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Analytical and Bioanalytical Chemistry

, Volume 410, Issue 22, pp 5431–5438 | Cite as

Ultra turrax® tube drive for the extraction of pesticides from egg and milk samples

  • Julia Sturm
  • Peter Wienhold
  • Thomas Frenzel
  • Karl Speer
Communication
Part of the following topical collections:
  1. Food Safety Analysis

Abstract

The Ultra turrax® tube drive, already successfully applied for the extraction of plant materials, has also proved to be suitable for the analysis of pesticides in eggs and milk. In comparison to the matrix solid-phase dispersion (MSPD), the extraction is less time-consuming at excellent extraction efficiency. Further advantages are the flexibility of the extraction conditions with respect to the pH value and water activity. So, even strongly acidic pesticides such as phenoxy carboxylic acids can be extracted. Eighty-nine GC-amenable and 75 LC-amenable pesticides, which had been detected successfully in whole chicken eggs following MSPD extraction and further processing according to Hildmann et al., could also be analyzed with the modified method. In addition, the analysis spectrum could be expanded by 4 GC- and 37 LC-amenable substances. Of the 208 pesticides tested, 205 substances could be detected in whole chicken eggs. Similar excellent results were achieved for the milk matrix. Furthermore, the modified extraction method allows a determination of the fat content from the same analysis approach.

Keywords

Pesticides Ultra turrax®tube drive MSPD Egg Milk GC-MS/MS LC-MS/MS 

Introduction

For the determination of pesticide residues in foods, several multi-methods such as the DFG S19 [1], the QuEChERS method [2, 3, 4, 5, 6, 7, 8, 9], as well as the ChemElut method [8] were developed. Despite the partially elaborate clean-up steps, the qualitative and the quantitative GC or HPLC measurement of the pesticides still proved to be difficult or even impossible, depending on the matrix. Primarily, this applies to fat-containing food of animal origin. Other matrix groups, i.e., tea, cereals, and spices, offer similar challenges as regards matrix interference. Recently, in the work-group, Hildmann et al. worked out and validated a method particularly suited to the analytics of residues in eggs [10]. The method involves matrix solid-phase dispersion (MSPD) followed by small-scale size exclusion chromatography and solid-phase extraction and results in two separate ready-to-use extracts for GC-MS/MS and LC-MS/MS measuremets, respectively. The method published by Hildmann et al. proved to be much faster compared to DFG S19 and employs clean-up steps, which are more selective than those applied in the QuEChERS procedure. Nevertheless, the MSPD used hereby is time-consuming due to the manually executed activities; besides, the spectrum of the extractable pesticides is limited so that this analysis step should be replaced by a suitable module based on a liquid-liquid extraction (LLE). The classical LLE is generally well established in pesticide residue analysis, mainly because of its simplicity and adaptability to the chemical characteristics of the analytes. The need for manual shaking of the samples and difficulties to achieve a clear phase separation is frequently taken as reasons for rejecting the LLE. Therefore, the Ultra turrax® tube drive by the IKA Company already employed by Rasche et al. [11] for dried fruits should be utilized for the extraction.

For the determination of pesticide residues in foods, several multi-methods such as the DFG S19 [1], the QuEChERS method [2, 3, 4, 5, 6, 7], as well as the ChemElut method [8] were developed. Despite the partially elaborate clean-up steps, the qualitative and the quantitative GC or HPLC measurement of the pesticides depending on the matrix still proved to be difficult or even impossible. Primarily, this applies to fat-containing animal foods. Recently, in the work-group, Hildmann et al. worked out and validated a method particularly suited to the analytics of residues in eggs [10]. Nevertheless, the MSPD used hereby is time-consuming due to the manually executed activities; besides, the spectrum of the extractable pesticides is limited, so that this analysis step should be replaced by a suitable module based on a liquid-liquid extraction. Therefore, the Ultra Turrax® Tube Drive by the IKA Company already employed by Rasche et al. [11] for dried fruits should be utilized for the extraction. With a rotor-stator system, this equipment allows for dispersing, homogenizing, mixing, and stirring in one-step as so-called “one-vessel solution.” With the one-way test vessels, cross contaminations (“carry over”) can be avoided, and there is no clean-up expenditure. Furthermore, it is an automation step by which better reproducible results can be achieved. Additionally, it offers the possibility to match the extraction conditions to the specific characteristics of the analytes so that the phenoxy carboxylic acids and even nicotine can be captured. The preshredded test material is placed in the sealable mixing vessel together with a solvent. After attaching the tubes to the propulsion, the disperging process is started. The test material is shredded by the rotor-stator unit. Hereby, the dispersion time, the number of revolutions, and the use of a turbor or the reverse function are variable. After centrifugation and phase separation, the organic phase is processed according to Hildmann et al. [10].

