QuEChERS – Where to start?22 Jan 2015
I am often asked for QuEChERS product recommendations. Usually the request is something like, “I need to test pesticides in _____” and that blank can be just about anything…sometimes not even food.
Unless it is something I have tested before, I usually take a quick look online to determine the approximate composition of the food. I do this because standard QuEChERS methods work best for high water, low lipid, and low carbohydrate commodities. The general recommendation is that the sample should contain about 80% water. If this isn’t the case, then adding some water is helpful and often needed for the extraction to work properly. In addition, high lipid samples can be challenging and you want to have a high level of C18 in your dispersive cleanup step.
The USDA Nutrient Database is a great resource for finding this information. Many foods are included and values for water, protein, lipid, carbohydrate, fiber and sugar levels are typically listed. The really great part is that these values are listed on a 100 g basis so the values are the same as percent weight.
I started checking this every time I test a different food because looks can be deceiving. The first time I tested spinach, I thought it looked “dry”. When I checked the USDA nutrient database, I saw spinach was listed at ~90% water. I was surprised. I did not add any water and the extraction worked perfectly.
In addition to water content, I take a look at the lipid and sugar values. If the lipid is “high” (above ~5%), I want to make sure to have some C18 sorbent in my dSPE tube to help remove coextracted lipids. Sugar is removed by PSA sorbent so knowing this value will help determine the amount of PSA in your cleanup.
U.S. Department of Agriculture, Agricultural Research Service. 2014. USDA National Nutrient Database for Standard Reference, Release . Nutrient Data Laboratory Home Page, http://www.ars.usda.gov/nutrientdata