Method parameters
The analysis was performed on a GC 7890 / MSD 5975C GC/MS system (Agilent Technologies) configured with a GERSTEL Cooled Injection System (CIS) PTV-type inlet and a GERSTEL MultiPurpose Sampler (MPS) sample preparation robot fitted with a 10 μL syringe for liquid injection.
Analysis conditions

CIS 4: splitless 25 °C; 12 °C/s; 280 °C (3 min)

Column: 30 m DB5-MS (Agilent) di = 0.25 mm; df = 0.25 μm

Carrier gas: He, constant flow 1.0 mL/min

GC oven: 60 °C (1 min); 10 °C/min; 300 °C (3 min)

Standards
A standard mixture of organochlorine- and organophosphorous pesticides at a concentration of 1000 μg/L was prepared in Acetonitrile.
Sample Preparation
For quantification purposes, fruit and vegetable extracts were spiked with diluted pesticide standards. (Concentrations: 20 μg/L and 200 μg/L in 500 μL extract).
DPX extraction
1 mL QuEChERS tips from DPX-Labs, LLC. 500 μL of the fruit- or vegetable extract was automatically transferred to a vial by the MPS fitted with a 2.5 mL syringe. A 1 μL aliquot of the extract was introduced to the GC.
Schematic diagram of the DPX clean-up.
Spinach and orange extracts before and after DPX clean-up.
Full scan chromatograms of orange extzract before (A) and after (B) DPX clean-up.
Full scan chromatograms of spinach extzract before (A) and after (B) DPX clean-up.
SIM chromatogram after DPX clean-up of spinach extract spiked to a concentration of 200 ppb with a standard pesticide mixture.
The sequence table for automated DPX sample preparation is easily and quickly set up by mouse-click in the MAESTRO.
Percent recovery and relative standard deviation for the pesticides.
Recovery of the pesticides with and without DPX clean-up (spiked to a concentration level of 200 ppb).

Food Safety

So long, troublemakers I

The QuEChERS method may be a recent arrival on the scene, but it is conquering the world. QuEChERS provides fast and reasonably priced extraction, enabling efficient determination of pesticide levels in agricultural samples. GERSTEL application chemists have used automated Disposable Pipette Extraction (DPX), a miniaturized dispersive SPE technique, combined with GC/MS to optimize the determination of pesticides in QuEChERS extracts. Interfering matrix compounds can be chased off quite easily - without laborious sample preparation.

Pesticides have long helped provide ample, affordable, and safe food supplies for billions of people across the globe. Even so, constant vigilance is needed in order to protect the environment and consumers from the consequences of improper application of pesticides to plants and crops. When it comes to food safety, the necessary first step is always to find efficient ways of controlling the quality of our food on a large scale in our globalized economy. Just the pesticides that can legally be used represent a list of hundreds of compounds with widely different characteristics. Multimethods are required, spanning both liquid chromatography (HPLC) and gas chromatography (GC) combined with Mass Spectrometry (MS). In theory, the analytical chemist can track down pretty much every pesticide known to man. In practice, the right sample preparation is critical if one wants to achieve accurate results; and it had better be automated, we’re talking about a big job. In just a few years, the QuEChERS method (Quick, Easy, Cheap, Efficient, Rugged, and Safe) has become the method of choice for extracting toxins from a variety of foods. Initially, QuEChERS was developed as a fast and inexpensive method to extract pesticides from various, mainly plant-based matrices. Validation studies have proven that the QuEChERS method results in good recovery and low standard deviation for a wide range of pesticides. Furthermore, the QuEChERS method is much less laborintensive and requires much less solvent than previously used methods. A wide range of pesticides can be extracted. In many laboratories, QuEChERS has caused a veritable productivity boom.

QuEChERS extracts the analytes – and more...

