Some consider moisture determination a dry subject. Yet, for food and pharmaceutical manufacturers, cosmetics producers, and others seeking to ensure consistent uniformity through each stage of production, moisture determination is a necessary step in product quality. The moisture content can reveal properties of the material relative to purity, concentration, stability, shelf life, and nutritional value.
Moisture determination balances (alternately referred to as a moisture balance, moisture analyzer, and moisture determination balance) all have a function that calculates the percentage of moisture by comparing an initial sample weight to the weight after drying.
The process used in these balances is referred to as Thermogravimetric Moisture Analysis, based on the principle of vaporization of water during the drying process. The moisture balance monitors the weight loss of a sample as the material is heated. The difference between the sample weight prior to heating and after reaching a steady-state mass after drying determines the moisture content.
Methods for Moisture Determination
Many different methods and instruments can be used for moisture determination:
A drying oven method still requires the use of a balance with sufficient sensitivity, typically recommended at 0.l% of the mass of the sample. This method involves these two devices and multiple steps.
Moisture meters require a known reference scale for the sample measurement to be related to a useful value; however, not all materials have a developed scale or can be measured properly with the pins or probes.
Titration, a method of chemical analysis which uses reagents to determine the concentration of the moisture, involves costly apparatus and skilled users. While capable of yielding excellent results, this method may not be practical for all applications or budgets. It includes purchasing, handling, and disposing of potentially hazardous reagents.
In contrast, the moisture balance provides users with a reasonably priced one-step method appropriate for many types of samples. The amount of moisture in a sample is related to other properties of density, weight, viscosity, refractive index, conductivity, and more. Manufacturers of pharmaceuticals, chemicals, cosmetics, paper, food, wastewater, paints, and coatings may find this instrument most useful.
Employing a “loss on drying” method, the moisture balance utilizes a built-in heating source to dry the sample, which is weighed before and after drying to obtain a moisture content value. The results are expressed in units such as parts per million (ppm), percentage of moisture, solids, volatiles, ATRO weight, as well as several others.
ATRO weight refers to a measurement of moisture content as compared to dry weight.
Typically moisture content = (wet weight - dry weight)/wet weight 100%
ATRO wet weight = (wet weight - dry weight)/dry weight 100%
For example, a 100-gram sample drying to 75 grams has a wet moisture content of 25/100=25%. However, ATRO moisture content is 25/75 = 33.3%.
Similarly, the dry content vs ATRO dry content:
Dry content = dry weight/wet weight
ATRO dry content = wet weight/dry weight
Users of this method employ an exacting procedure to ensure precision and repeatability. This procedure is either developed by the user or available from trade organizations and balance manufacturers. Compliance with regulatory agencies also will dictate the operational parameters and results. Balance models are available with the ability to store and develop methods.
The typical operational steps for using a moisture determination balance are: tare the sample pan, add the sample, and begin the drying process. The user takes the measurements and usually within minutes results appear. Sodium tartrate dihydrate can be used to verify the balance performance. In the industry, it is used as a standard because it contains 15.66% of chemically uniform moisture.
Heaters for Moisture Determination Balances
Select from several types of heaters:
Infrared: Infrared is less expensive than halogen or microwave type heaters but takes longer to dry samples and has less repeatability than either of the other two.
Halogen: Halogen heats up quickly and provides improved repeatability over infrared, plus it is not as costly as microwave types.
Microwave: Microwave is very expensive and while it has excellent drying time and repeatability, the cost may be prohibitive.
In contrast to other methods, moisture determination balances offer ease of use, safe methods, and various cost levels to fit budgets. The versatility and capabilities they offer in conjunction with software and PC connectivity make it a wise choice for most labs and applications.