How Moisture Analyzers Work: Loss-on-Drying, Explained Simply

The loss-on-drying principle

Moisture content is defined as the mass a sample loses on drying, expressed as a percentage of its starting (wet) mass. A moisture analyzer automates the classic oven-and-balance method: an integrated precision balance records the starting mass, a halogen lamp heats the sample — typically to between 50 °C and 200 °C — and the balance tracks the falling mass in real time. When the mass stops changing (or changes less than a set rate per unit time), drying is complete and the display shows the final moisture percentage.

Strictly speaking, LOD measures everything that evaporates — water, but also alcohols and other volatiles. For most food and industrial products that distinction doesn't matter, and LOD is the method written into countless internal QC specifications precisely because it is fast, simple, and needs no chemicals.

How the moisture percentage is calculated

Moisture analyzers report moisture on a wet basis by default: moisture % = (wet mass − dry mass) ÷ wet mass × 100. A 5.000 g sample that finishes at 4.650 g has lost 0.350 g, so the display reads 7.00%. Instruments can also report on a dry basis — the same loss divided by the dry mass, 7.53% in this example — which some food and feed specifications use, as well as solids content (100 − moisture %) for sludge and slurry work. The same drying run yields different numbers depending on the basis, so always check which one your specification means.

The other setting that defines the result is the end-point. A measurement can stop after a fixed time or, more repeatably, when the rate of mass loss falls below a set threshold per unit time — the way A&D's automatic ending modes work. A tighter end-point gives a drier, more repeatable final figure but a longer test; method development is largely the art of choosing temperature and end-point so the analyzer's result matches the reference oven method for your product.

Why halogen heating — and what SRA adds

Halogen lamps reach working temperature almost instantly and heat the sample surface evenly, which shortens measurements and improves repeatability compared with older infrared heaters. A&D's moisture analyzers add SRA — Secondary Radiation Assist — a filter structure around the lamp that redistributes the radiation for more uniform heating across the sample pan. Uniform heating matters because a scorched centre and a damp edge produce a moisture figure that is precisely wrong.

Sample preparation does the rest: a thin, even layer of roughly 5 g spread across the pan dries faster and more repeatably than a heap. For substances that skin over or scorch, a lower temperature over a slightly longer time gives more honest results than maximum heat.

The A&D range: from 0.001% research grade to routine QC

The flagship MS-70 reads moisture to 0.001% and achieves 0.01% repeatability with a 5 g sample — resolution suited to pharmaceutical development and low-moisture products such as plastics granulate. The MX-50 offers 0.01% readability for demanding production QC; the MF-50 covers general factory use; and the ML-50 provides an economical entry point for routine checks. All use the same halogen + SRA heating system.

The MS-70 and MX-50 also support memory programs for different products and connect to A&D's WinCT-Moisture software, whose RsTemp function automatically searches for the optimal drying temperature for a new sample and whose RsFig function graphs the drying curve in real time — the fastest way to develop and document a method for a new product. Output is GLP/GMP/GCP/ISO-compliant for audit-ready record keeping.

Moisture analyzer or Karl Fischer titration?

Karl Fischer titration measures water specifically, by chemical reaction, and can quantify trace levels far below what LOD resolves. It is the right method when the specification demands 'water content' rather than 'loss on drying', when moisture is very low (well below 0.1%), or when the sample is rich in other volatiles that would inflate an LOD figure.

For everything else — incoming ingredient checks, in-process control, finished-product QC on foods, powders, granulates, and sludges — LOD on a moisture analyzer wins on speed, cost per test, and simplicity: no reagents, no titration cell, no chemical handling, and any operator can run it after five minutes of instruction. Many labs run both: a moisture analyzer on the floor for speed, Karl Fischer in the lab for specification arbitration.

What loss-on-drying actually measures — and why it matters

This is the single most misunderstood point about the method, and it explains most disagreements between a moisture analyzer and a laboratory. Loss-on-drying does not measure water. It measures mass loss on heating — and reports it as moisture. Anything else that leaves the sample at the drying temperature is counted too: essential oils, alcohol, solvent residues, some fats, and other volatile components.

For most products that distinction never matters, because water is overwhelmingly what evaporates. It matters a great deal for spices, coffee, herbs, flavourings and high-fat foods, where volatile oils leave alongside the water and the reported figure reads high against a water-specific method. Karl Fischer titration, by contrast, reacts with water chemically and reports water only — which is why the two methods can legitimately disagree on the same sample without either being faulty.

The practical resolution is not to pick the 'more accurate' method in the abstract, but to define which one your specification is written against, and to keep your drying programme consistent so results are comparable batch to batch. Where a specification is written against an oven reference, the drying temperature and end-point criteria should be developed to agree with that oven.

Moisture analysis methods compared

Four methods cover almost all moisture determination. They differ in what they physically measure, how long they take, and what they demand of the operator — and those differences, not accuracy alone, usually decide which one belongs in a given plant.

A&D publishes a documented comparison worth knowing: on PET pellets, an MS-70 averaged 0.298% moisture in 6.8 minutes, against 0.307% by Karl Fischer titration in 19.1 minutes — with better repeatability (0.0045% standard deviation versus 0.0065%). For low-moisture work that would traditionally have gone to Karl Fischer, that is a meaningful result: comparable numbers, roughly a third of the time, and no reagents or hazardous waste.

Throughput: can it replace the oven for a full shift?

A halogen moisture analyzer tests one sample at a time, so throughput is set by the drying programme rather than by the instrument. At a typical 3–8 minutes per determination, a single unit comfortably handles the incoming-goods and in-process checks of most production lines — roughly eight to fifteen samples an hour, uninterrupted.

Where that is not enough, the constraint is usually the sampling plan rather than the instrument: a line needing continuous moisture data wants in-line NIR, not a faster bench analyzer. And one unit can serve several product types — the MS-70 and MX-50 store 20 sets of measurement conditions, so a plant running both food and plastics lines can keep validated programmes for each and switch between them without re-developing the method.

Typical applications in the Gulf

Food processors use moisture analyzers to check spices, milk powder, flour, rice, snacks, and dates — where moisture drives shelf life, texture, and compliance with product standards, and where over-drying wastes yield. Plastics converters check hygroscopic pellets (PET, nylon, ABS) before moulding, because wet granulate causes visible defects. Pharmaceutical plants verify granulate moisture before compression, and municipalities and contractors measure sludge dry-solids content.

A&D Gulf stocks the MS/MX/MF/ML moisture analyzer range in Jebel Ali (JAFZA), Dubai, with traceable calibration and application support — including help developing a drying method for your specific product.