INFICON HAPSITE ER Chemical Identification System - Manual (Page 546)

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HAPSITE ER Operating Manual
B.1.1.1 Using Cylinders Charged with Each Concentration
To perform a calibration, have the known gas mixtures on hand. 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 Target Compounds 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 Target Compounds will occur in the samples.
The mixtures received will be tagged with the precise value of the concentration of
each compound as delivered which will be approximately the concentrations
ordered. The concentration supplied will generally be within ±10% tolerance; this
is termed the blend accuracy.
The precise values are the ones to 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" 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 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