^{18}O/

^{16}O, and

^{17}O/

^{16}O Isotope Ratios in Water by Laser Absorption Spectroscopy at 2.73 μm

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This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license (

A compact isotope ratio laser spectrometry (IRLS) instrument was developed for simultaneous measurements of the D/H, ^{18}O/^{16}O and ^{17}O/^{16}O isotope ratios in water by laser absorption spectroscopy at 2.73 μm. Special attention is paid to the spectral data processing and implementation of a Kalman adaptive filtering to improve the measurement precision. Reduction of up to 3-fold in standard deviation in isotope ratio determination was obtained by the use of a Fourier filtering to remove undulation structure from spectrum baseline. Application of Kalman filtering enables isotope ratio measurement at 1 s time intervals with a precision (<1‰) better than that obtained by conventional 30 s averaging, while maintaining a fast system response. The implementation of the filter is described in detail and its effects on the accuracy and the precision of the isotope ratio measurements are investigated.

The stable isotopes of water (in the vapor or liquid phase) are powerful tracers for the study of the hydrological cycle, climate change, ecological process and paleoclimatic archives (see, e.g., [_{2} and equilibration with CO_{2} is commonly used for the determination of the ^{2}H (or D) and ^{18}O isotope ratios [^{17}O^{12}C^{16}O and ^{16}O^{13}C^{16}O molecules appear in the same mass channel, accurate and direct determination of the isotope ratio H_{2}^{17}O/H_{2}^{16}O is practically impossible. This ratio is usually inferred from the natural relation between the ^{17}O and ^{18}O abundance ratios [_{2} after fluorination of the water sample [

Measurements of the stable isotope ratio by optical spectroscopy, in particular by laser absorption spectroscopy (LAS), have attracted a growing interest in recent years [

In this paper, we report on the development of a compact IRLS instrument for simultaneous real-time measurements of the D/H, ^{18}O/^{16}O and ^{17}O/^{16}O isotope ratios in water by laser absorption spectroscopy at 2.73 μm. The 2.7 μm fundamental stretching mode is about one order of magnitude stronger than the band at 1.4 μm, and even the bending mode near 6.7 μm is not much stronger than that the 2.7 μm band. More important is that the selected absorption lines at 2.73 μm for measurements of water isotopologues (H_{2}^{18}O, H_{2}^{16}O, H_{2}^{17}O and HDO) may have relatively similar line strengths and similar ground state energies (

In the present work, special attention is paid to the spectral data processing by use of digital filtering techniques to improve the measurement precision. Oscillatory structure on spectrum baseline affecting the measurement precision was analyzed. Fourier analysis of spectral residuals resulting from a fit was performed, which enabled the identification and filtering of the noise in spectral signals with the help of Fourier filter. The paper also provides a detailed description of Kalman filtering of the data, as recently introduced by us to the field of isotope ratio measurements [

According to the Beer-Lambert law of linear absorption, the absorbance _{0}(^{3}), ^{2}/mol), and _{I}^{−1}) can be written as:
^{−1}/(mol·cm^{−2}) and _{0} = 296 K, E_{0} the lower level energy expressed in cm^{−1},

The isotope ratio can thus be determined from the ratio of the integrated areas _{I}_{2}^{17}O, H_{2}^{18}O, or HDO), _{2}^{16}O), and

The relative deviation of the isotope ratio in water with respect to the international standard reference known as Vienna Standard Mean Ocean Water (VSMOW), is expressed in terms of the δ-value:
_{vsmow}_{vsmow}^{18}O, 0.0003799 for ^{17}O and 0.00015576 for ^{2}H [^{18}O, δ^{17}O and δ^{2}H values, and thus indistinguishable _{vsmow}^{18}O, ^{17}O and ^{2}H [

In the present work, the laser instrument was calibrated against the working standards GS-49 (δ^{18}O = 0.39‰, δ^{17}O = 0.21‰, and δ^{2}H = 1.7‰, with respect to VSMOW, determined by repeated IRMS analyses at the Center for Isotope Research of the University of Groningen). The accuracy of the laser instrument was evaluated by measurement of another working standard GS-42 with different isotopic composition (δ^{18}O = −24.62‰, δ^{17}O = −13.1‰, and δ^{2}H = −187.7‰). Bottled water (Vittel, France) was used as unknown sample material.

