Ultratrace Determination of Atrazine and Its Six Major Degradation

Environ. Sci. Technol. 1997, 31, 1658-1663

Ultratrace Determination of Atrazine and Its Six Major Degradation Products in Water by Solid-Phase Extraction and Liquid Chromatography-Electrospray/Mass Spectrometry ANTONIO DI CORCIA,* CARLO CRESCENZI, ETTORE GUERRIERO, AND ROBERTO SAMPERI Dipartimento di Chimica, Universita` “La Sapienza” di Roma, Piazza Aldo Moro 5, 00185 Roma, Italy

Due to the widespread use of atrazine, this herbicide and its degradation products (DPs) can contaminate waters destined for human consumption. The overall impact of these compounds in aquatic ecosystems has not yet been assessed. This paper describes a very sensitive and specific method for ultratrace determination of atrazine and its six major DPs in environmental waters. This method is based upon solid-phase extraction with a new example of graphitized carbon black (Carbograph 4) followed by electrospray (ES) liquid chromatography-mass spectrometry (LC-MS). On extracting from 1 L of river water and 4 L of both groundwater and drinking water, analyte recoveries were better than 80%. Day-to-day precision was 2.9-9.8% at 25 ng/L (full-scan acquisition) and 2.3-7.7% at 3 ng/L (two-ion SIM acquisition). For all the analytes considered, raising the skimmer cone voltage in the desolvation chamber of the ES/MS system had the double effect of producing diagnostic fragment ions and enhancing the ion signal strength. In this way, highly specific full-scan LC-MS analysis of the seven analytes could be performed at levels down to 40-300 ng/L in river water samples. Timescheduled selected ion monitoring of the intact molecular ion plus one related fragment ion could afford specific determination of the analytes at sub-nanogram per liter levels in groundwaters and drinking waters. A short survey conducted by analyzing some river (Tiber) water samples ascertained the presence of all the atrazine DPs, except DEDIA, at a few nanogram per liter levels.

Introduction Atrazine is a very popular herbicide introduced about 35 years ago and applied to a variety of crops, including green vegetables. Laboratory (1-5) and field (6-8) studies have shown that atrazine is degraded through hydrolysis, UV radiations, and microbial activity leading mainly to the formation of dealkylated and hydroxylated species (Figure 1). As a result of the widespread use of atrazine and of the persistence of its degradation products (DPs), these have been frequently detected in surface waters and groundwaters as well as in water supplies (9-16). The determination of * To whom correspondence should be addressed. Telephone: +396-49913752; fax: +39-6-490631; e-mail: [email protected].

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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 6, 1997

FIGURE 1. Structures and acronyms of atrazine degradation products.

TABLE 1. Time-Scheduled SIM Conditions for Monitoring Atrazine and Its Major Six DPs in Watera compound

channel mass, m/zb (relative abundance)

retention window (min)

DIHA DEDIA DEHA DIA DEA HA atrazine

97 (60) ( 156 (100) 68 (40) ( 146 (100) 128 (100) ( 170 (70) 96 (60) ( 174 (100) 146 (100) ( 188 (50) 156 (100) ( 198 (90) 174 (100) ( 216 (50)

10-14.5 14.5-18 22-27 27-30 30-33 33-37

a Dwell time, 0.1 s; span, 1; skimmer cone voltage, 40 V. b H+ intact adduct ions are reported in boldface.

atrazine and its DPs in water is of huge interest as these compounds are suspected to be cancerogenic and studies focused on the fate and the degradation pathway of atrazine under natural conditions and after atrazine-contaminated waste treatments need to be validated. Ultratrace determination of atrazine and its derivatives in water poses two severe problems. First, the highly polar nature of atrazine metabolites results in poor extraction of them by using either liquid-liquid or solid-phase extraction (SPE) with C-18 bonded silica cartridges (14, 17, 18). By exploiting the higher basicity of atrazine hydroxylated DPs, selective extraction and good recovery of these species have been obtained by the use of a cation exchanger cartridge (18). The weakness of this approach is that it cannot be extended to determine atrazine and its dealkylated species. Graphitized carbon black (GCB), commercially referred to as Carbopack or Carbograph 1, is a valuable adsorbent able to give large enrichment factors even for well water-soluble organic contaminants in water (17, 19-21). Good recovery of HA was obtained on extracting it from 2 L of water with a 250-mg GCB cartridge (14). By means of the same cartridge, efficient isolation of nanogram per liter levels of DEA, DIA, and HA from 1 L of lake water has been achieved (15). Carbograph 4, a new type of GCB, has shown to be even abler than Carbograph 1 to extract very polar compounds from water (22).

