News from the BIPM laboratories—2016

In order to fulfil its mission to ensure and promote the global comparability of measurements, the BIPM operates laboratories in the fields of physical metrology, time, ionizing radiation and chemistry. These laboratories are a centre for scientific and technical collaboration between Member States providing capabilities for international measurement comparisons on a shared-cost basis. They coordinate international comparisons of national measurement standards agreed to be of the highest priority, and they establish and maintain appropriate reference standards for use as the basis of key international comparisons at the highest level and provide selected calibrations from them.

In the following sections, we provide highlights of the scientific work undertaken during 2016.

In the field of mass metrology, the BIPM is the pilot laboratory for the CCM Pilot Study of future primary realizations of the kilogram. The objectives are to test the consistency of future kilogram realizations based on different primary realization experiments, and continuity with the realization based on the present definition of the kilogram, which is the mass of the international prototype of the kilogram (IPK). Participants were the LNE (France), the NIST (USA), the NMIJ (Japan), the NRC (Canada) and the PTB (Germany). Three participated with Kibble/watt balances and two with ²⁸Si-Avogadro spheres. All measurements at the BIPM and by the participants have been completed and the Draft A report is under review by the participants. During the Pilot Study, the mass artefacts of the BIPM Ensemble of Reference Mass Standards (ERMS) were calibrated in their specific environments against the primary mass standards of the five NMI participants. The networks for storage under nitrogen and argon flow are now fully operational. The system for storage under vacuum is undergoing further development.

The demand for 1 kg Pt–Ir mass prototypes remains at a relatively high level. A new prototype has been manufactured for the NIM (China) and another is under fabrication for the KRISS. A stack of eight Pt–Ir disks and 21 spacers, to be used as a sorption artefact, has been manufactured for the NPL (UK).

The papers [1–3] related to the Extraordinary Calibrations using the IPK in 2014 have been published, along with a paper explaining the foundation for the redefinition of the kilogram [4].

Following several modifications, the BIPM Kibble balance has been fully reassembled and is operational in air, with the alignment of the system now optimized A number of complete measurements in air were carried out in June 2016. The signal-to-noise ratio of the voltage-to-velocity ratio was improved by a factor of five as a consequence of the improvements made on the interferometer. The Planck constant h was determined with an uncertainty of a few parts in 10⁶. The uncertainty is now dominated by noise on the force signal and the remaining misalignment of the coil with respect to the magnet, both of which will be the focus of future work. The operation of the apparatus under vacuum has been studied. The whole apparatus worked correctly, with the magnet position stability ensured through a more rigid and adjustable mechanical support that was fabricated and installed. The target uncertainty for mid-2017 is about 1 part in 10⁷.

The second on-site comparison using the transportable ac Josephson voltage standard (JVS), supplied to the BIPM by the NIST, was carried out in the laboratories of CENAM (Mexico) during 2016. Through participation in this comparison, the BIPM gained valuable experience in comparing stepwise approximated ac voltages at a frequency of 50 Hz, in preparation for a future on-site comparison programme. This will extend the present JVS comparisons (BIPM.EM-K10) at 1.018 V and 10 V to alternating signals. Both Josephson standards were compared indirectly by using a calibrator as a transfer standard. Both systems agreed within 3 parts in 10⁷ at 1 V_rms and within 7 parts in 10⁷ at 7 V_rms, which are satisfactory results. Since this comparison was considered as a trial, it was not registered as a key comparison. It is planned to develop the comparison technique further in 2017 with a secondee from the KRISS (Republic of Korea). In addition, an on-site Josephson comparison (BIPM.EM-K10) was carried out, however, the participant's standard did not behave satisfactorily, preventing a result from being published. Five bilateral comparisons were successfully arranged in the fields of voltage (DEFNAT, Tunisia), resistance (SMD, Belgium) and capacitance (NMIA, South Africa; NIS, Egypt; NSAI, Ireland) using BIPM transfer standards.

The BIPM is the coordinator of the CCEM key comparison of capacitance calibrations, CCEM-K4.2017. The technical protocol has been discussed with the participants and approved. Each participating institute will send its own capacitance standards to the BIPM where they will all be compared within the same period. Eight NMIs will participate, and the measurements will be carried out between March and September 2017. This comparison follows EURAMET.EM-S31, which was a supplementary comparison of capacitance and capacitance ratio, in which the BIPM participated. The first circulation of capacitance standards led to an unexpectedly large spread of the results. This required a second circulation of the capacitors and ac standard resistors. Investigations into systematic errors in the measuring bridges of several participants allowed the origin of these inconsistencies to be identified.

