Thermo Scientific 43i Instruction Manual page 311

Standards for Trace Level Analyzers
Geysitech Commands
ppb range, primarily due to the dark current of the photomultiplier tube,
note due to impurities in the zero gas source.
In conclusion, tests by the United States Environmental Agency, Tennessee
Valley Authority, Battelle National Labs, and the Thermo Fisher Scientific
Engineering Department have demonstrated that Trace Level analyzers are
readily calibrated at the low concentration levels require for sub-ambient
monitoring. Although extra care is required, primarily in zero air
generation, users familiar with normal compliance requirements should be
able with minimal additional effort to obtain valid concentration data.
Thermo Fisher Scientific Model 43i Trace Level-Enhanced Instruction Manual E-3