Thermo Fisher Scientific
Model 42
i
Trace Level Instruction Manual
E-1
Appendix E
Standards for Trace Level Analyzers
The development of ultra-sensitive analytical analyzers for the measurement
of trace quantities of pollutants such as NO, NO
2
, or SO
2
has raised a
number of questions concerning proper calibration procedures and
equipment. Analyzers that in theory have detection limits below 1 ppb,
may not in practice be useful due to inadequate and/or inaccurate
calibration practices.
Thermo Fisher Scientific has in its development of Trace Level
instrumentation used the same basic principles for calibration as for
standard analyzers. However, enhanced emphasis on the zero air supply
used for dilution of standard gases and for establishing a zero background
signal is made.
Chemiluminescence NO/No
x
and fluorescence SO
2
analyzers (when
properly designed) have bee shown in numerous research studies and
compliance monitoring situations to be inherently linear over a wide
dynamic range. Calibration is normally done using firs a zero gas then a
span gas generated by dilution of a calibration gas cylinder. Dilution is
done by dynamic mixing of accurately known flows of span gas and zero
gas. For Trace Level analyzers, mass flow controllers that are NIST
traceable are required. Nitric oxide and sulfur dioxide calibration standards
between 1 and 10 ppm are readily available in specially treated cylinders,
and have been shown to have excellent stability and accuracy. NIST
traceable mass flow controllers with dull scale ranges from 20 sccm to
20,000 sccm are also readily available. It is therefore relatively straight
forward to generate span gas concentration from below 1 ppb to 10 ppb
assuming a suitable zero gas source is used. For example:
[ ]
[ ]
Flow
Total
Flow
NO
x
NO
NO
SPAN
GENERATED
=
Assuming a calibration cylinder of 1 ppm, NO flow of 10 sccm, and a total
flow of 10,000 sccm, allows generation of span concentration of 1 ppb.
Using the specifications of the mass flow controllers and calibration
cylinder, this concentration should be accurate to within
±
5%. Multi-point
concentrations from 0.5 ppb to 10 ppb may be similarly generated,
establishing the linearity of the particular analyzer being calibrated.
