Sample preparation lipids/salts

Sample preparation used to extract proteins from cells prior to analysis is an important step that can have an effect on the accuracy and reproducibility of the results. Proteins isolated from bacterial cells will have co-extracted contaminants such as lipids, polysaccharides, and nucleic acids. In addition various organic salts, buffers, detergents, surfactants, and preservatives may have been added to aid in protein extraction or to retain enzymatic or biological activity of the proteins. The presence of these extraneous materials can significantly impede or affect the reproducibility of analysis if they are not removed prior to analysis. 

As discussed in Section 19.3.5.3, mimicking human digestion for the assessment of Se-bioavailability could be practically regarded as early examples of sequential enzymatic sample preparation applying, for example, proteolytic (pepsin), mixed (pancreatin), and non-proteolytic (amylase) enzymes, and digestive compounds like bile salts for lipid emulsification. On the other hand, the simultaneous use of enzymes was not considered a source of self-digestion problems as this phenomenon is common in the digestive tract. 

Other body fluids such as saliva or milk are also sometimes analyzed. Saliva contains approximately 0.3% protein (mostly enzymes), 0.3% mucin, and some salts. It can often be analyzed directly, without sample preparation or extraction. Human milk contains mainly proteins (3%), lipids (3%), and carbohydrates (mainly lactose, 6.8%). Lipids are suspended in the form of droplets, so homogeneity of the studied sample must be ensured. From the third week from the start of lactation, composition of human milk is quite constant, but that of the initially secreted colostrum is significantly different. Milk samples are commonly used for the trace analysis of pesticides, heavy metals, antibiotics, and some drugs. 

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