Method Reporting Limit (MRL) and Method Detection

There are always limitations to the sensitivity, accuracy, and precision of analytical instruments. It is essential to obtain data which is both accurate and precise. The terms “reporting limits” and “detection limits” represent the various limits that announce the lowest concentrations of compounds with a different degree of confidence. They describe the performance of a laboratory, operator, and test method. Figure 3.9 explains the concepts of accuracy and precision.

Figure 3.9: Concepts of Accuracy and Precision

(Source: Florida Department of Environmental Protection, 2009)

The method reporting limit (MRL) is the lowest amount of a chemical which can be quantitatively specified with acceptable accuracy and precision under stated analytical circumstances (ALS Environmental Lab). In fact, if a laboratory does not discover a substance in a sample, it does not indicate the absence of that substance in a sample. It only indicates that the amount of the substance is below the instrument sensitivity. Therefore, the smallest concentration of the compound which a laboratory can report is denominated MRL (LCS Laboratory Inc.). Sometimes, scientists use the phrase “Practical Quantitation Limit (PQL)” instead of MRL.

Giving an example can be useful. A water sample is tested for compound A and the regulatory limit for A is 0.5 µg/L. The method reporting limit for the laboratory is 1.0 µg/L. Then if the sample is contaminated by compound A with a concentration of 0.7

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µg/L, the experiment result shows the sample as clean, even though the amount of chemical A is above the regulatory limits and can be a health risk. Hence, it is important that an investigator initially informs the laboratory what MRL (PQL) is desired for the research; in this case, the laboratory may be able to select a more suitable test method to fulfill an investigator’s need (EPA, 2011).

The method detection limit (MDL) is the lowest concentration of a compound which can be quantified and reported with 99% confidence that the substance amount is greater than zero in the sample matrix (EPA, 2009). Therefore, MDL concentrations are not accurate or precise (USGS 1999). Figure 3.10 depicts the difference between MRL (PQL) and MDL.

Figure 3.10: Relationship between MRL (PQL) and MDL (Source: Florida Department of Environmental Protection, 2009)

When an analytical instrument analyzes the samples, it produces a signal even for a blank sample (matrix without analytes). This signal for a blank sample is called the

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instrument background level. Measurement of the fluctuation of the background level is referred to as noise. In the background signal, fluctuation measurement can be achieved by calculating the standard deviation of successive point measurements (Wells et al., 2011). The adequate concentration of the analyte in the matrix must exist to generate an analytical signal which can be recognized from analytical noise (Shrivastava and Gupta, 2011). Indeed, in situations when noise and analyte signal are indiscernible, MDL protects against faulty reporting of the availability of the analyte at low concentrations. When the instrument reports a detection of a chemical which is absent in the matrix, it is known as a “false positive.” Reporting the discovery of a compound at MDL amounts in a blank specimen or a sample which does not have the substance is rare. Thus, such a reporting is not presumably in error (USGS, 1999).

The United States EPA has developed a procedure to calculate the method detection limits. In this method, a minimum of seven replicate (n) spikes at low concentrations, usually 1 to 5 times the anticipated MDL, should be prepared and processed via the full analytical method (Figure 3.11) (USGS, 1999).

Figure 3.11: Relation between Spike Concentration and MDL

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Analysis of the spike samples is usually performed over a few days, and reagent water is typically the spiked matrix. By gathering data points at the spike concentration, a distribution of measured concentrations will be generated. Figure 3.12 shows an example which is related to distribution of measured concentrations of chlorobenzene for 50 injections spiked at 0.05 µg/L. The EPA procedure considers this distribution to be a normal distribution and is displayed by the bell-shaped curve. (USGS, 1999)

Figure 3.12: Frequency Distribution of Measured Concentrations of Chlorobenzene Spiked at 0.05 µg/L

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It is assumed that the frequency of the distribution and, therefore, the standard deviation of the distribution will be constant at some low concentration and stays constant down to zero concentration. Figure 3.13 represents the standard deviations for various spike concentrations. The EPA method suggests an iteration approach to decrease the spike concentration to lower concentrations in order to approximate the region of constant standard deviation to MDL (USGS, 1999).

Figure 3.13: Standard Deviations for Spike Concentrations, Presenting a Zone of Constant Standard Deviation at Low Concentrations

(Source: United States Geological Survey (USGS) 1999)

It is unfeasible to measure noise signal in repetitive blank samples. In an effort to simulate the distribution of measuring the noise signal or actual unspiked analyte or both in a series of blank samples, the frequency distribution of low concentration spikes will

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be centered on zero concentration and can be considered to be a hypothetical blank samples frequency distribution (Figure 3.14) (USGS, 1999).

Figure 3.14: Frequency Distribution of Spike Measurements is Superimposed on Zero Concentration

(Source: United States Geological Survey (USGS) 1999)

These hypothetical blank measurements are employed to compute the concentration at which no more than 1 percent of the blank samples will result in the reporting of a false positive, and that concentration is called the MDL. Accordingly, reported detections at concentrations equal to or greater than MDL concentrations should be real detections 99 percent of the time. The following formula is used to calculate EPA MDL (USGS, 1999).

51 Where:

n = number of replicate spike (1 to 5 times the estimated MDL)

s = standard deviation of measured concentrations of n spike

t = student’s t value at n-1 degrees of freedom and 1- ∝ (99 percent) confidence level. Student’s t value can be seen at Table 3.4.

∝ = level of significance

Table 3.4: Student’s t Value for Different Replicates and Degrees of Freedom

Number of Replicates Degrees of Freedom (n-1) 𝑡_{(𝑛−1,0.99)}

7 6 3.143

8 7 2.998

9 8 2.896

10 9 2.821

(Source: Environmental Protection Agency (EPA) 2009)

For this dissertation research, the method reporting limits (MRL) of each target analyte are presented in Appendix F.

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