Most of the important processes in a laboratory require measuring things with precision. A spectrophotometer is an important tool used to measure many things from the properties of a material to the consistency of a color; however, an instrument can only measure reliably if it is calibrated.
Calibration is very important, and many people fail to realize just how important it is. When a spectrophotometer is not calibrated, it tends to measure the same thing with a high level of inconsistency. A spectrophotometer that is calibrated in a reliable way is the first step to having reliable data.
What is Calibration?
Calibration of a spectrophotometer is a way of measuring how a spectrophotometer is with reference to a standard and then adjusting it if needed. The aging of a lamp, the dust in the air, and the condition of a spectrophotometer's optical components can all affect how accurately a spectrophotometer measures.
When a spectrophotometer is in need of calibration, it is because the reference value against which all measurement is compared is in the wrong position. This is just like needing to set a scale to 0 before using it to weigh something.
Why Calibration Is Important Today
More and more businesses are putting a greater focus on quality. In fields like automotive manufacturing, textiles, coatings, and even consumer electronics, tiny measurement discrepancies can spell disaster and lead to the rejection of a product.
Calibration positively impacts organizations by:
● Facilitating consistency of measurement across many instruments.
● Enhancing reproducibility and repeatability.
● Minimizing quality control failures.
● Achieving conformity with standards of the pertinent industry.
● Augmenting assurance of color and spectral measurements.
● Identifying instrument drift before it impacts the production process.
By having instruments calibrated on a regular basis, laboratories are also able to sustain traceability, making it possible to link measurement results to available standards and reference materials.
What You Should Know About Calibration
Calibration involves many steps that must be completed before the measurement is conducted.
● First, the Spectrophotometer needs to be placed in a stable environment. Temperature involved in the measurement should be stable. No vibrations, and no direct exposure to the sun.
● Second, the instrument needs to be checked. The edges for the aperture, the optical windows, the white calibration standard and the black traps should all be free of scratches and clean. Even the smallest contamination can alter measurements significantly.
● Lastly, some manufacturers recommend a warm up period of about 20 to 30 minutes to ensure the light and electronics reach a stable operating condition.
1: Black Calibration
Black calibration is used to eliminate stray light signals and set the zero point of the system.
For a black trap, our spectrometer will absorb all of the incoming light in order to define a dark background. If no light is reflected back to the spectrometer, then that is a measurement of a dark background, and any light reflected back will correspond to the measurement of the sample.
Stray light can contaminate our measurements, so a true dark background will increase the reliability of our measurement.
2: White Calibration
The next step after the black reference is white calibration. The highest amount of reflected light will be defined by a standard white calibration tile. The spectrometer will measure the standard tile and set the upper limit of the measurements.
The spectrometer interprets reflected values for light across all wavelengths. This will help ensure that measurements pertaining to color will be consistent.
The white calibration tile must also be maintained. White standard calibration tiles must not be scratched, stained, or changed in appearance for proper calibration.
3: Wavelength Accuracy
The accuracy of the measurement wavelengths is extremely important to the spectrometer. Incorrect measurement of light wavelengths means all subsequent measurements will also be incorrect. To check if the wavelengths are correct, labs might use reference materials, such as holmium oxide or filters, didymium.
These reference materials have specific spectral peaks with assigned wavelengths. The instrument scans the reference material, and the measured peaks are taken against the reference values. If there are notable differences, recalibration or repairs might be needed.
Wavelength calibration is important whenever there is an analysis of a spectrum, or when calibration is needed for compliance with a regulation.
4: Ensure Photometric Accuracy
Photometric accuracy determines if the spectrophotometer provides accurate absorbance or reflectance values. This is done by measuring reference materials with optical properties that are known. These can be neutral density filters, solutions of potassium dichromate, or other reference materials with traceability.
The measured values are then compared to the reference values. There may be minor differences, but the values should still be within the tolerances defined by either the manufacturer of the instrument, or the standards of the industry.
Photometric performance is crucial, as measurement of the optical intensity can affect the calculation of concentration, the assessment of color, and the evaluation of materials.
5: Use Certified Standards for Calibration
Internal verification of calibration is not enough. Now, certified color or spectral standards that are representative of the sample are measured. This gives an extra layer of assurance that the instrument is accurate in the relevant operating conditions.
This type of verification can show other calibration-related problems that may not have been seen before. It also shows that the instrument is compliant with the required standards prior to the first measurements.
Calibration Errors to Avoid
Calibration can be negatively affected, even when the operator is highly skilled. One mistake is not warming up the device first. Without a stable light source, calibrations can have inconsistent results.
Another classic error is not performing basic maintenance. Standard upkeep is cleaning and repairing the calibration tiles and reference materials. If neglected, tiles and reference materials can produce consistent errors.
Calibrations can also be disrupted by light, vibration, or air temperature. Not logging calibration results can prevent recognizing the instrument's drift and the same issues when the instrument is used repeatedly.
How Often Should a Spectrophotometer Be Calibrated?
How often a calibration is performed is determined by the amount of use of the instrument, the regulations of the industry, and the level of importance the measurements have.
As a general rule:
● Before daily measurements, perform routine black and white calibrations.
● Verification checks are performed when measurement accuracy is in doubt.
● Full calibrations are performed in one month or when it is indicated by the manufacturer.
● After the relocation of the instrument, full maintenance, lamp changes, or when the instrument is not used for a long time, a full recalibration is performed.
● To be compliant, organizations using strict quality systems can have more frequent calibrations.
Final Thoughts
The calibration of a spectrophotometer can determine its accuracy. Advanced technology and software can produce reliable measurements, but calibrations determine the reference measurements. Laboratories can determine quality control and scientific measurements by having black calibrations, white calibrations, and routine performance checks.
Calibrations should not be seen as routine maintenance. Calibrations can improve quality and enable confidence in the measurements and operations of the business. Consistently and correctly performed calibrations can ensure accurately analyzed measurements.
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