Filter Ratio (Meilenz) Method (USP)

These Reference Materials can be used to qualify the stray light (or Stray Radiant Energy) of Ultraviolet (UV) spectrophotometers according to the Filter Ratio or Mielenz method. This method is described in ASTM International E 387 and is cited for instrument qualification by the US Pharmacopeia in its Chapter (Method A). This method is useful for instruments with very low stray light, such as double monochromator systems.

Description and Discussion

A range of cut-off filter solutions that allow stray light to be checked at a range of wavelengths from 200 nm to 390 nm. Each liquid filter is permanently sealed by heat fusion into a 10 mm high quality far UV quartz cell and a 5 mm path length cell (in the conventional 10 mm format) of the same liquid. The 5mm cell is used to provide a differential reference to the measurement of the 10 mm cell (see explanation below). Starna alkali halide stray light Certified Reference Materials are prepared in accordance with ASTM International 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 within the monochromator itself or to light leaks or other effects in the rest of the optical system. As the detector cannot discriminate between the analytical wavelength and the stray light, the stray light contributes to the detector signal and introduces an error in the measured absorption. The stray 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 most quantitative determinations are based.

Stray light is wavelength and instrument dependant. It can be present at any wavelength but is most noticeable when the energy throughput of the system at the analytical wavelength is relatively low, for example in the far UV region, and any stray light will be comparatively more significant. At these wavelengths, any deterioration in the instrument optics or UV 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 other wavelengths. Any light detected by the instrument is then stray light. In the traditional method, known as the “Specified Wavelength” method, the cut-off filter appropriate to the analytical wavelength is scanned against a water blank.

This method was also accepted by the US Pharmacopoeia until May 2015, when ca new chapter <857> was issued,recommending the Solution Filter Ratio method, also
known as the Meilenz method. In this method the reference materials are measured not against water, but against a 5 mm path length cell containing the same solution. This has theeffect of “backing off” the measured absorbance, resulting in a peak. Spectra obtained with a Potassium Chloride reference material using the two methods are shown below:

The differential absorbance value (ΔA) at the peak is related empirically to the Stray-Light level (s) by

s = 0.25 x 10 –2*ΔA

Practically, this means that an instrument exhibiting 1% stray light would give a differential absorbance value at the peak of > 0.7A. This is the instrument qualification requirement of the USP.

Typical spectra

These typical spectra were recorded on three instruments with different stray light characteristics:

The peak wavelength is different on each instrument due to their different stray light characteristics, but in all three cases the absorbance maximum is greater than 0.7A, so all these instruments satisfy the USP stray light requirement.

For comparison purposes, the equivalence of the two test methods is indicated in the table:

An instrument giving an absorbance of >2.0 A using theSpecified Wavelength method should give a differentialabsorbance of >0.7A using the Filter Ratio method.

The working ranges of the solutions as cited in the USP are:

Filter Ratio Absorbance (Δ A)	Specified Wavelength Absorbance (A)
0.3	1.3
0.5	1.6
0.7	2.0
1.0	2.6
1.5	3.6
2.0	4.6
2.5	5.6

Material	CONCENTRATION	Usable range	Catalogue Number
Sodium Nitrite, 10mm and 5 mm cells	5% aqueous	300 - 385 nm	RM-SN/5
Acetone, 10mm and 5 mm cells	Spectroscopy grade	250 - 320 nm	RM-AC/5
Potassium Iodide, 10mm and 5 mm cells	1% aqueous	210 - 259 nm	RM-KI/5
Sodium Iodide, 10mm and 5 mm cells	1% aqueous	210 - 259 nm	RM-SI/5
Potassium Chloride, 10mm & 5 mm cells	1.2% aqueous	190 - 205 nm	RM-KC/5

Catalogue Number

Material	Catalogue Number
Universal Stray Light reference set, EP and USP compliant	RM-ACKCSISN/15

Material	Catalogue Number
Sodium Nitrite, 10mm and 5 mm cells	RM-SN/5
Acetone, 10mm and 5 mm cells	RM-AC/5
Potassium Iodide, 10mm and 5 mm cells	RM-KI/5
Sodium Iodide, 10mm and 5 mm cells	RM-SI/5
Potassium Chloride, 10mm & 5 mm cells	RM-KC/5

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Data sheet

Technique

UV-VIS References

Visible References

Stray Light Cut-off Filters – USP Filter Ratio Method

Purpose