John P Hammond reports on new certified reference materials for the determination of wavelength and photometric accuracy in far UV spectrophotometry.
Certified reference materials for spectrophotometric measurement (CRM) in the visible region (370680nm) have been available for over 30 years, from a variety of sources.
The use of neutral density glass filters for visible absorbance measurements was established initially by the National Institute of Standards and Technology (NIST) in the USA, and the National Physical Laboratory (NPL) in the UK. These materials are now also available from commercial sources, where calibration laboratory accreditation to ISO/IEC17025 has been achieved for the certification of these filters.
Good spectroscopic practice recommends that where possible, both the wavelength and absorbance scales should be validated by appropriate references. If measurements are to be performed in the UV region it is not advisable to use references certified in the visible region, as instrumental characteristics may vary significant between the two. ideally the certified values should 'bracket' ie have values above and below the wavelength or absorbance range required.
In recent years, sealed reference materials in far UV quartz cells have extended the lower limit of the wavelength range down in to the UV region to approximately 240nm. These materials, principally holmium oxide in perchloric acid for wavelength, and acidified potassium dichromate for absorbance, were again developed by NIST and sold as SRM2034 and SRM935a respectively.
Whilst SRM2034 is conveniently available in a sealed cell format, SRM935a is only available from NIST as the reference solid material, from which solutions have to be prepared according to approved procedures.
Many years ago a commercial vendor incorporated this solution preparation and cell sealing into their manufacturing procedures.
As a result, linearity series of concentrations from 20mgl-1 to 100mgl-1 acidified potassium dichromate are available from ISO/IEC17025 accredited calibration labs.
As procedures and techniques have developed to match the analytical requirements of the bio-chemical marketplace, significant applications have been developed for the 200-280nm region, routinely available on most quality UV-Visible spectrophotometers.
By definition, validation of these applications and the spectrophotometers used, now require the corresponding certified reference materials for this area of use to be made available.
If one looks at the general characteristics required of a certified reference material, these can be summarised as five simple statements: u Readily available, safe, and easy to handle. u Suit the optical properties of the instrument, eg spectral bandwidth, etc. u Be unaffected by environmental conditions, eg have a small temperature coefficient, good chemical stability, etc. u Be accompanied by a certificate, one or more of whose property values are certified by a procedure which establishes traceability to an accurate realisation of the unit in which the property values are expressed, and for which each certified value is accompanied by an uncertainty as a stated level of confidence.[1] u Be calibrated in a quality environment, endorsed by an independent third party.
With these specified requirements in mind, two certified reference materials have been produced to cover this far UV region of 200280nm.
The first of these is a wavelength reference, produced by permanently sealing by heat fusion an acidic rare earth solution into a far UV grade quartz cell.
The use of rare earth oxides solvated in acid is an established and well-recognised method for the validation of the wavelength scale of a spectrophotometer in the UV and visible regions, as previously described c.f. holmium oxide. This reference simply extends the usability of this elemental group in to the required spectral region.
The second is a series of sealed cells containing a solution of nicotinic acid.
The use of nicotinic acid solvated in dilute hydrochloric acid is a well-documented method[2] for the validation of the absorbance scale and linearity of a spectrophotometer in the far UV region.
When prepared in 0.1M hydrochloric acid, nicotinic acid gives a spectral scan containing characteristic peaks at approximately. 210nm and 260nm. Within the concentration range 525mg/l, if the absorbance scale of a narrow SBW (<2nm) spectrophotometer is linear, the apparent absorbances of a series of concentrations will be a linear function of concentration.
In both cases, permanent sealing of the solution into a quartz cell effectively provides a readily available, easy to use, safe material.
These products are designed for use with commercial UV-Visible spectrophtometers, and in practice the certificates detail the spectral bandwidth, etc at which the measured values were obtained.
Therefore it is essential that the measuring 'reference' spectrophotometer has essential characteristics that at least match, and ideally better those reported for most commercially available spectrophotometers.
For the above reasons, initial and on-going validation of the performance of this instrument is critical to the whole certification process, and forms one of the primary considerations of the ISO/IEC17025 accreditation.
The effect of environmental factors for example temperature, stability, can only be established by appropriate experimental studies. As both parameters input into the overall uncertainty budget for the references, minimisation of these values is essential to the production of a useable total uncertainty budget on the final certified values.
These studies have now been completed, and with both materials a small thermal expansion coefficient and excellent long-term chemical stability have been observed.
When these references are certified for appropriate parameter values, accreditation to ISO/IEC17025 provides the essential third party confirmation of: traceability, calculated uncertainty budgets, use of appropriately trained personnel, etc.
Overall, therefore these two new references conform to the established requirements, and will assist the validation of new or existing applications in the far UV (200280nm) region.
ENQUIRY No 50
John P Hammond is technical & marketing manager for analytical products at Optiglass Ltd, Hainault, Essex, UK. www.starna.com
