Section 7: Safe Work Practices and Procedures
7I: Laboratory Equipment
The potential
hazards posed by laboratory refrigerators and freezers involve vapors
from the contents, the possible presence of incompatible chemicals and
spillage.
Only
refrigerators and freezers specified for laboratory use should be utilized
for the storage of chemicals. These refrigerators have been constructed
with special design factors, such as heavy-duty cords and corrosion resistant
interiors to help reduce the risk of fire or explosions in the lab.
Standard
refrigerators have electrical fans and motors that make them potential
ignition sources
for flammable vapors. Do not store flammable liquids in a refrigerator
unless it is approved for such storage. Flammable liquid-approved refrigerators
are designed with spark-producing parts on the outside to avoid accidental
ignition. If refrigeration is needed inside a flammable-storage room,
you should use an explosion-proof refrigerator.
Frost-free
refrigerators should also be avoided. Many of them have a drain or
tube or hole that
carries water and possibly any spilled materials to an area near
the compression, which may spark. Electric heaters used to defrost
the freezing coils can
also spark.
Only chemicals
should be stored in chemical storage refrigerators; lab refrigerators
should not be used for food storage or preparation. Refrigerators should
be labeled for their intended purpose; labels reading “No Food or
Drink to be Stored in this Refrigerator” or “Refrigerator
For Food Only” are available from EHS by calling 8-5294.
All materials
in refrigerators or freezers should be labeled with the contents, owner,
date of acquisition or preparation and nature of any potential hazard.
Since refrigerators are often used for storage of large quantities
of
small vials and test tubes, a reference to a list outside of the
refrigerator could be used. Labels and ink used to identify materials
in the refrigerators
should be water-resistant.
All containers should be sealed, preferably
with a cap. Containers should be placed in secondary containers, or
catch
pans should be used.
Loss of electrical power can produce
extremely hazardous situations. Flammable or toxic vapors may be released
from refrigerators
and freezers as chemicals warm up and/or certain reactive materials
may decompose energetically upon warming. Proactive planning can avoid product loss and hazardous situations
in event of an extended power outage. Dry ice or alternate power sources can be
used to prevent refrigerator and freezer contents from warming.
The stirring and mixing devices
commonly found in laboratories include stirring motors, magnetic stirrers,
shakers, small pumps for fluids and rotary evaporators for solvent removal.
These devices are typically used in laboratory operations that are performed
in a hood, and it is important that they be operated in a way that precludes
the generation of electrical sparks.
Only spark-free
induction motors should be used in power stirring and mixing devices
or
any other rotating equipment used for laboratory operations. While
the motors in most of the currently marketed stirring and mixing devices
meet
this criterion, their on-off switches and rheostat-type speed controls
can produce an electrical spark because they have exposed electrical
conductors.
The speed of an induction motor operating under a load should not
be controlled by a variable autotransformer.
Because
stirring and mixing devices, especially stirring motors and magnetic stirrers,
are often operated for fairly long periods without constant attention,
the consequences of stirrer failure, electrical overload or blockage of
the motion of the stirring impeller should be considered.
Most labs use at least one
type of heating device, such as ovens, hot plates, heating mantles and
tapes, oil baths, salt baths, sand baths, air baths, hot-tube furnaces,
hot-air guns and microwave ovens. Steam-heated devices are generally preferred
whenever temperatures of 100o C or less are required because
they do not present shock or spark risks and can be left unattended with
assurance that their temperature will never exceed 100o C. Ensure the supply of water for steam generation is sufficient prior to leaving the reaction for any extended period of time.
A number of general precautions need
to be taken when working with heating devices in the laboratory. When
working
with heating devices, consider the following:
- The actual heating
element in any laboratory heating device should be enclosed
in such a fashion as to prevent a laboratory worker or any metallic
conductor
from accidentally touching the wire carrying the electric current.
- Heating device becomes so worn or damaged that its heating element
is exposed, the device should be either discarded or repaired before
it is used again.
- Laboratory heating devices should be used with a variable autotransformer
to control the input voltage by supplying some fraction of the total
line voltage, typically 110 V.
