International Journal of Environmental Analytical Chemistry
ISSN: 0306-7319 (Print) 1029-0397 (Online) Journal homepage: http://www.tandfonline.com/loi/geac20
Hyphenated and non-hyphenated chromatographic techniques for trace level explosives in water bodies – a review Cristina Veresmortean & Adrian Covaci To cite this article: Cristina Veresmortean & Adrian Covaci (2018): Hyphenated and nonhyphenated chromatographic techniques for trace level explosives in water bodies – a review, International Journal of Environmental Analytical Chemistry, DOI: 10.1080/03067319.2018.1478969 To link to this article: https://doi.org/10.1080/03067319.2018.1478969
Published online: 07 Jun 2018.
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INTERNATIONAL JOURNAL OF ENVIRONMENTAL ANALYTICAL CHEMISTRY https://doi.org/10.1080/03067319.2018.1478969
Hyphenated and non-hyphenated chromatographic techniques for trace level explosives in water bodies – a review Cristina Veresmorteana,b and Adrian Covacic a
Department of Chemistry, The Graduate Center of the City University of New York, New York, NY, USA; Department of Sciences, John Jay College of Criminal Justice, The City University of New York, New York, Wilrijk, USA; cToxicological Center, University of Antwerp, Belgium
b
ABSTRACT
ARTICLE HISTORY
Explosives are a class of xenobiotic, which pose a permanent and increasing concern on human and ecosystem health. These chemicals are highly toxic, some carcinogenic, and their detection in rural areas, surrounding military bases and weapon training facilities, became imperious. Quality of potable water particularly in rural areas, where wells are the primary water sources, is of great importance. More effective ways of extraction with less solvent consumption, coupled with chromatographic separation and mass spectrometry/ultraviolet detection techniques, are needed to quantify these trace level hazardous compounds at sub-ppb levels. The ultimate scope of measuring the concentration for these environmental contaminants is to build effective strategies for site protection, remediation and removal. This complex and original review brings together a vast amount of published work on conventional and modern explosives extraction approaches and their means of identification and quantification by hyphenated chromatographic techniques. With a strong focus on aqueous sample preparation and a multitude of analysis methods presented, this paper enables the researcher to a good assessment on past, present and future aspects. Latest progress in the highresolution instrumentation is also briefly discussed.
Received 15 January 2018 Accepted 8 May 2018 KEYWORDS
Explosives; mass spectrometry; chromatography; water contamination; sample preparation; ultraviolet detection
1. Information on identity, toxicity and properties of target explosives 1.1. Overview Listed as highly energetic compounds, explosives will rapidly decompose under chemical or physical stimuli, causing fast heat evolution and generation of high pressures of gases, e.g. NOx, H2O, CO and CO2. According to their susceptibility to detonation, explosives are classified into three categories: primary, secondary and tertiary. Primary explosives are highly sensitive to ignition by friction, shock, heating or spark and serve as detonators for secondary explosives. Secondary explosives are usually mixed with primary explosives and linked together with plasticizers, waxes or stabilizers. Organic secondary explosives are used for military operations, their detonation resulting in wide
CONTACT Cristina Veresmortean
[email protected]
© 2018 Informa UK Limited, trading as Taylor & Francis Group
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C. VERESMORTEAN AND A. COVACI
spreading of toxic debris over waterbodies and other environmental entities. Tertiary explosives do not self-detonate, unless a secondary explosive is present [1,2]. Belonging to the aforementioned category of organic secondary explosives and targeted in this review paper are: (a) nitroaromatics (NACs), a subclass of compounds holding a C-N-NO2 bond: NB (nitrobenzene), DNBs (dinitrobenzenes), TNB (trinitrobenzene), DNTs (dinitrotoluenes), TNT (trinitrotoluene); (b) nitroamines: RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine or hexogen) and HMX (octahydro-1,3,5,7-tetranitro1,3,5,7-terazocine or octogen); (c) nitroesters: PETN (pentaerythritol