Article pubs.acs.org/JAFC
Liquid Chromatography−Mass Spectrometry Method for the Quantitative Determination of Residues of Selected Veterinary Hormones in Powdered Ingredients Derived from Bovine Milk Stefan Ehling*,† and Todime M. Reddy† Abbott Laboratories, 3300 Stelzer Road, Columbus, Ohio 43219, United States S Supporting Information *
ABSTRACT: A rugged, quantitative liquid chromatography−tandem mass spectrometry method with modified QuEChERS (quick, easy, cheap, effective, rugged, and safe) sample preparation for 17 selected veterinary hormones in six different powdered ingredients derived from bovine milk was developed and comprehensively validated. A universal post-extraction spiked matrixmatching approach based on whole milk powder has been successfully implemented. Three validation runs based on four levels of pre-extraction spiked quality control (QC) samples have been conducted. Overall accuracy (86−117%), overall precision (99%), other ingredients derived from milk contain substantial amounts of lactose, e.g., whole milk powder at 37% and nonfat dry milk at 50%. All ingredients derived from milk contain certain amounts of inorganic matter (4−8%) and moisture (2−5%).20 The preferred instrumental approach for hormones analysis is liquid chromatography−tandem mass spectrometry as evidenced in the above referenced publications. The QuEChERS (quick, easy, cheap, effective, rugged, and safe) methodology, initially developed for pesticide analysis in agricultural products,21 is rapidly being adapted and implemented for the analysis of a host of other food contaminants such as veterinary drugs,22−25 mycotoxins,26−29 persistent © 2013 American Chemical Society
Received: Revised: Accepted: Published: 11782
September 20, 2013 November 7, 2013 November 12, 2013 November 12, 2013 dx.doi.org/10.1021/jf404229j | J. Agric. Food Chem. 2013, 61, 11782−11791
Journal of Agricultural and Food Chemistry
Article
Table 1. Veterinary Hormones Considered along with Certain Physical Properties, Regulated Concentration Levels in Milk, and Method Calibration Levels Used MRLd (μg/kg) no.
compound (classa)
CAS no.
formula
FWb
clogPc
EU
Japan
MIASSe (μg/mL)
matrix-based calibration levels (μg/kg) 0.5, 1, 2.5, 5, 12.5, 25, 37.5, 50 0.5, 1, 2.5, 5, 12.5, 25, 37.5, 50 0.5, 1, 2.5, 5, 12.5, 25, 37.5, 50 2, 4, 10, 20, 50, 100, 150, 200 2, 4, 10, 20, 50, 100, 150, 200 1, 2, 5, 10, 25, 50, 75, 100
1
boldenone (A)
846-48-0
C19H26O2
286.41
3.09
prohibited
prohibited
0.5
2
boldenone acetate (A)
2363-59-9
C21H28O3
328.45
4.05
prohibited
prohibited
0.5
3
clostebol (A)
1093-58-9
C19H27ClO2
322.87
3.64
prohibited
0.5
0.5
4
nandrolone (A)
434-22-0
C18H26O2
274.40
2.90
prohibited
prohibited
2.0
5
19-norandrosterone (A)
1225-01-0
C18H28O2
276.41
3.65
prohibited
prohibited
2.0
6
trenbolone (A)
10161-33-8
C18H22O2
270.37
3.17
prohibited
prohibited
1.0
7 8
dexamethasone (C) hydrocortisone (C)
50-02-2 50-23-7
C22H29FO5 C21H30O5
392.46 362.46
2.03 1.76
0.3
20 10
1.0 10.0
9
methylprednisolone (C)
83-43-2
C22H30O5
374.47
2.17
prohibited
10
2.0
10
prednisolone (C)
50-24-8
C21H28O5
360.44
1.64
6
0.7
1.0
11
altrenogest (P)
850-52-2
C21H26O2
310.44
4.18
3
0.5
12
chlormadinone acetate (P)
302-22-7
C23H29ClO4
404.93
3.80
2.5
3
0.5
13 14
fluorogestone acetate (P) melengestrol (P)
2529-45-5 5633-18-1
C23H31FO5 C23H30O3
406.49 354.48
2.82 3.06
1
15
melengestrol acetate (P)
2919-66-6
C25H32O4
396.52
3.35
16
norgestimate (P)
35189-28-7
C23H31NO3
369.50
5.13
0.12
0.1
17
diethylstilbestrol (E)
56-53-1
C18H20O2
268.35
5.33
prohibited
prohibited
1.0 0.5 0.5 0.5
10.0
1, 2, 5, 10, 25, 50, 75, 100 10, 20, 50, 100, 250, 500, 750, 1000 2, 4, 10, 20, 50, 100, 150, 200 1, 2, 5, 10, 25, 50, 75, 100 0.5, 1, 2.5, 5, 12.5, 25, 37.5, 50 0.5, 1, 2.5, 5, 12.5, 25, 37.5, 50 1, 2, 5, 10, 25, 50, 75, 100 0.5, 1, 2.5, 5, 12.5, 25, 37.5, 50 0.5, 1, 2.5, 5, 12.5, 25, 37.5, 50 0.5, 1, 2.5, 5, 12.5, 25, 37.5, 50 10, 20, 50, 100, 250, 500, 750, 1000
