News from the BIPM laboratories—2015

In order to fulfil its mission to ensure and promote the global comparability of measurements, the BIPM operates laboratories in the fields of mass, time, electricity, ionizing radiation and chemistry. All of the laboratory work addresses one or more of the agreed objectives for the BIPM's laboratories, which are:

• To establish and maintain appropriate reference standards for use as the basis of a limited number of key international comparisons at the highest level.
• To coordinate international comparisons of national measurement standards through the Consultative Committees of the CIPM; taking the role of coordinating laboratory for selected comparisons of the highest priority and undertaking the scientific work necessary to enable this to be done.
• To provide selected calibrations for Member States.

In the following sections, we provide highlights of the scientific work undertaken at the BIPM during 2015.

The BIPM Physical Metrology Department was created in October 2015 by merging the staff and activities of the Electricity and Mass Departments.

The measurement campaign of calibrations against the International Prototype of the Kilogram (IPK) in anticipation of the planned redefinition of the kilogram (Extraordinary Calibrations) was completed in 2015. Phase 1 [1] of this campaign was carried out in 2014, during which the BIPM working standards were recalibrated against the IPK. In Phase 2, the mass standards of National Metrology Institutes (NMIs) involved in determinations of the Planck constant have been calibrated. As a result of this work, all recent determinations of the Planck constant now have an improved uncertainty that is traceable to the IPK. During Phase 1 it had been observed that the BIPM as-maintained mass unit had developed an offset of 35 μg with respect to the IPK. The results of the analysis of the temporal evolution of this offset have been used to provide amendments for all mass calibration certificates issued between 2003 and early 2014. The BIPM was involved in the calculation of the correlation coefficient between the determinations of the Avogadro constant in 2011 and in 2015 [2], to determine how far both results can be seen as independent [3].

Another important element of the 'joint Consultative Committee for Mass and Related Quantities (CCM) and Consultative Committee for Units (CCU) roadmap towards the redefinition of the kilogram' is the pilot study of primary realizations of the mass unit. The BIPM is the pilot laboratory for this comparison and a technical protocol has been developed. The objectives are to investigate the coherence of realizations of the mass unit based on different primary realization experiments and continuity with the present definition of the kilogram.

As a consequence of the observation made during the Extraordinary Calibrations, that the masses of the BIPM working standards had changed, a new strategy for the use of the working standards has been developed. The main features are the creation of three hierarchical levels of working standards, which will be used at different periodicities, and the organization of the calibrations for NMIs during two dedicated periods of the year. In 2015, 25 Member States' mass standards were calibrated following the new scheme. Three new Pt-Ir prototypes were delivered during the year.

The Department uses a hydrostatic weighing apparatus for the determination of the density and the volume of new Pt-Ir prototypes and stainless steel standards. This system is being upgraded so that in the future Si cylinders will serve as the reference for density, instead of water, which should simplify the operation.

The Department is now equipped with a vacuum transfer system for the M_one mass comparator. This will allow mass standards to be brought into the comparator under vacuum or under inert gases; an important capability for the BIPM's future work for the Ensemble of Reference Mass Standards (ERMS). The work on the storage network for the ERMS is almost complete and it is planned that the mass standards will be placed in their containers in early 2016. The BIPM workshop is fabricating a new series of improved containers for storage under vacuum. With the recent arrival of the Si spheres and the silicon stack, all mass standards are now available. The mass evolution of the stainless steel and Pt-Ir mass standards in air continues to be monitored.

Assembly of the new watt balance apparatus is nearly complete. The new, open support structure allows convenient access to the apparatus for alignment. A novel technique for aligning the magnetic circuit has been developed [4] which has allowed alignment to be achieved to within 30 μrad. The new interferometer based on a space-separated heterodyning technique is being integrated into the watt balance. First measurements with the new apparatus are planned for early 2016. The demonstration of the principle of the watt balance to visitors has been greatly helped by the donation by the NIST, USA, of a watt balance made of Lego^® bricks to the BIPM. Dr Richard Davis developed a simple experiment to determine the value of the Planck constant to better than 1% by measuring the mass of a 20 g aluminium cube. The details have been published in the Journal of Chemical Education [5].

The BIPM's measurement services in electricity have been heavily used in 2015. Two on-site Josephson voltage comparisons and one on-site quantum Hall resistance (QHR) comparison have been carried out. The first of this new series of QHR comparisons, carried out in 2013, had led to interesting observations on the behavior of 1 Ω resistors [6]. Seven comparisons were carried out using BIPM transfer standards: Zener voltage standards (2), resistors (4) and capacitors (1). The BIPM is the pilot laboratory for the future CCEM-K4 capacitance comparison and the comparison protocol is under development. The Department participated in the second round of EURAMET.EM-S31, a comparison of capacitance calibrations to come to an understanding of the observed and unexpected differences between participants. The Department has provided 45 calibration certificates for resistors, 35 for capacitors and three for Zener voltage standards during 2015.

