Article pubs.acs.org/JAFC
Multiresidue Analysis of Pesticides in Straw Roughage by Liquid Chromatography−Tandem Mass Spectrometry Zihao Zhang,† Mengyuan Feng,† Kechen Zhu,† Lijun Han,*,† Yelena Sapozhnikova,§ and Steven J. Lehotay§ †
College of Science, China Agricultural University, Beijing 100193, China Eastern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, 600 East Mermaid Lane, Wyndmoor, Pennsylvania 19038, United States
§
S Supporting Information *
ABSTRACT: A multiresidue analytical method using a modification of the “quick, easy, cheap, effective, rugged, and safe” (QuEChERS) sample preparation approach combined with liquid chromatography−tandem mass spectrometry (LC-MS/MS) analysis was established and validated for the rapid determination of 69 pesticides at different levels (1−100 ng/g) in wheat and rice straws. In the quantitative analysis, the recoveries ranged from 70 to 120%, and consistent RSDs ≤ 20% were achieved for most of the target analytes (53 pesticides in wheat straw and 58 in rice straw). Almost all of the analytes achieved good linearity with R2 > 0.98, and the limit of validation levels (LVLs) for diverse pesticides ranged from 1 to 10 ng/g. Different extraction and cleanup conditions were evaluated in both types of straw, leading to different options. The use of 0.1% formic acid or not in extraction with acetonitrile yielded similar final outcomes, but led to the use of a different sorbent in dispersive solid-phase extraction. Both options are efficient and useful for the multiresidue analysis of targeted pesticides in wheat and rice straw samples. KEYWORDS: multiresidue pesticide analysis, rice and wheat straw, animal feed, QuEChERS, LC-MS/MS
■
INTRODUCTION Roughage in animal feed, such as straw, is a vital nutritional resource for livestock, and it has many advantages such as maintaining rumen function and increasing meat production. Roughage often comes from different low-cost sources, such as plant crop byproducts. As one of the main components of animal feed for cattle and other livestock, roughage refers to dietary fiber including hay, straw, pod, shell, rattan, and dry leaves that have about 20% crude fiber and 20% ≤20% >20%
51 6 0 11 1 0
54 5 5 3 1 1
57 4 1 4 1 2
56 4 5 2 1 0
120%
Figure 2. Distribution of recoveries and RSDs of 69 pesticides for different test portion amounts in (A) wheat and (B) rice straws. The boxes define the acceptable recoveries (70−120%) and RSD (≤20%).
aprop-p-ethyl (fenoxaprop-p), and prochloraz (2,4,6-trichlorophenol) were included in the target list of analytes. Method Development. Since its introduction in 2003, the QuEChERS approach has been widely applied in multiresidue analysis of pesticides on different matrices.20,21,25,26 For example, the method was adapted to analyze 14 diverse
Table 2. Number among 69 Analytes with Satisfactory Recoveries (70−120%) and/or RSDs (≤20%) Using Different Sorbents in d-SPE Cleanup of Wheat and Rice Straws Extracts (n = 3 Each) MgSO4
MgSO4 + PSA
MgSO4 + C18
MgSO4 + PSA + C18
MgSO4 + MWCNTs
sorbent
(30 mg)
(30 + 20 mg)
(30 + 20 mg)
(30 + 10 + 10 mg)
(30 + 10 mg)
no. of analytes 70−120% recovery no. of analytes ≤20% RSD no. of analytes for both
54 51 47
53 59 50
52 53 44
41 58 36
34 45 29
no. of analytes 70−120% recovery no. of analytes ≤20% RSD no. of analytes for both
59 57 53
60 56 50
52 63 60
53 60 49
20 51 17
Wheat Straw
Rice Straw
C
DOI: 10.1021/acs.jafc.5b05981 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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Journal of Agricultural and Food Chemistry
Table 3. Number among 69 Analytes with Respect to Matrix Effects (MEs) Using Different Sorbents in d-SPE Cleanup of Wheat and Rice Straws Extractsa sorbent
a
MgSO4
MgSO4 + PSA
(30 mg)
(30 + 20 mg)
ME < −20% |ME| < 20% ME > 20%
63 6 0
13 44 12
ME < −20% |ME| < 20% ME > 20%
29 31 9
63 6 0
MgSO4 + C18
MgSO4 + PSA + C18
MgSO4 + MWCNTs
(30 + 20 mg)
(30 + 10 + 10 mg)
(30 + 10 mg)
62 7 0
61 8 0
58 11 0
60 9 0
Wheat Straw 69 0 0 Rice Straw 10 46 13
ME < −20% involves ion suppression, |ME| < 20% is “no effect”, and ME > 20% relates to enhancement.