tetranitrate or penthrite). These pollutants were selected to be presented, due to their high probability to exist in waterbodies next to military establishments and based on numerous publications, reports and statistics about their negative impact on human health and environment [3]. TNT, RDX and HMX are being regularly investigated, as part of wastewater monitoring process. Due to the nitro group existence with electron-withdrawing properties, these compounds are not prone to aerobic attacks, making them persistent environmental pollutants. DNB, DNT and TNB will appear as co-pollutants, with faster mobility in waterbodies. As subjects of our interest, organic secondary explosives will be addressed in this paper, with the selection rationale as follows: (a) NB is an oxidizing agent used in explosives manufacturing. Owing a high biodegradability, it will not accumulate in soil or sediment, thus showing a potential to be present in surface water and groundwater [4]. NB is also listed as a transformation product of TNT [5]. (b) DNBs, by-products of TNT, can be formed in small quantities in the manufacturing process and accidentally released to the environment [6]. DNB is identified as a degradation product of TNT, in cyclic voltammetry studies [7]. Three isomers are encountered: ortho, meta and para DNB. The physical and chemical properties of the isomers are generally similar, with the difference that 1,2 isomer is more soluble in water [8]. (c) TNB is a compound resulting from TNT’s photodegradation, being itself resistant to further photodegradation. Biotransformation of TNB yields to dinitroaniline, a target contaminant on Environmental Protection Agency (EPA)’s list [9]. (d) DNTs are not specific explosives targets, being detected as impurities in TNT manufacturing or as degradation products of TNT [7]. DNTs are also used as propellants [9]. Among six isomers 2,4 and 2,6 DNT are the most common forms. The other forms make up only 5% of the technical grade [3]. (e) RDX and HMX are the most mobile contaminants present in groundwater [9]. The high explosive RDX production is limited to US Army ammunition plants solely [2]. RDX and HMX are chemical homologues having similar properties. HMX is used as booster in Octol (a TNT-based explosive) and as oxidizer in gun propellants. RDX and nitroglycerin form a plastic explosive mass [10]. (f) PETN is the strongest explosive known, less soluble in water and less persistent in the environment, compared to RDX. PETN is used in blasting caps, detonating fuses and manufacturing of pentolite [10]. Replacing plastic-based RDX explosives with plastic-based PETN might become a viable option for the US
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Department of Defense, in the efforts to minimise contamination, especially at military training ranges [11].
1.2. Toxicity Many explosives are very toxic, some mutagenic or possibly carcinogenic [12]. NB is one of the extremely toxic chemicals, detrimental to human health, and is listed as a contaminant of concern. At higher than 30 µg/L nitrobenzene concentration, symptoms include fatigue, weakness, dyspnoea, headache and dizziness. Potential human exposure at elevated levels of nitrobenzene might include depressed respiration, bluish-grey skin, disturbed vision and coma. Reproductive effects, such as a decrease in fertility, reduced testicular weights and decreased sperm production, have been found in animal studies. The criterion for protecting freshwater aquatic life is set at concentration level P > 1×10−9; different values are reported in literature [129]; b range: 1.0×104 > S > 1.0×10−3; c range: 7>log Koc>-3; d range: 7>log Kow>-3; e level dependent on the organic carbon fraction; f level dependent on the photolysis rate.
Nitroesters (less polar) Penta-erythrtol tetranitrate (PETN)
78-11-5
99-65-0
C6H4N2O4
1,3 Dinitrobenzene (DNB)
C5H8N4O12
98-95-3
C6H5NO2
Octahydro-1,3,5,7-tetranitro1,3,5,7-terazocine (HMX)
CAS
Formula
Category Nitroaromatic (polar) Nitrobenzene (NB)
Molecular weight MW (g/ mol)
Table 1. Physical properties of selected explosives and probability in environmental waters.
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The purpose of this review is to fill in lacking information of precursor reviews, on explosives extraction from water entities existent in the proximity of military establishments and its pertaining methods of analysis.