Selection of suitable absorption lines for water isotopic ratio measurements is one of the most important aspects in instrumental design since the choice of absorption lines has a direct impact on the instrumental performance in terms of measurement sensitivity, precision and selectivity.

Measurements of the isotopic ratios δ^{18}O, δ^{17}O, δ^{2}H of the stable isotopologues of water require probing of absorption lines of the four isotopologues H_{2}^{18}O, H_{2}^{16}O, H_{2}^{17}O and HDO within the laser tuning range. It is very desirable that the used absorption lines exhibit similar absorption depths at natural abundance with an absorption intensity as large as possible (for high sensitivity and high precision measurements), are free from interference from the same or other species (high selectivity consideration), and have similar ground state energy (in order to minimize the effects of temperature-dependent line intensities). The spectral region near 2.73 μm, covered by recently commercially available DFB lasers, meets these requirements to a high degree. Parameters of the four molecular lines selected in the present work are summarized in

The line absorption strength depends on the population of the ground state level and this population in turn depends on the temperature. Temperature drift during measurements may introduce a systematic error in the isotope ratio determination. The temperature coefficients defined as ΔS/S(T_{0}) for the selected lines are listed in ^{18}OH, H^{16}OH, H^{17}OH and H^{16}OD respectively. Therefore, realization of a temperature stability better than 0.1 K is essential for high precision measurements.

The experimental set-up, mounted on a 50 × 70 cm^{2} optical breadboard, is depicted in _{2} lens F1 (f = 200 mm) and an off-axis parabolic mirror PM2 (EFL = 50 mm).

The collimated beam was then divided into two parts. The first part (∼8%) was reflected by a beam splitter (CaF_{2}) and directed to a homemade Fabry-Perot etalon for spectral metrology. The frequency scale was linearized by means of the interference fringes produced by the etalon consisting of two air-separated uncoated CaF_{2} plates with a free spectral range (FSR) of ∼0.0283 cm^{−1}. Positions of the H_{2}O vapor absorption lines provided by the HITRAN database [_{2} lens F2 onto a LN_{2} cooled HgCdTe detector (J15D22-M204-S01M-60). A home-built bridge circuit was employed to realize DC coupling of the HgCdTe detector to a low noise preamplifier (Model 5113, EG&G, Albuquerque, NM, USA).

The signal background (corresponding to “laser off”), determined by the dark current level of the detector, was found to be fluctuating by about 1% over a 1 h time interval. This probably was due to variation of the detector temperature or its DC power supply. In order to correctly retrieve absorption spectra, a beam shutter (Thorlabs, SH05) was placed before the detector for background level acquisition at the beginning of each absorption spectral scan.

The laser frequency was periodically scanned at a rate of 10 Hz across the absorption lines of the water isotopologues H^{16}OH, H^{17}OH, H^{18}OH, and HDO by means of a triangular wave voltage. The output signal from the preamplifier was digitized with a laptop using a 16-bit analogue/digital data acquisition card (DAQ Card-6036E, National Instruments, (Austin, TX, USA) controlled with a Lab Windows-based program.

Careful attention has been paid in the present work to sample mass effects and sample memory effects that affect the determination of the isotopic ratios, as discussed in detail by, among others, Kerstel _{2}O molecule number density of 1.1 × 10^{17} mol/cm^{3} in the cell. Besides temperature fluctuations, variations in sample pressure inside the cell, resulting from sample injection by a syringe through a silicon membrane, will have a strong effect on the precision and accuracy of the measurements. Although constant volume was used for both sample and reference standard injections in our experiments, variances in the H_{2}O number density in the cell may occur due to H_{2}O leakage through the silicon membrane, over-tightening or under-tightening of the silicon membrane. In order to minimize the impact of this effect on the final results, only those injections resulting in a gas cell pressure within ±0.1 mbar of 4.5 mbar were accepted. Furthermore, in order to avoid sample memory effects due to the “stickiness” of water on the gas cell wall, the first three injections were discarded for each isotope ratio determination.

The temperature of the gas cell was actively controlled to 30 °C by the use of a heater band, and maintained constant within ±0.1 °C by a PID controller. The cell temperature was monitored with calibrated platinum resistors (Pt100) with an accuracy of 0.03 °C and a precision of 0.01 °C. No temperature gradient along the cell axis was observed within the measurement precision of the temperature sensors.

