Dry-down

Another aspect to consider in the design of closed-drying systems is the drydown time. The drydown time is the period requited for the system to dry down from its initial water concentration (or partial pressure) to a concentration that approaches equihbrium with the desiccant. During this time, the system is not fljlly protected from the negative effects of the moisture that the desiccant is designed to remove. In such a system, the instantaneous drying rate is proportional to the water content at any time (18). 

Lentges, G., Oetjen, G. W., Willemer, H., Wilmanns, J. Problems of measurement and control in freeze-drying down to -180 °C, p. 707-715. International Institute of Refrigeration (XIII Cong. Washington 1971)... 

Figure 2.8.1 shows a typical installation for flasks and other containers in which the product is to be dried. The condenser temperature for this plant is offered either as -55 °C or as -85 °C. For this type of plant, a condenser temperature of -55 °C is sufficient as this temperature corresponds with a water vapor pressure of approx. 2.1 10 2 mbar, allowing a secondary drying down to approx. 3 10-2 mbar. This is acceptable for a laboratory plant, in which the limitations are not the condenser temperature but the variation of heat transfer to the various containers, the rubber tube connections and the end pressure of the vacuum pump (2 stage pump, approx. 2 10 2 mbar). Figure 2.8.2 shows that these units are designed for very different needs. The ice condenser in this plant can take up 7.5 kg of ice at a temperature down to -53 °C. 

Stability — Samples remain stable for at least 468 days when frozen at -20°C. They are stable for at least five simulated freeze-and-thaw cycles and approximately 28 hr at room temperature. The analyte is viable for at least 6 days in a reconstitution solution stored in the autosampler (temperature set point at 10°C). A dried-down batch (sample process stopped at dry-down step) was stable at least 5 days in a refrigerator (temperature varied from 4 to 8°C). A stock solution of paricalcitol is stable for at least 11 months. Stock solution of internal standard is stable about 4.5 months under refrigeration. 

Recovery — Recovery control (RC) solutions were prepared in 10/90 v/v ACN/water. Recovery evaluation (RE) samples were prepared in human plasma. Aliquot of RC solutions into assay plates followed sample preparation procedure steps 1 and 2. Instead of adding 50 pL of diluent, wells containing RC solutions were dried down under a steady stream of room temperature N2. The dried wells were then reconstituted with 250 pL of diluent. Reconstituted RC solutions were directly injected onto an HPLC analytical column, bypassing the extraction column. RE samples were aliquoted into an assay plate following normal sample preparation. RE samples were analyzed using the full extraction procedure (with extraction column). The analyte was tested at three concentration levels and the internal standard was tested at one. Mean extraction recovery for fenofibric acid varied from 93.2 to 111.1%, and mean extraction recovery for the Pestanal internal standard was 105.2%. 

The great advantage of materials such as silica gel and activated alumina is that they enable the gas to be almost completely dried. Thus, with silica gel, air may be dried down to a dew point of 203 K. Small silica gel containers are frequently used to prevent moisture condensation in the low pressure lines of pneumatic control installations. 

J4. Hay dries, after being cut, at a rate which is proportional to the amount of moisture it contains. During a hot (90°F) July summer day, this proportionality constant is 0.30 b . Hay cannot be baled until it has dried down to no more than 5 wt % moisture. Higher moisture levels will cause heating and mold formation, making it unsuitable for horses. 

Add this to a well of the dry-down In-Fusion plate. Mix contents briefly by pipetting up and down. 

The multilamellar dispersions or vesicles were formed in the conventional manner (4). Drying down the lipid and suspending it in the desired aqueous solution (0.1M NaCl, 0.01M tris, pH 7.5) yielded, on gentle agitation, vesicles of the appropriate size for microelectrophoresis... 

To increase the deposition surface for phospholipids or to make much thinner Llms, a method has been developed in which the lipids are dried down onto a Lnely divided particulate support such as powdered sodium chloride, sorbitol, lactose, or polysaccharide, giving a product known as proliposomes. When hydrating the dry powder while mixing with a whirl mixer, the phospholipids swell whereas the support rapidly dissolves to give a liposomal suspension of MLVs in an aqueous solution. 

The quantitative determination of amperozide in plasma using MALDI-MS was demonstrated (Jespersen et al., 1995). This study required addition of a stable-isotope-labeled amperozide as an internal standard and a typical liquid-liquid extraction procedure with dry down and reconstitution. The sample was then mixed with the matrix and analyzed. Linearity was achieved over a range of 2.5-40 [iM. Other examples have been demonstrated (Duncan et al., 1993) which also required the use of stable-isotope analogs as internal standards. These examples also utilized TOF instruments with nitrogen lasers. 

After extraction, these fractions should be dried to remove water. When dry, the extraction solvent is removed by evaporation and the sample is reconstituted with a solvent or mobile phase before injection. Care must be taken that these evaporated samples go completely back into solution. Sonicating the sample with your starting mobile phase is usually sufficient. However, at least the first time you perform an extraction, it is always good technique to sonicate the dry-down tube with a strong solvent and reinject this wash as a check that everything redissolved. For gradient work, the stronger of the two mobile phases is an excellent choice for this second sonication solvent. 

Pellets must then be cooled and dried down to a safe moisture content as rapidly as possible after processing, using air cooling. 

DiesselE, Willmann S, Kamphaus P, Kurte R, Damm U, Heise HM. Glucose quantification in dried-down nanoliter samples using mid-infrared attenuated total reflection spectroscopy. Applied Spectroscopy 2004, 58, 442-450. 

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