Anal. Chem. 1993, 65, 21-28
21
Determination of Atrazine in Water at Low- and Sub-Parts-per-TriIIion Levels by Using Solid-Phase Extraction and Gas Chromatography/High-Resolution Mass Spectrometry Zongwei Cai, V. M. Sadagopa Ramanujam, Daryl E. Giblin, and Michael L. Gross' Midwest Center for Mass Spectrometry, Department of Chemistry, University of Nebraska, Lincoln, Nebraska 68588-0304 Roy F. Spalding Water Center, University of Nebraska, Lincoln, Nebraska 68588-0844
A caplllary columngas chromatography/hlgh-resolutlonmass sp&rwtry (GWHRMS) methodwas developedto determine atrazlne In water at low- and sub-parts-per-trllllon (ppt) levels. The method fllb a need when the detectlon llmlts of routlne GC, QC/MS, and high-performance llquld chromatography (HPLC) are Inadequate for studles to evaluate transport and fate of herblcldes In deep ground water systems. Atrazlne and the added Internalstandard [laCa]atrazlnewere extracted from water samples by wlld-phase extractlon uslng a Clo bonded rlWca cartrklge. A columnchromatographyprocedure wlth Florldl was developedto reduce when necessary further Interferences In the sample extract. The analysls was performedby GC/HRMS wkh the mass spectrometer operated In the selected lon monltorlng (SIM) mode: mass proflles were acqulred. The preclrion and accuracy of the method are 15% (relatlve standard devlatlon, n = 8) and >85%, respectlvely,for standard water samples contalnlng atrazine at 1 ppt. The method, whlch glves detectlon llmlts of 200500 parts-perquadrllllon(ppq), has been successfully used to analyzewater samplescontalnlngatrazlne at concentratlon levels of 1-500 ppt.
Atrazine (2-chloro-4-(ethylamino)-6-(isopropylamino)1,3,5-triazine) is one of the most widely used herbicides for a broad spectrum of crops. It was first marketed in 1957 and soon became a very popular herbicide for broadleaf weed control in corn and milo. Residues of atrazine are frequently found in environmental samples of soil and water owing to its persistence and relatively high solubility in water. Currently used methods for the analysis of atrazine in water involve sample enrichment by liquid-liquid'-3 or solid-phase extraction4-12 followed by reconcentration and determination (1)Graves, R. L. Method 507. Determination of Nitrogen- and Phosphorus-Containing Pesticides in Water by Gas Chromatography with Nitrogen-Phosphorus Detector, revision 2.0; 1989; pp 143-170. (2)Lee, H. B.; Stokker, Y. D. J. Assoc. Off. Anal. Chem. 1986,69, 568-572. (3)Lopez-Avila,V.; Hirata, P.; Kraska, S.; Flanagan, M.; Taylor, J. H.; Hern, S. C. Anal. Chem. 1985,57,2797-2801. (4)Popl, M.; Voznakova, Z.; Tatar, V.; Strnadova, J. J. Chromatogr. Sci. 1983,21,39-42. (5)Woodrow, J. E.; Majewski, M. S.; Seiber, J. N. J. Enuiron. Sci. Health 1986,B21, 143-164. (6)Richard, J. J.; Junk, G . A. Mikrochim. Acta 1986,1,387-394. (7) Junk G. A.; Richard, J. J. Anal. Chem. 1988,60,451-454. (8)Sherma, J. J.Liq. Chromatogr. 1986,9,3433-3438. (9)Naah, R. G. J. Assoc. Anal. Chem. 1990,73,438-442. (10)Mangani, F.; Bruner, F. Chromatographia 1983,17,377-380. (11)Bamati, R.: Benfenati, E.: Davoli, E.: Fanelli, R. Chemosphere 1988,17,5-9-65. (12)Di Corcia, A,; Marchetti, M.; Samperi, R. J. Chromatogr. 1987, 405,357-363. 0003-2700/93/0365-0021$04.00/0
