Basic Sampling Techniques and Statistics

1
Basic Sampling Techniques and Statistics

• WQT 134
• Aquatic Chemistry II
• Standard Methods 1020, 1060 Sampling
• Standard Methods 1010 Statistics

2
Week 1 Objectives
Reading assignment American Public Health
Association (APHA), American Water Works
Association (AWWA) Water Environment Federation
(WEF). 1999.   Standard Methods for the
Examination of Water and Wastewater, 20th
edition

1. Proper Sample Collection
2. Understand basic statistical concepts
3. Grasp QA/QC
4. Understand IDL, MDL, and LDL, and how to
   calculate them
5. Use Excel spreadsheet for lab analysis
6. Understand the role of alkalinity in water and
   waste water
7. Understand how to measure alkalinity (SM 2010)

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Sample Collection

• WQT 134
• Environmental Chemistry II
• STM. 20th edition. 1060

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Sample Collection 1060

• Grab Sample Grab samples are single samples
  collected at a specific spot at a site over a
  short period of time (typically seconds or
  minutes). Thus, they represent a snapshot in
  both space and time of a sampling area
• Composite Sample Composite samples should
  provide a more representative sampling of
  heterogeneous matrices in which the concentration
  of the analytes of interest may vary over short
  periods of time and/or space .

• Representative Sample sample should represent
  the whole distribution sample

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Sample Collection 1060

• Why is proper collection of a sample important?
• It is an old axiom that the result of any testing
  method can be no better than the sample on which
  it is performed.
• Why do we care?
• The analysis will govern sample collection.

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Sample Collection 1060

• Sample deterioration?
• Obtain a sample that meets the requirements of
  the sampling program and handle it so that it
  does not deteriorate or become contaminated or
  compromised before it is analyzed.
• Sample collection?
• Clean sample bottles especially when sampling and
  analyzing for very low analyte levels.
• Collect a 1-L sample for most physical and
  chemical analyses. Keep cool
• Representative sample
• Pre-rinse only if no preservative added
• Leave no head space

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Sample Collection 1060

• Field Sampling
• Noncomposite samples for acidity, alkalinity,
• chlorine residual, nitrate, dissolved oxygen,
• temperature, and pH. All on ice!
• BOD analysiscompositerefrigerate
• Microbial analysis no compositeson ice
• Note Field Observations are critical!
• water temperature, weather conditions, water
  level, stream flow, post-collection conditions

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Sample Collection 1060

• Things to keep in mind.
• Avoid areas of excessive turbulence
• Avoid sampling at weirs
• Collect samples beneath the surface in
• quiescent areas and open sampling container below
  surface with the mouth directed toward the
  current
• Composite samples are required, ensure that
  sample constituents are not lost during
  compositing.
• Be safety conscious
• Label samples adequately CHAIN OF CUSTODY!!

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Water Distribution Sample Collection

1. Samples must be representative of the water
   distribution system.
2. Water taps used for sampling should be free of
   aerators, strainers, hose attachments, mixing
   type faucets, and purification devices.
3. Cold water taps should be used.
4. The service line must be cleared before sampling
   by maintaining a steady water flow for at least
   two minutes (until the water changes
   temperature).
5. At least 100 mL of sample must be collected,
   allowing at least a 1-inch air space to
   facilitate mixing of the sample by shaking.
6. Immediately after collection, a sample
   information form should be completed CHAIN OF
   CUSTODY

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Water Distribution Sample Collection
Name of system (public water system site
identification number, if available)
Sample identification (if any) Sample site
location Sample type (e.g., routine
distribution system sample, repeat sample, raw
or process water, other special purpose sample)
Date and time of collection Analysis required
Disinfectant residual Name of sampler and
organization (if not the water system)
Sampler's initials Person(s) transporting the
samples from the system to the laboratory (if
not the sampler) Transportation condition
(e.g., lt10C, protection from sunlight). If a
commercial shipper was used, shipping records
should be available. Any remarks
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Which of the following terms refers to the
addition of chemicals to a sample in the field to
prevent water quality indicators from changing
before final measurements are performed?

1. Standardization
2. Buffer
3. Preservation
4. Titration

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A ____ sample is a discrete sample that is
collected manually.

1. Grab
2. Composite
3. Flow proportional
4. Temporal

13
A ________ sample consists of a collection of
individual samples collected at regular intervals
throughout the day

1. Composite
2. Grab
3. Weighted average
4. Final effluent

14
A single sample of water taken at one time from
one place is called a _________ sample.

1. Grab
2. Composite
3. Representative

15
Samples should only be collected where the water
is ______ ____________.

1. Well mixed
2. Overflowing weirs
3. Between processes
4. Standing quietly

16
_______ of ________ is a legal term for an
unbroken sequence of possession from sample
collection through analysis.