In the framework of the European Metrology Research Programme (EMRP) GraphOhm project, graphene samples developed by MIKES-VTT and Aalto University, Finland, have been measured at the BIPM. An equivalence of a few parts in 10⁹ was found with conventional GaAs samples. The Department was also involved in the investigation of a new generation of low-frequency current comparators that could form the basis of room temperature resistance bridges, to accompany new graphene references. An investigation of new comparators that were built by MIKES gave good results when substituted in the BIPM's existing bridge electronics. These developments have the potential to lead to a much simplified transportable system for on-site QHR comparisons.

A precision alignment probe for the calculable capacitor has been developed and fabricated and the laser for the interferometer has been re-designed for easier and more reliable use. In the coming months, the calculable capacitor will be moved and re-aligned in a new laboratory that is less subject to environmental disturbances, in particular due to ground vibrations. The capacitor will be re-installed together with the complete quadrature measuring chain including the quantum Hall resistance. Measurement of R_K will then resume in the new laboratory.

Work on the Josephson voltage standard (JVS) for the measurement of the induced voltage in the watt balance has progressed, with the first generation of a stepwise voltage ramp. However, some difficulties remain to be overcome, in particular related to trapping of flux. The a.c. JVS system, provided to the BIPM by the NIST, has been used for the first time during an on-site comparison at the NMIJ, Japan, in December 2015.

Major achievements in the Time Department included an improvement of time transfer uncertainty by a factor of about 2.5, with the implementation of calibrations of equipment in coordination with the Regional Metrology Organizations (RMOs) following the procedures described in the 'BIPM Guidelines for GNSS calibrations' [7]. This procedure involves repeated calibrations at intervals of about two years, allowing improved monitoring of the stability of the equipment and consequently a reduction of the uncertainty. The BIPM concluded the first measurement campaign at nine selected institutes from EURAMET, SIM, APMP and COOMET. Results of this campaign have been implemented in the computation of Coordinated Universal Time (UTC) since September 2015. The second campaign will begin in the first trimester of 2016. After completion of equipment calibration in other laboratories by the RMOs, all time links for UTC will be calibrated, with a positive impact on the uncertainty of the degrees of equivalence [UTC  −  UTC(k)] in the key comparison CCTF-K001.UTC. In parallel, work started on the algorithm for the evaluation of the uncertainties of [UTC  −  UTC(k)] to correct two drawbacks of the present algorithm: the absence of correlations and the non-optimal utilization of all available clock comparison data. An optimal solution using redundant links is being investigated, where the matrix of correlations will provide the uncertainties of [UTC  −  UTC(k)].

Studies continued on the implementation of a Kalman filter routine for the computation of UTC. Kalman filtering is a powerful mathematical tool for dealing with white phase noise that affects time transfer data. It is often applied for the computation of real-time time scales. This study is the first application to a clock ensemble for producing a post-processed time scale of the UTC-type. When implemented on a sub-set of the UTC contributing clocks, results show a significant improvement of the time scale frequency stability in the short- and mid-terms. This work has been developed in collaboration with the University of Torino, Italy.

A new technique called IPPP (PPP with integer ambiguity resolution) [8] that has the potential to revolutionize clock frequency comparisons using GPS signals has been developed by the BIPM Time Department in collaboration with the Centre National d'Etudes Spatiales (CNES) and the Collecte Localisation Satellites (CLS). The most significant result compares IPPP to a 420 km optical-fibre time link in Poland [8], demonstrating that the IPPP technique reaches a performance of 1  ×  10⁻¹⁶ in about five days when comparing the frequency of two clocks, and that the achieved accuracy continued to improve with longer averaging times.

BIPM Circular T continues to be published monthly, giving traceability to the SI second via Coordinated Universal Time (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 74 participants that together contributed data from about 470 atomic clocks at the end of 2015.

Regular publication of rapid UTC (UTCr) continued in 2015, 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 of the same order 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 BIPM Time Department actively contributed during 2015 to the preparations for the discussion on the future of the international reference time scale at the International Telecommunication Union (ITU) World Radiocommunication Conference (WRC-15) in November 2015. As a result, the WRC-15 called for stronger links between the ITU and BIPM in coming to a decision on the adoption of a continuous reference time scale by 2023.

