Skip to main content
NCBI home page
ACS Omega logoLink to ACS Omega
. 2021 May 21;6(22):14476–14480. doi: 10.1021/acsomega.1c01434

New Analytical Method for Measuring the Atomic Weight of Neon Using Gas Chromatography with a Thermal Conductivity Detector

Jeong Eun Kim †,‡, Inseok Yang †,‡, Dong Min Moon §, Jin Seog Kim §, Kiryong Hong §,*
PMCID: PMC8190878  PMID: 34124470

Abstract

graphic file with name ao1c01434_0005.jpg

The atomic weights of neon (Ne) gases were measured by gas chromatography with a thermal conductivity detector (GC-TCD). High-purity neon gas was used as the carrier and sample gases in this study, which is different from typical GC analysis. The peak signals from the GC-TCD appear when the thermal conductivity between the sample and carrier gases is different. In most gaseous molecules, the thermal conductivity has been assumed to be the same if the chemical species is the same. However, the thermal conductivity of neon gases shows different values among several manufacturers, because the relative abundance of the 22Ne isotope, which is quite large (∼10% in atmospheric neon), varies due to the mass fractionation during air separation. We identified the atomic weights of seven neon gases. Additionally, the absolute isotope ratios of all neon gases were measured using a magnetic sector type gas/mass spectrometer. The atomic weights of the seven neon gases were compared with the results obtained from GC-TCD, and the results agreed with each other within the expanded uncertainty (k = 2).

Introduction

Atomic weight is determined by two components: the isotope abundance and atomic mass of the elements that constitute the molecule. Numerous research groups have reported the molecular weight and atomic weight to be the basic physical properties of elements.1−7 The International Union of Pure and Applied Chemistry (IUPAC) reviews historical and current reported values and publishes a biennial report on the standard atomic weights of various elements. Because atomic or molecular weights are used for many studies in various ways, the studies on atomic weights continue to this day.

In many studies, the molecular weights or isotope ratios of noble gases are used, and neon (Ne) is widely applied in geology and cosmochemistry.8−11 For example, the production rates of stable 21Ne in cosmic rays on the surface of the Earth can be obtained by determining the production rate ratio of 21Ne in quartz.11 These ratios are then used to determine relevant geomorphological and glaciological information. Neon is also used as a material in the study of thermometry. The triple-point temperature of neon (Ttp-Ne) is one of the fixed-point temperatures defined by the International Temperature Scale of 1990 (ITS-90).12 The commercial neon used for research differs from the isotope-amount fraction of neon listed in IUPAC, and the isotope ratio is different depending on the extraction and purification process of neon.13−15 Therefore, there is a slight difference in Ttp-Ne depending on the isotopic composition of the neon used to measure it.13,16−18 Various studies have shown that a discrepancy in the isotope-amount fraction of 22Ne from natural neon sources causes a variation of 0.48 mK in Ttp-Ne.16 This variation is greater than the uncertainty of the realization of Ttp-Ne for one specific isotopic composition. Since the definition of ITS-90 was amended to describe the exact isotopic composition of neon to be used and the correction coefficients when the compositions are different from the defined ones,19 it is necessary to know the isotope ratio for the neon sample used to measure the triple point of neon so that it can be used to realize the ITS-90.20

In nature, neon has three stable isotopes: 20Ne, 21Ne, and 22Ne. The amount of neon is ∼18 μmol in one mol of atmosphere, and the isotope-amount fraction of 21Ne (21x) is 0.0027, the isotope-amount fraction of 22Ne (22x) is 0.0925, and the rest is 20Ne.21

Most methods of measuring atomic or molecular weights involve the use of mass spectrometry to measure the absolute isotope ratios and determine the atomic weights accordingly. The gas phase also uses a precision gas mass spectrometer (gas/MS, Finnigan MAT 271).22−24 However, we propose a new way to determine the atomic weights without using a gas/MS. This is because determining the atomic weight is a challenge in many laboratories, since it is difficult to measure the absolute isotope ratios without a gas/MS and standards assigned with an absolute isotope ratio. Therefore, we determined the atomic weight of neon using gas chromatography with a thermal conductivity detector (GC-TCD), instead of using a gas/MS.

