LC-MS (Liquid Chromatography Coupled with Mass Spectrometry) for Food Analysis

LC-MS instruments are usually associated with different types of mass analyzers to improve selectivity, to provide confidence in assigning the identity of the contaminants detected, and also to offer different approaches for analysis. From the available studies, the MS detectors used in mycotoxin analysis include triple quadrupole (QqQ), ion trap, time-of-flight (TOF), and orbital ion trap mass analyzers, as well as hybrid systems that combine two types of analyzers. The latter group includes quadrupole linear ion trap (QLIT), double quadrupole TOF (QqTOF), quadrupole orbital ion trap quadrupole Orbitrap; Q Orbitrap, and linear ion trap orbital ion trap systems. The LC-MS techniques are mostly applied in mycotoxin determination.

LC-QQQ MS/MS (Liquid Chromatography Coupled with Mass Spectrometry Occupied with Triple Quadrupole Detector)

Nowadays the approaches to the trace-level determination of food contaminants have changed considerably using selective detectors in mass spectrometry (MS) to increase the selectivity and the sensitivity of the test. For both quantification and semi-quantitative screening of food contaminants, such as pesticides, the combination of MS with chromatography, either GC or LC, is recognized as the 'gold standard' (Alder et al., 2006). Though GC-MS continues to be used in the detection and analyses of non-polar small molecules (e.g., dioxins, PCBs, pesticides and halogenated aromatic compounds) and volatile molecules, new developments in both LC and MS have made a very powerful instrumentation technique for the sensitive and selective determination of more polar or ionic contaminants at trace levels in food when considering the newly emerged food contaminants (Malik et al., 2010; O'Mahony et al., 2013). These contaminants mainly include 'emerging contaminants' (Farre and Barcelo, 2013), pesticides (Femandez- Alba et al., 2008; Botitsi et al., 2011), toxins (Suzuki et al., 2011; Caprioti et al., 2012), veterinary medicines which include antibiotics (Bizec et al., 2009; Bogialli et al., 2009), etc.

Ferrer et al. (2010) developed a precise and specific method for the detection of mycotoxins, especially aflatoxins, dyes and pesticides in a variety of spices using Reversed Phase (RP) Liquid Chromatography- Tandem Mass Spectrometry interfaced with Electrospray Ionization (LC-ESI-MS/MS). Here, the technique requires a simple sample treatment procedure using acetonitrile in the extraction step without further clean up. The mobile phase used was a Cl 8 column with an aqueous ammonium formate/methanol mixture, which is followed by gradient elution. The mass spectral acquisition was made in positive ion mode to provide a high degree of selectivity. This was done by applying multiple reactions and simultaneous monitoring of at least two fragmentation transitions per compound. They used this method and successfully performed in-house validation in terms of linearity, sensitivity, repeatability, recovery, and selectivity on six kinds of spices. The results obtained for all the analytes were satisfactory with quantitation limits acceptable for daily monitoring purposes. The recoveries obtained after extraction for a majority of the compounds present the spices ranging from 60-140 per cent (0.05 and 0.5 mg/kg). The large-scale applicability of this technique for the identification and analysis of pesticides, dyes, and aflatoxins simultaneously in several types of spices was also done.

Chen et al. (2017) developed a method for the multi-residue analysis of 117 pesticides in tea based on In-Syringe Dispersive Solid Phase Extraction (IS-D-SPE) and Ultra Performance Liquid Chromatography Orbitrap High Resolution Mass Spectrometry. A full scan mode was obtained at an m/z range of 100-800 with an orbitrap resolution at 70000. Confirmation was done at full scan/dd-MS2 mode. Identification criteria for retention time and accuracy of mass were found to be ±0.20 min. and ±5.0 ppm, respectively. In order to identify the pesticides with the same molecular mass weight, MS/MS fragment ions obtained dd-MS2 was necessary here. IS-D-SPE technique uses a mixture of C18 (100 mg), PSA (200 mg), and multi-walled carbon nanotubes (15 mg) to the cleanup of the tea sample. They obtained good linearity of about R2 > 0.99 for 117 pesticides. Recoveries obtained were in the range of 70-120 per cent for 105 pesticides and were found to be satisfactory. The intraday and interday precisions observed were below 20 per cent. LoQ was 10 pg kg A Later, this method was employed to analyze 117 pesticides in 70 tea samples. The accurate mass and high sensitivity is highly advantageous to the utility of HPLC-Orbitrap-MS for the quantification of a wide range of pesticides at trace residue in various food matrices, such as honey (Gomez Рёгег et al., 2012), fish (Farre et al., 2014), fruit and vegetables (Rajski et al., 2014) and baby foods 0ia et al., 2014).