pesticide recoveries were much better when using 20 mL of acetonitrile rather than 10 mL. Presumably due to matrix effects, similar sensitivities were achieved in each case despite the approximately 2-fold higher analyte concentrations in the 10 mL extracts. In a continuation of the sample/solvent ratio evaluation, sample test portions of 2 and 4 g straw samples were compared in an experiment. Water was added in the same ratio as before (5 mL for 2 g samples and 10 mL for 4 g) and either 20 or 30 mL of acetonitrile, respectively, was used for extraction followed by partitioning salt-out using 3 g of NaCl. The results shown in Figure 2 illustrate how recoveries increased slightly for most analytes when 4 g of sample was used, but, overall, more analytes (51 for wheat straw and 54 for rice straw) had satisfactory recoveries and RSDs for 2 g samples and fewer (49 for wheat straw and 47 for rice straw) achieved acceptable results for 4 g samples. Also, the 50 mL extraction tubes filled to the maximum when using 4 g of samples plus 10 mL of water, 30 mL of acetonitrile, and 3 g of NaCl. For these reasons, 2 g was chosen as the straw sample test portions, and 5 mL of water plus 20 mL of acetonitrile (with or without 0.1% formic acid) and 3 g of NaCl were chosen for extraction and partitioning in the final methods. Comparison of Sorbents for d-SPE Cleanup. As a fast, easy, and effective cleanup procedure, d-SPE is frequently used in the QuEChERS sample preparation approach.21 It is often effective and very flexible for many matrix types because many kinds of sorbents with different physicochemical properties can be applied individually or together to help remove coextracted matrix components. In the case of wheat and rice straw samples, different sorbents (PSA, C18, PSA + C18, and MWCNTs, in combination with anhydrous MgSO4 to retain coextracted water) were tested using d-SPE in the recovery of the 69 pesticides. The sorbent combinations and amounts along with the summarized results of the experiments are listed in Table 2. PSA and C18 are frequently used to help remove fatty acids, lipids, and similar components in food and environmental matrices.30,31 MWCNTs are a relatively new type of nanotube material that was reported to be a good sorbent for the cleanup of several matrices, such as vegetables and teas.32−34 In this study, the cleanup efficiency of five different sorbent combinations was investigated with respect to the fortification sample recoveries and RSDs and matrix effects (MEs). The numbers of analytes with acceptable recoveries and RSDs using the different cleanup sorbents in d-SPE of straws are presented in Table 2. In the case of wheat straw, the results show that MgSO4 + PSA achieved better results than the other
Figure 3. Matrix effects (MEs) of pesticides using five different d-SPE sorbents in (A) wheat and (B) rice straws.
