Monoterpenes play an important role in atmospheric chemistry due to their large anthropogenic and biogenic emission sources and high chemical reactivity. As a consequence, measurements are required to assess how changes in emissions of monoterpenes impact air quality. Accurate and comparable measurements of monoterpenes in indoor and outdoor environments require gaseous primary reference materials (PRMs) that are traceable to the international system of units (SI). PRMs of monoterpenes are challenging to produce due to the high chemical reactivity and low vapour pressures of monoterpenes and also their propensity to convert into other compounds, including other terpenes. In this paper, the long-term stability of gravimetrically prepared static monoterpene PRMs produced in differently passivated cylinders, including sampling canisters, was assessed. We demonstrate that static PRMs of multiple monoterpenes can be prepared and used as a suitable long-term standard. For the first time the effect of cylinder pressure and decanting from one cylinder to another on the chemical composition and amount fraction of monoterpenes was also studied. Gravimetrically prepared PRMs of limonene in high pressure cylinders were compared to a novel portable dynamic reference gas generator based on dilution of pure limonene vapour emitted from a permeation tube.
Terpenes are a large and diverse family of naturally occurring organic
compounds that are a major biosynthetic building block (de Meijere et al.,
1998; Nicklaus et al., 2013). Vegetation including forests and agricultural
crops (Curtis et al., 2014; Ormeño et al., 2010) emit substantial
quantities of isoprene (a hemiterpene (
Terpenes play an important role in atmospheric chemistry due to their high
reactivity influencing the
Terpenes are also known to be emitted from building materials and household products (Allen et al., 2016), in which they are primarily used as fragrances and flavourings (Lamorena and Lee, 2008; Steinemann et al., 2011; Wang et al., 2017; Wolkoff et al., 1998), impacting indoor air quality (Nazaroff and Goldstein, 2015; Singer et al., 2006). In particular, the exposure of the public to terpenes in indoor air quality is poorly understood due to a lack of available data, despite the toxicity of their photochemical products (Jones, 1999; Wolkoff and Nielsen, 2001; Wang et al., 2007; Wang et al., 2017).
A variety of techniques have been used for the sampling and analysis of complex mixtures of terpenes including active and passive sorbent tube loading and desorption (Sunesson et al., 1999), canister sampling (Batterman et al., 1998; Pollmann et al., 2005) followed by analysis using gas chromatography mass spectrometry (Birmili et al., 2003; Koch et al., 2000), proton transfer reaction mass spectrometry (Holzinger et al., 2005) or other spectroscopic techniques (Qiu et al., 2017). However, the accurate measurement of terpene amount fractions in indoor and outdoor air is highly dependent upon the availability of appropriate SI traceable gaseous PRMs (Rhoderick, 2010) and analytical methods (Helmig et al., 2013).
The World Meteorology Organisation (WMO) Global Atmosphere Watch (GAW) programme is a framework to provide reliable scientific data and information on the long-term trends in the chemical composition of the atmosphere. In WMO-GAW report no. 171 Global Long-Term Measurements of Volatile Organic Compounds (VOCs), new data quality objectives were created for priority VOC compounds including monoterpenes. These data quality objectives stipulated 20 % accuracy and 15 % precision for monoterpene measurements reported by GAW stations. Further recommendations by GAW's scientific advisory group for reactive gases have been made to lower these data quality objectives to 5 % and renamed as uncertainty and repeatability (Hoerger et al., 2015). In order to meet the 5 % uncertainty target, and prevent the reference material from dominating the uncertainty, stable PRMs of monoterpenes with uncertainties of better than 1.25 % (less than a quarter of the uncertainty) are required. There is also a requirement for performing reliable sampling or dynamic calibration methods for the in situ calibration of instruments during field campaigns or at long-term atmospheric monitoring stations and for independent verification of the gaseous PRMs.
PRMs containing monoterpenes are challenging because monoterpenes are highly reactive compounds and can isomerise, tautomerise or react to form a wide range of other compounds including other terpenes (Allahverdiev et al., 1998; Findik and Gunduz, 1997; Foletto et al., 2002). This has led to observations that the amount fraction of some monoterpenes increase overtime, including the observation of compounds that were not present when the mixture was first prepared, while the amount fraction of others declines (Rhoderick and Lin, 2013). Moreover, cylinder passivation (the coating applied to the internal surface of a cylinder to reduce adsorptive losses) has a big impact on the stability of monoterpene gas mixtures. Rhoderick and Lin (2013) demonstrated that specific passivation types, such as “Experis” (Quantum) manufactured by Air Products, looked the most promising for monoterpenes.