GC is a common type of chromatography used in analytical chemistry for separating and analyzing compounds that can be vaporized without decomposition.25 We used a TCD as a detector for GC. The TCD detects the changes in the thermal conductivity of the column effluent and compares it to a reference flow of carrier gas.26−29 The TCD consists of electrically heated filaments in a sample and reference cells, and the filaments are connected to a Wheatstone bridge circuit. Additionally, all of these are in a temperature-controlled body.26−29 The temperature of the filaments is kept constant in both cells, and the carrier gas always passes around the filament at a constant flow rate.26−29 At this time, only the carrier gas flows in both cells in the same way, and the baseline is formed. Then, the analyte (carrier gas with the sample) enters the sample cell, and the thermal conductivity becomes different from that of the reference cell.26−29 The filament changes the resistance value to maintain a constant temperature at all times, and the resistance value of the filament changes as the analyte passes through the filament.26−29 The resistance change is detected by the Wheatstone bridge, and then it converts the resistance change into an electrical signal. This electrical signal becomes the signal we can visualize.26−29

According to this concept, there is no electrical signal when the sample and carrier gases are the same chemical species. However, this only applies when the same chemical species have the same atomic weights. Therefore, the same chemical species with different atomic weights caused by different isotope ratios will produce electrical signals with different thermal conductivities.

In this study, we measured the atomic weight of commercial neon gases using GC-TCD. The atomic weights obtained by GC-TCD were compared to that obtained by a gas/MS, and the results agreed within their expanded uncertainty (k = 2). We propose a new method for determining the atomic weight of neon when a precise atomic weight is needed.

Results

Isotope Ratio of Various Samples

For validation of the neon atomic weights obtained by GC-TCD, each isotope-amount fraction of the neon samples was measured using a gas/MS (Finnigan MAT 271). The experimental procedure is the same method reported by Lee et al.23 and Min et al.,30 and the details and principles of MAT 271 are also described in the same references. For each sample, the isotope-amount fractions of neon for 21x and 22x were expressed as ratios for 20x, as R(21x/20x) and R(22x/20x), respectively. These isotope ratios of various samples are shown in Figure 1.

Figure 1.

Figure 1

Open in a new tab

Isotope ratios of various neon samples. The circle symbols mean that the isotope ratios had already been measured in 2015 (ref (16)). The square symbols mean that the isotope ratios were measured in this study. The same sample was represented by the same color.

The isotope ratios of all samples were measured in this experiment, including the three reported samples (Ne #1, #2, and #3 in Figure 1).16 The isotope ratios of Ne #1, #2, and #3 (square symbols) agreed with the previously reported ratios in 2015 (circle symbols).16 We also measured the isotope ratios of sample #7, which is used as the carrier gas in GC analyses. As shown in Ne #2 and #6 of Figure 1, commercial neon samples have different R(22x/20x) values because the abundance ratio of 22x varies. Thus, commercial neon has different atomic weights according to the various abundance ratios of 22x, and the difference is measured by GC-TCD.

To measure the absolute isotope ratios using a gas/MS, a mass discrimination factor (fMD) is required to compensate for drift in the equipment. To obtain the fMD, the absolute isotope ratio of the chemical species, like the enriched isotope pure materials or isotope references, is needed. The ratio of the neon isotope reference gas used in this study was 22x/20x = 0.032987(10), which was gravimetrically prepared by using 99.96% 20Ne (Isotech, USA) and 99.9% 22Ne (Iceblick, Romania). The gravimetric preparation procedure of the neon isotope reference gas was described by Min et al.30 and Yang et al.16 The fMD used in this experiment was −0.11%, and each atomic weight of the neon samples was corrected to this value. In addition, the background correction of Ar++ and CO2++ was performed, and it was negligible.