Sample/Solvent Extraction Ratios. In the next set of experiments, different solvent/sample ratios were evaluated. Use of less extraction solvent per gram of sample potentially yields more concentrated extracts and lower detection limits. For 2 g samples plus 5 mL of water, the volume of acetonitrile was reduced from 20 to 10 mL (see Figure 1 for summarized results). In this experiment, almost no differences were observed between wheat and rice straw results. Overall, 59 of 69 (85%) of the analytes had acceptable recoveries (70−120%) using 20 mL of acetonitrile (with 0.1% formic acid for rice straw), whereas 52 (76%) had acceptable results when 10 mL of acetonitrile was used for extraction. Moreover, RSDs of the D
DOI: 10.1021/acs.jafc.5b05981 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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Journal of Agricultural and Food Chemistry a
Table 4. Recoveries (RSDs) and LVLs of the Pesticides in Wheat Straw Using the Final Method concentration (n = 5) analyte 2,4,6-trichlorophenol acetamiprid anilofos atrazine azoxystrobin bentazone boscalid buprofezin butachlor carbendazim carbofuran carfentrazone-ethyl chlorpyrifos clodinafop clodinafop-propargyl clothianidin cyhalofop-butyl cyprodinil difenoconazole dinotefuran epoxiconazole fenoxaprop-p fenoxaprop-p-ethyl florasulam fluoroxypyr flutriafol halosulfuron-methyl hexaconazole hymexazol imidacloprid indoxacarb isoprocarb isoprothiolane isoproturon kresoxim-methyl mefenacet mepiquat chloride metalaxyl-M methomyl metolachlor nicosulfuron nitenpyram oxadiargyl oxyfluorfen paclobutrazol penoxsulam picoxystrobin pirimicarb pretilachlor prochloraz prometryn propamocarb propiconazole pymetrozine pyraclostrobin pyrazosulfuron-ethyl pyridaben pyrimethanil quinclorac
1 ng/g ND 102 73 102 ND 76 34 96 ND 96 ND ND ND 106 ND ND ND 95 75 90 112 ND 91 82 77 80 76 84 ND ND ND ND ND 102 ND 90 ND ND ND ND ND ND ND ND 90 92 ND 100 89 101 84 75 93 ND 85 88 96 ND 26
(7) (16) (12) (11) (35) (14) (8)
(12)
(18) (13) (23) (11) (25) (24) (23) (21) (5) (22)
(8) (11)
(16) (24) (8) (22) (20) (20) (19) (19) (23) (23) (10) (103)
5 ng/g ND 100 113 99 90 70 81 109 72 96 97 ND ND 103 73 113 ND 94 107 101 105 ND 78 102 97 95 89 101 ND ND 92 89 84 103 139 97 34 102 91 ND ND ND ND 101 107 101 76 101 88 120 97 88 109 ND 94 91 104 98 108
(3) (14) (13) (11) (9) (26) (3) (13) (10) (12)
(8) (30) (36) (20) (10) (16) (6) (16) (12) (21) (22) (25) (22)
(17) (16) (18) (15) (48) (11) (18) (13) (27)
(15) (4) (12) (7) (5) (10) (14) (8) (19) (10) (14) (15) (15) (11) (88)
10 ng/g ND 108 (5) 92 (5) 89 (9) 105 (10) 80 (20) 37 (81) 92 (5) 84 (20) 103 (6) 89 (16) 90 (20) ND 82 (12) 73 (35) 96 (23) 110 (9) 103 (20) 116 (7) 70 (6) 120 (9) 113 (44) 79 (20) 96 (15) 83 (20) 108 (7) 103 (6) 110 (11) 95(18) 91 (19) 74 (15) 98 (15) 120 (6) 93 (11) 209 (31) 91 (13) 37 (50) 70 (10) 104 (12) 86 (20) 34 (38) 93 (21) 23 (224) 94 (14) 84 (8) 86 (20) 91 (18) 99 (4) 105 (9) 87 (13) 111 (7) 95 (19) 99 (9) 57 (27) 97 (12) 95 (19) 117 (8) 99 (12) 93 (41)
E
50 ng/g 107 108 98 94 102 96 91 98 120 91 93 105 68 99 87 96 95 102 100 91 114 43 94 98 100 95 98 100 88 97 91 93 93 104 100 102 27 81 91 85 30 88 79 72 99 95 100 97 105 96 96 72 104 69 110 90 95 91 160
(38) (2) (16) (6) (2) (7) (14) (2) (7) (17) (10) (6) (16) (4) (21) (26) (11) (4) (2) (13) (13) (23) (3) (5) (14) (9) (9) (2) (115) (13) (15) (6) (4) (4) (7) (5) (12) (3) (17) (11) (28) (17) (101) (16) (2) (9) (2) (5) (4) (13) (3) (2) (3) (13) (3) (4) (3) (7) (53)
100 ng/g 208 119 92 97 98 100 48 100 102 109 104 101 106 98 97 79 102 98 100 114 115 41 98 101 94 96 99 93 70 99 93 100 96 105 71 97 25 105 94 96 30 91 75 75 98 94 100 99 103 98 98 80 105 71 103 91 118 100 19
(44) (12) (13) (2) (3) (7) (15) (3) (11) (1) (4) (18) (30) (7) (11) (26) (4) (9) (5) (11) (5) (8) (5) (8) (7) (3) (9) (6) (33) (5) (15) (9) (4) (6) (26) (4) (19) (9) (14) (15) (31) (15) (74) (14) (4) (10) (4) (8) (2) (6) (7) (4) (9) (8) (7) (4) (3) (8) (27)
overall − 111 98 96 99 85 58 99 95 99 96 99 102 97 75 93 102 98 100 93 113 66 88 96 90 95 93 98 − 96 87 95 98 101 − 95 31 89 95 89 46 91 − 80 96 94 92 99 98 100 97 82 102 62 98 91 106 97 −
