REPORTING QUESTIONS & ANSWERS

How are reporting limits determined?

There are specific EPA procedures that a laboratory must follow in the determination of its reporting limits. Before the practical quantitation limit (PQL) can be set, the method detection limit (MDL) must be determined. The most common method of determining a method detection limit is a statistical process involving the analysis of seven low level spikes on a single matrix (usually distilled water). A Student’s T analysis is applied to the results and a concentration at which the laboratory is 99% certain that the actual analyte concentration is not zero is calculated. The upper limit of the confidence interval of this number is the minimum concentration the laboratory can report, and is called the method detection limit. The PQL is the lowest level of analyte that can be reliably determined within specified limits of accuracy and precision during routine laboratory operating conditions. The PQL has usually been set to ten times the MDL. This is done to account for the effects of differing matrices in the real world. The PQL should generally be below any regulatory limits, but notifying the lab of reporting limit requirements is highly recommended.



How reliable are results just above the reporting limit?

The relative uncertainty of an analytical measurement varies substantially as the measured value approaches both the PQL and the MDL. The sample matrix may have a considerable effect on the sample preparation process and the ability of the instrument to detect an analyte at the usual PQL, as well. Also, the method detection limit (MDL) is set so that there is only a 1 % chance of reporting a false positive on the matrix on which the method detection limit was determined. This says nothing about the reliability of the actual reported value compared to a theoretical true value, nor does it account for matrix effects.



Why do your reports suggest confirming positive results by GC/MS?

While most positive values encountered during an analysis using a gas chromatograph are confirmed on a second column, it is always prudent to confirm a positive value using another detector. GC/MS is the preferred method of confirmation because the possibility of misidentification is almost non-existent.



What are J values and should they be used for regulatory purposes?

A J qualifier is used to flag a value that is between the method detection limit (MDL) and the practical quantitation limit (PQL). There is considerable uncertainty associated with values between the MDL and the PQL; therefore, these values should not be used for decision-making purposes.



Can I get lower reporting limits if I ask for drinking water methods on all matrices?

Some regulatory agencies are now requiring the use of EPA methods that provide the lowest PQL’s independent of the sample matrix. EPA 500 series methods were developed for the analysis of synthetic and volatile organic compounds in drinking water and yet some regulators are actually requiring that these methods be used on soils and groundwater... Method proficiency is initially demonstrated by analyzing replicate spikes on deionized water and does not take into account the effects of different sample matrices on the MDL. If a sample other than drinking water is analyzed using a drinking water method, it is almost certain that the PQLs will be much higher than those of a drinking water analyzed using the same method.



How long are samples routinely kept by North Coast Laboratories?

Our policy is to keep samples 60 days from the time we receive them. If you suspect that you may need additional analyses, it is important to request that your samples be held for possible additional analyses on your chain of custody. By doing this, your samples will stored under the proper conditions to maintain their integrity. Samples without this designation will be stored at room temperature following the completion of their analyses for thirty days.



How far in advance should I order sample containers?

We routinely keep a ready supply of sample containers on hand. If you know in advance that you will need sample containers it is wise to order them anywhere from 2 – 7 days in advance of the date that you would like to pick them up or have them delivered. With advance notice you are assured of having all of the requested containers ready when you need them.