The problem with placing samples in ice: non-uniform plate cooling
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Plate directly on crushed ice does not reach 4°C in any of the wells and well-to-well temperature is uneven.
The thermoconductive sink solution: uniform plate cooling
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Plate placed on a thermoconductive sink module and then placed on ice shows more uniform well-to-well temperature and all wells at or below 4°C (clue center plate wells are slightly warmer due to curvature of the underside of the plate).