Clearing up the confusion
PQLs, MDLs, EDLs, SQLs, EQLs, LLDs, MDCs, IDLs, LOQs, LODs, CRQLs, CRDLs - with the number of terms in use, it's easy to understand the confusion over environmental laboratory reporting limits. When a laboratory reports a target analyte as not detected (ND or <), they also give a numerical reporting limit. In other words, they are saying that the analyte is not present at or above the stated reporting limit. However, the analyte could be present at a lower concentration. In fact, the laboratory may even have positively identified it - though not reported it - at a lower concentration.
When making decisions based on laboratory results, it is critical that the engineer and scientist understands these terms. This is especially true for detailed environmental site assessments, risk assessment and statistics-based site monitoring.
Detection vs. quantitation limits
There are two distinct classes of reporting limits: detection limits and quantitation limits. Detection limits refer to a minimum concentration of an analyte that can be measured above the instrument background noise.
Analytical instruments have detectors that convert any target analyte entering the detector into an electrical signal that can be measured. Using a calibration curve, this signal can be converted into a concentration. Instrument noise is due to the normal, random generation of electrical signal from sources other than the target analyte. Some causes are radiation, magnetic fields, loose connections or static. They have nothing to do with the specific sample being analyzed.
There are different types of detection limits that arise from different methods of calculation and from taking different factors into account. For example, is the limit based on a statistically significant signal-to-noise ratio or a statistically determined confidence level? Is there a correction for the effects of sample handling and preparation? How about for sample matrix and dilution? In any case, detection limits are only a measure of the ability of the test procedure to generate a positive response and have nothing to do with the accuracy of that response.
On the other hand, quantitation limits refer to a minimum concentration of an analyte that can be measured within specified limits of precision and accuracy. They are generally five to 10 times the detection limit. Thus, when quantitation limits are used as reporting limits, the laboratory is saying that the analyte is not present in a sufficient amount to be reliably quantified - i.e., at a concentration above the quantitation limit. It may be present and even positively identified or detected at a lower concentration. There are different ways, discussed below, of handling analytes that are positively identified at a concentration below the reported quantitation limit.
Like detection limits, there are different types of quantitation limits that arise from different methods of calculation and from taking different factors into account. Unlike detection limits, there are both lower and upper quantitation limits. These can be thought of in terms of the calibration range of the instrument.
In fact, even though quantitation limits are defined in terms of precision and accuracy, they are often arbitrarily set at the lowest, or the highest, non-zero standard in the calibration curve. Alternatively, they may be set at some multiple of the detection limit.
Whether detection or quantitation, the reporting limits used by a laboratory may be derived specifically for that laboratory, instrument, sample matrix etc. Often, however, they are taken from the analytical method. Every analytical method gives either detection or quantitation limits that are typically attained under routine laboratory operating conditions.
For detection limits, if the laboratory uses the limits given in the analytical method, it must maintain documentation to show it is able to attain these limits. For many analytes, the laboratory can attain a lower detection limit that can be reported if specially requested for a project. Since quantitation limits are generally more arbitrary, they do not normally require documentation by the laboratory.
To better understand the different types of limits, let's look at the specific definitions. The definitions presented here are based on text in the analytical methods, as identified below. They may not agree with your laboratory's definition. If you are unsure, it is always best to check with the lab.
CLP - Contract laboratory program
EPA200- The EPA 200 Series of Methods for Chemical Analysis (Inorganic) of Water and Wastes
EPA500 - The EPA 500 Series of Methods for Organic Compounds in Drinking Water
EPA600 - The EPA 600 Series of Methods for the Analysis of Pollutants Under the Clean Water Act
SM- Standard Methods for the Examination of Water and Wastewater
SW846 - Test Methods for Evaluating Solid Waste
The following terms are generally used for inorganic analyses:
CRDL - Contract required detection limit
Minimum level of detection acceptable under the contract statement of work (SOW). The inorganic SOW for the Contract Laboratory Program gives CRDLs, but laboratory-derived IDLs - adjusted for sample size, dilution, and percent moisture - are used for reporting limits. The CLP CRDLs are based on typical instrument capabilities and should be attained by the laboratory. Inorganic analytes reported at a concentration above the laboratory's IDL but below the CLP CRDL are flagged with a "B."
IDL - Instrument detection limit
Lowest concentration that can be detected by an instrument without correction for the effects of sample matrix or method-specific parameters such as sample preparation. IDLs are explicitly determined and generally defined as three times the standard deviation of the mean noise level. This represents 99 percent confidence that the signal is not random noise.
