Specialty chemicals have to fulfill multiple quality requirements. One of these quality parameters, which can be found in almost all certificates of analysis and specifications, is the moisture content. The standard method for the determination of moisture content is Karl Fischer titration.
This method requires reproducible sample preparation, chemicals, and waste disposal. Alternatively, near-infrared spectroscopy (NIRS) can be used for the determination of moisture content. With this technique, samples can be analyzed without any preparation and without using any chemicals.
Ammonium nitrate samples were measured with a DS2500 Solid Analyzer in reflection mode over the full wavelength range (400–2500 nm). To minimize particle size effects, a rotating DS2500 large sample cup was employed. This accessory enables an automated measurement at different sample locations for a reproducible spectrum acquisition. As displayed in Figure 1, samples were measured without any preparation steps. The Metrohm software package Vision Air Complete was used for all data acquisition and prediction model development.
Table 1. Hardware and software equipment overview.
|DS2500 large sample cup||6.7402.050|
|Vision Air 2.0 Complete||6.6072.208|
The obtained Vis-NIR spectra (Figure 2) were used to create a prediction model for the moisture value determination. To verify the quality of the prediction model, correlation diagrams were created which display the correlation between Vis-NIR prediction and primary method values. The respective figures of merit (FOM) display the expected precision of a prediction during routine analysis.
Table 2. Figures of merit for the prediction of the moisture content using a DS2500 Solid Analyzer.
|Figures of merit||Value|
|Standard error of calibration||0.011 g/L|
|Standard error of cross-validation||0.012 g/L|
This Application Note demonstrates the feasibility of NIR spectroscopy for the analysis of low moisture content in specialty chemicals, specifically ammonium nitrate. In comparison to the standard titration method, the reduction of analysis time and chemicals is a major advantage of NIR spectroscopy.
Table 3. Comparison of running costs for the determination of the moisture content with titration and NIR spectroscopy.
|Lab method||NIR method|
|Number of analyses (per day)||10||10|
|Cost of operator (per hour)||$25||$25|
|Costs of chemicals for water determination||$2||$0|
|Time spent per analysis||5 min||1 min|
|Total running costs (per year)||$8,288||$938|