(6) (13) (8) (6) (11) (34) (6) (13) (8) (10) (15) (33) (8) (33) (32) (8) (14) (7) (14) (9) (25) (14) (13) (17) (13) (11) (13) (12) (15) (11) (8) (9) (9) (25) (9) (17) (17) (41) (18) (15) (7) (15) (8) (6) (9) (13) (9) (13) (10) (20) (12) (13) (8) (11)
LVL (ng/g) − 1 1 1 5 1 − 1 5 1 5 10 − 1 − − 10 1 1 1 1 − 1 1 1 1 1 1 − 10 5 5 5 1 − 1 − 5 5 10 − 10 − 10 1 1 5 1 1 1 1 1 1 − 1 1 1 5 −
DOI: 10.1021/acs.jafc.5b05981 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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Journal of Agricultural and Food Chemistry Table 4. continued concentration (n = 5) analyte tebuconazole thiabendazole thiamethoxam thifluzamide triazophos tricyclazole trifloxystrobin trifloxystrobin acid triflumuron trifluralin
1 ng/g 73 97 ND ND 70 102 88 43 ND ND
(10) (15)
(13) (7) (18) (22)
5 ng/g 86 91 95 278 117 96 92 65 97 ND
(16) (14) (25) (82) (4) (7) (20) (30) (24)
10 ng/g 98 107 100 202 95 101 93 40 79 ND
(12) (9) (18) (97) (6) (13) (12) (13) (5)
50 ng/g 100 104 82 77 100 102 104 61 95 104
(4) (10) (7) (20) (3) (4) (2) (10) (18) (23)
100 ng/g 103 98 100 124 92 84 99 56 95 159
(7) (9) (20) (45) (7) (7) (2) (4) (8) (106)
overall 92 100 92 − 95 97 95 53 92 121
(10) (11) (18) (6) (8) (11) (16) (14) (58)
LVL (ng/g) 1 1 5 − 1 1 1 − 5 −
Bold text indicates recovery >120%, or recovery 20%. ND, not detected; −, recoveries and RSDs were satisfied at fewer than three levels.
a
esters, and 6−15% triglycerides, with 0.2−0.3% diglycerides. The different extraction conditions make comparisons difficult, but the indication is that rice straw contains more lipids than wheat straw. PSA is reported to have significant retention of organic acids, including fatty acids and some sugars, whereas C18 is often helpful to retain lipophilic materials, such as sterols.31,37 Independent of straw sample composition, the different results may be due to the use of 0.1% formic acid for rice but not for wheat during extraction. The mechanism of PSA involves weak anion exchange, which in the case of fatty acids entails interaction of their carboxylic acid groups with the amines on the sorbent. Neutralized acid groups at lower pH are not as strongly retained by PSA. Furthermore, the formic acid in the extracts also interacts with the PSA rather than the active sites (amino groups) being available to retain the analytes. Thus, the findings are unsurprising that the PSA worked better in the neutral wheat straw extracts than in the acidified rice straw extracts. In the case of C18 sorbent, the long hydrocarbon chain functional groups interact with likewise long-chain hydrocarbons in the matrix extracts (e.g., fatty acids and other lipids). When the carboxylic acid groups are neutralized as in the acidified rice straw extracts, then the mechanism provided by the C18 sorbent dominates during cleanup, which explains the results observed during the d-SPE experiments. This entailed interesting chemistry with each method having advantages and similar end results; thus, they were used in method validation experiments for the sake of options for further study and comparisons. Method Validation. After the sample preparation experiments, method validation was conducted to evaluate the accuracy (trueness and precision) of the residue analysis of the 69 pesticides in wheat and rice straw samples. To reduce the error caused by MEs, matrix-matched calibration standards were prepared for quantification, and TPP was used as the IS. Almost all (64) of the analytes showed good linearity with coefficients of determination (R2) between 0.98 and 1.00. The analytes that did not achieve good linearity in one or both matrices included 2,4,6-trichlorophenol (R2 = 0.82−0.86) in both types of straw and oxadiargyl in wheat straw (R2 = 0.92). Hymexazol, boscalid, and trifluralin had acceptable linearity in rice straw extracts but had poor linearity in wheat straw (R2 = 0.50−0.61), which demonstrated problems in the analysis of those pesticides in wheat straw.
