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Water activity (aw), the partial vapor pressure of water in a substance divided by the vapor pressure of water in standard state, is expressed either as 0–100% equilibrium relative humidity (ERH) or scaled to 0–1 aw. It is used to assess the safety, quality, and strength of non-sterile drug pharma products. In compounded preparations, aw refers to water that is freely available to participate in reactions (e.g., hydrolysis) or provides an environment that supports microbiological growth. Typically, solid dosage pharmaceuticals are >0.70 aw, indicating that microbial growth is unlikely [1]. Elevated aw in powders affects flow, caking, compaction, and strength properties of solid dosage forms and it is used in the study of shelf-life, aging, and packaging requirements. Measuring aw in the pharma environment is described in USP<1112> and USP<922> [2]. Dedicated instruments that measure aw require up to 30 minutes per analysis, while the OMNIS NIR Analyzer delivers results in just a few seconds.

OMNIS NIR Analyzer Solid with 15 mm vial and Flexible holder OMNIS NIR.
Figure 1. OMNIS NIR Analyzer Solid with 15 mm vial and Flexible holder OMNIS NIR.

In this study, 17 tablets of paracetamol with varying water activity (0.23–0.85 aw) were measured on an OMNIS NIR Analyzer (Figure 1) to create a prediction model for quantification. Samples were measured in reflection mode (1000–2250 nm) in 15 mm vials using a flexible holder and single-point measurement.

The reference values were measured according to USP<922> Water Activity [3].

Table 1. Hardware and software equipment overview.
Equipment Article number
OMNIS NIR Analyzer Solid 2.1071.0010
Disposable vials, 15 mm, reflection 6.7402.110
Flexible holder OMNIS NIR 6.07402.300
OMNIS Stand-Alone license 6.06003.010
Quant Development software license
 6.06008.002

The measured NIR spectra (Figure 2) were used to create a quantification prediction model for the water activity in paracetamol tablets. The quality of the prediction model was evaluated using the correlation diagram (Figure 3) which displays a very high correlation between the NIR prediction and the reference values. The respective figures of merit (FOM) display the expected precision and confirm the feasibility during routine analysis.

Figure 2. Stacked NIR spectra of paracetamol tablets analyzed on an OMNIS NIR Analyzer Solid.
Figure 3. Correlation diagram and the respective figures of merit for the prediction of water activity using an OMNIS NIR Analyzer Solid. The reference water content was determined using a Novasina LabMaster-aw neo according to USP<922>.
 
R2 SEC (aw) SECV (aw)
0.958 0.0278 0.0322

This Application Note demonstrates the feasibility of determining aw in paracetamol tablets quickly and easily. NIR spectroscopy offers users a fast, cost-effective, and highly accurate alternative to other standard water activity measurement options. Additionally, NIRS analysis is non-destructive, completely reagent-free, and gives results in only a few seconds.

  1. Pharmaceutical Trends: Water Activity Measurement - International Pharmaceutical Industry, 2021.
  2. 〈922〉 Water Activity. DOI:10.31003/USPNF_M12475_02_01
  3.  USP 922 Water Activity Measurement - Novasina - Excellent new Method. https://www.novasina.ch/application/usp-922-water-activity/ (accessed 2024-08-27).
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