Environ. Sci. Techno/. 1995, 29, 2834-2841
stability of-0 ekoms Pesticides on Disposable Solid-Phase Extradon SILVIA LACORTE, N A D I A EHRESMANN, A N D DAMIA BARCELO* Department of Environmental Chemistry, CID-CSIC, cllordi Girona 18-26, 08034 Barcelona, Spain
The stability of 19 organophosphorus (OP) pesticides was performed using precolumns from the Prospekt (automated on-line solid-phase extraction system) filled with CIS. Several different storage conditions were tested, which included storage at 4 O C , at -20 "C, and a combination of two conditions where precolumns were stored at 4 "C for 1.5 months and held at room temperature until analysis (0.5 and 1 month). Complete recovery was observed in precolumns kept in a t -20 "C for 8 months except for mevinphos, dichlorvos, and phosmet, which had recoveries from 22 to 58%. Degradation also occurred for these same OP pesticides when precolumns were stored a t 4 "C for 2 months and at room temperature for 1 month, with losses varying from 21 to 78% and from 41 to 61%, respectively. Complete degradation of fenamiphos and fonofos occurred in precolumns stored at 4 "C and at room temperature. The stability of the pesticides in each storing treatment is related to the hydrolytic processes, to Koc,and to their solubility in water.
Introduction It is important to know the stability of the different contaminants in water during transport and storage. For example, Maskareinec et al. showed that acidificationwith hydrochloric acid effectively prevented degradation of volatile compounds and allowed sample storage for 112 days (1). The National Pesticide Survey (NPS)and the U.S. EPA appraise that all monitored pesticidesincluded in their programs should be stable in water at least during 14 days, after being inhibited biologically at pH < 3 and stored at 4 "C (2, 3). Organophosphorus (OP) pesticides have exhibited many stabilityproblems, and many of them have been eliminated from the NPS list. It is also surprising that the compounds parathion-ethyl, parathion-methyl, azinphos-methyl, fenitrothion, demeton, fenthion, and malathion have already been included in one of the European Community lists whereas the U.S. EPA has withdrawn them from their list due to their instability ( 4 ) . A recent review * Corresponding author fax: 34-3204 59 04.
2834 1 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 11, 1995
on preservationtechniques for organic compounds showed a 100% loss of many OP compounds after 14 days in biologically inhibited water at 4 "C, while they were stable in the stored sample extracts (5). Stability studies of OP pesticides spiked at 0.1 mg/kg in sediment stored at 4 "C reported losses of 82-100% after 7 months for metamidophos, terbufos, disulfoton, parathion-methyl,and fenthion (6).We have recently showed (7) that the degradation of various OP pesticides in estuarine water samples is fast, with half-lives varying from 2 to 12 days. The use of solid-phase extraction (SPE) material is an alternative of storage of the original matrix. For example, hydrocarbon samples have been found stable for a period of 100 days when stored on XAD-2 macroreticular resin and on CIS extraction columns (8). Fenitrothion was preconcentrated on an XAD-2 column, and the samples remained stable during 5 weeks at room temperature (9). Recent studies of stability have been carried out using Empore disks (4,10-12). Results indicated that pesticides had equivalent or greater stability on SPE disks compared to their storage in water at 4 "C. Additionally, the data obtained suggested that storage at -20 "C after pesticide loading was the most favorable storage option, with recoveries of 79-93% after 180 days of storage. Captan, one of the pesticidesstudied,exhibitedproblems of storage attributed to a combined process of hydrolysis and volatilization during storage at 4 and -20 "C due to the remaining water on the disk not removed by vacuum fdtration (11). Tomkins et al. (10) have shown that DDE, DDT, dieldrin, endrin, aldrin, isodrin, a-chlordane, and y-chlordane can be stored at least for 4 weeks in the disks after preconcentration of water samples at 50 ng/L level. Johnson et al. (12)indicated that storing the disk at -20 "C was the most proper way to obtain good stability data for pesticides but at 4 "C losses of 25-35% for carbofuran were noticed. Relationshipswere established between storage stability with the soil organic partition coefficient (K0J values and solubilityof the differentpesticides. Degradation of pesticideswas attributed either to hydrolysis or microbial breakdown. Microbial growth was observed when storing SPE disks at 4 "C. Some pesticides such as molinate and thiobencarb were fairly stable in the disks up to 180 days (12).