- The external cases of all variable autotransformers have perforations
for cooling by ventilation and, therefore, should be located where
water
and other chemicals cannot be spilled onto them and where they
will not be exposed to flammable liquids or vapors.
Fail-safe devices can prevent fires
or explosions that may arise if the temperature of a reaction increases
significantly
because of a change in line voltage, the accidental loss of reaction
solvent or loss of cooling. Some devices will turn off the electric
power if the
temperature of the heating device exceeds some preset limit or if
the flow of cooling water through a condenser is stopped owing to the
loss
of water pressure or loosening of the water supply hose to a condenser.
Ovens
Electrically heated ovens
are commonly used in the laboratory to remove water or other solvents
from chemical samples and to dry laboratory glassware. Never use laboratory
ovens for human food preparation.
- Laboratory ovens should be constructed such
that their heating elements and their temperature controls are physically
separated from their interior atmospheres.
- Laboratory ovens rarely have a provision for preventing the discharge
of the substances volatilized in them. Connecting the oven vent directly
to an exhaust system can reduce the possibility of substances escaping
into the lab or an explosive concentration developing within the
oven.
- Ovens should not be used to dry any chemical sample that might
pose a hazard because of acute or chronic toxicity unless special
precautions
have been taken to ensure continuous venting of the atmosphere
inside the oven.
- To avoid explosion, glassware that has been rinsed with an organic
solvent should be rinsed again with distilled water before being
dried
in an oven.
- Bimetallic strip thermometers are preferred for monitoring oven
temperatures. Mercury thermometers should not be mounted through
holes in the top
of ovens so that the bulb hangs into the oven. Should a mercury
thermometer be broken in an oven of any type, the oven should be
closed and turned
off immediately, and it should remain closed until cool. All mercury
should be removed from the cold oven with the use of appropriate
cleaning
equipment and procedures in order to avoid mercury exposure.
Hot
Plates
Laboratory hot plates are
normally used for heating solutions to 100o C or above when
inherently safer steam baths cannot be used. Any newly purchased hot plates
should be designed in a way that avoids electrical sparks. However, many
older hot plates pose an electrical spark hazard arising from either the
on-off switch located on the hot plate, the bimetallic thermostat used
to regulate the temperature or both. Laboratory workers should be warned
of the spark hazard associated with older hot plates.
In addition to the spark hazard,
old and corroded bimetallic thermostats in these devices can eventually
fuse shut
and deliver full, continuous current to a hot plate.
- Do not store volatile flammable
materials near a hot plate
- Limit use of older hot plates for flammable materials.
- Check for corrosion of thermostats. Corroded bimetallic thermostats
can be repaired or reconfigured to avoid spark hazards. Contact EHS
for more info.
Heating
Mantles
Heating mantles are commonly
used for heating round-bottomed flasks, reaction kettles and related reaction
vessels. These mantles enclose a heating element in a series of layers
of fiberglass cloth. As long as the fiberglass coating is not worn or
broken, and as long as no water or other chemicals are spilled into the
mantle, heating mantles pose no shock hazard.
- Always use a heating mantle with a variable autotransformer to control
the input voltage. Never plug them directly into a 110-V line.
- Be careful not to exceed the input voltage recommended by the mantle
manufacturer. Higher voltages will cause it to overheat, melt the
fiberglass
insulation and expose the bare heating element.
- If the heating mantle has an outer metal case that provides physical
protection against damage to the fiberglass, it is good practice
to
ground the outer metal case to protect against an electric shock
if the heating element inside the mantle shorts against the metal
case.
- Some older equipment might have asbestos insulation rather than
fiberglass. Contact EHS to replace the insulation and for proper
disposal of the
asbestos.
Oil, Salt
and Sand Baths
Electrically heated oil baths
are often used to heat small or irregularly shaped vessels or when a stable
heat source that can be maintained at a constant temperature is desired.
Molten salt baths, like hot oil baths, offer the advantages of good heat
transfer, commonly have a higher operating range (e.g., 200 to 425oC)
and may have a high thermal stability (e.g., 540oC).There are
several precautions to take when working with these types of heating devices:
- Take care with hot oil baths not to generate smoke or have the oil
burst into flames from overheating.