a
A, androgens; C, corticosteroids; P, progestagens; E, estrogens. bFormula weight. cPartition coefficient. dMaximum residue limit. eMixed intermediate analyte stock solution. (Fair Lawn, NJ). Acetonitrile (LC/MS grade) was from Honeywell Burdick & Jackson (Muskegon, MI). Formic acid ∼98% was from Sigma-Aldrich Corp. (St. Louis, MO). DisQuE QuEChERS extraction tubes (50 mL) containing 1.5 g trisodium citrate dihydrate, 0.5 g disodium hydrogencitrate sesquihydrate, 1 g sodium chloride, and 4 g magnesium sulfate (implementing method EN 1566240) and dispersive tubes (2 mL) containing 150 mg magnesium sulfate, 25 mg primary−secondary amine (PSA), and 25 mg C18 sorbent were purchased from Waters Corp. (Milford, MA). Samples. Whole milk powder (28.5% fat) was purchased from The Great American Spice Co. (Fort Wayne, IN) and from Franklin Farms East Inc. (Asbury, NJ). Nonfat dry milk was from Dairy America Inc. (Fresno, CA). Whey protein concentrate (75% protein) was from Leprino Foods Co. (Denver, CO). Milk protein concentrate (80% protein) was from Idaho Milk Products (Jerome, ID). Sodium caseinate (low viscosity) was from The Tatua Co-operative Dairy Company Ltd. (Tatuanui, New Zealand). Lactose was from Brewster Dairy Inc. (Brewster, OH). Stock Solutions. Individual analyte and internal standard stock solutions were prepared in methanol at concentrations of 100 μg/mL. Boldenone acetate (100 μg/mL) was used as supplied by the manufacturer. Stock solutions were stable for 6 months at +5 °C. A mixed intermediate analyte stock solution was prepared by diluting the individual analyte stock solutions in methanol at the concentrations listed in Table 1. A 1:20 dilution of the mixed intermediate analyte stock solution was also prepared in methanol. A mixed intermediate internal standard stock solution was prepared by diluting the individual
milk powder (the most complex matrix of all examined ingredients), followed by extension of the core method to the other ingredients derived from milk.
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MATERIALS AND METHODS
Safety. Solvents (methanol, acetonitrile) and formic acid should be dispensed in a hood, and appropriate laboratory safety glasses, coat, and gloves should be worn. Chemicals and Reagents. Betamethasone (≥98%), dexamethasone (≥97%), hydrocortisone (≥98%), methylprednisolone (≥98%), prednisolone (≥99%), altrenogest (≥99.9%), chlormadinone acetate (≥98%), diethylstilbestrol (≥99%), and melengestrol acetate (≥98.5%) were purchased from Sigma-Aldrich Corp. (St. Louis, MO). Melengestrol, fluorogestone acetate, 19-norandrosterone, and norgestimate (homogeneous by thin layer chromatography) were purchased from Steraloids Inc. (Newport, RI). Boldenone, nandrolone, and trenbolone (1 mg/mL in acetonitrile) were purchased from Cerilliant Corp. (Round Rock, TX). Boldenone acetate (100 μg/mL in acetonitrile) was purchased from EQ Laboratories Inc. (Atlanta, GA). Clostebol (1 mg/mL in acetonitrile) was purchased from Grace Davison Discovery Sciences (Deerfield, IL). Prednisolone2,4,6,6,21,21-d6 (98 atom % D), dexamethasone-4,6α,21,21-d4 (95% purity, 96 atom % D), and chlormadinone acetate-d3 (98 atom % D) were purchased from C/D/N Isotopes Inc. (Pointe-Claire, Quebec, Canada). Nandrolone-16,16,17-d3 (98 atom % D) (1 mg/mL in methanol) was purchased from Cambridge Isotope Laboratories Inc. (Andover, MA). Methanol (LC/MS grade) was from Fisher Scientific 11783
dx.doi.org/10.1021/jf404229j | J. Agric. Food Chem. 2013, 61, 11782−11791
Journal of Agricultural and Food Chemistry
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flow was diverted to waste between 0 and 3.5 min and 12 and 16 min of each run.
