Absorbance References

This Reference Material can be used to qualify the Absorbance scale, in the ultra-violet and visible region of the spectrum (250 nm to 850 nm) of spectrophotometers with spectral bandwidths not exceeding 6.5 nm.

These filters are equivalent to NIST SRM 2013a. A metallic coating is deposited on to a quartz plate at a thickness that determines the transmission characteristics of the filter are determined by the thickness of a metallic coating deposited on a quartz plate. The metallic surface is overlaid by an optically bonded cover plate, to protect it from oxidation, contamination, or physical degradation.

Note that SRM 2013a is no longer available from NIST, having been replaced by SRM 2013b, that differs from the prior series by using an evaporated silicon dioxide protective overcoating rather than a protective cover plate in the construction. We consider the bonded cover plate to be more durable than the overcoating method and have therefore retained the earlier design. The filters are mounted in stress-free black-anodized aluminium alloy holders, with front and rear sliding covers that protect the filters when not in use. All individual references and sets are supplied with an empty filter holder for use as a measurement reference.

The spectrum of these filters is essentially flat from 250 nm to 850 nm.

Absorbance and Transmittance values are certified at 250, 280, 340, 360, 400, 465, 500, 546.1, 590 and 635 nm.

These filters are based on a NIST SRM series whose values follow a sequence of Transmission rather than Absorbance values. Approximate values available are: 90% T (0.04 A), 50% T (0.3 A), 30% T (0.5 A), 10% T (1.0 A), 3%T (1.5 A) and 1% T (2.0A)

Instruments to be qualified should have a bandwidth of 5nm or less.

Note: The measurement values given in this document are for guidance only. The Calibration Certificate accompanying the Reference Material gives the actual values measured at a bandwidth of 1nm.

As these filters attenuate the incident radiation partly by reflection, inter-reflections could be generated within the sample compartments of some spectrophotometers. This could affect the accuracy of measurements, and it is recommended that potential purchasers contact the manufacturer of their instrument to check its compatibility with metal-on-quartz filters.

The references are also available individually. Nominal certified transmission/absorbance values and catalogue number are given below. Any combination of references can be ordered as a set by combining the catalogue numbers as in the example above. An empty filter holder is supplied with each reference or set of references to be used as a measurement blank.

These references can be used to qualify the absorbance accuracy and linearity of visible spectrophotometers with a spectral bandwidth of 6.5 nm or less, over the wavelength range 440 -635 nm.

Measurements made with these references will be accepted as evidence of instrument qualification by:

• • United States Pharmacopeia
• • American Society for Testing and Materials
• • Therapeutic Goods Administration (Australia)
• • British Pharmacopoeia

Glass filters mounted ‘stress free’ in an anodised aluminium holder. The use of Schott NG-type glasses has been used for over thirty years for the validation of the absorbance scale and linearity of spectrophotometers in the visible region. Filters with a wide range of transmission and absorbance values can be produced. The spectrum of these filters is essentially flat over the range of calibration.

A range of 18 filters is available from Starna, covering absorbance values from approximately 0.04 A to 3.5 A (see How to Order, below). Originally, these filters were manufactured with selected transmission values. Available transmission values and equivalent absorbances are: All Starna neutral density filters are manufactured and certified in accordance with the ‘Technical Specification for Certification of Spectrophotometric NIST Traceable Reference Materials’ (NIST Special Publication 260-140).

This recommends calibration at 440 nm, 465 nm 546.1 nm 590 nm and 635 nm, and Starna filters are certified at these wavelengths. NIST also lists three specifications for neutral density filter Standard Reference Materials (SRMs): SRM 930e, SRM 1930 and SRM 2930. These specifications also stipulate the maximum spectral bandwidth to be used at these wavelengths for certification measurements:

For those users wishing to work to these specifications, sets of filters equivalent to these SRMs are available (see How to Order). For convenience, but to comply with the SRM specifications, all certification measurements are made at a SBW of 1.0 nm. These sets consist of three filters together with an empty aluminium holder, to be used as a blank

These SRMs were originally specified in transmittance, the specified nominal transmittance and approximate absorbance values are given below: Certificate values are given in both %T and Absorbance.

NIST Equivalent SRM Sets

Three specifications for neutral density filter Standard Reference Materials (SRMs) are listed by NIST: SRM 930e, SRM 1930 and SRM 2930. For those users wishing to work to these specifications, sets of filters equivalent to these SRMs are available:

Visible Spectrophotometer Qualification Set

This is a convenient way of qualifying a visible spectrophotometer for photometric accuracy, linearity and wavelength accuracy.