Sample preparation degradation minimization

Except for bulk drug substances, samples can rarely be analyzed directly following simple preparation steps. In many cases, an active ingredient is found in a formulated form or in a physiological fluid or tissue. Likewise, interferences associated with the matrix and the presence of possible degradation products, metabolites, and other closely related compounds mean the target analyte is often highly diluted. Also, an analyte may be difficult to detect. Effective sample preparation steps serve up to three broad purposes (1) eliminate and/or minimize possible interferences, (2) concentrate the sample, and (3) render the analyte of interest into a more easily detectable form. 

Special care must always be exercised in the study of parts-per-billion concentrations of organics in water to ensure minimal losses due to sample degradation, adsorption or absorption to process materials, and other similar losses. These issues were addressed by dividing the measurement procedures into two parts (1) sample preparation and (2) analytical method, or finish. Because well-defined sample preparation steps were not available from the literature for the quantitative determination of parts-per-billion concentration levels of most of the model organic compounds in water, a considerable amount of effort was placed on the development of appropriate procedures for such measurements. In particular, each method was developed with the indent to have a procedure that could verify the presence of appropriate concen-... 

Because of the favorable sorptive properties of the reversed-phase supports, batch adsorption and desorption can be a very effective way to desalt a chromatographed sample or to partially fractionate a peptide mixture during a purification procedure. For example, 1-2 gm of an oc-tadecyl silica packed into a silanized glass or plastic pipette can be used for the batch fractionation of small amounts of a crude peptide extract from tissues, such as the pancreas or pituitary, or from a synthetic experiment. A number of commercial products, such as the Waters Sep-Pak, have found use in this manner 10) as a purification or sample preparation aid. Protocols for batch extraction procedures on alkyl silicas have been discussed 17a,b) and applied to neuropeptides 10, 158, 166) and other hormonal peptides 88, 162, 167, 168). With these methods recoveries of peptides present in a tissue extract are generally higher than those found with classical fractionation techniques due in part to the fact that proteolytic degradation is minimized. 

Recently, ICH guidance Q6A has simplified the development of specifications in several ways, not the least of which is the clarification that impurities if already controlled in the API do not have to be controlled in the dosage form unless they are also degradants. For the release assay, this paves the way for simpler, but no less sophisticated methods that require minimal sample preparation. Thus, the future may bring a return to spectroscopic techniques such as ultraviolet/visible (LJV/vis) spectroscopy. There also may be increased use of other high-speed and high-precision techniques such as flow injection analysis (FIA) and near infrared (NIR) analysis. 

The final goals of pesticide analyses are to obtain the cleanest possible samples, to determine the minimum possible concentration with the lowest limits of detection, and to avoid pesticide degradation during transfer to the laboratory. All this means that the accuracy and precision of a method for pesticide analysis will be directly dependent on the sample preparation procedure used. This operation is the most time-consuming and labor-intensive task in the analytical scheme. In response to the need for effective, robust, reliable sample preparation, a number of procedures have developed for fast, simple, and, if possible, solvent-free or solvent-minimized operation. Most such procedures, both conventional and new, are used for the analysis of pollutants in air, water, soils, sediments, and biota. ... 

During method development, it is critical that the derivatisation reaction is rapid, quantitative and produces minimal by-products or side reactions. These criteria are not always easy to achieve, hence, derivatisation as a sample preparation method is quite often chosen as a last resort. In pharmaceutical analysis, the use of derivatisation as a means of sample preparation is not usually the method of choice because the primary objective of any method development is to detect all major impurities and degradants in batches of the dmg substance. Quite often, many of these compounds will not have derivatisable functional groups or have inpurities that are present at such low levels that the derivatisation reaction is not optimised. 

Pyrolysis techniques for thermally degrading polymers have high sensitivity, allow trace analysis of all organics in liquid or solid state, require minimal sample preparation and small amounts of sample, and allow simultaneous identification and quantification in one run. However, they may have limited reproducibility and limited applicability where inorganic fillers are present (54). There have been various types of pre- and post-pyrolysis derivations done to simplify analysis by GC separation and reduce interferences from surrounding materials (55). 

Direct Inlet Probe (DIP) The DIP is one of the earliest techniques used to introduce nonvolatile or highly insoluble samples into mass spectrometers. This technique is still popular today because it provides a means to perform rapid sample analysis with minimal or no sample preparation [27, 28]. Applications of the direct-probe technique include quality-control analyses and the screening of drug, polymer, and synthesis samples. It has also been used to study the temperature programmed decomposition of synthetic polymers and inorganic materials, to characterize the molecular properties of materials under thermal degradation conditions. The same is also true of the studies with lAMS (see Sect. 6.4). 

Studies of the metabolism of vincristine and vinblastine have been complicated by chemical alterations of the drugs that occur during processing of samples for analysis. Extraction of these compounds under acidic conditions has been reported to minimize chemical degradation, and HPLC studies of the metabolism of vincristine indicate that microsomal preparations from mouse liver, but not those from human rhabdomyosarcoma tissue, convert vinblastine to 4-deacetyl vinblastine in vitro (38). 

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