Minimizing Sample Degradation

In electron microscopy a sample is bombarded with a finely focused beam of monochromatic electrons. Products of the interaction of the incident electron beam with the sample are detected. If the sample is sufficiently thin—up to 200 nm thickness—the beam is transmitted after interacting with the sample, leading to the technique of transmission electron microscopy (TEM). TEM is used to probe the existence of defects in crystals and phase distributions. Scanning TEM instruments have been recently developed to obtain images over a wider area and to minimize sample degradation from the high-intensity beams. 

Fig 3 a,b,c Fo, Fpl, and Fm signals from pea chloroplasts taken during one day from the same batch of plants. To minimize sample degradation in the flow cell, a separate liter of chloroplast suspension was used at each emission wavelength for the Fpl measurements. The sample was more stable during the Fo and Fm measurements. The amplitude of the very fast (13 - 17 ps) component was very high in these samples and has been reduced by a factor of 2 to fit on the same scale as the other components. 

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). 

Supercritical fluids, in general, are inexpensive, contaminant-free, and less costly to dispose of safely than conventional organic solvents. Extracts are obtained under mild conditions that minimize thermal degradation, they are usually solvent-free or in a concentrated form, and no evaporation steps are needed prior to the final assay. However, the disadvantages of SFE should not be also ignored. As with all extraction methods, there are analytes and matrices for which SFE is not suitable. Some compounds are insoluble and may need solvent extraction. Aqueous matrices can cause problems and samples may need to be freeze-dried. Like all single-step extractions, SFE has only limited selectivity, but some distinction can be made between analytes by careful selection of temperature and pressure. 

A base extraction procedure was developed to minimize the degradation of the performance of fused-silica capillary chromatographic columns used to analyze XAD resin extracts. The degradation of the capillary gas chromatographic column was apparently caused by acidic nonvolatiles called humic materials. The humic materials were absorbed on XAD resins and eluted by nonpolar solvents along with the nonpolar organic compounds of interest in the samples. The base extraction procedure removed approximately 84% of the humic materials present in the ether extract. 

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-... 

The coupling of two mass spectrometer systems has received attention in recent years. This system can be operated in an atmospheric pressure mode by passing the air matrix directly into the ionization source (II). This method minimizes sample contamination and degradation problems. Detection limits are compound-dependent and can vary over more than an order of magnitude for different families of hydrocarbons. For example, an aromatic hydrocarbon such as toluene cannot be detected at levels below 5 ppbv, whereas most aldehydes are detectable at levels as low as 50 pptrv. The tandem MS-MS system has the potential to be a useful detection system for organic compounds that do not store well in collection containers. 

Store purified anthocyanin extract at 4°C if subsequent analysis will be performed within 24 hr. Store sample for longer periods at -15°C or lower (preferably at -70°C) in a freeze-resistant container to minimize pigment degradation. 

We use a variety of preservation techniques in order to minimize the degradation of contaminant concentrations in soil and water samples. Preservation methods stem from the chemical nature of each contaminant and from the rate at which it will undergo irreversible changes in a sample. Preservation is achieved by the application of one or more of the following techniques ... 

Samples may also contain microorganisms, which may degrade the sample biologically. Extreme pH (high or low) and low temperature can minimize microbial degradation. Adding biocides such as mercuric chloride or pentachlorophenol can also kill the microbes. 

---