Optimizing the Formulation of Poorly Water-Soluble Drugs
2.2 Solid-State Characterization
2.2.4 Specifi c Surface Area
dry blend of the materials at the same ratio. While a decrease in peak intensity can be observed for the 50:50 physical mixture, the peaks are still present, indicating that processing indeed rendered the drug amorphous. Additionally, it can be seen that at higher drug loading levels the process did not generate or stabilize the drug in the amorphous form (Tobyn et al. 2009 ) .
2.2.4.1 BET Surface Area Analysis
Surface area (SA) analysis of pharmaceutical powders is often assessed via gas adsorption methods. In these methods, a nonreactive gas is adsorbed to the surface of the material at or near the boiling point of the liquid adsorptive at a single or multiple pressures. The quantity of gas molecules required to form a monolayer of adsorbed gas on the powder surface is determined and using the average diameter of a single molecule the SA is determined (Condon 2006 ) . This is done via mathemati-cal modeling of the adsorption isotherms (amount adsorbed vs. adsorptive pres-sure), with the most established model being BET analysis. For a detailed summary of the theory behind BET isotherm analysis, the reader is referred to Condon 2006 .
Sample Preparation
As vials for BET analysis are often extremely narrow at the opening, it is imperative that care be taken not to damage or alter the sample during loading. This is espe-cially true for porous materials which may be collapsed if pressure is applied and brittle systems which may fracture during loading. Fracture may yield inaccurate higher surface areas, while collapse results in substantially reduced values.
Degassing
The removal of gases and vapors physically adsorbed onto the surface of the powder being tested, termed degassing, is essential prior to determining the specifi c surface area (SSA, i.e., m 2 /g) of any sample. Failure to do so may result in a reduction in the calculated surface area or a high variability in obtained SSA values (USP 32–NF 27 General Chapter 846). It has been suggested that an adsorbed impurity will not alter the BET surface area calculation, provided its boiling point is at least three times higher than the adsorption temperature (Joy 1953 ) . However, such impurities may infl uence the calculated heat of adsorption (C-value). Therefore, in order to ensure accurate, reproducible surface area measurements, proper sample preparation must be employed.
Due to factors such as temperature sensitivity, variations in surface energies, particle sizes, and porosity, no single method may be applied universally that guar-antees complete removal of adsorbed gases and vapors to the powder surface; how-ever, two methods are often applied: vacuum pumping and purging by use of an inert gas (Lowell and Shields 1991 ) .
When applying a vacuum, a pressure of 10 −5 Torr has been stated as suffi cient in outgassing procedures. The application of elevated temperatures will increase the rate at which impurities/contaminants leave the powder surface, reducing the time required to hold the vacuum (Fagerlund 1973 ) . However, heat–liable products must be monitored with care, and glass vessels used in most BET equipment have a thresh-old of 400°C (Igwe 1991 ) . Additionally, elevated temperatures may cause rapid water loss from the sample surface, resulting in altered morphology or sample col-lapse. Samples that are sensitive to heat should be degassed at ambient temperatures
under vacuum for long periods of time, such as 12–24 h, in order to reach constant weight (Lowell and Shields 1991 ; Engstrom et al. 2007 ) .
Alternatively, fl ushing the powder sample with an inert gas may also be used to clean the surface. This purge gas must be of extremely high purity and dry so as not to introduce contaminants or induce moisture-related phenomena such as crystalli-zation. Heat may also be provided in this method as well to aid in desorption (Sing et al. 1985 ) . For samples in which heat may not be applied, multiple absorption–
desorption cycles may be employed to clean the surface with three to six cycles providing suffi cient cleanliness for reproducible measurements (Dios Lopez-Gonzalez et al. 1955 ) . This may also be done using gas blends such as nitrogen (10% v/v) in helium (Pendharkar et al. 1990 ) .