Other ways of stabilization have been reported in the literature, e.g., freeze-dryingwith the addition of glycine (131,which permitted the stabilization of fenamiphos and parathion for 1 month, but the system was not effective for fenitrothionand tetrechlorvinphos, For this study,we have concluded that freeze-drying is not the best option for stabilizing OP and other pesticides in water samples. In view of the current need to monitor OP pesticides in environmental water matrices (7,14,15), we have examined the stability of various pesticides under different storage conditions. Compound selectionwas based on the different problems encountered with the unstable OP pesticides mentioned above. In addition, our laboratory receives water samples each month containing OP pesticides (mevinphos, dichlorvos,azinphos-methyl and -ethyl, parathion-methyl and -ethyl, malathion, fenitrothion, chlorfenvinphos, diazinon, and fenthion) through the European organization Aquacheck (Medmenham, U.K.), which were included in this work. Other compounds such as py-
0013-936XJ95/0929-2834$09.00/0
0 1995 American Chemical
Society
ridafenthion or temephos were added in view of local problems in the area of study (the Ebre Delta, Tarragona, Spain) (7, 14-16'). The use of on-line disposable SPE precolumns has been reported for the assessment of OP pesticides in water (7, 15, 17); however, to our knowledge, no stability studies have been reported for disposable SPE precolumns. These precolumns are currentlyused for on-line SPE studies, such as the Prospekt coupled to liquid chromatography-diode array detection (LC-DAD). LC-DAD is currently used in the m i n e monitoring program and other European programs (7, 15, 17). The specific objectives of this paper were as follows: (i) to establish the storage stability of 19 OP pesticides selected from different priority lists, which generally offer degradation problems in water; (ii) to evaluate the disposable precolumns of Prospekt for current monitoring programs (stability,storage, transport of samples); and (iii) to carry out the stability study during up to a period of 8 months at three different temperatures (-20 "C, 4 "C, and room temperature) in order to assess the ideal storage conditions for pesticide stability. The final goal of the paper is to improve the quality assurance parameters in relation to sample handling, transport, and storage of pesticides in environmental water analysis and to apply the present approach to on-going monitoring programs. The present work follows our previous research on the analysis and degradation of OP pesticides in environmental water samples (7, 13-16).
Experimental Section Chemicals and Reagents. Pesticides mevinphos (CAS No. 7786-34-71,dichlorvos (62-73-71,fensulfothion (115-90-21, azinphos-methyl (86-50-01, azinophos-ethyl (2642-71-9), fenamiphos (22224-92-61, phosmet (732-11-61, pyridafenthion (119-12-01, parathion-methyl (298-00-0), parathion-ethyl (56-38-21,malathion (121-75-5),fenitrothion 122-14-51,chlorfenvinphos (470-90-61,fenthion (55-38-9), fonofos (944-22-91,coumaphos (56-72-41, EPN (2104-6451, chlorpyrifos(2921-88-21,and temephos (3383-96-8)were obtained from Promochem (Wesel, Germany), Pesticidegrade acetonitrile, methanol, and Milli-Q water were purchased from J. T. Baker (Deventer, The Netherlands). Sample Preparation. A mixture of all standards was prepared in acetonitrile at 20 pg/mL. The stability experiment was performed using groundwaterwith the following characteristics: pH = 7.38; conductivity 2.020 pQ1cm; alcalinity315mgof CaC03/L;chlor 369 mg of CUL; sulfates 387 mg of SOdL; nitrates 75 mg of NO3& and hardness 100mg of CaC03/L. Groundwater (not acidified)was spiked with the standard solution to approximately10 ng/mL. Online solid-phase extraction coupled to liquid chromatography with diode array detection (SPE-LC-DAD) was performed with the automated preconcentration system (Prospekt,Spark Holland, Emmen, The Netherlands). The automated SPE device (Prospekt)used in this work consists of a cartridgeexchange module, a pump (orsolvent delivery unit, SDU), and an electrically operated low-pressure sixport valve, which is connected to the gradient pumps. A total of 26 mL of the spiked water sample was preconcentrated on 10 x 2 mm i.d. disposable precolumns prepacked with 40 pm of octadecylsilica (Spark Holland) at a flow rate of 2 mllmin. Before extraction, the precolumns were conditioned via a solvent delivery unit (SDU) from Spark Holland with 10 mL of acetonitrile, 10