by gas chromatography (GC),1~~9~-7J3J4 gas chromatography/ mass spectrometry (GC/MS)13J0-12J6-28 and high-performance liquid chromatography (HPLC).2'+36 Improved methods with lower detection limits allow for the determination of ultratrace levels of this agrochemical in more pristine surface or near surface ground water. Such low levels also occur in semiconfined and/or deep formation waters. The presence of atrazine in even parts-per-quadrillion (ppq) levels in ground water indicates that the water is a product of relatively recent recharge (post-1960) from the surface. Low detection limits are also required to support research focused on environmental kinetics of the degradation of herbicides in both soil and water. Additional applications for methods with ultratrace detection limits include evaluations of the efficacy of activated carbon in purifying atrazinecontaminated waters and as an early warning indicator of herbicide breakthrough from activated carbon filters. (13)Grob, K.; Li, Z. J. Chromatogr. 1989,473,423-430. (14)Edgell, K. W.; Jenkins, E. L.; Lopez-Avila,V.; Longbottom, J. E. J. Assoc. Off. Anal. Chem. 1991, 74, 295-309. (15)Durand, G.; Barcelo, D. J. Chromatogr. 1990,502,275-286. (16)Rostad, C. E.; Pereira, W. E.; Leiker, T. F. Biomed. Enuiron. Mass Spectrom. 1989,18,820-827. (17)Huang, L. Q.;Mattina, M. J. 1.Biomed.Enuiron.Mass Spectrom. 1989,18,828-835. (18)Barcelo, D. Chromatographia 1988,25,295-299. (19)Eichelberger, J. W.;Behymer, T. D.; Budde, W. L. Method 525. Determination of Organic Compounds in Drinking Water by LiquidSolid Extraction and Capillary Column Gas ChromatographylMass Spectrometry, Revision 1.0, 2.0,2.1;1988; pp 325-356. (20)Thurman, E. M.; Meyer, M.; Perry, C.; Schwab, P. Anal. Chem. 1990,62,2043-2048. (21)Kadokami, K.; Morimoto, M.; Haraguchi, K.; Koga, M.; Shinohara, R. Anal. Sci. 1991,7. 247-252. (22)Benfenati, E.;Tremada, P.; Chiappetta, L.; Frassanito, R.; Bassi, G.; DiToro, N.; Fanelli, R.; Stella, G. Chemosphere 1990,21,1411-1421. (23)Huang, L. Q.J. Assoc. Off. Anal. Chem. 1989,72,349-354. (24)Schuette, S.A,; Smith, R. G.; Holden, L. R.; Graham, J. A. Anal. Chim.Acta 1990,236,141-144. (25)Pereira, W. E.; Rostad, C. E.; Leiker, T. J.Anal. Chim. Acta 1990, 228,69-75. (26)Davoln, E.;Benfenati, E.; Bagnati, R.; Fanelli, R. Chemosphere 1987,16,1425-1430. (27)Kraut-Vass, A.; Thoma, J. J. Chromatogr. 1991,538,233-240. (28)Durand, G.;Barcelo, D. Anal. Chim. Acta 1991,243,259-271. (29)Owens, D. S.;Sturrock, P. E. Anal. Chim. Acta 1986,188,269274. (30)Froehlich, D.;Meier, W. J. High Resolut. Chromatogr. 1989,12, 340-342. (31)Brown, D. F.; McDonough, L. M.; McCool, D. K.; Papendick R. I. J. Agric. Food Chem. 1984,32,195-200. (32)Reupert, R.; Ploeger, E. Fresenius' 2.Anal. Chem. 1988,331, 503-509. (33)Stahl, M.; Luehrmann, M.; Kicinski, H. G.; Ketrrup, A. Z. Wasser Abwasser Forsch. 1989,22,124-127. (34)Kicinski, H. G.Lebemm.- Biotechnol. 1990,7, 159-164. (35)Di Corcia, A.; Marchetti, M. Anal. Chem. 1991,63,580-585. (36)Battista, M.; Di Corcia, A.; Marchetti, M. Anal. Chem. 1989,61, 935-939. 0 1993 American Chemical Society
22
ANALYTICAL CHEMISTRY, VOL. 65, NO. 1, JANUARY 1, 1993
solvents were pesticide quality or HPLC grade and were Although ultratrace levels of atrazine are useful as environpurchasedfrom Fisher Scientific. Stock standardswere prepared mental traces for dating and in remediation, they likely have by dissolving 1.0 mg of native and labeled atrazine in 10 mL of no obvious health relevance. The maximum contaminant methanol, respectively. The solutions were further diluted to level for atrazine in potable water is 3 pg/L, and the first obtain the fortifyingstandard solutionsof 100 pg/pL in methanol. observable adverse effects are in the milligram per liter The standard solutionsfor GC/HRMS calibrationwere prepared rangemore than 1million times greater than detection levels with nine solutions at concentrationsof 10-1000pglpLfor native reported in this paper. atrazine and 200 pg/pL for [13C3]atrazinein methylene chloride. Increased analytical sensitivity and specificitycan be gained EnvironmentalSep-PakCle cartridge (1 g) was purchased from Waters