Mycotoxins (e.g., aflatoxin) are common food contaminants that cause poisoning and severe health risks to humans and animals. In a study conducted by Alsharif et al. (2019), achemometric approach was adopted in liquid chromatography-tandem mass spectrometry (LC-MS/ MS) optimization for simultaneous determination of mycotoxins, like aflatoxins B,, B„ G,, and G„ and ochratoxin A. Here, to study the occurrence of mycotoxins in 120 food matrices, a standardized Quick, Easy, Cheap, Effective, Rugged, and Safe (QuEChERS)-LC-MS/MS method was used after univariate and multivariate optimization methods. The (QuEChERS) technique, with or without dispersive SPE (Solid Phase Extraction), is one of the most widely used sample pre-treatment methods for mycotoxin analysis and other contaminants in food samples (Gonzalez-Jartin, 2019; Cao, 2018; Zhang, 2018; Rejczak and Tuzimski, 2015). For this, the recovery found to be ranged between 81.94-101.67 per cent with relative standard deviation (RSD) lesser than 11 per cent. Aflatoxins were detected in raisin, pistachio, peanut, wheat flour, spice, and chilli samples with quantity ranging between 0.45-16.93 pg/kg using this method. Traces of ochratoxin A detected in wheat flour and peanut samples ranged between 1.2-3.53 pg/kg.

LC-QToF—Quadrupole Time of Flight for Non-target Pesticide Analysis

It is already known that to identify, quantify, and resolve inexactness by selecting appropriate ionization and acquisition parameters, LC-MS can be equipped with several mass analyzers, each of which provides unique features for food analysis. Using LC in conjunction with LC-(Q)ToF-MS (time-of-flight mass spectrometry) to detect the presence of targeted and non-targeted pesticides in food and water is reported by Lacorte and Fernandez Alba (2006). This technique is characterized by a resolving power of 10,000 or more, thereby it gives accurate masses for both the parent and fragment ions. It enables the measurement of the elemental formula of a compound and also to achieve the identification of the compound. The combination of QToF (quadrupole-ToF) permits tandem MS to provide increased structural information and enhanced selectivity too. It gives LC-ToF-MS Liquid Chromatography Time-of-Flight Mass Spectrometry and LC-QToF-MS (Liquid Chromatography Quadrupole Time-of-Flight Mass Spectrometry) for detection and analysis of pesticides in food samples, its state of art, and applicability. The performance of such techniques is usually described in terms of precise mass quantification, selectivity, and fragmentation. LC-(Q)ToF-MS is highly applicable for the analysis of pesticides in food matrices routinely. This indicates those operational conditions and criteria used to screen, quantify, and identify the target and 'suspected' pesticides and their breakdown products in fruits, vegetables, and drinking water. Non-target chlorinated pesticides were determined in food using LC-ToF-MS without the use of analytical standards (Garcia-Reyes et al., 2005). Here the protonated molecule's full scan spectra and accurate mass measurements (better than two ppm accuracy) along with isotope clusters result in 1-2 elementary compositions. Together with the characteristic fragment ions of suspected species, chlortoluron, iprodione, and procymidone were identified in tomato, apples, and grapes, respectively. Confirmation and quantification can be finally performed with standards.

Yet another example is the identification of non-target contaminants by LC-QToF-MS at 7,000 resolution in surface waters, which enabled the detection and identification at levels of these contaminants less than 0.25 ug/L (Bobeldijk et al., 2001). The data obtained by LC-ToF-MS and LC-IT-MS (Ion Trap MS) can be related to developing a new identification strategy to detect unknown pesticides present on vegetables in the market.

 
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