Experimental

Reagents and apparatus

All pesticide and PCB standards used were of high purity purchased from Dr. Ehrenstorfer (Germany), Sigma-Aldrich (USA), Riedel-de Haen (USA), or Syngenta (Switzerland). The solvents were obtained from Merck (Germany) and VWR (USA) including LC-MS grade solvents for LC analysis. The exception was denatured ethanol which was offered by Berkel AHK (Germany). Acetic acid, formic acid, and molybdatophosphoric acid hydrate were purchased by Merck (Germany), and ammonium formate was supplied by Biosolve (USA). Air Liquide (France) offered helium, argon, and nitrogen, which were used for LC and GC analysis as well as for the evaporation, with high purity.

Individual stock solutions (1 mg/ml) and the working mixtures (8 ng/μl) were prepared in cyclohexane/ethyl acetate (CH/EA, 1:1, v:v) or acetonitrile and stored at −18 °C.

The apparatus for thin layer chromatography comprised an automatic TLC Sampler 3, developing chambers, and a TLC plate heater III, which were obtained from Camag (Switzerland). TLC silica gel 60 aluminum sheets (20 × 20 cm, thickness: 0.25 mm) were supplied by Merck (Germany). The IKA ultra turrax® tube drive was run with 50 ml vessels. For the other equipment, see the original paper presented by Hildmann et al. [10].

Methods

Extraction of egg and milk samples using the IKA Ultra turrax® tube drive

Ten grams of the homogenized sample are weighed in a 50-ml IKA Tube Drive DT-50. After appending 20 ml citrate puffer (pH = 5) and 20 ml solvent (cyclohexane/ethyl acetate (1:1), the sample is dispersed (2 min, 6.000 rpm). Then, the vessel stands for 10 min in a freezer (− 20 °C) before appending 2.5 g of a salt mixture existing of NaCl, sodium citrate-dihydrate, and disodium hydrogen citrate-sesquihydrate 1/1/0.5 (w/w/w) and 4 g magnesium sulfate. After shaking, the analytical preparation for 2 min, the vessel has to centrifuge at 7 °C for 10 min (1500 rpm). If there is no clear phase separation, the centrifugation time should be extended or the revolution speed should be increased (2000 rpm). Then, 2.5 ml of the clear supernatant are then used for the GPC cleanup step. For the further sample preparation, GPC and SPE parameters, as well as the GC-MS/MS and LC-MS/MS conditions, see the original paper published by Hildmann et al. [10]. The egg and milk samples were taken from the market within the monitoring programme.

Thin layer chromatography

The application as bands was carried out automatically onto a silica gel 60 sheet, which was used as stationary phase. The mobile phase consisted of petroleum ether/diethyl ether/acetic acid (85:15:1.5, v:v:v). The chamber conditioning was conducted with the mobile phase for at least 30 min. The development comprised 100 ml of the mobile phase and a run distance of the solvent front of 18 cm. The detection included the spraying with freshly prepared 10% molybdatophosphoric acid in ethanol (~20 ml) and the heating at 150 °C for ~2 min. With reference to egg extracts 10–20 μl extract with a sample equivalent of 2 g egg per ml extract were used for application [10].

