Environ. Sci. Technol. 2009, 43, 3020–3029
A Review of Methods for the Determination of Reduced Sulfur Compounds (RSCs) in Air SUDHIR KUMAR PANDEY AND KI-HYUN KIM* Atmospheric Environment Laboratory, Department of Earth & Environmental Sciences, Sejong University, Seoul 143-747 Korea
Received November 19, 2008. Revised manuscript received March 10, 2009. Accepted March 16, 2009.
The importance of reduced sulfur compounds (RSCs) in air is wellknown for its significant effect on global atmospheric chemistry and malodor and quality of life. In this review, methodological approaches commonly employed for the analysis of RSCs such as hydrogen sulfide, methane thiol, dimethyl sulfide, carbon disulfide, and dimethyl disulfide in air are described. To this end, we focus on gas chromatography (GC) because it is the most feasible, frequently used, and widely accepted approach for the analysis of RSC in air. The advantages and possible limitations related to sampling and/or preconcentration methods are also discussed. The relative performance of different GCbased detection methodologies is evaluated in terms of basic quality assurance. Some alternative methods (i.e., other than GC) that deal with the determination of RSCs in air matrices are also discussed briefly. Finally, this review addresses the methodological developments of RSC analysis by highlighting current limitations and future developments.
Introduction Considerable efforts have sought the precise determination of reduced sulfur compounds (RSCs) or volatile sulfur compounds (VSCs) in air because of their potent role in the global atmospheric chemistry (1). The most abundant RSCs in the environment include hydrogen sulfide (H2S), carbonyl sulfide (COS), methane thiol (MeSH), dimethyl sulfide (DMS), carbon disulfide (CS2), and dimethyl disulfide (DMDS) (2). The dominant fractions of RSCs originate from natural and/ or biogenic sources (3-7). As most RSCs (exception for COS) have a strong potential to be oxidized (e.g., formation of sulfate aerosols), they are often designated to exert influences on the Earth’s radiation budget and climate forcing (8). RSCs are normally present at very low concentration levels (i.e., below ppb levels). If present in excess quantities, these RSCs can also cause social and health problems (9). A number of RSCs are present in various matrices and exhibit large concentration differences (e.g., several orders of magnitude) among different species and/or different environments (10). It has been a challenge to measure RSCs in ambient air without bias from reactivity (e.g., absorption, adsorption, and photo oxidation 11-13) or instrumental instability (2). In this review, we focus on the developments of methodological approaches commonly available for the analysis of the most common RSCs in air (such as H2S, MeSH, DMS, CS2, and DMDS (Table 1)), with a special focus on gas chromatographic (GC) methods. In the course of this review, * Corresponding author e-mail:
[email protected]; tel: 82-2-4999151; fax: 82-2-499-2354. 3020
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we also discuss techniques most commonly applied for sampling and detection of RSCs from gaseous matrices with an aid of preconcentration or isolation methods. In addition, the analytical performance of different methodologies is discussed in terms of basic quality assurance (QA) criteria.
Background of Reduced Sulfur Analysis in Air In the analysis of RSCs in air matrices, the gas chromatography technique has been and is the most common methodology because of its excellent separation capability and quantitative recovery (14, 15). The GC-based analysis commonly involves a sample collection stage, injection, and separation on a chromatographic column. The final detection can then be carried out through various sulfur-selective (or universal) detectors. Direct chromatographic analysis or direct injection (DI) of RSCs into a GC injector is highly recommended when the concentration of samples is in the detectable range of a given GC-setup. The use of the DI method can reduce possible loss (or gain) due to contact with different surface types and analysis time by eliminating time-consuming procedures such as supplementary sample treatment by which contamination or loss of analytes can occur (16-18). However, the application of the DI approach is often limited, as most detectors of the GC method are not sensitive enough to cover ambient samples that are typically below a few ppb in concentration. Therefore, research has sought to improve and develop GC methods based on sulfur-specific detectors. As a result, a number of choices are currently available for detector types which include electron capture detector (ECD), flame photometric detector (FPD), pulsed flame photometric detector (PFPD), sulfur chemiluminescence detector (SCD), atomic emission detection (AED), Hall electrical conductivity detector (HECD), and photo ionization detector (PID). Combined application of a mass spectrometer (MS) and GC system has also been reported by numerous authors (19-22). However, these detectors vary in detection characteristics in terms of operation mode and relative response properties to individual sulfur compounds. To allow the analysis of increasingly smaller quantities of target compound(s), the range of instrumental detectability needs to be improved. As a means to extend the detectability of a given GC system, one can increase the total amount of analytes injected by adopting some preconcentration (or sample enrichment) stages: (1) sorption on certain metal surfaces such as gold, palladium, and platinum (23-25), (2) sorption on solid adsorbents such as activated charcoal, silica gel, aluminum oxide, graphitized carbon black, molecular sieves, and porous polymers (26-28), and (3) cryogenic trapping (18, 22, 29, 30). However, the analysis of low level 10.1021/es803272f CCC: $40.75
2009 American Chemical Society
Published on Web 04/03/2009
TABLE 1. Brief Description of the Reduced Sulfur Compounds (RSCs) Reviewed in This Study order
RSCs
1 2 3 4 5
hydrogen sulfide methane thiol dimethyl sulfide carbon disulfide dimethyl disulfide
6 7 8 9 10 11
carbonyl sulfide ethane thiol propane thiol pentane thiol iso-propane thiol iso-butane thiol
acronym
CAS no.
