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1 Quality assurance of sampling and analytical instruments Lecture Notes.

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1 1 Quality assurance of sampling and analytical instruments Lecture Notes

2 2 Three basic sources of variability Workplace Sampling Analytical

3 3 Sampling Variability Results from two types of error Random or Statistical Errors  can’t be eliminated - try to minimize  can be accounted for by statistical analysis Systematic Errors  can be eliminated - reduce chance of occurring  can’t be accounted for by statistical analysis

4 4 Workplace Random  Varying emission rates  Routine air currents  Process rate changes, etc... Systematic  Unexpected process upset  Winter “close-up” or summer “open-up”,  Work practices, etc…

5 5 Sampling Train Random  Fluctuations in pump flow rate  Sample stability,  Sample loss, etc… Systematic  Improper calibration  Sampling train leaks  Collection efficiency of media, etc…

6 6 Analytical Random  Extraction efficiency  Instrumentation fluctuation  Handling losses, etc… Systematic  Interfering chemical species  Calibration solutions  Appropriate transfer materials, etc…

7 7 Managing and minimizing systematic error Most important for IH to control Calibrate  timers - flows - etc. Check sampling train integrity Use blanks and control samples Periodic employee sampling Sample different conditions

8 8 Flow Calibration Primary standards - Best  bubble tube  timer Secondary standards - OK  wet gas meter  dry gas meter  hot wire anemometer  rotameters

9 9 Check sampling train integrity Properly assembled filter cassettes Tight connections Tubing with no leaks Pump diaphragms intact, etc…

10 10 Blanks and control samples Field blank  Handled exactly the same as the field samples, except no air is drawn through it  Used to estimate contamination in preparation for sampling, shipment and storage prior to measurement  Put right on worker

11 11 Blanks and control samples Media blank  An unexposed filter, sampling tube etc. not taken to the field, used for background correction of sample readings or for recovery studies.

12 12 Blanks and control samples Reagent blank  Reagent(s), without analyte or sampling media added, which are analyzed to determine their contribution to the total blank reading Spikes  A known mass of analyte added to a sampler for the purpose of determining recovery (analyst spikes), or for quality control (blind spikes).

13 13 Preparing spikes – Problem # 1 To complete the sampling campaign you've undertaken you desire to collect a “spiked” sample in your lab at a known SO 2 concentration and send it to the analytical lab with your field samples. In order to do this you must create a volume of air having a known SO 2 concentration. What volume of SO 2 gas must you add to a 100-liter gas- sampling bag to produce a SO 2 concentration of 500 ppm?

14 14 Solution to problem #1 Recall the relationship to determine the volume to add to a volume to create a known PPM concentration = 0.05L or 50 mL

15 15 Preparing spikes – Problem # 1b What mass of SO 2 would you expect the lab to report back to you for this sample if you had sampled 10 liters of the “standard SO 2 mixture” on the spiked filter?

16 16 Solution to Problem # 1b

17 17 Preparing spikes – Problem #2 We are sampling for methylene chloride and want to prepare a series of spiked samples that range in concentrations of 10% and 50% of the PEL value for a sample volume of 1L. We need to prepare a volume of methylene chloride at known concentration

18 18 Solution to problem #2 Determine the volume and concentration of methylene chloride we want  Select a volume of 100 L  Select a concentration of 2 x the PEL Based on concentration we want to determine the sample volume needed to get 10% and 50% of the PEL. Recall methylene chloride is a liquid

19 19 Solution to problem #2 - continued How much liquid methylene chloride do I need to evaporate in my 100 L volume to produce a concentration of 2 x PEL i.e. 50 PPM or 173.5 mg/m 3 ?

20 20 Solution to problem #2 - continued Determine volume of our known concentration to sample to get 10% or 50% of the PEL

21 21 In-class problem –spiked samples

22 22 Periodic employee sampling Regular intervals e.g. every 6 months Randomly select employees of the same SEG Sample as many workers as the budget allows – not 1 or 2 unless your budget restricts you to that Sample highest priority SEGs

23 23 Sample different conditions Sample different shifts and different days of the week especially if weekend shifts are different from those used during the week Sample different times of the year Sample under different run capacities within what is considered normal, etc…

24 24 Managing and minimizing random errors Can’t eliminate so we account for them in statements of uncertainty Use coefficients of variability Confidence intervals, etc…

25 25 Cumulative Error or Total Coefficient of Variation Typical CV for a sampling pump is assumed to be.05

26 26 Example of using CV T The NIOSH method 1005 for methylene chloride reports a method overall precision (CV A ) of 0.076 and if we assume a pump CV P of 0.05 then the total CV T will be?

27 27 Solution for CV T example

28 28 Application of the CV T You sample methylene chloride for 4 hrs at a flow rate of.15 LPM and have a reported mass of 35ug. Report your concentration and its relative standard deviation.

29 29 Answer

30 30 In class problem – reporting relative standard deviation

31 31


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