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Case Report
, Volume: 22( 9) DOI: DOI: 10.37532/0974-7419.2022.22(9).214

A Case Study on Impurity analysis of 5.5N Nitrogen gas and its impact on the preparation of Primary Reference Gas Mixtures

*Correspondence:
Daya Soni CSIR-National Physical Laboratory, New Delhi-110012, India, E-mail: [email protected]

Received:22- Sep-2022; Accepted:24- Sep-2022; Published:18- Nov-2022

Citation: Daya S. A Case Study on Impurity analysis of 5.5N Nitrogen gas and its impact on the preparation of Primary Reference Gas Mixtures. Anal Chem Ind J. 2022;22(9):214.

Abstract

Knowledge of impurities in matrix gas is one of the critical requirements for the preparation of Primary Reference Gas Mixtures (PRGMs) as per ISO 6142-1. PRGMs are prepared gravimetrically using high-purity component gas and high-grade nitrogen, here 99.9995% (5.5N) as matrix gas. In this paper, gaseous impurities such as moisture (H2O), methane (CH4), and carbon monoxide (CO) in 5.5N nitrogen are quantified using CRDS (cavity ring down spectroscopy), carbon dioxide (CO2), total hydrocarbons (THC) are analyzed using gas chromatography flame ionization detector (GC-FID) and nitrogen dioxide (NO2) impurity determined colorimetrically using UV-Vis spectrophotometer. Total impurities for 5.5N nitrogen are found to be (2945 ± 21.5) ppb. The impact of using this nitrogen is examined from the stability study of ambient range binary component gas mixtures of CH4 and CO in nitrogen and found that 5.5N nitrogen can be used as matrix gas for preparing PRGMs of CH4 and CO but the component gas impurity present in nitrogen can affect the accuracy of gravimetrically prepared gas mixture.

Keywords

Primary reference gas mixtures, gaseous impurity analysis, 5.5 N nitrogen, cavity ring-down spectroscopy (CRDS).

Introduction

In the present situation, ambient air pollution has become a subject of interest for every individual. All the strategic plans of regulatory bodies to control gaseous pollutants rely upon the measurement results getting from the instrument used for the measurement of pollutants. The results of measurement by any equipment are dependent on the quality of calibration standards used for the calibration of that equipment [1,2]. Gases with purity ranging from 99.996% (4.6 grades) to 99.99999% (7.0 grades) are used in various applications. Primary Reference Gas Mixtures (PRGMs) prepared in Ultra-High Pure (UHP) nitrogen (> 99.9995%) balance gas; can be used for the calibration of ambient air monitoring instruments. NMIs (National Metrology Institutes) is responsible for the realization of PRGMs which form the basis for propagating traceability for the analysis of gases. PRGMs are prepared by precisely weighing gases or volatile liquids into high-pressure cylinders following ISO 6142. In accordance with ISO 19229:2015 standard, critical examination of materials used in the preparation of calibration gas mixtures is required when the contamination is significant and contributes>10% to the target uncertainty of any of the components in the prepared gas mixture [3-5]. The accuracy of gas standards is now restricted by the ability to quantify trace-level impurities in the balance gases used to prepare standards in many NMIs. Trace gases impurities like moisture (H2O), Methane (CH4), Carbon Monoxide (CO), Carbon Dioxide (CO2), Total Hydrocarbon Concentration (THC), and Nitrogen Dioxide (NO2) effectively change the composition of standard gas mixtures by reacting with the component gases or by adsorbing on the cylinder surface [4,6].

The detection and measurement of trace gas impurities do not just influence the cost of high-purity gases itself but also the process and nature of the final products. So, the development of precise and reliable analytical methods for the estimation of gaseous impurities at trace levels (ppm and ppb) is immensely required. Many analytical techniques are available and used for trace level measurements e.g. electrochemical sensors, GC (Gas Chromatography) operated via different detectors like FID (Flame Ionization Detector), TCD (Thermal Conductivity Detector), PDHID (Pulse Discharge Helium Ionization Detector) and significant FTIR (Fourier Transform Infrared Spectroscopy) [79]. Moreover, remarkable attention was gained by the wavelength-dependent absorption of an analyte using optical spectroscopic methods like Differential Absorption Lidar (DIAL), Differential Optical Absorption Spectroscopy (DOAS), and CRDS (Cavity Ring Down Spectroscopy) [10, 11]. The functioning of CRDS is based upon regulating laser beam wavelength within the IR range where the contaminant absorption peak occurs [12]. Emitted light energy beam departs through a highly reflective mirror within the absorption cell where it is bounced between mirrors various times (almost 107 times, traveling around 20 km or more). This decay of energy from the cavity is recorded which provides a significant ppb detection limit to certain gases depending on the absorption strength at the used laser wavelength [13, 14].