chemical formula
molar mass (g mole-1)
[A] RSCs most frequently referred in this study H2S 7783-06-4 H2S MeSH 74-93-1 CH3SH DMS 75-18-3 (CH3)2S CS2 75-15-0 CS2 DMDS 624-92-0 (CH3)2S2
34.1 48.1 62.1 76.1 94.2
[B] Other RSCs 463-58-1 75-08-1 107-03-9 110-66-7 107-03-9 109-79-5
60.1 62.1 76.2 104 76.2 90.2
COS EtSH PrSH PeSH i-PrSH i-BuSH
sulfur species through such modifications can be subject to positive blank (e.g., memory effect) or sorptive loss in the chromatographic system (31, 32). A general outline of the analytical protocols commonly employed in RSC analysis is illustrated in Figure 1.
GC-Based Analysis Sampling and Preconcentration Strategies. RSCs from air matrices can be collected in vessels such as glass bulbs, canister bags, polymer bags, and Tedlar film bags. Considering the highly reactive nature of sulfur compounds, the sampling vessels should be inert enough to reduce adsorptive loss. Moreover, careful attention should be given to tubing and connecting materials used for sampling of sulfur compounds. This is because certain materials can act as significant sources of bias in the determination of RSC concentrations (33). For instance, the storage ability of different sampling containers (i.e., standard Tedlar sample bags, black/clear layered Tedlar sample bags, and Silcosteel sample cylinders) was investigated using a gaseous multi-
OCS CH3CH2SH CH3(CH2)2SH CH3(CH2)4SH CH3(CH)2SH CH(CH3)3SH
component standard (MeSH, EtSH, DMS, ethyl methyl sulfide, 2-PrSH, 1-PrSH, 2-BuSH, diethyl sulfide, and 1-BuSH) prepared at a concentration of 1 mg m-3 (for each compound) in nitrogen (31). The study revealed that RSC concentrations in the black/clear layered Tedlar sample bags decreased noticeably (e.g., up to 10% for MeSH) after 2 days of storage, whereas those in Silcosteel sample cylinders were stable. However, the RSC recoveries of the latter exceeded 100%, especially for compounds with higher boiling point. These authors later identified that silicone tubing used to transfer RSCs into a standard Tedlar sample bag was the source of the artifact (32). As the sorptive loss of the heavier RSCs (on such tubing material) proceeded less effectively than the lighter ones, the results of the former were overestimated in their early study (31). In continuation of these efforts, the loss patterns of four RSCs (H2S, MT, DMS, and DMDS) were investigated against five different tubing materials (stainless steel, silicone, PTFE Teflon, tygon, and copper) (12). The results indicated that the loss patterns of RSCs are distinct
FIGURE 1. General protocols of reduced sulfur compounds (RSCs) analysis in air matrices. The abbreviations used in figure are as follows: LC-AFS ) liquid chromatography-atomic fluorescence spectrometer, IC ) ion-chromatography, SPME ) solid phase microextraction, FPD ) flame photometric detector, PFPD ) pulsed flame photometric detector, MS ) mass spectrometer, AED ) atomic emission detector, SCD ) sulfur chemiluminescence detector, and TD ) thermal desorber. VOL. 43, NO. 9, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Summary of Preconcentration Methods Commonly Employed for the Analysis of RSC in Ambient Air Samples method
RSC type
collection media
source
(i) sorptive metals
DMS DMS DMS total sulfur
gold-coated glass wool gold coated glass beads gold plated sand metal foils
Curran et al. (48) Braman et al. (24) Braman et al. (24) Farwell et al. (47)
(ii) sorption on solid sorbent
DMS
activated charcoal
Lewis et al. (52)
COS, DMS
molecular sieve
DMS EtSH, DMS, CS2, PrSH, DMDS, and PeSH DMS
Tenax TA Tenax TA
Wylie et al. (53), Davison et al. (45), Bruyn et al. (54) Pio et al. (55) Rosa Ras et al. (22)
Tenax TA
Bruyn et al. (54)
H2S, MeSH, DMS, CS2, and DMDS
cryofocusing on cold trap packed with silica gel + carbopack B cryogenic trap silanized glass-lined steel tube filled with chromosorb W
Kim (30), Pandey and Kim (18)
(iii) cryogenic trapping
DMS and CS2 COS, MeSH, and CS2
(iv) solid phase microextraction (SPME)
H2S, MeSH, EtSH, DMS, and DMDS
100 µm PDMS, 65 µm DVB-PDMS, and 75 µm CAR-PDMS fibers in a Tedlar bag 100 µm PDMS and 75 µm carboxen-PDMS fibers from permeation tubes
