Detection Limits and Target Lists

Detection Limits are used to specify the lowest concentration of a compound that can be measured reliably by the analytical method.  There are several definitions for the detection limits used by laboratories so it is important to understand the differences, so that the objectives of your project are met.  On this web site, the term detection limit is used as a general term to collectively describe the more specific terms such as the Method Detection Limit (MDL), Lower Quantitation Limit (LQL), Reporting Limit (RL), and Method Reporting Limit (MRL), and Practical Quantitation Limit (PQL).  These terms are defined below.

IMPORTANT:  For air testing methods all of the definitions of the detection limit depend on the air volume loaded.  The detection limits listed in our tables, like other labs,  are calculated for a 1000 ml sample volume, which is our undiluted base volume.  Any volume less then 1000 ml this will result in a dilution factor being applied which raises the detection limit.  There are several factors that result in an adjustment that lowers the load volume in air samples, these are the canister pressurization factor, high moisture or carbon dioxide in sample, matrix interferences, target compounds at high concentrations. These effects are different then in water or soil samples where a diluted and an undiluted result can be reported.  In air, the matrix generally causes the undiluted sample to fail QC criteria so the result cannot be reported.  The client can then be left with unexpected high detection limits.   EAS (or any other air lab) has no way of knowing in advance what the dilution factor will be to successfully pass QC and analyze a sample.  At  EAS we are committed to providing our clients with the lowest possible detection limits, so we make every effort to to minimize dilution of the sample.

 WAYS TO AVOID UNEXPECTED HIGH DETECTION LIMITS

• For clean ambient air or clean indoor air samples plan on a dilution factor of 2 or 3 to allow for canister pressurization and slight matrix effects.

• For uncontaminated soil gas samples or clean source samples (low carbon dioxide), allow a dilution factor of 5 to 10 for can pressurization or moderate matrix effects.

• For contaminated soil gas samples or source samples with high carbon dioxide, allow a dilution factor of 10 to 500.

• Request a combined TO-15 Full Scan and SIM.  This way you get one full scan analysis for compounds with higher action limits and one SIM analysis for compounds with lower action limits.  The cost is only slightly higher then doing just the Full Scan analysis.

• Give the laboratory as much information about the levels that are expected in the sample.  This will minimize sample reruns, contaminated instruments, and other issues that will slow down analysis time.  Keeping the lab "blind" about the samples does not improve the project QC - all reputable labs have a full NELAP based QC program already - it just slows down analysis time.

Definitions of Detection Limits

Method Detection Limit (MDL)

MDL is defined as the minimum concentration of a substance that can be measured and reported with 99% confidence that the analyte concentration is greater than zero. Per the Code of Federal Regulations (CFR) (CFR 40, part 136, Appendix B, 1993), the practical protocol to determine MDL specifies mathematically to take a minimum of 7 replicates of given spiking concentration in a range of one to five times from the projected lowest concentration that detector in the analytical method can measure, and then calculate with the following equation:

MDL = SD x t0.99 (1)
Where SD = {åⁿ_i=1(xi –X)² / (n-1)}^½

MDL = method detection limit (mg/L), SD = standard deviation,
t_0.99 = t-distribution table value for 99% with the degree of freedom (n-1),
xi = spiking replicates concentration (mg/L ) (i = 1 . . . n) (n=7 in this case),

X = the mean of spiking concentrations (mg/L).

As we can see, the so-called 99% confidence is really based on the t-distribution in statistics. Of course, this assumes that the distribution of the low level spiking concentrations follows the t-distribution.

Lower Quantitation Limit (LQL)

The lower quantitation limit is defined as the lowest concentration at which an analyte can be detected in a sample and its concentration can be reported with a reasonable degree of accuracy and precision.  At EAS the LQL is the lowest point on the calibration curve and meets all of the laboratory QC requirements for the method.  Results between the LQL and the MDL are reported with a "J" flag to indicate that the results are an estimated concentration.

Reporting Limit (RL) and Method Reporting Limit (MRL)

The reporting limit (RL) is defined by the California DHS as the lowest concentration at which an analyte can be detected in a sample and its concentration can be reported with a reasonable degree of accuracy and precision.  The criteria for this would be the QC criteria used by the laboratory for the method.  The RL usually is about 3 to 5 times the MDL, but the only technical requirement is that it is above the MDL.  The RL or method reporting limit (MRL) is often the concentration of the lowest standard on the calibration curve.  In the past the RL has been used to reference project reporting requirements, which may be much higher.

Practical Quantitation Limit (PQL) and Limit of Quantitation (LOQ)

The practical quantitation limit (PQL), limit of quantitation (LOQ), and the estimated quantitation limit (EQL) are set to increase the confidence level in quantification. PQL or EQL is defined as 2 to 5 times above MDL. By raising MDL by a factor of 2 to 10, serving as a “safety factor,” laboratories hope to quantify the environmental sample concentrations with a degree of certainty. The degree of the factor (2-5) is decided by the quality of the instrument, and the nature of the sample objectives.