mL of methanol, and 10 mL of water at a flow rate of 2 mL/min. A total of 39 precolumns was used for preconcentrating the same spiked water sample. Immediately after preconcentration, three precolumns were eluted (time 0). The precolumns were removed from the holder without performingany dryingtreatment. The designinvolvedthree replicate analysis of each of the three storage treatments and a maximum of four storage periods. The storage treatments include the following: (i) one-third of the precolumns were stored at 4 "C until the day of the analysis, at 1.5, 2, 2.5, 3, and 3.5 months; (ii) precolumns kept at 4 "C for 1.5months followed by storage at room temperature (19-20 "C)for the remainder of the storage period. Those precolumns were analyzed 15 days, 1 month, and 1.5 months following the storage temperature change. This study was used to determine if pesticide degradation occurred in the precolumns once the precolumns were removed from cold conditions; (iii)storage at -20 "C with analysis at 1.5,2,2.5, and 8 months. The precolumns were brought to room temperature over 6-8 h prior to analysis in order to remove the frozen water matrix evolving the Cl* phase. In addition, blanks were prepared by percolating 26 mL of groundwater and storing the precolumns at 4 "C, at -20 "C, and at room temperature during a period of 1 month. In all instances,the precolumnswere stored immediately after the preconcentration step. Desorption of the analytes was carried out by placing the precolumn in the precolumn holder of the Prospektsuch that elutionwould be performed in the backflush mode. The elution step was done by coupling the precolumn "on-line" with the analytical column and starting the gradient. Statistical Evaluation. All results were converted to a percentage of the initial concentration. The precision and variability of the on-line SPE procedure for each pesticide was calculatedthrough the relative standard deviation. The sources of variation (storageconditions, time, compound) were measured with the analysisof variance using the Fisher test (a = 0.05). Variability for each time period within a storage condition was examined. Chromatographic Analysis. The LC analysis was performedwitha Waters 600-MS solventdeliveryunit equipped with a Waters 996 photodiode array detection (Waters, Milford, MA). A 250 x 4.6 mm cartridge column packed with 4 pm of Cg from Waters (Milford,MA) was used. The analysis of the precolumns kept at -20 "C for 8 months and recovery studies were performed with a 250 x 4.6 mm Waters Symmetry cartridge column packed with 5 pm of Cg. The gradient elution was in all cases from 15% acetonitrile and 85%water to 30% acetonitrile and 70% of water in 12 min, to 48% of acetonitrile and 52% of water in 6 min, and kept isocratic for 14 min, and from these conditions to 49% of acetonitrile,21% of methanol (30%of total organic phase), and 30%of water in 16min and finally to 90% of acetonitrile and 10% of methanol in 5 min, at a flow rate of 1 mL/min. Total run time was 63 min. Quantificationwas carried out at 220 nm for mevinphos, dichlorvos, azinphos-methyl, phosmet, malathion, EPN, and chlorpyrifos. Parathion-ethyl, parathion-methyl, and coumaphos were quantified at 280 nm. The rest of the pesticides were quantified at 254 nm. Breakthrough Volumes. The breakthrough of all the studied pesticides was carried out by percolating 20,40,60, 100, 150, 200, and 300 mL of groundwater through CIS precolumns in such a way that the amount injected was VOL. 29, NO. 11,1995 I ENVIRONMENTAL SCIENCE &TECHNOLOGY
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always 100 ng. Breakthrough of the analyte was calculated according to Pichon and Hennion (18). None of the compounds exhibited breakthrough when percolating 26 mL of water; however, breakthrough was observed at 60 and 100 mL for cis-mevinphos and trans-mevinphos, respectively,due to their high polarity and small retention upon the CISmaterial. Fenamiphos and dichlorvos,which are quite soluble in water also presented losses at 60 mL. Phosmet and fonofos presented a breakthroughvalueof 60 mL. Instrument Calibration. Calibration plots were constructed by percolating 26 mL of HPLC-grade water spiked at levels of 1, 3, 6, 9, 12, and 15 nglmL using on-line SPELC-DAD, following the methodology described in the Experimental Section, in such a way that the amount of each pesticide injected varied from 26 to 390 ng. These points were chosen in such a way that they covered the whole concentration range in which the stability study was carried out, taking into consideration the possibility of diminution of the concentration of pesticides due to degradation. All the pesticides under study showed a linear behavior within the 1-15 nglmL concentration range, as presumed since no breakthroughoccurs by percolating 26 mL of the water sample. Furthermore, the correlation coefficientswere at 0.99 level for all compounds except for cis-mevinphos (0.961, trans-mevinphos (0.981, phosmet (0.981, fenthion (0.941, chlorpyrifos (0.981, and temephos (not calculated due to bad peak shape). The former compound presents not only problems in quantification at low wavelengths but also a high polarity, which results in early elution time where interferences are more abundant. Fenthion is a rather unstable compound, and the time needed to carry out the preconcentration step is enough to hydrolyze this compound in water solution. Both chlorpynfos and temephos showed lower correlation coefficients due to coelution problems at a high percentage of acetonitrile.