Chromatography,Millipore. Anhydrous sodium sulfate by using a high-resolution mass spectrometry (HRMS) of analyticalgrade was purchased from Fisher Scientific and was technique involving a double-focusing magnet sector instrupurified by Soxhlet extraction with methanol for 48 h and by ment. Combined with capillary GC, HRMS has been heating overnight at 450 "C. Florisil (Fisher Certified ACS, 60successfully used for trace analysis of dioxins.3' GC/HRMS, 100 mesh) was activated overnight at 200 "C before using for the however, is yet to be applied to the ultratrace analysis of column chromatography procedure. atrazine and related herbicides. For standard water samples,distilled water was further purified A serious impediment to establish detection limits at lowby passing it through a four-moduleNANOpure cartridge system or sub-parts-per-trillion (ppt) levels is the increased back(FisherJBarnstead) installed in the Water Center, UNL. The standard water samples were prepared by adding native atrazine ground and interferences at low levels. The interferences standard solution into the water. The environmental water tend to grow exponentially as the detection limit is lowered. samples were collected by the Water Center, UNL, and two tap HRMS provides both high sensitivity and high resolving water sampleswere obtained from the Department of Chemistry, capacity for separating interfering compounds from the UNL, on November 11,1991, and January 30,1992. After the analytes. Moreover, the application of the mass profile mode internal standard [l3C31atrazinewas added at the 5 ppt level, the operated in selected ion monitoring (SIM) gives a high samples were directly submitted to the solid-phase extraction. certainty of identification because it has high diagnostic Procedure. A Cle bonded silica cartridge was sequentially capability.38 The quantification of atrazine is based on the prewashed with 5 mL each of methanol, methylenechloride,ethyl internal standard method in which [W3]atrazine is used as acetate, methanol, and clean water at a flow rate of 1-2 mL/min. an internal standard and added to the water prior to The [13C3]atrazinestandard solution (20 pL) was added to 500 mL of water. The sample was allowed to flow through the extraction. Thus, the method accuracy and precision should cartridge at approximately 10 mL/min under water aspirator not be adverselyaffected by problems with sample preparation vacuum. After the sample had been applied, the solid-phase and recovery of atrazine. These advantages of the internal cartridgewas allowed to flow for another 5 min to remove residues standard method are most important at low-concentration of water. The atrazine and the internal standard were then eluted levels and for complex matrices. by passing 2 mL of ethyl acetate through the cartridge at a flow The most often commonly reported isotopically labeled rate of 1 mL/min. The extract was then dried with approximately atrazine is atrazine-d5 in which hydrogen atoms of the 0.5 g of anhydroussodium sulfate and concentrated to 20-50 pL ethylamino group are replaced with deuterium.3~22-~ Atraunder a slow stream of nitrogen. zine-d5 is effective as an internal standard in the extraction The elimination of nonpesticide interferences from the tap water extracts was effected with a clean glass pipet filled with process. The ability to recognize the parent compound and 0.8 g of activated Florisil. After packing, the column was rinsed to gain insight into its degradation products, however, is with 6 mL of hexane. The water extract in ethyl acetate (0.5 mL) essential. The most frequently found decomposition comwas quantitatively transferred onto the column and rinsed with pounds of