6875D Freezer/Mill
1300-87052-1A, REV 1
SPEX SamplePrep LLC
Page 22 of 36
6.2 Manually Adding Liquid Nitrogen
(Cont’d)
Once more, slowly lower the lid and secure the latch.
Caution: When the lid is closed and
latched, nitrogen vapor which vents from the Freezer/Mill can displace the oxygen in a closed
room and cause asphyxiation.
If the tub is overfilled, liquid nitrogen will also exit from the vent
in the rear of the Freezer/Mill. A grinding program can now be run sample as outlined in
Section 5.2.
CAUTION: Never overly force an end plug (warm or cold) into a polycarbonate cylinder (warm
or cold). Polycarbonate shrinks when chilled, and can crack if stressed. If the
cylinder has cooled to the point that the end plug fit is too tight, allow the cylinder
to warm up before inserting the end plug.
6.2.1 Introduction and General Recommendations
As a rule of thumb, the sample pieces should be small enough to freely move inside the vial as
the impactor moves back and forth during grinding process. If sample is an irregular shape, it
may be necessary to cut sample pieces to smaller size. Place impactor in vial and fill the vial 1/3
to 1/2 the volume with sample pieces. Typical sample sizes for the various size of vials are
shown below in Table 1.
Table 1.
Typical Sample Sizes
Sample
Large Vial
6801, 6803
Mid-Size Vial
6881, 6883, 6885
Small Vial
6751, 6761,
6771, 6781S
Microvial
6757
Plant tissue, bone,
muscle, etc.
20-50 g
5-20 g
2-5 g
100-300 mg
Polymer pellet
10-20 g
2-10 g
1-2 g
NA
Virtually, no two materials will grind the same. Therefore, it is important to design the grinding
process to attain the desired particle size or smallest possible particle size for any material. Key
parameters in the design include precool time, grinding time, cycles, impactor rate as well as
sample weight and sample size. Through grinding trials and guidelines as described in this
Manual a custom procedure can be developed based on sample composition. Avoid overfilling
the vial as this greatly reduces grinding efficiency.
If at first the sample doesn’t grind as int
ended, try these strategies:
•
Reduce the overall weight of the sample. (Some fibers and thin films may grind best in
the Small vial with a sample weight of 1-2 g.)
•
Reduce the size of the individual pieces to 1/4 or 1/8 inch (6 to 3 mm).
•
Increase the Precool Time. (20 minutes of precooling is about the maximum necessary
for most samples to reach LN temperatures)
