The optimum volume, weight, grinding time, and impact frequency for any sample ground in a SPEX SamplePrep Freezer/Mill are determined by experimentation, the experience of the operator, and the requirements of the analyst. As a rule of thumb, the smaller the sample and the longer it is ground, the finer the particle size will be. In cryogenic grinding, temperature also affects the outcome: the colder a sample, the more finely it can be ground.

The 6750 Freezer/Mill with the 6751 Vial has a sample volume limit of about 5 ml; bulkier samples may restrict movement of the impactor. For efficient grinding, typical sample volume is 1 to 2 ml, corresponding to weights of 1 to 2 grams for polymers and 2 to 4 grams for bone.

The 6850 Freezer/Mill with the 6801 Vial has at least 50 ml maximum sample capacity, 10 times that of the 6750 Freezer/Mill. Typical sample volumes are 10 to 20 ml, equivalent to weights of 10 to 20 grams for polymers and 20 to 40 grams for bone. The 6850 Freezer/Mill can also hold four 6751 Vials for simultaneous grinding..

Limits of Cryogenic Grinding
Not every substance which is difficult to grind at room temperature can be pulverized by cryogenic milling. For this reason we at SPEX SamplePrep recommend that you discuss your samples and grinding requirements with one of our specialists before selecting a mill. If there is any question about whether a sample can be ground in a Freezer/Mill we will request that a portion be submitted for test-grinding.

Most naturally occurring materials of biological origin can be cryogenically ground, usually to a fine particle size: hair, muscle tissue, bone, wood, plant stems, cotton fiber, etc.

Polymers in general can be ground, but their physical form is important. Many flexible plastics which can routinely be milled in pellet form are difficult or impossible to grind as thin fibers and films, which remain flexible even at - 200°C. For the same reason, polymers in general cannot be ground to as fine a particle size as biological samples. As the particles become smaller they become more flexible, and progressively more difficult to grind. Many silicone compounds
remain elastic and hard-to-grind in any form.

The behavior of metals and metal alloys during cryogenic milling is highly variable. For example, impure copper as well as many copper alloys can be ground without much trouble, while pure copper remains malleable. Metal samples as a rule should be test-ground.

When a sample initially resists grinding, it may not be because it is “ungrindable.” Many such samples respond to the strategies of reducing the sample volume, increasing grinding time, and even lengthening the pre-cooling period to 30 minutes or so. Successes have also been scored by using “filler” grinding agents such as detergent and quartz sand.