Traceability in Molecular Spectrophotometry

This program, sometimes referred to as the Optical Filters program, has supported the development, certification, and (as specified) recertification of Standard Reference Materials® (SRMs®) for the verification of the transmittance (absorbance) and wavelength scales of spectrophotometers in the ultraviolet (UV) and visible (VIS) spectral regions for several decades. More recently, wavelength SRMs® have been developed and produced for the near infrared (NIR) spectral region, and transmittance (absorbance) SRMs® are under development for the NIR.

The original objective of supporting accuracy in spectrophotometry has evolved with the world metrology system to encompass the provision of traceability to references of the highest order in transmittance (absorbance) and wavelength [Refs 1, 2]. UV/visible transmittance traceability is to the second-generation High Accuracy Spectrophotometer (HAS II) patterned on the original instrument with modern automation. Wavelength traceability is through recognized atomic transitions utilized as secondary standards of length. NIR transmittance (absorbance) is traceable through other NIST calibration programs.

The production of spectrophotometric SRMs® may be discontinued as fit-for-purpose traceable reference materials are introduced by the commercial secondary standards sector. In particular, the oldest NIST transmittance standard, SRM® 930 (neutral density glass filters for spectrophotometry), was discontinued in favor of readily available equivalent products. The extended range companion SRMs® 1930 and 2930 have been discontinued for the same reason. However, we continue to  recertify NIST- and commercially-produced solid filters.

Secondary standards producers may provide (NIST) transmittance traceability through the use of our SRMs® or through the NIST Spectral Reflectance and Transmittance Program supported by the Sensor Science Division. A more important concept is that traceability should not be limited to a particular national metrology institute (NMI), inasmuch as the equivalence of numerous national transmittance scales is assured by interlaboratory comparisons.

The image shown on this page illustrates a number of SRMs® produced and supported by this program over the last two decades. The status of each of these products is elucidated under "Standard Reference Materials® Issued," including those that have been discontinued. We continue to support recertification of the four individually-certified solid filter SRMs®: 930x (x = blank, a, b....e) , 1930, 2930, and 2031x (x=blank, a, b, c).

Standard Reference Materials® Issued

SRM® 930x (x = blank, a, b,....e), "Glass Filters for Spectrophotometry" [Refs. 4, 5]. This set of three neutral density filters of 10 %, 20 %, and 30 % nominal transmittance was certified at five wavelengths from 440 nm to 635 nm in the visible spectral region. These filters were mounted in black-anodized aluminum holders simulating the dimensions of a 10-mm pathlength chemical cuvette in order to be used routinely in chemical spectrophotometers and were shipped and stored in machined aluminum canisters along with an empty filter holder (as a blank) in a fourth hole in the canister. Approximately 2500 sets were produced from 1970 to 2009, with the "series" ("x") designation advancing annually until SRM® 930d, at which time production was defined as "continuous." The only later series increment, to SRM® 930e, corresponded to a tighter production tolerance on the parallelism of the opposite faces of the filters. This product has been discontinued, though recertification of existing filters sets, required biannually, is supported. 

SRM® 1930, "Extended Range Glass Filters for Spectrophotometry" [Ref. 5]. Differing from SRM® 930x only in the nominal transmittances of 1 %, 3 %, and 50 %, about 250 sets of these filters were produced from 1987 to 2009 to extend the SRM® 930 coverage range to both lower and higher transmittance values. This product has been discontinued, though recertification of existing filters sets, required biannually, is supported. 

SRM® 2930, "Extended Range Glass Filters for Spectrophotometry." Differing from SRMs® 930x and 1930 only in the nominal transmittances of 0.1 %, 0.3 %, and 90 %, 10 sets of these filters were produced briefly in 2004-2005 to match the transmittance range supported by many international scales. This product has been discontinued, though recertification of existing filters sets, required biannually, is supported. 