CRDS is considered an excellent primary technique used for the application of impurity determination in the gas metrology field due to its high sensitivity and long-term stability. As of now, practically all NMIs utilize CRDS and related techniques to determine impurity in gases used for calibration gas mixtures preparation e.g. CRDS variants like OFCEAS (Optical Feedback Cavity Enhanced Absorption Spectroscopy) and OPO (Optical Parametric Oscillator) were used by VSL for hydrogen purity study [15]. In this paper, we accounted for trace gas impurities i.e. H2O, CH4, CO, CO2, THC, and NO2 quantification in 99.9995% nitrogen gas. CRDS method within IR region is operated for trace detection of moisture, methane, and carbon monoxide as tiny molecules provide fine-characterized strong absorption bands in this region [16, 17]. The presence of reactive gas NO2 affects the stability of Standard Gas Mixtures of NO [18]. Standard methods prescribed for NO2 determination are “Gas Phase Chemiluminescence” and “Sodium Arsenite (IS 5182)” as per ‘National Ambient Air Quality Standard (NAAQS)’ [19]. Herein, a wet chemical method IS 5182 is used to measure NO2 concentration [20]. Furthermore, CO2 and THC are measured via GC-FID. Besides the quantification of significant impurities, the impact of using 5.5 N nitrogen as matrix gas is studied as a stability study over a period of five years for lower amount fractions of gravimetrically prepared gas mixtures of CH4 and CO in nitrogen gas.

Experimental

Materials and Instruments

All the chemicals sodium hydroxide, sodium arsenite, hydrogen peroxide, N-(1-Naphthyl)-Ethylenediamine Di-Hydrochloride (NEDA), phosphoric acid, sodium nitrite, sulphanilamide of AR grade were purchased from Sigma-Aldrich. Ultra highly pure 5.5 N grade nitrogen cylinder is used in further study. Impingers used for sampling purposes were purchased from Merck. A constant flow rate is achieved using a rotameter with standardization using Mass Flow Controller (MFC, series 4000, TSI). UV spectrophotometer of the model (MS UV 134 Plus, MOTRAS Scientific) was operated to quantify NO2 impurity concentration in pure nitrogen gas. CRDS analyzer of Tiger Optics is featured for ‘Laser Trace’ moisture analyzer; methane (MTO-1000-CH4) and carbon monoxide (HALO3 Model with serial no. 4467-91-0) trace measurement. CO2 is determined by GC-FID (Agilent 6890 N) equipped with methanizer using Hysap-D column and hydrocarbon content is quantified using “Thermo scientific Model 55C” of GC-FID with Porapak Q column. The instrumentation used for impurity determination is shown in FIG. 1.

tsac-22-9-comparing

Figure 1:Systematic scheme of the cw-cavity ring down spectroscopy setup. CRDS involves a very long effective path length and compact absorption cell cavity.

Purity analysis in gas metrology

The PRGMs are used for traceability dissemination in gas metrology at CSIR-NPL (NMI of India, Internationally known as NPLI). High-purity nitrogen is used as a diluent gas for the preparation of PRGM in a nitrogen matrix. The purity analysis is a significant step for the preparation of PRGM. Corresponding to the principle of preparation of primary mixtures as per the standard guideline of ISO 6142, preparation cannot be fully executed without the purity information of the gases. Ultimately, the purity of gases can be evaluated from the amount fraction of every single critical impurity present in that pure gas. The amount fraction of pure gas can be determined conventionally by equation (1)

Where, xpure is the mole fraction of pure gas, xi mole fraction of impurity i, and N is the total number of impurities that can be determined in the pure gas.

The impurity information for the component of interest in balance gas is very much important and is used as a matrix for the PRGM preparation of a particular amount fraction of that component. Most gas manufacturers provide specifications for their pure gases with limited information on impurities. These specifications are based on non-traceable measurements so, cannot be used for critical impurities. Nitrogen gas with a purity of 99.9995% procured from a gas manufacturer is used as balance gas for the preparation of PRGMs. The impurity of the components used in the n preparation of PRGMs has an impact on the amount fraction of the component and hence on the uncertainty associated with the component as given in equations (2) and (3).