Results and discussion

Extraction optimization for the analysis of egg samples

In order to reach equal portions in the measuring solution as in the output method by Hildmann et al., 10 g of the test material were weighed out into the 50-ml IKA vessel instead of 5 g. This was simultaneously the highest possible weighed portion to still include the required solvent amounts. For 10 g of a weighed portion, a 20 ml solvent mixture (cyclohexane/ethyl acetate) and a 20 ml watery portion proved to be best. The highest number of revolutions reachable with the IKA Tube Drive System is 6000 rpm. Hereby, the test material is mixed most strongly with the buffer and the solvent mixture. Hooking up the turbo was dispensed with as it needs to be operated manually and works for maximally 30 s only. Ultimately, dispersing was carried out for 2 min at 6000 rpm. More extended dispersing times did not lead to any better recoveries. After adding the QuEChERS salts, a phase separation with an organic supernatant is reached. To optimize this, the IKA vessels are centrifuged for 10 min at 1500 rpm. Subsequently, there is still sufficient phase available for a fat determination as for a number of matrices the pesticide contents must be given in terms of fat. The pH value was lowered so that acidic pesticides can pass to the organic phase more easily. Anastassiades et al. recommend a pH value below 5.5; a pH value of 5.0 proved to be well-suited. The further processing then is carried out according to Hildmann et al. (Fig. 1).
Fig. 1

Schematic representation of the two methods

TLC detection of lipid compounds

To check the extraction for completeness, the fat content of the extract was determined according to the S19 regulation (Par.64 LFGB L 00.00-34). It was also investigated whether the extracted fats differ after the liquid-liquid extraction and the MSPD in respect to their lipid classes and concentration. For this purpose, a thin layer chromatography (TLC) is used. The extracts or rather the standards dissolved in solvent agents are sprayed onto a silicic plate with the Camag TLC sampler 3 in the form of bands. The mobile phase petrol ether/dietyl ether/glacial acetic acid (85:15:1.5; v,v,v) separated the lipid components at a migration distance of approx. 20 cm on the basis of different polarities.The spray reagent molydatophosphoric acid served for the detection. Figure 2 shows that there are no sizable differences relative to the chosen extraction.
Fig. 2

Extraction effectiveness of LLE (IKA) vs. MSPD determined for egg matrix by thin layer chromatography

Quality of extracts: LLE (IKA) versus MSPD

A comparison of the two methods was then carried out based on spiked samples (level 10.0 μg/kg) for 92 GC- and 81 LC-amenable analytes (Fig. 3). After the MSPD, for GC-MS/MS as well as for LC-MS/MS, more pesticides are determined in the 70 to 120% recovery range according to the SANTE document [12]. This may be caused by the fact that fresh solvent constantly flows the sample in the extraction column, and so the pesticides are extracted more easily from the sample matrix. Consequently, the recoveries of the analytes increase. In the liquid-liquid extraction (LLE), the solvent is added to the sample material only once (20 ml instead of 50 ml) and the extraction time is significantly shorter. Yet in contrast to the MSPD, more substances can be qualified with the IKA-tube drive system. Concerning the LC-amenable analytes, four additional substances could be integrated successfully, whereas concerning the LC-amenable substances, 37 additional pesticides could even be added (Fig. 4). The transitions of the additional 41 compounds are compiled in Table 1(a, b). This pertains foremost to the substance classes: Phenoxy carboxylic acids, benzoic acid derivates, and benzonitriles. Although the modified method according to Hildmann et al. in total shows lower recoveries of the analytes in comparison to the original method with MSPD, more substances can nevertheless be verified. Therefore, the modified method is especially well-suited for quick screening. Numerous analytes can be qualified but also quantified in a very short time.
Fig. 3

Recoveries of GC- and LC-amenable analytes in egg matrix; MSPD vs. LLE (IKA)

Fig. 4

GC- and LC-amenable analytes detectable after extraction in egg matrix; MSPD vs. LLE (IKA)

Table 1

Transitions of pesticides (extended to Hildmann et al.)

