How to consistently achieve top accuracy

Optical Emission Spectrometry (OES) is a fast and accurate way for foundries to accurately measure the exact chemical composition of the materials during their melt processes, but this accuracy and speed mean that spectrometers are also very sensitive.

Without regular verification, maintenance and recalibration, the performance of your OES can begin to suffer. When your business depends on the quality of your end products and maintaining tight control over the exact chemical composition, inaccurate analysis is something that needs to be remedied fast.

Because spark spectrometers use relative instead of absolute measurements, it is important to get the concentration or quantity of the analysed substance right. This is where calibration and certified reference materials (CRMs) come in.

In some cases, the standard calibration of the instrument might not fulfil your demands on the accuracy for different reasons. To get the accuracy of your OES analyser to the level you need, a Type Standardisation may be needed.


Do you need a Type Standardisation?

The big question here is: are you still seeing deviations despite being calibrated to a lot of CRMs?

There are a number of reasons why this could be happening:

  • The majority of CRMs are manufactured synthetically. This means it is not guaranteed that they will correspond to the composition or structure of the sample being analysed.
  • Alloys on the more exotic end of the spectrum can often deviate strongly from the matrix material.

The problem could also be in the status of your analyser itself. You may be asking too much of your OES by wanting a level of accuracy that is simply above the calibration of the instrument. You could also be feeling the effect of drift, which is the slow change in instrument sensitivity that can distort your results over time.

Hitachi High-Tech’s range of optical omission spectrometers have a total wavelength coverage of between 130 and 800 nm, making it really simple to pick up the early indicators of potentially disruptive drift. Monitoring the spectral positions of all relevant channels is also automated, so that correction occurs during each analysis.

While these measures greatly increase the stability of our OES analysers, even they are not immune to the need for an occasional type standardisation. If you use daily control samples to keep a detailed eye on your spectrometer’s performance, you will be able to spot this quickly so that operational disruption is kept to a minimum.

The benefits of Type Standardisation

If your OES is afflicted by any of the factors outlined above, Type Standardisation can help you achieve a bigger improvement in the accuracy of your OES results on top of a standard calibration. It is important to note, though, that it is only valid for the correction of unknown materials that are still similar in composition to the standardisation sample.

The process should be performed just before running one or more samples of an alloy type. This will allow you to really fine-tune the calculation. Once you have got the right kind of samples, you need to check the optimal level of accuracy of the basic calibration with the CRMs before you get started. Just as you would when using control samples for a recalibration, you also need to be sure that you have a lot of recently prepared and easy to measure materials ready to go.  

For all its benefits, Type Standardisation must never be used as a global correction method to analyse materials with significantly different chemical composition (That’s why it is called Type Standardisation).

The key to getting the best performance from your OES is understanding how your analyser works and the factors that can affect it over the long term. An OES analyser’s sensitivity is one of its great strengths, but it is also the thing that can lead to distorted results if you’re not careful.

Hitachi High-Tech OES analysers are built to be stable and durable. If you use control samples to regularly monitor the performance of your spectrometer, this underlying stability should mean that you can correct any imbalances before they get to the stage where you need a full-blown Type Standardisation.

Drawing on over 45 years of experience and expertise, the Hitachi High-Tech range of OES analysers are built to add value to industrial operations rather that slow them down. Whatever your quality control needs, we are confident we have the right solution for your business.  

Talk to a member of the Hitachi High-Tech team about our range of OES analysers and book a demo today.

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Date: 8 January 2019

Author: Wilhelm Sanders, OES Product Manager

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