Where, xi , prep, xi , grav, and Δxi , purity is the mole fraction correlated with absolute prepared primary mixture i, gravimetrically prepared mole fraction, and the sum of amount fractions of all the impurities present in i pure gas. Hereafter, ?2 corresponds to the uncertainty associated with xi , prep, xi , grav, and Δxi , purity

H2O, CH4, and CO impurities

CRDS technique is used for the determination of H2O, CH4, and CO impurities in nitrogen gas. Gravimetrically prepared calibration gas mixtures are used to calibrate CRDS using three-point calibrations. Traceability of CH4 and CO is established through mass and also through participation in international intercomparisons. [21, 22]. A detailed illustration of CRDS instrument is shown in FIG. 1 which primarily comprises a continuous wave laser system, ring-down cavity, rapid detector, and a data handling computer system. A CW diode laser emits a directed beam of light energy that enters into the cavity ringdown absorption cell containing two ultra-high reflective mirrors (>99.9%). The light reflects back and forth between two mirrors multiple times that it covers several kilometers between mirrors. Maximum light from the laser source is reflected back from input mirrors but a small amount of ‘light’ or ‘ring down signal’ emits through the second mirror on each successive pass that is sensed by the photo-sensitive solid-state detector. Once the detector sees a “preset level” of light energy, the light source is shuttered from the cavity. The intensity of such transmitted light decays exponentially in time with rising absorption path length following Beer-Lambert law and this energy decay of light is measured as a function of time (τ), named as “ring-down”. Precise concentration of absorbing molecules is determined by measuring the time taken for ringdown. So, CRDS provides the rapid detection of contaminants in pure gases [23-29]. Mathematically, contaminant concentration is determined by comparing this “tau zero” value to the “tau measured” ring-down time represented in equation (4).

Where,

c = speed of light (ms-1 )

σ( ) = absorption Cross Section (m2 )

d = cell length (m)

τ = ring down Time (s)

R = reflectivity of the mirror

ν = laser frequency (m-1 )

N = molecular density (concentration) in ppbv

NO2 impurity

A standard wet chemical Jacob and Hochheiser Method (IS 5182) is used for NO2 determination [19]. It is a manual monitoring method for concentration determination of NO2 in ambient air and is also known as “Sodium Arsenite” method having high collection efficiency for nitrogen dioxide. Concentration ranges from 3 ppb to 400 ppb can be easily determined and are insensitive to normal variations in operating parameters. NO2 was effervesced into a set of four impingers allied in series comprising 30 ml of absorbing reagent i.e. mixture of NaOH and NaAsO2 solution. NO2 content was determined colorimetrically by reacting it with phosphoric acid, sulfanilamide, and NEDA that results in the formation of azo-dye which was quantified via UV-Vis spectrometer at 540 nm. A complete depiction of the method followed is represented as a block diagram in FIG. 2.

tsac-22-9-block

Figure 2:Chemical procedure for NO2 impurity determination

CO2 and THC impurity

CO2 impurity is determined by GC-FID with methanizer using Hyseph D column. Traceability in measurement is ensured by using CO2 gas mixture standard used in CCQM K 120a and b [30]. Operating conditions include oven and detector temperature at 80°C, 250°C respectively, and methanizer at 350°C. Standard and sample is injected to GC through GSV using Mass Flow Controller (MFC). Helium gas with a flow rate of 25 ml/min is used as a carrier gas, H2 at 40 ml/min, and air at 300 ml/min is used as fuel for detection.

THC is also determined as an impurity in matrix gas (99.9995%) via GC-FID analyzer containing “Porapak Q” column for gas mixture separation. The sample is introduced to the inlet at 2 bar line pressure through a regulator. Operating parameters are optimized for quantification. The oven and detector temperatures were set to an isothermal condition at 70°C and 180°C individually. The high purity hydrogen and air are used as fuel and nitrogen is used as carrier gas in the detection of hydrocarbon content. It is a very sensitive detector but operational only for hydrocarbon analysis.

