Particulate sample preservation

Inorganic arsenic species, As111 and Asv, in natural water and anoxic seawater samples were not stable (Cutter et al., 1991). Rapid freezing and storage at —4°C was recommended as a means of preservation. Particulate samples were collected in acid-cleaned plastic bags, and then frozen. 

The obvious source of comet samples is by direct collection at a comet with Earth return. Stardust, the first comet sample mission (Brownlee et al., 2000), will collect the positively identified particulate samples from a comet and return them to Earth. Stardust, a NASA Discovery mission, will collect thousands of particles from the coma of SP comet Wild 2 and return them in 2006. Hopefully future sample return missions will, in addition, recover subsurface samples of ice and dust and return them to Earth with cryogenic preservation. 

The role of filtration of water samples at the time of collection and in relation to storage and preservation of the sample is often an important consideration. Many substances of interest may be present in a water sample in particulate as well as soluble form. Filtration removes particulate matter, so that a decision on whether to filter at the point of collection will depend on the objectives of the study. Another consideration relevant to filtration and the possible presence of particulate matter are the effects on such matter of adding a sample preservative such as acid. Generally, it is sound practice to filter before adding a preservative that may solubilize particulate matter or leach contaminants from it. 

In the methods for urea determination, filtration of the water samples is not strictly necessary, provided that the particulate matter does not interfere on the subsequent analytical steps. However, it is often carried out in order to improve the sample preservation and to standardize the sampling procedures. In these cases, precombusted glass microfiber filters are commonly used, as in the case of DON samples. 

Solutions containing Sbm and Sbv in deionised water at 0°C and 25°C in polyethylene containers were stable for 1 year (De la Calle Guntinas and Camara, 1992). However, samples of natural water, acidified to pH 2 or less, required rapid freezing to -4°C to avoid oxidation of Sbm. In anoxic seawater, concentrations of antimony species (Sbm and Sbv) in stored samples were about 49% lower than those determined at sea soon after the samples had been obtained (Cutter et al., 1991). Samples of particulate material were placed in acid-cleaned plastic bags or vials and then preserved by freezing. 

River, stream, and lake water groundwater as well as atmospheric precipitation are commonly analyzed by using ICP-MS [303], Often the samples can be run directly or after simple filtration or centrifugation to remove suspended particulates [304]. Typically samples can be preserved by the addition of 1% to 2% concentrated nitric acid by volume [305]. Nitric acid is preferable to hydrochloric or sulfuric acid because of the lack of molecular ion spectral overlaps from nitric acid or its reaction products in the ICP, as discussed earlier. In most cases preconcentration or separation is unnecessary. In some cases preconcentration and removal of chlorides from the sample are advantageous and can be done by using a simple flow injection approach [306]. 

Several potential problems with As speciation have been described [407]. ArCl+ causes a spectral overlap in ICP-MS detection. Speciation can change between the time the sample is obtained and when it is measured. Field separations or preservations can be developed to prevent that problem. Particulates in drinking water can contain a significant portion of the As, so filtering the sample without also analyzing the composition of the particulates can lead to measurements that are too low [407]. 

MW ultrafilter) were also collected. Two 50 unfiltered groundwaters samples were also collected and stored anaerobically in Al-containers with a special coating (Sigma Coating 9255-0300) to prevent contamination during transportation and storage. Further samples included 25 of 1 pm prefiltered and acidified water for U/Th isotopic analysis and 1 pm prefilters preserved for the U/Th analysis of particulates. 

Samples with particulate matter may present quite serious problems, and it may be desirable to remove particles, for example, by centrifugation, and examine this fraction by procedures applicable to solid phases which are discussed in Section 2.2.5. Tangential-flow high-volume filtration systems have been used for analysis of particulate fractions (>0.45 jum) where the analytes occur in only low concentration (Broman et al. 1991). Attention has already been drawn to artifacts resulting from reactions with cyclohexene added as an inhibitor to dichloromethane. It has also been suggested that under basic conditions, Mn2+ in water samples may be oxidized to Mn(III or IV) which in turn oxidized phenolic constituents to quinones (Chen et al. 1991). Serious problems may arise if mercuric chloride is added as a preservative after collection of the samples (Foreman et al. 1992) since this has appreciable solubility in many organic solvents, and its use should therefore be avoided. 

It is common practice to add preservatives to many classes of samples, for example, nitric acid to water samples collected for metals assessment, since this minimizes adsorption to the container surface and the precipitation of insoluble oxides and hydroxides. Note however, that if water samples are to be preserved with acid, they must first be filtered, otherwise the acid is likely to solubilize metals from particulate matter included in the sample and give biased results on analysis. Nitric acid is generally preferred to hydrochloric or sulfuric acids for preservation of metal ion concentrations in water samples because these could enhance precipitation due to the low solubility of some chlorides and sulfates. Other preservatives are required for specific analytes, for example, sodium hydroxide solution to preserve cyanides. 

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