S0013-936X(96)00494-4 CCC: $14.00

 1997 American Chemical Society

TABLE 2. Relative Abundances of Product and Precursor Ions of Atrazine and Its Major Six DPs by Varying the Skimmer Cone Voltage cone voltage (V) analyte

30

40

DIHA

156a

DEDIA

146

DEHA DIA

128 (40), 170 (100) 174

DEA

146 (15), 188 (100)

HA

156 (15), 198 (100)

atrazine

174 (25), 216 (100)

a

50

69 (25), 86 (40), 97 (60), 114 (25), 156 (100) 43 (65), 68 (40), 79 (40), 104 (60), 110 (20), 146 (100) 43 (15), 86 (50), 128 (100), 170 (70) 43 (15), 68 (15), 71 (50), 79 (25), 96 (60), 104 (25), 132 (40), 174 (100) 43 (15), 79 (10), 104 (15), 110 (10), 146 (100), 188 (50) 86 (30), 97 (20), 114 (35), 156 (100), 198 (90) 43 (15), 96 (20), 104 (15), 132 (15), 146 (10), 174 (100), 216 (50)

43 (30), 69 (100), 71 (40), 86 (70), 97 (80), 114 (30), 156 (80) 43 (100), 68 (60), 79 (20), 104 (25), 146 (25) 43 (30), 69 (10), 86 (100), 128 (70), 170 (20) 43 (40), 68 (60), 71 (100), 79 (30),96 (70), 104 (50), 132 (20), 174 (30) 43 (40), 68 (30), 79 (40), 104 (50), 110 (25), 146 (100), 188 (20) 43 (35), 69 (30), 71 (30), 86 (90), 97 (85) 114 (85), 156 (100), 198 (60) 43 (45), 68 (20), 71 (60), 79 (35), 96 (95), 104 (50), 132 (45), 146 (30), 174 (100), 216 (30)

Intact protonated adduct ions are reported in boldface.

TABLE 3. Recovery of Atrazine and Its Major Six DPs by Extracting Them from Spiked Drinking Water, Groundwater, and River Water Samples recoverya ( RSDb % drinking water, 4L

groundwater, 4L

river water, 1L

analyte

200c

3c

200c

200c

10c


98 ( 3 95 ( 4 98 ( 3 97 ( 2 96 ( 3 97 ( 2 98 ( 2

97 ( 5 92 ( 6 96 ( 4 97 ( 5 96 ( 4 96 ( 4 101 ( 3

92 ( 4 95 ( 5 93 ( 3 95 ( 3 97 ( 3 96 ( 3 98 ( 4

98 ( 3 97 ( 5 99 ( 4 98 ( 2 98 ( 3 101 ( 4 99 ( 4

84 ( 8 80 ( 7 82 ( 6 85 ( 6 88 ( 7 86 ( 6 89 ( 8

a

Mean values from six measurements. deviation. c Spike levels (ng/L).

b

RSD, relative standard

TABLE 4. Limits of Detection (ng) of the Method for Atrazine and Its Major Six DPs by Using Different Acquisition Modes analyte

full-scan CIDa

SIM (two-ion)b


4.9c 5.7 3.2 3.8 2.5 0.81 1.0

0.07 0.07 0.03 0.14 0.03 0.04 0.04

a Scan m/z range, 40-220 in 2 s; cone voltage, 50 V. b Cone voltage, 40 V; time-scheduled SIM of the MH+ ion + one related fragment ion. c Mean values obtained from three determinations.

