Amino acid analysis sample treatment

The product of the addition reaction, lysinoalanine, has been observed on numerous occasions of treatment of proteins under alkaline conditions (7,8,13,14,15). The amino acid is stable to the conditions of total protein hydrolysis and is easily accounted for in amino acid analysis. A segment of the chromatogram showing lysinoalanine found in the sample from a canned food product for human consumption is shown in Figure 15. 

An internal standard should always be used for every analysis carried out. This is an amino acid that is known to be absent from the sample under investigation. For instance in blood plasma analysis either of the non-physio-logical amino acids, nor-leucine or a-amino-/3-guanidinobutyric acid, may be used. This should be added in a known amount to the sample prior to any sample pre-treatment (for example, removal of protein). 

The analysis of methionine and cysteine is problematic. The sulfur containing side chains of these amino acids are prone to oxidation. The standard hydrochloric acid hydrolysis will cause the partial conversion of these amino acids into cystine, cysteine, cysteine sulfinic acid, cysteic acid, methionine, methionine sulfoxide, and methionine sulfone. The classic strategy (79) for dealing with this problem is simply to drive the oxidative process to completion (i.e., convert all the cyst(e)ine to cysteic acid) and then to analyze chromatographically for cysteic acid and/or methionine sulfone. This is traditionally accomplished by a prehydrolysis treatment of the sample with performic acid. While this method has sufficed over the years, the typical recovery (85 -90%) and precision (4% intra- and 15% interlaboratory) have been poor (80). 

The amount of the individual amino acids released during the enzymatic treatment of wool was monitored by the HPLC method. Moreover, an XPS analysis of enzymatic-modified wool fabric samples and contact angle measurements were performed. The data obtained by the XPS method allowed comparison of the changes in the elemental concentration on the wool surface after enzymatic treatment. The results of the contact angle measurements demonstrated an increase in the wettability of the modified wool surfaces. 

In recent years there has been a growing interest in the use of electrospray ionization-mass spectrometry (ESI-MS) either as a stand-alone technique, or following an analytical separation step like CE, to study and measure a wide variety of compounds in complex samples such us foods (Simo et al. 2005). ESI provides an effective means for ionising from large (e.g., proteins, peptides, carbohydrates) to small (e.g., amino acids, amines) analytes directly from solution prior to their MS analysis without a previous derivatization step. Santos et al. (2004) proposed the use of CE-ESI-MS for the separation and quantification of nine biogenic amines in white and red wines. More recently, the possibilities of two different CE-MS set-ups, namely, capillary electrophoresis-electrospray-ion trap mass spectrometry (CE-IT-MS) and capillary electrophoresis-electrospray-time of flight mass spectrometry (CE-TOE-MS) to analyze directly biogenic amines in wine samples without any previous treatment has been studied (Simo et al. 2008). 

In a related study, Latorre et al. applied exploratory rank analysis to ascertain the number of components of complex nonresolved electrophoretic peaks of some amino acid derivatives (32). The performance of EFA, WFA, and MCR-ALS for following the evolution of overlapping species in the system was compared. It was found that MCR-ALS provided the best results in the case of strongly overlapping contributions. The simultaneous treatment of the sample mixture with data from standards of interest permitted the analytes to be successfully quantihed. 

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