Chromatogram of GC-TCD Data

Figure 2 shows the chromatograms of six samples measured under the conditions shown in Table 2. In this experiment, six peaks at a retention time of 5.0 min represent a response of neon, and a peak at ∼4.65 min represents helium as an impurity in the neon cylinder. In the TCD principle, thermal conductivity is inversely proportional to the square root of molar mass. When the mass difference between the carrier gas and sample gas is large, the difference in thermal conductivity increases as much. As mentioned earlier, the response to the difference in thermal conductivity is expressed as a signal, and in GC, the area between the peak and baseline is usually used as the data. Therefore, a large difference in thermal conductivity indicates a large peak size.

Figure 2.

Figure 2

Open in a new tab

Chromatograms of six neon samples. The negative peaks in front of the neon peaks are peaks of helium present as an impurity in the sample Ne #3 and #5.

Table 2. Analytical Condition for GC-TCD.

instrument GC Agilent 6890A
detector TCD
detector temperature 200 °C
reference flow 30 mL/min
oven temperature 40 °C
column MS5A 9 ft. + 12 ft
carrier gas neon
sample flow 45 mL/min
sample loop 2 cm3 with a restrictor
Open in a new tab

Thus, the differences in the atomic weight are confirmed as proportional to the size of the area. Furthermore, the positive peaks have a heavier atomic weight compared to that of the carrier gas, whereas the negative peaks have a lighter atomic weight.

Calculation of Neon Atomic Weights

As shown in Figure 1, three previously reported atomic weights were consistent with those measured by a gas/MS within the uncertainty in this study. Therefore, a linear regression could be performed using the three reported atomic weights and the areas measured by GC-TCD, as shown in Figure S1a in the Supporting Information. The atomic weights of other commercial neon gases can be determined by putting the value of the area into the linear regression equation (see Figure S1b in the Supporting Information). The atomic weight of the carrier gas was also obtained by interpolating the value of the area, which is zero. This means that the value of the y-intercept is the atomic weight of the carrier gas. The details of the linear regression have been described in the Supporting Information. Furthermore, when the neon atomic weights obtained from GC-TCD and a gas/MS were compared, they were consistent within the uncertainty, as shown in Table 1. The black dashed line represents the linear regression made with three areas (Ne #1, #2, and #3) measured by GC-TCD and their reported atomic weights.16 The square symbols shown in Figure 3 are the atomic weights measured by a gas/MS. Because the atomic weights obtained by GC-TCD were obviously arranged at the dashed line (see Figure S1b in the Supporting Information), these values are not shown in Figure 3.

Table 1. Atomic Weights Determined by a Gas/MS and GC-TCDa.

samples atomic weight (g mol–1)b atomic weight from a gas/MS (g mol–1) area atomic weight from GC-TCD (g mol–1)
#1 20.1793(4) 20.1792(6) 1.154(58)  
#2 20.1830(4) 20.1832(6) 40.32(13)  
#3 20.1796(4) 20.1795(6) 4.364(32)  
#4   20.1790(6) –1.630(80) 20.1790(6)
#5   20.1803(6) 11.17(21) 20.1802(4)
#6   20.1829(6) 41.48(20) 20.1831(4)
#7 (carrier gas)   20.1793(6)   20.1792(4)
IUPAC standard valuec 20.1797(6)      
Open in a new tab
a

The uncertainties with the coverage factor of k = 2 are shown in parentheses.

b

Values of the neon standards’ atomic weight are reported in ref (16).

c

The IUPAC-recommended standard values are also shown for comparison (available reference materials for neon are air).

Figure 3.

Figure 3

Open in a new tab

Representative results of the linearity of seven commercial gases. The linear regression was performed with the area values of Ne #1, #2, and #3 and their reported atomic weights (ref (16)). The linear regression result is shown by the dashed line. The square symbols represent atomic weights of all neon samples including the carrier gas obtained by a gas/MS.

Table 1 shows the atomic weights of commercial neon gases obtained by GC-TCD and a gas/MS with their uncertainties. The uncertainties of the isotope ratios of the neon samples were propagated according to the reference mentioned in the previous section.16 The dominant factors in the uncertainties are fMD and the detection limit. The uncertainty in fMD mostly originates from the repeatability of the fMD measurement. The detection limit affects the determination of the ion current ratio, especially in the measurement of 21x, which is the smallest amount.

