Pharmaceutical solids sample preparation methods

In Chapter 18, we described solvent extraction and solid-phase extraction sample preparation methods, which are applicable to GC analyses as well as others. A convenient way of sampling volatile samples for GC analysis is the technique of head-space analysis. A sample in a sealed vial is equilibrated at a fixed temperature, for example, for 10 min, and the vapor in equilibrium above the sample is sampled and injected into the gas chromatograph. A typical 20-mL glass vial is capped with a silicone rubber septum lined with polytetrafluoroethylene (PTFE). A syringe needle can be inserted to withdraw a 1-mL portion. Or the pressurized vapor is allowed to expand into a 1-mL sample loop at atmospheric pressure, and then an auxiliary carrier gas carries the loop contents to the GC loop injector. Volatile compounds in solid or liquid samples can be determined at parts per million or less. Pharmaceutical tablets can be dissolved in a water-sodium sulfate solution... 

Solid-phase extraction, commonly abbreviated SPE, as a means to prepare environmental samples is a relatively recent alternative to LLE. SPE originated during the late 1970s and early 1980s as a means to preconcentrate aqueous samples that might contain dissolved semivolatile analytes that are amenable to analysis by gas chromatographic determinative techniques. SPE was first applied to sample preparation problems involving clinical or pharmaceutical types of samples and only much later evolved as a viable sample preparation method in TEQA. 

For a liquid or semi-solid pharmaceutical dosage form, it is crucial to include a preservative in the formulation. Commonly used preservatives in these systems include sodium benzoate, EDTA, sorbic acid, and parabens. A generic HPLC method is also recommended for the preservatives used in liquid formulations for routine monitoring to ensure the stability of the preservative itself and it must be validated specific to its use with the dosage form. (See chapters on Sample Preparation and Method Development.)... 

The use of solid-phase extraction (SPE) for the sample preparation of drugs and pharmaceuticals has increased over the last 15 years because of ease of operation, increased selectivity with many new phases, and interfacing of automation and robotics. A simple strategy now exists for SPE methods development of drugs and pharmaceuticals, which makes sample preparation extremely straightforward. This strategy is the use of generic mixed-mode SPE and is discussed in detail in this chapter. 

Complexation with PAN immobilized on a Dowex 50Wx4 cation exchanger makes a basis of the determination of Zn in human hair, natural water and pharmaceutical and cosmetic formulations [11]. Flow-injection solid-phase spectrophotometry using TAN immobilized on silica gel was proposed for the determination of Zn in pharmaceutical preparations [12]. Adsorption of the complex with PAR on Sephadex anion exchanger makes a basis of solid phase spectrophotometric method for the determination of Zn in environmental samples [13]. 

The optical microscope is a valuable tool in the laboratory and has numerous applications in most industries. Depending on the type of data that is required to solve a particular problem, optical microscopy can provide information on particle size, particle morphology, color, appearance, birefringence, etc. There are many accessories and techniques for optical microscopy that may be employed for the characterization of the physical properties of materials and the identification of unknowns, etc. Utilization of a hot-stage accessory on the microscope for the characterization of materials, including pharmaceutical solids (drug substances, excipients, formulations, etc.), can be extremely valuable. As with any instrument, there are many experimental conditions and techniques for the hot-stage microscope that may be used to collect different types of data. Often, various microscope objectives, optical filters, ramp rates, immersion media, sample preparation techniques, microchemical tests, fusion methods, etc., can be utilized. 

The ultimate development in the field of sample preparation is to eliminate it completely, that is, to make a chemical measurement directly without any sample pretreatment. This has been achieved with the application of chemometric near-infrared methods to direct analysis of pharmaceutical tablets and other pharmaceutical solids (74-77). Chemometrics is the use of mathematical and statistical correlation techniques to process instrumental data. Using these techniques, relatively raw analytical data can be converted to specific quantitative information. These methods have been most often used to treat near-infrared (NIR) data, but they can be applied to any instrumental measurement. Multiple linear regression or principal-component analysis is applied to direct absorbance spectra or to the mathematical derivatives of the spectra to define a calibration curve. These methods are considered secondary methods and must be calibrated using data from a primary method such as HPLC, and the calibration material must be manufactured using an equivalent process to the subject test material. However, once the calibration is done, it does not need to be repeated before each analysis. 

