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SAMPLE PRESERVATION Complete and uneqUIvocal preservatIOn of samples, eIther domestic sewage, mdustnal wastes, or natural waters, IS a practlcallmposslblhty Regardless of the nature of the sample, complete stablhty for every constItuent can never be achIeved At best, preservatIOn techmques can only retard the chemIcal and bIOlogiCal changes that mevltably contmue after the sample IS removed from the parent source The changes that take place m a sample are eIther chemIcal or bIOlogiCal In the former case, certam changes occur m the chemIcal structure of the constituents that are a functIOn of phySIcal condItIOns Metal catIOns may precIpItate as hydroxIdes or form complexes wIth other constituents, catIons or amons may change valence states under certam reducmg or oXldlzmg condItIons, other constItuents may dIssolve or volatlhze wIth the passage of tIme Metal catIOns may also adsorb onto surfaces (glass, plastIc, quartz, etc), such as, Iron and lead BIologIcal changes takmg place m a sample may change the valence of an element or a radIcal to a dIfferent valence Soluble constituents may be converted to orgamcally bound matenals m cell structures, or cell lYSIS may result m release of cellular matenal mto solutIOn The well known mtrogen and phosphorus cycles are examples of bIologIcal mfluence on sample composItIon TherefOIe, as a general rule, It IS best to analyze the samples as soon as possIble after collectIOn ThIS IS espeCIally true when the analyte concentratIOn IS expected to be m the low ug/l range Methods of preservatIOn are relatIvely hmlted and are mtended generally to (1) retard bIOlOgical actIOn, (2) retard hydrolysIs of chemIcal compounds and complexes, (3) reduce volatlhty of constItuents, and (4) reduce absorptIOn effects PreservatIOn methods are generally hmlted to pH control, chemIcal addItion, refngeratlon, and freezmg The recommended preservative for vanous constItuents IS gIven m Table 1 These chOIces are based on the accompanymg references and on mformatton supphed by vanous Quahty Assurance Coordmators As more data become avallable, these recommended holdmg tImes WIll be adjusted to reflect new mformatton Other mformatlon provIded m the table IS an estImatIon of the volume of sample reqUIred for the analysIs, the suggested type of contamer, and the maXImum recommended holdmg tImes for samples properly preserved xvPDF Image | Methods for Chemical Analysis of Water and Wastes
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