The inorganic methods in CLP, SW846, EPA200 and Standard Methods all give typical IDLs, but laboratory-derived IDLs - adjusted for sample size, dilution and percent moisture - are used for reporting limits. The IDL does not include the upward correction necessary to account for the effects of sample matrix or handling/preparation (minimal for inorganic water analyses). This is important to remember especially for risk assessments and highly contaminated samples.
LLD - Lower limit of detection
Generally the same as IDL.
LOD - Limit of detection
Generally the same as IDL.
The following terms are generally used for organic analyses:
CRQL - Contract required quantitation limit
Minimum level of reliable quantitation acceptable under the contract SOW. The organic SOW for the Contract Laboratory Program gives CRQLs, and they are used for reporting limits, after adjustment for percent moisture and dilution. The CLP CRQLs are arbitrarily set at the concentration of the lowest non-zero standard in the calibration curve. Organic analytes that are positively identified below the CLP CRQL are reported as present, but at an estimated concentration, with a "J" flag.
EDL - Estimated detection limit
Minimum concentration required to produce a specified signal-to-noise (S/N) ratio. The SW846 Method 8290 for dioxins/furans by GCMS requires that EDLs be used for reporting limits. The EDLs are explicitly determined by the laboratory for each analyte in each sample.
The noise in the vicinity of the absent analyte is measured, then multiplied by a S/N ratio of 2.5. This labor-intensive procedure is used in order to obtain the lowest possible reporting limits for these highly toxic compounds. It could be specially requested for other GCMS analyses as well.
EQL - Estimated quantitation limit
Lowest concentration that can be reliably achieved within specified limits of precision and accuracy during routine laboratory operating conditions. EQLs normally are arbitrarily set rather than explicitly determined. Most organic SW846 methods give EQLs. The SW846 EQLs are set at some multiple of typical MDLs for reagent water. Multiplying factors are given for various matrices such as groundwater, wastewater, soil and sludge.
Generally, laboratories use the SW846 EQLs - adjusted for sample size, dilution and percent moisture - for reporting limits, but they may use EQLs that they have generated. SW846 does not stipulate how to handle organic analytes that are positively identified at a concentration below the SW846 EQL. Generally, laboratories do not report these as present.
LLQ - Lower limit of quantitation
Generally the same as EQL.
LOL - Limit of linearity
Concentration at or above the upper end of the calibration curve at which the relationship between the quantity present and the instrument response ceases to be linear. The organic SOW for the Contract Laboratory Program defines the calibration range in terms of the region of demonstrated linearity. In other words, the LOL is set at the concentration of the highest standard analyzed even though it could extend beyond this. Organic analytes that are positively identified at a concentration above the LOL are flagged with an "E."
LOQ - Limit of quantitation
Generally the same as EQL.
MDC - Minimum detectable concentration
Generally the same as MDL.
MDL - Method detection limit
Lowest concentration that can be detected by an instrument with correction for the effects of sample matrix and method-specific parameters such as sample preparation. MDLs are explicitly determined as set forth in 40 CFR Part 136. They are defined as three times the standard deviation of replicate spiked analyses. This represents 99 percent confidence that the analyte concentration is greater than zero.
The organic methods in EPA500, EPA600 and Standard Methods all give typical MDLs for clean water samples. Generally these clean-water MDLs, corrected for percent moisture, sample size and dilution, are used for reporting limits, but the laboratory may use MDLs that they have generated. MDLs generated by the laboratory using the sample matrix of interest are the most reliable. If MDLs using reagent water with no significant contamination are used, remember that they do not include the upward correction necessary to account for the effects of sample matrix. This is important to remember especially for risk assessments and highly contaminated samples.
MQL - Method quantitation limit
Generally the same as EQL.
PQL - Practical quantitation limit
Generally the same as EQL. This term was used in SW846 prior to 1994.
SDL - Sample detection limit
The MDL adjusted to reflect sample-specific actions such as dilution or use of smaller aliquot sizes, or to report results on a dry-weight basis.
SQL - Sample quantitation limit
The EQL adjusted to reflect sample-specific actions such as dilution or use of smaller aliquot sizes, or to report results on a dry-weight basis.
UCL - Upper calibration limit
Generally the same as LOL.
This article originally appeared in the 05/01/1999 issue of Environmental Protection.