DNA Extraction

Human bones and teeth, especially when fresh, are difficult to pulverize with normal mills. However, skeletal material can be ground cryogenically and DNA extracted from the powder, whether the remains are recent or very old. Forensics labs use Freezer/Mills to isolate the DNA of crime victims or war casualties when medical and dental records are inadequate. Recently this approach was used to help identify the bones of Russia’s Czar Nicholas II and his family.
Paleoarchaeologists are beginning to plot the DNA of more ancient human remains, again using cryogenic grinding and PCR (polymerase chain reaction) amplification. A SPEX SamplePrep Freezer/Mill was also employed to prepare bone fragments from the 5300-year-old “Ice Man” found in the Alps in 1991.

Trace Element Analysis

Low levels of toxic heavy metals such as cadmium, chromium, and mercury are known to have profound effects on human health, whether present in substances we ingest or in paint, fabric, plastic, and other materials with which we (or our food and water) have contact. Many such samples, from fish scales and bovine liver to the plastics used in food packaging and children’s toys, have to be prepared for analysis; grinding with SPEX SamplePrep Freezer/Mills is often the simplest way. Medical examiners, pathologists, and even anthropologists may be concerned with clues such as the concentration of chromium in bones or the accumulation of arsenic in hair and fingernails; here again, cryogenic milling is often the best way of pulverizing such samples for extractions, dissolutions, or direct analysis.

Screening of Asbestos-Containing Materials (ACMs)

Typically the floor tiles, ceiling tiles, and pipe insulation being checked for harmful asbestiform minerals are composite materials which are both flexible and tough. The challenge is first to free the mineral fibers from their matrix and then to determine whether they are among the species most damaging to human health. Freezer/Mills are useful not only to break up the matrix and free included fibers for polarized-light microscopy, but also to grind those fibers finer for X-ray diffraction analysis.

Pharmaceutical Analysis and Drug Testing

The complex molecules present in drugs, and their metabolites, are of considerable interest to both pharmaceutical researchers and crime labs. There are many cases where it is important to be able to distinguish between different closely related isomers; unfortunately, many such com-pounds are quickly degraded by the pressure and heat accompanying room-temperature milling. Preparation of key samples, whether the compounds themselves or tissues which may contain
them, is greatly facilitated by cryogenic grinding in SPEX SamplePrep Freezer/Mills.

Recovery of Volatile Compounds

Coal, petroleum shale, waxes, and many other organic materials contain light-molecular-weight fractions which are quickly driven off when their matrix is pulverized at room temperature. Grinding these materials in Freezer/Mills insures the retention of volatile components, which can be concentrated later by heating the ground sample in a gas trap.

Mineralogical Studies

The structures of certain minerals, notably clays and fibrous amphiboles, can be difficult to distinguish by X-ray diffraction because they are subject to matting and preferred orientation when pulverized by normal means; with unusually soft minerals there can also be distortion of the crystal lattice due to impact. When chilled during grinding in SPEX SamplePrep Freezer/Mills, these minerals can often be broken up more completely, exhibiting more nearly random
orientation of grains and maintaining greater structural integrity.

Medical Research

Medical schools and research hospitals have long used Freezer/Mill technology in a variety of unconventional projects. One is the pulverizing of fresh, sterilized bone to use as a cement in joint-replacement surgery; the tiny bone fragments act as growth nuclei to stimulate the formation of bone around prosthetic implants. Another is the homogenizing of heart tissue to smear on slides as a culture medium. Freezer/Mills have even been used to grind implant materials to a fine powder which can be injected into live subjects for allergy and toxicity testing. At dental schools cryogenic grinding is used to pulverize fresh teeth without degrading them through heating; the overall possibilities in the medical field seem endless.

SPEX SamplePrep, LLC. 6750 and 6850 Freezer/Mills

SPEX SamplePrep Freezer/Mills are cryogenic laboratory mills which chill samples in liquid nitro-gen and pulverize them with a magnetically driven impactor. Each sample is placed in a separate grinding vial which is immersed in a liquid nitrogen bath inside the mill. Thus there is no cross-sample contamination, and the low temperature of the sample is maintained during grinding. These features have made the SPEX Freezer/Mill the most effective in the world, the “mill of last resort” for many normally ungrindable samples.

