search.noResults

search.searching

note.createNoteMessage

search.noResults

search.searching

orderForm.title

orderForm.productCode
orderForm.description
orderForm.quantity
orderForm.itemPrice
orderForm.price
orderForm.totalPrice
orderForm.deliveryDetails.billingAddress
orderForm.deliveryDetails.deliveryAddress
orderForm.noItems
29


through 0.45 µm PTFE syringe filters into vials for subsequent analysis by LC/MSMS.


Pesticides to be detected by LC/MSMS were purchased as neat reference standards of purity ≥ 98% from either QMX Laboratories (Thaxted, UK), LGC standards (Teddington, UK) or Greyhound Chromatography (Birkenhead, UK). Single stock solutions of each pesticide were prepared in-house in methanol at concentrations of approximately 400 µg/ml. Newly prepared single stock solutions were compared against old stock solutions as per SANTE guidelines. Single stock solutions were combined into mixed standards at approximately 5 µg/ml for most pesticides and stored at 4-7°C until required for analysis. Pesticides were grouped into two sets of mixtures (A and B) to safeguard that pesticides known to convert to one another were in separate mixtures, for example thiophanate- methyl and carbendazim, ensuring that quantitative results could be achieved for positive samples without the need for analysis with separate standards. HPLC grade methanol and acetonitrile were supplied by Rathburn Chemicals (Walkerburn, UK). Ammonium acetate was supplied by Fisher Scientific UK, Loughborough, UK. Matrix- matched calibration standards at 5 levels were prepared in appropriate (organic) fruit or vegetable matrix that had been extracted using the citrate QuEChERS extraction method. A mixture of internal standards containing carbendazim D4, methomyl D3 and pendimethalin D5 was added to each sample and standard as an injection internal standard.


A Shimadzu Nexera UHPLC system (Shimadzu, Milton Keynes, UK) was coupled with a Sciex 6500 QTRAP mass spectrometer (Sciex, Warrington, UK). A Genius 3031 nitrogen generator from Peak Scientific, Inchinnan, UK was used to supply gas.


UHPLC Set-up Run time:


Column: Gradient elution:


Time (min) % Eluent A 0.1 0.7


11.8 13.8 14.0


MS Set-up


Ionisation mode:


Electrospray in positive and negative ion mode with polarity switching. Multiple Reaction Monitoring (MRM).


Ion Source Temperature:


425°C


Collision (CAD) gas: High Ionspray voltage: switching between +4500 and -4500V 35 psi 60 psi 50 psi


Curtain gas:


Ionspray gas 1: Ionspray gas 2:


Entrance potential: 10 V (positive ion) and -10 V (negative ion)


Results and Discussion 2 x 17 min


Kinetex 2.6 µm, C18, 50 x 4.6 mm with Security Guard cartridge


Injection volume:


(Phenomenex, Macclesfield, UK) 3 µl


Column temperature: 40°C Flow rate: Eluent A:


0.4mL/min


Methanol/Water 5/95 v/v + 5mM


ammonium acetate Eluent B:


Methanol + 5mM ammonium acetate


There are many considerations when deciding upon a workflow for pesticide residue analysis. Hundreds of pesticides and their metabolites must be screened for, in complex matrices down to low levels and positive residues must be quantified and confirmed with sufficient identification points in accordance with SANTE guidelines. Detection limits are typically 10 ppb for most pesticides although some pesticides need to have lower detection limits since they have much lower MRLs due to their toxicity. For example the MRL for carbofuran in some crops is 0.001 mg/kg necessitating a detection limit of at least 1 ppb. The desired throughput of the laboratory is also important when selecting the analytical approach.


Most surveillance monitoring samples require to be reported either monthly or quarterly therefore high throughput was not crucial. The aims of the method described here were to obtain fully quantitative, robust results for all pesticides in positive and negative ionisation modes and to generate sufficient confirmation data by acquiring at least two MRMs per pesticide in the first analytical run. In this way samples only needed repeat analysis if residue levels exceeded the calibration range (5 - 80 ppb). Calibration curves in this range (and higher for many pesticides) were linear. Typically for positive samples where residues exceeded


75 40 2 2


75


%Eluent B 25 60 98 98 25


200 ppb these were diluted into the linear range of the instrument.


The position of the probe in the Turbo VTM source can greatly affect the sensitivity of the analysis. It is desirable to ensure that for typical LC flow rates the position of the electrode is not too close to the orifice of the mass spectrometer to avoid undesirable components fouling the system. In this work the probe vertical micrometer position was set to 3 mm and the probe horizontal micrometer position was set to 6 mm.


The use of the advanced Scheduled MRMTM Pro algorithm in Analyst software aided the acquisition of the large number of MRMs whilst still ensuring that there were sufficient data points across each peak (>10 points required for quantitation) even in congested areas of the acquisition method.


Figure 1a shows the total ion current (TIC) in positive ion mode (red) and negative ion mode (blue) of a strawberry matrix standard at 10 ppb. Figure 1b shows the TIC of a 2016 strawberry sample that contained 16 pesticide residues in the range 10 – 300 ppb. 15 of these pesticides were acquired in positive ion mode (red) and one was acquired in negative ion mode (blue). The extracted ion chromatograms (XIC) presented Figure 1c show boscalid (+MRM) quantifier and qualifier transitions for the strawberry matrix standard at 10 ppb and 2016 strawberry sample containing 16 pesticide residues including boscalid (+MRM) at 80 ppb. XIC of fludioxonil (-MRM) quantifier and qualifier transitions for strawberry matrix standard at 10ppb and 2016 strawberry sample containing 16 pesticide residues including fludioxonil (-MRM) at 100 ppb are displayed in Figure 1d.


The figure is typical of the good peak integrity with excellent signal to noise and sufficient data points in both ionisation modes for all pesticides which was maintained throughout 2016 without the need for remedial maintenance activities. Simple cleaning of the mass spectrometer’s curtain plate was sufficient to maintain this consistent performance without the need to vent the MS for cleaning in between each annual preventative maintenance visit by the service engineer.


Consistent retention times were achieved but Scheduled MRMTM


Pro has a function


where acquisition of an MRM is extended by up to one minute if the response is still above a threshold at the end of the acquisition window. The Security Guard cartridge was replaced if backpressures increased or peak shapes deteriorated but this was only required after several thousand injections. Column life was also up to several


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68