25
Starna had previously worked closely with NIST and its UK equivalent the National Physical Laboratory (NPL) for many years and had developed the technology to permanently heat-fuse seal reference solutions into far UV quartz cuvettes, similar technology being a concept used by NIST for its own sealed-cell products.
Ideally, reference materials used for instrument qualifi cation should have certifi ed values as close as possible to the conditions used for the proposed analysis. The primary goal of the NTRM program was for commercially produced NTRM fi lters to supplement or in some instances replace the coverage afforded by SRMs. For example, as mentioned above, Pharmacopoeias and other regulatory bodies recommend the references to be used for instrument qualifi cation. For wavelength, this is invariably holmium oxide solution, which certainly covers the widely-used region between 240 and 640 nm, and an instrument qualifi ed with this fi lter could in principle be described as ‘pharmacopoeia compliant’. If the analytical wavelength is outside this range the qualifi cation is less relevant, and indeed best practice would be that the qualifi cation uses references whose certifi ed wavelengths ‘bracket’ the analytical wavelength. Under these circumstances, the pharmacopoeias allow the use of other certifi ed materials, and today, a whole series of commercially available liquid references allow instrument qualifi cation in the wavelength range, from the deep UV to the near-Infrared:
Clearly any commercially produced reference materials would only be useful if they were accepted by regulatory authorities as being equivalent to NIST SRMs. The NTRMTM
(NIST Traceable Reference
Material) program [6] was developed to provide a mechanism whereby the measurement values provided with commercially produced reference materials would be traceable to NIST primary standards, via an accredited process.
Traceability is defi ned in ISO/IEC Guide 99:2007 [7] as the “property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty”. This normally means that the instrument used to generate the calibration values has itself been qualifi ed using NIST SRMs. Such materials can then be referred to as ‘Certifi ed Reference Materials’, defi ned by the International Standards Organisation (ISO) Technical Committee on Reference Materials (ISO/REMCO) as a “Reference Material, characterised by a metrologically valid procedure for one or more specifi ed properties, accompanied by a certifi cate that provides the value of the specifi ed property, its associated uncertainty, and a statement of metrological traceability.”[8]
At the inception of the NTRM program, the range of SRMs for UV/ Visible spectrophotometry was rather limited, see Table 1.
Table 1. NIST SRMs for UV/Visible spectrophotometry (1998) [9] SRM 930e 931e 935a 1930 2030a
2031a 2032
Parameter Transmittance Absorbance Absorbance Transmittance Transmittance Type Neutral Density glass
Co/Ni in nitric/perchloric acids
Potassium dichromate powder
Neutral Density glass Neutral Density glass
Reference material Starna Deep UV (DUV)
Starna ‘Rare Earth’ solution Samarium perchlorate Holmium oxide Didymium oxide
Combined Holmium/Didymium oxides Starna Near IR reference
Usable range (nm) 190 – 230 200 - 270 230 - 500 240 - 650 290 - 870 240 - 795 930 -2550
Furthermore, where competence has been demonstrated by the private sector, NIST has been able to discontinue the manufacture of many of the SRMs in Table 1, as certifi ed traceable equivalents are now available commercially, via the appropriate accredited standards.
Most certifi ed values are given at specifi ed spectral bandwidths. Other variables, such as temperature, could affect the certifi ed values of reference materials. An accredited Reference Material producer should be able to provide certifi ed values under any reasonable customer- specifi ed operating conditions.
Wavelength range (nm) Unit size
440-635 302-678
Examination of Table 1 also reveals that to use some of the SRMs, users are obliged to conduct dissolution or liquid transfer operations, with their attendant risks. This is largely overcome by the purchase of ready- made solutions in fl ame-sealed cuvettes that have been prepared under carefully controlled conditions – which also avoids such risks as evaporation or contamination.
3 fi lters, 1 ‘blank’ holder
Set of 12 ampoules
235-350 15 g 440-635 465
Transmittance Metal-on-quartz fi lters 250-635 Stray light
2034 Wavelength 2035 Wavelength
3 fi lters, 1 ‘blank’ holder
1 fi lter, 1 ‘blank’ holder
3 fi lters, 1 ‘blank’ holder
Potassium iodide solid 240-280 25 g Holmium oxide solution 240-650 Rare-earth glass
1 sealed cuvette 1000-2000 2.5 cm disk Accreditation
A basic requirement of the NTRM program was that commercial reference material suppliers be accredited. Accreditation is not just about the ability to make accurate calibration measurements, it also covers aspects such as documentation, record keeping and the supplier’s quality control systems. There are several detailed and continuously evolving international standards that impact directly or indirectly on the production of reference materials, and not all commercially offered products comply with them. Suppliers will usually claim accreditation to one or more of the following ISO Standards, but caution should be exercised when evaluating potential CRM suppliers as accreditation to a standard may not indicate the necessary competence for the job in hand?
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