Results and Discussion Recovery Studies. Recovery studies of 19 pesticides were performed by spiking groundwater at a concentration of 10 ng/mL, bypreconcentratingof 26 mL on CISprecolumns, and by analyzingon-line SPE-LC-DAD. Quantificationwas done by external standard comparison using HPLC water spiked at the same concentration and analyzed with the same technique. Table 1 shows the mean recoveries of the pesticides under study and the standard deviation using spiked HPLC water as standard and on-line SPE analysis. Recoveries are in all instances around loo%, and low standard deviations are found. However, up to 10% recovery variability was accepted for this analytical procedure. Recoveries obtained by on-line SPE provide equivalent results as compared to off-linetechniquesusing either cartridges (19, 20) or disks (21). These analytical conditions were found suitable to carry out stability studies because losses of the analytes should only be originated by degradation of such compounds during the storage conditions. In addition, the study was performed at a spiking level of 10 nglmL, in such a way that the amount of each pesticide injected was 260 ng at 100%recovery. This was done to monitor the presence of possible transformation products (TPs) that might be formed during the storage conditions. A complete separation of the 19 pesticides was achieved with the column Symmetry from Waters except for feni2836
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29. NO. 11. 1995
TABLE 1
% Mean Recovery and Relative Standard Deviation (n = 3) of Pesticides in Qroundwater Using On=Lire SPE-LC-DAD and Mean % Recovery (n = 3) of Pesticides Stored at 4 O C during 1.5, 2, 2.5, and
3 Month9 compound cis-mevinphos trans-mevinphos dichlorvos fensulfothion azinphos-methyl fenamiphos phosmet pyridafenthion parathion-methyl malathion fenitrothion azinphos-ethyl chlorfenvinphos fenthion parathion-ethyl coumaphos fonofos
EPN chlorpyrifos temephos a
ID no.
A
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
220 220 220 254 220 254 220 254 280 220 254 254 254 254 280 280 254 220 220 254
months stored at 4 "C YOmean recovery& SD 1.5 2 2.5 3
106 i 0.8 102 4 1.0 1 0 6 i 1.4 1 0 4 i 1.1 98 i 0.2 101 i . 2 . 4 98i0.2 101 i 0.6 98 i 1.4 101 i 1.5 99i2.1 99 i 2.0 100 i. 1.6 101 i 1 . 6 100 i.0.7 101 i 1 . 3 101 f 1.8 10853.7 114f4.4 111 f 9 . 2
32 45 117 101 115
nfb
nf 35 ncc 19 nc nf
22 79 97 98
99 74 114 100 nf
nf
nf
86 106 127 88 110 110 110 93 114 118
51 nc 93 108 116 133 nc 65 93 107 93 107 98 107 90 91 1 1 1 114 108 115
61
nf
nf
nf
nf
nf nc
109 nc
91 91 90 85 104 103
128 102 107 103 132 130 121 104 148 141 123 128
Volume of water preconcentrated = 26 mL, spiked at 10 ng/mL. nc, unable to integrate.
* nf, not found.
trothion and azinphos-ethyl, which coeluted (Figure 1A). Hereafter, both compounds will be quantified in conjunction. The various pesticides were quantified at the wavelength which enhanced selectivity. As can be seen in Table 1,few compounds (parathion-ethyl,parathion-methyl, and coumaphos) were quantified at 280 nm. This was necessary to avoid a contribution of a coeluting peak that could be observed when quantifying at a lower wavelength. In these cases, quantificationwas possible by wavelength selection. Figure 1B,C represents two examples of the spectra of two coelutingcompounds. In Figure l B , the spectra offenthion and parathion-ethyl are shown. While parathion-ethyl can be quantified at 280 nm without any contribution of the coeluting peak, fenthion, quantified at 254 nm, always presents a contribution of parathion-ethyl. Similarly, temephos and chlorpyrifos could be quantified at 254 and 220 nm, respectively,taking advantageof the stretch where the coeluting compound does not absorb (Figure1C).Using this UV selectivity,the recoveries of all compounds turned out to be satisfactory (see Figure 21. As can be observed in Table 1, mevinphos appears as isomers cis and trans, which differ by 3.6 min in retention time. The presence of these two isomers has been reported earlier, and the difficultyto discern both isomers is obvious from works that have been published (221 and also from an intercalibration study concerning OP pesticides (15). LC-DADoffers the possibilityof distinguishingboth of them and quantifying them with an error below 10%. The recoveries of both isomers are loo%,and therefore,no losses due to breakthrough or losses due to their high polarity are observed. The problem of analyzing this compound relies more in its low UV absorption (
