Freeze-drying is an excellent way of drying thermally sensitivematerials and preserving thermally labile compounds. It is ex-tensively used in the pharmaceutical and biotechnology indus-try to produce active pharmaceutical ingredients (e.g., smallmolecules and proteins), which then can be conveniently dis-tributed and stored. Freeze-drying is also used in other sec-tors, for instance, in food industry (instant coffee), bacteriology(strain conservation), or chemical synthesis. Moreover, freeze-drying is a versatile means for processing of reference materi-als (RMs) for various application fields (e.g., clinical chemistry,food, and environment). Such RMs and especially certified RMs(CRMs) are important tools enabling laboratories all over theworld to deliver accurate measurement results of demonstratedreliability.1–3Considering the major efforts required for planning, process-ing, characterizing, and certifying a RM, the production of largeand stable batches is preferred, so that the CRM is available forseveral years after production. To achieve the desired long-termstability of biological materials, removal of water is essential.Freeze-dried materials can be kept for many years for most ma-trix/analyte combinations, provided that they are stored at anappropriate storage temperature. Obviously, similar require-ments apply for many pharmaceutical products with respect to stability and therefore freeze dryers are also widely used inpharmaceutical industry.A freeze-drying cycle normally consists of the following threesteps:1. Freezing of the water present in the matrix at ambientpressure.2. Sublimation or primary drying, whereby the water isevaporating from the solid ice in the material and cap-tured on the condenser under vacuum.3. Secondary drying where most of the remaining water isremoved under hard vacuum and temperatures above0◦C.During freezing, it is of interest to produce as large ice crys-tals as possible in the matrix.4,5Large crystals contribute tothe formation of larger pores in the material to be dried, whichresults in a higher flux of water vapor from the material duringprimary drying. However, high freezing rates result in smallerice crystals and smaller pores.6Hence, different ice crystalstructures result in different flux of water vapor.During sublimation, the chamber pressure is normallyaround 0.2 mbar (for the equipment used in this study) andthe water vapor that leaves the ice is captured on the ice con-denser placed under the freeze-drying chamber. When the sub-limation step is approaching the end, the pressure is furtherreduced in the chamber to about 0.005 mbar, and the sec-ondary drying is started. At the same time, the shelf temper-ature is further increased on the shelves so that most of thelast free water can be removed from the matrix. Monitoring