The calculable capacitor has been relocated to a dedicated laboratory where it is now installed on a concrete block for improved vibration isolation. A removable clean room enclosure has been built around the capacitor so that the precision alignment of the electrode bars can be carried out in a clean environment. Using a set of capacitive probes, the electrode bars have been aligned so that the contributions of the residual misalignments are now 3 parts in 10⁹, compared to 3 parts in 10⁷ previously. Completion of the reassembly and the start of a new series of measurements are planned for early 2017.

As part of the project to develop a readily transportable quantum Hall system, measurements of the quantized Hall resistance have been made on the BIPM's first commercial graphene sample. Three Hall bars on the same chip have been investigated. Quantization of the resistance could be achieved, but required the application of a high magnetic field, probably due to non-uniformity in the samples. The development of transportable QHR standards will need operation at lower magnetic field values. In addition, an on-site comparison of quantum Hall resistance standards had to been rescheduled for 2017 at the request of the participant.

The archiving and dissemination of services by the Time Department has been improved in 2016. A new HTML version of monthly Circular T that allows improved access to the data, results and plots supporting the values published in its sections has been launched. User access to the FTP server has been redesigned to allow easier consultation, and at the same time historical information on timescales has been added to provide long-term tracking of laboratories' contributions. A database has been developed that gives an inventory of the complete set of elements relevant to the time scales calculated at the BIPM; it is linked to the processes involved in the computation of time scales within the department. Since September 2016 the database has been accessible to users at http://webtai.bipm.org/database/html/.

The Time Department has continued to work on improvements to the estimation of time transfer uncertainty for coordinated universal time (UTC). The department's proposal that aims to optimize the process of GNSS equipment calibration received an excellent response from the regional metrology organizations (RMOs). Results of GPS calibrations organized by EURAMET laboratories have been implemented in the computation of UTC. The BIPM has started the second calibration campaign in selected institutes; measurements have been completed at the APMP and are in progress within EURAMET. With this continuous process in place, the number of laboratories with calibration uncertainty at or below 2.5 ns is increasing. The BIPM is coordinating a pilot study that aims to reduce the additional noise, with diurnal signature, observed in most two-way satellite time and frequency transfer (TWSTFT) links. The first results obtained with the operation of a 'software designed radio' (SDR) in the TWSTFT stations has shown a substantial reduction of the diurnal noise, which may help to understand the origin of the perturbation.

Research into a new approach to UTC calculation proved that by using a Kalman filter, the frequency stability of UTC can be significantly improved [5]. A revision of the algorithm for the uncertainty of [UTC  −  UTC(k)] has started with the aim of correcting undesired effects coming from the present procedure. A new approach is under development that takes into account the correlations, which are not fully considered in the current algorithm.

Advances in optical frequency standards indicate that in the next decade, the definition of the second will most probably be revised. In preparation, the CCTF is identifying candidate transitions for recommendation by the CIPM as secondary representations of the second (SRS). However, before any individual atom/ion species can be chosen among these candidates, a thorough investigation of their properties and aptitude to become the base of a redefined second must be undertaken. To this end 'direct frequency ratio measurements between standards' is a major tool and such measurements are already in progress at several NMIs. The BIPM has developed a novel method to allow the study of consistency within the set of frequency ratio measurements to be compared to the estimated individual uncertainties of the standards [6]. A technique based on 'graph theory concepts' readily isolates inconsistencies and allows for evaluation of the full set of ratio measurements, providing an interesting and simplified alternative to the non-linear least squares approach.

BIPM Circular T continues to be published monthly, giving traceability to the SI second via UTC to its local realizations in national laboratories. It is the most frequent key comparison, with one evaluation of the key comparison reference value UTC and the degrees of equivalence [UTC  −  UTC(k)] every five days for the 77 participants that together contribute data from about 500 atomic clocks.

Regular publication of rapid UTC (UTCr) continued in 2016, providing a weekly solution based on data collected over four weeks and which represents about 70% of the clocks in UTC. Consequently, the frequency instability is similar to that of UTC. This rapid solution supports the quality of the representations of UTC in national laboratories and the steering of the Global Navigation Satellite Systems' times to local representations of UTC.

The World Radiocommunication Conference (2015) has called for stronger links between the ITU and the BIPM in coming to a decision on the adoption of a continuous reference time scale by 2023. In this direction, the Time Department has been actively contributing to the work of a Task Group created by the Consultative Committee for Time and Frequency (CCTF) in 2016 to provide formal definitions of the current time scales International Atomic Time (TAI) and UTC and to recommend actions to be proposed to the CGPM in 2018.

