INFICON HAPSITE ER Chemical Identification System - Manual (Page 448)

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HAPSITE ER Operating Manual
Concentrator exits the analytical column and crosses the membrane into the Mass
Spectrometer, the MS pressure reading rises. Therefore, the amount of moisture
left on the Concentrator, prior to thermal desorption, must be managed to keep the
MS pressure within the software controlled limits. The proper dry purge time should
be empirically determined for the sampling conditions.
14.11 Calibration Standards
14.11.1 Introduction
Custom and ready-to-purchase calibration mixes may be purchased from a variety
of vendors. These calibration mixes are prepared in a water-soluble matrix such as
methanol for easy dilution into a known volume of water. The standard matrix must
be water soluble to evenly disperse the analytes. The low molecular weight of
methanol is below the normal scan range and therefore it is not detected by the
HAPSITE’s Mass Spectrometer.
14.11.2 Standard Preparation
Commercially available calibration mixes are a reliable and convenient source for
calibration standards. These standards are normally received in flame sealed glass
vials that are easily opened by snapping the flame sealed end and transferring the
contents to a screw-cap vial with a septum seal.
The water standard is prepared for immediate use by transferring a measured
quantity of the standard mix, with a syringe, into a known volume of VOC free
water. The transfer volume of the typical spiking process is a few microliters of the
standard mix into one liter of water. Two acceptable spiking techniques are:
1 Prepare the standard directly in the purging vessel. See Figure 14-5 .
1a Transfer a known volume of water into the purging vessel. The water level must
meet depth specifications with the probe immersed.
1b Draw the required volume of standard mix into a syringe.
1c With a stirrer, begin swirling the water. Remove the stirrer and rapidly dispense
the standard below the water surface.
1d Quickly withdraw the syringe and allow a few seconds for the standard to
disperse with the water’s movement.
1e For Internal Standard calibration, a second spike of the Internal Standard mix
can also be added.
1f Immerse the probe and immediately begin the analysis to avoid loss of VOCs.
NOTE: The sample purging process removes VOCs from the water sample.
Therefore, there can only be one analysis per water standard.