Analysis f Pesticide Residues in Citrus Oils by GCxGC-TOFMS with Minimal Sample Preparatin LECO Crpratin; Saint Jseph, Michigan USA Key Wrds: GCxGC-TOFMS, Classificatins, Decnvlutin, Quantificatin, Sample Preparatin, Pesticides 1. Abstract Citrus ils are widely used in cnsumer prducts. Cntaminatin with pesticides is f particular cncern because the citrus ils are derived frm the uter prtin f the fruit. GCxGC-TOFMS prvides fr analysis fr many pesticides in citrus ils with minimal sample preparatin. This wrk shws the ability f this technique t sufficiently separate these pesticides frm the citrus il cmpnents, t prvide cnfirmatin f identity, and t quantify these residues ver several rders f magnitude cncentratin, reaching as lw as part-per-billin levels. 2. Intrductin Citrus ils are traded internatinally and are included in prducts that are traded internatinally. These ils are derived frm the uter prtin f the fruit, as byprducts in the much mre ecnmically imprtant prductin f citrus juices. And, it is the utside f the fruit that is expsed t the pesticides that are applied t prtect the citrus crp and trees. The analysis f citrus ils fr pesticides has becme increasingly imprtant as pesticides that are acceptable in ne market may be banned in anther. Even in markets where specific pesticides may be allwed, there are increasing restrictins n the permissible levels f pesticides t which humans r animals may be expsed. While the greater prtin f citrus il may be limnene, the ther cmpunds present frm a highly cmplex mixture that includes many natural prducts. Althugh injectin f the citrus il directly int a GC wuld be highly desirable, the cmplexity f the il is such that GC-MS analysis fr a large number f pesticides at ppb levels is nt practical. The higher-biling cmpunds in the citrus ils celute with the pesticides. These cmpunds are present in significant cncentratin (relative t pesticide residues) and prvide many ins in cmmn with the pesticides, cnfunding many MS detectin and cnfirmatin techniques. Additinally, the lading f sme f the citrus il cmpnents n the chrmatgraphic clumn is sufficiently high that GC retentin times may be shifted, again cnfunding the prcess f lcating and identifying the pesticides. This wrk demnstrates a GCxGC separatin f citrus il cntaining pesticides, and subsequent quantificatin using TOFMS. The first chrmatgraphic separatin is a biling pint separatin, prviding a grss separatin f the cmplex mixture. A secnd separatin, using a 50% phenyl phase, separates the mre plar r mre armatic pesticides frm the bulk f the matrix that celutes. Cmpunds celuting in the first chrmatgraphic separatin include sesquiterpines, ther hydrcarbns, r derivatives f these such as alchls, ketnes, and ther cmpunds. Even with the extremely high peak capacity prvided by GCxGC, individual pesticides still may celute with ther cmpunds in the mixture. Mass spectral detectin is required t btain distinguishable signals fr the pesticides. TOFMS is used t lcate, identify, and quantitate the cmpunds. 