[6] Protein concentration measurements: The dry weight

A dry-weight protocol consists of pipetting protein solution into standard preweighed, clean, and heat-dried weighing bottles; weighing to 0.1 rag; heating in an air oven; and cooling in a desiccator and reweighing. The heating and weighing operations are repeated until constant weights, +0.1 mg, are achieved. This chapter focuses on the precision of an operationally defined dry weight, rather than on accuracy in terms of an absolute value. The procedure described in the chapter to obtain better precision on this basis is one which can be practiced in almost any well-equipped laboratory; no unusual equipment or specialized expertize are required. It is assumed that a small vacuum oven and a well-maintained analytical balance, sensitive at least to 10μg, are available. Balances with maximum loads of the order of 20 g and graduated to 5 μg are very suitable. Protection against drafts and unusual vibrations is required and the temperature and relative humidity of the room should not change rapidly during a series of weighings.