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Analytical artifacts, sample handling and preservation methods of environmental samples of synthetic pyrethroids
Trends in Analytical Chemistry, Vol. 30, No. 11, 2011
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Analytical artifacts, sample handling and preservation methods of environmental samples of synthetic pyrethroids Saeed S. Albaseer, R. Nageswara Rao, Y.V. Swamy, K. Mukkanti Sample handling and preservation methods of environmental samples of synthetic pyrethroids (SPs) are very important and must be controlled to maintain sample integrity during analytical determinations. However, published literature has treated this issue only partly, and, in many instances, with contradictory conclusions. The tendency of SPs to adsorb – to varying degrees under different conditions – to surfaces and solid particulates with which they come in contact may be responsible for this situation. It has become evident that SPs discharged to water bodies are present mainly in the adsorbed state, and that affects their bioavailability and the reliability of analytical results. Refrigeration and storage in the dark are prerequisites for stabilization of SPs in environmental samples. Several other factors that contribute to SPs instability include: (1) matrix composition; (2) container material; and, (3) sample acidity. Sample agitation prior to analysis may be useful to reduce losses due to adsorption. There are several chemical reagents that inhibit the degradative processes of SPs, but the efficiency of preservation depends – to a large extent – on the characteristics of sample matrix. This article reviews various aspects related to preservation of SPs and puts forward a preliminary guideline for proper practice during sampling, storage and sample preparation of SPs. ª 2011 Elsevier Ltd. All rights reserved. Keywords: Adsorption; Analytical artifact; Environmental analysis; Preservation; Sample handling; Soil analysis; Stability; Suspended particle; Synthetic pyrethroid; Water analysis
1. Introduction Saeed S. Albaseer, K. Mukkanti Centre for Chemical Sciences and Technology, Institute of Science and Technology, JNT University, Kukatpally, Hyderabad 500085, India R. Nageswara Rao* HPLC group, Analytical Chemistry Division, Indian Institute of Chemical Technology, Tarnaka, Hyderabad 500007, India *
Corresponding author. Tel.: +91 40 2719 3193; Fax: +91 40 2716 0387; E-mail: [email protected], [email protected]
Y.V. Swamy Bioengineering and Environmental Centre, Indian Institute of Chemical Technology, Tarnaka, Hyderabad 500007, India
0165-9936/$ - see front matter ª 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.trac.2011.05.010
Synthetic pyrethroids (SPs) are a class of pesticides widely used for agricultural and household purposes. SPs are synthetic esters derived from the naturally-occurring pyrethrins. Commercially-available SPs include allethrin, bifenthrin, bioresmethrin, cyfluthrin, cyhalothrin, cypermethrin, deltamethrin, esfenvalerate (fenvalerate), flucythrinate, flumethrin, fluvalinate, fenpropathrin, permethrin, phenothrin, resmethrin, tefluthrin, tetramethrin, and tralomethrin (Fig. 1). Their use has increased in recent years because they are: 1771
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(1) selective in action, which is mainly due to differences in uptake and distribution [1]; and, (2) easily degradable in the environment when compared to other classes of pesticides [2]. However, some of the most recently developed generations of SPs can persist in the environment for a few months before they are degraded [3]. Use of these toxic compounds is expected to increase because they are increasingly used in place of other classes of insecticides (e.g., organophosphates) [4]. But, the increase in SPs use has subsequently raised concerns about their effects on the fate of aquatic life, as SPs are highly toxic to aquatic invertebrates [5], although toxic effects on human health are still unclear. Also, data on long-term effects and chronic toxicity on the environmental species is very limited. The EU Directive on Drinking Water Quality (98/83/ CE) established 0.10 lg/L as maximum contaminant level (MCL) for individual SPs and 0.50 lg/L for total SP pesticides [6]. For various reasons, analysis of most of the environmental trace contaminants cannot be done on
site, so it is essential to collect a representative sample (sampling) and to preserve it until analysis. Several factors have been reported to affect the stability of SPs during sampling and storage. It is, therefore, important to consider the influence of these factors on SP stability to get representative analytical samples of the real situation, and, subsequently, accurate assessment of their impact on the environment. A recent review [7] showed that there is a plethora of analytical methods for analysis of SPs in environmental matrices, but the ability to preserve the collected samples until they can be analyzed in the laboratory is a challenging task. Published studies on the stability of SPs in environmental samples are, in many instances, contradictory. In this article, we review these studies, and attempt to identify the sources of contradiction. Besides, on the basis of published literature and our experience, we make suggestions concerning the optimum conditions for maintaining the environmental samples of SPs during handling and storage.
Figure 1. Chemical structures of selected synthetic pyrethroids.
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Table 1. Physico-chemical properties of selected synthetic pyrethroids Common name Acrinathrin Bifenthrin Cyfluthrin Cyhalothrin Cypermethrin Deltamethrin Esfenvalerate Fenpropathrin Permethrin Resmethrin Tralomethrin Tetramethrin
Log Kow 5.6 6 5.9 6.9 6.6 6.1 4 6 6.5 5.4 7.6 4.6
Koc
Water solubility (mg/L)
a
48,231 237,000 124,000 326,000 310,000 46,000–1,630,000 252,000 5000–340,000 277,000 310,000 43,796–675,667 790
60.02 0.1 0.002 0.003 0.004 <0.002 0.0002 0.014 0.0055a 0.037 0.08 1.83
Hydrolysis half-life (days) 5–100 >30 1.84–183 8.66– >30 1.9–619 23 >30 18.9 >30–242 >89 2.5a No data
All information obtained (with some modification) from [3], except where noted. a : Koc values were taken from [63,37].
2. Physico-chemical properties of synthetic pyrethroids With the exception of deltamethrin, SPs are a complex mixture of isomers rather than one single pure compound [4], and insecticidal activities of individual SPs differ based upon their optical isomerism [3,8]. The wide divergence in reported assessments of the toxicities of these compounds may therefore be due, in part, to the production of SPs with different enantiomeric ratios. Generally, SPs have low vapor pressures, low HenryÕs law constants and high molecular weights. SPs are nonionic in neutral solution, and extremely hydrophobic. Octanol/water partition coefficients (Kow) are of the order of 106 and water solubilities are as low as lg/L (Table 1). Although, high log KOC values in the range 4–7 suggest that SPs have low risk of leaching to groundwater, they can enter lakes, ponds, rivers and streams with rainfall or run-off from agricultural fields.
3. Analyte adsorption Sorption is one of the key processes controlling the fate of SPs in the environment and poses a real challenge during storage and extraction of SPs from environmental samples. SPs are rapidly and strongly sorbed to organic matter, suspended particulates and even sample collection and storage containers. It has become evident that SPs discharged to water bodies are present mainly in the adsorbed state [2,4,9]. Hence, careful treatment of the samples should be performed. 3.1. To container walls Several authors have demonstrated that SPs adsorb strongly – and instantly – to container walls [10,11]. The analytes adsorbed to container walls may, under
certain conditions, constitute more than 60% of the initial sample concentration [12]. Hence, there could be a potential loss of analytes, due to which rapid extraction methods may only minimize adsorption-derived errors but not prevent them. However, the extent of analyte loss due to adsorption to container walls depends on a number of factors, e.g.: (1) volume-to-contact area ratio; (2) initial sample concentration; (3) sample content of suspended particulates and organic matter; (4) container material; and, (5) holding period. 3.1.1. Ratio of volume to contact area. An increase in the ratio of volume to contact area is directly associated with a decrease in loss of SPs during sampling and storage caused by adsorption to container walls. For example, permethrin losses of 70%, 63% and 42% were reported with volume-to-contact-area ratios of 0.32 mL/cm2, 0.38 mL/cm2 and 0.44 mL/cm2, respectively [13]. The loss percentage decreased to about 36% of the initial concentration when the volume-to-contact area ratio increased to 0.62 mL/cm2 [14]. As most recently introduced extraction methods involve the use of only a small quantity of the sample with a small volume-to-contact area ratio, this adds an additional burden on analysts to develop new procedures that could prevent, or at least minimize, analyte adsorption to container walls. 3.1.2. Sample concentration. Sample concentration affects directly the loss percentage of SPs due to adsorption to container walls {e.g., the loss percentage of [14C] fenvalerate decreased as its initial concentration increased [15]}. Hence, it is expected that adsorption effect will be more significant at low concentrations of SPs. The fact that the loss percentage decreases as the concentration increases indicates that the adsorption http://www.elsevier.com/locate/trac
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capacity of container walls is limited and the quantity of the analytes adsorbed in different cases may be comparable. 3.1.3. Container material. The material of the container used for sampling and storage of SPs is more than an issue of sample collection and storage. Different materials show different adsorption capacities for SPs. The amount of SPs adsorbed to the walls of a 20-mL beaker was found to increase in the order polycarbonate < glass < HDPE < Teflon containers [14,16]. These findings agree with a previous report that showed that the amount of SPs retained on the bottle walls was 90–95% for Teflon and 70–95% for glass of the same size after holding time of 66 h [17]. It has been suggested that there is a relationship between the Kow of individual SPs and the magnitude of pyrethroid association to container walls – the higher the Kow value, the stronger SP affinity to adsorption will be [11]. The influence of SP adsorption to container walls on the integrity of samples and the reliability of analytical results could be crucial when water samples undergo several treatments that involve the use of several containers before the final extract is obtained. However, the loss will vary greatly, depending upon the volume and the type of the container used as well as the protocol applied for extraction and preconcentration. 3.1.4. Sample content of organic matter and suspended materials. When distilled water or filtered real-water samples are stored, SPs are adsorbed only onto container walls. However, unfiltered real water samples contain suspended solids and dissolved organic matter (DOM), which constitute additional adsorption surfaces, leading to an increase in the amount of SPs adsorbed, which, according to some studies [18,19], may amount to 70–90% of the overall SP-sample concentration. In such a case, SPs would exist in four phases: (1) adsorbed to suspended solids; (2) adsorbed to DOM; (3) adsorbed to container walls; and, (4) freely dissolved in water. Competition between various adsorption surfaces is also possible. For example, it was found that the quantity of SPs adsorbed to walls of 1-L glass bottles decreased from 40% of initial concentration for samples of deionized water to only 3% with the presence of suspended solids and organic matter [16,20]. In stream-water samples with higher content of suspended solids, approximately 95–97% of bifenthrin or permethrin in the equilibrated state was adsorbed on the suspended solids [13,19], suggesting that sediment and organic-matter-bound pyrethroids are unlikely to associate with glass containers. Further, in the presence of both suspended particulates and DOM, SPs prefer to partition into the organic carbon phase 1774