atrazine in the environment are deethylatrazine 3 mL of 6% ethyl ether in hexane. The atrazine was eluted from and deethylatrazine-type compounds in which the ethyl group the column by using 5 mL of 50% ethyl ether in hexane, and the of atrazine is metabolized or replaced by other g r 0 ~ p e . ~ ~ @ 2 8fraction ~ ~ ~ was evaporated to approximately 20 pL for GC/HRMS Therefore, the choice of atrazine-ds as an internal standard analysis. is not satisfactory to simulate the native atrazine in the study GC/HRMS Analysis. The GC/HRMS analyses were perof atrazine transport in the environment because the degformed on a Carlo-ErbaGC/KratosMS-50double-focusing mass spectrometer system. An aliquot (2 pL) of a sample extract was radation products of both native atrazine and atrazine-d5 are injected into a DB-1 fused-silica capillary GC column (30 m X not differentiated. For environmental fate and transport 0.32 mm, J&W Scientific) with on-column injection at 50 OC, studies, an isotopically stable compound is recommended. hold for 1 min, followed by a programmed increase in temperature Atrazine with isotopicallylabeled carbons on the ring structure to 250 "C at a rate of 15 "C/min. The entire column effluent was ('3C3) was synthesized as the internal standard in this research. passed directly into the source via an interface operated at 250 Because it has similar chemical and physical properties as "C. The GC carrier gas was helium at a head pressure of 1.5 the native atrazine and because it possesses chemical stability kg/cm2. The retention time of atrazine by the GC conditions in the GC separation and MS analysis processes, it is an ideal was 10.50 min. It is important that the atrazine retention time internal standard. In addition, [W3]atrazine is stable in be kept small to avoid decomposition on the GC column. environmental conditions. The mass spectrometerwas operated in the electron ionization (EI)mode at 70 eV, source temperature 250 "C and resolution 8000-10000 (10% valley definition). Data acquisition and EXPERIMENTAL SECTION processing for the mass spectrometerwere controlled by a Kratos Reagents and Samples. Native atrazine standard was MACH-3computer system. A selected ion monitoring group in purchased from AccuStandard Inc., New Haven, CT. The the mass profile mode was used for the qualitative and quaninternal standard [13C3]atrazine (isotope purity of more than titative analyses. Three ions were selected for the analysis of 99 % ) was specially synthesized for the Water Center, University atrazine: mlz 200.0703 and m/z 203.0805 as the quantification of Nebraska-Lincoln (UNL)by Merck Frosst (Canada). Organic ions for native atrazine and [W3]atrazine, respectively,and m/z 202.0674 as the confirmationion for the identification of atrazine. The mass profiles of the selected ions were obtained during GC (37) Gross, M. L.; Sun, T.; Lyon, P. A.; Wojinski, S. F.; Hilker, D. R.; Dupuy, A. E.; Heath, R. G. Anal. Chem. 1981,53, 1902-1906. elution. The identification was based on examination of mass (38) Tong,H.Y.;Giblin, D. L.; Lapp, R. L.; Monson,S. J.; Gross, M. profile peak shape, central mass shift, isotopic ratio of mlz L. Anal. Chem. 1991,63, 1772-1780. 202.0674 to m/z 200.0703, and retention time of GC separation. (39) Durand,G.; Forteza, R.; Barcelo, D. Chromatographia 1989,28, The area of the mass profile peaks of the quantification ions was 597-604. used for the quantitative analysis of atrazine; specifically, a (40) Vermeulen, N. M.; Apostolides, Z.; Potgieter, D. J. J. J . Chromatogr. 1982,240, 247-253. relative response factor (RRF) of native atrazine signal to that
ANALYTICAL
'71
'g"'
A
i
I
58.1
215.1
I
1
1 1 sb
bo
Id0
120
lb
Ibo
160
260
203.1
'7
I
B
1 !