internal standard stock solutions in methanol at the following concentrations: prednisolone-d6, 0.5 μg/mL; dexamethasone-d4, 0.1 μg/mL; nandrolone-d3, 0.1 μg/mL; chlormadinone acetate-d3, 0.1 μg/ mL. Sample Preparation. Sample portions of 1.00 ± 0.01 g were weighed into 50-mL polypropylene centrifuge tubes. Quality control (QC) samples were spiked with appropriate aliquots of the mixed intermediate analyte stock solution and 100 μL of the mixed intermediate internal standard stock solutions at this stage. In the next step 10 mL of 90/10 water−methanol containing 1% formic acid, pH 2.3 was added to each tube (except for sodium caseinate where 10 mL of 1% formic acid in water was used). The tubes were vortexed to allow thorough mixing of contents. Acetonitrile (10 mL) was added to the tubes followed by shaking for 30 s. The contents of the tubes were poured into 50-mL DisQuE QuEChERS extraction tubes, followed by brief shaking. The tubes were next shaken on an orbital shaker at 1000 rpm for 2 min, followed by centrifugation at 1300 rcf for 5 min. The supernatant layer (8.5−10.5 mL) in each tube was transferred to a 15mL glass centrifuge tube with a glass pipet. The extracts were concentrated to 1 mL under a gentle stream of nitrogen at 55 °C. Concentrated extracts (1 mL) were transferred to 2-mL DisQuE QuEChERS dispersive tubes with a glass pipet, followed by brief shaking. Dispersive tubes were next shaken vigorously by hand for 30 s, followed by centrifugation at 800 rcf for 5 min. The supernatant layer (0.5−0.7 mL) in each tube was transferred to a clean 15-mL glass centrifuge tube with a glass pipet and concentrated to 0.1 mL under a gentle stream of nitrogen at 55 °C. Calibration standards were similarly prepared by spiking the extracted blank whole milk powder matrix (as described above), with appropriate aliquots of the mixed intermediate analyte stock solution and 100 μL of the mixed intermediate internal standard stock solution at this stage (in order to achieve the concentrations listed in Table 1), followed by addition of a small aliquot of methanol (up to a total volume of 0.3 mL). QC sample extracts were reconstituted with 0.9 mL of 60/40 water−methanol containing 0.1% formic acid. Spiked calibration standard extracts were reconstituted with 0.7 mL of 77/23 water− methanol containing 0.13% formic acid. All samples were vortexed briefly. Whole milk powder extracts were filtered through stacked PTFE filters (0.45 μm and a 0.2 μm). All other sample extracts were filtered through a single 0.2 μm PTFE filter. Instrumentation. Analysis was performed on a Waters ACQUITY Ultra Performance LC coupled to a Xevo-TQMS triple quadrupole mass spectrometer. Chromatographic separation was carried out on a Waters ACQUITY UPLC BEH C18 column (1.7 μm, 2.1 mm × 100 mm) maintained at 60 °C, according to the gradient described in Table 2. The injection volumes were 10 μL. The mass spectrometer was operated in the positive electrospray (ESI+) mode with both quadrupoles tuned for unit resolution. Selected operating parameters were capillary voltage (3 kV), desolvation temperature (350 °C), desolvation gas (900 L/h), cone gas (50 L/h), collision gas (0.15 mL/ min). Two multiple reaction monitoring (MRM) transitions were monitored for each compound, with cone voltages and collision energies were optimized for each transition (Table 3). The solvent
■