In this paper, multicomponent monoterpene static gaseous PRMs containing
PRMs containing the four monoterpenes,
All “pure” liquid compounds were purchased from commercial suppliers (Fluka and Sigma Aldrich) and were purity analysed following the guidelines stipulated in ISO 19229:2015 by gas chromatography with a flame ionisation detector (GC-FID) prior to use. Impurities were identified and quantified by percentage area. The purity of all the monoterpenes was between 93.5 % and 99.5 % (Table S1 in the Supplement).
A PRM of nominally 100 nmol mol
Gravimetric compositions of monoterpene PRMs made by dilution of the
parent mixtures (mixtures A–E). Amount fractions are all in
nmol mol
All of the measurements were performed using a GC-FID (Varian CP-3800). The
system uses a sample pre-concentration trap containing glass beads cooled by
liquid nitrogen and held at
The PRMs were connected to the GC using a minimal dead volume connector and
the flow rate was set to 50 mL min
A schematic illustrating the decanting procedure is shown in Fig. 1. The
decanting experiments were performed in 10 L aluminium Luxfer cylinders that
had been treated with different types of cylinder passivation, these included
Experis, sometimes referred to as Quantum (Air Products), SPECTRA-SEAL (BOC)
and “in-house” treated BOC SPECTRA-SEAL. It has been observed that this
propriety “in-house” passivation provides improved stability for a wide
range of compounds at low amount fractions. All cylinders had a 10 L
internal volume. Initially, a new PRM, identified as cylinder 1 in Fig. 1 was
prepared gravimetrically (as described in Sect. 2.2) at an amount fraction of
nominally 2 nmol mol
Once a new PRM (cylinder 1) had been prepared at 120 bar (day 1), the mixture was analysed by GC-FID and compared against the reference PRM, mixture BB (day 2). The following day (day 3), approximately 50 bar of cylinder 1 was decanted by direct fill (a short well-purged transfer line) to cylinder 2 leaving 70 bar in cylinder 1. Both cylinder 1 and 2 were then analysed by GC-FID and compared against reference PRM, mixture BB. Finally (day 4), approximately 20 bar of cylinder 2 was decanted to cylinder 3 leaving 30 bar in cylinder 2 and both cylinder 2 and 3 were then analysed by GC-FID and compared against reference PRM, mixture BB (differences in the gravimetric values between the PRM and the reference standard were normalised). All of the cylinders were evacuated and the decant procedure was repeated for a second time.
All of the analyses were performed using GC-FID as described in Sect. 2.2.
The amount fraction of each compound in the decanted cylinder was determined
through a comparison with a nominal 2 nmol mol
Schematic of the decanting procedure that was performed for the monoterpenes using 10 L Luxfer cylinders treated with different passivation types (Experis, SPECTRA-SEAL and an in-house treated SPECTRA-SEAL).
To determine the short and long-term stability of the four component
monoterpene reference PRM, mixture BB was regularly analysed over a 3-month (75 day) period. GC peak area responses of each terpene were ratioed to
A large number of samples are collected in the field during measurement
campaigns. It is imperative that these samples can be collected and stored in
a way that preserves the contents until they are analysed. One commonly used
option is the use of sampling canisters or vessels that have been evacuated
prior to use. Previous work has shown that the use of stainless steel
canisters for sampling terpenes in dry or humidified air can be problematic
(Batterman et al., 1998). Here we decant a portion of our 2 nmol mol
An alternative to PRM preparation in high pressure cylinders is dynamic preparation using permeation. The ReGaS2 is a mobile generator that can produce traceable reference gas mixtures of a number of species, including terpenes (Pascale et al., 2017).
The method is based on permeation and subsequent dynamic dilution: a permeation tube containing the pure terpene is stored in an oven used as permeation chamber. The pure substance permeates at a constant rate into the matrix gas and can be diluted to give the desired amount fraction. The mass loss over time of the permeation tube is precisely calibrated using a traceable magnetic suspension balance. All parts in contact with the reference gas are coated with SilcoNert2000®.