There are many sample preparation procedures published in the scientific literature, and within the scope of this chapter, only the most current and popular methods will be discussed. By far, the commonest and most popular method used for pretreatment of liquid samples is solid phase extraction (SPE) [40,41]. For solid samples, several techniques are available including supercritical fluid extraction (SFE) [42,43], microwave-assisted solvent extraction (MASE) [44,45] and accelerated solvent extraction (ASE) [46,47]. Solvent extraction methods have long been established as the standard approach to sample preparation, but the increasingly demanding needs of industries like the pharmaceutical, agrochemical and petrochemical for greater productivity, faster assays, and increased automation have led to the development of newer ways of sample preparation summarised in Fig. 2.3. 

Solid-phase extraction (SPE) is the method of sample preparation that concentrates and purifies analytes from solution by sorption onto a disposable solid-phase cartridge, followed by elution of the analyte with an appropriate solvent. The SPE technique was developed in the mid-1970s as an alternative means of liquid-liquid extraction but become particularly attractive for its automation, parallel purification, and pre-concentration. Since 1995, SPE has been applied in various fields, environmental, food sciences, biomedical analyses, pharmaceutical analyses, and organic synthesis. " There are a numbers of publications and reviews on the subjects of development of new solid-phase supporting materials, instrumentation and device, techniques, and theoretical aspect. ... 

The use of NIR reflectance has gained considerable interest for process analysis in the food, pharmaceutical, petrochemical, and other chemical industries. This method requires little or no sample preparation and involves measurement of the reflectance of a liquid or solid sample relative to that of a standard in the same wavelength series. As illustrated in Table 3, process NIR has gained considerable use for moisture determination in coal, cosmetics, and detergent powders, as well as for protein contents of cereal and grain, and hydrogenation of unsaturated fats and oils. 

Photoacoustic spectroscopy (PAS) [92] - [95] is a fast, nondestructive method for analyzing various materials in the gas, liquid, or solid state with virtually no sample preparation. In cases where samples are insoluble, difficult to grind into a powder, or of irregular shape—such as coal, carbon-filled and conducting polymers, pharmaceutical preparations. spin coatings on fibers, and coalings on irregular surfaces—photoacouslic spectroscopy... 

The earliest applications for quantitative analysis of liquid samples and solid preparations entailed sample dissolution in an appropriate solvent. A number of moisture determinations in APIs and pharmaceutical preparations based on both reflectance and transmission measurements have been reported. Their results are comparable to those of the KF method. The high sensitivity provided by the NIR technique has fostered its use in the determination of moisture in freeze-dried pharmaceuticals. ° The noninvasive nature of NIR has been exploited in determination of moisture in sealed glass vials. " " ... 

Within the pharmaceutical industry there has always been a need for sample purity. Any compound that is a potential drug candidate can only be fully characterised and tested once it is available in a pure form. There are many purification tools available for sample clean-up, e.g. flash chromatography, solid phase extraction, etc. 1-31. However, for the more complex purification problems where the desired compound and its associated contaminants have very similar polarities, structures, etc., preparative chromatography is the method of choice due to its superior separative capabilities. Preparative chromatography can also be scaled up from lens of milligrams to tens or even hundreds of grams of compound. The other main factor in favour of this technique is its ability to be tailored for most classes of compound. 

Because throughput continues to drive pharmaceutical analytical chemistry, future challenges in the automated solid-phase extraction workstation realm could involve a combination of parallel 96-well liquid handling (as demonstrated by the Tomtec Quadra-96) with fully automated extractions. This total automation could include the method development steps such as standard preparation and sorbent/solvent selection, removal of operator interventions such as positioning of collection tubes, and injection of samples into the chromatographic or electro-phoretic system. Along this line, it seems inevitable that a dedicated solid-phase extraction workstation with all of these features will appear by the year 2003. 

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