Methodology for impurity determination

Analysis of impurities was done in controlled environmental conditions at 23°C ± 3 °C temperature and 45% ± 10% RH. The temperature stability of gas mixtures was reassured by storing them within the laboratory for overnight prior to analysis. A pressure regulator and stainless steel tubing is used for the sample gas introduction into the analyzer to avoid any chances of permeation of air from the ambient. For determination of impurities, N2 cylinder was connected to each CRDS analyzer in which all kinds of trace impurities are to be quantified. The wavelength positioned at 1392.53 nm and 1653.45 nm absorption line of continuous wave-laser corresponds to the H2O and CH4 in N2 gas respectively. The lowest detection limit for H2O, CH4, and CO are 200 ppt, 2 ppb, and 4 ppb respectively. The N2 cylinder having pressure 180 bars were connected to a sample line of CRDS analyzers maintaining the flow of 0.5 lpm-1 lpm in the sample line. The sample was then introduced into the ringdown cavity via a pressure regulator at around 2 bar line pressure for continuous 2 hours to get a steady concentration of trace impurity was obtained. The same sample cylinder was connected to the CRDS sample line for a further three days.

In a similar way, 5.5 N grade N2 cylinder is connected to GC-FID hydrocarbon analyzer to determine THC content. And the same cylinder is also connected to GC-FID through Gas Sampling Valve (GSV) for CO2 determination. Moreover, NO2 content was quantified following the colorimetric method as per IS 5182.

Results and Discussion

Impurities determination

Specifications provided by gas manufacturers for the purity of gases do not have sufficient information of the amount of impurities. H2O, CH4, and CO impurities in high-purity N2 is determined by CRDS analyzer. At the same time, CO2 and THC is quantified by means of GC-FID and NO2 by colorimetric wet chemical method IS 5182 (part 6).

FIG. 3(a) shows the graphical representation of one-day measurement data of H2O, CH4, CO, and THC. Four days' data of all the impurity measurements are shown in the figure below. Repeatability assessment is necessary for the stability and reliability of the instrument via multiple measurements. FIG. 3(b) demonstrates that the variation range for the concentration of H2O, CO, CH4, THC, and NO2 impurities is 2500 ppb-2700 ppb, 2 ppb-4 ppb, 340 ppb-420 ppb, 12 ppb-16 ppb, and 3 ppb-5 ppb, respectively. The results clearly show that methods employed for trace measurement of impurities are reproducible and provide long-term stability of measurement results. TABLE 1 represents consecutive four days of measurement data of all the impurities analyzed along with their Type A measurement uncertainty [31].

tsac-22-9-measuremen

Figure 3(a): One day measurement data (b) Four days repeatability data of H2O, CH4, CO, THC, and NO2 impurity in highly pure N2 cylinder. Error bar in (b) representing the standard deviation of each measurement. Five measurement data are shown for NO2

Moisture present in the gas cylinder will react with other impurities like SOX and NOX which leads to the formation of H2SO4 and HNO3, may corrode the inner surfaces of the gas cylinder, and hence may affect the stability of the gas mixture. Methane calibration gas mixture standards can be prepared in nitrogen matrix or synthetic air matrix as per the need of measurement. So, it is important to have an estimation of the methane impurity in matrix gas so that we can have accurate estimates of the uncertainties in the final composition of the standard mixture. CO measurement in highly pure N2 gas is not only important for the preparation of PRGMs of CO in ambient range but also for the purpose of food packaging industries.

NO2 being a reactive gas affects the stability of calibration gas mixtures e.g. standard gas mixture of NO must not contain more than 1 ppm of NO2 as an impurity according to USEPA. For the preparation of 100 ppm NO in a nitrogen cylinder impurity of only 1 ppm NO2, will introduce an error greater than 20% in the calibration results of NO2 analyzer hence, NO2 impurity determination is important [18]. The chemical reaction engaged for quantification that results in the formation of azo-dye whose absorbance is determined at wavelength maxima 540 nm, is represented in FIG. 4.

tsac-22-9-wavelength

Figure 4:Chemical reaction of nitrite ion produced during sampling with sulphanilamide and NEDA reagent

A series of ten samples were analyzed to quantify the NO2 amount fraction in pure nitrogen gas. And data is given in S6 Average of ten measurements of NO2 content in 5.5 N grade nitrogen is 4.15 ppb ± 0.29 ppb (TABLE 1).

TABLE 1. Day-wise average concentration for component impurities in 5.5 N nitrogen gas cylinder.

