Thermo Fisher Scientific (China) Co., Ltd.
Foreword
International regulations set the maximum limits (MRLs) of thousands of pesticide residues in foods to extremely low levels – 10 ppb or less1. At present, more than 300 defined pesticide residues can be detected by gas chromatography single quadrupole mass spectrometry (GC/MS), and liquid chromatography mass spectrometry (LC/MS) can detect multiple polarities. Pesticide compound 2. The use of gas chromatography single quadrupole mass spectrometry (GC/MS) for the analysis of the maximum residue limit of a large number of pesticides, gradually fail to meet the needs of quality control and government control decisions. This article describes an analytical method for rapid detection of multiple pesticide residues using the unique capabilities of the Thermo Scientific TSQ QuantumTM Gas Chromatography-Triple Quadrupole Mass Spectrometer for quantitative analysis and final confirmation of positive results.
The difficulty in detecting and quantifying multiple pesticide residues in one run is mainly due to the co-elution of a large number of compounds. Since the efflux time of many compounds is partially overlapped or even identical, the analytical instrument is required to have a sufficiently high velocity to obtain sufficient quantitative data points to ensure the accuracy of integration of overlapping chromatographic peaks.
Thanks to the unparalleled acquisition speed of the TSQ Quantum GC mass spectrometer, rapid analysis of multiple pesticide residues has become a routine task in modern food safety testing laboratories, while addressing the main challenges encountered in this type of analysis:
a. Narrow peaks require fast enough mass spectrometry acquisition speeds to obtain sufficient data points for qualitative and quantitative analysis.
b. Multi-component simultaneous analysis methods must take into account multiple components that are partially or completely co-eluting. The SRM conversion speed needs to be fast enough to monitor multiple co-eluting compounds, providing enough data points for reliable peak integration results.
c. Multi-component simultaneous analysis methods need to cover as many compounds as possible, requiring the mass spectrometer to simultaneously monitor hundreds of SRM ion pairs in one analytical run.
d. Due to the small difference in retention time of multiple components in a matrix with complex matrix, the mass spectrometry method should be able to satisfy the detection of multiple targets with as few acquisition fragments as possible. This requires the mass spectrometer to determine as many ion pairs as possible in a single fragment window without loss of sensitivity.
e. Under these conditions, short SRM dwell time and scan interval are necessary.
f. Short dwell time and scan interval time will result in false positive results for targets that generate the same product ions, and “crosstalk†generated in sample analysis must be avoided.
Experimental condition
The purpose of simultaneous analysis of multi-residue pesticides is to reduce the results of multiple compound analyses and the time required to view and generate reports, thereby increasing analytical efficiency and meeting increasing sample throughput requirements. Although this method includes a wide variety of pesticide residues, a short capillary column is used to increase chromatographic separation speed and increase sample throughput. Using the TSQ Quantum Gas Chromatograph, the data acquisition method achieved the acquisition of all 175 SRM ion pairs in a 20-minute run. One run is divided into 11 retention time windows (acquisition segments), and a single window includes up to 25 SRM ion pairs. The dwell time is as short as 25ms, ensuring the high sensitivity and data acquisition speed required to accurately quantify the target. No crosstalk was found even in a segmented window containing 25 SRM ion pairs.
Thermo Scientific TRACE GC UltraTM Gas Chromatography provides fast chromatographic separation and the equipped Thermo Scientific TriPlusTM autosampler provides automated, reproducible liquid injection. Using a Thermo Scientific TRACETM TR-5MS column with a column length of 15 m speeds up GC analysis and increases sample throughput. Table 1 lists the instrument parameters for the gas chromatograph, autosampler, and mass spectrometer.
Table 1. Instrument parameters for the TRACE GC Ultra gas chromatograph, TriPlus autosampler, and TSQ Quantum GC mass spectrometer
Thermo Scientific QuanLabTM Forms 2.5 software provides tailor-made data viewing and reporting programs for pesticide residue analysis laboratories to meet workflow needs and increase the efficiency of multi-residue analysis. QuanLab Forms software includes an integrated user interface for managing a total inventory of pesticide residues, automated data viewing, custom viewing, and multiple reporting formats.