3. Experimental Unflded citrus ils (Flrida midseasn range il and Califrnia cld-pressed lemn il) and limnene were btained frm cmmercial surces. A prtin f each il was spiked t give a slutin cntaining nminally PPB (w/v) f each pesticide. Prtins f the spiked il were mixed with unspiked il t btain desired cncentratins. A 200 µl prtin f the spiked il was placed in a GC vial insert and 20 µl f a 20 ng/ul slutin f 2-flurbiphenyl was added as an internal standard. The sample was mixed by drawing it int a Pasteur pipette and expelling it int the insert several times. Calibratin curves were generated frm pesticides spiked int limnene at cncentratins f 10 t ppb t demnstrate linearity f pesticide respnse in an uncntaminated matrix. The citrus ils were run spiked at 100 ppb, 25 ppb, and unspiked. The 25 ppb spiked sample was run at least five times t shw relative standard deviatins (RSDs) at this level f spike. Where pesticides were bserved in the unspiked ils, the backgrund cncentratin was determined by using the added spikes as standard additins. Analyses were perfrmed n a LECO Pegasus 4D GCxGC- TOFMS system under the fllwing analytical cnditins. Injectin Vlume: 1 µl Liner: Splitless Liner with Wl (Restek #22401) Temp: 250 C Mde: Splitless Clumn #1 J&W Scientific DB-PONA (50 m x. 0.2 mm x 0.5 µm) #2 SGE BPX-50 (2 m x 0.1 mm x 0.1 µm) Carrier: He Flw Rate: 0.5 ml/minute Life Science and Chemical Analysis Slutins
Delivering the Right Results Temperature Ramp Primary Oven Initial Temp: Ramp: Final Temp: Secndary Oven Initial Temp: Ramp: Final Temp: Mdulatr Temp Offset: Mdulatin Time: Ht Pulse Time: 150 C; Duratin: 3 minutes 3 C/minute 330 C; Hld: 5 minutes 160 C; Duratin: 3 minutes 3 C/minute 335 C; Hld: 5 minutes 30 C 10 secnds 1.5 secnds Mass Spectrmeter Acquisitin Range: 29 t 390u Acquisitin Rate: 100 spectra/secnd Surce Temp: 200 C Transfer Line Temp: 300 C 4. Results Chrmatgraphy f the pesticides in limnene shws separatin f the pesticides frm the higher biling materials nrmally fund in cmmercial limnene (Figure 1). The GCxGC chrmatgram f range il shws the presence f significant quantities f materials celuting with the pesticides in the first chrmatgraphic dimensin. The additin f the secnd chrmatgraphic dimensin prvides sufficient separatin t allw the identificatin and quantitatin f the pesticides. Figure 3 shws the lcatins f pesticides in an range il chrmatgram. Tw peaks, identified as chinmethinate and fenprpathrin, were nt included in any standard, but were identified in a review f the chrmatgraphic peak table generated by the Autmated Peak Find and Spectral Decnvlutin capability f LECO's ChrmaTOF sftware. Chinmethinate (als knwn as Mrestan) is used as an acaride, fungicide, fumigant, and insecticide n citrus. The mass spectrum frm this peak and its librarymatched spectrum are shwn in Figure 4. Fenprpathrin, a pyrethrid, is used as an acaride and insecticide. The mass spectrum frm this peak and its library-matched spectrum are shwn in Figure 5. Figure 1. Pesticides in the ppb standard made in cmmercial limnene. Nte that limnene elutes frm the clumn set befre 1100 secnds under the cnditins used. Higher biling impurities frm the limnene are seen in the cntur plt. Peak markers are shwn nly fr pesticides present in the chrmatgram. The respnse f the cmpunds is linear ver the range f interest. A typical calibratin curve is shwn in Figure 2. Rati 0.0160 0.0140 0.0120 0.0100 Figure 3. Pesticides in chrmatgram f spiked range il, 1 ppm spike. Peak True - sample "Flrida Mid Seasn Orange Oil - ppb spike:1", peak 2 861, at 2510, 3.670 sec, sec (Spec # 151367) Library Hit - similarity 761, "Chinmethinate" 64 89 116 148 173 206 234 206 234 272 329 0.00800 0.00600 0.00400 39 64 89 116 174 148 0.00200 0.00 100 300 700 900 Cncentratin y= +1.75158e-005x - 8.07267e-005 r = 0.99994 Figure 4. Sample and library mass spectra fr peak identified as chinmethinate. Figure 2. Calibratin plt fr Parathin in limnene. Cncentratin range is 10 t ppb.