The highly sensitive and specific gas chromatography/ mass spectrometry technique with capillary columns has been advantageously exploited for trace analysis of atrazine and its dealkylated metabolites in environmental waters (6, 23). Nonvolatile hydroxylated atrazine metabolites cannot be determined however unless affording incomplete derivatization procedures (24). Liquid chromatography (LC) with a diode array detector (DAD) has been successfully employed for identification of atrazine derivatives, including hydroxylated forms, generated by photolysis experiments (1). However, LC/DAD can fail to determine unambiguously trace levels of target compounds in complex matrices. LC-MS-MS with a thermospray interface has been successfully applied to the characterization of atrazine and its DPs in aquatic photodegradation studies and in real polluted soil samples (25). LC-MS-MS with an electrospray

(ES) ion source has been used for identification of atrazine derivatives produced by Fenton’s reagent (26) and for confirmative analysis of hydroxylated DPs of atrazine in river water samples (16). So far, however, no LC-ES/MS procedure has been elaborated for simultaneously monitoring atrazine and its six major metabolites in environmental waters. The object of this work has been that of developing a simple method for determining trace levels of atrazine and six related DPs in surface water, groundwater, and drinking water samples. This method involves SPE of the analytes by a Carbograph 4 cartridge followed by separation and identification by LC with a benchtop ES/MS detector.

Experimental Section Reagents and Chemicals. Atrazine and its six major metabolites were purchased from Alltech, Sedriano, Italy. Individual standard solutions of DIA, DEA, and atrazine were prepared by dissolving 100 mg of them in 100 mL of acetonitrile. A standard solution of DEDIA was prepared by dissolving 20 mg of it in 200 mL of methanol. Finally, individual standard solutions of DIHA, DEHA, and HA were prepared by dissolving 50 mg of them in 100 mL of acetonitrile acidified with HCl, 5 mmol/L. Composite working standard solutions were freshly prepared each week by suitably mixing the standard solutions mentioned above. When unused, all standard solutions were stored at 4 °C. For LC, distilled water was further purified by passing it through the Milli-Q Plus apparatus (Millipore, Bedford, MA). Methanol “plus” and acetonitrile “plus” of gradient grade was obtained from Carlo Erba, Milano, Italy. All other solvents were of analytical grade (Carlo Erba), and they were used as supplied. Apparatus. Extraction cartridges filled with 0.5 g of Carbograph 4 (surface area, 210 m2/g; 120-400 mesh size, Carbochimica Romana, Rome, Italy) were prepared as previously reported (27). The Carbograph 4 cartridge was fitted into a side-arm filtration flask, and liquids were forced to pass through the cartridge by vacuum (water pump). Before processing water samples, the cartridge was washed with 6 mL of the eluent phase for the analytes (see below), followed by 2 mL of methanol and 5 mL of distilled water. Sampling. Grab samples of surface waters and groundwaters were collected in brown bottles from various rivers and wells located near Rome and kept at 4 °C in the dark until analysis. To avoid possible sample alteration, storage time was no longer than 2 days. Unless they contained algae and debris able to plug the cartridge, river water samples were extracted unfiltered (although with restricted flow rates). Procedure. For recovery studies, aqueous samples were fortified with known amounts of the composite standard

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FIGURE 2. TIC LC-MS CID chromatogram obtained by analyzing 1 L of a river water sample spiked with the seven analytes at the individual level of 0.4 µg/L. solution. Before spiking with analytes, hypochlorite in drinking water was eliminated by the addition of Na2S2O3‚ 5H2O, 0.5 g/L. Water samples were firmly agitated for about 1 min and, after 2-3 min, poured in a glass reservoir connected to the sorbent cartridge. Water was forced to pass through the cartridge at flow rates of 70-100 mL/min by reducing the pressure in the vacuum apparatus to the minimum. After the sample was passed through the column, the pump was disconnected and the cartridge was filled with 7 mL of distilled water, which was allowed to pass through the cartridge at flow rates of 5-7 mL/min. Any void space created by some shrinking of the sorbent bed occurring during the passage of the sample was eliminated by pushing the upper frit against the sorbent bed. Most water was removed from the cartridge by forcing room air through it for 1 min. The pump was disconnected, and 0.5 mL of methanol was poured into the cartridge, which was slowly passed through the sorbent bed to eliminate part of the residual water. Following the passage of methanol, the pressure was reduced to the minimum for 1 min. Thereafter, a suitably drilled cylindrical Teflon piston with one conically indented base and a Luer tip was forced to enter the cartridge until it reached the upper frit (27). The trap was then turned upside down, a 1.4 cm i.d. glass vial with a conical bottom was placed below the cartridge, and analytes were back-eluted by passing 1.5 mL of methanol through the trap followed by 6 mL of a methylene chloride/methanol (80:20, v/v) mixture containing HCl, 5 mmol/L. When the eluent phase was not acidified, poor recovery of DIHA and DEDIA was observed. The flow rate at which the eluent phase was percolated through the cartridge was ca. 6 mL/min obtained by suitably regulating the vacuum in the apparatus. The last drops of this solvent mixture were collected by a further decrease in the pressure inside the flask. The eluate was neutralized by 70 µL of a