Within the dosimetry programme, eight ongoing comparison series, BIPM.RI(I)-K1 to BIPM.RI(I)-K8, are supported and the project to develop an absorbed-dose standard for medium-energy x-rays is nearing completion. A pilot comparison with the PTB, Germany, using this primary standard was conducted in November 2015 and should result in a new comparison series beginning in early 2016. However, minor problems with instabilities in the transfer instruments remain under investigation and a fourth transfer chamber is currently being charcterized. The ninth comparison in the series BIPM.RI(I)-K6 for absorbed dose to water in high-energy photon beams, was carried out with the NMIJ/AIST, Japan, on-site using their 6 MV, 10 MV and 15 MV beams.

A comparison for the reference air kerma rate for HDR ¹⁹²Ir brachytherapy sources BIPM.RI(I)-K8 was carried out on-site at the NMIJ. The results and reports of two previous comparisons were published. The design study to establish a new laboratory for this activity is complete and is being implemented.

In total, ten dosimetry comparisons and eighteen dosimetry characterizations of national secondary standards were carried out, which are underpinned by a significant effort in equipment calibration and maintenance. In addition, fifteen comparison reports were approved and published in the Metrologia Technical Supplement.

Within the radionuclide measurements programme, the Système International de Référence (SIR) received eight ampoules of five different radionuclides from seven laboratories, five were oriented to generate equivalence values in the ongoing BIPM.RI(II)-K1 comparison. Three ampoules of ⁶⁸Ge from three laboratories were received during 2015 and will serve to evaluate the key comparison reference value (KCRV) for this radionuclide and to provide a link for the CCRI key comparison CCRI(II)-K2.Ge-68. ²²³Ra (T_1/2  =  11.43 d, u_c  =  0.03 d) is a promising radionuclide for the therapy of some cancers, for which two ampoules from two laboratories have been evaluated in 2015, constituting a new SIR entry. An earlier submission of ¹¹¹Ag has been evaluated and the Draft A report issued following receipt of the corresponding reporting form. A further ampoule of ⁵⁷Co had slightly deteriorated when it arrived at the BIPM, however it was possible to recondition the solution in an undamaged ampoule and measure it using the SIR.

SIR Transfer Instrument (SIRTI) comparisons (BIPM.RI(II)-K4) of ^99mTc and ¹⁸F took place on-site at the NMISA, South Africa. In addition, the first calibration measurements of the SIRTI against the SIR were carried out for ⁶⁴Cu by measuring a solution from the CNRS-Orléans, France, in both systems.

In total, twelve radionuclide activity comparisons were undertaken and five updated reports of BIPM.RI(II)-K1 comparisons were approved and published in the Metrologia Technical Supplement covering ⁵⁶Mn, ⁶⁵Zn, ⁸⁵Sr, ²⁰⁷Bi and ^166mHo, including the link from the CCRI(II)-K2.Zn-65 comparison. In addition, the first three results in the BIPM.RI(II)-K4.F-18 comparison were presented at the ICRM-2015 conference in Vienna, Austria, on 8–11 June 2015 and submitted for publication in Applied Radiation and Isotopes. The radioactivity group contributed to three chapters in the Metrologia special issue on 'Uncertainties in Radionuclide Metrology' [9–12], as well as to the guest editors' team.

In summary, twenty-six comparisons, eighteen calibrations and twenty comparison reports were produced by the Ionizing Radiation Department in 2015. In addition, five articles were published or accepted in peer-review journals, and one CCRI report and six RMO comparison reports were reviewed and published.

Laboratory activities have been undertaken in the core areas of international comparisons and equivalence of gas standards for air quality and climate change monitoring and the international comparison of primary organic calibrators. Planning started for a laboratory-based Capacity Building and Knowledge Transfer Programme on Mycotoxin Metrology, which is scheduled to begin in 2016.

A paper demonstrating equivalence between methane standards made in whole and synthetic air measured by cavity ring-down spectroscopy (CRDS) and gas chromatography—flame ionization detection (GC-FID) for atmospheric monitoring applications was published in Analytical Chemistry [13] in February 2015, completing the Department's activities related to the CCQM-K82 comparison (Methane in air). Validation work continued on the CCQM-K120 comparison (carbon dioxide in air), with validation of Fourier transform infrared spectroscopy (FTIR) and isotope ratio infrared spectrometer (IRIS) measurement systems with a set of 23 standards produced by the NIST, USA; NPL, UK; and the National Oceanic and Atmospheric Administration (NOAA), USA, (the World Meteorological Organization—Global Atmosphere Watch (WMO-GAW) Central Calibration Laboratory, CCL, for CO₂). The standards were produced using five different sources of CO₂ that resulted in a diversity of δ¹³C and δ¹⁸O values for the standards. Their isotopic composition was value assigned on the VPDB scale with traceability to the JRAS06 standards of the Max Planck Institute for Biogeochemistry (MPI-BGC) in Jena, Germany. In parallel, the assembly of a manometric system for CO₂ measurements has progressed, supported by the secondment of Dr Stephen Maxwell from the NIST. The all-glass apparatus was installed in a temperature-controlled oven and the connections to a cryostat and Residual Gas Analyser were tested, allowing the cryogenic separation of CO₂ from air. Full automation and validation of the system is anticipated in 2016. The importance of these comparisons for the monitoring of essential climate variables (ECVs) was presented and discussed at the BIPM Workshop on Global to Urban Scale Carbon Measurements, which was held on 30 June to 1 July 2015. Discussions on oceanic ECVs, which started in 2013, have resulted in a set of review papers in Metrologia on 'Metrological challenges for measurements of key climatological observables: oceanic salinity and pH, and atmospheric humidity' [14].