Compound

tRet (min)

Fragmentor (V)

SRM transition (m/z) for quantification (collision energy [eV])

SRM transition (m/z) for qualification (collision energy [V])

Polarity

a) LC-MS/MS acquisition parameters for analytes additionally covered by means of IKA extraction

2,4-D

9.55

50

219 → 161 (0)

219 → 125 (20)

Negative

2,4-dB

12.57

50

247 → 161 (0)

247 → 125 (25)

Negative

Amidosulfuron

9.30

93

370 → 261 (8)

370 → 218 (20)

Positive

BAC 10

11.70

125

276 → 91 (28)

276 → 58 (28)

Positive

Bentazone

8.03

110

239 → 132 (25)

239 → 197 (20)

Negative

Bromoxynil

9.52

100

274 → 79 (40)

274 → 81 (20)

Negative

Carbosulfan

19.29

108

381 → 118 (16)

381 → 76 (40)

Positive

DDAC 8

13.0

130

270 → 158 (30)

270 → 71 (30)

Positive

Dicamba

7.33

60

219 → 175 (0)

219 → 145 (5)

Negative

Dichlorprop-P

10.90

50

233 → 161 (0)

233 → 125 (20)

Negative

Florasulam

7.59

110

360 → 129 (25)

360 → 192 (10)

Positive

Fluopyram

12.0

145

397 → 145 (60)

397 → 173 (32)

Positive

Fluroxypyr

8.10

90

255 → 209 (15)

255 → 181 (25)

Positive

Ioxynil

10.38

110

370 → 127 (40)

370 → 215 (30)

Negative

MCPA

9.98

60

199 → 141 (0)

201 → 142 (15)

Negative

MCPB

12.68

50

227 → 141 (15)

229 → 143 (5)

Negative

Phoratsulfone

10.0

82

293 → 97 (32)

293 → 171 (4)

Positive

Phoratsulfoxid

9.5

70

277 → 97 (36)

277 → 143 (16)

Positive

Prochloraz

15.0

60

376 → 266 (10)

376 → 308 (5)

Positive

Quizalofop

12.98

110

345 → 299 (15)

345 → 163 (40)

Positive

Saflufenacil

10.78

150

501 → 349 (25)

501 → 198 (45)

Positive

Spirotetramat

12.52

108

374 → 216 (32)

374 → 302 (12)

Positive

Spirotetramat-ketohydroxy

10.35

102

318 → 300 (8)

318 → 214 (24)

Positive

Spirotetramat-monohydroxy

8.37

148

304 → 211 (16)

304 → 254 (16)

Positive

Triclopyr

10.52

50

254 → 196 (5)

254 → 218 (0)

Negative

Quinmerac

7.05

90

222 → 204 (15)

222 → 141 (35)

Positive

Topramezone

6.26

119

364 → 334 (8)

364 → 125 (24)

Positive

b) GC-MS/MS acquisition parameters for analytes additionally covered by means of IKA extraction

Dichlofluanid

15.22

 

224 → 123 (15)

226 → 123 (15)

 

Fenpropidin

14.51

 

98 → 55 (15)

98 → 70 (15)

 

Indoxacarb

28.34

 

203 → 106 (20)

203 → 134 (20)

 

Tolylfluanid

17.46

 

238 → 137 (15)

240 → 137 (15)

 

a) LC-MS/MS acquisition parameters for analytes additionally covered by means of IKA extraction

b) GC-MS/MS acquisition parameters for analytes additionally covered by means of IKA extraction

In relative standard deviations below 20%, the 70–120% recoveries do not necessarily have to be reached according to SANTE document. For Imazalil, for example, a recovery of 44% at a relative standard deviation of 6% was determined after the threefold determination. If this substance is validated at various concentrations and the 44% are thereby recovered dependably, it is SANTE-conform.

Those analytes, which could not be verified via LLE, could also not be captured via the Hildmann et al. method. This concerns nicotine, which needs alkalized processing and the LC-amenable substances, the pyridincarbon acids, aminopyralid, and clopyralid. Whereas clopyralid is lost explicitly in the extraction step, aminopyralid losses (50%) which occur by means of GPC and SPE in addition to the extraction losses.The validation data are shown in Table S1 (see Electronic Supplementary Material, ESM).