0
1
220
70.0
60
80
100
120
140
160
180
200
220
FIgwr 1. Hlgtuesolution E1 mass spectra of (A) native atrazine and (6) [13C3]atrazine.
of ita corresponding internal standard [Wslatrazine was used for the quantitative analysis to improve the accuracy of the method:
RRF = (A,C,)/(A,C,) (1) where A, and Ah are the mass profile peak areas of atrazine at m/z 200.0703 and [l3C31atrazineat m/z 203.0805 and C,and C, are the concentrations of native atrazine and [Walatrazine in the calibration standards, respectively. The concentration of atrazine in an unknown water sample was calculated by the formula C, = [(C~)(A,/A~)l/RRF (2) where C, is the concentration of analyzed atrazine in sample, C, is the concentration of the added [Wslatrazine, and A. and Ai, are the mass profile peak areas for signals at m/z 200.0703 and m/z 203.0805,respectively.
RESULTS AND DISCUSSION Qualitative and Quantitative Analysis. High-resolution E1 (70eV) maas spectra for atrazine and [1SC31atrazine were obtained at 10 OOO resolution (10% valley definition) (Figure 1)to provide the nec8888Ty information for the analysis of atrazine by using SIM. In the SIM technique, ions for quantification were chosen on the basis of maximum abundance, maximum selectivity, and low susceptibility to interference from other compounds that may be present. Thus, consideration was given primarily to the most abundant ions in the mass spectra to achieve the maximum instrument reponse. As indicated by the spectra, it is possible to select the ions of rnlz 200.0703 and rnlz 203.0805 as quantification ions for native atrazine and [%]atrazine, respectively. The
CHEMISTRY,
VOL. 65, NO. 1, JANUARY 1, 1993
28
ion of rnlz 202.0674, which is the W1-containing fragment ion, was selected to provide confirmation for the presence of atrazine. The mass spectrum of [13C31atrazine illustrates that the labeled atrazine gives no significant ions at rnlz 200 and rnlz 202. Thus, the internal standard does not interfere with the analysis of native atrazine when both compoundsare present. The criteria we use for determiningthe presence of atrazine at low levels are as follows. 1. The retention time of the native atrazine, as determined from the mass chromatogram of the rnlz 200.0703 ion, must be equal to within 2 s to that of the internal standard rnlz 203.0805 ions. 2. The masses of the rnlz 200.0703 and rnlz 202.0674 ions must be within f5 ppm of the true values. 3. The isotope abundance ratio of the rnlz 202.0674 and the rnlz 200.0703 ions must be 0.35 f 0.10. The mass profile mode is better-suited for meeting these criteria than the peak top monitoring mode.% The ability to establish accurate maw measurementsand isotope abundance ratios as well as to detect the presence of interferences is particularly useful for determiningwith certainty that atrazine is present in the sample. The spectrum of native atrazine shows a signal at rnlz 203.0696 with an abundance of 3.25% ,which results from 13C and 3'Cl contribution to the [M - 15]+ fragment ion at rnlz 200.0703. Thus, this ion contributes to the quantification ion abundance. To separate the interference ion (mlz 203.0696)from the quantification ion of [W31atrazine (mlz 203.08051,a resolving power of more than 19 OOO would be required. Although this higher resolution can be reached by using many magnet sector mass spectrometers, the sensitivity will be decreased by at least a factor of 2 compared to that at resolving power 10 OOO. Because the percentage of the isotopic contribution from the native atrazine base peak is determinant, the analytical error caused by the interference ion can be minimized by an area correction according to the following equation: A(["C,]atrazine) = A(203.0805) - A(200.0703) X 0.032 (3) where A( [W3]atrazine) is the actual area of [lSC3]atrazine peak after the correction and A(203.0805)and A(200.0703) are areas of the rnlz 203.0805 and rnlz 200.0703 peaks, respectively. By using this equation, the area of the rnlz 203.0805ion for the internal standard was corrected for the 3.2 % contribution from the ion rnlz 200.0703 of native atrazine. When the concentration of native atrazine is similar to or less than the concentration of the added [W31atrazine, the correction is not necessary because the presence of native atrazine does not significantly interfere with the determination of the labeled atrazine. Better accuracy, however, was achieved after using the correction, especially when the concentration of atrazine is more than 5 times higher than the concentration of the added [W3latrazine. The calibration curve, presented in Figure 2,shows a linear dynamic range over 3orders of magnitude. For the calibration data points, the concentration of [lSCslatrazine was kept at 200 pglbL whereas the concentration of atrazine was varied from loto 1OOOpglpL. The area of [13C3latrazinewas adjusted only for the atrazine concentration of 1OOO pg/bL because no significant changes were obtained by the correction for the other calibration points. For the internal standard method, a concentration ratio of less than 10of analyte-to-internal standard was recommended
ANALYTICAL CHEMISTRY, VOL. 65, NO. 1, JANUARY 1, 1993
24
Table 11. Recoveries of Atrazine by Solid-Phase Extraction of 500 mL of Water at Three Concentration Levels
Internal Standard: 200 pg 13C-Atrazine
recoverv ( % ) 1 2 3 4 5 6 av f SD,O % RSD,b % (n = 6)'
$ 3 m % 2 1: L
0
.-n- O
z
U
o
200
400 600 800 1000 Injected amount of ATR-H (pg)
1200
ATR-H: native atrazine AIR-C: 13C-atrazine
Fburo 2. Linear dynamic range for the QCIHRMS analysis of atrazine when the injected amount of [13C3]atrazlnewas 200 pg.