In our experiment, a slight drift of the laser wavelength with time has been observed. In order to minimize the effects of the instrumental drift on the determination of _{I}

_{2}O isotopologues around 2.73 μm. The spectrum, resulting from an average of 10 laser scans, was recorded at a pressure of 4.5 mbar and a temperature of 30 °C. Spectral data processing for isotopic composition determination is discussed in detail in the following subsections.

In order to determine the integrated absorbance _{I}^{18}OH, H^{16}OH, H^{17}OH and H^{16}OD, two H^{16}OD lines at 3663.2250 cm^{−1} and 3663.3879 cm^{−1} on either side of the H^{17}OH line were also taken into account in the spectral fitting program. As indicated by the fit residuals shown in _{I}. In order to significantly minimize the uncertainty in the determination of the integrated area under the absorption features, a more sophisticated model could be adopted in the future (for instance, the speed-dependent Galatry profile), to take into account the narrowing due to the speed-dependence of relaxation rates as well as the averaging effect of velocity-changing collisions.

A periodic oscillatory structure on the baseline was observed in both the Voigt (_{0}(m) is a 4th-order polynomial representing the laser power ramp, A_{k} and B_{k} are the oscillation amplitudes, and ω_{k} is the oscillation frequency determined from the Fourier transform. In our experiments, only the most prominent three frequencies have been taken into account: ω_{1}= 3.662 × 10^{−4} Hz, ω_{2} = 4.882 × 10^{−4} Hz and ω_{3} = 7.324 × 10^{−4} Hz. G_{i}(m) is the Galatry function which describes the direct absorption line shape for the H^{18}OH (3662.9196 cm^{−1}), H^{16}OH (3663.0452 cm^{−1}), H^{16}OD (3663.2250 cm^{−1}), H^{17}OH (3663.3213 cm^{−1}), H^{16}OD (3663.3879 cm^{−1}) and H^{16}OD (3663.8419 cm^{−1}) lines. The Galatry profile can be defined by:
_{I}^{−1}), _{0} (cm^{−1}) is the line center frequency, _{D}^{−1}) the Doppler half-width, _{L}^{−1}) the collisional half-width, and _{C}^{−1}) representing the average effect of collisions on Doppler broadening.

To obtain the integrated absorbance _{I}_{fit}_{D}_{I}_{0}, the Lorentzian pressure broadening width _{L}_{C}

Though application of Fourier filtering can improve the measurement precision by removing oscillatory structure from the baseline, it is hard to completely account for the exact baseline structure with only three Fourier components. In fact, the residuals of

The measurement precision is usually affected by the instrument instability and measurement errors related to sample handing and injection (e.g., incomplete evacuation of the gas cell between two consecutive measurements will lead to memory effects and thus affect the measurement precision). Another important limiting factor to achieving high precision is the signal-to-noise ratio (SNR) of the spectral data. In LAS, high SNR can be obtained by enhancing the signal (by selecting stronger absorption lines and using long absorption path length or cavity enhanced spectroscopy) and reducing the noise (by averaging N laser scans and using modulation or/and balanced-beam detection techniques). With the signal averaging approach, the optimal averaging number N, limited by the stability of the instrument, can be determined by an Allan variance analysis [^{18}O, 6.6‰ for δ^{17}O, and 8.0‰ for δ^{2}H of a bottled water (Vittel, France).

In order to further improve the measurement precision, the 1 s raw δ values are further averaged. Allan variance analysis has been performed to determine the optimum averaging number. As can be seen in ^{18}O, 1.1‰ for δ^{17}O, and 1.5‰ for δ^{2}H, respectively.

Though signal averaging enables one to improve the measurement precision [

Using a linear stochastic difference model, the true isotope ratio _{k}_{+1} at time _{k}

At time _{k}_{k}_{k} and _{k} are uncorrelated random variables related to the process variability and the measurement noise with corresponding covariance of σ^{2}_{w}^{2}_{v}

The time update procedure projects forward in time current isotope ratio and error variance estimates to obtain

The measurement update incorporates a new measurement into the ^{−}_{k}_{+1} is the predicted isotope ratio estimate at step k+1, _{k}^{−}_{k}_{+1} is _{k}_{k}_{k}^{−}_{k}

