Typically 10% of the total number of samples analyzed consist of laboratory duplicates. The mean of the two values is reported as the concentration for that sample. If a difference of >10% is observed between replicates, then all of the replicates for that particular analytical run are carefully reviewed. If only one of the duplicate pairs is in question, then only that sample is re-analyzed. If all show a similar trend, then instrumentation/reagent problems are suspected and the analytical run is halted until such time as the problem is resolved. This procedure is practiced for all dissolved analytes that are not consumed completely in the analytical procedure. For those that are completely consumed and for particulate analytes, duplicate samples constitute actual duplicate samples collected in the field and analyzed in the same analytical run.
Values for each duplicate analyzed are recorded in a separate QA/QC data file along with the sample number, sample collection date and analysis date. The mean concentration and standard deviation of the replicates are calculated in this data file.
In the case of particulate carbon and nitrogen, total suspended solids and chlorophyll a, 10% of the total number of samples are analyzed as duplicates. This generates sufficient quality assurance data to compensate for the omission of laboratory spikes for these non-aqueous samples.
Laboratory duplicates serve as an indicator of instrument stability, consistency in laboratory sample preparation and analysis, as well as an estimate of field proficiency.
Typically 10% of the total number of samples analyzed consist of laboratory spikes. A spike is prepared by adding a known volume of standard to a known volume of pre-analyzed sample. We routinely add enough concentrated standard to provide a significant response on our instruments that is distinguishable from the original concentration of the sample. This concentrated standard is used to minimize any possible change in sample matrix by the addition of spike.
The spiked sample is analyzed and its expected concentration calculated as the sum of the original concentration and the spike concentration, normalized for the constituent volumes. A comparison is made between the actual value and the expected value. These concentrations (original, expected and actual) are recorded in a separate QA/QC data file along with sample number, sample collection date, analysis date and the amount of spike added. In the case of particulate phosphorus, the volume filtered is not used in the calculation to determine percentage recovery.
If a value of >120% or <80% is observed for percentage recovery of the spike, then all of the spikes for that particular analytical run are carefully reviewed. If only one of the spikes is in question, then only that sample is re-analyzed. If all show poor recovery, then instrumentation/reagent problems are suspected and the analytical run is halted until such time that the problem is resolved. This procedure is adhered to for all dissolved analytes and for particulate phosphorus and biogenic silica.
Documentation of Slopes
A running record of the slopes of the standard curves (the so-called "F," "S" and "K" factors) is maintained for each analysis. Random up and down movement within a predetermined range as a function of time indicates the analysis is under control. Consistent upward or downward movement of these factors indicates the analysis is out of control and requires immediate attention.
Limits of Detection
Limits of detection, the lowest concentration of an analyte that the analytical procedure can reliably detect, have been established for all parameters routinely measured by Nutrient Analytical Services. The limit of detection is student's t value times the standard deviation of a minimum of seven replicates of a single low concentration sample.
Statements on Instrument Comparability
The Nutrient Analytical Services Laboratory develops a data quality maintenance program for each analyte whenever new instrumentation is acquired. It is the policy of the Nutrient Analytical Services Laboratory to report any data from new instrumentation only after thorough and satisfactory side-by-side comparisons with existing instrumentation are performed.
No predetermined number of data pairs are used to make the assessment on data comparability between new and existing methodology. Even in the case of instrumentation with similar methods of detection (i.e., automated colorimetric), no specific number of data pairs is used. Comparability at low and high concentrations, salinity and other possible matrix interferences, sensitivity and precision are all factors in determining the number of pairs that must be addressed before bringing an instrument on-line and in determining instrument comparability.
The analyst who performs these comparisons should be experienced, open-minded and impartial. This person can give an evaluation of ease of instrument operation and a very important general statement of comparability. This statement on comparability must then be substantiated via statistical analysis of the data. As previously mentioned, these data must encompass the entire concentration range, matrix interferences, percent recovery, results of standard reference material analyses, etc. The data interpretation must support comparability. The analyst and laboratory QA/QC officer must concur and finally, some sort of presentation (written or verbal) must be given to the contractor explaining what procedure was followed and the results that were obtained to bring this instrument online.
Standard Reference Materials
Particulate Carbon, Nitrogen and Phosphorus: PACS-2 is a marine sediment reference material prepared by the National Research Council of Canada. It is certified by the Council for carbon content, gives a non-certified range of results for phosphorus, but no information for nitrogen. We have analyzed this sediment for many years and maintain a substantial database for nitrogen and phosphorus, as well as carbon values. We analyze this sediment with every run and compare these results to the certified value, non-certified range of values and our historical values. Dissolved Analytes and Hardness: Standard reference materials for ammonium, nitrite + nitrate, nitrite, orthophosphate, dissolved nitrogen, dissolved phosphorus, dissolved organic carbon, sulfate and chloride and hardness are supplied by SPEX, a US EPA certified company. The samples arrive in ampules and we prepare final concentrations to approximate typical estuarine concentrations. The samples are then placed in pre-cleaned poly bottles, frozen and analyzed at the beginning and end of every run. The analysis of these frozen standard reference materials as a function of time also provides data on the effect of our preservation technique (freezing) on the integrity of the concentration of samples. The US EPA recommends a holding time of 28 days for many of the parameters we routinely analyze.
The Chesapeake Bay Blind Audit Program was initiated in 1998 to help agencies and laboratories identify and correct analytical and/or field sampling problems before monitoring data are applied to a long term data base and subsequently used to make management decisions. Twice a year, in summer and winter, low-level nutrient samples are prepared by Nutrient Analytical Services Laboratory. The samples are then sent to participating laboratories with a set of instructions for analyzing. After results are submitted and reviewed a final report and laboratory comparison is prepared. Each laboratory is then sent a copy of the final blind audit report. Discrepencies are noted and laboratories check their results if they do not have a reported value within the expected range. The Blind Audit Program is funded by the Chesapeake Bay Program and Maryland Department of Natural Resources.
Split Sample Program
Chesapeake Bay Quarterly split samples are collected and analyzed throughout the year at institutions such as Old Dominion University (ODU), Division of Consolidated Laboratory Services (DCLS), Maryland Department of Health and Mental Hygiene (DHMH), and Virginia Institute of Marine Science (VIMS).