Development of gas standard reference material containing thirty volatile organic compounds at 1μmol/mol in nitrogen

Abstract

A gas certified reference materials containing 30 volatile organic compounds (VOCs) including alkane, alkene and aromatic hydrocarbons in nitrogen at 1 µmol/mol level were developed using gravimetric method with four-step dilution and home-made liquid gasifying and filling apparatus. All components were analyzed using gas chromatograph with hydrogen flame ionization detector (GC-FID) and the repeatability of the analytical method was between 0.2-1.0%. Three types of cylinders were involved to test the adsorption/desorption/reaction of components on the inner wall and domestic treated cylinder were chosen for preparation. The results of within-homogeneity demonstrated that all the components remain stable above 2MPa. Furthermore, no significant change of the concentration was tested at least 16 months. Meanwhile, low temperature freezing test showed that extreme low temperature in transportation had no effect on the concentration of the gas standard material. Finally, the relative expended uncertainty (k = 2) of 30 components were evaluated using gravimetric method which were calculated as 2.2%~4.3% .

Introduction

Volatile organic compounds (VOCs) as the important precursor of ozone and PM_2.5 pollutions have caused widespread concerns in recent years (Derwent et al. 2010; Wei et al. 2008; Wu and Xie 2018). Alkanes, alkenes, aromatics and other VOCs contributed most to the formation of ground-level ozone(Li et al. 2015; Liu et al. 2010; Shao et al. 2009; Sun et al. 2016; Tang et al. 2012). Meanwhile, epidemiologic studies indicate that VOCs with toxic have adverse impacting on human health(Huang et al. 2019; Malherbe and Mandin 2007). Ministry of Ecology and Environment of China has paid more and more attentions to the pollutions of ambient air and stationary pollution source caused by VOCs since 2010 and established a national atmospheric photochemical monitoring network gradually.

The metrological traceability of certified reference materials (CRMs) is an important factor to ensure the traceability of monitoring data(Ihara et al. 2009; Neves et al. 2015). VOCs CRMs with accurate concentrations, within-cylinder homogeneity and stability are urgently needed for environmental monitoring (Koziel et al. 2004; Li et al. 2017; Yarita et al. 2007). National institutes of metrology all around the world have developed multi-component VOCs gas CRMs (Grenfell et al. 2010; Nohr et al. 2015; Rhoderick and Yen 2006).

In order to satisfy the needs of routine environmental monitoring in China, corresponding gas CRMs have been developed gradually. In this research, a gas CRMs containing 30 VOCs at 1µmol/mol was developed and the components were selected based on the ozone precursors measured by the U.S. Environmental Protection Agency Photochemical Assessment Monitoring Stations (PAMS) and European Union Air Quality Directive 2002/3/EC. The gravimetric method with four-step dilution was adopted to the preparation of 30VOCs CRMs. The results of this research indicated that 30VOCs gas CRMs at 1µmol/mol with relative expanded uncertainties (k = 2) of 5% could be remain stable above 2 MPa in domestic treated cylinder at least 16 months. The developed 30VOCs CRMs can be used for routine environmental monitoring, pollution source apportionment, proficiency testing and other research.

Materials And Methods

Reagents

The 30 VOCs raw materials with purities range from 99.0–99.97% and the nitrogen as diluent gas with a specified purity of 99.99995% were obtained from commercial suppliers. In order to confirm the purity information, the purities of all raw material were identified and determined by gas chromatography mass spectrometry (GC-MS) and chromatography flame ionization detector (GC-FID) respectively. The nitrogen was analyzed using GC-FID equipped with preconcentrator. The purity information determined by laboratory of suppliers and author’ were shown in Table 1. 

Cylinders

Three types of cylinders including imported treated, domestic treated and domestic non-treated were obtained commercially and selected properly to be used in the preparation of 30 VOCs CRMs. The imported treated cylinders with an internal volume of 6 L were treated using a proprietary technology called Aculife IV to inactivate the inner wall of the cylinder and equipped with CGA-350 brass valves. The domestic treated cylinders were treated with electroless Ni-P alloy plating and the non-treated cylinder had no further processing steps. The internal volume of the domestic cylinders were 2L/4L/8L and equipped with YQF-1 stainless steel valve.

Apparatus

The 81-V-HCE-30G series double plate balance with sensitivity of 0.001g and 30kg (HNU Systems, Inc., USA) and a Mettler-Toledo 1D1 Plus KA32 balance with a sensitivity of 0.001 and 32Kg capacity (Mettler Toledo, Switzerland)) were used to weigh cylinders. The AE240 series electrical balance (Mettler Toledo, Switzerland) with a capacity of 200 g and a readability of 0.01 mg was used to weigh the VOCs raw material.