Peak True - sample "Flrida Mid Seasn Orange Oil - ppb spike:1", peak 4168, at 3090, 3.240 sec, sec 41 55 97 181 125 209 229 Library Hit - similarity 698, "Fenprpathrin" 55 83 97 The ability t separate a cmpund frm interfering matrix is clearly demnstrated by Methyl Parathin. The mass spectrum f Methyl Parathin shws three intense ins (109, 125, and 263), as well as many weaker ins. GCxGC surface plts (Figure 6) shw the well-reslved peak in the chrmatgraphic plane. A recnstructed ne-dimensinal trace is shwn in each f these figures (as the chrmatgram in the backgrund). The recnstructed ne-dimensinal trace shws that this ppb standard wuld be difficult t detect, cnfirm, and quantify based n examinatin f in traces and in ratis frm ne-dimensinal chrmatgraphy. And, this cmpund wuld be even mre difficult t detect and quantify at lwer cncentratins. 265 181 125 141 208 265 349 Figure 5. Sample and library mass spectra fr peak identified as fenprpathrin. Figure 6. Methyl Parathin separated frm interferences in GCxGC, shwn fr m/z 263, 125, and 109. 349 Table 1 shws the variability f the 25 ppb spike. (In cases where a 50 ppb spike was used, it was because that was the spike level affrded by the standard slutins used.) With five replicatins, the RSDs btained when there was n pesticide riginally present ranged frm abut 10% t 20%. Examinatin f the results f the spiking study shwed a number f pesticides t be present in the range il prir t spiking. When pesticides were already present in the il, the spike, crrected fr additinal material present, gave RSDs in the same range. In cases where pesticides were already present in the il, the regressin thrugh the results fr the spike at 100 ppb, the multiple results at 25 ppb, and the unspiked sample gave resulted in the measurement f the pesticide already present in the il. With backgrund cncentratins included in the calculatin, RSDs fr the measured pesticides typically ranged frm abut 5% t abut 20%. Brmprpylate is clearly present in the range il, but ther cmpunds present in the il ffer t cnfund in ratis if they are t be used in ne-dimensinal GC- MS t cnfirm identity. Figure 7 shws the surface plt fr mass 183 with a peak fr brmprpylate. In the GCxGC plane, the brmprpylate peak is well reslved frm ther peaks. The recnstructed in trace shws that in a ne-dimensinal trace, celutins wuld adversely affect the ability t integrate the m/z 183 peak. The in fr m/z 185 des nt shw similar interference in the ne-dimensinal trace, thus the rati f intensities f m/183 and 185 in this sample wuld nt be expected t match the rati fr these masses in the pure cmpund, with the risk f reprting brmprpylate as being undetected. Brmprpylate Figure 7. Extracted in surface plt fr m/z 183 shwing Brmprpylate in range il. Nte first dimensin celutin that wuld cnfund in rati calculatins fr ne-dimensinal chrmatgraphy and selected in mnitring MS. The celutin f fenprpathrin with brmprpylate wuld nt have been likely t have been nticed with techniques such as selected in mnitring (SIM) r MS/MS unless this cmpund was n a list f target analytes. Including ins fr this cmpund in SIM r MS/MS target lists culd reduce the verall sensitivity in this regin, as detectin time wuld have t be divided between these tw celuting cmpunds. Life Science and Chemical Analysis Slutins
Delivering the Right Results Table 1. Results f spiking range il with pesticides. With backgrund levels included in the calculatin, RSDs fr five injectins range frm 4% t 24%. Average Fund value Pesticide Spike Standard deviatin RSD % Cncentratin in blank (by regressin line, ppb) -Phenylphenl 25 25 1 Appears t be High ppb r lw ppm level RSD % fr crrected cnc. 2,4-D methyl ester 25 25 1 2.0 7.9 Ethprp 25 25 1 2.6 10 Sulftepp 25 25 1 2.1 8.5 Phrate 25 27 6.4 23 5 20 Dimethate 25 25 1 3.0 12 Methyl parathin 50 50 1 2.6 5.2 Methyl chlrpyrifs 50 50 1 2.2 4.4 Metalaxyl 25 25 1 0.95 3.8 Rnnel 25 20 2.4 12 3 11 Pirimiphs methyl 25 25 1 1.5 6.0 Malathin 50 51 13 25 13 (peak is t prly shaped fr practical use) 20 Fenthin 25 27 3.8 14 8 11 Parathin 25 26 6.7 26 7 (peak is t prly shaped fr practical use) 20 Chlrpyrifs 50 45 15 33 217 5.6 Trichlrnate 25 23 4.1 17 3 15 Thiabendazle 25 22 5.0 22 16 13 Methidathin 25 25 1 Present in "blank" at level abve 100 ppb Stirifs 25 25 1 3.9 29 6 