Analyze Samples with a Total Surface Area of 1.0m2 or Less

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 measurement 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 measurement 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 — produces even better results. This, however, will depend on the type of sample you are analyzing.

Filler rods and glass beads are included in the 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.
  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.
  1. Determine the volume (v) of the sample in cm3:

where

m = mass of sample (g)
ρ

=

density of sample (g/cm3); if density is unknown, refer to your laboratory handbook.
  1. Determine the number (n) of glass beads needed to equal the sample volume:

where

0.014cm3 = approximate volume of one bead
  1. Outgas the sample in the sample tube at an appropriate temperature for an appropriate amount of time.
  2. Install the sample tube (containing the outgassed sample) onto the analysis port and the sample tube (containing the glass beads) onto the balance port.
  3. Specify a one-point measurement (P/P0 = 0.05 to 0.1) so that the initial free-space measurement can quickly be determined; then perform the measurement.
  4. Using the measured free space absolute value and the following relationship, determine the mass of glass beads to remove from (or add to) the balance tube to reduce the free-space imbalance:

where

2.515 g/cm3 = density of glass beads
3.53

=

thermal correction (no units)
  Note that the volume of one glass bead is approximately 0.014 cm3. Therefore, if the measured free space is less than 0.02 cm3, it is unnecessary to correct the free space.
  1. Use a beaker of warm water to bring the balance tube to room temperature before removing it from the balance port of the Gemini to remove (or add) glass beads. This prevents condensation of moisture from the laboratory atmosphere onto the cold glass beads.
  2. Remove the balance tube:
  • If the measured free space is negative (–), add the calculated mass of glass beads into the balance tube.
  • If the measured free space is positive (+), remove the calculated mass of glass beads from the balance tube.
  1. Reinstall the balance tube onto the balance port of the Gemini analyzer.
  For subsequent samples of the same material, use the same mass of sample that was used for the initial sample so that the original bead quantity may be left undisturbed on the balance port.
  1. Prepare the analysis Dewar and place it on the elevator.
  2. Close the sample compartment door and start the analysis.

Using Filler Rods

  1. Clean the sample tube, balance tube, and filler rods; label your sample and balance tubes.
  1. Prepare the sample and place it into the sample tube; insert the shorter filler rod into the sample tube.

If using bulbous sample tubes, use the same size filler rod as used in the balance tube.

  1. Ensure that the filler rods are equidistant from the top of each tube. If they are not, add or remove sample until they are the same distance from the top of the tube.

 

Packing of some powders may restrict gas access to the powder and cause slower equilibration and/or lower results.

  1. Attach the sample tube to the analysis port and the balance tube to the balance port.
  2. Prepare the analysis Dewar and place it on the elevator.
  3. Close the sample compartment door and start the analysis.

Using Glass Beads and Filler Rods

This method may further improve results, depending on the sample material being analyzed.

  1. After determining and loading the amount of glass beads to use, insert a filler rod into the balance tube.
  2. Insert a filler rod into the sample tube. Ensure that the filler rods are equidistant from the top of each tube. If they are not, add or remove sample until they are the same distance from the top of the tube.
  3. If using bulbous sample tubes, use the same size filler rod as used in the balance tube.
  4. Attach the sample tube to the analysis port and the balance tube to the balance port.
  5. Prepare the analysis Dewar and place it on the elevator.
  6. Close the sample compartment door and start the analysis.