The ReGaS2 mobile gas generator was fitted with a limonene permeation tube
and set to dynamically generate an output of nominally 4 nmol mol
The evaluation of measurement uncertainty was in accordance with the “Guide to the expression of uncertainty in measurement” (Joint Committee for Guides in Metrology, 2008).
In the Supplement there is a description of an uncertainty evaluation when comparing the response of an unknown mixture against a validated calibration standard, e.g. a PRM (Eqs. S1–S4).
The adsorption of the monoterpenes to the internal surfaces of the cylinder and valve were investigated through a series of decanting experiments as detailed in Sect. 2.3. The results for the different passivation types at 120 bar are shown in Fig. 2. There is a tabulated summary of the results of the decanting experiments in Tables S2–S7.
The relative difference between the amount fraction of the decanted mixtures and the expected amount fraction based on gravimetric value of the mixture before any decanting. Each decantation was performed twice for each passivation type.
Decant losses of monoterpenes in the 10 L cylinders internally passivated
with Air Products Experis treatment were minimal (Tables S2 and S3). No
statistically significant differences were observed, therefore it can be
confirmed, in agreement with Rhoderick and Lin (2013) that Experis cylinders
are the most suitable for containing monoterpene PRMs. Figure 3 shows that
the amount fraction does not appear to be influenced by the pressure within
the cylinder, down to low pressure at 30 bar, as all agree within the
measurement uncertainty and there is no overall directional trend. Below
30 bar we observe that the ratio is less than 1 for all components. While
the results are within the measurement uncertainty, wall factors could have
an influence on composition at low pressures (
Figure 2 and Tables S3 and S4, show the initial decant, and repeat decant at
120 bar, in 10 L cylinders passivated internally with BOC SPECTRA-SEAL
treatment. Aside from the
The relationship between cylinder pressure and monoterpene amount
fraction after normalisation to
To investigate potential degradation components, a sample of a monoterpene
mixture in an internally treated SPECTRA-SEAL cylinder was loaded onto a set
of Chromasorb-106 and Tenax sorbent tubes (both packed in-house) and analysed
on a Thermal-Desorption Gas Chromatograph Mass Spectrometer (TD-GC-MS).
Similarly, a portion of the reference PRM (mixture BB) was also loaded onto
Chromasorb-106 and Tenax sorbent tubes and analysed by TD-GC-MS. Five major
peaks were consistently observed in the chromatograms of the desorbed tubes
(Fig. 4). The additional peaks observed in the sample from the SPECTRA-SEAL
cylinder were identified as the following monoterpenes: (a)
Interestingly,
Typical chromatograms for a stable (pink) and an unstable (grey)
terpene mixture. The nominally 2 nmol mol
The short-term and long-term stability of mixture BB was determined through a
series of experiments as detailed in Sect. 2.4. Over the first 3-month
period that mixture BB was analysed the ratio of the monoterpene to
The short-term stability of reference PRM (mixture BB) at nominally
2 nmol mol
Mixture BB was prepared on 2 June 2015 and mixture CC was more than 2 years
later (904 days) on the 22 November 2017. A set of measurements were run to
compare mixture BB and CC. This was repeated twice in the space of 2 days.
Gravimetric values were normalised and the peak areas of the monoterpenes
were then compared and the differences recorded (Table 2). It was found that,
unsurprisingly,
Mixture F and G containing
Comparison showing the percentage difference between PRM mixtures prepared more than 2 years apart to assess the long-term stability of mixture BB and mixture F. Gravimetric values were normalised and the peak areas compared. There are two columns for the comparison of mixture BB and CC as the comparison was repeated on 2 consecutive days.
Field campaign measurements require the short-term storage of VOC samples. Sampling canisters made from electropolished steel are frequently used despite losses being observed (Batterman et al., 1998). Another solution is to use SilcoNert 2000® treated canisters (silanisation treatment, Silcotek). However, the SPECTRA-SEAL cylinders that performed poorly in the decant experiments also use a silanisation surface treatment, therefore it was important to determine the suitability of SilcoNert 2000® treated canisters for short-term storage of monoterpenes. Following decant of mixture BB into the SilcoNert 2000® treated canister the contents were compared against mixture BB after 1, 8 and 83 days. The results of this are shown in Fig. 6.