  Components concentration (ppbv)  
  H2O CH4 CO THC NO2*
Day 1 2498 ± 3.97 2.1 ± 0.15 400 ± 4.5 14.6 ± 0.31 4.15±0.3
Day 2 2579 ± 14.73 2.9 ± 0.18 362 ± 5.6 16.1 ± 0.27  
Day 3 2474 ± 9.35 2.2 ± 0.14 409 ± 4.23 16.2 ± 0.32  
Day 4 2624 ± 4.76 1.8 ± 0.12 346 ± 5.51 14.6 ± 0.31  

* NO2 concentration represents an average of 10 independent sample measurements.

The absolute concentration of all the impurities examined is shown in TABLE 2. It shows the total impurity concentration (2945 ppb ± 21.5 ppb) is present in 99.9995 % pure nitrogen gas cylinder where ± 21.5 ppb indicates the combined uncertainty of measurement results at coverage factor k=1 from all the impurities. Very few laboratories reported the measurement of impurities in matrix gas for the preparation of calibration gas mixtures prepared by them [6, 7, 32]. A summary of the literature survey on determination of impurities in Ultra-High Pure (UHP) nitrogen along with the techniques used is described in Table 3 where the results of this paper are also compared with the reported results.

TABLE 2. Amount fractions of impurities present in the Nitrogen.

99.9995 % Pure Nitrogen Gas
Analyte Average Concentration  (ppb) Total impurity* (ppb)
H2O 2544 ± 19  
CH4 2.2 ± 0.3  
CO 379 ± 10 2945 ±21.5
THC 15.4 ± 0.6  
NO2 4.15 ± 0.3  
CO2** < DL  

Impact of impurity on PRGM composition and stability

To highlight the impact of impurity measurement in diluent nitrogen gas used for the preparation of gas standards, a PRGM of methane in nitrogen (cylinder number M1064003062) was prepared gravimetrically (amount-of-substance fraction; 2945 nmol/mol) following ISO 6142-1 and analyzed using GC-FID (porapak Q column) and CRDS techniques independently almost every year after its preparation. The analytical accuracy of the measurement using two instrumental techniques is ± 5% relative. The mixture was prepared from a premixture of methane in nitrogen with an amount fraction 2200 µmol/mol in two-step gravimetric dilution. The traceability of premixture is established from participation in international comparison APMP.QM-S7.1 [21]. The five-year measurement result of the cylinder is plotted to ensure the stability of the prepared composition and represented in FIG. 5(a) which clearly shows the stability of methane in nitrogen composition from 2018 to 2022. An impurity present in nitrogen gas doesn’t affect the stability of the methane cylinder. in high-purity nitrogen, methane was observed above the limit of quantification of the CRDS instrument and hence using this 5.5 N nitrogen only has a minor impact on the gravimetric assigned amount fraction of methane in nitrogen. So, 5.5 N nitrogen gas can be used for the preparation of ambient methane cylinders ensuring the significant impurities are known

tsac-22-9-diluent

Figure 5: Measurement data of gravimetrically prepared PRGMs using CRDS and GC-FID (a) CH4 (b) CO

Average concentration of the component is estimated as combined uncertainty (at k=1) using the following equation;

**DL - detection limit of the instrument

TABLE 3. A summary of literature on impurity determination and technique used for UHP N2 cylinder impurity analysis

Matrix gas

 

Component concentration in ppb

 

(Technique/Method Used)

 

H2O

 

CH4

 

CO

 

THC

 

CO2

 

NO2

 

Ref.

 

5.5N N2

 

1500 ± 0.9 (GC-PDHID)

 

3163 ± 0.5 (GC-PDHID)

 

1289 ± 0.2 (GC-PDHID)

 

-

 

22 ± 0.01(GC-PDHID)

 

-

 

[6]

 

N2 CYL

 

16 ± 4 (CRDS)

 

0.5 ± 0.5 (CRDS)

 

<1 ppb ± 1(CRDS)

 

-

 

4.3 ± 0.7 (FTIR)

 

-

 

[7]

 

N2 CYL

 

-

 

< 0.2

 

-

 

-

 

2

 

-

 

[31]*

 

O2 CYL

 

-

 

27-43

 

-

 

-

 

-

 

-

 

[31]*

 

Whole air

 

-

 

1.1-1.6

 

-

 

-

 

-

 

-

 

[31]*

 

This paper

 

2544 ± 19 (CRDS)

 