sorbents; 50 (72%) of 69 pesticides had satisfactory recoveries and RSDs. In rice straw using acid in extraction, however, MgSO4 + C18 had the best results with 60 (87%) analytes achieving acceptable recoveries and RSDs. When MgSO4 + PSA + C18 was applied, the results were not as satisfactory; 36 (52%) and 49 (71%) analytes had good recoveries and RSDs in wheat and rice straws, respectively. As for MWCNTs only 29 (42%) and 17 (25%) analytes yielded satisfactory recoveries and RSDs in wheat and rice straws, respectively. Furthermore, >30 analytes were retained by the MWNTs, resulting in 40− 70% recoveries. MEs for the analytes from the different cleanup sorbents in wheat and rice straws were measured, which gives a good indication of the amount of cleanup achieved. A summary of the resulting MEs is illustrated in Table 3 and Figure 3. As usual in ESI, ion suppression (ME < −20%) was common, not enhancements (ME > 20%), and |ME < 20%| (absolute value) was considered to be insignificant. In the case of wheat straw (Figure 3A), 44 (64%) of the analytes were not significantly affected by MEs when MgSO4 + PSA was used. When MgSO4 + C18 was employed in d-SPE, the MEs were quite high, with 47 (68%) of the analytes showing significant effects of |ME > 50%|. MgSO4 + PSA + C18 in the case of wheat straw reduced MEs versus MgSO4 + C18, but the combination of PSA + C18 sorbents still gave overall more MEs than MgSO4 + PSA, as shown in Table 3. In the case of rice straw, however, MgSO4 + C18 had the best cleanup efficiency, as shown in Table 3 and Figure 3B, whereas MgSO4 + PSA had the most analytes with significant MEs. When the combination of MgSO4 + PSA + C18 was used for dSPE cleanup of rice straw, the results fell in between, not better than using the sorbents individually. In the final methods, MgSO4 + PSA and MgSO4 + C18 were chosen to be the best sorbents for the cleanup of wheat and rice straws, respectively. Wheat and rice straws are both similar in water and fiber contents; thus, it is expected that the same extraction and cleanup method would be established. However, the sorbents comparison tests showed significantly different cleanup efficiencies for the two matrices, which may be due to lipids, particularly fatty acids. It was reported that wheat straw extracted by acetone contains 25% long-chain free fatty acids, as well as free fatty alcohols (≈20%) and high molecular weight fatty acid and alcohol esters (≈11%).35 In the case of rice straw, Xiao36 found that lipophilic extracts from petroleum ether, dichloromethane, or chloroform consisted mainly of 16−24% free fatty acids, 4−28% sterols, 5−14% waxes, 6−12% steryl F
DOI: 10.1021/acs.jafc.5b05981 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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Journal of Agricultural and Food Chemistry Table 5. Recoveries (RSDs) and LVLs of the Pesticides in Rice Straw Using Its Final Method
a
concentration (n = 5) analyte
1 ng/g
5 ng/g
