The near-infrared (NIR) spectral region lies approximately from 780 nm to 2500 nm (4000 cm^–1 to 12,800 cm^–1) bridging the more well known and analytically used regions of the UV–vis (190–780 nm) and the infrared (4000–600 cm^–1). Until relatively recently, it has been called the "forgotten" region. However, with the arrival of Fourier transform (FT)–NIR instruments in the 1990s and the increasing awareness of the NIR's unique information content in the signal, which can provide both physical and chemical information, deployment of NIR spectrometry systems for both qualitative and quantitative purposes has become widespread, particularly in the pharmaceutical industry. This application in the NIR has led to new standards being generated by ASTM (1,2).

This renewed interest has led to the need for traceable standards for the calibration and qualification of the wavelength scale of NIR spectrometers in the regulated environment. The purpose of this article is to review the current position with respect to wavelength standards availability and traceability.

Because of the versatility in the sample presentation modes in the NIR, there is a need to ensure wavelength accuracy in transmittance, reflectance, and transflectance. For dispersive instruments, wavelength precision is also important because of the use of mechanical wavelength drives. However, for modern FT instruments, the precision is so good that only the accuracy of the wavelength scale is important.

The wavelength scale accuracy is important because of the excellent signal-to-noise ratio (S/N) of the NIR, especially in the region above 1200 nm. This facet means that small changes in the spectral response function are spectroscopically significant; hence, transfer of spectral information between different NIR systems becomes problematic if the wavelength shifts, among others, are not controlled. Regulatory interest has led to the promulgation of monographs for the specification, calibration, and control of NIR spectrometers in both the U.S. (3) and European Pharmacopoeias (4). Unfortunately, these monographs have not been harmonized, and there are significant differences.

In addition, some of the wavelength requirements are unsound scientifically. For example, USP General Chapter <1119> recommends the use of NIST SRM 2034, which is not certified by NIST for use in this region, and EP 2.2.40 gives a value of a water vapor line of 7299.86 cm^–1 without reference to its source and is demonstrably wrong (see section on water vapor for more details). One other source of wavelength calibration information is ASTM E275-01 (5) (currently under revision). This document recommends a third and different set of wavelength standards for the NIR.

Table I: Comparison of the three major documents on wavelength calibration

These three documents are summarized in Table I. In addition, other standards have been proposed for transmission mode, including trichloromethane (6), SRM 2036 (7), and polystyrene (8) for quality control.

This article will review some of these standards from the perspective of traceability, "fitness for purpose," and practical ease of use.

Atomic and Molecular Band Spectra

The use of atomic and molecular lines for the calibration of spectrometers is attractive scientifically because they often are linked to primary standards. However, the use of atomic line sources can present practical difficulties for end-users, especially for instruments designed to work in a routine environment. Instrument manufacturers often use these sources during instrument building and commissioning.