Monitoring the resin loading during peptide synthesis gives useful information about the progress of the assembly. This can easily done by quantifying the amount of Fmoc removed at each deprotection/coupling cycle via spectrophotometry.
The Kaiser test1 is a qualitative test for the presence or absence of free primary amino groups, and it can be a useful indication about the completeness of a coupling step. The test is based on the reaction of ninhydrin with primary amines, which gives a characteristic dark blue colour. The test requires minimal amounts of analyte and is completed within a few minutes.
When Fmoc removal data show anomalies, before performing a step that requires the use of particularly expensive materials, or after a critical step, and in general when assembling longer sequences, it may be useful to cleave and analyse a small amount of intermediate product.