Measurement results have to be internationally comparable. This demand is assured by the relation of the measurements results to standard references, normally maintained by a national standard body. Accuracy of those standards is assured by an unbroken chain of international comparison measurements. Our calibration standards and calibration standard measurement equipment is traceable to international accepted standards of NIST, USA (National Institute of Standards and Technology).
”Traceability“ describes the procedure of comparing the display of a measuring device with the original reference source for the measurement in one or more steps. Each step has to be calibrated against a standard whose metrological quality was also certified against a standard of higher order, creating a calibration hierarchy from the national working standards through a laboratory.
Traceability is characterised by some basic essentials:
Does the setting of the slit match with the specifications on the certificate? Other slit settings can cause errors in the peak maxima.
The absorbance values of the neutral density glass filters do depend on their thickness, and so they can be available on customer request.
The holmium glass is slightly hygroscopic, so the covering is a kind of water film. Measurements are not affected. The filter may be wiped with alcohol and a soft tissue to remove the film. As a basic principle the filters should be stored in a dry place.
The lifespan of the filters can last several years, depending on the number of applications, on the storage conditions, and the maintainance. To acknowledge the wearout ahead of time, we recommend to recertify the filters regularly.
To check the correctness of the values given on the certificate, the certified reference materials should be recertified in regular intervals. The periodicity of these intervals should be determined by the user and depends on the laboratory environment and the conditions of use, just like the total lifetime of the filters. To define a statistical basis for establishing the recertification interval it is recommended to have all reference materials recalibrated every 12 months in the first two years of use. Afterwards an interval of recalibration should be chosen which seems to be suitable based on the values obtained this way.
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’).
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.
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.
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.
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.
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.
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.
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.).