Basics of Environmental Sampling and Analysis - Fundamentals of Environmental Sampling and Analysis - page 17

Two other terms are relative standard deviation (RSD) and relative percent
difference (RPD):

RSD ¼ðs = xÞ100 ¼ CV100 ð2 : 8Þ

RPD ¼½ðABÞ=ðAþBÞ= 2100 ð2 : 9Þ

where CV denotes coefficient of variation ðCV ¼ s x) and RPD is the difference

between the duplicate values (A and B) divided by the average of the dupli-
cate values and multiplied by 100. RSD is used for the evaluation of multiple
replicate measurements, whereas RPD is used for measuring precision between two
duplicate measurements.
P recision and A ccuracy along with three other qualitative descriptors
( R epresentativeness, C omparability, and C ompleteness), or PARCC, are termed
data quality indicators (DQIs). The PARCC parameters enable us to determine the
validity of environmental data.
Both accuracy and precision are needed to determine the data quality in a
quantitative way. In an analogy, accuracy is how close you get to the bull's-eye,
whereas precision is how close the repetitive shots are to one another. Hence, a good
precision does not guarantee an accurate analysis. On the contrary, it is nearly
impossible to have a good accuracy without a good precision.

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2.1.4 Detection Limit and Quantitation Limit

The method detection limit is one of the secondary DQIs. The U.S. EPA defines
method detection limit (MDL) as ''the minimum concentration that can be measured
and reported with 99% confidence that the analyte concentration is greater than
zero'' (EPA, 1984). To determine MDL, an analyte-free matrix (reagent water or
laboratory-grade sand) is spiked with the target analyte at a concentration that is 3-5
times the estimated MDLs. This sample is then measured at a minimum of seven
times. From these measurements, a standard deviation (s) is calculated (Eq. 2.7), and
the MDL is calculated according to the formula:

MDL ¼ s t ð2 : 10Þ

where t is obtained from ''Student's t value Table'' (Appendix C2), corresponding to
t 0.98 and degree of freedom df ¼ n1, where n is the number of measurements. For
example if n ¼ 7 (i.e., df ¼ 6), then t ¼ 3 143 at a 99% confidence level.

The MDLs are specific to a given matrix, method, instrument, and analytical
technique. The MDL, however, is not the lowest concentration we can accurately
measure during a routine laboratory analysis. EPA, therefore, uses another term,
practical quantitation limit (PQL), and defines it as ''the lowest concentration that
can be reliably achieved within specified limits of precision and accuracy during
routine operating condition'' (EPA, 1996). Typically, laboratories choose PQL value
at 2-10 times its MDLs. Therefore, the PQL may be also defined as ''a concentration
that is 2-10 times greater than the MDL and that represents the lowest point on the

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