ASTM E1655-05

These practices can be used to establish the validity of the results obtained by an infrared (IR) spectrometer at the time the calibration is developed. The ongoing validation of estimates produced by analysis of unknown samples using the calibration model should be covered separately (see for example, Practice D 6122).

These practices are intended for all users of infrared spectroscopy. Near-infrared spectroscopy is widely used for quantitative analysis. Many of the general principles described in these practices relate to the common modern practices of near-infrared spectroscopic analysis. While sampling methods and instrumentation may differ, the general calibration methodologies are equally applicable to mid-infrared spectroscopy. New techniques are under study that may enhance those discussed within these practices. Users will find these practices to be applicable to basic aspects of the technique, to include sample selection and preparation, instrument operation, and data interpretation.

The calibration procedures define the range over which measurements are valid and demonstrate whether or not the sensitivity and linearity of the analysis outputs are adequate for providing meaningful estimates of the specific physical or chemical characteristics of the types of materials for which the calibration is developed.

1.1 These practices cover a guide for the multivariate calibration of infrared spectrometers used in determining the physical or chemical characteristics of materials. These practices are applicable to analyses conducted in the near infrared (NIR) spectral region (roughly 780 to 2500 nm) through the mid infrared (MIR) spectral region (roughly 4000 to 400 cm^-1).

Note 1—While the practices described herein deal specifically with mid- and near-infrared analysis, much of the mathematical and procedural detail contained herein is also applicable for multivariate quantitative analysis done using other forms of spectroscopy. The user is cautioned that typical and best practices for multivariate quantitative analysis using other forms of spectroscopy may differ from practices described herein for mid- and near-infrared spectroscopies.

1.2 Procedures for collecting and treating data for developing IR calibrations are outlined. Definitions, terms, and calibration techniques are described. Criteria for validating the performance of the calibration model are described.

1.3 The implementation of these practices require that the IR spectrometer has been installed in compliance with the manufacturer's specifications. In addition, it assumes that, at the times of calibration and of validation, the analyzer is operating at the conditions specified by the manufacturer.

1.4 These practices cover techniques that are routinely applied in the near and mid infrared spectral regions for quantitative analysis. The practices outlined cover the general cases for coarse solids, fine ground solids, and liquids. All techniques covered require the use of a computer for data collection and analysis.

1.5 These practices provide a questionnaire against which multivariate calibrations can be examined to determine if they conform to the requirements defined herein.

1.6 For some multivariate spectroscopic analyses, interferences and matrix effects are sufficiently small that it is possible to calibrate using mixtures that contain substantially fewer chemical components than the samples that will ultimately be analyzed. While these surrogate methods generally make use of the multivariate mathematics described herein, they do not conform to procedures described herein, specifically with respect to the handling of outliers. Surrogate methods may indicate that they make use of the mathematics described herein, but they should not claim to follow the procedures described herein.

1.7 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.