Purpose
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. They are accepted for this purpose by most pharmacopoeias and other regulatory bodies
Description and Discussion
Schott NG type glass filters, from which a wide range of transmission and absorbance can be produced, have been available for over forty years. 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. Starna absorbance references are calibrated at a bandwidth of 1 nm, however other bandwidths are available on request They can also be certified at other wavelengths to suit customer requirements with the provision that the 440 nm, 546.1 nm and 635 nm wavelengths are included for traceability.
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:
|
% Transmission |
Absorbance |
|
92 |
0.04 |
|
79.4 |
0.10 |
| 73 | 0.14 |
| 60 | 0.22 |
| 56.5 | 0.25 |
| 50 | 0.30 |
| 40 | 0.40 |
| 30 | 0.52 |
| 25 | 0.60 |
| 20 | 0.70 |
| 10 | 1.00 |
| 6 | 1.22 |
| 3 | 1.52 |
| 1.5 | 1.82 |
| 1 | 2.00 |
| 0.3 | 2.52 |
| 0.1 | 3.00 |
| 0.06 | 3.22 |
| 0.03 | 3.52 |
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:
|
Wavelength(nm) |
440 |
465 |
546.1 |
590 |
635 |
|
SBW (NM) |
2.2 |
2.7 |
6.5 |
5.4 |
6.0 |
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.
|
|
SRM 930e |
SRM 1930 |
SRM 2930 |
|||
|
|
%T |
A |
%T |
A |
%T | A |
|
Filter 1 |
10 |
1.0 |
1 |
2.0 |
0.1 | 3.0 |
|
Filter 2 |
20 |
0.7 |
3 |
1.5 |
0.3 | 2.5 |
|
Filter 3 |
30 |
0.5 |
50 |
0.3 |
92 | 0.04 |
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.
