APPENDIX
D
Special Application Notes
SAMPLING VISCOUS AND/OR
NONHOMOGENEOUS SPECIMENS
The broad range of specimen materials amenable to testing in the
vapor pressure osmometer may require you to adapt your sampling
technique to suit the physical characteristics of unusual samples.
Using the micropipettor will assure the application of uniform vol-
umes of both test specimen and calibrating solutions, but if the vis-
cosity of the sample is extremely high, a positive-displacement
micropipettor may be preferable for sampling. These devices are
not recommended for routine use, however, due to their propensity
toward carry-over error.
If the sample material does not readily saturate the paper sample
disc or does not spread out over the whole disc naturally, it may be
preferable to eliminate the sample disc and use the pipettor tip to
apply the material as uniformly as possible over the central depres-
sion of the sample holder.
In other situations, materials can be sampled successfully by
immersing the paper sample disc, which is held in the forceps, into
the specimen to be tested, then carefully transferring the wet disc to
the central depression of the sample holder. Caution must be exer-
cised when using this "disc immersion" technique to avoid any con-
tact of the wet sample disc with the outer portion of the sample
holder, since this would result in solute material being transferred to
the thermocouple mount and would rapidly contaminate the sample
chamber.
In any event, when working with unusual specimens, make certain
the sample occupies the full diameter of the central depression in
the sample holder, as it would if saturated into a paper sample disc.
The thickness of the specimen should be as small as possible.
88
To Next Page
Loading...