Time consumption MSPD versus LLE (IKA)

In comparison to the MSPD, using the modified method saves a considerable amount of time. If processing is begun in the early afternoon, the completed GPC eluates of eight samples are available the next morning. After solvent exchange and SPE, the measuring solutions can be measured via GC-MS/MS or rather LC-MS/MS so that first results are obtained within 24 h. For eight samples, adding the solvent and the buffer requires 10 min, the extraction takes 20 min, adding salt and shaking is carried out in 24 min, and the centrifugation takes 20 min. Overall, for the extraction step for eight samples by means of the modified method, 1 h and 15 min must be calculated. The original method according to Hildmann et al., on the other hand, takes 45–60 min per sample alone for crushing the sample in the mortar, packing the MSPD column, and the extraction, thus 6–8 h for eight samples. Furthermore, the extract must be concentrated before the GPC cleanup step. For the modified method, this amounts to a time-saving of at least 6 h.

Using the tube drive system for the matrix milk

Based on the “homogeneity” of the whole milk matrix, it is thinkable not to extract the sample with the IKA tube drive but to fill the milk directly into a centrifuge tube and to shake it after adding the solvent and the QuEChERS-salts. Afterwards, the milk samples could then be centrifuged at 4000 rpm for 5 min and the supernatant, which has formed, processed according to Hildmann et al. [10]. In the gravimetric fat determination following the shaking extraction, however, a noticeably lower fat content was established (2.2%) than after the IKA tube drive extraction (5.4%). According to the nutritional value table (Souci-Fachmann-Kraut [13]), a fat content of at least 4.3% is expected for raw milk; the residual peak contents in VO (EG) No. 396/2005 for milk are based on 4.0%; consequently, the value obtained after shaking is too low. Using the IKA system, the emulsion of the milk is so “destroyed,” presumably due to the impact of shear forces that the fat can enter into the organic phase more easily.

As the fat content obtained of a UHT-milk sample corresponded quite well with the declaration on the milk container during the IKA extraction, it can be assumed that the fat content obtained from the raw milk sample was determined reliably as well. In comparison to the shaking extraction, the better extraction of the milk fat via the IKA extraction might also be responsible for the altogether better recoveries in respect to the unipolar analytes (Fig. 5).
Fig. 5

Recoveries from whole milk matrix for GC-amenable analytes after extraction; LLE vs. LLE (IKA)

Table 2 lists the substances, which obtained better recoveries with the IKA system. Only dichlofluanid was recovered better with the QuEChERS method than with the IKA system (recovery 71% instead of 57%). The lower recovery, however, is acceptable in view of the achieved precision.
Table 2

Recoveries from whole milk matrix for selected GC-amenable analytes after extraction; LLE vs. LLE (IKA)

Compound

LLE

LLE (IKA)

Rec (%)

Rec (%)

Bromocyclen

49

100

Chlorbensid

16

120

Chlorpropham

62

98

Chlorpyrifos-methyl

66

103

Diallat

42

95

Dicloran

54

86

Diphenylamin

34

80

Etridiazol

16

87

HCB

23

93

HCE-cis

66

94

HCH-alpha

52

96

HCH-gamma

63

93

Heptachlor

56

100

PCB 028

56

95

Pentachloranilin

68

97

Phosalon

67

101

Phosmet

52

104

Pyridaphenthion

67

101

Quintozen

38

91

Resmethrin-trans

67

87

Tecnazen

20

93

Dichlofluanid

71

57

LC-amenable analytes

The recoveries of the LC-amenable analytes with and without the use of the IKA-system are compiled in Fig. 6. For both kinds of extraction, the distribution of the recoveries is similar.
Fig. 6

Recoveries from whole milk matrix for LC-amenable analytes after extraction; LLE vs. LLE (IKA)