Table I. Relative Response Factors of Native Atrazine Relative to [WslAtrazine iniected amt (DE) native atrazine
10 20 40 50
100 200 250 500 lo00 av SDb RSD: % (n = 9)d
*
[Wdatrazine
RRFa
206 206 206 206 206 206 206 206 206
0.99 0.97 1.19 1.23 1.17 1.12 1.20 1.22 1.21 1.14 f 0.10 8.7
*
Relative response factor. b Average RRF standard deviation. Relative standard deviation. Number of determinations. a
to obtain the best accuracy.41 For unknown environmental samples,the actual concentrationratio may exceed this factor. If the analysis of a water sample indicated that the concentration of native atrazine was a factor of more than 10 times that of the added [W31atrazine,asecond analysis was carried out for this sample by using a more suitable concentration of the internal standard, which had been estimated from the first analysis. To improve further the method accuracy, a relative response factor (RRF) was used for the analysis. The RRF of native atrazine to [W3latrazine was obtained from the GC/HRMS results for standard solutions of native atrazine and [13C31atrazine (Table I). Under ideal conditions, the RRF should be equal to 1. The deviation from unity may be due to the different purities of standards and to errors introduced in the preparation of the standard solutions and in the GC/ HRMS analysis. Thus, the RRF was determined with standards containing at least three different concentration ratios of the native atrazine to [13Cs]atrazine each day samples were analyzed. If a standard solution was changed, the RRF was determined again with at least six points in the dynamic range shown in Figure 2. Recovery and Detection Limit. Because the internal standard, which is the stable isotopically labeled form of the analyte, was added into the water samplesprior to the sample preparation, the quantification of the native compound is based on the instrument response and has minimal dependence on recovery. The detection limit, however, can be improved when good recovery is achieved. Nevertheless, the (41) Colby, B. N.; Rosecrance, A. E.; Colby, M.E.Anal. Chem. 1981, 53, 1907-1911.
67 77 78 84 81 75 77.0 f 5.8 7.6
79 79 78 78 81 68 77.2 f 4.6 6.0
80 77 91 80 78 81 81.2 k 5.0 6.2
Average recovery f standard deviation. Relative standard deviation. Number of determinations.
Table 111. Accuracy and Precision Data for Atrazine Analysis by Solid-Phase Extraction and GC/HRMS 1.04 1.04 1.04 1.04 1.04 1.04 1.04 1.04 av f SDc relative error, % RSD,d % (n = 8)e
1.05 0.81 0.78 1.05 1.00 0.95 1.05 0.73 0.93 f 0.13 -10.6 14.3
0.20 0.22 0.38 0.20 0.24 0.14 0.15 0.29 0.23 & 0.08 34
a Order of data presentation was the order of analysis. Detection limit. Average concentration of atrazine f standard deviation. Relative standard deviation. e Number of determinations.