Freezer/Mill technology, fieldproven since the 1960s, has been built  into two new models, the 6750 and 6850 Freezer/Mills. Both include significant safety features, and electronic controls which allow an entire grinding sequence to be programmed. The 6750 Freezer/Mill has the capacity to grind several grams of most samples; the 6850 Freezer/Mill can handle about ten times that much. (See the Guide to Freezer/Mill Sample Capacity on p. 135) All SPEX SamplePrep Freezer/Mills are capable of grinding many different sample types to analytical fineness.

Chilling materials in liquid nitrogen (at temperatures approaching -200° C) has two important consequences for sample preparation: it embrittles flexible samples so they can be pulverized by impact milling, and it preserves structural and compositional aspects usually damaged or lost during roomtemperature pulverizing. The list of samples ground in SPEX SamplePrep Freezer/Mills is very long, and includes NASA’s moon rocks, bone samples from Russia’s royal family, asbestos-containing materials, and in short, nearly every natural or synthetic material which for one reason or another is difficult to grind properly with conventional mills. Freezer/Mills are used by polymer chemists, forensic experts, environmental biologists, geneticists, botanists, doctors and dentists, geologists and mineralogists, and many others. Applications include DNA extraction, polymer structure studies, trace element monitoring, food analysis, mineral diffraction research, implant surgery, drug testing, textile identification, etc.

SPEX SamplePrep Freezer/Mills are compact benchtop laboratory mills which utilize liquid nitrogen as a coolant. A Freezer/Mill incorporates an insulated tub into which liquid nitrogen is poured. The grinding mechanism is a magnetic coil assembly suspended in the nitrogen bath. Each sample is place in a closed grinding vial along with a steel impactor, and the vial is then inserted in the coil assembly and lowered into the liquid nitrogen. When the sample is thoroughly
chilled—usually a matter of 10 to 15 minutes—grinding begins. The magnetic coil shuttles the impactor rapidly back and forth, pulverizing the sample against the end plugs of the vial. When the grinding cycle is complete, the vial is removed from the Freezer/Mill, emptied, and cleaned.

The 6750 Freezer/Mill is an updated version of the classic 6700 Freezer/Mill with electronic controls, a safety interlock, and an improved overall design. It uses sample vials the same size as those for the 6700, and its sample capacity is the same: as much as 4 ml (3-5 g of many samples) for the 6751 vial, and around 0.5 ml for the 6753 microvial. The 6850 Freezer/Mill is a new, larger mill which incorporates the same principles and technology as the 6700 and 6750
Freezer/Mills. Its sample capacity is much greater: as much as 50 ml for many materials (e.g. 50 g of some polymers, 100 g of bone). See Guide to Freezer/Mill Sample Capacity and Limits of Cryogenic Grinding, beginning on the next page.

A typical milling cycle after pre-cooling consists of several grinding periods interrupted by pauses for re-cooling. An example is fifteen minutes of cooling followed by three two-minute grinds with two-minute rests in between each grind. With the new Freezer/Mills such a cycle can be programmed. Impact frequency can also be altered to suit the sample. Programming and manual control are done through a keyboard with LCD display in the lid of the mill.

The basic grinding vial for SPEX SamplePrep Freezer/Mills consists of a steel impactor and end plugs with a polycarbonate center section. Steel is necessary for the impactor and end plugs as it is the only magnetic material which is easily manufactured and holds up well at liquid nitrogen temperature. The plastic center section is surprisingly durable, and has two practical advantages: when chilled it forms a tight seal with the end plugs, and allows visual inspection of the
ground sample. Stainless steel center sections are available for cases where contamination with polycarbonate is not acceptable.

These are some of the distinctive features which make SPEX SamplePrep Freezer/Mills uniquely effective:
• Because the sample vial is immersed in liquid nitrogen while the Freezer/Mill is operating, grinding takes place at consistently low temperatures.
• Each sample is completely retained within its vial, insuring control of critical or hazardous samples.
• Vials are self-contained and can be cleaned after each use, eliminating both cross-sample contamination, and contamination of the Freezer/Mill itself.
• Electronic controls allow programming of a full grinding cycle, increasing sample-to-sample consistency.
• The only moving part is the impactor; this greatly reduces the mechanical stress which is expected in low-temperature equipment.