RESULTS AND DISCUSSION Method Development. A modified QuEChERS (quick, easy, cheap, effective, rugged, and safe) procedure was implemented. Instead of dispersal of the powder in pure water, it was found that 90/10 water−methanol (containing 1% formic acid) offers substantial gains in terms of partitioning efficiency in the extraction step. The only exception was sodium caseinate, for which strong ion suppression was noticed in several areas of the chromatogram when 10% methanol was used in the extraction solvent. By omitting methanol and using 1% formic acid in water for sample dispersal this problem was eliminated. The raw extract was concentrated to 1 mL before dispersive cleanup in order to achieve maximum sensitivity. The overall dilution ratio during the entire procedure is 1:1, i.e., 1 mL of extract is obtained from 1 g of sample. The reconstitution solvents for calibration standards and QCs were chosen in such a way as to achieve equal concentrations of organic solvent and formic acid in the final extracts of both and achieve compatibility with the starting mobile phase composition. In a previous report on hormones analysis in milk,2 an enzymatic hydrolysis step has been used to release conjugated forms (glucuronide and sulfate) of hormones that could potentially be present. The usefulness of the enzymatic hydrolysis step remains controversial. There are multiple limitations and pitfalls associated with this procedure, such as incomplete hydrolysis of hormone conjugates and chemical conversion of one steroid into another.39,41 No significant difference was reported for hydrocortisone in milk with or without enzymatic hydrolysis.2 For all other hormones included in the scope of this method, it is unknown what fraction of each (if any) may be found in their conjugated forms in milk. Analytical standards for most conjugated hormones are presently not available commercially, and this makes the validation of the enzymatic hydrolysis procedure impossible. Furthermore, the effects of processing of milk into powdered ingredients on hormone conjugates are not known either. On the basis of the above, an enzymatic hydrolysis step was not included in the present method. However, the issue of enzymatic hydrolysis of conjugated hormones is relevant to powdered ingredients, although it is beyond the scope of the present work. Future work is needed to elucidate the fate of hormone conjugates during the processing of milk to powdered ingredients and to evaluate the efficacy of enzymatic hydrolysis of individual hormone conjugates (when they become commercially available). The modified QuEChERS procedure was found to provide sufficient cleanup. In a relevant report2 matrix effects in milk could not be totally overcome even after sequential cleanup using two different solid-phase extraction cartridges. Compared to such a laborious procedure, the modified QuEChERS-based methodology coupled with a matrix-matching approach offers obvious advantages. The hormones included in the scope of this method cover a wide range of polarities with partition coefficients (clogP) between 1.6 and 5.3 (Table 1). Chromatographic separation was carried out on a sub-2 μm C18 stationary phase with an acidic water−methanol gradient in which the organic component was varied between 40% and 100%. The column temperature was maintained at 60° in order to reduce back
Table 2. Mobile-Phase Conditions Used for Chromatographic Separation
a
time (min)
flow (mL/min)
% Aa
% Bb
curve
0.00 1.00 8.00 10.00 11.00 13.50 13.51 16.00
0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30
60 60 40 20 0 0 60 60
40 40 60 80 100 100 40 40
6 6 6 6 6 6 6 6
0.1% formic acid in water. b0.1% formic acid in methanol. 11784
dx.doi.org/10.1021/jf404229j | J. Agric. Food Chem. 2013, 61, 11782−11791
Journal of Agricultural and Food Chemistry
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Table 3. Optimized Multiple Reaction Monitoring (MRM) Transitions for Considered Veterinary Hormones no.
