To achieve the precision specification mentioned earlier required a very robust peak assessment algorithm. After extensive
testing, a center-of-gravity (COG) method was chosen. This method has been criticized for defining a band value significantly
different from the visual peak maximum, especially for asymmetrical peaks (17).
While the debate continues, NIST does provide on the SRM 2035 certification "information values" for the peak locations using
a peak amplitude method. However, there are no uncertainty budgets associated with these values, as NIST considers these to
be "a value that will be of interest and use to the user, but insufficient information is available to access the uncertainty
associated with the value." While the temperature coefficient associated with the band assignment of 0.1 cm–1 °C–1 (< 0.02 nm/°C) will not make a significant contribution to laboratory usage of this reference, the effect in environments
not at 25 °C must be taken into account (18).
Table IV: Comparison of dispersive wavelength maxima values with three FT–NIR instruments
There are significant discrepancies from these informational values for FT–NIR instruments even though the water vapor calibration
data are within ± 0.1 nm. For example, at 8 cm–1 the following data, rounded to ± 0.1 nm, were observed (NIST Table IV data from reference 18 are given in brackets for the
seven absorbance maxima in nanometers; 975.9 [975.8], 1077.3 [1075.7], 1151.9 [1151.7], 1224.1 [1222.5], 1368.9 [1366.8],
1471.8 [1469.4], and 1945.4 [1945.7]).
NIST SRM 2065 is essentially the same physical standard as SRM 2035 and is certified for the location of the seven absorbance
bands (COG) in the spectral region from 10,300 cm–1 to 5130 cm–1 at 4-cm–1 resolution. SRM 2065 is also certified for the location of seven absorbance bands in the spectral region from 970 nm to
1946 nm and 13 additional transmittance peaks spanning the spectral region from 334 nm to 805 nm.
McCrone M-27 and Nelson M-42
Figure 6
In the late 1980s, a series of new rare earth-doped materials in a monocrystalline host material were developed (19). These
doped yttrium aluminum garnet–type crystals have the property of having sharper and more numerous absorption peaks than the
previous "didymium" type of glasses. The first one, M-27 (neodymium doped), had excellent lines within the UV-vis region but
only had three lines in the NIR, at 869.2, 1486.2, and 1735.2 nm at a spectral bandwidth of 1 nm.