2,4,6-trichlorophenol acetamiprid anilofos atrazine azoxystrobin bentazone boscalid buprofezin butachlor carbendazim carbofuran carfentrazone-ethyl chlorpyrifos clodinafop clodinafop-propargyl clothianidin cyhalofop-butyl cyprodinil difenoconazole dinotefuran epoxiconazole fenoxaprop-p fenoxaprop-p-ethyl florasulam fluoroxypyr flutriafol halosulfuron-methyl hexaconazole hymexazol imidacloprid indoxacarb isoprocarb isoprothiolane isoproturon kresoxim-methyl mefenacet mepiquat chloride metalaxyl-M methomyl metolachlor nicosulfuron nitenpyram oxadiargyl oxyfluorfen paclobutrazol penoxsulam picoxystrobin pirimicarb pretilachlor prochloraz prometryn propamocarb propiconazole pymetrozine pyraclostrobin pyrazosulfuron-ethyl pyridaben pyrimethanil quinclorac
ND 80 (17) ND 120 (5) 91 (7) ND ND 103 (10) ND 95 (25) 96 (22) ND ND 96 (14) 105 (18) ND ND 126 (53) ND ND 94 (26) ND 100 (6) 94 (9) 105 (15) 97 (9) 105 (12) 132 (9) ND ND ND 96 (9) 103 (21) 95 (21) ND 107 (14) 35 (9) 84 (27) ND ND 51 (24) ND ND ND 109 (9) ND 92 (13) 114 (4) 102 (6) ND 111 (7) 96 (41) 83 (18) ND ND 91 (17) 93 (9) 87 (26) ND
161 (49) 103 (18) 68 (8) 104 (7) 91 (11) 106 (13) ND 91 (10) 116 (20) 84 (19) 117 (20) ND ND 93 (16) 73 (20) 173 (43) ND 80 (16) 98 (5) ND 86 (26) ND 109 (5) 98 (2) 109 (13) 96 (10) 87 (9) 153 (13) ND ND 82 (19) 109 (23) 96 (19) 88 (11) 69 (29) 99 (9) 57 (8) 113 (19) ND ND 53 (29) ND ND ND 115 (15) ND 97 (5) 103 (6) 116 (10) 78 (18) 107 (8) 85 (20) 101 (17) ND 70 (11) 101 (6) 103 (7) 113 (6) ND
10 ng/g 29 97 94 86 96 75 61 100 114 101 102 86 130 94 70 230 85 97 102 87 114 77 102 96 118 103 99 101 ND 87 120 120 110 93 120 102 22 107 89 68 47 117 ND 102 100 114 111 98 98 87 98 117 103 125 90 107 101 117 ND
(53) (24) (19) (9) (11) (11) (29) (7) (7) (13) (17) (29) (31) (10) (25) (35) (26) (9) (12) (22) (17) (29) (1) (4) (23) (15) (17) (8) (29) (15) (14) (13) (10) (18) (8) (25) (11) (24) (19) (37) (29) (26) (3) (20) (4) (4) (7) (5) (6) (12) (5) (24) (18) (8) (9) (29)
G
50 ng/g 87 93 99 88 99 92 83 88 89 89 95 95 91 84 102 54 105 87 92 103 91 84 95 96 97 86 100 89 25 95 98 103 94 99 94 100 20 93 101 95 46 74 52 99 93 101 100 99 93 93 89 74 95 76 98 91 83 102 49
(44) (7) (9) (2) (4) (6) (15) (3) (11) (1) (5) (17) (28) (7) (8) (26) (7) (8) (5) (9) (4) (8) (5) (8) (13) (8) (7) (6) (20) (5) (14) (9) (4) (3) (16) (4) (19) (5) (10) (7) (31) (12) (27) (15) (4) (8) (4) (5) (3) (6) (7) (4) (4) (8) (4) (4) (5) (8) (26)
100 ng/g 172 85 102 94 106 103 86 92 96 92 104 90 126 84 95 75 83 99 97 92 98 84 99 101 93 94 100 87 27 104 97 109 94 95 83 99 20 105 101 91 41 101 66 76 91 98 97 103 98 102 93 75 94 69 90 97 97 101 96
(34) (15) (12) (4) (5) (5) (12) (5) (9) (2) (5) (16) (31) (6) (9) (26) (5) (10) (6) (12) (5) (6) (7) (7) (6) (5) (8) (8) (23) (4) (16) (7) (2) (8) (16) (3) (21) (2) (15) (13) (34) (9) (67) (18) (5) (11) (4) (6) (3) (7) (9) (5) (9) (9) (8) (5) (5) (4) (23)
overall 120 91 109 99 96 94 77 94 104 92 103 90 122 90 89 − 91 98 97 94 97 80 101 97 116 95 98 113 26 95 102 108 99 94 91 101 35 109 97 84 57 97 59 92 102 104 100 103 111 90 99 89 95 114 87 97 95 104 72
(58) (16) (13) (5) (8) (9) (19) (7) (12) (12) (14) (20) (37) (10) (16) (12) (19) (7) (14) (16) (14) (5) (6) (14) (9) (11) (9) (36) (13) (16) (12) (12) (11) (20) (7) (18) (13) (17) (13) (31) (17) (47) (20) (7) (13) (6) (5) (6) (9) (8) (16) (11) (25) (10) (8) (7) (15) (26)
LVL (ng/g) − 1 5 1 1 5 10 1 1 1 1 10 − 1 1 − 10 5 5 10 1 10 1 1 1 1 1 10 − 10 5 1 1 1 5 1 − 1 10 10 − 10 − 10 1 10 1 1 1 5 1 5 1 − 5 1 1 1 −
