Importance of Sample Preservation Stacie Metzler Sample Preservation • What is it? – Chemical or physical treatment of a sample to assure continued presence of the target analytes at the same level as when the sample was first taken • Why do it? – Sample preservation retards chemical and biological changes that continue after sample collection The Importance of Sample Preservation Representative Sampling + Preservation + Handling + Documentation + Analysis + Reporting = VALID Results The Importance of Sample Preservation VALID Results • Valid analytical results generated from valid samples are used to make compliance decisions • Preservation is an essential element of sample collection and validation • Approved sample preservation methods listed in 40 CFR part 136 Types of Sample Changes • Sample changes can cause loss of target analytes: – Adsorption or ion exchange with walls of sample containers • Metals – pH changes can cause some constituents to dissolve and others to precipitate • Calcium Carbonate • Hardness Types of Sample Changes – Biological and Microbiological activity can change oxidation states of constituents either releasing them into solution of binding them • Nutrients • BOD – Head space can result in loss of compounds • Volatile Organics Preservation Techniques • Proper Sample preservation may require one or more of these: – – – – – Cooling to 6oC pH Control Chemical Addition Use of amber and opaque bottles Filtration Preservation Techniques • Cooling to 6oC – Microbiological, BOD, Solids • Slows Biological activity • Minimal change in sample matrix- Suspended Solids remain in suspension • Oxidation state of components remains the same – Holding times are short • Microbiology: 8 to 12 hours • BOD: 48 hours • Solids: 7 days Preservation Techniques • pH Control – Acidify to pH<2- Metals, Mercury, Nutrients, Phenol, Oil and Grease and COD – Keeps the target analyte in solution • Retards Biological Activity • Prevents adsorption to sample container walls • Holding Times vary • Metals: 6 months • Mercury, Nutrients, Phenol, Oil and Grease and COD : 28 days Preservation Techniques • pH Control continued: – Adjust to pH>10 : Cyanide • Cyanide is most stable in solution at high pH • Holding time is 14 days due instability Preservation Techniques • Chemical Addition – Sulfide • Precipitates sulfide to prevent volatilization • Holding time is 28 days Preservation Techniques • Use of amber and opaque bottles – Organics • Protects light sensitive compounds from exposure • Holding times range from 7 to 28 days • Filtration – Ortho- Phosphorous • Removes solids which may go into solution releasing additional phosphorous into solution • Also requires acidification and cooling to 6oC Interference Removal – Interference removal- removes or neutralizes compounds that can interfere with stability of the target analyte • • • • Cyanide- Sulfides, oxidizers Phenol- oxidizers Organics- chlorine, oxidizers Microbiology- chlorine, metals Samples not requiring Preservation • Samples not requiring preservation are not required to be transported on ice or stored at 6oC • Transporting on ice of storing at 6oC does not invalidate these samples: – Chloride – Fluoride • Something to keep in mind when collecting large numbers of samples Conclusions: • Sample preservation is an essential element of sample collection and storage • Sample preservation retards chemical and biological changes that continue after collection • Proper preservation assures target analytes remain at the same level as when the sample is collected