Tandem Mass Spectrometry (MS/MS)
Tandem mass spectrometers allow rapid analysis of individual compounds in complex mixtures. Two mass spectrometers are coupled, separated only by a collision cell, all within one instrument. In general, the first (MS1) or second analyzer (MS2) can be set either to scan a mass range or to select one or more individual ions of a specific mass-to-charge ratio (m/z). The collision cell is utilized to further breakdown the ions by use of a neutral gas (e.g., nitrogen) and to transmit them to MS2 (Figure). This design enables four different analytical or scan modes, three of which are used by Mayo's Biochemical Genetics Laboratory for the purpose of newborn screening (Table 2).
For acylcarnitine profiling, MS1 scans a defined mass range and MS2 transmits fragment ions with a specific m/z value following collision-activated fragmentation. In this mode, the data system then correlates each detected ion to its respective precursor scanned in MS1 (precursor or parent ion scan). In a neutral loss experiment MS1 and MS2 are both scanned at the same rate with a constant m/z difference. The resulting spectrum includes only those compounds among precursor ions that fragment with a common neutral loss (a behavior indicating that they belong to a family of structurally related compounds. This scan is used for the generation of amino acid profiles.
The concentrations of a few, selected amino acids and acylcarnitine species are more accurately measured when taking advantage of an MS/MS analysis in selected reaction monitoring (SRM) mode, where the selection of a parent ion in MS1 is followed by a similar process for a specific fragment ion in MS2. The resulting signal corresponds exclusively to the transition from parent to product ion, a process virtually free of any interference irrespective of the specimen analyzed.