DOI: 10.1021/acs.jafc.5b05981 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
Article
Journal of Agricultural and Food Chemistry Table 5. continued concentration (n = 5) analyte tebuconazole thiabendazole thiamethoxam thifluzamide triazophos tricyclazole trifloxystrobin trifloxystrobin acid triflumuron trifluralin
1 ng/g 151 113 ND ND 99 96 104 83 79 ND
(8) (5)
(22) (13) (13) (17) (88)
5 ng/g 143 79 ND 124 104 109 100 70 115 ND
(30) (20) (48) (12) (13) (17) (30) (30)
10 ng/g 84 109 106 100 99 99 99 93 80 ND
50 ng/g
(25) (18) (17) (58) (5) (12) (19) (19) (19)
91 89 79 73 99 102 78 113 98 61
(17) (9) (9) (41) (7) (7) (2) (2) (4) (106)
100 ng/g 97 94 120 85 86 101 90 90 97 159
(9) (7) (17) (38) (9) (5) (5) (6) (6) (88)
overall 113 97 102 96 97 101 94 90 94 −
(18) (12) (14) (46) (11) (10) (11) (15) (29)
LVL (ng/g) 10 1 10 − 1 1 1 1 5 −
Bold text indicates recovery > 120% or recovery < 70% or RSD > 20%. ND, not detected; −, recoveries and RSDs were satisfied at fewer than three levels.
a
were optimized for use in the final method. If acidified acetonitrile is used for extraction, then C18 sorbent was found to be more applicable in d-SPE cleanup (as in the case of rice straw), and PSA was better in d-SPE if acid was not used during extraction (wheat straw). Acceptable validation analyte recoveries ranged from 70 to 120% with RSDs within 20%. Ultimately, 77% (53) of the pesticides in wheat straw and 84% (58) of the pesticides in rice straw met validation criteria with LVLs of 1−10 ng/g, which meets straw roughage monitoring needs.
Fortified recovery experiments were conducted by spiking the standard solution into blank wheat and rice straw test portions at five levels (1, 5, 10, 50, and 100 ng/g). The requirements of 70−120% recoveries and RSD ≤20% should be met in least at three levels for an analyte to be validated satisfactorily, with the lowest acceptable concentration defined to be the LVL. The results at each spiking level in wheat and rice straws are listed in Tables 4 and 5, respectively, and the overall results are summarized in Figure S1 (Supporting Information), which illustrates the overall recoveries of the 69 pesticides in the order of their retention times. Analytes that did not yield acceptable results are listed in the tables and Figure S1. For example, 2,4,6-trichlorophenol, oxadiargyl, hymexazol, and trifluralin were not able to be validated due to their poor sensitivities in both matrices. Another four pesticides (quinclorac, trifluzamide, kresoximmethyl, and clothianidin) had variable responses and high RSDs in one or both of the straw matrices. Pesticides with unique physicochemical properties had lower recoveries,38 such as mepiquat chloride, which is a kind of quaternary ammonium salt with low solubility in acetonitrile and high solubility in water (>500 g/L, 20 °C), and quinclorac, which has very low solubility in both acetonitrile and water. As expected, the sulfonyl urea herbicide, nicosulfuron, was also not well extracted by acetonitrile under neutral and alkaline conditions.38 Although the overall recovery of nicosulfuron was a little higher (57%) in rice straw than in wheat straw (46%), which may be due to the addition of 0.1% formic acid, the recoveries were