Results and discussion
Figure 1 shows a complete chromatogram of 170 pesticide residues at a concentration of 50 pg/μL, showing the complexity of the elution conditions. The elution time of the last chromatographic peak was 20 min. Target pesticide residues cover a wide range of compound species: organochlorines, organophosphorus, carbamates, pyrethroids and triazine pesticides, which can be detected with 175 corresponding SRM ion pairs.
Figure 1 Total ion chromatogram of 170 pesticide residues detected by GC-MS/MS
The selection of SRM ion pairing is primarily based on the fact that the parameter settings apply to all compounds in the group, ensuring a 25ms dwell time per compound. Taking a segmentation window with a retention time from 5.97 min to 8.27 min as an example, Table 2 lists all 25 SRM ion pairs for 27 pesticide residues.
Table 2. All 25 SRM ion pairs of 27 pesticide residues ( 5.97-8.27 min retention time window)
* Due to the overlap of retention time windows, diazinon appears in two groups of 5.97-8.27min and 8.27-0.117min.
Zero "crosstalk"
The memory effect of the collision cell of the triple quadrupole mass spectrometer is called "crosstalk". If crosstalk occurs, compounds that are not actually present in the sample may be detected, resulting in false positive results. The residence time and scan interval of all 25 SRM ion pairs in the 5.97min to 8.27min segmentation window are very short, which requires a fast scan sequence to effectively clear the collision pool before the next data set is acquired. The zero crosstalk characteristics of the TSQ Quantum GC mass spectrometer are illustrated by monocrotophos ( m/z 127 → 109) and parathion ( m/z 137 → 109). The two compounds are in the same segmentation window and produce the same The daughter ion m/z 109, but got two different SRM ion pairs, as shown in Figure 2. Another similar example is hexachlorocyclohexane (m/z 219 → 183) and pyrimethanil ( m/z 198 → 183). The TSQ Quantum GC mass spectrometer is completely immune to crosstalk and provides very reliable data without false positive results.
Figure 2. Crosstalk signal is not seen in the fast SRM acquisition sequence at 8.09min of high-concentration monocrotophos ( m/z 127 → 109, bottom) to oxalophos ( m/z 137 → 109, top)
Data viewing and report generation
The QuanLab Forms software package provides a natural, fluid workflow wizard for quantitative analysis of target compounds, from building methods and setting up sequences to data viewing and reporting. All pesticide residues covered by the method are associated with peak integration results and standard curves (see Figure 3). In this experiment, a cocktail containing 10 ppb of pesticide residue was added to a leek sample. Peak integration results show concentrations at the highest limit (MRL) level. Quickly view data with QuanLab Forms software, including samples, standards, QCs, blanks, and spike recovery (%). QuanLab Forms provides a quantitative report of tight typography, as shown in Figure 4. This type of report helps to report a wide variety of compounds to a small amount of paper.
Figure 3. QuanView Forms software data viewing window (with a 10 ppb amaranth sample of MRL level)
Figure 4. Example of a tight integration result report for QuanLab Forms software (analysis of a sample of amaranth samples with 10 ppb of pesticide residues added to the MRL level using the H-SRM mode of the TSQ Quantum GC mass spectrometer)
in conclusion
In order to effectively control the pesticide residue at the MRL level in international regulations, the Thermo Scientific TSQ Quantum GC mass spectrometer provides the highest efficiency for multi-residue analysis.
The fast GC method ensures high sample throughput. With one injection, Quantum GC achieved a highly sensitive screening of 170 pesticide residues in a 20-minute analysis period. By dividing the compound into 11 segmented windows, a single window contains up to 25 ion pairs for superior sensitivity, selectivity and flexibility. Even in the cycle of quickly collecting data, false positive results due to crosstalk can be completely avoided. Quantum GC is expected to further reduce dwell time without sacrificing overall sensitivity to meet the requirements of adding more target compound analysis when necessary. QuanLab Forms software provides a workflow wizard format that makes it easier for analysts to create and manage analytical methods, sample data, view results, and print reports.
references
1. EU Commission Directives 91/414/EEC, 86/362/EEC, 86/363/EEC, 90/642/EEC, Regulation (EC) N.396/2005 and subsequent amendments.
2. Ministry of Health, Labour and Welfare Japan, Department of Food Safety, The Positive List System for Agricultural Chemical Residues in Foods, June 2006, see: http://
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