Lestremau et al. (82)
RSC (Not specified)
PTFE tubes
Dincer et al. (20)
H2S, MeSH, EtSH, and PrSH
trapping/ preconcentration in alkaline aqueous phase collection on diffusion scrubbers
Bramanti et al. (106)
MeSH, BuSH, i-PrSH, and i-BuSH
(v) other methods (non-GC)
sulfide, methanethiolate, sulfite, and sulfate
among different tubing types due to the unique loss mechanisms of each RSC against those tubings. A major difficulty in sampling of RSCs is interference caused by atmospheric oxidants such as SO2, O3, and NOx which are common in ambient air. To help overcome these problems, many substances were developed as scrubbers including PTFE, Tygon, glass fiber filters, chromosorb, anakrom, and glass beads with a coating of Na2CO3 or MnO2 (24). The Na2CO3 based scrubbers were successfully used, showing reliable results in field applications (34, 35). KOH or NaOH-based filters were also used as oxidant removing materials by impregnation on a prefilter device (36, 37). Some studies have also evaluated the scrubbing efficiency of the aforementioned materials. For instance, Saltzman and Cooper (38) reported that a carbonate-based Anakrom scrubber was superior to KOH filter. In another intercomparison study, a KI/glycerol/Vitex filter was found to be superior to filter scrubbers with Na2CO3 and KOH/NaOH (39). Furthermore, Na2CO3, FeSO4.7H2O, KI, and KBr were ineffective for oxidant removal in moderately polluted air, if applied as either pure salts or coated on Anacom P (40). Inomata et al. (28) used ascorbic acid as a potent scrubbing agent to remove atmospheric oxidants for the simultaneous analysis of five RSCs (COS, H2S, CS2, MeSH, and DMS) with flame photometric detection (FPD). Another considerable problem in determining RSCs is relative humidity. This is because water significantly lowers the capacity of adsorbents and can clog cryogenic traps (41). Moreover, it can cause baseline perturbations and retention 3022
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Ivey and Swan (51) Hobe et al. (8)
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Haberhauer-Troyer et al. (81)
Ohira and Toda (105)
time shifts in chromatography to deteriorate detection (41). To overcome the humidity problem, a number of approaches have been attempted. For instance, the use of the drying agent CaCl2 increased the sampling capacity of Molecular Sieve 5A markedly (42). Commercially available Nafion dryers (Perma-Pure) combined with a cotton oxidant scrubber was tested under laboratory and field conditions. This approach was found to be suitable for the measurements of most RSCs (such as H2S, COS, MeSH, DMS, and CS2) in the pptv-range in both dry and humid air (43). The applicability of two different types of Nafion membrane dryers (based on countercurrent flow and desiccant drying) was tested at ambient concentration levels (1-5 ppbv) in combination with a new ozone scrubbing material (polyphenylene sulfide wool (noXon-S)) for adsorptive sampling of selected VSCs (MeSH, DMS, i-PrSH, and i-BuSH) (41). No analyte losses occurred with either type of dryer at relative humidity (RH) of e50%, while between 6 and 32% of the thiols tended to be lost at higher RH values (>50%) even after conditioning. The combined use of a Nafion membrane dryer and a noXon-S ozone scrubber can thus produce artifact-free sampling for many sulfur compounds (41). Sample enrichment or the preconcentration step is desirable to analyze samples with low concentration levels (i.e., subppb level) (Table 2). Because many types of enrichment approaches have been tested previously, these issues will be discussed briefly in the following subsections. Sorption on Metal Surfaces. The extent of RSC sorption can be affected greatly by the type of metals (mainly gold,
palladium, and platinum) used for such reaction (23-25, 44). These metallic materials are also found in modified forms such as glass or quartz tubes filled with gold wool, goldplated sand, or metal foils (24, 40, 45-48). The most common target for these approaches is dimethyl sulfide (DMS). For instance, preconcentration of ambient DMS on gold wool was applied to achieve a lower detection limit of