- Always monitor oil baths by using a thermometer or other thermal
sensing devices to ensure that its temperature does not exceed the
flash point
of the oil being used.
- Fit oil baths left unattended with thermal sensing devices that
will turn off the electric power if the bath overheats.
- Mix oil baths well to ensure that there are no “hot spots” around
the elements that take the surrounding oil to unacceptable temperatures.
- Contain heated oil in a vessel that can withstand an accidental
strike by a hard object.
- Mount baths carefully on a stable horizontal support such as a
laboratory jack that can be raised or lowered without danger of the
bath tipping
over. Iron rings are not acceptable supports for hot baths.
- Clamp equipment high enough above a hot bath that if the reaction
begins to overheat, the bath can be lowered immediately and replaced
with a cooling bath without having to readjust the equipment setup.
- Provide secondary containment in the event of a spill of hot oil.
- Wear heat-resistant gloves when handling a hot bath.
- The reaction container used in a molten salt bath must be able
to withstand a very rapid heat-up to a temperature above the melting
point
of salt.
- Take care to keep salt baths dry since they are hygroscopic, which
can cause hazardous popping and splattering if the absorbed water
vaporizes
during heat-up.
Hot Air
Baths and Tube Furnaces
Hot air baths are used in
the lab as heating devices. Nitrogen is preferred for reactions involving
flammable materials. Electrically heated air baths are frequently used
to heat small or irregularly shaped vessels. One drawback of the hot air
bath is that they have a low heat capacity. As a result, these baths normally
have to be heated to 100oC or more above the target temperature.
Tube furnaces are often used for high-temperature reactions under pressure.
Consider the following when working with either apparatus:
- Ensure that the heating element is completely enclosed.
- For air baths constructed of glass, wrap the vessel with heat resistant
tape to contain the glass if it should break.
- Sand baths are generally preferable to air baths.
- For tube furnaces, carefully select glassware and metal tubes and
joints to ensure they are able to withstand the pressure.
- Follow safe practices outlined for both electrical safety and pressure
and vacuum systems.
Heat
Guns
Laboratory heat guns are constructed
with a motor-driven fan that blows air over an electrically heated filament.
They are frequently used to dry glassware or to heat the upper parts of
a distillation apparatus during distillation of high-boiling materials.
The heating element
in a heat gun typically becomes red-hot during use and the on-off switches
and fan motors are not usually spark-free.
For these reasons, heat guns almost always pose a serious spark hazard.
See Anecdote.
- Household hair
dryers may be substituted for laboratory heat guns only if they have
a grounded plug or are double insulated.
- Any hand-held heating device of this type that will be used in
a laboratory should have ground-fault circuit interrupter (GFCI)
protection to ensure
against electric shock.
- Never use a heat gun near flammable materials including open containers
of flammable liquids, flammable vapors or hoods used to control flammable
vapors.
Microwave
Ovens
Microwave ovens used in the laboratory may pose
several different types of hazards.
- As with most electrical apparatus, there is the risk
of generating sparks that can ignite flammable vapors.
- Metals placed inside the microwave oven may produce an
arc that can ignite flammable materials.
- Materials placed inside the oven may overheat and ignite.
- Sealed containers, even if loosely sealed, can build pressure upon
expansion during heating, creating a risk of container rupture.
To minimize the risk of these hazards,
- Never operate microwave ovens with doors open in order
to avoid exposure to microwaves.
- Do not place wires and other objects between the sealing
surface and the door on the oven’s front face. The
sealing surfaces must be kept absolutely clean.
- Never use a microwave oven for both laboratory use and
food preparation.
- Electrically ground the microwave. If use of an extension
cord is necessary, only a three-wire cord with a rating equal
to or greater than that for the oven should be used.
- Do not use metal containers and metal-containing
objects (e.g., stir bars) in the microwave. They can cause
arcing.
- Do not heat sealed containers in the microwave oven.
Even heating a container with a loosened cap or lid poses
a significant risk since microwave ovens can heat material so quickly
that the
lid
can seat upward against the threads and containers can explode.
- Remove screw caps from containers being microwaved. If the sterility
of the contents must be preserved, use cotton or foam plugs.