Within the dosimetry programme, eight ongoing comparison series (BIPM.RI(I)-K1 to BIPM.RI(I)-K8) are supported. The project to develop an absorbed-dose standard for medium-energy x-rays, which will form the basis for a ninth comparison series (BIPM.RI(I)-K9) was completed with further measurements of photon attenuation in air and the characterization of three more transfer instruments. Measurements for the first full comparison, also with the PTB, were made during November and December 2016. The new standard will be presented to the CCRI(I) at its 2017 meeting, with the aim of gaining approval to launch the new comparison series and publish the results of the PTB comparison.

New software for data acquisition and analysis was developed for the BIPM.RI(I)-K6 comparison series of absorbed dose to water in high-energy photon beams. The new system was used for the tenth comparison in the series, which was carried out on-site at the NIM, in November 2016, using their 6 MV and 10 MV linear accelerator beams. The measurements for 25 MV could not be made because of the long delays necessary to fulfil appropriate conditions for radiological protection.

The new laboratory to support measurements for the BIPM.RI(I)-K8 comparison series for high dose rate (HDR) ¹⁹²Ir brachytherapy sources was completed. A new system for digital radiography has also been installed in the new laboratory.

During 2016, eight dosimetry comparisons were undertaken and twenty-one characterizations of national secondary dosimetry standards were carried out for five NMIs and the IAEA. Comparisons and calibrations are underpinned by a significant effort in equipment calibration and maintenance.

Within the radionuclide measurements programme, the Système International de Référence (SIR) received four ampoules of three different radionuclides, ⁸⁸Y, ¹³³Ba and ¹³⁴Cs from three laboratories (LNE-LNHB (France), NRC and PTB), all of which were sent to establish equivalence values in the ongoing BIPM.RI(II)-K1 comparison.

The second calibration measurements of the SIR transfer instrument (SIRTI) against the SIR were carried out for ⁶⁴Cu by measuring a solution from the NPL in both systems. The analysis is in progress. The SIRTI comparisons (BIPM.RI(II)-K4) took place on-site at the NIST for ¹⁸F, ⁶⁴Cu and at POLATOM (Poland) for ¹⁸F, ⁶⁴Cu and ^99mTc. These laboratories were the first participants in the BIPM.RI(II)-K4.Cu-64 comparison.

The pilot study for the extension of the SIR to beta emitters by liquid scintillation counting will resume with the collaboration of a secondee from the NIST. The newly purchased balance is now fully in service and has been used to prepare suitable 10 ml sources of ³H with three scintillators from the six ampoules previously received from the IFIN (Romania), IRMM, NIM, NMIJ, POLATOM and PTB. Measurements have been completed in the Beckman LSC TA 1000 counter and are in progress in the Quantulus 1220. Measurements with the Perkin Elmer TriCarb 2910 TR and with the BIPM-designed triple-to-double coincidence ratio (TDCR) counters will soon follow.

In summary, the Ionizing Radiation Department carried out sixteen comparisons, twenty-one calibrations and submitted sixteen comparison reports for publication in 2016. In addition, three RMO comparison reports were reviewed and published.

Laboratory activities for the capacity building and knowledge transfer (CB&KT) programmes for 'Metrology for Clean Air' and 'Safe Food and Feed' started in the Chemistry Department in 2016. In addition, the coordination of seven key comparisons in the field of metrology in chemistry and biology has progressed during the year, with the support of nine visiting scientists from NMIs.

In the area of air quality measurement standards, new values of the ozone absorption cross-section, based on gas phase titration have been published in Analytical Chemistry [7]. The BIPM has contributed to the review and recommendations on absorption cross-sections of ozone in the ultraviolet and visible spectral regions published in the Journal of Molecular Spectroscopy [8] as part of an initiative of the International Ozone Commission (IO3C), the World Meteorological Organization (WMO) and the IGACO (Integrated Global Atmospheric Chemistry Observations) O3/UV subgroup. This initiative will study, evaluate, and recommend the most suitable ozone absorption cross-section laboratory data to be used in atmospheric ozone measurements. The BIPM is contributing to the CCQM GAWG Ozone Cross Section Task Group, created within the CCQM Gas Analysis WG (GAWG), which will review and recommend the best value and uncertainty for the ozone cross-section to be used in the key comparison BIPM.QM-K1. The task group is expected to publish its recommendation in 2017. Measurements for the CCQM-K90 comparison on formaldehyde standards were completed. The final report is in preparation and will be published in early 2017. The first skills broadening secondment for FTIR Measurements on Gas Standards (NO₂, HCHO, HNO₃, CO₂) was completed by M de Jesus Avila, CENAM (Mexico) in the BIPM laboratories, as part of the Metrology for Clean Air CB&KT programme.