In the area of air quality, four laboratories brought their national ozone standards to the BIPM for comparison with the BIPM-SRP27 reference standard in 2015. The new value of the ozone absorption cross-section, measured by UV absorption on pure ozone samples at the BIPM in 2014, was published in 2015 in Atmospheric Measurement Techniques [15]. Gas phase titration measurements of ozone with nitrogen monoxide have been completed, and a paper is in preparation, confirming the agreement of these measurements with those made by UV absorption in 2014. The CCQM-K90 comparison on formaldehyde standards has continued. Following completion of measurements at the BIPM, eight cylinders were selected and sent to participants. Six cylinders remain at the BIPM, and these cylinders have been measured regularly in order to monitor their stability. Participants performed their own measurements between June and October 2015. Cylinders are being returned to the BIPM for final measurements and the results will be presented to the CCQM Working Group on Gas Analysis (GAWG) in April 2016. Planning has started for a key comparison (CCQM-K137) on nitrogen monoxide (NO) at 30 μmol mol⁻¹–70 μmol mol⁻¹ in nitrogen, which is scheduled for late 2016. Validation measurements have been carried out on the BIPM's comparison facilities and twenty laboratories have expressed their interest in this future comparison.

The BIPM's organic small-molecule purity programme has continued with the transfer of ownership of a 400 MHz nuclear magnetic resonance (NMR) spectrometer to the BIPM, following successful qualification and acceptance testing of the spectrometer, which was completed at the beginning of 2015. The visit by Dr Takeshi Saito (NMIJ, Japan) to the BIPM in February 2015 marked the start of the BIPM-NMIJ collaborative projects in this area and provided further training to BIPM staff. Subsequently, Dr T Huang (NIM, China) and Dr I Un (UME, Turkey) have worked on secondment at the BIPM in 2015 to develop and validate methods for performing high accuracy quantitative NMR (qNMR) measurements in various deuterated solvents. This work has allowed the qNMR facility to be used for the first time to assign a purity value to the CCQM-K55.d comparison material, folic acid, in addition to mass balance methods. The comparison samples were distributed to participants in September 2015 and the submission of results is scheduled for January 2016. Validation studies of methods for the assignment of the mass fraction content of amino acids in solution using liquid chromatography-UV (LC-UV), liquid chromatography-charge aerosol detector (LC-CAD), liquid chromatography-tandem MS (LC-MS/MS) and ion chromatography (IC) in preparation for the CCQM-K78 comparison were completed in 2015, during the secondment of Dr B Garrido (INMETRO, Brazil). A candidate material consisting of a batch of 200 ampoules of a multi-component amino acid solution has been prepared and will be evaluated as a suitable candidate for the CCQM-K78 comparison material. The BIPM has continued to coordinate the drafting of technical guidelines on 'Methods for the SI Value Assignment of the Purity of Organic Compounds for use as Primary Reference Materials and Calibrators' as an International Union of Pure and Applied Chemistry (IUPAC) project. The final Technical Report will be produced in 2016.

Measurements on the first CCQM key comparison on peptide purity (CCQM-K115/P55.2) coordinated by the BIPM in collaboration with the NIM, China, were completed and presented at the first meeting of the new CCQM Working Group on Protein Analysis (PAWG) in October 2015. High-resolution mass spectrometry methods for related structure impurities were developed by Dr M Li from the NIM, during his secondment to the BIPM. Method development for future comparisons has continued, with the secondment of Ms P Bros from the LNE, France, to work on the method development for high-resolution mass spectrometry coupled to liquid chromatography (LC-hrMS) for the purity determination of hepcidin, a key regulator of iron homeostasis and a promising clinical biomarker for iron deposition in the brain, a possible causal agent of Alzheimer's disease. High-purity oxytocin and calcitonin, both synthetic therapeutic peptides, have been produced in collaboration with the NIM, China, to serve as future candidate key comparison materials for small peptides with disulphide bonds. A paper describing the development and comparison of mass spectrometric methods for the quantification of angiotensin (ANG I) has been published in Rapid Communications in Mass Spectrometry [16].