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rather than to adsorb onto the solid particulates [18,21]. However, concentration and properties of organic matter significantly affect the phase partitioning of SPs between DOM and water [9]. The freely dissolved phase is of particular importance because of its mobility, bioavailability and extractability by ‘‘mild’’ extraction procedures. It has been demonstrated that, at naturally occurring levels, SPs adsorbed onto DOM, suspended particulates and container walls are not bioavailable to organisms [18,22]. Nevertheless, further studies are needed to elucidate the role of other factors regulating uptake of SPs in aquatic systems. Although the revised European legislation on environmental quality standards in the field of water policy [23] considers only total concentrations in the whole water sample for assessment of organic compounds, improper sample transportation and storage may prevent some extraction methods [e.g., solid-phase microextraction (SPME)] from expressing the sample concentration, as such ‘‘mild’’ extraction methods are unable to extract adsorbed SPs [24]. Hence, a complete report of sampling and storage conditions of real SP samples is of great significance. 3.2. Desorption methods Several studies have demonstrated that association of SPs with the container walls could be reclaimed. Vortex of scintillation vials before sampling was claimed to allow complete recovery of k-cyhalothrin to solution from vial walls [14]. Shaking or agitation of samples has been reported to be an effective tool to reduce the SP adsorbed phase [20,25,26]. However, these techniques do not prevent adsorption, but rather re-suspend adsorbed SPs. Re-suspension of adsorbed analytes by vortex or shaking suggests that the analytes are not chemisorbed on container walls. Nevertheless, it seems difficult to report exactly how vigorous the shaking or vortex was in order to release the adsorbed SPs. Again, the type of container material acts here by affecting the rate of re-suspension. SPs adsorbed to glass containers were more easily reclaimed by shaking than those bound to Teflon, polyethylene and PVC [13,16]. For quantification of SPs adsorbed onto container walls, rinsing the sample bottle with dichloromethane or methanol after pouring out the water sample was sufficient to achieve extraction efficiency of 87–95% [12,25]. One approach to the problem of SP adsorption to containers is to coat the container with a substance that will prevent adsorption. Although Carbowax-polyethylene glycol (PEG) coating is relatively lipophilic and SPs are likely to adhere to it, it has been used to prevent adsorption of SP compounds to glass containers [27]. However, the efficacy of this treatment varied considerably between individual SPs; this clearly indicates that surface-treatment conditions developed for one SP cannot be assumed to be equally effective for others.
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4. Filtration and centrifugation The boundary between dissolved and particulate matter has been operationally defined as the material passing through a 0.45-lm filter. Because SPs in environmental samples are mainly present in the adsorbed phase, that implies that filtration will cause a significant decrease in the recovery of total SPs, especially at high concentration of suspended solids. It is also expected that significance of bias may increase due to filtration with increasing lipophilicity of the individual SPs. In addition to holding back suspended particulates, thus separating adsorbed SPs from freely dissolved SPs, filter papers may also become an additional adsorption surface causing further reduction in initial sample concentration. Factors that affect adsorption of SPs on filter papers include sample salinity and DOC content. At high sample salinity and DOC, adsorption of SPs on glass-fiber filters increased as a result of increasing SP solubility in water due to the salting-out effect and the ability of DOC to pass through glass-fiber filters [28]. Also, it is expected that the reduction in concentration due to filtration may increase if filtration was performed using filter papers with small I.D. (e.g., 0.22 lm), as DOC will be retained. So, the effect of retention of dissolved SPs on the filter papers should not be underestimated, especially at very low concentrations, filtration should not be recommended unless unavoidable, and interpretation of analytical results should be treated carefully. However, centrifugation is involved in some extraction methods of water samples, and it is expected to precipitate suspended particulates present in the original sample matrix. This process will reduce the concentration of adsorbed SPs, but not that of the freely dissolved SPs. It is also expected that the reduction of the concentration of adsorbed SPs would, to some extent, depend on the centrifugation speed. It is obvious that concentration obtained by exhaustive extraction methods will be directly affected if filtration or/and centrifugation were part of sample-preparation protocol. For non-exhaustive extraction methods, no significant difference in concentration would be observed before and after centrifugation or filtration. This assumption has been studied experimentally using SPME (non-exhaustive extraction method) and liquid-liquid extraction (LLE) (exhaustive extraction method) and found valid [19].
5. Stability of synthetic pyrethroids in environmental samples At trace levels, almost any surface with which SPs come in contact becomes a possible source of contamination and/or instability. The method chosen for SP sampling and/or extraction from aqueous medium will certainly
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affect the extent of sample stability. However, the ability to preserve the concentration of native analytes until the sample can be analyzed in the laboratory is a somewhat more challenging task and requires the person handling the sample during transport and subsequent storage to have adequate knowledge about the requirements of proper practice for preserving SPs. Between the time of sample collection and analysis, several chemical and physical processes may occur in the sample affecting the concentration of SPs. Thus, reporting SPs with artificially low concentration is possible. These processes include adsorption, biodegradation, thermal decomposition, photodegradation, and chemical reaction, among others. This mean that samples analysis should be carried out as early as possible to minimize analyte losses. Stability of SPs in environmental water was enhanced by the presence of suspended sediment. Gan [29] reported that bifenthrin degradation by S. acidaminiphila was significantly inhibited in the presence of suspended sediment, and the effect was probably caused by strong adsorption to the solid phase indicating the stability of SPs adsorbed on solid media. In any case, however, stability of SPs sample is affected by several other factors that include holding time, storage temperature, sample acidity, kind and quantity of microorganisms and light, among others (Fig. 2). 5.1. Holding time Holding time or period can be defined as the length of time the sample was kept after sampling until extraction or analysis. It has become evident that storage conditions affect sample holding time, so it is important to determine the length of the safe holding time before degradative interactions begin or adsorption of SPs to the container walls becomes irreversible. A study found that the amount of SPs retained on walls of glass bottles increased from around 50% in the first 24 h to 70–95% after a 66-h storage period [17]. However, variations in loss percentage of permethrin were reported after one week in storage; the loss was 24– 70% [13,30]. This apparent contradiction in reported results may be attributed to the composition of environmental samples of SPs differing from one source to another, and most studies did not take into account the differing sample content of suspended particulates and DOM. As for sediment samples, high moisture content (>75%) has been reported to decrease the stability of SP samples [31]. This could be explained as a result of reduction in analyte adsorption, due to the competition between water and analytes for adsorptive sites on the solid matrix. Interestingly, higher stability was observed and the holding period was extended up to one month when SPs were extracted using adsorbents (quartz-fiber filter disk and Empore disk), and stored in the adsorbed form at 4C protected from light [32]. This technique http://www.elsevier.com/locate/trac
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Figure 2. Possible sources of instability of synthetic pyrethroids during various analytical stages.
Table 2. Effect of storage conditions on stability of synthetic pyrethroids (SPs) in environmental samples SPs
Matrix
Holding period
b-Cyfluthrin k-Cyhalothrin Cypermethrin Deltamethrin.
Agricultural soil: organic C content (25 g/kg), total N content (1.7 g/kg). soil pH 4.9
14 d
Fenpropathrin fenvalerate
Non-sterilized alkaline soil Sterilized alkaline soil Run-off water, suspended solids (25.4 mg/L)
55 d
River water
1d
Lake water
8d 7d
Bifenthrin cis-permethrin trans-permethrin deltamethrin Esfenvalerate cis-permethrin trans-permethrin Bifenthrin Fenpropathrin k-Cyhalothrin Permethrin Cyfluthrin Cypermethrin Esfenvalerate Deltamethrin
1d
Storage conditions
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Analytes loss (%)
Ref.
Glass containers kept in dark, temp. 4 ± 1C. After incubation, samples were air-dried overnight at room temperature before extraction. Methanol-buffered solutions of soil samples. pH: 8.4, temp. 22 ± 1C. Glass bottles, pH 7.01. Temp. 4 ± 0.5C
LLE
8–17
[64]
Matrix Solid-phase Dispersion (MSPD)
75–90
[36]
Amber glass at 4C
SPE
Stored at 4C in glass bottle under three holding conditions: (I) water samples only; (II) with water acidified to pH 2 with HCl; (III) with 20 mL hexane as a keeper
LLE
could be a great development if appropriate adsorbents and conditions were carefully chosen. Table 2 provides some examples of the effect of storage conditions on the holding period of SP samples.