Otherwise plug the container with kimwipes to reduce splash potential.
Human exposure to ultrasound
with frequencies between 16 and 100 kilohertz (kHz) can be divided into
three distinct categories: airborne conduction, direct contact through
a liquid coupling medium, and direct contact with a vibrating solid.
Ultrasound
through airborne conduction does not appear to pose a significant health
hazard
to humans. However, exposure to the associated high volumes of audible
sound can produce a variety of effects, including fatigue, headaches,
nausea and tinnitus. When ultrasonic equipment is operated in the laboratory,
the apparatus must be enclosed in a 2-cm thick wooden box or in a box
lined with acoustically absorbing foam or tiles to substantially reduce
acoustic emissions (most of which are inaudible).
Direct contact
of the body with liquids or solids subjected to high-intensity ultrasound
of the sort used to promote chemical reactions should be avoided. Under
sonochemical conditions, cavitation is created in liquids, and it can
induce high-energy chemistry in liquids and tissues. Cell death from
membrane
disruption can occur even at relatively low acoustic intensities.
Exposure
to ultrasonically vibrating solids, such as an acoustic horn, can lead
to rapid frictional
heating and potentially severe burns.
Centrifuges should be
properly installed and must be operated only by trained personnel.
It is important
that the load is balanced each time the centrifuge is used and that
the lid be closed while the rotor is in motion. The disconnect switch
must
be working properly to shut off the equipment when the top is opened,
and the manufacturer’s instructions for safe operating speeds
must be followed.
For flammable
and/or hazardous materials, the centrifuge should be under negative
pressure
to a suitable exhaust system.
Glass components of the rotary
evaporator should be made of Pyrex or similar glass. Glass vessels should
be completely enclosed in a shield to guard against flying glass should
the components implode. Increase in rotation speed and application of
vacuum to the flask whose solvent is to be evaporated should be gradual.
The use of an autoclave
is a very effective way to decontaminate infectious waste. Autoclaves
work
by killing microbes with superheated steam. The following are recommended
guidelines when using an autoclave:
- Do not put sharp or pointed contaminated objects into an autoclave
bag. Place them in an appropriate rigid sharps disposal container.
- Use caution when handling an infectious waste autoclave bag, in
case sharp objects were inadvertently placed in the bag. Never lift
a bag
from the bottom to load it into the chamber. Handle the bag from
the top.
- Do not overfill an autoclave bag. Steam and heat cannot penetrate
as easily to the interior of a densely packed autoclave bag. Frequently
the outer contents of the bag will be treated but the innermost part
will be unaffected.
- Do not overload an autoclave. An overpacked autoclave chamber does
not allow efficient steam distribution. Considerably longer sterilization
times may be required to achieve decontamination if an autoclave
is
tightly packed.
- Conduct autoclave sterility testing on a regular basis using appropriate
biological indicators (B. stearothermophilus spore strips) to monitor
efficacy. Use indicator tape with each load to verify it has been
autoclaved.
- Do not mix contaminated and clean items together during the same
autoclave cycle. Clean items generally require shorter decontamination
times (15-20
minutes) while a bag of infectious waste (24" x 36")
typically requires 45 minutes to an hour to be effectively decontaminated
throughout.
- Always wear personal protective equipment, including heat-resistant
gloves, safety glasses and a lab coat when operating an autoclave.
Use
caution when opening the autoclave door. Allow superheated steam
to exit before attempting to remove autoclave contents.
- Be on the alert when handling pressurized containers. Superheated
liquids may spurt from closed containers. Never seal a liquid container
with a cork or stopper. This could cause an explosion inside the
autoclave.
- Agar plates will melt and the agar will become liquefied when autoclaved.
Avoid contact with molten agar. Use a secondary tray to catch any
potential
leakage from an autoclave bag rather than allowing it to leak
onto the floor of the autoclave chamber.
- If there is a spill inside the autoclave chamber, allow the unit
to cool before attempting to clean up the spill. If glass breaks
in the
autoclave, use tongs, forceps or other mechanical means to recover
fragments. Do not use bare or gloved hands to pick up broken glassware.
