Sample Preparation Selection

With the increasing awareness of food safety in both developed and developing countries, analysis of a variety of imported and exported commodities is a priority concern for the national competent authorities. Sample preparation of foods such as vegetables, fruits, dairy, and meats followed by downstream high-pressure liquid chromatography (HPLC), liquid chromatography with tandem mass spectrometry (LC/MS/MS), or gas chromatography with mass spectrometry (GC/MS) analysis is a practice that helps ensure the safety of consumers. Sample preparation is a huge challenge in this analytical process for two primary reasons. First, typical food testing assays can include many analytes with widely varying chemical properties, and second, sample matrices are complex and often contain compounds that can interfere with analysis. For instance, the avocado matrix is rich in lipids that cause ion suppression in mass spectrometry (MS) analysis, leading to inaccurate results when tested for a particular set of analytes.

All analysis begins with sample preparation, whether it’s a simple dilution or filtration or uses more targeted techniques such as LLE (liquid-liquid extraction), QuEChERS (quick easy, cheap, effective, rugged, and safe), or SPE (solid phase extraction). Because accurate analysis is required in food safety testing, sample preparation plays an integral role and directly affects downstream analytical results.

Sample preparation helps ensure that accurate and reproducible results are produced across a wide variety of food sample matrices. As regulations change and become more strict, analysts are challenged to develop robust analytical methods that reach even lower detection and quantitation levels. More selective sample preparation methods are employed in some cases; while in other situations, a less specific technique focusing on simple matrix removal may be more effective.

Matrix interferences such as lipids, proteins, and carbohydrates have become a limiting factor because they can cause ion suppression or enhancement, making it difficult to accurately identify and quantify the target analytes. Further, without adequate cleanup, these troublesome components can damage or shorten the lifetime of laboratory instrumentation. Thus, sample preparation for matrix interference removal in food samples is extremely important in order to achieve proper performance requirements and to improve the “shelf life” of instrument systems.

Selecting the most effective sample preparation technique to achieve your overall analysis goals saves time that would otherwise be spent on trying different techniques based on trial and error.

Sample preparation can also represent a major bottleneck in the analytical laboratory. It’s estimated that the sample preparation step can make up 60 to 70 percent of the total time required for analysis. Estimates also show that 30 percent of analytical errors originate from the sample preparation step. Most food testing labs follow approved official/standard methods that often specify a validated sample preparation procedure. In some cases, when special needs must be met, analysts are granted flexibility to deviate from the official method and select a more appropriate sample preparation technique.

Straightforward, non-specific sample preparation techniques such as weighing, dilution, or filtration typically work well and can adequately achieve the goals of sample preparation for simple sample matrices.

For more complex and dirty sample matrices, more intricate and selective extraction/cleanup methods such as LLE, QuEChERS, and SPE are required to transform samples into compatible formats for GC/MS and LC/MS/MS analyses. (See Figure 1.)

Each approach has its own benefits, drawbacks, and specific uses and no one approach is superior to the other. However, choosing the correct approach can significantly help in analysis goals.

Figure 1

LLE

Probably the simplest and most typical sample preparation approach for a variety of sample matrices is LLE. In LLE, homogenized sample is added to a biphasic system containing an aqueous phase and an organic phase. The target analytes will partition into either layer, which can then be isolated for further analysis.