2. Experimental considerations on sample preparation Knowledge about sample and analyte type, matrix of provenience, amount of sample available, previous existence of analytical methods or the need to develop new ones, instrumentation, materials available, etc., are among important factors to be evaluated prior to planning an analysis [24]. Nowadays, as groups are switching towards a greener and cheaper method, improved sample preparation methods are essential [25].
2.1. Sample collection and preservation Trace evidence of explosives in environmental samples such as, soil, sediment, air, water, dust and melted snow [26] has become an intensely researched topic. Collection of samples from the contaminated sites, such as military establishments, practice ranges, ammunition plants, dumping areas and burials, demolition zones, snow avalanche control areas, etc., must follow well-established guidelines [27]. The guidelines must emphasise on conservation of sample’s chemical, physical and biological attributes and be processed with minimal contamination [28]. Solutions that contain trace level quantities of organic explosives must be kept in silanised glass containers, at below 4°C, in the absence of light. A water effluent sample should be acidified to pH 3.5 and 10% acetonitrile added. Some of the compounds such as DNT will be retained by plastic containers, not determined with TNT, RDX and HMX [29]. A useful approach for surfaceand groundwater sampling is given in [30]. Passive sampling is a novel method of analyte enrichment from environmental water, presenting many advantages over the already established sampling procedures: larger volumes, rapid collection, reduced field sampling variability, ease of handling, etc. The sampler coating polymer can be tailored to selectively concentrate TNT and RDX from marine systems [31].
2.2. Preparation techniques for water samples in environmental forensics 2.2.1. Preliminary processing Schramm et al. developed an LC/MS method for DNB, TNB, TNT, DNT, HMX, RDX and PETN analysis in groundwater samples from a military site, without a pre-concentration step. Samples were filtrated through syringe filters and placed directly into the autosampler [22,32]. Mu et al. (2012) followed the same approach of filtration using a 0.22 µm nylon membrane, with direct injection into the LC/MS system [33,34]. Similarly, other groups have opted for minimal sample preparation, in quantifying TNT and RDX present in water samples. Analytes are extracted with an organic solvent, which then is separated from aqueous phase and injected directly into the chromatographic system [35]. Other preliminary processing steps include centrifugation, solvent exchange, desalting, evaporation, freeze drying, evaporation, distillation, microdialysis, sieving and grinding (for solid samples) [36].
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2.2.2. Conventional extraction methods overview Among the extraction methods of high usage for liquid samples (statistical data of 2002) are conventional SPE (solid-phase extraction), conventional LLE (liquid–liquid extraction), supercritical fluid extraction (SFE) or SFC (supercritical fluid extraction) and column chromatography. USE (ultrasonic extraction) technique is mainly employed for soil and sediment extraction, although some publications list sonication coupled with HF-LLME (hollow fiber liquid–liquid microextraction) to monitor the fate of explosives in water samples [37]. Following on another publication, an ultrasonic titanium probe was dipped into a reaction mixture of water sample and extracting solvent, to aid in extraction of NB and TNT [38]. Conventional SLE (solid-liquid extraction), a seldom used replacement method for LLE, will solve some of the disadvantages of LLE by using a highly purified, high-surface area, inert DE (diatomaceous earth) sorbent. SFE can be directly applied to either air, solid or liquid matrices. A combination of SPE and SFE had been used for DNT extraction from well and surface water samples [39]. All of the common, old-fashioned extraction techniques employ high or relatively high volumes of solvent. They had gradually been replaced by safer techniques, with minimal solvent usage. A comprehensive collection of sample preparation techniques for chromatography is presented in Scheme 1, to aid the reader in a better understanding of abbreviations and classifications used throughout the literature.
Scheme 1. Classification of Extraction Techniques.