In the area of greenhouse gas standards, validation work was completed for the CCQM-K120 comparison (carbon dioxide in air), with Fourier transform infrared spectroscopy (FTIR), isotope ratio infrared spectrometer (IRIS) and GC-FID methods being fully characterized. A method for measuring isotopic abundances in CO₂, which was required for corrections to be applied to the comparison method, was developed. A paper on Calibration strategies for FTIR and other IRIS instruments for accurate δ¹³C and δ¹⁸O measurements of CO₂ in air, in collaboration with the University of Wollongong (Australia) has been submitted to Analytical Chemistry. Forty-four standards will be compared as part of the comparison and have started to arrive at the BIPM, with measurements expected to be completed by the end of May 2017. Development and validation of a manometric system for CO₂ measurements has progressed with secondments from the NIST (USA) and the SP (Sweden). This work demonstrated that the first all-glass prototype can produce results within 1% of certified gravimetric values. Encouraging results on treated steel surfaces have permitted the design of a more robust second prototype, with testing and validation of the system planned for the first half of 2017. Preparation for a comparison on CO₂ isotope ratio standards, coordinated jointly by the BIPM and the International Atomic Energy Agency (IAEA), has started with the development of a stable isotope reference mixture generation facility at the BIPM supported by a secondee from the NIM.

The department's work in the area of supporting measurements for key climatological observables advanced with the publication of a paper in Metrologia [9].

Within the BIPM's organic primary calibrator programme, the CCQM-K55.d comparison on folic acid purity was completed. The final report will be published in 2017. The comparison demonstrated the increasing use of quantitative NMR (qNMR) at NMIs for the value assignment of this category of standard. The universal calibrator programme for qNMR at the BIPM, an activity initiated together with the NMIJ (Japan), was supported by secondees from NIM (China) and INMETRO (Brazil), with characterization of the performance of standards soluble in deuterated chloroform and methanol. Preparations for the CCQM-K78.a comparison on multi-component amino acid calibration solutions were completed. Ampouled samples were distributed to participants in November–December 2016, with results expected in the first quarter of 2017. The final report of the key comparison on C-peptide purity (CCQM-K115/P55.2), coordinated by the BIPM in collaboration with the NIM, was completed. A paper written in collaboration with the University of Missouri (USA), NIM, NMIJ and NIBSC (UK) on the evolving calibration hierarchies for C-peptide measurements has been submitted to Clinical Chemistry. The general approach to pure peptide calibrator value assignment was presented at Protein and Peptide Therapeutics and Diagnostics: Research and Quality Assurance (PPTD-2016), which was held in Chengdu (China) in June 2016. This workshop was organized jointly by the BIPM and the NIM. The work undertaken on Impurity Determination for Hepcidin by Liquid Chromatography—High Resolution and Ion Mobility Mass Spectrometry for the Value Assignment of Candidate Primary Calibrators, which was supported by a secondment from the LNE (France) was submitted to Analytical and Bioanalytical Chemistry. The development of methods to characterize calcitonin calibrators was undertaken by a secondee from the HSA (Singapore), with the material intended as a future candidate key comparison material for small peptides with disulfide bonds.

The startup meeting for the CB&KT programme for 'Metrology for Safe Food and Feed', focusing on mycotoxin metrology and standards, was held at the BIPM in April 2016, with the participation of INMETRO, INTI (Argentina), KEBS (Kenya), NIM, NIMT (Thailand), NMISA, PTB, UME (Turkey) and the United Nations Industrial Development Organization (UNIDO). Laboratory activities started with a secondment from the NIM. Pure materials for aflatoxin B1 and zearalenone (ZEN) have been characterized. The mass fraction of ZEN in the pure material was determined by qNMR and structurally-related impurities were quantified by liquid chromatography-ultraviolet spectroscopy-charged aerosol detection (LC-UV-CAD) and liquid chromatography-tandem mass spectrometry (LC-MS/MS) techniques. Stock and calibration solutions of ZEN have been ampouled for homogeneity and stability testing, in preparation for CB&KT activities that will take place at the BIPM in 2017.