Sample collection and storage
Between March 2017 and September 2019, 51 samples of corn oil were obtained randomly from markets, supermarkets, and local retailers in Jiaxing, China. The samples were transported to the laboratory in an insulated container and analyzed upon arrival.
Chemicals and reagents
All standards and reagents used were of the highest purity commercially available. Reagent-grade water was obtained using a Milli-Q Ultrapure Water Purification System (Millipore, Bedford, MA, USA). HPLC-grade methanol and acetonitrile were purchased from Merck (Darmstadt, Germany). ZEN in acetonitrile (50 mg L-1), was purchased from ANPEL Laboratory Technologies (Shanghai, China), and used to prepare calibration curves and recovery experiments. 13C-ZEN in acetonitrile (25 μg mL-1), was purchased from ROMER(Beijing, China).
Apparatus
The LC–MS/MS system consisted of a 30AD LC instrument (Shimadzu, Kyoto, Japan) coupled with a QTRAP5500 triple quadrupole mass spectrometer (AB SCIEX instruments, Foster, CA, USA). The LC-30AD system had two interconnected pump units, one with an integrated degasser and the other with a mixer, and was comprised of an UHPLC gradient system, a refrigerated autosampler, and a column oven compartment. A Waters BEH C18 column (100 mm ´ 2.1 mm, 1.7 μm) was used as the analytical column. The mass spectrometer was equipped with an electrospray ionization source and spectra were acquired in negative ion multiple reaction monitoring (MRM) mode and enhanced product ion (EPI) scan mode. Nitrogen was used as the nebulizer, heater, and curtain gas, and the collision-activated dissociation gas.
LC–MS/MS conditions
The gradient elution solvent comprised acetonitrile (A) and water (B). The gradient was programmed as follows: 0–1 min, 95–80% B; 1–4 min, 80–75% B; 4–6 min, 75% B; 6–8 min, 75–0% B; 8–8.5 min, 0% B; 8.5–10 min, 0–95% B; and 10–12 min, 95% B. The column temperature was set at 40 °C, the flow rate was 0.3 mL min-1, and the injection volume was 10 mL.
LC–MS/MS with electrospray ionization (ESI) was operated in negative mode. Tandem MS analyses were performed in MRM acquisition mode, with two precursor-to-product ion transitions monitored for simultaneous detection of all analytes. The optimized MS/MS parameters were as follows: Source temperature, 500 °C; ion spray voltage (IS), 5500 V; ion source gas 1 (GS1) pressure, 50 psi; ion source gas 2 (GS2) pressure, 50 psi; curtain gas (CUR) pressure, 20 psi; collision gas (CAD), medium. Table 1 shows the MRM parameters used in the optimized survey scan. The peak area of the most intense MRM transition was used for quantification.
Table 1
Retention time and MRM parameters of ZEN
Analytes
|
Retention Time(min)
|
Precursor Ions(m/z)
|
Product Ions(m/z)
|
DP(v)
|
CE(v)
|
ZEN
|
7.48
|
317.1
|
175.1*/131.1
|
170
|
24/30
|
13C-ZEN
|
7.48
|
335.1
|
185.1
|
170
|
26
|
* Quantitative ion.
MRM: multiple reaction monitoring.
|
In this study, an IDA (information-dependent acquisition) experiment was used to automatically trigger EPI scans by analyzing MRM signals. The EPI scans were operated in ESI- mode for product ions at a scan rate of 10,000 Da/s, with dynamic fill in the linear ion trap and a step size of 0.12 Da. The collision energy (CE) spread of EPI was set at 40 eV, with a CE spread of 10 eV to provide rich EPI spectra. The CAD was set to high. The IDA criteria included selecting the most intense peak after dynamic background subtraction of the survey scan, for ions greater than m/z 50 and smaller than m/z 350 that exceeded 1000 counts per second (cps).
Calibration curve of the IDA–MRM–EPI method
The ZEN standard was used to prepare standard solutions by pipetting appropriate volumes into a set of 20-mL calibrated volumetric flasks and diluting with acetonitrile/H2O(20:80, v/v) to volume. The ZEN concentrations were 0.05, 0.1, 0.2, 0.5, 1.0, 2.0, 5.0, 10.0, and 20.0 ng mL-1. 13C-ZEN was diluting with acetonitrile to 0. 1 μg mL-1. The internal standard 13C-ZEN(0.1 μg mL-1) 10 μL was added to each 990 μL ZEN standard concentration respectively.
Calibration curves were established using peak area ratio(ZEN/13C-ZEN) as the dependent variable (y-axis) and the concentration of each analyte as the independent variable (x-axis). The linearity was evaluated from the correlation coefficient (r) value of each calibration curve. The LODs and LOQs for each analyte were calculated at signal-to-noise (S/N) ratios of 3 and 10, respectively, by analyzing several decreasing concentrations of each analyte until the relevant S/N ratio was reached.
Extraction
The sample preparation procedure was developed with reference to the method of Xu28. The corn oil (5 g) was accurately weighed into a 50-mL centrifuge tube, 60 μL 13C-ZEN(1 μg mL-1) was added, vortexing, sit for 5 mins. Then 20 mL extraction solvent (acetonitrile/water, 80:20 (v/v)) was added. ZEN was extracted in a turbine mixer for 1 min, followed by vigorous vortexing for 30 mins in an automatic vibrator. The extract was centrifuged for 5 min at 10,000 rpm and the resulting supernatant was collected. A 1-mL aliquot of the supernatant was transferred into a 5-mL centrifuge tube and diluted to 3 mL with water. The sample solution (1 mL) was transferred directly into vials for LC–MS/MS analysis.
Quality assurance of the IDA–MRM–EPI method
Specificity, selectivity, and accuracy
Corn oil samples were selected to evaluate the specificity and selectivity of this method. To verify the absence of interfering substances, the MRM chromatograms of the blank solvent, solutions of standard, and sample solutions were compared with regard to the retention time of the target analytes.
The accuracy and precision of the method were measured using the intra- and inter-day recoveries and RSDs. The standard solution of the ZEN was spiked into the corn oil samples to obtain spiked samples (three concentrations, 0.6, 6, 60 μg kg-1). All spiked samples were detected three times a day on five different days.