Every measurement comes with uncertainty. If you’re measuring with a ruler that is only marked to the millimeter, or a with balance using only one-gram standard weights, there is going to be some point at which that measurement becomes inexact. You can get pretty close to exact, but the question is, how close? How certain can you be that your measurement is correct? This calculation of the uncertainty in measurement is where reference materials play a key role, especially in a field as specific as spark spectrometry.
Spark spectrometers (also referred to as spark emission spectrometers or spark OES) analyze metallic samples through the application of an electric arc or spark, causing analytes to emit light and thereby reveal their presence and concentration via distinctive wavelength signatures. Spark spectrometers are relatively easy to use and maintain, and they provide the fastest determination of present elements with high accuracy and precision. But how accurate and precise they are, is a question that needs to be answered, for each and every measurement performed.
With spark spectrometry of metals knowledge of the level of uncertainty in measurement is hugely important, and is necessary to properly validate measurement results. According to DIN EN ISO 17025, the worldwide standard for laboratory accreditation in the area of testing and calibration, it is necessary to calculate the measurement uncertainty for each measurement result.
Uncertainty in measurement for reference materials
Uncertainty is generally defined as a range of values centered on the measurement value, and it includes an asserted true value with the stated probability. [To put it in more technical terms, 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. The parameter of uncertainty is specifically associated with the result of a measurement.]
Like most analytical instrumentation, spark spectrometry is comparative, meaning that it requires a sample of a known composition for accurate calibration. These known samples are called reference materials, and measurements of unknown samples can be compared to them. Uncertainty in measurement for reference materials needs to be accounted for and applied to measurements of unknowns. For this reason, it is important to understand what type of reference materials might be in use for any given experiment.
Spark spectrometry reference materials
Four types of reference materials can be used for spark spectrometry: Setting Up samples, reference materials (RMs), Certified reference materials (CRMs), and Traceable CRMs.
- Setting Up Samples are homogeneous but do not have certified exact concentration values. These samples can be used for control cards, instrument recalibration, or homogeneity testing. 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.
- Reference materials (RMs) is often the term used for a group of either pure substances or matrix materials which are used for calibration, method validation, method development, and for various quality control purposes. 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) are produced according to relevant standards and have their concentrations calculated through a round-robin test. A CRM fulfils numerous requirements including proven homogeneity and stability. 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.
- Traceable CRMs are the highest quality reference materials, 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.
Regardless of the system operator or the sample preparation method, or what your exact application of spark OES spectrometry may be having the right reference materials for the job is essential to calculate the measurement uncertainty for each result and keep compliant with laboratory accreditation standards.
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