1. Quality assurance of sampling and analytical instruments
Lecture Notes

2. Three basic sources of variability
Sampling
Three basic sources of variability
Analytical
Workplace

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
Random errors and fluctuations are sometimes called statistical errors since they can be accounted for statistically even thought they can’t be eliminated
Systematic errors can’t be accounted of statistically They can either remain constant through a series of samples ( e.g. improper calibration) or vary abruptly (e.g. a process change).

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

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

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. Flow Calibration Primary standards - Best Secondary standards - OK
bubble tube
timer
Secondary standards - OK
wet gas meter
dry gas meter
hot wire anemometer
rotameters
Primary standards generally are those devices which allow a direct measurement of the parameter of interest (e.g. volume) on basis of the physical principles of the measuring device. Bubble tube based on diameter and length. The volume is the cross sectional area x length
Secondary standard is one which can trace its calibration to a primary standard
They usually measure a parameter indirectly related to parameter of interest - hot wire anometer measure temperature change which is related to air flow across the thermocouple

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

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
Field blank: A sampler 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, but not actually subtracted from sample readings (see media blank).

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.
Media blank An unexposed sampler, not taken to the field or shipped, used for background correction of sample readings or for recovery studies.

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

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 SO2 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 SO2 concentration. What volume of SO2 gas must you add to a 100-liter gas-sampling bag to produce a SO2 concentration of 500 ppm?

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. Preparing spikes – Problem # 1b
What mass of SO2 would you expect the lab to report back to you for this sample if you had sampled 10 liters of the “standard SO2 mixture” on the spiked filter?

16. Solution to Problem # 1b

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. 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. 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 mg/m3?

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

21. In-class problem –spiked samples

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

26. Example of using CVT
The NIOSH method 1005 for methylene chloride reports a method overall precision (CVA) of and if we assume a pump CVP of 0.05 then the total CVT will be?

27. Solution for CVT example

28. Application of the CVT
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. Answer

30. In class problem – reporting relative standard deviation

31. The End