Preparation of high precision standards (with ± 1 ppm) using a gravimetric method for measuring atmospheric oxygen

Transcription

1 Preparation of high precision standards (with ± 1 ppm) using a gravimetric method for measuring atmospheric oxygen Nobuyuki Aoki 1, Takuya Shimosaka 1 Shigeyuki Ishidoya 2, Shohei Murayama 2 1.National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology 2.Environmental Management Research Institute, National Institute of Advanced Industrial Science and Technology 1

2 Background In atmospheric observation, small variation of O 2 concentration has been observed with the precision of 1 µmol/mol or less. The concentration should be measured using standard gases in which its standard uncertainty is less than 1 µmol/mol, to be able to directly compare each laboratory s data. However, the preparation technique under the uncertainty of 1 µmol/mol is not established. Recommended compatibility of measurements Component Target O 2 /N 2 ±2 per meg (0.4 µmol/mol) Range per meg (vs. SIO scale) First of all, we aimed for the uncertainty of 1 µmol/mol (4.8 per meg). In next step, we will aim for the uncertainty of 0.4 µmol/mol (2 per meg). 2

3 Previous study About ten years ago, Tohjima et al. developed standard gases for O 2 observation by a gravimetric method. The gravimetrically calculated standard uncertainty was 1.6 µmol/mol. The measured variability of O 2 concentration was 2.9 µmol/mol which was larger than gravimetric uncertainty (1.6 µmol/mol). This difference means that there was unknown uncertainty. They pointed out that the unknown uncertainty was derived from change in cylinder weight. This change causes deviation of source gas amounts. Measured variability 2.9 µmol/mol Gravimetric uncertainty Unknown uncertainty 1.6 µmol/mol 3

4 Determination of source gas amount Before Sample cylinder - Reference cylinder = g g g CO 2 /Ar,O 2,N g Sample cylinder Reference cylinder After g g = g Source gas amount is calculated by the weight difference of the sample cylinder before and after filling source gases. A sample cylinder weight is estimated by weight difference between the sample cylinder and the reference cylinder by alternately measuring both cylinders. The filled amounts are actually calculated using the weight differences

5 Measurement method of cylinder weight NMIJ weighing system Weighing a reference cylinder by lowering a arm reference sample kg Reference cylinder Sample cylinder Exchange of a reference cylinder and a sample cylinder by turn sample reference kg Mass comparator: XP26003L(mettler Toledo) Resolution : 1 mg Max: 26 kg Weighing a sample cylinder by lowering the arm sample reference kg 5

6 Significant uncertainty factors 1. Cylinder weighing uncertainty 1. Repeatability of cylinder weighing Evaluation by repeatedly measuring weight difference 2. Reproducibility of cylinder weighing Measuring cylinder weight after cylinder temperature change 3. Leak of source gases from a cylinder valve leak rate is negligible 2. Molar masses of source gases (N 2 and O 2 ) determined by atomic weights calculated using the isotope ratios of N and O based on the corresponding atmospheric value 3. Purities of source gases (CO 2, Ar, O 2 and N 2 ) measuring the purities of source gases 6

7 Repeatability of cylinder weight Result to repeatedly measure weight difference for three days Weight difference (mg) weight difference between both cylinder Standard deviation of cylinder weight for 3 days was 0.8 mg Elapsed time (day) Repeatability of cylinder weight measurement was 0.8 mg 7

8 Equilibrium of cylinder surface conditions The temperature of sample cylinder increases to up to 8 ºC due to adiabatic compression of source gases Nonequilibrium of cylinder surface conditions will cause the deviation of cylinder weight. Room temperature 26 ºC 26 ºC Before filling Sample Cylinder temperature sample kg After filling sample 26 ºC 34 ºC Weight difference (mg) Change of weight difference after filling Nonequilibrium equilibrium kg :00 2:00 4:00 6:00 8:00 10:00 12:00 Elapsed time after filling source gases (h) We measured the equilibrium time of each source gas in order to optimize the measurement method of weight difference. 8

