External control for SPME in solid food samples: analysis of volatile compounds in raw beef

Abstract

Solid phase microextraction (SPME) is the most used technique for direct analysis of volatiles in solid samples due to its easy automation and implementation, but especially to its high sensitivity. For complex matrices, such as meat, comparisons among samples are usually found using just raw areas or area percentages. In the first case, there is no control over changes related to the fiber adsorption‐desorption or the detector sensitivity. In the second case, the problem arises when all the compounds vary in the same way and, therefore, no differences between samples are observed. The difficulty in quantification lies on the heterogeneity of solid samples, associated with unspecific variations in sample‐gas and gasfiber distribution coefficients. Furthermore, fiber has to be changed among experiments that last for a long period of time. Little variations in fiber´s coating have a direct impact on the number of molecules adsorbed on it. Therefore, SPME is extremely sensitive to matrix variations and variations between fibers. Problems might be solved with an internal standard (IS) with similar distribution coefficients. Moreover, there are other considerations to be taken into account such as finding an appropriate IS with similar volatility and no coelution with any other compound with the same mass to charge ratio, and the development of the procedure for the addition of the IS to the sample. This work presents a new strategy to control irreproducibility and to quantify 25 volatile compounds in raw beef by HS‐SPME. In the proposed method, 4 g of raw beef knuckles (rectus femoris) were transferred to a 20 mL headspace vial and a PDMS/DVB fiber was exposed to the headspace of the sample for 40 min at 37 °C. Volatiles were analyzed in a GCMS using a DB‐WAXETR column. In the final developed strategy, dipropylene glycol control solution was analyzed by HS‐SPME‐GC‐MS every 16 meat samples. This control solution contained one representative compound of each studied family in beef (alcohols, furans, saturated ketones, unsaturated ketones, saturated aldehydes, alkenals and acids) and two IS for monitoring control solution stability. Using the most selective mass for each control compound, response factors were calculated for each family. These response factors were subsequently applied to each compound that belonged to the same family. This strategy was able to control the quantification procedure even if the fiber, the column or the control solution changed, with DSR (%) below 12% in the control solution.