Free amino acids can be obtained from cell cultures, tissue cultures, tissue biopsies, etc. Physiological fluids present challenges because of their complex matrices. In addition, platelets in plasma, contamination of plasma from hemolysis, contamination of cerebrospinal fluid with blood, and drug metabolites in urine make analysis of physiological samples difficult. Whatever the source, sample preparation is the most important factor in physiological AAA and starts at the point of sample collection. For example, the tubes used for blood collection may include heparin or EDTA as anticoagulants, and these may interfere with analysis.
Amino acids can differ in the stability during storage.
The quality of buffers used for analysis is more important for physiological samples than for hydrolyzed samples. Their purity and consistency are critical, and all precautions must be taken to prevent mold growth. Prepackaged buffers may be preferred for ease of use and standardization. The availability of standards for quantification and detection of physiological samples is a problem. Because synthetic mixtures are poor mimics of real physiological samples, biological reference standards should be used.