Micromeritics or Particulate Systems Instrument(s):
- TriStar II 3020
Personal Experience with Micromeritics:
We operate in a highly variable humidity region of the US, and produce materials that are highly variable in hygroscopicity. While previous studies indicated our out-gassing conditions were sufficient to remove adsorbed moisture from our samples, we were surprised [and relieved] to finally understand and recognize the root cause of a leading contributor to variability in our surface area reproducibility and repeatability results. We’ve learned that complete removal of moisture [in the form of condensate] from the sample cells requires an even more rigorous sample preparation regimen.
While working with high free moisture [the term we use for adsorbed water] samples, we’ve frequently noticed the opaque film of condensed moisture that formed around the inside of the top of the sample cell stems, during the sample out-gassing step. We also noticed how this opaque water film would move up the stem, and finally disappear, during the course of the out-gassing step. We had previously perform numerous metrology capability studies with the instrument, the analytical balances used for weighing samples, new and previously sample cells, and technicians, using NIST traceable standards and random selections of our various products and intermediates, so we had data to support our work instructions and standard operating procedures.
One of our more hygroscopic products is known to adsorb as much as 25+ weight percent moisture, upon exposure to > 60 % relative humidity ambient, in as little as a few days. We routinely dry such samples for at least 1 hour, at 250 C, prior to performing surface area and other analytical testing. The surface area measurement out-gassing procedure, for this product, calls for a minimum of 45 minutes of out-gassing, with a sample temperature of 250 C and under a minimum dry [< -150 C dew point] nitrogen flow rate of 40 ml/minute. The known surface area of this particular sample is 350, +/- 12, square meters/gram. During a very recent laboratory testing competency exercise, we’ve determined that the sample had adsorbed 20 weight percent water. We also determined, based participation of 13 laboratory technicians [most of which are highly trained and demonstrably reliable], that significantly longer drying and/or degassing times are required in order to sufficiently remove the 20 weight percent water [even though the opaque condensate film had disappeared well before the end of the 45 minute out-gassing time], such that all laboratory personnel are able to measure the sample surface area within the acceptable/known value range.
As “time is money”, we are investigating more efficient/faster ways to achieve a reliably anhydrous state of such samples, but are limited to a maximum out-gassing temperature of 250 C due to the nature of the product.