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FIGURE 3. Full-scan background-subtracted mass spectra taken from the average of chromatographic peaks for DIHA and DEHA on analyzing 4 L of a drinking water sample spiked with the analytes at the individual level of 20 ng/L. water/methanol solution containing ammonia, 0.4 mol/L, and concentrated down to about 10 µL in a water bath at 30 °C under a nitrogen stream. After diluting with 90 µL of water and measuring the exact final extract volume, 20 µL of it was injected into the LC column. When analyzing river water samples, the injected extract volume was 10 µL. On injecting larger extract volumes, anomalously weak signals were observed for the early-eluting analytes, i.e., DIHA and DEDIA. This signal weakening was presumably due to saturation effects of the ES/MS detector by coeluted fulvic acids. LC-ES/MS Analysis. Liquid chromatography was carried out with a Varian (Walnut Creek, CA) Model 9010 equipped with a Rheodyne Model 7125 injector having a 25-µL loop. The analytes were chromatographed on an Alltima 25 cm × 4.6 mm i.d. column filled with 5-µm C-18 reversed phase packing (Alltech). For fractionating the analytes, the phase A was methanol and the phase B was water. Both solvents contained formic acid, 10-5 mol/L. The addition of this concentration of formic acid to the mobile phase resulted in an about 60% increase of the ion signal for all the analytes. At higher acid concentrations, ion signals showed a slight but steady tendency to a decrease. The initial composition of the mobile phase was 0% A that was first increased linearly to 35% within 16 min and then to 84% within 20 min. The flow rate of the mobile phase was 1 mL/min, and 40 µL/min of the column effluent was diverted to the ES source. A Fisons VG Platform benchtop mass spectrometer (Fisons Instruments/VG BioTech, Milano, Italy) consisting of a pneumatically assisted ES interface and a single quadrupole was used for detecting and quantifying target compounds in the LC column effluent. This was introduced into the ES interface through a 40-cm length of 75 µm diameter PEEK capillary tube. The MS was operated in the positive-ion mode by

standard solution in the eluent phase used for eluting analytes from the Carbograph 4 cartridge and then following the rest of the procedure reported above. Concentrations of the analytes in river water samples were measured by using the external standard quantification procedure. Following the above procedure, calibration graphs were constructed by adding to the eluent phase respectively 2, 5, 10, and 20 ng of each analyte from a composite working standard solution. The mass spectrometry data handling system used was the Mass Lynx software from Fisons Instruments.

Results and Discussion

FIGURE 4. Two-ion SIM LC-ES/MS chromatogram obtained by analyzing 4 L of a drinking water sample spiked with the analytes at the individual level of 1 ng/L. applying to the capillary a voltage of 4.0 kV. The source temperature was maintained at 70 °C. MH+ ions were generated using highly pure nitrogen as drying and nebulizing gases at flow rates of 250 L/h and 13 mL/min, respectively. By suitably controlling the potential difference between sample and skimmer cones, structurally significant fragment ions resulting from collision-induced dissociation (CID) between MH+ ions and residual drying gas molecules in the intermediate pressure region could be obtained (28-36). These CID spectra are comparable to those obtained by tandem MS spectrometry. Unless otherwise specified, the skimmer cone voltage was set at 50 V. Full-scan LC-MS chromatograms were obtained by scanning the quadrupole from 40 to 220 m/z with 2-s scan. Time-scheduled selected ion monitoring (SIM) LC-MS analysis of the seven analytes was performed following conditions reported in Table 1. On a daily basis and with the system beyond the sample cone still under vacuum, the sample cone was cleaned with use of a methanol-imbibed paper, while the counterelectrode was cleaned with a concentrated nitric acid/water (50:50, v/v) mixture and 5-min sonication. Thereafter, it was extensively washed with distilled water, followed by acetone and methanol. This operation did not take more than 15-20 min. By the same procedure, the sample cone and the skimmer lens were cleaned on a weekly basis. Analyte recoveries were generally assessed by full-scan acquisition. The concentrations of the analytes in spiked samples were calculated by measuring peak areas from selected extracted-ion-current profiles and comparing them with those obtained from standard solutions. In particular, when assessing analyte recoveries from a drinking water sample spiked with 3 ng/L of each analyte, acquisition in the SIM mode was adopted. Standard solutions were prepared by dissolving known and appropriate volumes of the working

