|
Complete Article from Edition 50 APG eNewsletter
Article by Kristel Fijolek, QC Chemist at APG
Holding time refers to the maximum amount of time samples can be held after collection and preservation before the analysis is carried out. For the purpose of proficiency testing samples, the holding time is the maximum time a sample can be kept after sample preparation (with proper storage) before analysis. Some analytical methods, which require an extraction prior to analysis, may also have holding times for the extract. Sample extraction and analysis must be completed within the respective maximum holding time for each step. Most organic extractions are performed immediately following PT sample preparation instructions, so the extraction hold time is seldom an issue. The holding time for some of the inorganic PT samples require more attention and planning.
Holding times vary depending on the analysis method and the sample matrix. Many PT samples can be used for multiple methods, and there may be different holding times for the individual methods. Whether you are using Standard Methods or EPA methods, the analyte with the shortest holding time should be your guideline as to when the analysis on the entire sample should be completed. The Minerals PT (both WS and WP) sample is probably the most demanding in terms of scheduling analysis after PT sample preparation because of the number of analytical methods to be performed on one sample. The Alkalinity as CaCO3 has an EPA hold time of 14 days, however some methods recommend 24 hours. The alkalinity is likely to be influenced by absorption of carbon dioxide from the air and therefore should be the first analysis performed after PT sample preparation. The Minerals PT sample is likely to be handled several times by different people throughout the analysis of all analytes and the Alkalinity as CaCO3 is most likely to be affected. The Total Dissolved Solids must then be analyzed in 7 days and the remaining analytes in 28 days to 180 days depending on which analytical method is used.
APG WS Nutrients includes Fluoride, Nitrate Nitrogen as N, Nitrite as N, and Orthophosphate as P. The Standard Methods maximum storage recommendation times for these analytes is 28 days for Fluoride, and 48 hours for the remaining analytes. The formulation of APG’s WS Nutrients is provided in three separate vials; Orthophosphate as P and Nitrate Nitrogen as N are is sample 1, Fluoride is in sample 2, and Nitrite as N is in sample 3. This presentation will help avoid potential conflicts in scheduling of analysis for the PT samples due to different holding times.
Complete preservation of every analyte in a sample is impossible once the sample has been opened, and some analytes are more unstable than others. Preservation and storage can, at best, reduce or slow down oxidation, precipitation, volatilization, and other chemical, physical, and biological changes that can occur within a sample after it has been exposed to light, air, and possible contaminants within the laboratory.
To achieve the best results, it is best to analyze a sample as soon as possible, but it is essential that published holding times and storage methods be adhered to.
References:
Environmental Protection Agency
Standard Methods for the Examination of Water and Wastewater, 20th Ed.
|
 |
|
|