The short-term stability of mixture BB decanted into a SilcoNert
2000® treated canister compared as a ratio of
the area of each monoterpene normalised relative to the
No statistically significant trends were observed for the stability,
although higher than normal relative standard deviations in the GC peak areas
were observed (
It appears that unlike the SPECTRA-SEAL passivated cylinders, the SilcoNert 2000® treated canisters would allow the storage of multi-component monoterpene standards for up to 3 months and still meet the data quality objective criteria recommended by GAW and its scientific advisory group (Hoerger et al., 2015). Nevertheless, this does not mean that a whole air sample containing terpenes or a broad array of terpenes together would behave in the same way due to the impact of humidity, therefore more work is required to determine this. However, it would suggest that decanting of PRMs for transport into the field in small SilcoNert 2000® treated canister should be possible.
Two SI traceable preparation techniques for producing reference gas mixtures
were compared. One was the preparation of static gravimetric PRMs, the other
the generation of a dynamic reference standard from ReGaS2 using a permeation
tube. From the weighing of the limonene permeation tube and from the data
that was logged for the nitrogen flow and subsequent dilution it was
calculated that the ReGaS2 mobile gas generator was outputting
The static PRM that was used in this comparison (mixture BB) was also one of
the mixtures used as part of the CCQM-K121 monoterpene key comparison at
nominally 2.5 nmol mol
One of the reasons for the systematic bias between the two approaches can be
attributed to the temperature at which the permeator was operated, as the
temperature was observed to have a strong influence on the reproducibility of
the permeation rate. At lower temperatures, such as 30
The second reason is the 15 %–20 % decrease in the permeation rate. To investigate this further the permeation rate of limonene from the ReGaS2 dynamic system was measured over an 11-month period between March 2017 and February 2018. The decrease in the permeation rate was determined to be 35 % over this temporal period (Fig. S3) for the same temperature. The measurement of the permeation rate in the magnetic suspension balance lasted between 2 and 7 days with an associated uncertainty between 0.5 % and 1.5 % for one measurement at one temperature thus suggesting that the uncertainty assigned to ReGaS2 during the comparison was too low.
A decrease in the permeation rate of this magnitude coupled to the high uncertainties at such low temperatures would be enough to compensate for the systematic bias observed between the two approaches. Despite the systematic bias observed between the two methods at this trace level, the results of this first comparison are encouraging and show that state-of-the-art developments are being made with dynamic systems capable of delivering reliable outputs suitable for calibrating systems in the field.
In this paper we have investigated the short-term and long-term stability of monoterpenes in differently internally passivated cylinders. The choice of cylinder passivation is critically important in the preparation of monoterpene gas mixtures. We have demonstrated that Experis treated cylinders are the most appropriate for containing low amount fraction monoterpene PRMs and that the amount fraction is not influenced by pressure between 30 and 120 bar.
The need for suitable storage and transport of PRMs into the field has driven us to investigate the suitability of using SilcoNert 2000® treated canisters for monoterpenes. It was discovered that SilcoNert 2000® treated canisters could hold monoterpenes for up to 3 months with an uncertainty of 10 %, in line with GAW data quality objectives.
We compared the ReGaS2 dynamic mobile generator against high pressure static
PRMs gravimetrically prepared at NPL. It was found that the output of
limonene from dynamic ReGaS2 was 15 %–20 % lower than calculated.
These differences correspond to less than 0.5 nmol mol
PRM gravimetric data are provided in the paper. Decanting data
are provided in the Supplement as percentages. GC-MS data were only used
qualitatively; however, the spectra are also available in the supplement.
Further information is available in the KeyVOCs EMRP final reports which can
be found at:
The supplement related to this article is available online at:
NDCA and DRW wrote the paper. NDCA made the PRM gas mixtures, performed the labwork and designed the experiments. CP built the ReGaS2 dynamic system. DRW made the plots and led the decant work as part of the EMRP funded KeyVOC project. DRW, PJB and BN supervised. All authors discussed the results and commented on the manuscript.
The authors declare that they have no conflict of interest.
NPL and METAS were both funded as part of the European Metrology Research Programme (EMRP) “Metrology for VOC indicators in air pollution and climate change (KEYVOC)”. The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union. Edited by: Eric C. Apel Reviewed by: two anonymous referees