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SPE
8–10 37 28 21 32. 16–19 45–48 (I): 19–35 (II)&(III: no significant loss
[20]
[38]
[30]
5.2. Storage temperature The degradation rate of SPs is influenced by temperature. Increase of temperature from 15C to 30C caused an increase in degradation rate of permethrin in lake
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samples [25]. In addition, stability of SP water samples improved when samples were frozen at 20C [2]. However, Lee et al. [20] reported that, with the exception of transpermethrin, the loss of other SPs studied may generally be independent of temperature in the range 4–20C for samples kept in the dark. Nevertheless, other incubation studies showed that increasing temperature significantly reduced the t1/2 values of SPs in laboratory-controlled samples [33]. As for sediment samples, the effect of temperature on stability of bifenthrin and permethrin was reported [29], and showed higher stability at 4C than at 20C. 5.3. Biodegradation Microorganisms play an important role in the degradation of SPs in natural waters and soil [34,35]. Fenpropathrin and fenvalerate were degraded readily in non-sterilized alkaline soil and nearly 90% and 75% of initial concentrations, respectively, disappeared after 55 d of incubation [36]. However, in sterilized alkaline soil, only 10% and 8% losses of total fenpropathrin and fenvalerate, respectively, were observed after 55 d of incubation. This indicates that the degradation of fenpropathrin and fenvalerate in non-sterilized alkaline soil was mainly biologically mediated. In addition, stereospecificity in SP degradation by soil microorganisms was demonstrated under experimental conditions [35]. Based on these studies, it is expected that the extent of degradation of individual SPs will vary from sample to sample due to variations in the microbial population and types from one place to another. Hence, data on one SP will give only an indication of the degradability of other SPs, although the degradation rate may be different. 5.4. Photodegradation Photodegradation is probably the dominant process in the environment and has the advantage of having a higher degradation rate than biodegradation and hydrolysis. It is of great significance to take into account the effect of photodegradation on the stability of SPs in real samples. Photodegradation studies on three SPs showed that the rate of degradation of individual SPs varied greatly and yielded half-lives from as low as 0.7 d for cyfluthrin to as high as 408 d for bifenthrin and 603 d for fenpropathrin [37]. The attenuation of incident light in natural-water samples and the intensity of radiation can affect the rate of photodegradation. Ueda and co-workers reported large differences in the rate of photodegradation of individual SPs under different conditions [38]. In addition, studies [37,39] showed that the SP photodegradation in natural light is slower than in artificial light, and that can be of particular importance for SP samples brought to laboratories for analysis. 5.5. Matrix acidity Acidity of sample matrix affects stability and/or integrity of SPs both directly and indirectly. The indirect effect of
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sample acidity comes from microbial growth and activity being mostly pH-dependent. At low pH, most organisms have inhibited or reduced activity, so it was expected that stability of SPs could be improved at low pH and this assumption has been proved experimentally, and some studies [40,41] revealed that the stability of SPs in water samples was enhanced at pH 4 and reduced at pH 8. In addition, SP transformation by hydrolysis has been found to be restricted to alkaline medium [42], so it is clear that SP environmental samples must be acidified in situ. Unacidified environmental water samples of SPs cannot be stored for more than 5 d without affecting the reliability of the results [40] and the half-life of fluvalinate (e.g., at 25C) decreased from 48 d to only 1.13 d when the acidity of the solution increased from pH 5 to pH 9 [43]. In addition to its indirect effect on the stability of SPs, acidity may also act directly by affecting the SP-sample integrity. A study by Perschke and Hussain [44] suggested that acidification of SP-water samples may block the ground-state epimerization reaction of SPs in solution [44]. In contrast to acidic soil, fenpropathrin and fenvalerate present in alkaline soil underwent racemization in both non-sterilized and sterilized soils, suggesting that the racemization was chemically mediated and pH dependent [43]. In addition, in sterilized methanol– buffer solutions, isomer conversion of fenpropathrin and S,2R-fenvalerate took place at a faster rate at lower pH values, and as much as 49.1% and 49.8% of S-fenpropathrin and S,2R-fenvalerate were converted to R-fenpropathrin or aR,2R-fenvalerate, respectively [36]. The effect of medium acidity on SP adsorption to container walls was shown to be insignificant for a 17-d holding period [9]. The maximum recovery of SPs from groundwater samples by SPME was obtained when the sample pH was adjusted to 3 and the lowest extraction levels were at pH 10 [9], which could be attributed to the higher stability of SPs at lower pH. In addition, for unspecified reasons, working at higher than pH 7 was reported to cause broadening of peaks, as well as peak interference between others [45]. 5.6. Analyte preservatives Some chemicals, when added to the SP samples in small volumes, may act as preservatives protecting SPs from, or delaying, degradative reactions. Several compounds have been reported to be useful for preserving SPs in aqueous samples {e.g., ethyl glycerol, gulonolactone, d-sorbitol, corn oil, olive oil, L-gulonic acid c-lactone and acetic acid [46,47]}. A combination of olive oil and L-gulonic acid c-lactone was found to be quite effective for most of the SPs in soil [48]. Also, several authors have demonstrated that these additives counteract, to some extent, the matrix effects [48,49] during the determination of pyrethroids using LC/MS with electrospray http://www.elsevier.com/locate/trac
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ionization in positive mode (LC/ESI-MS), so the use of fortified blank samples as calibration standards could be avoided. 5.7. Enantioselective degradation and isomeric conversion Enantioselective degradation of pesticides in environmental samples is well documented, and is also applicable to SPs, which are chiral compounds and are usually degraded by one or more processes enantioselectively [3,35,50,51]. This is of particular importance when the toxicity of individual isomers is of concern. In general, trans-isomers of SPs degrade faster than the cisisomers [52]. In addition, it is expected that sampling location as well as environmental conditions may influence the direction of the enantioselective degradation of SPs because there exist variations in the population and the types of microbial species from one place to another. Analysis of pure cis-permethrin after application on soil and sediments resulted in an enantiomeric fraction (EF) in the range 0.412–0.535, which proved the enantioselective degradation [53]. The (–) enantiomer of cis-bifenthrin and cis-permethrin was preferentially degraded, resulting in relative enrichment of the (+) enantiomer in water [54,55]. Also demonstrated was a preferential degradation of 1S,cis enantiomer over 1R,cis enantiomer for (Z)-cis-bifenthrin and cis-permethrin in water [55]. The most important point here is that the same degradation trends of several SPs have been observed under laboratory-controlled conditions [56,57]. The pH dependence of the racemization rate of SPs was also reported {e.g., racemization of fenpropathrin and fenvalerate was observed in acidic soil but not in alkaline soil [36]}. Also, photoinduced isomerization of permethrin was reported by Holmstead et al. under artificial light as well as sunlight [58].
6. Stability of reference standards As customary, SP reference solutions are prepared in organic solvent, usually acetonitrile or methanol. These reference solutions are used for instrument calibration and preparation of spiked samples. Thus the stability of such solutions is a prerequisite for reliable interpretation of analytical results. Studies showed that the nature of the organic solvent and storage conditions affect the stability of SP reference solutions. For example, deltamethrin solutions kept in the dark experienced isomer conversion in methanol, ethanol, 1-propanol and 2-propanol and with slower rate in 1-butanol, 2-butanol, 1-pentanol, acetone and acetonitrile [59]. Similarly, degradation of stock solutions of cypermethrin occurred at a fast rate with 50% degradation occurring within the first two weeks of storage when prepared in methanol, while no degradation was observed in acetonitrile under the same 1778
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storage conditions [60]. Although isomeric conversion of SPs was not observed in solutions of n-hexane, ethyl acetate, benzene, toluene, 1-octanol and 2-octanol, it was reported that isomer conversion in hexane took place when the hexane solution was exposed to bright sunshine in summer for 5 d, indicating that isomer conversion in this case was photoinduced [59,61]. Also, artificial photochemical degradation of deltamethrin and fenvalerate took place much faster in hexane solution than in methanol/water solution (50:50, V/V), [39] (i.e. degradation in the non-polar organic solvent was faster than in polar methanol/water medium). The point here is that, stability of individual SPs in organic solvents is not identical for all compounds and that necessitates that data on one SP may, in some cases, not be applicable for other members of the class. For example, while deltamethrin experienced interconversion in the secondary alcohol 2-butanol, although at a slow rate, cypermethrin stereoisomers did not show any significant interconversion under the same conditions [59,62]. By contrast, permethrin, whose structure is similar to cypermethrin except for the absence of an R-carbon chiral center, was found to be stable in all solvents in the dark at room temperature (25 ± 2C) [62]. Also, enhancement of interconversion of cypermethrin stereoisomers was observed in methanol/water mixtures unlike pure-solvent solutions [62] These observations suggest that the ability of an organic solvent to dissolve SPs is not the only important feature to consider when choosing a solvent for preparing SP reference solutions. Further, as methanol and acetonitrile are the most reported solvents for preparing stock solutions of SPs, it seems that methanol should be avoided in favor of acetonitrile and secondary alcohols.
7. Conclusions SP adsorption onto the walls of most containers to varying degrees, depending on the container material and the composition of samples, should be addressed and assessed when developing new sample-preparation and extraction protocols. Although shaking the container appears to be sufficient to resuspend the SPs to a large extent, further work should clearly define how vigorous shaking should be. Published literature shows considerable differences in terms of stability and degradation rates in both real samples and standard samples for individual SPs, so generalization of experimental observations on one or more SPs to other members of the class is not recommended, and it is important to consider such differences when reporting analytical results. An individual SP may, in some analytical aspects, be considered as a disconnected, stand-alone entity. In summary, based on the above discussions and the study conducted by US Geological Survey [16], the
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following practice may be recommended during sampling, storage and sample preparation of SP environmental samples for analysis (1) Glass containers should be used whenever possible, and, if plastics containers are used, then it is optimal to transfer the sample to a glass bottle as soon as possible. (2) Regardless of the container material, samples should be agitated vigorously for at least 1 min immediately before being transferred to another container. (3) Volume-to-contact-area ratio should be maximized. (4) SP samples must be kept in ice immediately after sampling until reaching the laboratory where they should be kept refrigerated in the dark until analysis. (5) While sediment samples can be frozen for a few months (prior to extraction), with insignificant or no changes in analytical integrity, water samples should be analyzed within 3 d of collection (while considering other storage conditions). (6) Acidification of water sample immediately after collection to minimize hydrolytic processes. In addition, the following precautions may also be considered for reliable analysis of SPs. (1) Extract obtained after the preconcentration step should be analyzed immediately, as its high concentration may cause analyte loss. (2) Blank sample injection should be done at reasonable intervals, if several samples are to be analyzed in the same session. (3) Injection syringe should be washed after each injection by HPLC methanol.