1. Chain Custody
2. Chain Command
3. Chain Sample

17
One mL is what fraction of a L?

1. 1/10
2. 1/100
3. 1/1000
4. 1/100000

18
What is the most common preservation method for
samples?

1. Cooling to 4 deg. C
2. Dechlorination
3. Reduction
4. Acidification

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9/5(degrees C) 32 ______

1. Celsius
2. Fahrenheit
3. Kelvin

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Convert 16oF to oC?
oC 5 (oF 32) 9 oC 5
(16-32)/9 oC -9
16 oF oC 5 (oF 32) 9 oC
5 (16-32)/9 oC -9

• Given
• Formula
• Solve

1. -23 OC
2. -9 OC
3. 26 OC

21
Samples taken for routine analysis should be
preserved by

1. refrigerating
2. filtering
3. boiling
4. sterilizing

22
What is the maximum recommended holding time for
a sample that is to be analyzed for pH?

1. None it must be analyzed immediately
2. 48 hours
3. 7 days
4. 14 days

23
What is the minimum number of pH standards needed
for calibration of a pH meter?

1. 1
2. 2
3. 3
4. 4

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Basic Statistics

• WQT 134
• Environmental Chemistry II
• STM. 20th edition. 1010 1020

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Statistics Key Words/Equations

• Mean is simply the sum of all values divided by
  the number of values
• µ (?ixi)/n
• Standard Deviation is a measure of the spread of
  values
• s ?(x-µ)2/n1/2.
• Coefficient of Variation normalizes the standard
  deviation and sometimes facilitates making direct
  comparisons among analyses that include a wide
  range of concentrations s/µ, with its estimate
  s/x
• Median middle value of a sample population
• Mode most frequently occurring number in a
  sample population

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Another word for the average is the
______________?

1. Mean
2. Norm
3. Geometric mean
4. Highest probability

27
The mode is the most frequently occurring number
in a data set?

1. True
2. False

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Statistics Key Words/Equations

• Accuracy combination of bias and precision of an
  analytical procedure, which reflects the
  closeness of a measured value to a true value.
• Precision measure of the degree of agreement
  among replicate analyses of a sample, usually
  expressed as the standard deviation

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Statistics QA/QC Key

• Quality assessment procedure for determining the
  quality of laboratory measurements by use of data
  from internal and external quality control
  measures
• Quality assurance a definitive plan for
  laboratory operation that specifies the measures
  used to produce data of known precision and bias.
  
• Quality control set of measures within a sample
  analysis methodology to assure that the process
  is in control.

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QA Steps

1. Appropriate signatures
2. Test Performed by Laboratory
3. Sample Handling Procedures Checklist
4. Standard Operating Procedures
5. Data Verification Practices
6. Chain of custody

QC Steps

1. Show yearly results of MDL
2. Run Internal Blank
3. Run duplicates
4. Internal Standards
5. Blind Samples
6. Routine Calibration

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Quality control samples are analyzed for all of
the following reasons EXCEPT

1. Check for Contamination
2. Verify precision
3. Verify accuracy
4. Determine if interferences are present.
5. To boost the ego of the analyst.

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Normal Distribution

• Normal Distribution If a measurement is
  repeated many times under essentially identical
  conditions, the results of each measurement, x,
  will be distributed randomly about a mean value
  (arithmetic average) because of uncontrollable or
  experimental error.

68.27 of the measurements lie between µ
1s 95.45 between µ 2s, 99.70 between µ 3s.
It is sufficiently accurate to state that 95
of the values are within 2s and 99 within 3s.
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Average

• Average Two feet in bucket of ice water, two
  feet on hot coals.
• On average I feel
  great?

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Accuracy and Precision

• Accuracy refers to how closely a measured value
  agrees with the correct value.
• Precision refers to how closely individual
  measurements agree with each other.

