Mass spectrometry amino compounds detection

As a more sensitive detection method, MS can be very useful in amino acid determinations. For example, S-carboxymethyl-(R) cysteine or SCMC, is a mucolytic agent used in the treatment of respiratory diseases. The development of a method utilizing high performance IEC and atmospheric pressure ionization (API) mass spectrometry to quantify SCMC in plasma has been described.66 This method is simple (no derivatization needed), rapid (inn time 16 min.), sensitive (limit of quantification 200 ng/mL in human plasma), and has an overall throughput of more than 60 analyses per day. API-MS was used successfully with IEC to determine other sulfur-containing amino acids and their cyclic compounds in human urine.67 IEC has also been used as a cleanup step for amino acids prior to their derivatization and analysis by gas chromatography (GC), either alone or in conjunction with MS.68 69... 

GC-MS Gas chromatography-mass spectrometry is the most versatile method. It can be used for pyrolysates (see above), but also for the detection of PAHs in extracts. In combination with commercially available derivatization protocols, amino acids can be analyzed as well. If chiral columns are used, enantiomeric separation of chiral compounds is possible. Usually, either quadrupole or ion trap mass spectrometers are used as detectors, but Time-of-flight (ToF) MS can also be used. 

Many reported methods for BA and AA involve pre- or postcolumn (or capillary) derivatization of these compounds before detection using 3-(4-carboxybenzoyl)-2-quinoline-carboxaldehyde," 5-(4,6-dichloro-s-triazin-2-ylamino) fluorescein, l,2-naphthoquinone-4-sulfonate," < -Phthalaldehyde (OPA), fluorescamine, and many other procedures reported in Table 30.2. OPA is disadvantageous in that it reacts only with primary amines, and the fluorescent derivatives are associated with significant instability. Dabsyl- and dansylchloride are better in this respect as they react with both primary and secondary amino groups, and provide stable derivatives. Indirect UV, indirect fluorescence detection, conductivity," and electrochemical detection have been utilized after CE separation as well as mass spectrometry Kvasnicka et al. " developed a direct,... 

In early mass spectrometry applications of lasers, the sample was irradiated directly by a laser beam to desorb intact sample-related ions [27]. In this direct mode, termed laser desorption/ionization (LDI), the extent of energy transfer is, however, difficult to control and often leads to excessive thermal degradation. Also, not all compounds absorb radiation at the laser wavelength and thus are not amenable to LDI. Only those compounds that have mass below 1000 Da can be analyzed by LDI. Analytical sensitivity is also poor. A key contribution of LDI experiments is the observation that the desorption efficiency of amino acids and peptides that absorb the laser fight beam is greater than those without the chromophore [28]. IR lasers (e.g., an Nd YAG laser at 1.06 p m and a pulsed CO2 laser at 10.6 pm) and UV lasers (frequency-quadrapled Nd YAG laser at 266 nm) have aU been used. The detection of malaria parasites in blood by LDI with an N2 laser has been demonstrated [29]. 

AQC is one of the best precolumn fluorescence derivatization reagents for amino compounds [29]. Currently, MS/MS detection is frequently used for the selective determination of biological substances in complex mixtures. The reagent AQC reacts with primary and secondary amines to form aminoquinoline-labeled compounds via a carbamide linkage. These derivatives are separated by reversed-phase liquid chromatography and can be monitored by electrospray ionization—mass spectrometry. The loss of the aminoquinoline tag occurs readily and can be monitored by MS/MS detection, thus, metabolite analysis of amino compounds can be carried out [35]. 

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