Acknowledgements S.S. Albaseer would like to thank Thamar University, Thamar, Yemen, for financial support. The authors wish to thank J.S. Yadav, Director, Indian Institute of Chemical Technology, Hyderabad, India, for encouragement and permission to communicate this article for publication.
References [1] J. Stenersen, Chemical Pesticides: Mode of Action and Toxicology, CRC Press, Boca Raton, FL, USA, 2004 p. 146. [2] V. Casas, M. Llomparts, C.G. Jares, R. Cela, T. Dagnac, Anal. Bioanal. Chem. 387 (2007) 1841. [3] Agency for Toxic Substances and Disease Registry (ATSDR), Toxicological profile for pyrethrins and pyrethroids, US Department of Health and Human Services, Public Health Service, Atlanta, GA, USA, 2003. [4] Department of Pesticide Regulation, Environmental Monitoring Branch, A Preliminary Assessment of Pyrethroid Contamination of Surface Waters and Bed Sediments in High Pyrethroid-Use Regions of California, Sacramento, CA, USA, 2004.
Trends [5] C. Cox, J. Pestic. Reform. 22 (2002) 12. [6] European Commission, European Directive on the Quality of Water Intended for Human Consumption, 98/83/EC, EU Council, Brussels, Belgium, 1998. [7] S.S. Albaseer, R.N. Rao, Y.V. Swamy, K. Mukkanti, J. Chromatogr., A 1217 (2010) 5537. [8] D.E. Ray, in: W.J. Hayes, E.R. Laws (Editors), Handbook of Pesticide Toxicology, Vol. 2, Academic Press, Inc., 1991, p. 585. [9] M.A. Ali, P.J. Baugh, Int. J. Environ. Anal. Chem. 83 (2003) 923. [10] C.S. Brunete, R.A. Pe´rez, E. Miguel, J.L. Tadeo, J. Chromatogr., A 823 (1998) 17. [11] H. Oudou, H.C. Hansen, Chemosphere 49 (2002) 1285. [12] L.J. Zhou, S. Rowland, R.F.C. Mantoura, Water Res 29 (1995) 1023. [13] M.S. Sharom, K.R. Solomon, J. Agric. Food Chem. 29 (1981) 1122. [14] C.E. Wheelock, J.L. Miller, M.J. Miller, B.M. Phillips, S.J. Geea, R.S. Tjeerdema, B.D. Hammock, Aquat. Toxicol. 74 (2005) 47. [15] K. Day, N.K. Kaushik, Aquat. Toxicol. 10 (1987) 131. [16] M.L. Hladik, J.L. Orlando, K.M. Kuivila, Collection of pyrethroids in water and sediment matrices: development and validation of a standard operating procedure, US Geological Survey Scientific Investigations Report, Reston, VA, USA, 2009. [17] W.A. House, Z. Ou, Chemosphere 24 (1992) 819. [18] W. Yang, J. Gan, W. Hunter, F. Spurlock, Environ. Toxicol. Chem. 25 (2006) 1585. [19] W. Liu, J.J. Gan, S. Lee, J.N. Kabashima, Environ. Toxicol. Chem. 23 (2004) 7. [20] S. Lee, J. Gan, J. Kabashima, J. Agric. Food Chem. 50 (2002) 7194. [21] M. Rebhun, R. Kalabo, L. Grossman, J. Manka, Ch. Rav-Acha, Water Res 26 (1992) 79. [22] W.C. Yang, W. Hunter, J. Gan, J. Environ. Qual. 36 (2007) 1678. [23] European Commission (EC), European Directive 2008/105/EC of the European Parliament and of the Council on environmental quality standards in the field of water policy, EC, Brussels, Belgium, December 2008. [24] S. Bondarenko, F. Spurlock, J. Gan, Environ. Toxicol. Chem. 26 (2007) 2587. [25] M.S. Sharom, K.R. Solomon, Environ. Pollut., B 4 (1982) 269. [26] M.L. Hladik, K.M. Kuivila, J. Agric. Food Chem. 57 (2009) 9079. [27] D.W. Helmuth, S.M. Ghiasuddin, D.M. Soderlund, J. Agric. Food Chem. 31 (1983) 1127. [28] A.G. Gutie`rrez, E. Jover, J.M. Bayona, J. Albaige´s, Anal. Chim. Acta 583 (2007) 202. [29] J.J. Gan, Technical Completion Reports, Pyrethroid insecticides in nursery runoff: Transport and impact on aquatic invertebrates, University of California Water Resources Center, UC Berkeley, CAQ, USA, 2006. [30] D. Wang, D.P. Weston, M.J. Lydy, Talanta 78 (2009) 1345. [31] G.F. Pang, Y.Z. Cao, C.L. Fan, J.J. Zhang, X.M. Li, J. AOAC Int. 80 (1997) 63. [32] T. Yoshida, I. Matsunaga, H. Oda, J. Chromatogr., A 1023 (2004) 255. [33] M.Y. Tariq, S. Afzal, I. Hussain, Environ. Res. 100 (2006) 184. [34] R.A. Chapman, C.M. Tu, C.R. Harris, C. Cole, Bull. Environ. Contam. Toxicol. 26 (1981) 513. [35] S. Sakata, N. Mikami, H. Yamada, J. Pesticide Sci. 17 (1992) 181. [36] Z. Li, Z. Zhang, L. Zhang, L. Leng, Chemosphere 76 (2009) 509. [37] D.A. Laskowski, Rev. Environ. Contam. Toxicol. 174 (2002) 49. [38] K. Ueda, L.C. Gaughan, J.E. Casida, J. Agric. Food Chem. 22 (1974) 212. [39] P. Liu, Y. Liu, Q. Liu, J. Liu, J. Environ. Sci. 22 (2010) 1123. [40] M. Lyytika´inen, J.V.K. Kukkonen, M.J. Lydy, Arch. Environ. Contam. Toxicol. 44 (2003) 437. [41] K. Ma´stovska´, S.J. Lehotay, J. Chromatogr., A 1040 (2004) 259.
http://www.elsevier.com/locate/trac
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Trends in Analytical Chemistry, Vol. 30, No. 11, 2011
[42] S.J. Stangroom, C.D. Collins, J.N. Lester, Environ. Technol. 21 (2000) 865. [43] C.D.S. Tomlin (Editor), The Pesticide Manual - World Compendium, 11th Edition, British Crop Protection Council, Alton, Hants, UK, 1997. [44] H. Perschke, M. Hussain, Chemosphere 22 (1991) 677. [45] P.P. Vazquez, A.R. Mughari, M.M. Galera, J. Chromatogr., A 1188 (2008) 61. [46] M. Anastassiades, K. Mastovska, S.J. Lehotay, J. Chromatogr., A 1015 (2003) 163. [47] E.R. Bennett, M.T. Moore, C.M. Cooper, S. Smith, Bull. Environ. Contam. Toxicol. 64 (2000) 825. [48] C. Sa`nchez-Brunete, B. Albero, G. Martin, J.L. Tadeo, Anal. Sci. 21 (2005) 1291. [49] M. Anastassiades, S.J. Lehotay, D. Stajnbaher, F.J. Schenck, J. AOAC Int. 86 (2003) 412. [50] M.K. Morgan, L.S. Sheldon, C.W. Croghan, P.A. Jones, J.C. Chuang, N.K. Wilson, Environ. Res. 104 (2007) 266. [51] S. Qin, R. Budd, S. Bondarenko, W. Liu, J. Gan, J. Agric. Food Chem. 54 (2006) 5040. [52] E.I. Ulrich, Chim. Oggi 25 (2008) 37. [53] S. Qin, J. Gan, J. Agric. Food Chem. 55 (2007) 5734.
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[54] W. Liu, J. Gan, D. Schlenk, W.A. Jury, Proc. Natl. Acad. Sci. USA 102 (2005) 701. [55] W. Liu, J. Gan, J. Agric. Food Chem. 52 (2004) 736. [56] S. Qin, W. Liu, J. Gan, in: J. Gan, F. Spurlock, P. Hendley, D.P. Weston (Editors), Synthetic Pyrethroids: Occurrence and Behaviour in Aquatic Environments, ACS Symposium Series, OUP USA, New York, USA, 2008, p. 238. [57] W. Liu, J. Gan, S. Lee, I. Werner, J. Agric. Food Chem. 52 (2004) 6233. [58] R.L. Holmstead, J.E. Casida, L.O. Ruzo, D.G. Fullmer, J. Agric. Food Chem. 26 (1978) 590. [59] H. Perschke, M. Hussain, J. Agric. Food Chem. 40 (1992) 686. [60] E.R. Brouwer, E.A. Struys, J.J. Vreuls, U.A.T. Brinkman, FreseniusÕ J. Anal. Chem. 350 (1994) 487. [61] R.J. Maguire, J. Agric. Food Chem. 38 (1990) 1613. [62] M.G. Nillos, S. Qin, C. Larive, D. Schlenk, J. Gan, J. Agric. Food Chem. 57 (2009) 6938. [63] US National Library of Medicine, 2011 (http://toxnet. nlm.nih.gov/). [64] W. Amelung, A. Nikolakis, V. Laabs, J. AOAC Int. 90 (2007) 1659.
