INFICON HAPSITE Smart Plus Chemical Identification System - Manual (Page 572)

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HAPSITE Smart Plus Operating Manual
B.1.1 How to Establish the Desired Concentrations
There are two basic ways to obtain several concentrations of a given mix of
compounds. The most obvious is to buy the compounds, premixed to specification,
in cylinders containing the several concentrations desired. The second is to buy a
master cylinder of the compounds at the highest concentration needed, and dilute
to the lower concentrations required. Each of these options is discussed below.
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 samples.
The mixtures received will be tagged with the precise value of the concentration of
each compound as delivered; these 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.
1.Scott Specialty Gases: (215) 766-8861