Direct analysis and monitoring of organosulphur compounds in the gaseous phase using portable mass spectrometry

Organosulphides are ubiquitous in the natural world and are important in the agriculture, pharmaceuticals and petrochemical sectors. For the first time a lightweight (12 kg), manportable membrane inlet mass spectrometer (MIMS) has been employed to analyse volatile organosulphur compounds (VOSCs) in the gaseous phase. Monitoring of such compounds in field conditions (outside the chemical laboratory). Representative compounds tested include: 2-methyl-2-propanethiol, 1-propanethiol, diethyl disulphide, 1-butanethiol, ethanethiol, thiophene, methyl ethyl sulphide and dimethyl disulphide. Experiments in the gas phase were performed at low parts-per-billion (ppb) analyte levels. The results obtained showed low limits of detection (high parts-per-trillion ppt), very good linear regression within the examined concentration range, fast membrane response times and good repeatability with relative standard deviation, RSD < 4%. Analysis of a complex multi-component gaseous mixture of organosulphur compounds was also demonstrated.


INTRODUCTION
The continuous increase of air and water pollution poses a major concern for public health as well as contributing to climate change. The extensive use of volatile organic compounds (VOCs) is well known to contribute to environmental pollution [1][2][3][4][5][6][7][8][9][10]. Volatile organosulphur compounds (VOSCs), a subclass of VOCs, are mainly released into the atmosphere from industrial waste, waste management facilities and from natural sources (e.g. volcanoes, wildfires, vegetation, fossil fuels, oceans) [11]. The removal of organosulphides is also a major activity in the petrochemical sector and monitoring of organosulphide compounds is important in the pharmaceutical sector and in agriculture. VOSCs have been previously investigated in the gaseous phase using headspace gas chromatography (GC) with flame ionisation detection (FID) and/or GC coupled with an electrolytic conductivity detector (ELCD) [12]. Benchtop MIMS [13,14] and proton-transfer reaction time-of-flight mass spectrometry (PTR-ToF- MS) have also been utilised [15].
Sample collection of sulphur compounds in the gas phase is a complicated procedure as organosulphur compounds are highly reactive, can be adsorbed onto surfaces, reacting with them. They may also undergo photooxidation reactions and are prone to catalytic oxidation [16,17]. Thus, special containers (e.g. glass or stainless steel sorbent tubes, glass canisters, or gas sample bags) are required for their collection and transportation to the chemical laboratory, to avoid absorption phenomena or any type of chemical reactions. To prevent photochemical reactions, transparent glass canisters or sampling bags can be covered with light preventing surfaces e.g. aluminium foil. The complex reactive nature of the organosulphur compounds alongside with the time-consuming collection, transportation and storage of the samples demonstrate the need for in-situ and real-time chemical analysis using reliable systems with high sensitivity.
Portable MS and more specifically field-deployable MIMS  could be a possible solution for VOSCs detection and monitoring. MIMS has been previously utilised in the detection and on-line monitoring of VOCs and semi-VOCs for various applications; i.e. homeland security, forensics, environmental monitoring (air quality and water analysis), industrial processes, health and life science, molecular communications, etc. MIMS is a well investigated analytical methodology which employs a membrane interface to introduce sample molecules into the vacuum system of a mass spectrometer for analysis. It is based on a threestage pervaporation process. Sample molecules (from the gaseous, liquid or solid phase) absorb onto the surface of a membrane, diffuse through it and finally desorb into the vacuum system.
MIMS is a selective technique, property which depends on the membrane material (e.g. silicone, polytetrafluoroethylene -PTFE, polyethersulfone -PES, nylon, etc.), its characteristics (e.g. hydrophobic, hydrophilic, thickness, porosity, etc.) and the operational conditions (e.g. sample flow, suction flow, applied temperature, etc.). A widely used membrane material is polydimethylsiloxane (PDMS) which allows non-polar or medium polarity molecules to pass through it. In this work, a portable system based on triple filter quadrupole mass spectrometry (QMS) provided by Q-Technologies Ltd. UK [50] was utilised. The chemical sensor weighs less than 12 kg and has power consumption of 75 W. It can detect compounds with mass range m/z 1 -200 amu, and its limits of detections are in the ppt range. The triple filter QMS has several advantages over single QMS. It offers enhanced ion focusing, high sensitivity and resistance to contamination. In this study, we focus on the analysis (detection and monitoring) of low molecular weight VOSCs in the gas phase with potential focus in industrial systems and environmental applications.