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Analytical artifacts, sample handling and preservation methods of environmental samples of synthetic pyrethroids Saeed S. Albaseer, R. Nageswara Rao, Y.V. Swamy, K. Mukkanti Sample handling and preservation methods of environmental samples of synthetic pyrethroids (SPs) are very important and must be controlled to maintain sample integrity during analytical determinations. However, published literature has treated this issue only partly, and, in many instances, with contradictory conclusions. The tendency of SPs to adsorb – to varying degrees under different conditions – to surfaces and solid particulates with which they come in contact may be responsible for this situation. It has become evident that SPs discharged to water bodies are present mainly in the adsorbed state, and that affects their bioavailability and the reliability of analytical results. Refrigeration and storage in the dark are prerequisites for stabilization of SPs in environmental samples. Several other factors that contribute to SPs instability include: (1) matrix composition; (2) container material; and, (3) sample acidity. Sample agitation prior to analysis may be useful to reduce losses due to adsorption. There are several chemical reagents that inhibit the degradative processes of SPs, but the efficiency of preservation depends – to a large extent – on the characteristics of sample matrix. This article reviews various aspects related to preservation of SPs and puts forward a preliminary guideline for proper practice during sampling, storage and sample preparation of SPs. ª 2011 Elsevier Ltd. All rights reserved. Keywords: Adsorption; Analytical artifact; Environmental analysis; Preservation; Sample handling; Soil analysis; Stability; Suspended particle; Synthetic pyrethroid; Water analysis
1. Introduction Saeed S. Albaseer, K. Mukkanti Centre for Chemical Sciences and Technology, Institute of Science and Technology, JNT University, Kukatpally, Hyderabad 500085, India R. Nageswara Rao* HPLC group, Analytical Chemistry Division, Indian Institute of Chemical Technology, Tarnaka, Hyderabad 500007, India *
Corresponding author. Tel.: +91 40 2719 3193; Fax: +91 40 2716 0387; E-mail: [email protected], [email protected]
Y.V. Swamy Bioengineering and Environmental Centre, Indian Institute of Chemical Technology, Tarnaka, Hyderabad 500007, India
0165-9936/$ - see front matter ª 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.trac.2011.05.010
Synthetic pyrethroids (SPs) are a class of pesticides widely used for agricultural and household purposes. SPs are synthetic esters derived from the naturally-occurring pyrethrins. Commercially-available SPs include allethrin, bifenthrin, bioresmethrin, cyfluthrin, cyhalothrin, cypermethrin, deltamethrin, esfenvalerate (fenvalerate), flucythrinate, flumethrin, fluvalinate, fenpropathrin, permethrin, phenothrin, resmethrin, tefluthrin, tetramethrin, and tralomethrin (Fig. 1). Their use has increased in recent years because they are: 1771
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(1) selective in action, which is mainly due to differences in uptake and distribution [1]; and, (2) easily degradable in the environment when compared to other classes of pesticides [2]. However, some of the most recently developed generations of SPs can persist in the environment for a few months before they are degraded [3]. Use of these toxic compounds is expected to increase because they are increasingly used in place of other classes of insecticides (e.g., organophosphates) [4]. But, the increase in SPs use has subsequently raised concerns about their effects on the fate of aquatic life, as SPs are highly toxic to aquatic invertebrates [5], although toxic effects on human health are still unclear. Also, data on long-term effects and chronic toxicity on the environmental species is very limited. The EU Directive on Drinking Water Quality (98/83/ CE) established 0.10 lg/L as maximum contaminant level (MCL) for individual SPs and 0.50 lg/L for total SP pesticides [6]. For various reasons, analysis of most of the environmental trace contaminants cannot be done on
site, so it is essential to collect a representative sample (sampling) and to preserve it until analysis. Several factors have been reported to affect the stability of SPs during sampling and storage. It is, therefore, important to consider the influence of these factors on SP stability to get representative analytical samples of the real situation, and, subsequently, accurate assessment of their impact on the environment. A recent review [7] showed that there is a plethora of analytical methods for analysis of SPs in environmental matrices, but the ability to preserve the collected samples until they can be analyzed in the laboratory is a challenging task. Published studies on the stability of SPs in environmental samples are, in many instances, contradictory. In this article, we review these studies, and attempt to identify the sources of contradiction. Besides, on the basis of published literature and our experience, we make suggestions concerning the optimum conditions for maintaining the environmental samples of SPs during handling and storage.
Figure 1. Chemical structures of selected synthetic pyrethroids.
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Table 1. Physico-chemical properties of selected synthetic pyrethroids Common name Acrinathrin Bifenthrin Cyfluthrin Cyhalothrin Cypermethrin Deltamethrin Esfenvalerate Fenpropathrin Permethrin Resmethrin Tralomethrin Tetramethrin
Log Kow 5.6 6 5.9 6.9 6.6 6.1 4 6 6.5 5.4 7.6 4.6
Koc
Water solubility (mg/L)
a
48,231 237,000 124,000 326,000 310,000 46,000–1,630,000 252,000 5000–340,000 277,000 310,000 43,796–675,667 790
60.02 0.1 0.002 0.003 0.004 <0.002 0.0002 0.014 0.0055a 0.037 0.08 1.83
Hydrolysis half-life (days) 5–100 >30 1.84–183 8.66– >30 1.9–619 23 >30 18.9 >30–242 >89 2.5a No data
All information obtained (with some modification) from [3], except where noted. a : Koc values were taken from [63,37].
2. Physico-chemical properties of synthetic pyrethroids With the exception of deltamethrin, SPs are a complex mixture of isomers rather than one single pure compound [4], and insecticidal activities of individual SPs differ based upon their optical isomerism [3,8]. The wide divergence in reported assessments of the toxicities of these compounds may therefore be due, in part, to the production of SPs with different enantiomeric ratios. Generally, SPs have low vapor pressures, low HenryÕs law constants and high molecular weights. SPs are nonionic in neutral solution, and extremely hydrophobic. Octanol/water partition coefficients (Kow) are of the order of 106 and water solubilities are as low as lg/L (Table 1). Although, high log KOC values in the range 4–7 suggest that SPs have low risk of leaching to groundwater, they can enter lakes, ponds, rivers and streams with rainfall or run-off from agricultural fields.
3. Analyte adsorption Sorption is one of the key processes controlling the fate of SPs in the environment and poses a real challenge during storage and extraction of SPs from environmental samples. SPs are rapidly and strongly sorbed to organic matter, suspended particulates and even sample collection and storage containers. It has become evident that SPs discharged to water bodies are present mainly in the adsorbed state [2,4,9]. Hence, careful treatment of the samples should be performed. 3.1. To container walls Several authors have demonstrated that SPs adsorb strongly – and instantly – to container walls [10,11]. The analytes adsorbed to container walls may, under
certain conditions, constitute more than 60% of the initial sample concentration [12]. Hence, there could be a potential loss of analytes, due to which rapid extraction methods may only minimize adsorption-derived errors but not prevent them. However, the extent of analyte loss due to adsorption to container walls depends on a number of factors, e.g.: (1) volume-to-contact area ratio; (2) initial sample concentration; (3) sample content of suspended particulates and organic matter; (4) container material; and, (5) holding period. 3.1.1. Ratio of volume to contact area. An increase in the ratio of volume to contact area is directly associated with a decrease in loss of SPs during sampling and storage caused by adsorption to container walls. For example, permethrin losses of 70%, 63% and 42% were reported with volume-to-contact-area ratios of 0.32 mL/cm2, 0.38 mL/cm2 and 0.44 mL/cm2, respectively [13]. The loss percentage decreased to about 36% of the initial concentration when the volume-to-contact area ratio increased to 0.62 mL/cm2 [14]. As most recently introduced extraction methods involve the use of only a small quantity of the sample with a small volume-to-contact area ratio, this adds an additional burden on analysts to develop new procedures that could prevent, or at least minimize, analyte adsorption to container walls. 3.1.2. Sample concentration. Sample concentration affects directly the loss percentage of SPs due to adsorption to container walls {e.g., the loss percentage of [14C] fenvalerate decreased as its initial concentration increased [15]}. Hence, it is expected that adsorption effect will be more significant at low concentrations of SPs. The fact that the loss percentage decreases as the concentration increases indicates that the adsorption http://www.elsevier.com/locate/trac
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capacity of container walls is limited and the quantity of the analytes adsorbed in different cases may be comparable. 3.1.3. Container material. The material of the container used for sampling and storage of SPs is more than an issue of sample collection and storage. Different materials show different adsorption capacities for SPs. The amount of SPs adsorbed to the walls of a 20-mL beaker was found to increase in the order polycarbonate < glass < HDPE < Teflon containers [14,16]. These findings agree with a previous report that showed that the amount of SPs retained on the bottle walls was 90–95% for Teflon and 70–95% for glass of the same size after holding time of 66 h [17]. It has been suggested that there is a relationship between the Kow of individual SPs and the magnitude of pyrethroid association to container walls – the higher the Kow value, the stronger SP affinity to adsorption will be [11]. The influence of SP adsorption to container walls on the integrity of samples and the reliability of analytical results could be crucial when water samples undergo several treatments that involve the use of several containers before the final extract is obtained. However, the loss will vary greatly, depending upon the volume and the type of the container used as well as the protocol applied for extraction and preconcentration. 3.1.4. Sample content of organic matter and suspended materials. When distilled water or filtered real-water samples are stored, SPs are adsorbed only onto container walls. However, unfiltered real water samples contain suspended solids and dissolved organic matter (DOM), which constitute additional adsorption surfaces, leading to an increase in the amount of SPs adsorbed, which, according to some studies [18,19], may amount to 70–90% of the overall SP-sample concentration. In such a case, SPs would exist in four phases: (1) adsorbed to suspended solids; (2) adsorbed to DOM; (3) adsorbed to container walls; and, (4) freely dissolved in water. Competition between various adsorption surfaces is also possible. For example, it was found that the quantity of SPs adsorbed to walls of 1-L glass bottles decreased from 40% of initial concentration for samples of deionized water to only 3% with the presence of suspended solids and organic matter [16,20]. In stream-water samples with higher content of suspended solids, approximately 95–97% of bifenthrin or permethrin in the equilibrated state was adsorbed on the suspended solids [13,19], suggesting that sediment and organic-matter-bound pyrethroids are unlikely to associate with glass containers. Further, in the presence of both suspended particulates and DOM, SPs prefer to partition into the organic carbon phase 1774