Table 1 and Figure 3 signify that the atomic weights of neon gases are diverse, and their deviation is up to 4 mg mol–1. This appears to be a mass fractionation during the sampling of every manufacturer. In particular, the atomic weights of Ne #2 and #6 are very different from the neon atomic weight value of the IUPAC. In the case of the determination of the Ttp-Ne value, without the atomic weight of the commercial high-purity neon, a relatively large effect of uncertainty occurs when using only the IUPAC value.

Conclusions

A new analytical method was developed to determine the neon atomic weight using the principle of GC-TCD. A linear regression can be obtained from two or more neon references which have quite different atomic weights such as Ne #1 and #2. Then, the area measured by GC-TCD can be put into the equation of linear regression to determine the unknown neon atomic weights. The determined neon atomic weights using the GC-TCD method were verified by measuring the absolute isotope ratio using a gas/MS, and the results were consistent within the uncertainty. Therefore, even without the absolute isotope ratios of chemical species, the atomic weights of neon can be obtained more easily using GC-TCD when the atomic weight of commercial neon gases is needed.

Experimental Section

Sample of Neon Gases

A total of seven bottles of commercial high-purity neon gases were used; their purities were between 99.995 and 99.9999%. These samples were not further purified.

Analytical Conditions for GC

To maximize the sensitivity, the experimental conditions shown in Table 2 were applied. For a better resolution, connected columns with 9 and 12 feet were used in this experiment. As the column filler, a molecular sieve with a size of 5 Å was used. The detector temperature was set as 200 °C, and the oven temperature was set as 40 °C. Oven temperature affects the flow rate. Therefore, molecular mobility is slower at lower temperatures and increases at higher temperatures. In the case of neon, the molecular weight is low, and neon comes out of the column early. At this time, it was measured at a low temperature (40 °C) to prevent overlapping with helium. The reference flow was set to 30 mL per minute, and the sample flow was allowed into the instrument at 45 mL per minute using a mass flow controller (MFC). The sample loop was about 2 cm3. In GC analysis, helium or nitrogen is generally used as the carrier gas. However, in this experiment, the highest-purity neon sample among seven commercial neon cylinders was used as the carrier gas. The carrier gas pressure was set to 60 psi. Additionally, a restrictor (length of ∼20 cm, inner diameter 0.1 mm) was attached at the end of the vent line to improve sensitivity.

Acknowledgments

This research was supported by Research on Measurement Standards for Redefinition of SI Units funded by Korea Research Institute of Standards and Science (KRISS–2021–GP2021-0001).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.1c01434.

  • Detailed description of the linear regression with the regression equation (PDF)

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.

Supplementary Material

ao1c01434_si_001.pdf (183.1KB, pdf)

