Freezing samples

Samples collected from the field should ideally be placed in boxes in order to prevent damage to the crop sample and to aid storage, although this often depends on the freezing facilities of the organization concerned. Where samples are boxed, untreated samples should not be mixed with treated samples. When freezing, samples should be separated by space or by using separate freezers for treated and untreated samples. 

In nature, the lowest temperature existing in the universe is 2.7 K. This background temperature depends on the presence of fossil photons from the big bang . On the other hand, in laboratory, it is possible to freeze samples of materials down to about 10 6 K. Moreover, in condensed matter laboratories, it is nowadays possible to cool a small number of atoms or molecules (MO6) down to MOOpK. 

The use of an Intense laser light source with biological materials Is accompanied by the concomitant problems of localized sample heating and the possibility of protein denaturetlon. A further complication Introduced by resonance Raman spectroscopy Is the Increased potential for photochemical destruction of chromo-phorlc metal centers as a result of the absorption of large amounts of Incident radiation. Both of these situations may be ameliorated by freezing samples to liquid nitrogen temperature ( 90 K), while the even lower temperatures made possible with a closed-cycle... 

With a Pasteur pipet, pick individual colonies into 1 mL of HT medium contained m 24-well plates. Feed with 1 mL of HT medium, and split when good growth commences Freeze samples in liquid nitrogen... 

Rescreen the picked colonies and expand positive cultures Freeze samples of these in liquid nitrogen and clone twice... 

Evaluations of various soil and sediment samplers have been reported [56, 57]. The sediment shovel proved highly practical, but was limited because small particles tend to be lost when the shovel is lifted [56]. A cryogenic sediment sampler was less convenient to use, but allowed the collection of almost undisturbed samples. Houba described a different device for the automatic subsampling of soil, sediment and plant material for proficiency testing [57]. In another study, Thoms showed that freeze-sampling collects representative sediment samples, whereas grab-sampling introduces a bias into the textural composition of the 120 mesh fraction, due to washout and elutriation of the finer fractions [58]. 

Isotherm data at high equilibrium concentration for modification from organic solvent, have also been obtained by the freeze sampling method. The adsorption isotherm of APTS on dehydrated mesoporous silica from toluene solution is given in figure 9.10. 

Figure 9.10 Adsorption isotherm of APTS on dehydrated (673K) mesoporous silica gel from toluene solution, by freeze sampling method.

Fish and sediment Add water containing acrolein and acrylonitrile to sample freeze sample, extract in vacuum GC/MS 0.025 /Jg/8 Sediment matrix 1011 recovery. Fish matrix 90% recovery Hiatt 1981... 

Comparisons of the two methods found that the urease method tends to underestimate concentrations in seawater and so we recommend the monoxime method for analysis of marine samples (Price and Harrison, 1987 ReviUa et al, 2005). With respect to estuarine samples, the monoxime method is more accurate and less affected by salinity than the urease method (ReviUa et al, 2005). There has been some concern that freezing samples may result in lower urea concentrations (Mulvenna and Savidge, 1992). Other researchers, however, have shown that freezing does not alter the urea concentration measured (Cochlan and Bronk, 2001 Price and Harrison, 1987). 

A preferred method for freezing samples is to use strips of aluminum foil (7- to 8-mm wide and 40-mm long) with an identification pressed in at one end with a pencil and the sample sitting on the other. Remove the tissue from the 20% sucrose solution, blot on a piece of paper towel, and set on the other end of the foil. Immediately with a forceps, plunge the foil strips with the tissue into the cold isopentane (Fig. 4.2). The tissue will stick to the foil and it can be placed in a vial in the liquid nitrogen bath and is subsequently stored in a -70°C freezer in individual small tubes. For sectioning, the frozen tissue is mounted later in O.C.T. on a chuck. 

Another method for freezing samples uses plastic molds. There are many varieties of small disposable plastic molds for one or more pieces of tissue to be positioned next to one another. Rather than freezing the tissue blocks individually... 

Freezing samples after sampling for the convenience of scheduling analysis is not an acceptable practice because it may cause delay in finding and responding to out-of-specification test results or may adversely affect the stability of a product that does not withstand freezing. 

The four main components of any fluorescence or phosphorescence instrument are the source of excitation, the sample cell, the detector, and the filters or gratings used to select the exciting and emitted radiation. Phosphorescence instruments must also include a Dewar flask for the liquid nitrogen used to freeze samples. Since phosphorescence instruments have the same four main components as fluorescence instruments, the common components of both will be discussed first. 

Freezing samples or initially filtering liquids can reduce these problems. In the absence of freezing, a preservative should be added to prevent decomposition of biota and accumulation of decomposition products. The analyst should plan to minimize the problem by prompt analysis and either mix the sample before taking an aliquot or sample the liquid and solid phases in proportion. 

Freeze samples and blanks in liquid nitrogen and store at —70°C until further analysis. 

Soil samples stored in an air-dry condition are relatively stable particularly when total contents are to be determined. Samples must be well mixed after long-term storage to minimize the affect of size separation. Storage of soil extracts is much more problematical due to precipitation from solution and sorption processes on the container walls and should be avoided when possible. This is particularly the case for soil solutions or when weak electrolytes are used as extractants. The common method of preserving samples is acidification, but is only suitable when speciation is not required. Biological activity can be minimized by freezing samples or by adding small amounts of toluene. 

Retest, pick six positive clones, grow up and freeze samples,... 

The olanzapine liquid-liquid extraction differs from the SPE extraction in that the optimal pH for olanzapine is 9. This extraction targets olanzapine alone (8). Furthermore, it is necessary to freeze samples that are suspected of containing olanzapine due to the oxidation of olanzapine to olanzapine-S-oxide in vitro. The freezing of the specimens prior to analysis and the addition of ascorbic acid during the extraction work to limit this reaction. 

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