9 Equilibrium time of cylinder surface conditions Change amount the drift amount between the measured weight difference and the weight difference after the equilibrium Change amounts (mg) After nitrogen After oxygen Agter argon After vaccum 0:00 2:00 4:00 6:00 8:00 10:00 12:00 Elapsed time after filling source gases(h) Source gases Ar(CO2/Ar), O 2 N 2 Equilibrium time > 6 h > 10 h These results indicate that it takes a few days to prepare the standard gases. In addition, the equilibrium time may not be constant all the time due to change of temperature and humidity in range of ±0.5 ºC and ±5 % by a season A condition indicating the equilibrium is necessary. 9

10 Factors of the weight change 1. Adsorption water effect weight change by adsorption or desorption of water vapor due to cylinder temperature increase or decrease H 2 O H 2 O H 2 O H 2 O Cylinder temp: 26 ºC (Equilibrium) H 2 O H 2 O Cylinder temp: 34 ºC (Nonequilibrium) H 2 O H 2 O H 2 O H 2 O H 2 O H 2 O kg kg 2. Thermal effect weight change by up and down flow of air cooled or heated by the sample cylinder Cylinder temp: 26 ºC (Equilibrium) Cylinder temp: 34 ºC (Nonequilibrium) kg kg The change amount of weight difference between both cylinders depend on their temperature difference because only temperature of a sample cylinder changes in preparation process. 10

11 Change amounts (mg) Condition indicating the equilibrium y = x Sample cylinder temperature lower higher (Sample Temp.) (Reference Temp.)(K) Change amounts were proportional to temperature difference between a sample cylinder and a reference cylinder. Although thermal effect depend on only temperature difference, adsorption water effect depend on temperature, humidity, surface area. If the proportional relation is caused by adsorption water effect, its slope will depend on temperature,humidity, and surface area. The change amounts are less than repeatability (0.8 mg) if there is below the temperature difference of 0.06 ºC. 11

12 Condition indicating the equilibrium Change amounts (mg) C, 50 % 26 C, 50 % 29 C, 50 % 26C, 35% 26 C, 50 % 26C, 65 % 26 C, 80 % difference surface_26c, 50% Sample cylinder temperature lower higher (Sample Temp.) (Reference Temp.)(K) Experiment conditions 1. Air humidity 35 %, 50 %, 65 %, 80 % 2. Air temperature 22 ºC, 26 ºC, 29 ºC 3. Surface area Using two type of cylinder Two type cylinder used in this experiment We didn t detect humidity, temperature and surface area dependences 12

13 Condition indicating the equilibrium These results mean that the change in the sample cylinder weight is caused by thermal effect. It is important that there is no difference of temperature between the sample cylinder and the reference cylinder when we weigh the cylinders. We determined measuring the weight difference between both cylinders after the temperature difference became negligible. 13

14 Reproducibility of cylinder weight Weight difference (mg) After cooling in filling room After heating at 60 ºC SD:0.4mg Measurement order Reproducibility (0.4 mg)<repeatability (0.8 mg) 14

15 Budget table of weighing uncertainty Uncertainty budget of cylinder weight measurement Component Repeatability Reproducibility Leak Total Standard uncertainty 0.8 mg 0.0 mg 0.0 mg 0.8 mg Typical uncertainty budget of O 2 concentration Component Standard uncertainty of cylinder weight measurement was 0.8 mg. Main factor of total uncertainty was the uncertainty of source gas masses. Standard uncertainties of O 2 concentrations were 0.6 ppm to 0.8 ppm. Standard uncertainty amounts of source gases Molar masses of source gases Purities of source gases Total 0.67 µmol/mol 0.03 µmol/mol 0.03 µmol/mol 0.67 µmol/mol 15