In practical applications, the measurement noise σ^{2}_{v}^{2}_{w}

Information on the measurement noise variance σ^{2}_{v}^{2}_{w}^{2}_{v}^{2}_{w}^{2}_{v}^{2}_{w}

The parameter q is known as the filter tuning parameter. The choice of q depends on the particular instrument and its application environment. For a larger q value, the system's response time is longer to follow real-time variation in isotopic composition. Conversely, the filtering is less efficient in removing shot-to-shot real-time noise when a smaller q value is used. ^{18}O is toward a steady state value for q > 800. For δ^{17}O and δ^{2}H, the decreasing trend slowed down for q > 800, while toward a steady state value for q > 2000. This may be explained by small drift in δ^{17}O, δ^{2}H resulting a Gaussian distribution, high q value is needed to get steady state, because the larger q value, the more efficient is the filtering δ-value fluctuations due to random noise. In the present work, a value of q = 800 was chosen as a compromise between fast temporal response and high filtering efficiency (thus high measurement precision) for the developed laser instrument. In the present work, a value of q = 800 was chosen as a compromise between fast temporal response and high filtering efficiency (thus high measurement precision) for the developed laser instrument. In our experiment, σ^{2}_{v}^{2}_{w}^{2}_{v}

The effects of the Kalman filtering on the measurement accuracy were investigated in the present work. For this purpose, the working standard GS-42 with well known isotope ratios was used. The relative deviation in water (GS-42) isotope ratio measurement accuracy, (^{18}O and δ^{2}H are almost constant in the range of ±1% with increased q-value. Though the relative deviation for δ^{17}O varies evidently with q-value, the values are under a range of ±0.7%. The Kalman filtering is more effective to remove fluctuations due to random white noise. However, apart from the measurement random noise, temperature fluctuations of the gaseous sample and baseline drift may result in drift over time in δ values, which cannot be removed by the Kalman filtering. Because of slow drift in δ^{17}O values, the fluctuation of δ^{17}O mean for different q-value is almost two times larger than δ^{18}O and δ^{2}H. The maximal differences in relative deviation for the measurement accuracy are 1.6 × 10^{−3} for δ^{18}O, 1 × 10^{−2} for δ^{17}O, and 5 × 10^{−4} for δ^{2}H in

The deviation of the measured values from the reference was −0.2‰ for δ^{18}O, 1.4‰ for δ^{17}O and 1.5‰ for δ^{2}H. The large measurement deviation, especially for δ^{17}O and δ^{2}H, would be mainly caused sample mass effects. Though careful attention has been paid to sample amount effects in the present work, more precise control of injection volume and sample pressure was still needed. The 1σ standard deviation is 1.2‰ for δ^{18}O, 0.7‰ for δ^{17}O, and 1.4‰ for δ^{2}H for five injections of water (GS-42). Moreover, the precision of 0.8‰ for δ^{18}O, 0.6‰ for δ^{17}O and 0.9‰ for δ^{2}H with 1 s Kalman filtering also had certain influence on the measurement deviation. It should be noted that the δ^{17}O value in water could not be directly determined with IRMS; the value given in the table is not a measured value, but was calculated based on a natural relation between the ^{17}O and ^{18}O abundance ratios [^{17}O = (1 + δ^{18}O)^{0.5281} − 1.

The measurement precisions using 1 s Kalman filtering (q = 800) are given in parentheses in ^{1/2}). We converted the standard deviation into SE with the same N values for the reason of comparison with the standard working reference GS-42.

^{18}O, 0.57‰ for δ^{17}O, and 0.91‰ for δ^{2}H, with a measurement time of 1 s. Comparison of the measurement precisions is given in