Preparation of gas standard

Based on the boiling point and targeted concentration of each component, the 30 VOCs were divided into three categories which were shown in Table 2. The 30 VOCs CRMs were prepared by four-stage dilution gravimetric method and a home-made liquid gasifying and filling apparatus was used to ensure the complete gasification of liquid raw materials and transferring into cylinder. The preparation procedure was shown in Fig. 1. 

The first step was to prepare individual intermediate gas of 9VOCs in category Ⅰ. The pre-calculated of each 9VOCs raw materials and nitrogen was filled into their individual cylinder through gas filling apparatus. Their gravimetric concentrations were then calculated as approximately 0.02 ~ 0.04mol/mol using the data from the weighting of cylinders.

The second step was to develop the pre-mix 1 and pre-mix 2. Pre-mix 1 with targeted concentration at 500 µmol/mol level was prepared by adding individual intermediate gas of 9VOCs one by one into a cylinder and then adding pure nitrogen into the same cylinder. Pre-mix 2 was prepared by adding the mix intermediate liquid of 5VOCs in category Ⅱ which was taken out by an injection syringe with shut-off value to prevent loss of liquid compounds and transferred into cylinder by liquid gasifying and filling apparatus. Pre-mix 2 was calculated as about 400 µmol/mol.

The third step was to prepare pre-mixt 3 at 20µmol/mol level. The pre-mix 3 was prepared by adding pre-mixture 1, pre-mixture 2 and nitrogen. The forth step was to develop 30VOCs CRMs at 1µmol/mol by adding pre-mix 3, mix intermediate liquid of 16VOCs in category Ⅲ and nitrogen.

Analysis method

30 VOCs CRMs were determined using Agilent 7890A GC-FID equipped with a heated six-way valve and 2mL gas sample loop. A 60m by 0.32mm i.d. capillary column of GS-GasPro, a proprietary, bonded silica-based phase from J&W, was used to separate the mixtures. The oven temperature is 80℃ for 2min, and programmed to 175℃ at 18℃ per minute, then to 230℃ at 25℃ per minute. The final oven temperature is maintained for 19 minutes. A typical chromatogram of the 30 VOC gas reference material was shown in Fig. 2.

The repeatability of analytical method was calculated by the relative standard deviation (RSD) of response values of 6 repeated measurements which were between 0.2%-1.0%.

Results And Discussion

Selection of cylinder type for preparation

As VOCs may adsorb, desorb or react with each other in the active site of the cylinder inner wall, it is necessary to select the suitable cylinder before preparation. Imported treated cylinders, domestic treated cylinders and domestic non-treated cylinders were included in this research. The cylinder containing 30VOCs CRMs was prepared and stored as the mother cylinder while the cleaned and evacuated cylinder was used as sub-cylinder. The 30VOCs CRMs was transferred from the mother cylinder to the sub-cylinder by pre-treated pipeline as short as possible until the pressure balanced between the two cylinders. After waiting for 7 days, the 30VOCs CRMs in the mother cylinder and sub-cylinder were determined by GC-FID and the relative deviations(E) of the response values were calculated to evaluate the applicability of sub-cylinder. Consider the high cost and long transportation time of the imported treated cylinders, only domestic treated and non-treated cylinders were selected as sub-cylinders.

Relative deviations(E) of domestic treated cylinders (A-type) and non-treated cylinders (B-type) were listed in Table 3, only one set of test data was listed considering that others have shown the same trend. The relative deviations of A-type were less than 1.2% for all components which were closed to the repeatability of analytical method. The relative deviations of B-type showed that the components with heavier molecular mass, such as n-propylbenzene, 1,3,5-trimethylene and 1,2,4-trimethylene were more than 2% which indicated adsorption reaction may occur in the inner wall. Therefore, only A-type can be used to prepare 30 VOCs CRMs.

Within-cylinder homogeneity test

In the view of that VOCs with high molecular mass may be stratified in the cylinder which may cause the decrease or increase of their concentration under different pressure, within-cylinder homogeneity was carried out to study the stable of concentration while the pressure decreased. 30VOCs CRMs were prepared with a pressure of 10 MPa and determined three times at the pressure of 10, 8, 6, 4, 2 and 1 MPa.

F-test was applied to evaluate the within-cylinder homogeneity of all the components and the uncertainty of within-cylinder homogeneity\({(u}_{bb})\) was computed according to Eq. 1–4. The results were displayed in Table 4.

Where m is the number of pressure tested setting, n is the determine time under the same tested pressure, \({\stackrel{-}{X}}_{j}\) is the concentration at each tested pressure, and N is the number of all determinations.