16 Residual from calibration line (ppm) cylinder N 2 (µmol/mol) O 2 (µmol/mol) Ar (µmol/mol) CO 2 (µmol/mol) CPB ± ± ± ± 0.06 CPB ± ± ± ± 0.06 CPB ± ± ± ± 0.06 CPB ± ± ± ± 0.06 CPC ± ± ± ± 0.06 CPB ± ± ± ± Evaluation of preparation error Results evaluated using a paramagnetic analyzer Gravimetric O2 concentration (µmol/mol) Residual from calibration line : less than ±1 µmol/mol Measured residuals The results means that uncertainty factors were valid. Gravimetric uncertainty 16

17 Comparison with previous O 2 /N 2 data Tohjima et. al.(2005) Observation value of Scripps Value of Tohjima et. al.(2005) δ(o 2 /N 2 ) in 2000 based on absolute O 2 /N 2 ratio at Hateruma in 2000 (Tohjima et al. (2005)) δ ( O N ) ( O2 N2 ) ( O N ) = standard This work Value of this work The difference between δ (O 2 /N 2 ) at Hateruma in 2015 determined using standard gases prepared in this work and absolute O 2 /N 2 value at Hateruma in 2000 (Tohjima et al. (2005)) Observation value of Scripps Observation results of Scripps at La Jolla. (Keeling and Manning, 2014) The difference between values of Tohjima et. al. and this work were consistent with the observed δ (O 2 /N 2 ). 17

18 Summary We prepared the O 2 standard gases with standard uncertainty of 0.8 µmol/mol by a gravimetric method. We found out that change amounts of cylinder weight depended on only temperature difference between a sample cylinder and a reference cylinder. We optimized the method to measure the sample cylinder weight after source gas filling. We showed that uncertainty factors were valid by comparing the residuals and the gravimetric uncertainty. We showed that the O 2 /N 2 ratios in standard gases prepared in this work were consistent with the values of previous studies. 18

19 Thank you for your attention! 19

20 Uncertainty for cylinder weihgt determination We can achieve the cylinder mass determination with sufficient accuracy, as long as temperature and humidity of ambient air are stable and in equilibrium with balance body and cylinders. The conditions around our weighing system are sufficiently stable because temperature and humidity in weighing room are controlled with 26 ± 0.3 ºC and 45 ± 2 %. It is difficult to achieve the equilibrium of ambient air with the sample cylinder in short time, because the temperature of the sample cylinder increases according to adiabatic compression when source gases are filled into the sample cylinder. In addition, we have to fill source gases into the sample cylinder in the other room (filling room) of which air conditions are different from the weighing room. We examined the equilibrium time to precisely weigh it. 20

21 Leak from cylinder valve Gases permeate through seat materials of high pressure valves which make from resin, for example, PTCFE, PTFE etc.. We confirmed the leak by monitoring the cylinder mass at pressure of 10MPa. Change of cylinder mass (mg) Leak rate: 0.10 mg /day ( 10MPa) Elapsed time (day) CPC16184 Change of cylinder mass (mg) Leak rate: mg/day (10MPa) Elapsed time (day) CPC00555 The gas leak didn t contribute to the weighing uncertainty although leak rates were different among cylinder valves 21

22 Atomic weight of source gases Atomic weight of O and O were determined by measuring precisely the difference of isotope ratios from the corresponding atmospheric value using IRMS. Isotope ratio of N and O in the source gases Isotope Mass 14 N Isotope abundance in source gases Isotope ratio to atmosphere 15 N δ 15 N = O O δ 17 O= O δ 18 O= 7.5 Atomic weight of source gases Source Atomic weight of nitrogen Standard uncertainty Atomic weight of oxygen Standard Uncertainty IUPAC Source gases

23 Compare of Ar concentration Tohjima et. al. Park et. al. This work Sample Ar concentration Reference Institute Hateruma (2000) 9333±2 µmol/mol Tohjima et. al. (2005) NIES Anmyeon (2002) Niwot Ridge (2001) 9332±3 µmol/mol Park et. al. (2004) KRISS Hateruma (2015) 9334±0.6 µmol/mo This work NMIJ Values less than ± express standard uncertainty 23