Specificity. For the seven analytes considered, the effect of increasing the skimmer cone voltage on the extent of fragmentation of the MH+ ions was investigated. This experiment was conducted by injecting 100 ng each of the seven analytes from a standard solution into the LC column and separating them following the conditions reported in the Experimental Section. At any cone voltage increasing from 30 to 50 V, background-subtracted spectra were taken from the average of chromatographic peaks. Relative abundances of parent and product ions were calculated by averaging four determinations. Fragment ions having relative abundances not higher than 10% were not considered. Results are reported in Table 2. At 30 V cone voltage, only spectra from DEHA, DEA, HA, and atrazine displayed a fragment ion generated by the loss of the isopropyl group (-42 Da). By raising the cone voltage at 40 V, several fragment ions were produced by cleavage of the triazine ring. Structures of these ions were postulated in two previous works aimed at characterizing CID spectra acquired by tandem MS with either TS (25) or ES (26) ion sources. At 60 V cone voltage, only weak or no appreciable signals for intact protonated molecular ions were observed. Interestingly, as the collision energy was increased by increasing the cone voltage, the production of fragment ions for the analytes considered was steadily accompanied by an enhancement of the ion signal intensity. This positive effect has been already observed with several other compounds (36) and may be traced to a more efficient adduct ion sampling occurring in the sample cone region. Recovery Studies. In order to achieve sufficiently high enrichment factors, which enable target compounds to be monitored in water samples in the nanogram per liter range, the extraction of large volumes is a prerequisite. In this respect, the ability of the Carbograph 4 cartridge to effectively extract atrazine and its major DPs from 4 L of drinking water and groundwater and 1 L of river water was evaluated. For both drinking water and river water, recovery studies at two different spike levels were conducted. Data reported in Table 3 show that, except for river water at 10 ng/L spike level, recoveries of the analytes were better than 90%, even when they were present in drinking water at the 3 ng/l level. This demonstrates that no effect of irreversible adsorption was produced by the extraction device. When the spike level in river water was decreased from 200 to 10 ng/L, a 10-17% loss of the analytes was observed. Some effects of adsorption on suspended particles and/or strong interactions between humic acids and analytes were presumably responsible for this loss. When the cartridge was not reversed before analyte re-extraction, up to about 70% of DIHA, DEDIA, and DEHA were not eluted from the cartridge by the eluent phase. Limits of Detection (LODs). Under two different acquisition modes and based on the peak to peak noise measured on the base line close to the analyte peak, LODs (S/N ) 3) were calculated for the seven analytes by eluting them as reported in the Experimental Section and measuring peak heights against average background noise (Table 4). When operating in the full-scan mode, LODs were estimated by


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TABLE 5. Day-to-Day Precision of the Method at Two Concentrations of Atrazine and Its Major Six DPs in Drinking Water

TABLE 6. Concentration Levels of Atrazine and Its Major Six DPs in Three Samples of a River (Tiber) Water Collected between June and August 1995

measured concna (ng/L) ( SD

b

analyte

25b

3b


24 ( 1.2 24 ( 2.3 25 ( 1.0 24 ( 0.9 24 ( 0.7 24 ( 1.1 25 ( 2.2

2.9 ( 0.22 2.7 ( 0.19 2.9 ( 0.21 3.0 ( 0.12 2.9 ( 0.12 2.9 ( 0.21 3.1 ( 0.07

a Mean values from five determinations performed over 8 days. Spike level (ng/L).