References

  1. Kipphardt H.; Valkiers S.; Henriksen F.; De Bièvre P.; Taylor P. D. P.; Tölg G. Measurement of the isotopic composition of germanium using GeF4 produced by direct fluorination and wet chemical procedures. Int. J. Mass Spectrom. 1999, 189, 27–37. 10.1016/S1387-3806(99)00047-0. [DOI] [Google Scholar]
  2. Gonfiantini R.; De Bievre P.; Valkiers S.; Taylor P. D. P. Measuring the molar mass of silicon for a better Avogadro constant: Reduced uncertainty. IEEE Trans. Instrum. Meas. 1997, 46, 566–571. 10.1109/19.571916. [DOI] [Google Scholar]
  3. Chang T.-L.; Li W.-J.; Zhao M.-T.; Wang J.; Qian Q.-Y. Absolute isotopic composition and atomic weight of dysprosium. Int. J. Mass Spectrom. 2001, 207, 13–17. 10.1016/S1387-3806(00)00451-6. [DOI] [Google Scholar]
  4. Aregbe Y.; Valkiers S.; Poths J.; No̷rgaard J.; Kipphardt H.; De Bièvre P.; Taylor P. D. P. A primary isotopic gas standard for krypton with values for isotopic composition and molar mass traceable to the Système International d’Unités. Int. J. Mass Spectrom. 2001, 206, 129–136. 10.1016/S1387-3806(00)00397-3. [DOI] [Google Scholar]
  5. Ding T.; Valkiers S.; Kipphardt H.; De Bievre P.; Taylor P. D. P.; Gonfiantini R.; Krouse R. Calibrated sulfur isotope abundance ratios of three IAEA sulfur isotope reference materials and V-CDT with a reassessment of the atomic weight of sulfur. Geochim. Cosmochim. Acta 2001, 65, 2433–2437. 10.1016/S0016-7037(01)00611-1. [DOI] [Google Scholar]
  6. Zhao M.; Zhou T.; Wang J.; Lu H.; Xiang F. Absolute measurements of neodymium isotopic abundances and atomic weight by MC-ICPMS. Int. J. Mass Spectrom. 2005, 245, 36–40. 10.1016/j.ijms.2005.06.006. [DOI] [Google Scholar]
  7. Wieser M. E.; De Laeter J. R. Absolute isotopic composition of molybdenum and the solar abundances of the p-process nuclides 92,94Mo. Phys. Rev. C 2007, 75, 055802 10.1103/PhysRevC.75.055802. [DOI] [Google Scholar]
  8. Lippmann-Pipke J.; Sherwood Lollar B.; Niedermann S.; Stroncik N. A.; Naumann R.; van Heerden E.; Onstott T. C. Neon identifies two billion year old fluid component in Kaapvaal Craton. Chem. Geo. 2011, 283, 287–296. 10.1016/j.chemgeo.2011.01.028. [DOI] [Google Scholar]
  9. Xue C.; Chen Y.; Wang D.; Yang J.; Yang W.; Zeng R. Geology and isotopic composition of helium, neon, xenon and metallogenic age of the Jinding and Baiyangping ore deposits, northwest Yunnan, China. Sci. China Ser. D Earth Sci. 2003, 46, 789–800. 10.1007/BF02879523. [DOI] [Google Scholar]
  10. Yokochi R.; Marty B. A determination of the neon isotopic composition of the deep mantle. Earth Planet. Sci. Lett. 2004, 225, 77–88. 10.1016/j.epsl.2004.06.010. [DOI] [Google Scholar]
  11. Niedermann S.; Graf T.; Kim J. S.; Kohl C. P.; Marti K.; Nishiizumi K. Cosmic-ray-produced 21Ne in terrestrial quartz: the neon inventory of Sierra Nevada quartz separates. Earth Planet. Sci. Lett. 1994, 125, 341–355. 10.1016/0012-821X(94)90225-9. [DOI] [Google Scholar]
  12. Guide to the Realization of the ITS-90 https://www.bipm.org/utils/common/pdf/ITS-90/Guide_ITS-90_1_Introduction_2018.pdf (accessed 14–09-2020).
  13. Steur P. P. M.; Pavese F.; Fellmuth B.; Hermier Y.; Hill K. D.; Kim J. S.; Lipinski L.; Nagao K.; Nakano T.; Peruzzi A. Isotopic effects in the neon fixed point: uncertainty of the calibration data correction. Metrologia 2015, 52, 104–110. 10.1088/0026-1394/52/1/104. [DOI] [Google Scholar]
  14. Pavese F.; Fellmuth B.; Head D. I.; Hermier Y.; Hill K. D.; Valkiers S. Evidence of a systematic deviation of the isotopic composition of neon from commercial sources compared with its isotopic composition in air. Anal. Chem. 2005, 77, 5076–5080. 10.1021/ac048083f. [DOI] [PubMed] [Google Scholar]
