NIST has produced Standard Reference Materials for calibrating the wavelength scale and verifying the absorbance accuracy of UV/Visible spectrophotometers for several decades. Recently, the NIST production capacity has become insufficient to meet demand and a programme has been developed to leverage NIST measurement capability through the private sector. By Lesley DeLong.
In today's world of increasing laboratory regulation, the need to verify, validate and certify analytical results has become an essential part of a laboratory's routine operations. A principle requirement for the spectrophotometer operator is to prove that the system meets the regulations. The most common method for proving spectrophotometric performance is through the use of calibrated standards that measure absorbance and wavelength accuracy. These standards must have sufficient accuracy and traceability ascribed to them by their vendor.
For more than 30 years, the National Institute of Standards and Technology (NIST) has been manufacturing and re-certifying Standard Reference Materials (SRMs) for the verification and calibration of UV-Visible spectrophotometers worldwide. However, demand for these 930e and 1930 series has outstripped NIST's ability to deliver these products. To meet the increased demand for certified standards, commercial vendors have offered standards that are certified as atraceable to NIST', meaning that they have been traced via NIST-certified reference materials to the NIST National Reference Spectrophotometer. Unfortunately, thein-house processes of some manufacturers have not always been tightly controlled and aNIST traceability' has become inconsistent.
In an effort to accommodate a rapidly increasing demand for their standards and to provide a process for stringent certification, NIST developed a programme (National Voluntary Laboratory Accreditation Program) to allow commercial manufacturers to offer these materials under tightly controlled conditions.
Firstly, NIST trademarked the name NTRM (NIST Traceable Reference Material) to designate private vendor-certified reference materials (CRMs) that are commercially produced with a well-defined traceability linkage to existing NIST standards. This linkage is established via criteria and protocols defined by NIST. Thermo Spectronic was in an advantageous position to participate in this process since we were already accredited under United Kingdom Accreditation Service (UKAS) and ISO 17025 via National Voluntary Laboratory Accreditation Program (NVLAP). Theseprogrammes ensure that processes, values and uncertainties ascribed to the standards have been independently checked and audited by a thirdaparty.
So, together with NIST, Thermo Spectronic began the process of production and calibration of filters that could be offered as NTRMs. A dedicated NTRM Transfer Spectrophotometer was identified at our facility and was approved by NIST. This instrument has undergone repeated qualification of performance and accuracy by NIST. In addition, NIST SRMs and in-house working standards are checked daily to monitor the performance of this instrument. All data obtained on the Transfer Spectrophotometer are checked by NIST to monitor long-term performance.
As a result, Thermo Spectronic is now offering a number of NTRM products that can validate absorbance measurements and wavelength accuracy of UV-Visible spectrophotometers. A total of six absorbance filters are available in values of 0.3, 0.5, 0.7, 1, 1.5 and 2 Absorbance. They can be measured at five wavelengths: 440, 465, 546.1, 590 and 635nm. Prior to certification measurements, the glass filters have been aged at Thermo Spectronic for at least six months, and each filter is examined for surface defects and is thoroughly cleaned.
Included with the Thermo Spectronic NTRM products is a holmium oxide filter, traceable to both NIST and NPL (National Physical Laboratory). This is essential for certifying wavelength accuracy of a spectrophotometer before checking its photometric accuracy. NTRM Absorbance Filter Sets are available in three configurations containing a variety of absorbance value filters. Filters are supplied in metal anodized containers designed to minimise contamination of glass surfaces. They are supplied with a certificate of calibration and a recommended date of recalibration.
According to NIST recommendations, NTRM filters that are part of a laboratory's quality control procedure should be re-certified annually. Thermo Spectronic offers this service for all their NTRM Absorbance Filter Sets.
These new NTRM Absorbance Filter Sets from Thermo Spectronic provide a failsafe method for monitoring and recording the performance of a laboratory spectrophotometer. They are convenient, unaffected by instrument-generated stray light, show virtually no change in optical properties with temperature, and are easy to operate. These filters, together with a routine aclean and calibrate' regime will ensure accurate and consistent results from your spectrophotometer and provide regulator-required documentation.
ENQUIRY No 47
Lesley DeLong is product manager for NTRM at Thermo Spectronic, Rochester, New York, USA. www.thermospectronic.com
