Sample Preparation Selection

The flexibility and strength of SPE comes from the users’ ability to choose the sorbent that selectively interacts with the analyte(s) of interest or with the matrix interferences that could affect the recovery. Thus, SPE cartridges can also be used in a nonretentive approach as a “chemical filter” that removes interference from the sample while target analytes pass through the sorbent and are collected for further analysis. Unwanted matrix interferences will remain in the SPE cartridge while the analytes of interest are collected.

SPE is performed by using a tube filled/packed with a chemically derivitized sorbent. By varying the chemical nature of the sorbent and the buffer conditions used during the loading, washing, and elution stages, a method can be developed that can be very selective to clean and isolate the target analytes from complex sample matrices.

The steps for SPE include:

• Pretreat sample (via LLE, homogenization, buffering, etc.);
• Choose appropriate SPE sorbent and protocol;
• Condition sorbent to prepare for interaction with sample;
• Load pretreated sample onto SPE sorbent (target analyte will be retained on sorbent);
• Wash sorbent to remove unwanted interferences that are not retained on the sorbent;
• Elute target analytes (using a combination of organic strengths and buffers); and
• Analyze clean eluent by GC or HPLC.

Although SPE is highly selective, produces high recoveries, and provides repeatable results, there are a few drawbacks that prevent labs from implementing the technique. SPE requires method development, special equipment, and is much more expensive than its alternatives due to sorbent packing and media costs. When the analysis of complex sample is required, SPE would be the ideal choice of sample preparation technique because it reliably and repeatedly provides high recovery.

Real-World Instances

The following are some real-world examples on how each approach was chosen as the most effective technique.

Example 1: Multiresidue Pesticide Analysis in Spinach using QuEChERS AOAC Kits. With the strong presence of pesticides in the food cycle, the purpose of this analysis is to detect concentrations of pesticides below the maximum residue limits because global legislations are quickly becoming more concerned. It is imperative that sensitive and efficient analytical techniques are used to detect low levels of the variety of pesticides.

The primary challenge in this analysis is to eliminate the naturally occurring pigments, fatty acids, nutrients, and fats that are present in spinach samples in order to achieve lower limits of detections (LOD) of pesticides. Although a traditional technique such as LLE can be used, it employs the use of hazardous solvents and cannot remove all matrix interferences that can prevent reaching the desired LOD.

roQ QuEChERS Kits (Phenomenex) were employed. The combination of buffering salts, magnesium sulfate, and organic solvent induced delivered clean separation and extracted all pesticides, while PSA and GCB dSPE sorbents were used to remove the remaining matrix interferences

(Figure 2 and Figure 3). Because the AOAC 2007.01 method had already been validated and established, this procedure was the best alternative to achieve a combination of low limits of detection, easy and quick processing, and analysis of a wide range of pesticides. SPE would also be an acceptable cleanup option for this work, however, because pesticides are of varying polarities, SPE method development would have been quite intensive and may not have produced the high recoveries of all of the varying pesticides that can be achieved with QuEChERS, which provides a wide analyte screening.

Figure 2. Spinach extracts after liquid partitioning step with 1 percent acetic acid in ­acetonitrile and magnesium sulfate. The organic phase was heavily pigmented in dark green.

Figure 3. Spinach extracts after dSPE cleanup. GCB removed a majority of the pigment from the sample matrix and the extracts were clear with a light green tint.