1. How does the recertification of my filters work?


Hints for the return and recertification of your reference materials can be found in the product catalog "Certified Reference Materials for UV / Vis Spectroscopy" on page 55.

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2. Why do holmium glass filters become cloudy? Will this interfere with the measurement?


The glass material used for this filter is somewhat hygroscopic, which means that the filters become coated with a kind of water film. The film does not interfere with measurements or change the characteristic peak positions of holmium. The filter can be easily wiped down using alcohol and a soft cloth. The filter should generally be stored in a dry place.


3. How long can a calibration standard be used for in total?


Depending on the conditions in which they are used and stored, as well as how they are maintained, filters usually last for many years. We recommend having filters regularly recertified so that any signs of deterioration can be recognized at an early stage.


4. How often should filters be recertified?


Certified reference materials should be recertified at regular intervals to check that the values stated on the calibration certificate are still valid. It is up to the user to decide on the regularity of these intervals, which should take into account the use, storage and usage conditions of the filter in the laboratory.

To establish a statistical database for determining recertification intervals, we recommend having all reference materials recertified at least every 12 months during their first two years of use, and then selecting a suitable recertification interval based on the values measured.

Further information can be found in chapter 5 in the product catalog "Certified Reference Materials for UV / Vis Spectroscopy".

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5. What do the tolerances on the calibration certificate tell us and how can they be correctly interpreted?


The measurement uncertainties that appear on calibration certificates only refer to measurements conducted by Hellma Analytics and apply solely to the measurement conditions at the company (spectrophotometer used, environmental influences such as temperature, air humidity, user influence, reference materials used, etc.). Consequently, the measurement uncertainties of the NIST reference materials used to ensure traceability have been mathematically combined with the measurement uncertainty statistics calculated by Hellma Analytics. The value provided is therefore an expanded measurement uncertainty (double standard deviation, coverage factor k=2). This means that the actual value is 95% certain to fall within this range. To correctly calculate the measurement uncertainties valid for their measuring system, reference material users should follow the same steps, mathematically/statistically combining the measurement uncertainties provided with the measurement uncertainty statistics they have calculated themselves for a particular spectrophotometer and relevant conditions (see ISO/IEC Guide 98-3:2008 ‘Guide to the Expression of Uncertainty in Measurement’).


6. What is a baseline correction?


Baseline corrections are carried out with an empty cuvette holder to compensate for the lamps. Since lamps emit light at different strengths at various wavelengths, baseline corrections (also known as auto zero) are carried out to determine a zero value. Baseline corrections are usually performed automatically when the spectrophotometer is started up, but can also be carried out manually.


7. What is background correction?


Background correction is carried out to eliminate any influences that extend beyond the sample’s properties. In double beam photometers, background correction is performed by simultaneously measuring the comparison cuvette in the reference beam path. This comparison cuvette usually contains pure solvent. In single beam photometers, background correction is carried out before the actual sample measurement is taken by measuring the comparison cuvette. The values obtained for the comparison cuvette are then deducted from the values of the sample measurement.


8. Why does the calibration certificate for the filter set used to determine spectral resolution look different to other calibration certificates?


Determining spectral resolution does not fall within our scope of accreditation. The filter set for determining spectral resolution therefore cannot be issued with a DAkkS calibration certificate or calibration mark. That is why this calibration certificate looks different from other calibration certificates for filter sets.


9. Why does Hellma Analytics no longer offer potassium dichromate filters for checking photometric accuracy with sulfuric acid as a solvent, as described in the European Pharmacopoeia?


In the past, certified reference materials for checking photometric accuracy in the UV range contained a solution of potassium dichromate in sulfuric acid and were manufactured in strict compliance with European Pharmacopoeia requirements. Over a number of years, Hellma Analytics noticed a continuous decrease in the absorbance values of the ‘potassium dichromate dissolved in sulfuric acid’ filter during daily calibrations.

We do not have a sufficient explanation for why this happens, but we assume that the comparatively high ionic strength of sulfuric acid causes mixed chromium (VI) complexes to form. To compensate for this behavior, over which we have no control, filters would need to be recertified much more regularly.

Another possibility would be preparing new solutions every time the spectrophotometer is checked. As a simple alternative, we offer a liquid filter that uses ‘potassium dichromate dissolved in perchloric acid’. This type of liquid filter for checking photometric accuracy has proven itself as a reliable and very stable standard for many years. No changes in absorbance properties comparable to those of the sulfuric acid model are known for this filter.

Hellma Analytics cuvettes are permanently sealed, eliminating concerns about the toxicity of perchloric acid. Furthermore, the European Pharmacopoeia states that “suitable certified reference materials” may also be used, which undoubtedly applies to our perchloric acid solvent model. This model also contains a formulation described by NIST.


10. Why does the weight of potassium dichromate filters seem to change after every recertification?


Due to measurement uncertainties, measurement values may fall within a specific range. This leads to an apparent change in weight from qualification to qualification, as the initial weight is calculated directly from the measured absorbance values. Earlier versions of regulatory codes stipulated that filters for checking photometric accuracy had to contain 60.06 mg/l potassium dichromate, and allowed a tolerance of 0.01 Abs. More current versions of the European Pharmacopoeia have replaced this very strict provision, now accepting weights between 57.0 mg/l and 63.0 mg/l. The specific absorbance calculated (see European Pharmacopoeia, chapter 2.2.25) is now stated with a margin of tolerance.


11. Why are these peaks measured for certifying holmium glass and didymium glass filters?


Measurement errors are low in medium to high transmittance ranges. As a result, peaks in the range from 0 Abs to 1.0 Abs (corresponds to 100% -T to 10% T) are preferred for certification.


12. How do I calculate my measurement uncertainty?


The measurement uncertainties stated on the calibration certificate only refer to measurements conducted by Hellma Analytics and apply solely to the measurement conditions at the company (spectrophotometer used, environmental influences such as temperature, air humidity, user influence, reference materials used, etc.).

The smallest possible measurement uncertainty that can be achieved by the user can then be derived by statistically combining the measurement uncertainty stated on the calibration certificate with all the user’s uncertainty contributions, such as the wavelength scale tolerance of the spectrophotometer used and other influences on measurement accuracy (environmental factors such as temperature, air humidity, user influence, etc.).

Further information can be found in chapter 6 in the product catalog "Certified Reference Materials for UV / Vis Spectroscopy".

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Hellma USA Inc.
120 Terminal Drive
11803 Plainview, NY
United States

Phone: +1 5169 3908 88


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