User Guide

For an introduction to the application and use of Certified Reference Materials in absorption spectrophotometry

Purpose

These Reference Materials can be used to qualify the Stray Light (or Stray Radiant Energy) of ultraviolet spectrometers according to the ASTM387 Specified Wavelength Method.This method is accepted for instrument qualification by most regulatory authorities, including the EuropeanPharmacopoeia and ASTM. The USPharmacopeia now recommends the Filter Ratio, or Meilenz method.

Description and Discussion

A range of liquid cut-off filters that allow stray light to be checked at a range of wavelengths from 200 nm to 390 nm.Starna liquid stray light references are supplied permanentlysealed by heat fusion into 10 mm high quality far UV quartz cells. Starna alkali halide stray light Certified Reference Materials are prepared in accordance with ASTM E-387.

Stray light, also called Stray Radiant Energy or Power, is any light reaching the detector that is outside the Spectral BandWidth selected for analysis by the monochromator. It can be due to optical imperfections or stray reflections withinthe monochromator itself or to light leaks or other effectsin the rest of the optical system. As the detector cannotdiscriminate between the analytical wavelength and the stray light, the stray light contributes to the detector signaland introduces an error in the measured absorption. Thestray light is not absorbed even at high concentrations of the absorbing species, so its effect is a negative deviation from the linear relationship between concentration and absorbance (the Beer-Lambert law) on which mostquantitative determinations are based.

Stray light is wavelength and instrument dependant. It can bepresent at any wavelength but is most noticeable when the energy throughput of the system at the analytical wavelengthis relatively low, for example in the far UV region, and any stray light will be comparatively more significant. At thesewavelengths, any deterioration in the instrument optics orUV light source will exaggerate the apparent stray light, so It is desirable to check it even if the instrument is not to be used in the far UV, as it is an excellent way of monitoring the condition of the instrument optics.

The usual way of assessing stray light is to measure, at the desired analytical wavelength, a sample that totally absorbs the radiation at that wavelength, but transmits at all otherwavelengths. Any light detected by the instrument is then stray light.

Practically, the usual method is to use cut-off filters orsolutions that cut off all light near the analytical wavelength,and transmit at all higher wavelengths. Starna stray light CRMs have very sharp transitional (cut-off) spectra, giving excellent filtering characteristics. The certified wavelength is that at which the spectrum transitions 2.0 A. Below thiswavelength, within the indicated usable range, any indicationof light transmission must be stray light

Typical spectra

stray-light

 

Material  CONCENTRATION  Cut-off  Usable range Catalogue Number 

Sodium Nitrite

5% aqueous

391 nm  300 - 385 nm  RM-SN 

Acetone

Spectroscopy grade

326 nm  250 - 320 nm  RM-AT 

Potassium Iodide

1% aqueous

260 nm  210 - 259 nm  RM-KI 

Sodium Iodide

1% aqueous

260 nm  210 - 259 nm  RM-SI 

Lithium Carbonate

Saturated aqueous

227 nm  210 - 225 nm  RM-LC 

Sodium Chloride

1% aqueous

201 nm  175 - 200 nm  RM-SC 

Potassium Chloride

1.2% aqueous

201 nm  175 - 200 nm  RM-KC 

 

Potassium Iodide and Sodium Iodide have practicallyidentical cut-off characteristics and may be regarded asinterchangeable. Several of these filters are approved
for instrument qualification by Pharmacopoeias and other standardisation bodies, including the EuropeanPharmacopoeia. Potassium Chloride is the most frequently used, and is included in the Starna EP Instrument Qualification Set.

The European Pharmacopoeia states that at a path length of 10mm, a 12g/l aqueous solution of potassium chloride must have an absorbance ≥ 2A at 198 nm.

 

Catalogue Number

Material  Catalogue Number 

Universal Stray Light reference set, EP and USP compliant

RM-ACKCSISN/15 

Material  Catalogue Number 

Sodium Nitrite

RM-SN 

Acetone

RM-AT 

Potassium Iodide

RM-KI 

Sodium Iodide

RM-SI 

Lithium Carbonate

RM-LC 

Sodium Chloride

RM-SC 

Potassium Chloride

RM-KC