B - 2
HAPSITE ER Operating Manual
B.1.1.1 Using Cylinders Charged with Each Concentration
A gas supplier (such as Scott Specialty Gases
1
) can provide a choice of cylinder
sizes with the compounds of interest mixed in a suitable matrix at the requisite
concentrations. The matrix (or balance gas) for the mixture should be specified as
"VOC-free Nitrogen" or "VOC-free Air", to minimize the level of background VOC's
in the calibration mix.
The concentrations for calibration of the various compounds of interest will
probably be defined in the method being followed. The method may specify, for
example, 0.1 ppm, 1 ppm, and 10 ppm of each compound. Calibrate the HAPSITE
to bracket the concentrations at which the compounds of interest will occur in the
samples.
The mixtures received will be tagged with the precise value of the concentration of
each compound as delivered. The concentration supplied will generally be within
±10% tolerance; this is termed the blend accuracy.
The precise values should be used in the course of building the calibration libraries
and are accurate to +2 -20%, depending on the target concentration levels and the
certification methods used. This is termed the analytical accuracy. The certified
concentrations in each cylinder mixture will generally be stable at room
temperature conditions for about six months.
The selected gas supplier should be able to advise about the reactivity of the
compounds needed, and the materials of cylinder construction to provide the best
long term stability of the concentration. The supplier will recommend the use of
stainless steel regulators with stainless diaphragms. To minimize stagnant volumes
where VOCs can accumulate, the regulator body should be designed with
minimum internal dead-volume. Use 1 in. diameter gauges, or eliminate the
gauges altogether. The regulators and the tubing following should be rated for high
purity, mildly corrosive (or corrosive) service if any halogenated VOC's are to be
delivered.
NOTE: A regulator/transfer line system must be well purged with pure nitrogen or
air to remove any residual VOCs prior to use with a cylinder containing a
lower concentration mix.
Transfer fittings should be composed of the stainless steel Swagelok
2
type, and
transfer lines should be clean, stainless steel or nickel 1/8 inch tubing. Teflon tubing
should be avoided due to its permeability. Ideally, regulators and transfer lines
should be heat-traced to maintain above ambient temperatures (35-55 °C) and to
reduce adsorption of the higher boiling VOC's.
1.Scott Specialty Gases: (215) 766-8861
2.Swagelock (Crawford Fitting Company): (216) 248-4600
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