Basics of Environmental Sampling and Analysis - Fundamentals of Environmental Sampling and Analysis - page 40

7. An analytical method is being developed to measure trichloracetinitrile in water. The
analyst has performed a number of spiked and recovery experiments in different water
matrices to determine the method detection limits (MDLs). The data are reported in the
following table.

Trichloroacetonitrile Recovery and Precision Data

Relative

Number True Mean standard Standard

of sample conc. conc., x deviation deviation

Matrix (n) (mg/L) (mg/L) (%RSD) (s)

Distilled Water 7 4.0 2.8 6.8 0.190

River Water 7 4.0 4.55 6.9 0.314

Ground Water 7 4.0 5.58 2.6 0.145

(a) Which water matrix yields the most precise results? Why?
(b) Which water matrix yields the most accurate results? Why?
(c) Calculate the MDLs with a 98% confidence level for each water matrix.
8. A Cr 6þ solution of 0.175 mg/L was prepared and analyzed eight times over the course of
several days. The results were 0.195, 0.167, 0.178, 0.151, 0.176, 0.155, 0.154, 0.164 mg/L.
The standard deviation of this data set was calculated to be 0.0149 mg/L. (a) Calculate the
method detection limit (MDL), (b) What is the estimated practical quantitation limit
(PQL)? (c) Is it accurate to measure samples containing Cr 6þ around 0.1 mg/L?
9. Describe in what cases the use of ''median'' can be preferred over that of ''mean'' in
environmental data reporting?
10. Define Type I error and Type II error. Explain why both ''false positive'' and ''false
negative'' should be avoided in the analysis and monitoring of environmental contaminants?
11. Define ''outliers'' and also describe how the ''outlier'' data should generally be dealt
with and be removed?
12. Evaluate the pros and cons when using mean or median to evaluate the following two
data sets (unit in mg/L) - Data A: 1, 1, 1,1, ....1, 10 6 (n ¼ 1; 000). Data point 10 6 could
be an outlier. Data B: 0.11, ND, 0.13, 0.15. ND denotes ''not detected.''
13. For the following set of analyses of an urban air sample for carbon monoxide: 325, 320,
334, 331, 280, 331, 338 mg/m 3 , (a) determine the mean, standard deviation, median, mode,
and range using Excel's Descriptive Statistics. (b) calculate the coefficient of variation and
RSD. (c) is there any value in the above data set that can be discarded as an outlier?
14. The following absorbance data at wavelength 543 nm were obtained for a series of
standard solutions containing nitrite (NO 2 ) using a colorimetric method: A¼0 (blank),
A¼0.220 (5 mM), A¼0.41 (10 mM), A¼0.59 (15 mM), A¼0.80 (20 mM). Using
Excel (a) to plot the calibration curve, and (b) to determine the calibration equation
and the regression coefficient (R 2 ).
15. For the raw data set x i ¼ 1; 100; 1000 and its logarithmic (base 10) transformation data
set y i ¼ 0; 2; 3, what are the means and medians for these two data sets (Hint: For