Thermo Scientific 49iQ Instruction Manual page 161

Calibration
Periodic Zero and Span Checks
outside of the limits of ±4% for zero and ±6% for span drift, the
multipoint calibration above is indicated to isolate the problem.
For detailed guidance in setting up a quality assurance program, refer to
the Code of Federal Regulations and the EPA Handbook on Quality
Assurance.
Thermo Scientific 49iQ Instruction Manual 4-13