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rather than to adsorb onto the solid particulates [18,21]. However, concentration and properties of organic matter significantly affect the phase partitioning of SPs between DOM and water [9]. The freely dissolved phase is of particular importance because of its mobility, bioavailability and extractability by ‘‘mild’’ extraction procedures. It has been demonstrated that, at naturally occurring levels, SPs adsorbed onto DOM, suspended particulates and container walls are not bioavailable to organisms [18,22]. Nevertheless, further studies are needed to elucidate the role of other factors regulating uptake of SPs in aquatic systems. Although the revised European legislation on environmental quality standards in the field of water policy [23] considers only total concentrations in the whole water sample for assessment of organic compounds, improper sample transportation and storage may prevent some extraction methods [e.g., solid-phase microextraction (SPME)] from expressing the sample concentration, as such ‘‘mild’’ extraction methods are unable to extract adsorbed SPs [24]. Hence, a complete report of sampling and storage conditions of real SP samples is of great significance. 3.2. Desorption methods Several studies have demonstrated that association of SPs with the container walls could be reclaimed. Vortex of scintillation vials before sampling was claimed to allow complete recovery of k-cyhalothrin to solution from vial walls [14]. Shaking or agitation of samples has been reported to be an effective tool to reduce the SP adsorbed phase [20,25,26]. However, these techniques do not prevent adsorption, but rather re-suspend adsorbed SPs. Re-suspension of adsorbed analytes by vortex or shaking suggests that the analytes are not chemisorbed on container walls. Nevertheless, it seems difficult to report exactly how vigorous the shaking or vortex was in order to release the adsorbed SPs. Again, the type of container material acts here by affecting the rate of re-suspension. SPs adsorbed to glass containers were more easily reclaimed by shaking than those bound to Teflon, polyethylene and PVC [13,16]. For quantification of SPs adsorbed onto container walls, rinsing the sample bottle with dichloromethane or methanol after pouring out the water sample was sufficient to achieve extraction efficiency of 87–95% [12,25]. One approach to the problem of SP adsorption to containers is to coat the container with a substance that will prevent adsorption. Although Carbowax-polyethylene glycol (PEG) coating is relatively lipophilic and SPs are likely to adhere to it, it has been used to prevent adsorption of SP compounds to glass containers [27]. However, the efficacy of this treatment varied considerably between individual SPs; this clearly indicates that surface-treatment conditions developed for one SP cannot be assumed to be equally effective for others.
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4. Filtration and centrifugation The boundary between dissolved and particulate matter has been operationally defined as the material passing through a 0.45-lm filter. Because SPs in environmental samples are mainly present in the adsorbed phase, that implies that filtration will cause a significant decrease in the recovery of total SPs, especially at high concentration of suspended solids. It is also expected that significance of bias may increase due to filtration with increasing lipophilicity of the individual SPs. In addition to holding back suspended particulates, thus separating adsorbed SPs from freely dissolved SPs, filter papers may also become an additional adsorption surface causing further reduction in initial sample concentration. Factors that affect adsorption of SPs on filter papers include sample salinity and DOC content. At high sample salinity and DOC, adsorption of SPs on glass-fiber filters increased as a result of increasing SP solubility in water due to the salting-out effect and the ability of DOC to pass through glass-fiber filters [28]. Also, it is expected that the reduction in concentration due to filtration may increase if filtration was performed using filter papers with small I.D. (e.g., 0.22 lm), as DOC will be retained. So, the effect of retention of dissolved SPs on the filter papers should not be underestimated, especially at very low concentrations, filtration should not be recommended unless unavoidable, and interpretation of analytical results should be treated carefully. However, centrifugation is involved in some extraction methods of water samples, and it is expected to precipitate suspended particulates present in the original sample matrix. This process will reduce the concentration of adsorbed SPs, but not that of the freely dissolved SPs. It is also expected that the reduction of the concentration of adsorbed SPs would, to some extent, depend on the centrifugation speed. It is obvious that concentration obtained by exhaustive extraction methods will be directly affected if filtration or/and centrifugation were part of sample-preparation protocol. For non-exhaustive extraction methods, no significant difference in concentration would be observed before and after centrifugation or filtration. This assumption has been studied experimentally using SPME (non-exhaustive extraction method) and liquid-liquid extraction (LLE) (exhaustive extraction method) and found valid [19].
5. Stability of synthetic pyrethroids in environmental samples At trace levels, almost any surface with which SPs come in contact becomes a possible source of contamination and/or instability. The method chosen for SP sampling and/or extraction from aqueous medium will certainly
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affect the extent of sample stability. However, the ability to preserve the concentration of native analytes until the sample can be analyzed in the laboratory is a somewhat more challenging task and requires the person handling the sample during transport and subsequent storage to have adequate knowledge about the requirements of proper practice for preserving SPs. Between the time of sample collection and analysis, several chemical and physical processes may occur in the sample affecting the concentration of SPs. Thus, reporting SPs with artificially low concentration is possible. These processes include adsorption, biodegradation, thermal decomposition, photodegradation, and chemical reaction, among others. This mean that samples analysis should be carried out as early as possible to minimize analyte losses. Stability of SPs in environmental water was enhanced by the presence of suspended sediment. Gan [29] reported that bifenthrin degradation by S. acidaminiphila was significantly inhibited in the presence of suspended sediment, and the effect was probably caused by strong adsorption to the solid phase indicating the stability of SPs adsorbed on solid media. In any case, however, stability of SPs sample is affected by several other factors that include holding time, storage temperature, sample acidity, kind and quantity of microorganisms and light, among others (Fig. 2). 5.1. Holding time Holding time or period can be defined as the length of time the sample was kept after sampling until extraction or analysis. It has become evident that storage conditions affect sample holding time, so it is important to determine the length of the safe holding time before degradative interactions begin or adsorption of SPs to the container walls becomes irreversible. A study found that the amount of SPs retained on walls of glass bottles increased from around 50% in the first 24 h to 70–95% after a 66-h storage period [17]. However, variations in loss percentage of permethrin were reported after one week in storage; the loss was 24– 70% [13,30]. This apparent contradiction in reported results may be attributed to the composition of environmental samples of SPs differing from one source to another, and most studies did not take into account the differing sample content of suspended particulates and DOM. As for sediment samples, high moisture content (>75%) has been reported to decrease the stability of SP samples [31]. This could be explained as a result of reduction in analyte adsorption, due to the competition between water and analytes for adsorptive sites on the solid matrix. Interestingly, higher stability was observed and the holding period was extended up to one month when SPs were extracted using adsorbents (quartz-fiber filter disk and Empore disk), and stored in the adsorbed form at 4C protected from light [32]. This technique http://www.elsevier.com/locate/trac
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Figure 2. Possible sources of instability of synthetic pyrethroids during various analytical stages.
Table 2. Effect of storage conditions on stability of synthetic pyrethroids (SPs) in environmental samples SPs
Matrix
Holding period
b-Cyfluthrin k-Cyhalothrin Cypermethrin Deltamethrin.
Agricultural soil: organic C content (25 g/kg), total N content (1.7 g/kg). soil pH 4.9
14 d
Fenpropathrin fenvalerate
Non-sterilized alkaline soil Sterilized alkaline soil Run-off water, suspended solids (25.4 mg/L)
55 d
River water
1d
Lake water
8d 7d
Bifenthrin cis-permethrin trans-permethrin deltamethrin Esfenvalerate cis-permethrin trans-permethrin Bifenthrin Fenpropathrin k-Cyhalothrin Permethrin Cyfluthrin Cypermethrin Esfenvalerate Deltamethrin
1d
Storage conditions
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Analytes loss (%)
Ref.
Glass containers kept in dark, temp. 4 ± 1C. After incubation, samples were air-dried overnight at room temperature before extraction. Methanol-buffered solutions of soil samples. pH: 8.4, temp. 22 ± 1C. Glass bottles, pH 7.01. Temp. 4 ± 0.5C
LLE
8–17
[64]
Matrix Solid-phase Dispersion (MSPD)
75–90
[36]
Amber glass at 4C
SPE
Stored at 4C in glass bottle under three holding conditions: (I) water samples only; (II) with water acidified to pH 2 with HCl; (III) with 20 mL hexane as a keeper
LLE
could be a great development if appropriate adsorbents and conditions were carefully chosen. Table 2 provides some examples of the effect of storage conditions on the holding period of SP samples.
