Electrically charged samples and receivers: Dry, finely divided powders may be
charged with static electricity that can make the powder either attracted to or repelled by
the receiver or the balance, causing inaccurate weight measurements and specimen loss
during transfer. A drift in the balance readings should alert the operator to the possibility
that the material has a static charge. Commercially available balances with a built-in
antistatic device can be used to remedy the problem. Such devices may use piezoelectric
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components or a very small amount of a radioactive element (typically polonium) to
generate a stream of ions that dissipate the static charge when passed over the powder
being weighed. Antistatic weigh boats, antistatic guns, and antistatic screens also are
commercially available. The static charge depends also on the relative humidity of the
laboratory, which in turn depends on atmospheric conditions. Under certain conditions,
static charge is caused by the type of clothing worn by the operator and this charge can
cause large errors in the weighing. Borosilicate glassware and plastic receivers have a
well-known propensity for picking up static charge, especially at low relative humidity. The
gloves used to protect the operator also may increase the potential for a static charge
problem. Placing the container in a metal holder may help to shield the static charge, and
antistatic gloves also can help to alleviate the problem.
Volatile samples: When weighing a liquid that has a low boiling point, analysts must
receive the specimen in a vessel with a gas-tight enclosure of small diameter. The analyst
then tares the vessel and enclosure, adds the desired amount of sample, and replaces
the enclosure. After the balance display stabilizes, the analyst records the specimen
weight.
Warm or cool samples: Samples that are warm or cool should be equilibrated in the
laboratory, or the weight readings may be erroneous. With regard to warm samples, the
apparent weight is smaller than the true weight because of heat convection. For example,
a flask that is warmer than ambient air warms up this air, which then flows upward along
the flask and reduces the apparent weight of the contents by viscous friction.
Hygroscopic samples: Hygroscopic materials readily absorb moisture from the
atmosphere and steadily gain weight if left exposed. Therefore, hygroscopic samples
must be either weighed promptly or placed in a vessel with a gas-tight enclosure. For a
gas-tight vessel, analysts should tare the vessel and enclosure, add the desired amount
of sample, and replace the enclosure. After the balance display stabilizes, the analyst can
record the specimen weight.
Aseptic or biohazardous samples: The weighing of sterile or biohazardous samples
should take place within the confines of a clean bench, biosafety cabinet, isolator, or
similar containment device. Air flow within the hood potentially can cause balance
instability, so after a balance has been installed under a hood, analysts should perform a
rigorous qualification study with suitable weight artifacts (see 41 ) in order to determine
the acceptability of the balance performance in this environment.
Weighing corrosive materials: Many chemicals, such as salts, are corrosive, and
materials of this nature should not be spilled on the balance pan or inside the balance
housing. Extra care is essential when materials of this nature are weighed. Analysts
should consider the use of sealed containers such as weighing bottles or syringes. In the
event of a spill, requalification of balance may be necessary, depending on the nature of
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the spill.