SRM® 2031x, "Metal on Fused Silica Filters for Spectrophotometry" [Ref 6]. Similar to SRM® 930x in form factor, these filters are produced with a protected thin metal film on fused-silica in order to perform in the UV as well as the visible. They are certified at ten UV/visible wavelengths from 240 nm to 635 nm, at nominal transmittance values of 10 %, 30 %, and 90 %. The SRM® 2031a series is characterized by a tighter specification on the opposite face parallelism than SRM® 2031. SRM® 2031b has a protective layer of evaporated silicon dioxide over the metal rather than the optically contacted cover plate of SRMs® 2031/2031a. SRM® 2031c returns to the optically contacted protective approach of the 2031/2031a.  Over 400 sets of these filters have been produced since 1979 and production continues.  Recertification of existing filters sets, required biennially, is supported. 

SRM® 931x (x= blank, a, b,....,h), "Liquid Absorbance Filters" [Refs. 7, 8] Each unit contains three sets of liquid filters in break-open 10-mL ampoules. Each set contains three levels of a nickel-cobalt solution and a blank. The net absorbance (sample corrected to blank) for a 10-mm pathlength cuvette is certified at four wavelengths from 302 nm to 678 nm. This SRM® was first released in 1972 and production continues. 

SRM® 1935x (x= blank, a), "Liquid Absorbance Standards for Ultraviolet Spectrophotometry." Each unit of SRM® 1935x contains five sets of liquid filters, each set consisting of a blank solution and a sample solution, for a total of ten 10-mL ampoules. This material replaced the earlier SRM® 935x potassium dichromate offered in powder form [Ref. 9].  This material has been discontinued and certification of all existing units has expired. 

SRM® 2034, "Holmium Oxide Solution Wavelength Standard from 240 nm to 650 nm" [Ref. 10] is certified for the wavelength location of minimum transmittance for fourteen bands in the spectral range from 240 nm to 650 nm and at six spectral bandwidths from 0.1 nm to 3 nm. For an approach using commercial CRMs and intrinsic properties to meet traceability needs formerly provided by this SRM®, see "Intrinsic Wavelength Standard Absorption Bands in Holmium Oxide Solution for UV/visible Molecular Absorption Spectrophotometry" [Ref. 11]. The model spectrum shown in Figure 1 in the paper may be downloaded in digital form here (*.xls) or here (*.csv). 

SRM® 2035x (x=blank, a, b), "Ultraviolet-Visible-Near-Infrared Wavelength/Wavenumber Transmission Standard".  This material supersedes SRMs® 2035 and 2065.  It is a certified transfer standard intended for the verification and calibration of the wavelength/wavenumber scale of ultraviolet (UV)–Visible–Near-Infrared (NIR) spectrometers operating in transmission mode. SRM® 2035x is certified for the location of the seven absorbance bands in the spectral region from 10 300 cm1 to 5 130 cm1 at 8 cm1 resolution in vacuum wavenumber.  The same seven bands in the spectral region from 970 nm to 1 946 nm are certified in wavelength (air) at 1 nm and 3 nm spectral bandwidth. In addition, 13 transmittance peaks spanning the spectral region from 334 nm to 805 nm are certified for peak location at 1 nm and 3 nm spectral bandwidth. The optical filter is 25 mm in diameter and 1.5 mm thick. The combination of constituent rare earth oxide concentrations and filter thickness yields absorption bands between 0.1 and 0.6 absorbance units in the NIR and 5 % to 60 % transmittance in the UV–Visible. 

SRM® 2035 (superseded by SRM® 2035x series), "Near Infrared Transmission Wavelength Standard." SRM® 2035 is certified for the location of the center of gravity (COG) of seven absorbance bands in the spectral range from 10 300 cm-1 (971 nm) to 5 130 cm-1 (1949 nm) at six spectral bandwidths from 4 cm-1 to 128 cm1. The filter is 25 mm in diameter and 1.5 mm thick in an optical mount. The combination of rare earth oxide constituent concentrations and filter thickness yields absorption bands between 0.1 and 0.6 absorbance units.  Certification of all existing units has expired. 