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Extraction method
SPE
8–10 37 28 21 32. 16–19 45–48 (I): 19–35 (II)&(III: no significant loss
[20]
[38]
[30]
5.2. Storage temperature The degradation rate of SPs is influenced by temperature. Increase of temperature from 15C to 30C caused an increase in degradation rate of permethrin in lake
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samples [25]. In addition, stability of SP water samples improved when samples were frozen at 20C [2]. However, Lee et al. [20] reported that, with the exception of transpermethrin, the loss of other SPs studied may generally be independent of temperature in the range 4–20C for samples kept in the dark. Nevertheless, other incubation studies showed that increasing temperature significantly reduced the t1/2 values of SPs in laboratory-controlled samples [33]. As for sediment samples, the effect of temperature on stability of bifenthrin and permethrin was reported [29], and showed higher stability at 4C than at 20C. 5.3. Biodegradation Microorganisms play an important role in the degradation of SPs in natural waters and soil [34,35]. Fenpropathrin and fenvalerate were degraded readily in non-sterilized alkaline soil and nearly 90% and 75% of initial concentrations, respectively, disappeared after 55 d of incubation [36]. However, in sterilized alkaline soil, only 10% and 8% losses of total fenpropathrin and fenvalerate, respectively, were observed after 55 d of incubation. This indicates that the degradation of fenpropathrin and fenvalerate in non-sterilized alkaline soil was mainly biologically mediated. In addition, stereospecificity in SP degradation by soil microorganisms was demonstrated under experimental conditions [35]. Based on these studies, it is expected that the extent of degradation of individual SPs will vary from sample to sample due to variations in the microbial population and types from one place to another. Hence, data on one SP will give only an indication of the degradability of other SPs, although the degradation rate may be different. 5.4. Photodegradation Photodegradation is probably the dominant process in the environment and has the advantage of having a higher degradation rate than biodegradation and hydrolysis. It is of great significance to take into account the effect of photodegradation on the stability of SPs in real samples. Photodegradation studies on three SPs showed that the rate of degradation of individual SPs varied greatly and yielded half-lives from as low as 0.7 d for cyfluthrin to as high as 408 d for bifenthrin and 603 d for fenpropathrin [37]. The attenuation of incident light in natural-water samples and the intensity of radiation can affect the rate of photodegradation. Ueda and co-workers reported large differences in the rate of photodegradation of individual SPs under different conditions [38]. In addition, studies [37,39] showed that the SP photodegradation in natural light is slower than in artificial light, and that can be of particular importance for SP samples brought to laboratories for analysis. 5.5. Matrix acidity Acidity of sample matrix affects stability and/or integrity of SPs both directly and indirectly. The indirect effect of
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sample acidity comes from microbial growth and activity being mostly pH-dependent. At low pH, most organisms have inhibited or reduced activity, so it was expected that stability of SPs could be improved at low pH and this assumption has been proved experimentally, and some studies [40,41] revealed that the stability of SPs in water samples was enhanced at pH 4 and reduced at pH 8. In addition, SP transformation by hydrolysis has been found to be restricted to alkaline medium [42], so it is clear that SP environmental samples must be acidified in situ. Unacidified environmental water samples of SPs cannot be stored for more than 5 d without affecting the reliability of the results [40] and the half-life of fluvalinate (e.g., at 25C) decreased from 48 d to only 1.13 d when the acidity of the solution increased from pH 5 to pH 9 [43]. In addition to its indirect effect on the stability of SPs, acidity may also act directly by affecting the SP-sample integrity. A study by Perschke and Hussain [44] suggested that acidification of SP-water samples may block the ground-state epimerization reaction of SPs in solution [44]. In contrast to acidic soil, fenpropathrin and fenvalerate present in alkaline soil underwent racemization in both non-sterilized and sterilized soils, suggesting that the racemization was chemically mediated and pH dependent [43]. In addition, in sterilized methanol– buffer solutions, isomer conversion of fenpropathrin and S,2R-fenvalerate took place at a faster rate at lower pH values, and as much as 49.1% and 49.8% of S-fenpropathrin and S,2R-fenvalerate were converted to R-fenpropathrin or aR,2R-fenvalerate, respectively [36]. The effect of medium acidity on SP adsorption to container walls was shown to be insignificant for a 17-d holding period [9]. The maximum recovery of SPs from groundwater samples by SPME was obtained when the sample pH was adjusted to 3 and the lowest extraction levels were at pH 10 [9], which could be attributed to the higher stability of SPs at lower pH. In addition, for unspecified reasons, working at higher than pH 7 was reported to cause broadening of peaks, as well as peak interference between others [45]. 5.6. Analyte preservatives Some chemicals, when added to the SP samples in small volumes, may act as preservatives protecting SPs from, or delaying, degradative reactions. Several compounds have been reported to be useful for preserving SPs in aqueous samples {e.g., ethyl glycerol, gulonolactone, d-sorbitol, corn oil, olive oil, L-gulonic acid c-lactone and acetic acid [46,47]}. A combination of olive oil and L-gulonic acid c-lactone was found to be quite effective for most of the SPs in soil [48]. Also, several authors have demonstrated that these additives counteract, to some extent, the matrix effects [48,49] during the determination of pyrethroids using LC/MS with electrospray http://www.elsevier.com/locate/trac
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ionization in positive mode (LC/ESI-MS), so the use of fortified blank samples as calibration standards could be avoided. 5.7. Enantioselective degradation and isomeric conversion Enantioselective degradation of pesticides in environmental samples is well documented, and is also applicable to SPs, which are chiral compounds and are usually degraded by one or more processes enantioselectively [3,35,50,51]. This is of particular importance when the toxicity of individual isomers is of concern. In general, trans-isomers of SPs degrade faster than the cisisomers [52]. In addition, it is expected that sampling location as well as environmental conditions may influence the direction of the enantioselective degradation of SPs because there exist variations in the population and the types of microbial species from one place to another. Analysis of pure cis-permethrin after application on soil and sediments resulted in an enantiomeric fraction (EF) in the range 0.412–0.535, which proved the enantioselective degradation [53]. The (–) enantiomer of cis-bifenthrin and cis-permethrin was preferentially degraded, resulting in relative enrichment of the (+) enantiomer in water [54,55]. Also demonstrated was a preferential degradation of 1S,cis enantiomer over 1R,cis enantiomer for (Z)-cis-bifenthrin and cis-permethrin in water [55]. The most important point here is that the same degradation trends of several SPs have been observed under laboratory-controlled conditions [56,57]. The pH dependence of the racemization rate of SPs was also reported {e.g., racemization of fenpropathrin and fenvalerate was observed in acidic soil but not in alkaline soil [36]}. Also, photoinduced isomerization of permethrin was reported by Holmstead et al. under artificial light as well as sunlight [58].
6. Stability of reference standards As customary, SP reference solutions are prepared in organic solvent, usually acetonitrile or methanol. These reference solutions are used for instrument calibration and preparation of spiked samples. Thus the stability of such solutions is a prerequisite for reliable interpretation of analytical results. Studies showed that the nature of the organic solvent and storage conditions affect the stability of SP reference solutions. For example, deltamethrin solutions kept in the dark experienced isomer conversion in methanol, ethanol, 1-propanol and 2-propanol and with slower rate in 1-butanol, 2-butanol, 1-pentanol, acetone and acetonitrile [59]. Similarly, degradation of stock solutions of cypermethrin occurred at a fast rate with 50% degradation occurring within the first two weeks of storage when prepared in methanol, while no degradation was observed in acetonitrile under the same 1778
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storage conditions [60]. Although isomeric conversion of SPs was not observed in solutions of n-hexane, ethyl acetate, benzene, toluene, 1-octanol and 2-octanol, it was reported that isomer conversion in hexane took place when the hexane solution was exposed to bright sunshine in summer for 5 d, indicating that isomer conversion in this case was photoinduced [59,61]. Also, artificial photochemical degradation of deltamethrin and fenvalerate took place much faster in hexane solution than in methanol/water solution (50:50, V/V), [39] (i.e. degradation in the non-polar organic solvent was faster than in polar methanol/water medium). The point here is that, stability of individual SPs in organic solvents is not identical for all compounds and that necessitates that data on one SP may, in some cases, not be applicable for other members of the class. For example, while deltamethrin experienced interconversion in the secondary alcohol 2-butanol, although at a slow rate, cypermethrin stereoisomers did not show any significant interconversion under the same conditions [59,62]. By contrast, permethrin, whose structure is similar to cypermethrin except for the absence of an R-carbon chiral center, was found to be stable in all solvents in the dark at room temperature (25 ± 2C) [62]. Also, enhancement of interconversion of cypermethrin stereoisomers was observed in methanol/water mixtures unlike pure-solvent solutions [62] These observations suggest that the ability of an organic solvent to dissolve SPs is not the only important feature to consider when choosing a solvent for preparing SP reference solutions. Further, as methanol and acetonitrile are the most reported solvents for preparing stock solutions of SPs, it seems that methanol should be avoided in favor of acetonitrile and secondary alcohols.
7. Conclusions SP adsorption onto the walls of most containers to varying degrees, depending on the container material and the composition of samples, should be addressed and assessed when developing new sample-preparation and extraction protocols. Although shaking the container appears to be sufficient to resuspend the SPs to a large extent, further work should clearly define how vigorous shaking should be. Published literature shows considerable differences in terms of stability and degradation rates in both real samples and standard samples for individual SPs, so generalization of experimental observations on one or more SPs to other members of the class is not recommended, and it is important to consider such differences when reporting analytical results. An individual SP may, in some analytical aspects, be considered as a disconnected, stand-alone entity. In summary, based on the above discussions and the study conducted by US Geological Survey [16], the
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following practice may be recommended during sampling, storage and sample preparation of SP environmental samples for analysis (1) Glass containers should be used whenever possible, and, if plastics containers are used, then it is optimal to transfer the sample to a glass bottle as soon as possible. (2) Regardless of the container material, samples should be agitated vigorously for at least 1 min immediately before being transferred to another container. (3) Volume-to-contact-area ratio should be maximized. (4) SP samples must be kept in ice immediately after sampling until reaching the laboratory where they should be kept refrigerated in the dark until analysis. (5) While sediment samples can be frozen for a few months (prior to extraction), with insignificant or no changes in analytical integrity, water samples should be analyzed within 3 d of collection (while considering other storage conditions). (6) Acidification of water sample immediately after collection to minimize hydrolytic processes. In addition, the following precautions may also be considered for reliable analysis of SPs. (1) Extract obtained after the preconcentration step should be analyzed immediately, as its high concentration may cause analyte loss. (2) Blank sample injection should be done at reasonable intervals, if several samples are to be analyzed in the same session. (3) Injection syringe should be washed after each injection by HPLC methanol.