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2.2.3. Microtechniques overview Flowing with the new trend towards solvent-less applications, LLE has been scaled down to LPME (liquid-phase microextraction) by using microvolumes of extracting phase, and SPME has been introduced as a fast, solventless extraction method for trace level analysis. LLEs comprise of a multitude of microtechniques (membrane-assisted, single drop, drop in drop, fibers, ionic liquids, vortex/microwave assisted, continuous flow, directly suspended and floating drop), where the ratio of sample/solvent is very high. Enrichment factor (analyte concentration in the processed sample/analyte concentration in the initial sample) can reach 100 times higher values. Included in the same class are: membrane-assisted, salting-out, electrochemically modulated and cloud-point extractions [24,36]. Similarly, SPEs include microtechniques (fibers, stir-bar, thin-film, in tip, syringe, intube) with diverse materials used as sorbents and other techniques such as matrix dispersion and QuEChERS. SPME can be automated and hyphenated with GC/MS or LC/MS techniques [40]. The fibre is either immersed into a liquid, where from the analyte is adsorbed, or is placed in the headspace of the vial filled with sample. Volatile analytes sorb onto fibre of diverse polarities and either thermally desorb inside a GC injector or solvate into a mobile phase of an LC system [41]. A comprehensive diagram with different modes of SPME is outlined by Bagheri et al. in 2014 [42]. LPME and SPME are extensively used for NB, DNB, TNB, TNT and DNT extraction in soil and water samples (Table 2). Variations of these techniques are outlined in a review of Padron et al. [43]. 2.2.4. LLE and LLME functionalised interfaces Conventional or common LLEs have been described earlier in this paper. A timeline review of LPME and a very detailed description of the variations of the technique are given in a chapter written by Fernández and Vidal [44]. Choice of solvents as extraction phases in LPME is illustrated by Dadfarnia and Haji-Shabani [45]. 2.2.4.1. Conventional LLE method. Nitrobenzene has been extracted from water samples by the means of funnel extraction separation. The organic layer was collected and a small volume directly injected into a GC/MS instrument. Calculated recovery reported was 108% [46]. EPA method 3650B lists DNBs and 2,4 DNT being extracted in organic solvents, by liquid–liquid partitioning method [47]. Discontinuous and continuous LLE has been applied for enrichment of water samples, in the area of former ammunition plants in Germany, by the German group of Lewin et al. in 1997. Their data show recoveries of 76–78% for DNT isomers. The limit of detection for HMX and RDX is 20ug/L and 35ug/L, respectively. By-products and metabolites of TNT had been evaluated [48]. 2.2.4.2. Membrane assisted. This type of extraction involves two liquid phases, organic and aqueous separated by a membrane, which can also function as sensor/ biosensor and are suitable for compounds containing nucleophilic groups, such as TNT and RDX. Detection range for TNT and RDX in this displacement immunoassay used is at the femtomole level [49,50].
2,4 DNT
1,3,5 TNB
1,3 DNB
Target NB HPLC/UV GC/MS HPLC/UV GC/MS HPLC/DAD _
SPE SPME D-µSPE DUSA-DLLME
IL-LPME
Filtration SPE SPE
APCI(-) LC/MS APCI(-) LC/MS GC/MS ESI(-) LC/MS HPLC/UV GC/MS
Filtration SPE MW-CNT, HF-SLPME SPE SPME, SPE HF-LPME _ APCI(-) LC/MS APCI(-) LC/MS APCI(-) LC/MS
GC/MS APCI(-) LC/MS HPLC/UV HPLC/UV _
MW-CNT, HF-SLPME Filtration SPME, SPE Amberlite XRD _
_ -
GC/ECD APCI(-) LC/MS APCI(-) LC/MS HPLC/UV GC/MS
Amberlite XRD SPE SPE SPE HF-LPME
_
GC/MS
Analysis
SPE (EPA 3535)
Extraction
_ 0.5 0.08 0.092; 0.022
0.02 0.01 0.03–0.94 0.142; 0.013 3.3; 0.1 0.57
0.03–0.94 0.5 7.2; 0.03 50 _