The concentrations of most components were stable in the within-cylinder homogeneity test, expect that styrene, p-xylene, o-xylene, n-propylbenzene, 1,3,5-trimethylene and 1,2,4-trimethylene were increased significantly when the pressure decreased to 1 MPa. Therefore, the concentrations of 30 VOCs CRMs can remain stable above 2 MPa and the uncertainty caused by with-in homogeneity was range from 0.23–1.13%.

Long-term Stability test

Long-term stability is an important factor to monitor the change of concentration with storage time. According to ISO Guide 35, 30VOCs CRMs were measured by GC-FID at different monitor time (0, 3, 6, 9, 12, 16 months).

T-test was used to determine whether there is a significant difference in concentration over monitor time. And then, a linear curve of monitor time and concentration was fitted as Eq. 5, where X denotes time and Y denotes the concentration. The uncertainty caused by long-term stability\({(u}_{lts})\) were calculated by Equations 6–9 and shown in Table 5.

\({\text{u}}_{\text{lts}}\text{=s}\left({\text{b}}_{\text{1}}\right)\text{*t}\) , where t is the shelf life (9)

As a consequence, the concentrations of 30VOCs CRMs have no significant trend within 16 months which indicate it can main stable for at least 16 months and the uncertainty caused by long-term stability\({(u}_{lts,rel})\) ranged from 1.0 to 2.1%.

Short-term Stability test

The short-term stability test was carried out to determine whether the extreme low temperature during transportation affected the concentrations of 30 VOCs CRMs. The freezing operation procedure of 30VOCs CRMs was: frozen to -20℃ for 24 hours and then returned to room temperature. The 30VOCs was determined by GC-FID before and after freezing operation procedure and the relative deviation (E) was calculated to estimate the short-term stability. The results were shown in Table 6 and the E is close to \({u}_{lts}\), which indicates that the low temperature during transportation has no effect on 30VOCs CRMs.

Uncertainty evaluation of 30VOCs CRMs

The certified value of 30VOCs CRMs was calculated through gravimetric method which was recommended by ISO 35 and its uncertainty mainly comes from: preparation, within-cylinder homogeneity and long-term stability. The preparation uncertainty (\({\text{u}}_{\text{prep}}\)) was evaluated by identifying, evaluating and synthesizing all the uncertainty factors related to preparation which was shown in Fig. 3

The uncertainty of 30VOCs CRMs (\({u}_{CRM,grav}\)) was certified by Eq. 10 and the relative expanded uncertainty Ugrav (where k = 2) was 2.2%-4.1%. The results were all listed in Table 7.

Comparison of certified value

The 30VOCs CRMs were compared with other related gases CRMs by Linde Spectra Environment Gases and Air Liquid Company to verify its accuracy of certified value. The determination data which can be confirmed the consistency of certified value was evaluated by Formula 11.

Where x_CRM is the certified value of 30VOCs CRMs, x_meas is the determined concentration of 30VOCs CRMs, u_CRMs is the uncertainty of 30VOCs CRMs and u_meas is the uncertainty caused by determination.

The comparison results were shown in Table 8 and the differences between certified value and determined value were less than their uncertainty which illustrated the consistency of 30VOCs CRMs.

Conclusions

The 30VOCs CRMs at 1µmol/mol in nitrogen was prepared by gravimetric method combined with four stage dilution and the domestic treated cylinder was used in the research. The concentration of 30VOCs CRMs could be homogeneous and stable above 2MPa in cylinder for at least 16 months. Meanwhile, the low temperature during transport had no effect on concentration of the 30 VOC CRMs. The certified value of 30VOCs CRMs was evaluated by gravimetric method with its relative expanded uncertainty (k = 2) calculated as 2 ~ 5%.

Declarations

The authors have no relevant financial or non-financial interests to disclose.

Authors Contributions

Du Jian: develop of 30 VOCs gas standard materials, cylinder selection, analysis method, within-cylinder homogeneity test, stability test, uncertainty evaluation, writing (original draft preparation) and writing (reviewing and editing). 

Yang Jing: develop of 30 VOCs gas standard materials, cylinder selection, analysis method, stability test, uncertainty evaluation and writing (reviewing and editing). 

Tian Wen: writing (original draft preparation) and writing (reviewing and editing).

Li Ning: cylinder selection, analysis method, within-cylinder homogeneity test, stability test, uncertainty evaluation, writing (original draft preparation) and writing (reviewing and editing). 

Funding

No funding was obtained for this study. 

Competing Interests

Not applicable
Availability of data and materials

Not applicable

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