2.2 ± 0.3 (CRDS)

 

379 ± 10 (CRDS)

 

15.4 ± 0.6 (GC-FID)

 

<DL (GC-FID)

 

4.2 ± 0.3 (IS 5182)

 

 
 

* OPO-Based CRDS technique is used for all matrix gases in reference [31]

Similarly, CO in nitrogen (cylinder No JJ108706) with an amount fraction 5019 nmol/mol was prepared from premixture by gravimetric dilution and analyzed via GC-FID with methanizer (molecular sieve 13x column) and CRDS from 2018 to 2022. Measurement data is represented in FIG. 5(b) which shows that the concentration of CO achieved from CRDS is a little higher than the gravimetric calculated amount fraction of CO. Because of the high sensitivity of CRDS as compared to GC, it can even detect ppb level of CO which is introduced because of impurities of CO in matrix gas. Furthermore, 5.5 N nitrogen gas can be used for CO calibration gas mixtures in lower amount fractions but the amount of CO must be known in the diluent nitrogen gas

TABLE 4 shows all the measurement concentrations obtained for methane and carbon monoxide with a standard deviation of measurements. Traceability to SI amount-of-substance is achieved through mass by using calibration gas mixtures (PRGMs) prepared gravimetrically at NPL India and also through participation in APMP.QM-S7.1 (CH4 in nitrogen; 2000 µmol/mol) and APMP.QM-S9.2017 (CO in nitrogen; 100 µmol/mol) [21,22].

TABLE 4. Amount fractions of methane and carbon monoxide obtained from CRDS and GC-FID with methanizer.

  Amount fraction (nmol/mol)
Years Methane CRDS GC Carbon monoxide CRDS GC
2018 2900 ± 21 3042± 75 5335 ± 87 5091± 221
2019 2869 ± 23 2957 ± 23 5292 ± 42 ND
2020 ND ND ND 5167 ± 159
2021 2853 ± 20 2998 ± 50 5343 ± 32 ND

* ND - measurement not done.

 

Conclusion

Purity analysis turns into the most limiting factor as quality requirements for primary reference gas mixtures are getting progressively stricter. The quality of gas standards is currently regulated by means of the capability to quantify trace-level impurities in the matrix and component gases used to prepare PRGMs by many NMIs. The purity determination of N2 is of great importance for ensuring an accurate composition of the prepared gas mixture standards. Also, the uncertainty associated with the measurement of impurities defines the uncertainty of the certified PRGMs. For this purpose, advantageous techniques are used depending upon their sensitivity and limit of detection that can measure trace concentrations of impurities. This paper reports the H2O, CH4, CO, THC, and NO2 concentrations in highly pure nitrogen gas (5.5 N). The average concentration is found to be 2544 ppb ± 19 ppb, (2.2 ppb ± 0.3 ppb) , (379 ppb ± 10 ppb), (15.4 ppb ± 0.6 ppb), and (4.15 ppb ± 0. ppb 3) respectively. CO2 concentration was below the detection limit of GC-FID. The total quantified impurity is (2945 ppb ± 21.5 ppb) for 5.5 N pure nitrogen. This impurity profile can also help to certify the high-purity nitrogen gas provided by the supplier. Also, the impact of the use of 5.5 N nitrogen for the preparation of PRGMs in the range of nmol/mol was studied with the help of stability analysis of gravimetrically prepared gas mixtures of CH4 in nitrogen and CO in nitrogen over five years after its preparation. The analysis of both the mixtures over a period of 5 years using GC FID and CRDS proves that use of 5.5 N nitrogen does not affect the stability of prepared gas mixtures over period of time but the presence of the target component impurity may impact the accuracy of the gravimetrically prepared value of gas mixture as can be observed in PRGM of CO in nitrogen and these variations are detectable only if the measurement technique is highly sensitive like CRDS.

Acknowledgment

The authors, Gazal and Poonam Kumari are thankful to the Council of Scientific and Industrial Research (CSIR) for providing the fellowship under the CSIR-SRF scheme. The authors are thankful to the Director, CSIR-NPL for providing all support to carry out gas metrology work, and further extend their thanks to the Head of Environmental Science and Biomedical Metrology Division (ESBMD) for their encouragement and support. The authors are also thankful to Dr. T K Mandal and Dr. Sudheer Kumar Sharma of ESBMD, NPL for providing an instrument for THC measurements

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