rta (min)
cone (V)
MRM1b (quantifier)
collision energy (eV)
MRM2b (qualifier)
collision energy (eV)
dwell time (ms)
1 2 3
prednisolone-d6 (ISc) prednisolone hydrocortisone
3.99 4.05 4.07
30, 15 30, 15 30
367.4 > 150.2 361.4 > 147.1 363.4 > 121.1
15 15 15
349.4 > 151.1 343.4 >147.1 363.4 > 309.3
20 25 15
50 50 50
4 5
dexamethasone-d4 (ISc) dexamethasone
5.33 5.38
15 15
397.4 > 359.3 393.4 > 355.3
10 10
397.4 > 341.3 393.4 > 337.3
10 10
25 25
6 7 8 9 10 11 12
methylprednisolone trenbolone boldenone fluorogestone acetate nandrolone-d3 (ISc) nandrolone diethylstilbestrol
5.63 5.70 6.26 6.45 6.45 6.49 7.37
15 35 25 35 35 35 25
375.4 271.3 287.4 407.5 278.4 275.4 269.2
> > > > > > >
357.3 199.2 121.1 267.2 260.3 239.2 135.1
10 25 20 20 15 15 15
375.4 271.3 287.4 407.5 278.4 275.4 269.2
> > > > > > >
339.3 227.2 147.1 309.3 242.2 257.3 107.1
10 25 15 20 15 15 25
25 25 25 25 25 25 25
13 14 15 16 17
melengestrol clostebol altrenogest 19-norandrosterone chlormadinone acetate-d3 (ISc) chlormadinone acetate melengestrol acetate boldenone acetate norgestimate
8.40 8.98 9.04 9.50 9.66
30 30 30 25 25
355.4 323.4 311.4 259.4 408.4
> > > > >
236.3 143.0 227.2 145.1 309.3
30 20 25 20 15
355.4 323.4 311.4 259.4 408.4
> > > > >
279.3 131.0 269.2 241.3 345.3
20 20 15 20 15
25 25 25 25 25
9.68 9.90 10.01 10.75
25 25 20 40
405.4 397.4 329.4 370.5
> > > >
309.3 279.2 135.1 124.1
15 20 15 25
405.4 397.4 329.4 370.5
> > > >
345.3 337.3 121.1 310.3
15 15 25 25
25 25 25 25
18 19 20 21 a
compound
Retention time. bMultiple reaction monitoring. cInternal standard.
pressure at the used flow rate (0.3 mL/min). Under these conditions analytes eluted at 4−11 min, with a total run time of 16 min (Figure 1). Three different mass acquisition windows were used (3.5−5, 5−8, and 8−12 min, respectively). The four stable isotope-labeled internal standards were assigned to one or more of the 17 analytes based on retention time proximity (Table 3). Norgestimate is a mixture of stereoisomers and elicits two peaks of which the later eluting (10.75 min) and most intense peak was used for quantitation. Betamethasone (regulated in milk37,38) is a stereoisomer of dexamethasone and was not included in the present method because only partial resolution between the two could be achieved, and inclusion of both would have prevented satisfactory integration of either peak. However, since the two compounds have identical MRM transitions, similar sensitivities, and identical maximum residue limits (MRLs), betamethasone can be analyzed by the same method in place of dexamethasone. The method offers sufficient chromatographic resolution to distinguish between the two stereoisomers in an incurred sample. In the method described all analytes were detected in the positive electrospray ion mode with an acidic water−methanol eluent. While others have described the analysis of corticosteroids in the negative ion mode due to enhanced sensitivity,2,42−46 hereby excellent sensitivity was achieved in the positive ion mode for the [M + H]+ ion of each compound. Diethylstilbestrol elicited lower sensitivity in the positive ion mode than all other compounds. Indeed, the negative ionization mode is preferred for this compound due to its phenolic structure.47 Method Validation. Core method validation was carried out in whole milk powder, which has the most complex composition among all ingredients derived from milk and was
expected to be the most challenging from an analytical standpoint. The core method was then cross-validated on the other ingredient matrices. Selectivity was verified by inspecting retention windows of all analytes for interfering matrix peaks. Hydrocortisone (a natural component of milk) was present in some ingredients at a low μg/kg level. Hence the lowest calibration level used for this compound was 10 μg/kg, which gave an instrumental response ≥5 times that of the background level. Monitoring residue levels of hydrocortisone below this threshold in powdered ingredients derived from milk is not justified given the high MRL for this compound established in liquid milk at 10 μg/ kg.38 For all other compounds no significant chromatographic interferences were found. Post-extraction matrix-matched calibration curves were established in whole milk powder. Eight calibration levels were used spanning a 100-fold concentration range (Table 1), with the lowest level close to the limit of quantitation. Calibration standards were analyzed at the beginning and at the end of each run. A quadratic regression model (1/x2-weighted) was used for all compounds. The coefficients of determination (R2) were ≥0.99 for all compounds except for diethylstilbestrol and norgestimate, for which they were ≥0.98. Residuals were ≤20% at the lowest calibration level and ≤15% at all other levels. The slopes of calibration curves were reproducible over 3 days with relative standard deviations (RSDs) of
