Equipment
and
Technique |
Hood Vapors
Prolonged or repeated inhalation of low concentrations of certain vapors can cause a variety of types of irreversible damage, and fume hoods are used to prevent contact with these dangerous vapors.
Certain fume hoods are designed for specific vapors, and investigators that require the use of fume hoods must insure that they employ the proper fume hood.
For example, when using perchloric acid the hood must be specifically designed such that it will not cause an explosion when it is turned on.
Deadly and catastrophic explosions can result from perchlorate compounds that precipitate out onto the motors as their vapors are evacuated.
Also, when using heavy liquids or solvents with vapors heavier than ambient air, the fume hood must have adequate power to evacuate the heavy vapors.
Fume hood users must be aware that certain compounds and their vapors are incompatible and should not be placed under the same fume hood.
Some examples of procedures that require the use of fume hoods are described below.
Located below each fume hood are storage areas specifically designed for the various wet chemicals employed in the laboratory.
These include flammable liquids, organic solvents, acids, bases, oxidizers, and heavy liquids.
These storage areas allow the user to properly segregate chemicals from one another.
Heavy Mineral Separation
The heavy mineral component of sediment or sedimentary rock consists of all clastic grains with specific gravities greater than about 2.9.
Minerals with a specific gravity lower than 2.9 are considered light minerals.
The heavy and even the light mineral suite is a significant feature of a sediment or sedimentary rock, and these minerals can provide clues as to provenance of a sedimentary rock or sediment, as well as transport and sedimentation history and the environment of deposition.
Separating the light minerals from the heavy minerals assists investigators in diagnosing and deciphering the geologic history of a region or a deposit.
Most standard laboratory techniques for heavy-mineral separation are based on mass separation in a liquid with a specific gravity between the specific gravities of the minerals or groups of mineral to be separated.
Generally the minerals that have been prepared and cleaned are placed into a separatory funnel or centrifuge tubes filled with a bromated or halogenated liquids, typically tetrabromoethane and tribromoethane.
Methylene iodide is another common liquid.
There exists a family of non-toxic, water soluble heavy liquids that can negate the need for fume hoods and halogenated or bromated heavy liquids.
The separation procedure is simple.
Prepared minerals are placed in the fluid and all minerals with a specific gravity lower than that of the fluid float while minerals with a specific gravity greater than that of the liquid sink.
Generally this separation is accomplished by gravity settling or by centrifugation.
These separations must be performed under a fume hood owing to the toxicity of most heavy liquids and their resulting vapors.
Once the minerals have been separated they can be poured or drained off into filter paper that allows the heavy liquid to drain into another clean reservoir for subsequent reuse.
The separated minerals are rinsed with an appropriate solvent, usually acetone, dried, and are then ready for analysis.
Insoluble Residue Determinations
Insoluble residues are materials remaining from the digestion of a sample in a fluid solvent, normally an acid.
Insoluble residues may be extracted from sediment or rock with several different solvents.
The specific solvent employed is determined by sample composition and by the purpose of the study.
Acids commonly used are hydrochloric, perchloric, formic, hydrofluoric, and acetic, but other solvents are used for special purposes.
Carbonates are the most commonly dissolved rocks, but silicates can be dissolved with hydrofluoric acid.
Typical insoluble residues are siliceous materials such as chert, quartz, and siliceous fossils, or aggregates of clay, silt, sand, or rock.
The residues remaining can be used for correlation of strata, especially where fossils are absent.
Simply the procedure involves placing the material to be dissolved in a beaker filled with an appropriate solvent.
The investigator must wait for dissolution or digestion to be complete, then decant the solvent into an appropriated container and then wash, rinse, and dry the insoluble residue.
Oil and Resin Mounts
Oil and resin mounts are used for optical identification or two-and three- dimensional shape analysis.
Grains that have been separated from a sample and mounted onto a slide require mounting mediums like refraction index oil, epoxy, and resins that allow the grains to be imbedded in-and stick to- the glass slide.
Many of these mounting substances release harmful vapors while curing and require the use of a fume hood.
Staining and Etching Techniques
Staining and etching allows the investigator to identifying and differentiate between minerals that can be difficult to distinguish when in their natural state.
Many of the substances used to stain or etch minerals release harmful fumes, therefore these techniques require the use of a fume hood.
The procedure involves placing rocks or grains into a specific compound long enough for them to stain or etch the minerals.
Knowing how various minerals stain or etch, the investigator can then identify the specific mineral components.
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