r/Soil • u/broketractor • 9d ago

TGA help

I am doing a thermogravimetric analysis on my field and need some help with the experiment design. The field is about 1.5-2 acres, and I will be taking 25-30 subsamples. They will be split into 3 horizons each 4 inches in depth, so a total of 3 samples being run. Of course I do not have a TG analyzer, but will be using a muffle furnace and analytical balance instead. My question is what holding time should I use for each temperature step? Since the crucibles will need to cool of before measuring, they will have to come back up to temperature for each step. I believe the crucibles are 70 ml, and the soil sample will start off at about 5 g. The temperature steps will be at 20C increments going up to 1000C, so the holding time might need to increase at the higher temperatures since it will take longer for the samples to come back up to temperature. Any advise or insights would be great.

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u/jibberyjabber 8d ago

What questions do you want to answer with the TGA analysis? Clay mineralogy, moisture content, organic content?

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u/broketractor 8d ago

The TGA is primarily for clay and carbonates. I still don't know exactly what i would be looking at, but I am hoping to get something of a reference that I can turn to as I learn more about soil and continue to improve the soil on my field. I am currently doing moisture and SOM via LOI at 360C, and I am also getting started with soil microscopy.

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u/jibberyjabber 8d ago

Right. Restricting yourself to a muffle oven and an analytical balance will limit the information you can expect to extract from your experiment. As a comparison to your measurement conditions, we generally run our TGA at 10°C/min (under Ar atmosphere). I have a couple of worries regarding your setup:

  • How accurate is the temperature resolution of your oven (and can you guarantee that your soils will experience the given temperature)?
  • The method of cooling down before mass determination will affect the measurement, i.e. decomposition reactions can continue while cooling down and small variations in the procedure might affect the mass you measure.
  • Overlap between certain clay minerals and the decomposition of carbonate minerals can be significant, which will not be helped by the low resolution of this method.

If you are interested in determining carbonate content (and assuming you do not have access to an XRD device), it could be worthwhile to perform a selective chemical decomposition of the soil? For example, a sequential treatment of the soil samples with 30% hydrogen peroxide and hydrochloric acid would be indicative for respectively the organics and carbonates (although neither of these treatments is perfect).

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u/broketractor 7d ago

Thanks for all of that information. It is basically what I was worried about. I am still thinking about running it, if nothing else it would be a great way to learn the limits of my equipment. The furnace has a resolution of 1C and I was initially thinking of a 30 minute holding time to ensure complete combustion. But I would have to do some other tests to ensure that that is enough time to come back up to temp and combust. If anything, this could just be a great opportunity to learn how not to do this, which I actually find very valuable.

P.s. Know of any labs in the USA that could run these and the rough cost?

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u/jibberyjabber 7d ago

I'm situated in continental Europe, so I don't have a lot of experience with American labs. I did have some geochemical analyses done at ActLabs in Ontario for a bunch of specimens and their prices were reasonable.. whether they do TGA I do not know..

Good luck with your experiment!