In the years 2016 to 2019, the chapter USP <857> was subjected to a further revision; the resulting USP <857> (42nd edition, update from Dec. 2019) is showing the following major changes:

1. CONTROL OF WAVELENGTH ACCURACY

to date:
A Holmium filter is used to check wavelength accuracy from 240 nm to 650 nm.
A Didymium filter is used to check wavelength accuracy above 650 nm (730 nm to 870 nm)

New supplements in USP <857>:
For wavelength accuracy below 240 nm spectrophotometers must be checked with a Cerium filter (200 nm to 255 nm)!
 

Here you can find more information about Wavelength Filters.

2. CONTROL OF ABSORBANCE ACCURACY

to date:
UV range:
Potassium dichromate solutions are used over the intended operational range up to 3 Abs.
Vis range:
Neutral density glass filters are used over the intended operational range up to 3 Abs.

New supplement in USP <857>:
UV range 0 - 1 Abs:
Potassium dichromate solutions with concentrations from 20 – 60 mg/l,
or Nicotinic acid solutions with concentrations from 6 – 24 mg/l may be used.
UV range 1 - 3 Abs:
Potassium dichromate solutions with concentrations from 80 – 200 mg/l may be used.

Here you can find more information about Absorbance Filters.

3. CONTROL OF PHOTOMETRIC LINEARITY

to date:
Not required.

New in USP <857>:
“Verification of photometric response (linearity) is required and it should be evaluated using a standard type appropriate for the wavelength(s) required, where at least three different absorbance levels appropriate to and spanning the required operational range are measured”.

Following reference materials (filters) are suggested:
UV range:
Nicotinic acid solutions 6 – 60 mg/l
or Potassium dichromate solutions 20 – 200 mg/l
Vis range:
Neutral density glass filters

Note:
To check the photometric linearity, at least 3 filters with different concentrations are required per spectral range. According to the USP <857> regulation it’s not necessary to calculate the correlation coefficient of the standard response to demonstrate linearity, just demonstrate that at the different absorbance levels selected the acceptance limits have been met.
 

Here you can find more information about Photometric Linearity.

4. LIMIT OF STRAY LIGHT

to date:
Stray light levels have to be monitored in the wavelength range: 200 – 400 nm.
Measuring stray light in accordance with USP <857> requires a 5 mm reference filter and a 10 mm filter, both filled with the same solution..

New supplement in USP <857>:
In USP <857> (42th edition, update December 2019), two possible procedures are described for checking limit of stray light.

Procedure A: the already given and discribed method.

Procedure B: The analysts can measure the stray light filter with a path length of 10 mm against the reference filter filled with pure water (10 mm path length), with the exception of acetone, which is measured against air. The measured absorbance value per filter should be greater than 2.0 at the recommended wavelength listed in the table:

5. CONTROL OF RESOLUTION

to date:
A liquid filter of Toluene in Hexane and a Hexane reference filter are used to check the resolution of the spectrophotometer..

New USP <857>:
No changes!.

Here you can find more information about Control of Resolution.