injecting 20 ng of each of the analytes three times into the LC column from a standard solution. Considering this method involves extraction of 4 L of both drinking water and groundwater, and injection of one-fifth of the final extract; limits of quantification (LOQs, defined as 5 times the limits of detection) by full-scan LC-MS-CID analysis for the compounds of interest in these two types of water ranged between about 5 (HA) and 36 ng/L (DEDIA). For river water, the above limits have to be increased by a factor 8 (see Experimental Section). As an example, Figure 2 shows a chromatogram obtained by full-scan LC-MS-CID analysis of an extract relative to a river water sample spiked with the analytes at the individual concentration of 400 ng/L. For DIHA and DEHA, full-scan background-subtracted mass spectra taken from the average of chromatographic peaks (not shown here) obtained by analyzing 4 L of a drinking water sample spiked with 25 ng/L each of the analytes are shown in Figure 3. It appears that all the major peaks for fragment and parent ions are present in the spectra (see Table 2) from the two analytes injected into the LC column at the individual level of 20 ng. This demonstrated that the high specificity of the method was still retained nearby the calculated limits of quantification. Acquisition in time-scheduled two-ion SIM mode affords the highest sensitivity. Related LODs (Table 4) were calculated in the same way as reported above but injecting analytes at the individual level of 0.8 ng. Figure 4 shows a typical time-scheduled, two-ion SIM LCMS chromatogram obtained by analyzing 4 L of a municipal water sample spiked with the seven analytes at the individual level of 1 ng/L. Reproducibility. The day-to-day precision of this method at very low analyte concentrations in water was estimated. A drinking water sample was spiked with the analytes at two concentration levels, i.e., 25 and 3 ng/L, and analyzed five times over 8 days by following the conditions reported in the Experimental Section, including the cleaning procedures of the ES/MS system. When analyzing water spiked at the level of 25 ng/L, the ES/MS system was operated in the full-scan mode. Under this condition and for each analyte, quantification was performed by extracting the ion current profile relative to the most abundant ion. On the contrary, the twoion SIM acquisition mode was used for analyzing the most diluted water sample. Results are shown in Table 5. Application to Environmental Samples. For some years, atrazine has been banned in Italy for agricultural use. The actual levels of atrazine and its DPs in environmental waters was the object of a short investigation. This survey was conducted by analyzing three river (Tiber) water samples collected between June and August 1995. Samples were analyzed in duplicate, and the results are reported in Table 6. For DIA, concentrations were reported with only one significant figure, as they were below the related LOQ (7 ng/

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concentrationa (ng/L)

DIHA DEDIA DEHA DIA DEA HA atrazine a

sample 1 June 1995

sample 2 July 1995

sample 3 August 1995

7.6 ndb 2.4 3 4.3 3.7 5.4

16 nd 6.6 2 4.3 6.0 6.7

3.9 nd 5.8 5 6.5 9.8 3.4

Mean values from duplicate analysis.

b

nd, not detected.

FIGURE 5. Two-ion SIM LC-ES/MS chromatogram obtained by analyzing 1 L of a river (Tiber, August 1995) water sample. As measured by this method, the concentrations (ng/L) of the analytes were as follows: DIHA, 4.5; DEHA, 5.8; DIA, 5; DEA, 6.5; HA, 9.8; atrazine, 3.4. L). Except for DEDIA, all the other atrazine metabolites were present in the three samples analyzed. Therefore, our data confirm that even DIHA and DEHA can contaminate surface waters (16). Concentration levels of atrazine measured in the Tiber River were only 3-4 times lower than that measured 8 years before (37), and the metabolite concentrations were comparable with that of the parent compound. These results give further evidence for the persistence of atrazine and its metabolites in environmental compartments. As an example of the potentiality of this method for determining a few nanograms per liter of the analytes of interest in a complex aqueous matrix, a LC-MS SIM chromatogram obtained by analyzing a Tiber water sample (August 1995) is shown in Figure 5. The peak for protonated DEA was obscured by a tailed peak for an unknown compound. In this case,

quantitation of DEA was carried out by exploiting the ion current profile relative to its fragment ion at 148 m/z.

Acknowledgments This work has been financially supported by the Consiglio Nazionale delle Ricerche (Contract CNR CTB 95.02833.74).

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Received for review June 10, 1996. Revised manuscript received November 26, 1996. Accepted December 24, 1996.X ES960494T X

Abstract published in Advance ACS Abstracts, March 1, 1997.


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Ultratrace Determination of Atrazine and Its Six Major Degradation

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