FAQ: "We use the ColorQuest XE with instrument manual version 1.4. The instrument manual states that purified water will have tristimulus values of X=98.074, Z=118.232 and Y=100.0. Current USP has these values at X=98.0, Y=100.0 and Z=118.1. Can you please let me know why there is a difference in the X and Z values? Thanks for your time."
By the current USP method, I believe you are referring to:
USP 24 Monograph 1061 Color – Instrumental Measurement
USP - United States Pharmacopoeia Inc., Rockville, MD USA www.usp.org
There are 2 United States Pharmacopoeia monographs that address color (631) and color measurement (1061). USP 1061 describes the CIE system of color measurement and USP 631 references the visual USP liquid color standards and Munsell color standards.
USP 1061 is a concise, well-written description of color measurement but by my guess was written in the 1990s.
Color measurement is defined by the CIE - Commission internationale de l´éclairage www.cie.co.at and by industry standards groups such as ASTM E12. In the 2000s there were slight improvements in the CIE and ASTM protocols to add precision by increasing the number of decimal places in the calculations. The differences you note are editorial and related to when the USP method was last edited.
For the most current method on measuring color and for best definition of the white point or top-of-scale tristimulus values, I would refer you to:
ASTM E308 Standard Practice for Computing the Color of Objects by Using the CIE System
ASTM - American Society of Testing and Materials, West Conshohocken, PA USA www.astm.org