# Basic Sampling Techniques and Statistics

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Basic Sampling Techniques and Statistics
WQT 134 Aquatic Chemistry II Standard Methods #1020, #1060 Sampling Standard Methods #1010 Statistics

Week 1 Objectives Proper Sample Collection
Reading assignment: American Public Health Association (APHA), American Water Works Association (AWWA) & Water Environment Federation (WEF)   Standard Methods for the Examination of Water and Wastewater, 20th edition Proper Sample Collection Understand basic statistical concepts Grasp QA/QC Understand IDL, MDL, and LDL, and how to calculate them Use Excel spreadsheet for lab analysis Understand the role of alkalinity in water and waste water Understand how to measure alkalinity (SM #2010)

WQT 134 Environmental Chemistry II STM. 20th edition. #1060
Sample Collection WQT 134 Environmental Chemistry II STM. 20th edition. #1060

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

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.

Sample Collection #1060 Sample deterioration? Sample collection?
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

Sample Collection #1060 Field Sampling
Noncomposite samples for acidity, alkalinity, chlorine residual, nitrate, dissolved oxygen, temperature, and pH. All on ice! BOD analysis=composite=refrigerate Microbial analysis= no composites=on ice Note: Field Observations are critical! water temperature, weather conditions, water level, stream flow, post-collection conditions

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!!

Water Distribution Sample Collection
Samples must be representative of the water distribution system. Water taps used for sampling should be free of aerators, strainers, hose attachments, mixing type faucets, and purification devices. Cold water taps should be used. The service line must be cleared before sampling by maintaining a steady water flow for at least two minutes (until the water changes temperature). 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. Immediately after collection, a sample information form should be completed= CHAIN OF CUSTODY

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., <10°C, protection from sunlight). If a commercial shipper was used, shipping records should be available. · Any remarks

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? Standardization Buffer Preservation Titration

A ____ sample is a discrete sample that is collected manually.
Grab Composite Flow proportional Temporal

A ________ sample consists of a collection of individual samples collected at regular intervals throughout the day Composite Grab Weighted average Final effluent

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

Samples should only be collected where the water is ______ ____________.
Well mixed Overflowing weirs Between processes Standing quietly

_______ of ________ is a legal term for an unbroken sequence of possession from sample collection through analysis. Chain Custody Chain Command Chain Sample

One mL is what fraction of a L?
1/10 1/100 1/1000 1/100000

What is the most common preservation method for samples?
Cooling to 4 deg. C Dechlorination Reduction Acidification

9/5(degrees C) + 32 = ______ Celsius Fahrenheit Kelvin

Convert 16oF to oC? oC= 5 * (oF – 32) 9 oC= 5 * (16-32)/9 oC= -9 16 oF
Given Formula: Solve: -23 OC -9 OC 26 OC

Samples taken for routine analysis should be preserved by:
refrigerating filtering boiling sterilizing

What is the maximum recommended holding time for a sample that is to be analyzed for pH?
None; it must be analyzed immediately 48 hours 7 days 14 days

What is the minimum number of pH standards needed for calibration of a pH meter?
1 2 3 4

WQT 134 Environmental Chemistry II STM. 20th edition. #1010 & 1020
Basic Statistics WQT 134 Environmental Chemistry II STM. 20th edition. #1010 & 1020

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 σ = [∑(x−µ)2/n]1/2. Coefficient of Variation: normalizes the standard deviation and sometimes facilitates making direct comparisons among analyses that include a wide range of concentrations σ/µ, with its estimate s/x Median: middle value of a sample population Mode: most frequently occurring number in a sample population

Another word for the average is the ______________?
Mean Norm Geometric mean Highest probability

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

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

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.

QA Steps QC Steps Appropriate signatures Test Performed by Laboratory
Sample Handling Procedures Checklist Standard Operating Procedures Data Verification Practices Chain of custody QC Steps Show yearly results of MDL Run Internal Blank Run duplicates Internal Standards Blind Samples Routine Calibration

Quality control samples are analyzed for all of the following reasons EXCEPT:
Check for Contamination Verify precision Verify accuracy Determine if interferences are present. To boost the ego of the analyst.

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 ± 1σ 95.45% between µ ± 2σ, 99.70% between µ ± 3σ. It is sufficiently accurate to state that 95% of the values are within ±2σ and 99% within ±3σ.

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

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 accurate and precise inaccurate not precise precise not accurate

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

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.

Both of these quality control samples can be used to measure accuracy and precision.
Standards and Duplicates Blanks and Standards Blanks and Duplicates Duplicates and Spikes

This type of quality control sample is used to check for interferences.
Spikes Duplicate Standards Blanks

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= σ/µ CV = σ/µ *100 CV % Rule of Thumb For WQT 134 0-5% =excellent precision 5-10% =acceptable precision 10-20%= Poor precision 20%>=Houston we have a problem!

Duplicates should agree with their original samples by this margin.
Plus or Minus 20% Relative Percent Difference Plus or Minus 10% Relative Percent Difference Plus or Minus 20% Plus or Minus 10%

The statistical calculation done to determine the spread of a set of measurements is called the ______________. Standard Deviation Relative Pervasive Difference Percent Deviation Standard Error

Problem #1 Calculate mean= 7.2+7.0+6.9+5.5+7.3 = 6.78 5
Some WQT 134 students measure pH on the same sample! The results are as follows: AB= 7.2 DO=7.0 KO=6.9 ML=5.5 EC=7.3 Calculate mean= = 6.78 5

Problem #1 AVG, Excel

Problem #1, STDEV, Excel continued
Calculate standard deviation= σ = [∑(x−µ)2/n]1/2.

Problem #1, RSD, Excel continued
Calculate RSD RSD= σ/µ

Problem #1, CV, Excel continued
Calculate CV CV= σ/µ * 100

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 20%>=Houston we have a problem!

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= 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 IDL=1.645* = mg/L

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 = Student’s t value (from a table) x standard deviation.

Problem #3 MDL A IDL for a phosphate procedure is 0.045 mg/L.
Make a solution of mg/L phosphate, which is approximately four times the estimated limit 2. Results of multiple analysis (mg/L) 0.190, , 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 = Student’s t value (from table) x std. deviation = x mg/L= mg/L

Problem #3 Excel MDL

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 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!

Today’s objective: to become proficient with sample collection and statistical analysis has been met. Strongly Agree Agree Neutral Disagree Strongly Disagree

To improve my knowledge pertinent to the subject matter I would like to??
Have the instructor provide more math/stats blackboard homework assignments Have data available on the webpage in Excel to practice my analysis Have the instructor provide more blackboard reading assignments Have a quiz to review the subject matter None of the above

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