If the recovery of a substance was above 70% in an IKA extraction, as a rule, it was also above 70% without the IKA system. The same applies to the substances whose recoveries were under 70% or rather above 120%. Even so, in addition to aminopyralid and clopyralid, the compounds carbosulfan, disulfoton, fenthion, and fenthion-oxon could not be detected. It is possible that these substances were transformed completely into their metabolites. For example, fenthion sufoxide shows a very high recovery, so a conversion of fenthion and fenthion oxon might be conceivable. For disulfoton and carbosulfan, this could not be ascertained according to the recoveries of the metabolites; however, not all metabolites possible were available as standards. An overview of the results is presented in Table 3.
Table 3

Recoveries from whole milk matrix for selected LC-amenable analytes after extraction; LLE vs. LLE (IKA)

Compound

LLE

LLE (IKA)

Rec (%)

Rec (%)

Cymiazol

23

91

Flumethrin

18

72

Methacrifos

32

74

Phoxim

60

85

Spinosyn A

54

72

Spinosyn D

51

74

Tepraloxydim

56

81

Bifenazat

70

11

Imazalil

75

48

The RSD of the flumethrin agent was already at 20% in the extraction using the IKA-system. Without the use of the IKA-system, it is still even higher (87%) and points toward fluctuating individual results. The same also applies to methacrifos. Seven analytes show better recoveries via IKA extraction, whereby the RSD of cymiazol is not SANTE-conform. Only two substances (bifenazat and imazalil) had higher recoveries without the IKA system. The recoveries of spiked samples (10 μg/kg; n = 3) are summarized for milk in the Table S2 (see ESM).

Matrix milk: Ultra turrax tube drive versus QuEChERS

The “QuEChERS” multi-method is predominantly suitable to analyze polar analytes. For the separation of the lipids, the extract is frequently deep-frozen. Hereby, a fairly significant portion of the unpolar substances is lost. The GC-amenable analytes, which could not be validated after QuEChERS-processing (concentration 10 μg/kg), are listed as follows: aldrin, bromophos-ethyl, DDE, brompropylat, chinomethionat, chlorbenzilat, chlorfenapyr, cyhalothrin-lambda, deltamethrin, dicloran, diphenylamin, endosulfan-alpha, endosulfan-beta, endosulfansulfat, etridiazol, lindan, pentachloranilin, quintozen, tetradifon, trifluralin, vinclozolin.

Comparing the lab-internal validation data of the QuEChERS method for the LC-amenable analytes in the milk matrix with the recoveries of the modified Hildmann method, as expected, the recovery rates for the milk samples processed with the Hildmann et al. method are generally higher. Only amidosuforon and bentazon reach a higher recovery via the modified method using the tube drive. Dicamba, however, could not be validated. In the investigation of the polar analytes in the milk matrix, the QuEChERS method proved to be less time-consuming. In case of analyzing also unpolar pesticides, the QuEChERS method is less suitable. Based on the losses for unpolar pesticides due to fat separation by freezing and the use of the medium-polar solvent acetonitrile, these do not completely arrive in the organic phase. Furthermore, the fat portion is of great importance if the agent specification concerns the fat content of a sample (e.g., PCBs). Determining the fat content in the framework of the QuEChERS method, however, is not possible.

Transferring the IKA-Ultra-Turrax-drive onto other animal matrices

Whereas very good results could also be obtained for cream, transferring the modified method onto matrices such as turkey, pork butt, halibut, and carp has so far been unsatisfactory as the supernatants of organic solvents turned out to be very low due to the relatively low rpm number during the centrifugation process, and the knives were not sufficently sharp for separating the meat fibers. Currently, a technical solution to solve the problem is being attempted.

Notes

Compliance with ethical standards

Conflict of interest

The authors declare that they have no conflict of interest.

Supplementary material

216_2018_1254_MOESM1_ESM.pdf (158 kb)
ESM 1 (PDF 157 kb)

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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2018

Authors and Affiliations

  • Julia Sturm
    • 1
  • Peter Wienhold
    • 2
  • Thomas Frenzel
    • 2
  • Karl Speer
    • 1
  1. 1.TU Dresden, Chair of Special Food Chemistry and Food ProductionDresdenGermany
  2. 2.Saxon State Institute of Health and Veterinary AffairsDresdenGermany

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