Halogen Moisture Analysis: Speed and Simplicity Benefits over Drying Oven Method  

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Moisture affects the quality, shelf-life and usability of everything from plastics and pharmaceuticals to foods, making the analysis of moisture content a critical step in product development and manufacturing. The measurement of moisture content is often carried out by loss on drying utilising a drying oven, but this takes a significant amount of time - a precious commodity in busy labs and production environments. Much faster determination of moisture content can be achieved using halogen moisture analysis. Industry guidelines typically permit the use of alternative analytical methods providing that the results obtained by the two methods are comparable. Ensuring results equivalency takes initial effort, but long-term productivity enhancements are worth it. Here’s how you can make the most of halogen moisture analysis’ speed while maintaining the required accuracy.

Moisture content is a key quality parameter in many industries, including chemicals, pharmaceuticals and food. It can be used to determine raw materials’ quality and cost, it affects product quality (for example shelf life), and it often influences the financial margin of finished goods. Moisture is also a key parameter for in-process control. In this situation, moisture determination must be carried out rapidly and reliably so that necessary intervention in the production process can be carried out promptly, to avoid costly interruptions or loss of product.
Historically, drying ovens have been used as the reference method for loss on drying (LOD) tests; however, whilst accurate, this process is slow and labour-intensive – taking in region of 2-3 hours per sample. Techniques such as halogen moisture analysis (HMA) can perform the same function in just 5-15 minutes, with the additional benefits of ease of operation and delivery of a direct result with no subsequent calculations necessary. This makes halogen moisture analysis ideally suited for conducting fast and reliable measurements both in the laboratory environment and at the production line. In some industries, such as plastics, the HMA method has already been established as a new standard test method [1].
The biggest obstacle to utilising faster technologies is often changing methods, which involves proving that the HMA results are comparable with established methods. Determining comparability of results between the two methods takes a little time—usually about a day’s worth of testing of fewer than 20 samples. However, taking the time to demonstrate the suitability of HMA for moisture content determination of a given sample or process is well worth it – both for the time saved down the line, as well as for the improved quality and productivity of the production line. 

Comparison Based on Process Requirements

Practically speaking, an HMA can replace a drying oven if comparability between the two methods can be proven. Two approaches to establish results equivalency are in use today: evaluation based on process requirements and evaluation based on statistical data comparison.
An example of comparability based on process requirements can be gleaned from the pharmaceutical industry. The United States Pharmacopeia (USP) chapter <1010> ‘Analytical Data – Interpretation and Treatment’ states that an alternative method—in this case, HMA—is comparable if results do not differ from the reference method by more than ‘an amount deemed important’ [2]. To evaluate equivalency, method precision and accuracy should be compared [3]. The decision on whether differences between the two methods are within an acceptable range must be taken in context of the application. This is based on acceptable tolerances in moisture control (%MC tolerances) of a production process and can be determined by statistical process control, for example [4]. 
The typical and well-proven approach for comparison of drying oven and HMA is to apply a range of acceptance to the mean value and standard deviation of drying oven results, subsequently verifying that HMA results are within this range (see example Table 1). 

Table 1. Exemplary tolerances applied as acceptance criteria for samples within a moisture range of ~2%MC to ~15%MC

Definitions

%MCDO = mean of at least 6 measurements utilising the drying oven method
%MCHMA = mean of at least 6 measurements utilising the HMA method
SDDO = the standard deviation of at least 6 measurements utilising the drying oven method
SDHMA = the standard deviation of at least 6 measurements utilising the HMA method 
Q = the quotient of SDHMA and SDDO

Note: these values are exemplary and it is the responsibility of the operator to verify their suitability for a specific process. For samples outside this moisture range other values may become applicable.