Acknowledgements S.S. Albaseer would like to thank Thamar University, Thamar, Yemen, for financial support. The authors wish to thank J.S. Yadav, Director, Indian Institute of Chemical Technology, Hyderabad, India, for encouragement and permission to communicate this article for publication.
References [1] J. Stenersen, Chemical Pesticides: Mode of Action and Toxicology, CRC Press, Boca Raton, FL, USA, 2004 p. 146. [2] V. Casas, M. Llomparts, C.G. Jares, R. Cela, T. Dagnac, Anal. Bioanal. Chem. 387 (2007) 1841. [3] Agency for Toxic Substances and Disease Registry (ATSDR), Toxicological profile for pyrethrins and pyrethroids, US Department of Health and Human Services, Public Health Service, Atlanta, GA, USA, 2003. [4] Department of Pesticide Regulation, Environmental Monitoring Branch, A Preliminary Assessment of Pyrethroid Contamination of Surface Waters and Bed Sediments in High Pyrethroid-Use Regions of California, Sacramento, CA, USA, 2004.
Trends [5] C. Cox, J. Pestic. Reform. 22 (2002) 12. [6] European Commission, European Directive on the Quality of Water Intended for Human Consumption, 98/83/EC, EU Council, Brussels, Belgium, 1998. [7] S.S. Albaseer, R.N. Rao, Y.V. Swamy, K. Mukkanti, J. Chromatogr., A 1217 (2010) 5537. [8] D.E. Ray, in: W.J. Hayes, E.R. Laws (Editors), Handbook of Pesticide Toxicology, Vol. 2, Academic Press, Inc., 1991, p. 585. [9] M.A. Ali, P.J. Baugh, Int. J. Environ. Anal. Chem. 83 (2003) 923. [10] C.S. Brunete, R.A. Pe´rez, E. Miguel, J.L. Tadeo, J. Chromatogr., A 823 (1998) 17. [11] H. Oudou, H.C. Hansen, Chemosphere 49 (2002) 1285. [12] L.J. Zhou, S. Rowland, R.F.C. Mantoura, Water Res 29 (1995) 1023. [13] M.S. Sharom, K.R. Solomon, J. Agric. Food Chem. 29 (1981) 1122. [14] C.E. Wheelock, J.L. Miller, M.J. Miller, B.M. Phillips, S.J. Geea, R.S. Tjeerdema, B.D. Hammock, Aquat. Toxicol. 74 (2005) 47. [15] K. Day, N.K. Kaushik, Aquat. Toxicol. 10 (1987) 131. [16] M.L. Hladik, J.L. Orlando, K.M. Kuivila, Collection of pyrethroids in water and sediment matrices: development and validation of a standard operating procedure, US Geological Survey Scientific Investigations Report, Reston, VA, USA, 2009. [17] W.A. House, Z. Ou, Chemosphere 24 (1992) 819. [18] W. Yang, J. Gan, W. Hunter, F. Spurlock, Environ. Toxicol. Chem. 25 (2006) 1585. [19] W. Liu, J.J. Gan, S. Lee, J.N. Kabashima, Environ. Toxicol. Chem. 23 (2004) 7. [20] S. Lee, J. Gan, J. Kabashima, J. Agric. Food Chem. 50 (2002) 7194. [21] M. Rebhun, R. Kalabo, L. Grossman, J. Manka, Ch. Rav-Acha, Water Res 26 (1992) 79. [22] W.C. Yang, W. Hunter, J. Gan, J. Environ. Qual. 36 (2007) 1678. [23] European Commission (EC), European Directive 2008/105/EC of the European Parliament and of the Council on environmental quality standards in the field of water policy, EC, Brussels, Belgium, December 2008. [24] S. Bondarenko, F. Spurlock, J. Gan, Environ. Toxicol. Chem. 26 (2007) 2587. [25] M.S. Sharom, K.R. Solomon, Environ. Pollut., B 4 (1982) 269. [26] M.L. Hladik, K.M. Kuivila, J. Agric. Food Chem. 57 (2009) 9079. [27] D.W. Helmuth, S.M. Ghiasuddin, D.M. Soderlund, J. Agric. Food Chem. 31 (1983) 1127. [28] A.G. Gutie`rrez, E. Jover, J.M. Bayona, J. Albaige´s, Anal. Chim. Acta 583 (2007) 202. [29] J.J. Gan, Technical Completion Reports, Pyrethroid insecticides in nursery runoff: Transport and impact on aquatic invertebrates, University of California Water Resources Center, UC Berkeley, CAQ, USA, 2006. [30] D. Wang, D.P. Weston, M.J. Lydy, Talanta 78 (2009) 1345. [31] G.F. Pang, Y.Z. Cao, C.L. Fan, J.J. Zhang, X.M. Li, J. AOAC Int. 80 (1997) 63. [32] T. Yoshida, I. Matsunaga, H. Oda, J. Chromatogr., A 1023 (2004) 255. [33] M.Y. Tariq, S. Afzal, I. Hussain, Environ. Res. 100 (2006) 184. [34] R.A. Chapman, C.M. Tu, C.R. Harris, C. Cole, Bull. Environ. Contam. Toxicol. 26 (1981) 513. [35] S. Sakata, N. Mikami, H. Yamada, J. Pesticide Sci. 17 (1992) 181. [36] Z. Li, Z. Zhang, L. Zhang, L. Leng, Chemosphere 76 (2009) 509. [37] D.A. Laskowski, Rev. Environ. Contam. Toxicol. 174 (2002) 49. [38] K. Ueda, L.C. Gaughan, J.E. Casida, J. Agric. Food Chem. 22 (1974) 212. [39] P. Liu, Y. Liu, Q. Liu, J. Liu, J. Environ. Sci. 22 (2010) 1123. [40] M. Lyytika´inen, J.V.K. Kukkonen, M.J. Lydy, Arch. Environ. Contam. Toxicol. 44 (2003) 437. [41] K. Ma´stovska´, S.J. Lehotay, J. Chromatogr., A 1040 (2004) 259.
http://www.elsevier.com/locate/trac
1779
Trends
Trends in Analytical Chemistry, Vol. 30, No. 11, 2011
[42] S.J. Stangroom, C.D. Collins, J.N. Lester, Environ. Technol. 21 (2000) 865. [43] C.D.S. Tomlin (Editor), The Pesticide Manual - World Compendium, 11th Edition, British Crop Protection Council, Alton, Hants, UK, 1997. [44] H. Perschke, M. Hussain, Chemosphere 22 (1991) 677. [45] P.P. Vazquez, A.R. Mughari, M.M. Galera, J. Chromatogr., A 1188 (2008) 61. [46] M. Anastassiades, K. Mastovska, S.J. Lehotay, J. Chromatogr., A 1015 (2003) 163. [47] E.R. Bennett, M.T. Moore, C.M. Cooper, S. Smith, Bull. Environ. Contam. Toxicol. 64 (2000) 825. [48] C. Sa`nchez-Brunete, B. Albero, G. Martin, J.L. Tadeo, Anal. Sci. 21 (2005) 1291. [49] M. Anastassiades, S.J. Lehotay, D. Stajnbaher, F.J. Schenck, J. AOAC Int. 86 (2003) 412. [50] M.K. Morgan, L.S. Sheldon, C.W. Croghan, P.A. Jones, J.C. Chuang, N.K. Wilson, Environ. Res. 104 (2007) 266. [51] S. Qin, R. Budd, S. Bondarenko, W. Liu, J. Gan, J. Agric. Food Chem. 54 (2006) 5040. [52] E.I. Ulrich, Chim. Oggi 25 (2008) 37. [53] S. Qin, J. Gan, J. Agric. Food Chem. 55 (2007) 5734.
1780
http://www.elsevier.com/locate/trac
[54] W. Liu, J. Gan, D. Schlenk, W.A. Jury, Proc. Natl. Acad. Sci. USA 102 (2005) 701. [55] W. Liu, J. Gan, J. Agric. Food Chem. 52 (2004) 736. [56] S. Qin, W. Liu, J. Gan, in: J. Gan, F. Spurlock, P. Hendley, D.P. Weston (Editors), Synthetic Pyrethroids: Occurrence and Behaviour in Aquatic Environments, ACS Symposium Series, OUP USA, New York, USA, 2008, p. 238. [57] W. Liu, J. Gan, S. Lee, I. Werner, J. Agric. Food Chem. 52 (2004) 6233. [58] R.L. Holmstead, J.E. Casida, L.O. Ruzo, D.G. Fullmer, J. Agric. Food Chem. 26 (1978) 590. [59] H. Perschke, M. Hussain, J. Agric. Food Chem. 40 (1992) 686. [60] E.R. Brouwer, E.A. Struys, J.J. Vreuls, U.A.T. Brinkman, FreseniusÕ J. Anal. Chem. 350 (1994) 487. [61] R.J. Maguire, J. Agric. Food Chem. 38 (1990) 1613. [62] M.G. Nillos, S. Qin, C. Larive, D. Schlenk, J. Gan, J. Agric. Food Chem. 57 (2009) 6938. [63] US National Library of Medicine, 2011 (http://toxnet. nlm.nih.gov/). [64] W. Amelung, A. Nikolakis, V. Laabs, J. AOAC Int. 90 (2007) 1659.