Several different qualitative screens are available. In a qualitative screen only the presence or absence of a compound is determined, not its concentration. This is a useful way to screen for a large number of chemicals and then to proceed to accurately quantitate the level of only the compounds that are present in the sample.
In a qualitative screen, compounds with similar chemical or physical properties are grouped together, allowing the screening of a sample for a large number of different compounds in a single analytical run. Group 1 compounds, for instance, can be qualitatively assessed by LS/MS/MS with the instrument in positive mode. Group 2 compounds require the instrument to be in negative mode. Group 3 compounds are non-polar, and are screened by GC/MS/MS. Within each group, compounds are further divided into functionaly and chemically related packages. In Group 1, for example, there are packages of phosphorous pesticides or hepatotoxic microcystins. Chlorinated pesticides fall in group 3. This approach ensures that the maximum number of compatible compounds are analyed in a single run, providing the best value to the client. |