Micromeritics Gemini VII 2390a, Operator'S Manual

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Gemini VII Appendix E
239-42828-01 - Dec 2012 E-1
E. ANALYZING SAMPLES WITH TOTAL SURFACE AREA OF
1.0M2 OR LESS
Introduction
The unique balanced measurement method used in the Gemini permits small amounts of surface area
to be measured with nitrogen gas that otherwise would be measurable only with krypton. Low surface
area samples often displace many times more nitrogen than they adsorb, especially if composed of
low-density materials of large particle size. The standard, built-in, helium free-space difference mea-
surement and mathematical compensation routine typically removes the effects of more than 99% of
this sample displacement, but the small amount remaining uncompensated can still be significant.
A technique for adding a compensating volume with negligible surface area into the balance tube has
been developed. It can reduce the size of the initial imbalance to low levels and allow precise measure-
ment of adsorbed gas.
This technique can be performed by the following methods:
• Using glass beads in the balance tube
• Using filler rods in the sample and balance tubes
Some users find that a combination of the two methods — using glass beads and filler rods — pro-
duces even better results. This, however, will depend on the type of sample you are analyzing.
Filler rods and glass beads are included in your accessories kit. Either straight-wall or bulb sample
tubes may be used.
Using Glass Beads
This method typically produces the best results.
1. Place an appropriate quantity of sample in a clean sample tube.
v
w

----
=
2. Load a second sample tube of the same size with glass beads that have a total volume
approximately the same as the sample volume.
a.) Determine the volume (
v
) of the sample in cm
3
:
where
w
= mass of sample (g)

= density of sample (g/cm
3
); if density is unknown, refer to your laboratory
handbook