Comparison of Different Experimental Techniques for the Measurement of Crystal Growth Kinetics

The relative merits and domains of application of three observation techniques (atomic force microscopy, Michelson interferometry, and laser confocal microscopy with differential interference contrast microscopy) for the investigation of crystal growth kinetics are discussed in the context of protein crystallization. Growth rate measurements on the same system under identical experimental conditions using different techniques show differences up to 5-fold in growth rate and a different behavior of growth rate as a function of supersaturation. These results are discussed in terms of differences in mass transport at the crystal interface during data collection in the first case and as data processing artifacts in the second. Guidelines are provided for the selection of the optimal observation technique in crystal growth studies as a function of the specific problem under investigation, showing that AFM is best suited for nanometere-size, slow processes; LCM-DIM for nanometer-micrometer size, slow and medium processes; and PSMI for micrometer size, fast processes.