QuEChERS approach optimization for low-moisture matrices – case of honey and brown rice flour

Last month, Nancy published a blog summarizing how to approach samples with less than 80% water. Today, I want to go into more detail on how to deal with different commodities with less than 20%. As Nancy said, QuEChERS was first developed for high-moisture matrices such as strawberries and spinach. However, the method is very adaptable for a variety of other commodities, even dry goods with some simple adjustments. So, why do we need the moisture?

QuEChERS is a technique based on the extraction of analytes of interest from the matrix using acetonitrile or similar water-miscible solvent, followed by separating the water and matrix from the solvent, aided using salts (salting-out effect). Water needs to be present to hydrate the sample so it is accessible to the solvent for extraction. Without sufficient water, extraction will be incomplete and result in poor recoveries. Samples rich in moisture have enough water already present to start the extraction process. In low moisture samples, water needs to be added to make up for the lack of moisture native in the sample. Ideally, the water and solvent amounts should be the same (e.g. 10 mL of each).

With that in mind, I’d like to focus on two matrices I worked with: brown rice flour and honey. These two have some similarities, such as moisture content below 20% and high carbohydrate content. On the other hand, there are many differences. For example, the carbohydrates present are different, with rice flour mostly consisting of starches, while honey contains mostly simple sugars, such as fructose and glucose. Also, honey contains almost no other nutrients (i.e. fats and proteins) while the rice flour has about 5-8% protein and 1-3% of fat. Another big difference is honey is a super-saturated liquid while rice flour is a fine, dry powder.

Assuming that hydrating flour will be more challenging than hydrating honey (after all, honey is a great sweetener for tea), I tested two main approaches for analysis of low-moisture food commodities using the original unbuffered QuEChERS salts (#25847). The first approach is to add the same amount of water as of extraction solvent (10 mL) and keep the commodity loading the same (10 g). The second approach is also to add 10 mL of water but to cut the commodity in half (5 g). The matrix and water were shaken for 30 minutes before extraction. The methods were tested with QuEChERS Performance Mix (#31152) and the comparison is shown in Figure 1 for selected pesticides (commonly occurring in rice). The raw recoveries (as area/area of internal standard) show very little difference between the methods, with exception of p,p’-DDT, which had a significantly higher response with method 2 (5g of flour with 10 mL of water). It is worth mentioning that the method 1, samples had more materials in the centrifuge tube which made it harder to manipulate, mainly shaking and removal of the supernatant.
Figure 1: Comparison of selected pesticide's recoveries in brown rice flour with 2 different extraction methods. Method 1: 10 g of brown rice flour and 10 mL water. Method 2: 5 g of brown rice flour and 10 mL of water.

Further tests of extraction salts showed that either original unbuffered or EN salts were preferable to AOAC salts (Fig 2A) for brown rice flour. The clean-up dSPE tubes tested were mostly with a high load of PSA and end-capped C18 (C18-EC). Namely, I picked #26125 (50 mg PSA, 50 mg C18-EC), #26124 (50 mg PSA), #26243 (50 mg PSA, 50 mg C18-EC, 7.5 g of GCB) and #26216 (25 mg PSA, 25 mg C18-EC). While most of the responses were similar for all dSPEs, the higher load of both PSA and C18-EC (#26125, Fig 2B) performed better for p,p’-DDT and endosulfan sulfate.

Figure 2: Comparison of selected pesticide's recoveries in brown rice flour. A: Comparison of recoveries after extraction with different extraction salts. b: Comparison of recoveries after cleanup with various dSPE.

With honey I used what I’ve learned with the brown rice flour for hydrating the sample; i.e. to use 5 or 7.5 g of honey with 10 or 15 mL of water. The evaluation of extraction salts showed that the more acidic environment of AOAC salts resulted in higher recoveries (Fig. 3A), especially for dicofol. Due to the possible presence of sugars and waxes, the tested dSPE had either PSA (#26124) or C18-EC (#26242) or a combination of both (#26125 and #26243) added. The best performing dSPE tube was #26124 with 50 mg PSA (Fig. 3B).

Figure 3: Comparison of selected pesticide's recoveries in honey. A: Comparison of recoveries after extraction with different extraction salts. b: Comparison of recoveries after cleanup with various dSPE.

To summarize, hydration is an important step in the extraction of low-moisture commodities. Reducing the matrix load may result in improved sample handling and better recoveries. While we can use the same approach to hydration to various commodities with a similar moisture level, optimization of extraction salts and clean-up methods should always be tailored to individual matrices.