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Basic Sampling Techniques and Statistics

Basic Sampling Techniques and Statistics. WQT 134 Aquatic Chemistry II Standard Methods #1020, #1060 Sampling Standard Methods #1010 Statistics. Week 1 Objectives. Reading assignment:

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Basic Sampling Techniques and Statistics

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  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 • 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)

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

  4. 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

  5. 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.

  6. 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 bottlesespecially 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

  7. 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

  8. 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!!

  9. 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

  10. 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

  11. 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

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

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

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

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

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

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

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

  19. 9/5(degrees C) + 32 = ______ • Celsius • Fahrenheit • Kelvin

  20. 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: • -23 OC • -9 OC • 26 OC

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

  22. 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

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

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

  25. 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

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

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

  28. 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

  29. 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.

  30. QA 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

  31. 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.

  32. 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.

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

  34. 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 inaccuratenot precise precisenot accurate

  35. 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

  36. 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.

  37. 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

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

  39. 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!

  40. 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%

  41. 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

  42. Problem #1 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= 7.2+7.0+6.9+5.5+7.3 = 6.78 5

  43. Problem #1 AVG, Excel

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

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

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

  47. 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!

  48. 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 IDL=1.645*.000158= .0026 mg/L

  49. 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.

  50. 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 = Student’s t value (from table) x std. deviation = 2.821 x 0.019 mg/L= 0.056 mg/L

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