- Do not to leave an autoclave operating unattended for a long period
of time. Always be sure someone is in the vicinity while an autoclave
is cycling in case there is a problem.
- Autoclaves should be placed under preventive maintenance contracts
to ensure they are operating properly.
Precautions to prevent electric shock must be followed
when conducting procedures involving electrophoresis. Lethal electric
shock can result when operating at high voltages such as in DNA sequencing
or low voltages such as in agarose gel electrophoresis (e.g., 100 volts
at 25 milliamps).These general guidelines should be followed:
- Turn the power off before connecting the electrical leads
- Connect one lead at a time, using one hand only
- Ensure that hands are dry while connecting leads
- Keep the apparatus away from sinks or other water sources
- Turn off power before opening lid or reaching inside chamber
- Do not override safety devices
- Do not run electrophoresis equipment unattended.
- If using acrylamide, purchase premixed solutions or pre-weighed
quantities whenever possible
- If using ethidium bromide, have a hand-held UV light source available
in the laboratory. Check working surfaces after each use.
- Mix all stock solutions in a chemical fume hood.
- Provide spill containment by mixing gels on a plastic tray
- Decontaminate surfaces with ethanol. Dispose of all cleanup materials
as hazardous waste.
Although glass vessels are frequently used in low-vacuum
operations, evacuated glass vessels may collapse violently, either spontaneously
from strain or from an accidental blow. Therefore, pressure and vacuum
operations in glass vessels should be conducted behind adequate shielding.
It is advisable to check for flaws such as star cracks, scratches and
etching marks each time a vacuum apparatus is used. Only round-bottomed
or thick-walled (e.g., Pyrex) evacuated reaction vessels specifically
designed for operations at reduced pressure should be used. Repaired glassware
is subject to thermal shock and should be avoided. Thin-walled, Erlenmeyer
or round-bottomed flasks larger than 1 L should never be evacuated.
Vacuum pumps are used in the lab to remove air and
other vapors from a vessel or manifold. The most common usages are
on rotary
evaporators, drying manifolds, centrifugal concentrators (“speedvacs”),
acrylamide gel dryers, freeze dryers, vacuum ovens, tissue culture
filter
flasks and aspirators, desiccators, filtration apparatus and filter/degassing
apparatus.
The critical factors in vacuum pump selection
are:
- Application
the pump will be used on
- Nature
of the sample (air, chemical, moisture)
- Size
of the sample(s)
When using a vacuum pump on a rotary evaporator,
a dry ice alcohol slurry cold trap or a refrigerated trap is recommended.
A Cold Trap should be used in line with the pump when high vapor loads
from drying samples will occur. Consult manufacturer for specific situations.
These recommendations are based on keeping evaporating flask on rotary
evaporator at 400 C. Operating at a higher temperature allows the Dry
Vacuum System to strip boiling point solvents with acceptable evaporation
rates.
Vacuum pumps can pump vapors from air, water to toxic and corrosive materials
like TFA and methylene chloride. Oil seal pumps are susceptible to excessive
amounts of solvent, corrosive acids and bases and excessive water vapors.
Pump oil can be contaminated quite rapidly by solvent vapors and mists. Condensed solvents will thin the oil and diminish its lubricating poroperties, possibly seizing the pump motor. Corrosives can create sludge by breaking down the
oil and cause overheating. Excess water can coagulate the oil and promotes corrosion within the pump. Proper trapping (cold trap, acid trap) and routine oil changes
greatly extend the life of an oil seal vacuum. Pump oil should be changed when it begins to turn a dark brown color.
Diaphragm pumps are virtually impervious to attack from laboratory
chemical vapors. They are susceptible to physical wearing of the membrane
if excessive
chemical vapors are allowed to condense and crystallize in the pumping
chambers. A five minute air purge either as part of the procedure or
at
day’s end will drive off condensed water vapors and further
prolong pump life.
Hazardous chemicals can escape from the vacuum pump and pump should be
place in the hood. Cold traps and acid traps can be helpful, but if allowed
to thaw or saturate, they can lose their effectiveness.
 Section 7J: Particularly Hazardous Substances
 Section 7H: Pressure and Vacuum Systems
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