20 Tkuthin 25 25 1 5.1 22 4 19 Merphs 25 25 1 9.1 34 Backgrund peak clearly nt Merphs, but cntributes t signal. Fensulfthin 25 25 1 1.8 8.9 6 6.8 Ethin 25 25 1 6.4 27 67 7.1 Sulprfs 25 25 1 6.1 27 9 19 Brmprpylate 25 25 1 In excess f ppb in backgrund Small peak present in blank, but requires additinal standards t accunt Tetradifn 25 25 1 0.46 1.8 fr curvature in respnse curve. Prchlraz 25 25 1 4.6 18 Examinatin f the GCxGC plts fr individual ins (Figure 11) shws a clearly identifiable peak fr several ins, while in the ne-dimensinal trace, m/z 159 might be a distinguishable peak. Cnfirming ins wuld be difficult t find in a ne-dimensinal analysis, and if fund, in ratis may be cmprmised. The GCxGC separatin ffers the ability t identify and quantify Tetradifn at even lwer cncentratins than fund here. 0 8000 6000 4000 2000 1st Time (s) 2nd Time (s) 3243-3 3263-2 3.6 3.8 4 4.2 159 227 227 111 127 Figure 9. Extracted in chrmatgrams fr Tetradifn in unspiked lemn il. Peak True - sample "Califrnia cld Pressed lemn Oil Oil - Unspiked:1", peak 3274, at 316 0, 3.830 sec, sec 4.4 4.6 1 In cases where n pesticide was present in the blank, there were nly tw levels in the regressin, the lw and high spikes. In this case, deviatin frm linearity will nt be bserved. 75 111 159 Lemn il again shws significant quantities f material celuting with the pesticides in the first chrmatgraphic dimensin. With the additin f the secnd chrmatgraphic dimensin, the pesticides are separated frm the bulk f the matrix. Figure 8 shws the lcatin f pesticides in lemn il. 50 127 Reference Spectrum - Calibratin "Pesticides in Cld Pressed Lemn Oil mdified", Analyte ">Tetradifn " 61 75 111 227 145 175 356 201 159 50 127 227 175 356 199 254 Figure 10. Mass spectrum f Tetradifn fund in unspiked lemn il with cmparisn t Reference Spectrum. The evaluatin f pesticides in lemn il was fcused primarily n thse pesticides nted as already present in the il befre spiking. RSDs are similar t thse btained fr pesticides in range il. Again, when backgrund levels are included in the calculatin, RSDs fr seven replicates are typically in the 10% t 20% range. Figure 8. Pesticides in the chrmatgram f spiked lemn il, 1 ppm spike. As with the range il, pesticides were fund t be present in the unspiked lemn il. Tetradifn is present in the il at abut 44 ppb. The GCxGC separatin gives in traces that clearly allw fr accurate integratin f the peak (Figure 9). The spectrum btained unequivcally identifies the cmpund (Figure 10).
Figure 11. Peaks fr Tetradifn shwn amng interferences in lemn il. Masses shwn are 159, 356, and 127. Table 2. Results fr selected pesticides in lemn il. With backgrund cncentratins taken int accunt, RSDs fr seven replicates range frm 5% t 22% fr the pesticides measured. Average Fund value Cncentratin in blank (by regressin line, ppb) Pesticide Spike Standard deviatin RSD % -Phenylphenl 25 25 1 14 22 RSD % fr crrected cnc. 2,4-D methyl ester 25 25 1 5 21 Ethprp 25 25 1 3 12 Phrate 25 25 1 1.3 5 Methyl parathin 50 25 1 9 18 7 Methyl chlrpyrifs 25 26 5 18 1 17 Parathin 25 25 1 2 17 1 Chlrpyrifs 50 40 12 31 115 8.0 Methidathin 25 20 11 54 34 20 Stirifs 25 25 1 1 5 Tkuthin 18 3 16 8 11 Ethin 25 25 6 31 51 8.5 Brmprpylate 25 25 7 49 39 11 Tetradifn 25 25 10 41 44 22 Prchlraz 25 25 1 2 9 1 In cases where n pesticide was present in the blank, there were nly tw levels in the regressin, the lw and high spikes. In this case, deviatin frm linearity will nt be bserved. 5. Cnclusins GCxGC-TOFMS prvides the capability t determine pesticides in citrus ils with n sample preparatin ther than the additin f internal standard. Detectin limits belw 25 ppb fr many pesticides are clearly achievable. Even at lw cncentratins, when spectral infrmatin becmes difficult t btain, the retentin times btained n tw statinary phases serves t cnfirm pesticide identity. The full mass range capability f the TOFMS, cupled with spectral decnvlutin, affrds the ability t identify pesticides nt riginally included in a target analysis list. LECO Crpratin 3000 Lakeview Avenue St. Jseph, MI 49085 Phne: 800-292-6141 Fax: 269-982-8977 inf@lec.cm www.lec.cm ISO-9001:2000 N. FM 24045 LECO is a registered trademark f LECO Crpratin. Frm N. 203-821-293 2/08-REV1 2008 LECO Crpratin Life Science and Chemical Analysis Slutins