Assume aim is directed at bulls eye
accurate(the average is accurate)not precise
accurateandprecise
precisenot accurate
inaccuratenot precise
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Analytical Precision
Instrumental detection level (IDL) the
constituent concentration that produces a signal
greater than five times the signal/ noise ratio
of the instrument. 1.645 times the
standard deviation of the blank Method
detection level (MDL) For seven replicates of
the sample, the mean must be 3.14s above the
blank where s is the standard deviation of the
seven replicates. The method detection limit
(MDL) is the only one designed to be determined
in your laboratory using your chemicals,
equipment, and technicians. Compute MDL from
replicate measurements one to five times the
actual MDL. The MDL will be larger than the LLD
because of the few replications and the sample
processing steps and may vary with constituent
and matrix. Lower level of detection (LLD) the
constituent concentration in reagent water that
produces a signal
3.219s above the mean of blank analyses
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Analytical Precision
minimum quantitation level (MQL) the
constituent concentration that produces a signal
sufficiently greater than the blank that it can
be detected within specified levels by good
laboratories during routine operating conditions.
Typically it is the concentration that produces
a signal 10s above the reagent water blank
signal.
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Both of these quality control samples can be
used to measure accuracy and precision.

1. Standards and Duplicates
2. Blanks and Standards
3. Blanks and Duplicates
4. Duplicates and Spikes

38
This type of quality control sample is used to
check for interferences.

1. Spikes
2. Duplicate
3. Standards
4. Blanks

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Relative Standard Deviation
Relative standard deviation (RSD) is a measure
of precision, calculated by dividing the standard
deviation for a series of measurements by the
average measurement.
RSD s/µ CV
s/µ 100 CV Rule of Thumb For WQT
134 0-5 excellent precision 5-10
acceptable precision 10-20 Poor
precision 20gtHouston we have a problem!
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Duplicates should agree with their original
samples by this margin.

1. Plus or Minus 20 Relative Percent Difference
2. Plus or Minus 10 Relative Percent Difference
3. Plus or Minus 20
4. Plus or Minus 10

41
The statistical calculation done to determine the
spread of a set of measurements is called the
______________.

1. Standard Deviation
2. Relative Pervasive Difference
3. Percent Deviation
4. Standard Error

42
Problem 1
Some WQT 134 students measure pH on the same
sample! The results are as follows AB
7.2 DO7.0 KO6.9 ML5.5 EC7.3 Calculate mean
7.27.06.95.57.3 6.78
5
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Problem 1 AVG, Excel
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Problem 1, STDEV, Excel continued

• Calculate standard deviation
• s ?(x-µ)2/n1/2.
• 
  

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Problem 1, RSD, Excel continued
Calculate RSD RSD s/µ

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Problem 1, CV, Excel continued
Calculate CV CV s/µ 100

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Problem 1, Final Analysis, Excel continued
CV Rule of Thumb For WQT 134 0-5
excellent precision 5-10 acceptable
precision 10-20 Poor precision 20gtHousto
n we have a problem!
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Problem 2, IDL
Instrumental detection level (IDL) the lowest
limit that the instrument can detect. It is
determined on samples which have not gone through
any sample preparation steps. AB .0024
mg/L DO.0023 mg/L KO.0026 mg/L ML.0027
mg/L EC.0025 mg/L Calculate IDL
1.645 X standard deviation of the blank
IDL1.645.000158 .0026 mg/L
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Problem 3 MDL

• One of the procedures outlined in 40 CFR 136 for
  determining the MDL is to prepare the analyte
  concentration at a level one to five times the
  estimated detection limit and run it multiple
  times.
• Make solution of the analyte that is one to five
  times the estimated detection.
• 2. Test this solution seven or more times, then
  determine the standard deviation of the data set.
• 3. The method detection limit is calculated
  according to the formula
• MDL Students t value (from a table) x standard
  deviation.

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Problem 3 MDL
A IDL for a phosphate procedure is 0.045
mg/L. Make a solution of 0.175 mg/L phosphate,
which is approximately four times the estimated
limit 2. Results of multiple analysis (mg/L)
0.190, 0.194, 0.166, 0.174, 0.149, 0.183,
0.153, 0.144, 0.173 3. Determine STDEV
0.019 mg/L. 4. Use MDL
equation MDL Students t value (from table)
x std. deviation
2.821 x 0.019 mg/L 0.056 mg/L
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Problem 3 Excel MDL
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Problem 4 LDL
The lowest limit is actually 2.5 times higher
than the MDL. 1. Use LDL equation LDL 2.5
X MDL 2.5 X 0.056 mg/L
0.14 mg/L The lowest possible value that can be
detected with this method is 0.14 mg/L anything
below this value is suspect!
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Todays objective to become proficient with
sample collection and statistical analysis has
been met.

1. Strongly Agree
2. Agree
3. Neutral
4. Disagree
5. Strongly Disagree

54
To improve my knowledge pertinent to the subject
matter I would like to??

1. Have the instructor provide more math/stats
   blackboard homework assignments
2. Have data available on the webpage in Excel to
   practice my analysis
3. Have the instructor provide more blackboard
   reading assignments
4. Have a quiz to review the subject matter
5. None of the above