  15. Pavese F.; Fellmuth B.; Hill K. D.; Head D.; Hermier Y.; Lipinski L.; Nakano T.; Peruzzi A.; Sakurai H.; Szmyrka-Grzebyk A. Progress Towards the Determination of the Relationship of Triple-Point Temperature versus Isotopic Composition of Neon. Int. J. Thermophys. 2008, 29, 57–66. 10.1007/s10765-007-0329-1. [DOI] [Google Scholar]
  16. Yang I.; Gam K. S.; Joung W.; Kim Y.-G. Triple-Point Temperature and the Isotopic Composition of Three Commercial Neon Gases. Int. J. Thermophys. 2015, 36, 2072–2084. 10.1007/s10765-015-1883-6. [DOI] [Google Scholar]
  17. Pavese F.; Valkiers S.; Steur P. P. M.; Ferri D.; Giraudi D. An accurate determination of the triple point temperature of pure 20Ne and 22Ne. J. Chem. Thermodynamics 2010, 42, 1222–1229. 10.1016/j.jct.2010.04.024. [DOI] [Google Scholar]
  18. Pavese F.; Steur P. P. M.; Kim J. S.; Giraudi D. Further results on the triple point temperature of pure 20Ne and 22Ne. J. Chem. Thermodynamics 2011, 43, 1977–1983. 10.1016/j.jct.2011.07.011. [DOI] [Google Scholar]
  19. Technical Annex for the International Temperature Scale of 1990 (ITS-90). https://www.bipm.org/utils/en/pdf/MeP_K_Technical_Annex.pdf.
  20. Hill K. D.; Fahr M. Triple-Point Temperatures of 20Ne and 22Ne. Int. J. Thermophys. 2011, 32, 173–188. 10.1007/s10765-011-0939-5. [DOI] [Google Scholar]
  21. De Laeter J. R.; Böhlke J. K.; De Bièvre P.; Hidaka H.; Peiser H. S.; Rosman K. J. R.; Taylor P. D. P. Atomic weights of the elements. Review 2000 (IUPAC Technical Report). Pure Appl. Chem. 2003, 75, 683–800. 10.1351/pac200375060683. [DOI] [Google Scholar]
  22. Yang I.; Pitre L.; Moldover M. R.; Zhang J.; Feng X.; Kim J. S. Improving acoustic determinations of the Boltzmann constant with mass spectrometer measurements of the molar mass of argon. Metrologia 2015, 52, S394–S409. 10.1088/0026-1394/52/5/S394. [DOI] [Google Scholar]
  23. Lee J.-Y.; Marti K.; Severinghaus J. P.; Kawamura K.; Yoo H.-S.; Lee J. B.; Kim J. S. A redetermination of the isotopic abundances of atmospheric Ar. Geochim. Cosmochim. Acta 2006, 70, 4507–4512. 10.1016/j.gca.2006.06.1563. [DOI] [Google Scholar]
  24. Steur P. P. M.; Kim J. S.; Giraudi D.; Pavese F. Experimental verification of ideality of 22Ne in 20Ne mixtures at the triple point by means of certified artificial mixtures. J. Chem. Thermodyn. 2013, 60, 87–93. 10.1016/j.jct.2013.01.011. [DOI] [Google Scholar]
  25. Darwish M. S. A.; Kunz U.; Peuker U. Preparation and catalytic use of platinum in magnetic core/shell nanocomposites. J. Appl. Polym. Sci. 2013, 129, 1806–1811. 10.1002/app.38864. [DOI] [Google Scholar]
  26. Colón L. A.; Baird L. J.. Detectors in Modern Gas Chromatography. In Modern practice of gas chromatography; 4th ed.; Grob R. L.; Barry E. F., Eds.; John Wiley & Sons, Inc.: New Jersey, 1977; 275–337. [Google Scholar]
  27. Harris D. C.Gas Chromatography. In Quantitative Chemical Analysis; 5th ed.; W. H. Freeman and Company: New York, 1999; 675–712. [Google Scholar]
  28. Higson S. P. J.Analytical Chemistry, Oxford University Press: Oxford, 2004. [Google Scholar]
  29. Swinley J.; de Coning P.. A Practical Guide to Gas Analysis by Gas Chromatography; 1st ed.; Elsevier: Oxford, 2019; 247–290. [Google Scholar]
  30. Min D.; Lee J. B.; Lee C.; Lee D. S.; Kim J. S. Estimation of Mass Discrimination Factor for a Wide Range of m/z by Argon Artificial Isotope Mixtures and NF3 Gas. Bull. Korean Chem. Soc. 2014, 35, 2403–2409. 10.5012/bkcs.2014.35.8.2403. [DOI] [Google Scholar]

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

ao1c01434_si_001.pdf (183.1KB, pdf)

Articles from ACS Omega are provided here courtesy of American Chemical Society

RESOURCES