It is also worth noting, as can be seen in

We have described a compact IRLS instrument for simultaneous measurements of the D/H and ^{18}O/^{16}O, ^{17}O/^{16}O isotope ratios in water by laser absorption spectroscopy at 2.73 μm. We demonstrated the potential of application of Kalman filtering for fast and high precision isotope ratio measurements. The measurement precision achieved at 1 s Kalman filtering time intervals is better than that obtained by conventional 30 s averaging. The Kalman filter can be optimized to filter out the maximum amount of shot-to-shot real-time noise while following the real variation in measured physical quantity. The impact of the Kalman filtering on the measurement precision and accuracy were investigated. The measurement precision was improved clearly with high q value, while measurement accuracy was less impact by the Kalman filtering and mainly determined by calibration with known standard materials. The Voigt and Galatry profiles were used to investigate the possible influence of the choice of the line shape model on the determination of the integrated absorbance. The Galatry profile fit resulted in a smaller residual than using the Voigt profile. However, relatively big residuals around absorption peaks are still evidenced even using Galatry model, which leads to a bigger uncertainty in the determination of the integrated area A_{I}. A more sophisticated model should be adopted in the future (for instance, the speed-dependent Galatry profile), to take into account simultaneously the narrowing due to the speed-dependence of relaxation rates and to the averaging effect of velocity-changing collisions. Fourier analysis of fitted spectral residuals allowed for determination of the Fourier frequency components in the undulation structure on the baseline. Reduction of up to three times in standard deviation in the isotope ratio determination was obtained by application of Fourier filtering to remove the undulation structure from spectrum baseline. However, a further reduction by a factor of 3.7 was obtained by the elimination of the electrical noise source. Measurement accuracy comparison of isotope ratios in water reference GS-42 between IRLS and IRMS was performed. Measured value of δ^{18}O was more close to reference value, while measured values δ^{17}O and δ^{2}H presented a deviation from reference value. The measurement deviation would be mainly caused sample mass effects. Though careful attention has been paid to sample amount effects in the present work, more precise control of injection volume and sample pressure was still needed. Moreover, the precision of δ^{18}O, δ^{17}O and δ^{2}H with 1 s Kalman filtering also had certain influence on the measurement deviation.

Further improvements in isotope ratio determination precision and accuracy can be envisaged as follows: (1) precise control of injection volume and sample pressure; (2) real-time calibration by alternating the introduction into the cell of a reference and the sample [

This work is mainly supported by the IRENI program of the RĂ©gion Nord-Pas de Calais. The support of the Groupement de Recherche International SAMIA between CNRS (France), RFBR (Russia) and CAS (China) is acknowledged. This research was supported by National Natural Science Foundation of China (No. 41265011), International Cooperation of Jiangxi Province of China (No.20132BDH80006) and Natural Science Foundation of Jiangxi Province, China (No. 20114BAB212008). We acknowledge helpful discussions with Peter Werle and Frank Tittel on signal processing and the Kalman filtering technique and with Erik Kerstel on isotope issues. The authors thank Francis Leveugle for prompt technical help.

The authors declare no conflict of interest.

^{18}O/

^{16}O Isotope Ratio for Atmospheric and Ecological Applications

^{2}H and δ

^{18}O measurement for water and volatile organic compounds by continuous-flow pyrolysis isotope ratio mass spectrometry

^{18}O content of waters from natural sources

^{17}O and δ

^{18}O isotope measurements in water

^{17}O/

^{16}O and

^{18}O/

^{16}O of O

_{2}in H

_{2}O

^{2}H/

^{1}H,

^{17}O/

^{16}O, and

^{18}O/

^{16}O isotope ratios in water by means of tunable diode laser spectroscopy at 1.39 μm

^{2}H/

^{1}H,

^{17}O/

^{16}O, and

^{18}O/

^{16}O isotope abundance ratios in water by means of laser spectrometry

^{18}O/

^{16}O,

^{17}O/

^{16}O in and out of clouds map dehydration pathways

^{2}H and δ

^{18}O) measurements in atmospheric moisture using an optical feedback cavity enhanced absorption laser spectrometer

^{2}H,

^{17}O, and

^{18}O) spectrometer based on optical feedback cavity-enhanced absorption for

^{18}O/

^{16}O measurements of microliter natural water samples

^{18}O, δ

^{17}O, and

^{17}O excess in Water by Off-Axis Integrated Cavity Output Spectroscopy and Isotope Ratio Mass Spectrometry

^{18}O measurements by off-axis integrated cavity output spectroscopy

^{18}O and δ

^{2}H values of fluid inclusion water in speleothems using cavity ring-down spectroscopy compared with isotope ratio mass spectrometry

^{17}O

_{excess}measurements using laser-current tuned cavity ring-down spectroscopy

^{18}O) in water samples using cavity ring down spectrometry: Application to bottled mineral

_{2}O Isotopologue Ratios by Laser Absorption Spectroscopy at 2.73 μm

_{2}isotopologue spectrometer with a difference-frequency generation laser source

_{2}and

^{13}CO

_{2}/

^{12}CO

_{2}isotopic ratio using a lead-salt laser diode spectrometer

Optical layout. PM1 and PM2: parabolic mirrors with effective focal length of 25 mm and 50 mm, respectively; F1 and F2: lenses of focal length of 200 mm and 50 mm, respectively.