SRM® 2036, "Near-Infrared Wavelength/Wavenumber Reflection Standard." This material is a certified transfer standard intended for the verification and calibration of the wavelength/wavenumber scale Near-Infrared (NIR) spectrometers operating in diffuse reflectance mode. SRM® 2036 is a combination of a glass that is compositionally identical to SRM® 2065 Ultraviolet-Visible-Near–Infrared Transmission Wavelength/Wavenumber Standard physically contacted with a piece of sintered polytetrafluoroethylene (PTFE). The combination of rare earth oxide glass with a nearly ideal diffuse reflector provides reflection-absorption bands that range from 15 % R to 40 % R. SRM® 2036 is certified for the 10 % band fraction centroid of seven bands spanning the spectral region from 975 nm to 1946 nm (air wavelength). In addition, it is certified for the 10 % band fraction centroid location of the same seven bands in the spectral region from 10 300 cm-1 to 5 130 cm-1 at 8 cm-1 resolution (vacuum wavenumber). The optical filter is 25 mm in diameter and 1.5 mm thick. The sintered PTFE is 25 mm in diameter and approximately 6 mm thick. A unit of SRM® 2036 consists of the optical filter-PTFE assembly mounted in an optical holder, contained in a wooden box. 

SRM® 2065 (superseded by SRM® 2035x series), "Ultraviolet–Visible–Near-Infrared Transmission Wavelength/Vacuum Wavenumber Standard." This Standard Reference Material® (SRM®) is a certified transfer standard intended for the verification and calibration of the wavelength/wavenumber scale of ultraviolet (UV)–Visible–Near-Infrared (NIR) spectrometers operating in transmission mode. SRM® 2065 is certified for the location of the seven absorbance bands in the spectral region from 10 300 cm1 to 5 130 cm1 at 4 cm1 resolution. In addition, SRM® 2065 is certified for the location of seven absorbance bands in the spectral region from 970 nm to 1 946 nm and 13 additional transmittance peaks spanning the spectral region from 334 nm to 805 nm. The optical filter is 25 mm in diameter and 1.5 mm thick. The combination of constituent rare earth oxide concentrations and filter thickness yields absorption bands between 0.1 and 0.6 absorbance units in the NIR and 5 % to 60 % transmittance in the UV–Visible. 

References

1. Mavrodineanu, R., An Accurate Spectrophotometer for Measuring the Transmittance of Solid and Liquid Materials, NBS Journal of Research 76A, No. 5, pp. 405-425, (1972).
2. SP 260-81 - Accuracy in Analytical Spectrophotometry, pp. 46 - 66.
3. J. C. Travis, M. V. Smith, S. J. Choquette, and Hung-kung Liu, "Certified Transmittance Density Uncertainties for Standard Reference Materials using a Transfer Spectrophotometer," NIST Technical Note 1715, (2011)
4. SP 260-51 - Glass Filters as a Standard Reference Material for Spectrophotometry: Selection, Preparation, Certification, Use: SRM 930 (November 1975).
5. SP 260-116 - Glass Filters as a Standard Reference Material for Spectrophotometry: Selection, Preparation, Certification and Use of SRM 930 and SRM 1930 (March 1994).
6. SP 260-68 - Metal-On- Quartz Filters as a Standard Reference Material for Spectrophotometry SRM 2031 (April 1980).
7. Burke, R.W., Deardorff, E.R., and Menis, O., J. Res. Nat. Bur. Stand. 76A, pp. 469-482 (1972).
8. SP 260-81 - Accuracy in Analytical Spectrophotometry, pp. 67 -80.
9. SP 260-54 - Certification and Use of Acidic Potassium Dichromate Solutions as an Ultraviolet Absorbance Standard SRM 935 (August 1977).
10. SP 260-102 - Holmium Oxide Solution Wavelength Standard from 240 to 650 nm, SRM 2034 (July 1986).
11. "Intrinsic Wavelength Standard Absorption Bands in Holmium Oxide Solution for UVÕvisible Molecular Absorption Spectrophotometry", JPCRD, Vol 34, No. 1, 2005.