Regardless of the chosen degassing method, the sample must be monitored by some means to ensure degassing is complete. With regard to vacuum pumping, lit-erature often describes the method of ensuring all adsorbed gases and vapors are removed as monitoring of the system pressure. Webb and Orr ( 1997 ) state that iso-lated samples displaying a pressure rise of <10 −3 Torr/minute are indicative of ade-quate degassing. Similarly, Gregg and Sing ( 1982 ) state a pressure of 10 −4 Torr as suffi cient, while Sing states ~10 MPa is satisfactory (Gregg and Sing 1982 ; Sing et al. 1985 ) . Allen ( 1997 ) states outgassing to be complete if, following 15 min of isolation from the vacuum, no pressure increase is observed upon reintroduction to vacuum, while Igwe deems completion as maintaining a pressure between 10 −4 and 10 −5 mmHg for a substantial period of time following isolation from the pumping line and cold trap (Igwe 1991 ; Allen 1997 ) .
As mentioned earlier, purging with an inert gas may be used to clean the sample.
In this case, the effl uent gas may be monitored via mass spectroscopy, or a thermal conductivity detector which may detect impurities in the range of a few parts per million (ppm). Once the level of contaminants and foreign material is below detec-tion, adequate removal of the adsorbed material has been achieved (Sing et al. 1985 ; Lowell and Shields 1991 ) .
Again, no single method can be employed universally to properly degas a sample prior to analysis. Understanding the morphology (i.e., porosity) and thermal sensi-tivity of a powder will aid in determining degassing time and temperature as a more porous network will require increased times. However, once a sample has been run under a given set of parameters, it is recommended that a second sample be run using an extended degassing time (i.e., 1.5–2× the original time). If the results are statistically signifi cantly different, the fi rst results may be discarded and the second set of parameters selected. Additionally, it is recommended that powders be ana-lyzed at least in triplicate using three individually prepared samples, especially for powders with a large particle-size distribution.
Sample Analysis
Prior to any sample analysis, the BET unit to be used should be properly calibrated for accurate results. This is can be performed using commercial reference standards
such as silica, garnet, or kaolinite (Antila and Yliruusi 1991 ) . These can most readily be purchased from the manufacturer of the equipment in use, such as the alumina pellets offered by Quantachrome, and range in SSA values from below 1 to greater than 150 m 2 /g. Standard selection will depend upon the samples to be analyzed, as the standard SSA should be near that of the samples. For example, Swinkels et al.
( 1994 ) used a kaolinite reference standard with a specifi c surface area of 16.2 m 2 /g to verify that no drift had occurred prior to sample analysis of the experimental samples (SSA 26–32 m 2 /g.
Following calibration, sample analysis can be performed. As mentioned earlier, care must be taken when adding the sample to the analysis bulb and the weight of the added powder must be known. The analysis bulb should be weighed empty on an analytical balance, the powder then added, and the fi nal weight of the bulb mea-sured and recorded. Subtraction of the bulb weight from the fi nal weight of the fi lled bulb allows accurate determination of the true amount of sample added. The amount of powder added will depend on density, as most commercial BET units require a minimum surface area rather than minimum weight. For example, the Nova ® Series offered by Quantachrome requires a minimum of 0.01 m 2 for accurate analysis.
While sample sizes may be low for high surface area powders such as those produced by spray freezing into liquid (200 mg (Rogers et al. 2002 ) ), higher amounts are necessary for lower surface area materials (5 g (Swinkels et al. 1994 ) ). Sample degassing can then be performed utilizing the appropriate method from those outlined above. Post degassing, the weight of the sample should again be taken to ensure accuracy when calculating SSA as some weight will be lost during the degas-sing process (Sing et al. 1985 ) . Analysis is then carried out with nitrogen often used as the adsorptive gas; however, for powders exhibiting extremely low SSA values it is necessary to use an adsorptive gas of lower vapor pressure. In such cases, krypton can be employed. It must be noted that differences in SA values will be obtained based on the gas used for the study as the size of the gas molecules forming the monolayer will differ resulting in differences in pore penetration (Sandell 1993 ) .
During analysis, the sample is repeatedly lowered into a liquid nitrogen bath.
While this process is automated by today’s units, the researcher is still required to maintain the level of liquid nitrogen in the Dewar to ensure proper submergence of the sample; failure to do so will result in inaccurate results. Data reduction can then be performed and the results reported as m 2 /g material.