24 Calibration of balance (AX2600) 傾き Slope /4/ /5/2015 6/4/2016 2/2/2017 Date 24

25 Adsorption water effect 58.4cm Cylinder surface Area: 3700 cm 2 Diameter of H 2 O: cm Area of H 2 O : cm 2 Avogadro number: Molar mass of H 2 O : 18 g/mol 17.5cm Adsorption water amount: 0.16 mg ステンレス表面の水分吸着量 µg/cm µg/cm 2 高圧容器の表面積に換算 mg-1.37 mg (S. Mizushima et al., Metrologia, 2007) 25

26 Evaluation of preparation error O 2 standard gases in this work Cylinder number N 2 O 2 Ar CPB ± ± CPB ± ± ±0.7 CPB CPB ± ± ±0.6 CPB Residual (µmol/mol)) 1.5 Results evaluated using a paramagnetic analyzer O 2 concentration (µmol/mol) Within ±1 µmol/mol Measured variability is comparable to the gravimetric uncertainty. 26

27 Free convection effect on weighing value 1 Drift amount of weighing values(µg) cm 17.5cm Influence of free convection (M.Schreiber et al.,metrologia 2015) 58.4cm Force with vertical surface of cylinder Δm = k 1 A v d 1/4 ΔT ΔT = mg/k Force with Horizontal surface of cylinder Δm=k 2 A h dδt = mg/k -1.5 Temperature difference between weight and ambient air (K) Experiment of 1kg standard weight (M.Glaser and J.Y.Do,Metrologia 1993) k 1 = g cm -9/4 K -3/4 k 2 = g cm -3 K -1 27

28 回帰直線からの残差 (ppm) 巡回比較実験の酸素標準ガス cylinder N 2 O 2 Ar CO 2 CPB ± ± ± ± 0.06 CPB ± ± ± ± 0.06 CPB ± ± ± ± 0.06 CPB ± ± ± ± 0.06 CPC ± ± ± ± 0.06 CPB ± ± ± ± 0.06 磁気式酸素計の測定結果 Set3 dev. From fitted line:±0.47 ppm Set1 dev. From fitted line:±0.62 ppm 質量比混合法で決定した酸素濃度 (µmol/mol) 質量比混合法で決定した酸素濃度 (µmol/mol) ( O N ) δ 質量分析計の測定結果 ( O2 N ) ( O2 N2 ) standard 標準の酸素濃度 2 2 = 10 y = x y = x

29 Calibration of mass spectrometry y = ( ±0.0004)x O 2 /N 2 measured with IRMS O 2 /N 2 for standard gases The sensitivity (span) of mass spectrometry deviates 0.3% O 2 /N 2 for standard gases 29

30 Change amounts (mg) Condition indicating the equilibrium y = x C, 50 % 26 C, 50 % 29 C, 50 % 26C, 35% 26 C, 50 % 26C, 65 % 26 C, 80 % difference surface_26c, 50% Sample cylinder temperature lower higher (Sample (Sample Cylinder Cylinder Temp.) Temp.) (RoomTemp.)(K) Temp.)(K) If the proportional relation is caused Change by of water cylinder adsorption weight effect, is its caused slope by depend water adsorption on humidity, or temperature, desorption and and by surface up and area. down We flow examined of air cooled these or dependences heated by the as sample following cylinder. experiment conditions Experiment The relation condition between weight 1. change Humidity and dependence temperature of cylinder 35 %, was 50 %, examined. 65 %, 80 % 2. Temperature dependence 22 ºC, 26 ºC, 29 ºC Change amounts are 3. Surface area dependence proportional two type of cylinder to temperature difference between a sample cylinder and room temperature. This we didn t results detect identify humidity, that we temperature can measure and surface cylinder area weight dependences less than repeatability (0.8mg)

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