Loss on ignition (LOI) – Inorganic/organic analysis
Loss on ignition (LOI) is a test used in inorganic analytical chemistry and soil science, particularly in the analysis of minerals and soil. It involves heating or “igniting” a sample at a high temperature to remove combustable and volatile materials.
In water treatment systems, it’s used to analyse and identify deposits and determine their composition and is a key part of our RO Membrane Autopsy, Low Pressure (UF/MF/MBR) autopsy and ion exchange testing programs.
The simple test typically consists of placing 2-10 grams of the material in a furnace for a set time to burn off any volatile material and then cooling it and determining the mass. Thermogravimetry analysis (or TGA) is a variant of the test in which mass-change is continually monitored by a specialized instrument .
Typically Loss on Ignition is used to determine water content levels, carbon levels, and the amount of organic matter in a sample.
- Water – Either free water, water tied into hydrates (water of crystallization) or hydroxy compounds is quantified by drying at 110C for 120 minutes
- Organics – Any organic matter is either volatilized or oxidized by heating up to 550C
- Inorganic degradation – Carbonates and iron oxidation as well as some other species that degrade at high temperatures can be identified by heating up to 950-1000C
To conduct an LOI, we must be able to scrape and isolate a reasonably large amount of foulant. Firstly, we will scrape a known area of membrane to isolate the deposit. We then weigh the sample and dry at 110C to measure the moisture content. After this we burn it at 550C for to remove all organic material before allowing it to cool for re-weighing.
This procedure can also be conducted on ion-exchange resins where the goal isn’t so much to measure the lost material, but to burn away the resin so that the residue can be analysed (ash analysis). However, this procedure has largely been superseded by the use of Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS).
LOI Muffle Furnace and crucibles
Use and importance of LOI
LOI is one of the best methods for determining the balance between organic and inorganic fouling through direct measurement rather than through a calculation (such as when using extractable inorganic and organic analysis). For us it gives us a key indication of what is happening in situations with complex fouling that can occur commonly in UF/MF systems.
The limitation of the technique is that it does need to have a sufficiently large amount of fouling to isolate a sample.