Owner manual

Chapter 1: Introduction

Product Service 1-800-522-7658

vapor is from the sample to the collector. Since this vapor diffusion process

occurs very slowly under normal atmospheric conditions, a good vacuum is

essential to maintain an efficient rate. In many applications, the maintenance of a

vacuum of 0.133 mBar or less is recommended.

The rate of freeze drying is directly proportional to the vapor pressure and the

vapor pressure is dependent upon both eutectic temperature and solute

concentration of the sample. For example, a solution of sodium chloride and

water would freeze dry at a slower rate than pure water. The eutectic temperature

of a sodium chloride solution is about –21°C and at this temperature the vapor

pressure is about 1/16 that of water at 0°C. Although the eutectic temperature is

not dependent upon the concentration of sodium chloride, the vapor pressure of

the water would decrease as the concentration of sodium chloride increased. This

is due to the fact that as the solute concentration increases, less of the surface area

of the frozen sample is occupied by water. In general, most solutions or

biological samples will have a eutectic temperature of –10° to –25°C. However,

if the sample contains a simple sugar such as glucose or if the sample is animal or

plant tissue, the eutectic temperature may be as low as –30° to –50°C.

Freeze Dry Capacity

The volume of a sample that can be freeze dried at one time is related to factors

discussed previously and the size and design of the freeze dry system. With any

given instrument, the capacity is based on the surface area of the sample, the

eutectic temperature and concentration of the sample and the rate and amount of

heat transferred to the frozen sample. Of these factors, the eutectic temperature is

the most important factor in determining the amount of sample that can be freeze

dried at one time, particularly when flasks are used. This is because as the

eutectic temperature decreases, the vapor pressure decreases but the rate of heat

absorption by the sample does not change. This tends to promote melting of the

sample, which leads to a marked increase in vapor pressure and ultimately

overloads the collector and vacuum pump. Samples that have eutectic

temperatures of –20°C or lower should be placed on the freeze dry system one

flask at a time so that the vacuum in the system may recover before adding

another sample to the system. If the vacuum does not recover, the capacity of the

freeze dry system has been exceeded and the sample should be removed.

If there is a problem with a particular type of sample melting when placed on the

freeze dry system, dilution of the sample with more water or providing some

insulation around the flask to decrease the rate of heat absorption by the sample

may help. If the eutectic temperature of the sample is –40 to –60°C, the freeze

dry system selected for use must be equipped with cascade type refrigeration so

that the collector temperature can be cooled to below –75°C, or a dry ice/solvent

trap may be used between the collector and the vacuum pump.