Motivation
The motivation behind this study is to investigate the direct chemical analysis of volatile organosulphur compounds in the gas phase using a lightweight portable MS system. The target of this study is to provide a complete user-friendly analytical solution that will enable the measurement (detection, monitoring, alarming, etc.) of environmental pollutants and toxic industrial compounds in field applications. Selected chemicals for analysis are presented in Table 1, whereas Table 2 gives an overview of their main uses and possible health effects. The high toxicity of the investigated compounds and the human exposure to them may lead to severe health issues [1,2,4,6,7,8,9,10]. The following chemical analytes: 2-methyl-2-propanethiol (99% purity), 1-propanethiol (99% purity), diethyl disulphide (99% purity), 1-butanethiol (99% purity), ethanethiol (97% purity), thiophene (99% purity), methyl ethyl sulphide (96% purity) and dimethyl disulphide were purchased from Sigma Aldrich Co. LLC., U.K. Standard stock solutions of the above reagents were prepared in methanol at concentrations of 200 μg/mL and 1000 μg/mL. Methanol (HPLC grade, purity >99.9 %) was also provided by Sigma Aldrich Co. LLC., UK. All reagents were provided in the liquid phase and the stock solutions were stored in the fridge at 4 o C until their use.

Experimental Procedure.
Experiments were done using a hand-portable (12 kg) membrane inlet mass spectrometer  to check the repeatability of the tests and the stability of our system.

Organosulphur compound experiments
This experimental series was done to investigate the mass spectrometric detection and monitoring of volatile organosulphur compounds in the gas phase using our

Multi-compound experiment
In order to examine that our system can operate reliably with complicated samples (which commonly occur in practice), we generated a gas mixture of 5 compounds: 1-butanethiol, 2methyl-2-propanethiol, thiophene, dimethyl disulphide and methyl ethyl sulphide at 100 ppb each. Sample preparation was done within a stainless-steel mixing chamber connected with five pieces of Teflon tubing (150 mm length with 6.35 mm ID) to five 1.2 L glass flasks (one compound per flask) containing the gas standards. Gaseous samples were prepared following the same process as discussed above. A representative mass spectrum of a multi-compound mixture is shown in Figure 4. Characteristic mass fragments of the examined compounds are clearly shown. There is an overlapping peak (m/z 90) common in the mass spectrum of 1butanethiol and 2-methyl-2-propanethiol, which can deconvoluted to the individual components by calculating the contribution of individual compound components to the peak intensity.

Evaluation of the method
This section summarises the analytical characteristics of Liverpool MIMS system. We examine the following analytical criteria: a) membrane response, b) linearity of the data, c) limits of detection (LoD), d) repeatability and stability of the results obtained. Table 3

Validation experiments
To validate the performance of our system, we run simulation experiments in a test chamber with a volume of 16 m 3 to simulate an indoor environment [38]. The test chamber was purged overnight and filled with synthetic air (BOC Ltd. UK) before the start of the experiment. A vapour generator [53][54][55][56] was used to produce a mixture of 2-methyl-2-propanethiol, thiophene and dimethyl disulphide at 100 ppb each in a controllable way. The vapour generator (based on controlled evaporation of liquid analytes and their diffusion in a carrier gas stream) was connected with the test chamber and vapour samples were introduced by a sidewall hole with diameter of 6.35 mm. Injection time was 10 sec. In another sidewall, our MIMS system was sampling continuously the chamber air (as described above). A fan inside the chamber was providing a homogeneous distribution of the sample mixture. A representative mass spectrum of the injected mixture, 2 minutes after sample injection, is presented in Figure 6. At this time, the detected concentrations of 2-methyl-2-propanethiol, thiophene and dimethyl disulphide were calculated (based on the calibration curves) to be 2.26, 3.01 and 2.05 ppb respectively.

CONCLUSSIONS
In this paper we report a lightweight (12 kg) portable MS is able to detect and monitor both qualitatively and quantitatively organosuphur compounds in the gaseous phase for air quality monitoring purposes and/or industrial emissions. Proof-of-principle for trace detection of low molecular weight VOSCs using MIMS was demonstrated. During measurements, fast membrane response times (few seconds) were observed, whereas we got good linear calibration curves for the compounds tested and repeatability with RSD calculated to be 3.9 %. Good peak discrimination and separation were also obtained when a complex mixture of organosulphide compounds was tested. Detection experiments within a test chamber with a volume of 16 m 3 were also performed. These positive results allow future exploitation of this technology for example in environmental monitoring.
Future work includes field testing and validation of the technique described above. We also plan to further miniaturize our sensor using a lighter vacuum system (e.g. Pfeiffer HiPace 10 Turbo Pump and MVP-006 Diaphragm) and to improve its technical characteristics for higher pressure operation. Characteristically, for field testing, we plan to utilize a quadrupole mass analyser with mass range up to 500 amu in order to expand the range of the VOCs which we can detect on-site. A heated membrane sampling probe could allow detection of compounds at lower concentrations levels compared to the current LoD with faster response times (e.g. by a factor of 5), whereas integration of signal processing algorithms in our data analysis software would boost sensitivity. We also plan to benefit from machine learning and advanced chemometrics to allow our system to interpret data itself, generate alarms and make decisions autonomously. The later in combination with our additional miniaturization plans will enhance the capabilities of the system and will allow us to integrate it on autonomous robotic platforms for remote chemical sensing in various application areas.