All Hellma Analytics certified reference materials are prepared in accordance with NIST (National Institute of Standards and Technology, USA), ASTM (American Society for Testing and Materials) or European Pharmacopeia. Liquid filters are filled under controlled conditions, then the cells are permanently sealed. Solid filters are carefully cleaned and mounted stress free into their holders. The manufacturing process of the filters is completed by certifying them individually.
The manufacturing of the calibration standards is accomplished through the use of a high performance UV/Vis/NIR spectrophotometer. This instrument is used exclusively for calibration purposes and is checked for its accuracy at fixed intervals of time.

For calibration of the spectrophotometer which is used to calibrate the Hellma Analytics filters, among others, the following sets of primary standards from the “National Institute of Standards and Technology” (NIST, USA) are used:

Photometrical Accuracy

• SRM 930e Neutral Density Glass Filter

Wavelength Accuracy:

• SRM 2034 Holmium Oxide Liquide Filter
  

Primary instrumental wavelength calibration is established using the emission lines from mercury and deuterium sources. Linearity is established using double aperture method.