAkkS symbol for spark spectrometry.

 

About Dr. Ulrike Corradi

In 2003, Ulrike finished studying geology at the Technical University of Munich. As life does not always follow a straight line, she got the possibility to work on a research project on lead-free solder alloys as a scientific assistant and head of a laboratory at the Applied University of Augsburg. During this time her major task was the investigation of solder alloys and intermetallic phases in solder joints with SEM/EDX/EBSD.

 

In cooperation with the Freie University of Berlin, Ulrike received her Ph.D. in 2017 for her work on this research project.

In 2015 she started at TAZ GmbH as an application engineer for damage analysis. In recent years the focus of work changed more and more to quality management topics. So now she is in charge of the whole quality management and accreditation in accordance with the ISO/IEC 17025 standard of the testing laboratory TAZ GmbH. For about 4 years now Ulrike is also the quality manager for the calibration laboratory for optical emission spectrometers TAZ Servicetechnik GmbH & Co. KG, which is also accredited in accordance with the ISO/IEC 17025 standard.

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers.

For more information on this source, please visit Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers.

Disclaimer: The views expressed here are those of the interviewee and do not necessarily represent the views of AZoM.com Limited (T/A) AZoNetwork, the owner and operator of this website. This disclaimer forms part of the Terms and Conditions of use of this website.

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