Every solution results in new, unexpected and interesting challenges and there is a price to pay for going QuEChERS: Fruit and vegetable extracts produced using the QuEChERS method contain large amounts of matrix residue. But „quick and dirty“ is fine if we can get rid of the dirt afterwards. There are two ways to accomplish this: Clean up the extract or use analytical instruments that can handle samples with matrix residue. If we take a separate look at each of these alternatives, further clean-up of QuEChERS extracts is typically performed using manual dispersive solid phase extraction (SPE) followed by centrifugation to remove solids. These steps are not easily automated, but the same clean-up effect can be accomplished using Disposable Pipette Extraction (DPX), a dispersive SPE technique that is fully automated. In this article, examples are presented, which show that DPX is an attractive and efficient alternative for clean-up of spinach and orange extracts. Should the analyst not want to perform further clean-up of such extracts prior to GC/ MS analysis, the only remedy is to replace the GC inlet liner at regular intervals. Liner exchange is normally a time-consuming task, but GERSTEL‘s Automated Liner EXchange (ALEX) performs Liner EXchange automatically, enabling the analysis of samples with undissolved sample matrix. Matrix build-up has negative consequences especially for the GC/MS system: If raw QuEChERS extracts are injected directly into the GC, residue will accumulate in the inlet. This build-up will lead to compound loss by adsorption on active surfaces and increased variability, negatively impacting the results. In combination with the MultiPurpose Sampler (MPS), ALEX performs automated liner exchange at user-defined intervals.

Disposable Pipette Extraction (DPX)

DPX is an SPE technique that relies not on packed adsorbents in standard cartridges, but on adsorbent powder placed inside disposable pipette tips. In the case of spinach and orange extracts, graphitized Carbon Black was used among other adsorbents (DPX Qg-tips, as specified in DIN EN 15662). Plant colorants, such as chlorophyll, and free acids were successfully removed. The transport adapter at the top and a frit at the bottom help contain adsorbent and sample inside the pipette tip while enabling highly efficient airbubble induced mixing. The transport adapter also serves the dual purpose of allowing the MPS to get a grip on the cartridge in order to transport it and to introduce the syringe needle into the cartridge for liquid transfer. The DPX process is clever yet simple: The MPS picks up a DPX tip from the tray. Depending on the method, the adsorbent can be washed with a suitable solvent, which is taken from a solvent reservoir. The solvent can either be aspirated into the tip from below or added to the top using the autosampler syringe. A 500 μL sample of the extract in question was aspirated into the tip. Extracts had been spiked with organochlorine- and organophosphorous pesticide standard mixtures at different concentration levels. Samples were aspirated into the DPX tip from below, which means they were never in contact with the syringe needle or piston. „There is no sample-to-sample cross contamination or carry-over“ said Carlos Gil, Manager, Analytical Services at GERSTEL Headquarters, while adding: „Since DPX is a dispersive SPE technique, the extraction efficiency is not influenced by the flow path or the flow rate through the adsorbent, making the technique highly rugged and reliable“. Once the sample has entered the DPX tip, the syringe pulls air through the tip and the sample from below. The liquid suspension undergoes highly efficient turbulent mixing leading to optimal contact between the phases and highly efficient and fast extraction. „The efficiency of the cleanup is clearly demonstrated by the fact that the final spinach extract is almost completely colorless“, says Carlos Gil. The extraction takes place in less than two minutes. Then the cleaned QuEChERS extract is then transferred directly to a clean autosampler vial for analysis or to undergo further liquid sample preparation steps prior to the analysis as needed. The used pipette tip is discarded. As soon as the prepared sample has been introduced to the GC system, a clean pipette tip is picked up and the next sample prepared. „Analysis and sample preparation are performed in parallel, ensuring best possible utilization and return on investment for the entire instrument set-up“, says Carlos Gil.

The power of DPX - conclusion

Apart from the visible removal of spinach and orange matrix, the analysis results bear testament to the efficiency of the DPX process. Carlos Gil: „The results were convincing, we had excellent recovery of the organochlorine- and organophosphorous pesticides that were determined in the study“. The relative standard deviation (n=3) was under 10 % both for the extract spiked at 20 ppb and for the extract spiked at 200 ppb. Average recoveries were 119 % for the orange sample and 91 % for the spinach. The study proved that automated DPX is useful for second stage clean-up of QuEChERS extracts prior to GC/MS analysis. The DPX tips efficiently removed interfering matrix material, improving overall system reliability and productivity, and reducing the need for maintenance since there was less residue build-up in the GC/MS system.

 

Literature:

AppNote-2009-01
Automated Multi-Residue Pesticide Analysis in Fruits and Vegetables by Disposable Pipette Extraction (DPX) and Gas Chromatography / Mass Spectrometry