data in Table I1 demonstrate excellent recoveries of atrazine in water at the concentration levels of 1, 5, and 10 ppt by employing the solid-phase extraction procedure with the CU cartridge. The average recoveries and the relative standard deviations (RSD) were determined from six GC/HRMS detections of three sample extractions. The sample volumes for the recovery testa were 500 mL. The C18 cartridge (1g) was tested for the extraction of atrazine at ppt to ppb levels in larger water volume. No significant breakthrough at up to loo0 mL water, however, was found. The capacity of the same kind of CU(360mg) for some triazine herbicides in water was determined, and the breakthrough for atrazine was 3200 mL of water.20 The typical instrument detection limit for atrazine at a signal-to-noise (S/N) ratio of 3 is 10 pg/injection, which corresponds to a procedure detection limit of 200 ppq if the sample volume is 500 mL and 2 of 20 pL from the final extract is injected into the GC column. The procedure detection limit, however, depends upon the nature of the samples and the quality of extraction. The detection limits for the standard water samples are defined as the concentration of atrazine required to produce a signal with an amplitude of at least 3 times the baseline noise; they are listed in Table I11 for the analyses conducted in this research. Detection limits can be lowered further if a large volume of sample is used, if the sampleextract is further concentrated, or if the mam resolving power is reduced. It is noted that contemporary magnet sedor instruments have higher sensitivities than the 16-year-old instrument used here. To measure lower ppq levels of atrazine and other related herbicides, a preconcentration step (e.g., evaporation of the water) could be used before the solidphase extraction. The background noise from a larger volume of water samples, however, will also increase and possibly cause interferences. In such cases, column chromatography with Florisil can be used to eliminate or reduce the interferences in the sample extracts.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 1, JANUARY 1, lQg3 26
1
A
484 (1252)
196 (10:31)
m/z
i
Table IV. Results of the Analyses of Water Samples from the Aquifer in the Southeast of Grand Island, Nebraska (MLE), and Tap Water Samples from the Department of Chemistry. UNL (TAP)
557(13:26)
~~
202.1
I,
526 (13:ll)
100
200
300
400
500
600
700
scan number
B
195 (1030)
samples MLE 75 MLE 80 MLE 85 MLE 90 MLE 95 MLE 100 MLE 105 MLE 110 blank 1 MLE 105
blank 2 TAP 1 TAP 2
cx (PPt)b 132 107 21.5 12.5 22.4 15.7 2.5 11.5 ND (0.59)‘ 3.4 ND (0.48) 463 428
4 (PPW 52 52 52 52 52 52 52 52 52 5.2 5.2 52 104
*
a Concentration of added internal standard. Detected concentration of native atrazine. Not detected (detection limit).
203.1,
I 100
200
300
400
500
600
700
scan number
Fburo 3. TIC chromatograms of the Cle carbidge eluants for a tap watersample(A)befweand(8)aftertheFlorlsllcdumnchromatogaphy procedure.
Interferences. The interferences include method interferences caused by contaminants in solvents, glassware, CIS cartridge, and other sample processing apparatus and matrix interferences caused by contaminants that are coextracted from the water samples. Alkanes, alkenes, plasticizers, and antioxidants were identified as possible interferences by extracting componentsof the C18bonded silica cartridge that interfered with the analysis of herbicides by GC with highsensitivityelectron capture detection42and by GC/quadrupole MS.43 The interferences from the C18 cartridge, however, do not affect the determinationof atrazine by GC/HRMS because this method has higher selectivity than GC/low-resolution MS. The need for an additional chromatography step for application of the solid-phase extraction and GC/HRMS analysis depends on the cleanliness of the matrix. The extent of matrix interferences will vary considerablywith the source of the water sample. The cartridge eluants of the standard water samples and the relatively clean groundwater samples in this study did not have significant interferences so that the Florisil chromatography step was not used for those eluants. The chromatography procedure, however, waa required for the eluant of a tap water sample. Figure 3 shows the TIC chromatograms (containing the three selected ions) of the sample extract of tap water before and after the chromatography step. The final volume as well as the injection volume of both samples were kept similar so that the background noise and interferences peaks could be reliably compared witheach other. Because the GC/HRMStechnique has high selectivity, the background noise and interference peaks are small relative to the intense peak of atrazine, as (42)Junk, G.A.; Avery, M.J.; Richard, J. J. Anal. Chem. 1988, 60, 1347-1350. (43)Johnson, W. E.; Fendmger, N. J.; Plimmer, J. R. Anal. Chem. 1991,63, 1510-1513.
