Figure of merits for Spectrometer selection. what you need to know??🤔

4 min readAug 1, 2019

Quality analysis technology has elevated the role of sensor and spectroscopy in many industry including likes of Food industry, pharmaceutical manufacturing industry and agricultural industry. Often the ideal technology must be selected from many suitable candidates based on limited data available. For a analytical measurement instrument regular calibration is very essential. Calibration allows setting a baseline for the instrument and making sure that the baseline is maintained over time. Many tests are conducted for measuring wavelength accuracy and repeatability, photo-metric accuracy and repeatability as well as the signal to noise ratio of the instrument. When the spectrometers are mass-produced it is essential to have a few sets of parameters those should be validated against each instrument in order to achieve smoother transferability of the calibration model. There should be ideal values for these parameters which are also called figure of merits. Every instrument produced should have these parameters within the acceptable range.

The figure of merits are used to qualify a instrument. Deviation in these parameters could be because of deficiencies in instrument design features or loose manufacturing tolerances. These test are related to similarity among the mass produced spectrometers and reference standards. Ideal instruments should be alike in all parameters and should give accurate and reproducible results to reference standards. We consider Iso Propyle Alcohol (IPA) as a standard reference solution for the evaluation of figure of merit parameters. Standard reference can be selected based on convenience.

Why figure of merits are used?🤓

To qualify a instruments.
To check similarity among the mass produced spectrometers and reference standards.
To set acceptable criteria for an spectrometer instruments.

What are figure of merits?

a. SNR
b. Absorbance accuracy
c. Wavelength accuracy
d. Repeatability

SNR:

In spectroscopy SNR is considered as ability of the spectrometer to make accurate measurements depends on the quality of the signal obtained from the detector and the subsequent electrical circuits. The SNR compares the average power available in the signal to the average power contained in the noise. General formula for SNR is,

SNR = signal/noise

Higher the SNR value, the spectrometer can provide useful and reliable information about the sample. Lower the SNR value, the spectrometer cannot separate the signal containing information about the sample from the noise signal, and thus the measurement provides no useful data.

How to improve the SNR in a spectrometer: The design choices must increase the power in the measurement signal while at the same time minimize the noise sources as much as possible.

Wavelength accuracy:🎯

Accuracy is a general term that describes the agreement between a measurement and a true value.

Wavelength accuracy can be determined by analyzing a standard reference material with stable, narrow and well defined peaks and comparing the calculated peak position on instrument with the known reference position.

Consider figure 1 and figure 2 in the above images, figure one has two peaks and where as figure 2 has only one peak at rounded region, it can be a best example for worst wavelength accuracy.

Absorbance accuracy:🎯

Similar to wavelength accuracy, we need to find the absorbance accuracy of the device. Absorbance accuracy can be understood as standard deviation of the measured absorbance from reference standard absorbance.

Repeatability(Precision):🔁

Wavelength repeatability is the standard deviation of each peak position with lower values being best. Wavelength precision describes the repeatability on the X axis position and is important for calibration models and transferability.

All these parameter setting depends on your requirement and need and spectrometer selection depends on these parameters most of the time

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