Experimental spectrum of H_{2}O isotopologues around 2.73 μm. At the bottom is shown the fringe from an etalon that was used for frequency metrology in combination with the H_{2}O line positions provided by the HITRAN database.

H_{2}O isotopologue absorption spectrum measured in this work (raw data). Spectral data were fitted to Voigt and Galatry profiles. Residuals resulting from each fit are shown: (

Discrete Fourier transform of the fit residuals resulting from different spectral fitting algorithms.

The ^{18}O/^{16}O, ^{17}O/^{16}O and D/H isotope ratios obtained with different baseline modelings. Left panel: baseline described with a 4th-order polynomial. Central panel: baseline modeled using a 4th-order polynomial and a Fourier series function (with three main frequency components). Right panel: no oscillation structure in original spectrum baseline.

Measurement results of a bottled water (Vittel, France). The upper three panels show raw measurement of the δ-value for ^{18}O, ^{17}O, ^{2}H (dots) and the corresponding Kalman filter output for a q-value of 800 (lines). The Allan variance plotted in the lower panel shows an optimal averaging time of ∼30 s for the present IRLS system.

(

Comparisons of line intensities, ground state energies and temperature coefficients between different IRLS operating at different wavelengths in the infrared spectral region.

^{−1}) |
^{−23} cm·mol^{−1}) |
^{−1}) |
^{−1}) | ||
---|---|---|---|---|---|

H^{18}OH |
7183.5858 | 0.62 | 733.7 | 6.9‰ | |

[ |
H^{16}OH |
7183.6858 | 0.31 | 661.6 | 5.7‰ |

H^{17}OH |
7183.7354 | 0.12 | 95 | −3.4‰ | |

H^{16}OD |
7183.9727 | 0.035 | 156.4 | −2.5‰ | |

H^{18}OH |
1483.9261 | 8.4 | 550.5 | 4.1‰ | |

H^{16}OD |
1484.1064 | 2.3 | 225.9 | −1.3‰ | |

[ |
H^{16}OH |
1484.2573 | 1.8 | 1899.0 | 26.2‰ |

H^{17}OH |
1484.5109 | 2.0 | 205.5 | −1.6‰ | |

H^{18}OH |
1484.9716 | 10.0 | 325.2 | 0.3‰ | |

H^{16}OH |
1485.1336 | 6.2 | 1907.6 | 26.4‰ | |

H^{18}OH |
3662.9196 | 2.1 | 398.3 | 1.5‰ | |

[ |
H^{16}OH |
3663.0452 | 8.5 | 586.4 | 4.6‰ |

H^{17}OH |
3663.3213 | 7.2 | 224.3 | −1.4‰ | |

H^{16}OD |
3663.8419 | 1.2 | 100.4 | −3.4‰ |

IRLS ^{1/2}, is the standard error of the mean values for N measurements. IRLS results were obtained using 1 s Kalman filtering with q = 800.

^{18}O (SE) |
^{17}O (SE) |
^{2}H (SE) | ||
---|---|---|---|---|

IRMS (CIO) | 3 | −24.62 (0.02) | −13.1 (0.1) | −187.7 (0.3) |

IRLS (present work) | 5 | −24.83 (0.50) | −11.7 (0.3) | −186.2 (0.6) |

Measurement precision comparison: Raw measured δ-value from the average of 10 laser scans in 1 s, data from conventional averaging of 30 measured δ-values within the optimal average time (30 s) determined by an Allan variance analysis, and 1 s Kalman filtering data with q = 800.

Method / Time | Measurement precision | |||||
---|---|---|---|---|---|---|

δ^{18}O (SD) |
δ^{17}O (SD) |
δ^{2}H (SD) | ||||

Raw measurement / 1 s | 7.8 ‰ | 6.6 ‰ | 8.0 ‰ | |||

Kalman filtering / 1 s | 1.4 ‰ | 1.1 ‰ | 1.5 ‰ | |||

Averaging 30-δ / 30 s | 0.8 ‰ | 0.6 ‰ | 0.9 ‰ |