shown in Figure 3. The S/N ratio for the sample analysis after the chromatography step is better than that of the sample analysis before the chromatography step, and interference peaks (scan numbers 484,526, and 557 in Figure 3A) are not seen. The recoveries of atrazine in the Florisil column chromatography procedure were 83% and 80% for 1 and 10 ng of atrazine, respectively. This indicates that the procedure detection limit will be affected minimally (decreased by less than 20%) by introducing the chromatography procedure. This additional chromatography step was found to be necessary when a dirty sample was analyzed because intense interferences shorten the lifetime of the GC column and contaminate the ion source of a system operated at high sensitivity, reducing GC resolution, S/N ratio, and ultimately the sensitivity. Method Accuracy and Precision. Eight standard samples containing 1 ppt atrazine in 500 mL of water were analyzed by GC/HRMS after solid-phase extraction. The accuracy and precision data are presented in Table 111. The average error for the determination of atrazine is less than &15%. The method precision given as relative standard deviation (RSD)is better than 15% (n = 8) for the standard water samples. Ground water samples from a Pleistocene age sand and gravel aquifer in southeast Grand Island, NE44 and two tap water samples containing ground water from the city of Lincoln municipal supply line were analyzed (see Table IV for results). The Grand Island samples were obtained from a specially installed multilevel sampling system capable of extracting ground water from discrete 5-ft. levels throughout the 100-ft. saturated 20118.~5 The concentrationa of atrazine in the samples from Grand Island range from low ppt in the deep samples to more than 100 ppt in the shallower samples. The data indicate a decrease in atrazine concentration with depth, and this is in accord with the hypothesisthat the deeper water is older. Recharge to these deep ground water zonea may have contained less atrazine, and the residual atrazine may have undergone more retardation and degradation. The concentration of the added internal standard was 52 ppt for all unknown samples and one of the blank samples. A second round of analysis with a fortifying concentration of 5.2 ppt was performed for the sample MLE 105 as well as the (44)Spalding,R.F.;Exner,M.E.;Martin,G.E.;Snow,D.D.J.HydroZ., in press. (45)Spalding, R. F.;Exner, M. E.; Burbach, M.E. GroundwaterResidue Sampling Design;ACS Symposium Series; American Chemical Society: Washington, DC, 1991;Chapter 15,255-261.
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 1, JANUARY 1, 1993
other blank sample. These samples were also analyzed by a GC/quadrupole MS at the Water Center, UNL.4 For samples MLE 75, MLE 80, MLE 85, and MLE 95, in which atrazine concentrations were above the detection limit of the method using low-resolution GUMS, the results from the two methods are in close agreement.48 Results for the blank samples show high reliability of the solid-phase extraction and GC/HRMS for the analysis of atrazine. Conclusion. The method using solid-phaseextraction and GC/HRMS is satisfactory for the analysis of atrazine in water at ultratrace levels and gives good accuracy and precision at these levels. The use of the high-resolutionmass spectrometry technique supported by an optional Florisil column chromatography procedure can eliminate most of the interferences (46) Caesada, D. A.; Z. Cai; Spalding, R. F.; Cross,M. L. Manuscript in preparation.
that are introduced from samples and the method itself. The method, which already has been successfullyused to analyze ground water samples, can be incorporated in other studies such as the metabolism of atrazine and related herbicides in the environment. Moreover, it is expected that this GC/ HRMS method will be applicable to investigation of movement and degradation of selected herbicides in the environment.
ACKNOWLEDGMENT This study was supported by the University of Nebraska Water Center by the Nebraska Research Initiative and the National Science Foundation (Grant No. DIR 9017262). RECEIVED for review July 20, 1992. Accepted Odober 5, 1992.
