Sample Prep.

Any sample that is to be analysed under vacuum must be prepared to remove adsorbed contaminants before starting a physisorption or chemi experiment.

Such contaminants can partially cover surfaces and any reactive sites – leading to a reduction in the measureable surfaces and to lower than expected results.

But the insidious damage is that, when the sample is exposed to the vacuum of the instrument, contaminants will boil off from the sample. This outgassing is seen as drift in the analytical transducer pressure reading, likely measurement failures if the required low pressures are not reached and possible estimation errors.

The normal approach to degassing a sample (remove adsorbed volatiles) is to heat the sample, to energise the molecules so they boil into the gas phase. To hasten removal of these molecules you then apply a stream of dry gas (purge) or vacuum (suction).

Some instruments have sample prep stations built in (like the ASAP 2020 shown below right) to help the analyst. But given that most degas procedures will take nearly 12 hours, especially if vacuum is used, the sample prep is often the rate limiting step. In these cases a stand alone multi port sample prep sation is the ideal solution.

The action of heat and gas purge is akin to the effectiveness of a simple hair drier, the water is “blown away” quickly and efficiently. And this is how the simple FlowPrep 060 shown above left works…with up to six sample tubes at once.  Each sample is prepared in the same tube that will be transferred to the Surface Area analyser, saving time, handling and hence money.

Alternatively you can apply vacuum to remove the boiled volatiles – as with  the VacPrep shown in the next photo. This  is more traditional,  akin to the operation and effectiveness of a vacuum oven, but tends to be a lot slower in cleaning down samples.

Whichever approach is used, the analyst must study the temperature needed to degas the sample  and the time/dose effect on the measureable surface and on your sample stability.

For most organics we use as low a temperature as possible. Even 40C is enough to hasten degassing. With thermally stable materials like ceramics we can maybe go to 350C and achieve clean down in an hour or two. But in some cases with organic materials even 60C can adversely affects the sample and the surface area! The example in the graph is for Magnesium Stearate that was prep’d at temperatures from 30 through 125C and there is a clear and continuous loss of surface with each temperature rise used.