Quality assurance of sampling and analytical instruments

IH&S 725 Dr. Myers, C.I.H.

Quality assurance of sampling and analytical instruments

Lecture Notes

IH&S 725 Dr. Myers C.I.H.

Three basic sources of variability

Workplace

Sampling

Analytical


Sampling Variability Results fromtwo types of error

Random or Statistical Errors

Systematic Errors


Workplace

Random

Systematic


Sampling Train

Random

Systematic


Analytical

Random

Systematic


Managing and minimizing systematic error

Most important for IH to control Calibrate

Check sampling train integrity Use blanks and control samples Periodic employee sampling Sample different conditions


Flow Calibration

Primary standards - Best

Secondary standards – OK


Check sampling train integrity

Properly assembled filter cassettes Tight connections Tubing with no leaks Pump diaphragms intact, etc…


Blanks and control samples

Field blank



Media blank



Reagent blank

Spikes


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?


Solution for problem #1


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?


Solution to Problem # 1b


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


Solution to problem #2


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


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…


Managing and minimizing random errors

Can’t eliminate so we account for them in statements of uncertainty

Use coefficients of variability Confidence intervals, etc…


Cumulative Error or Total Coefficient of Variation

CV CV CV CVT 12

22

n2

Typical CV for a sampling pump is assumed to be .05


Example of using CVT

If the NIOSH method 1005 for methylene chloride reports a method overall precision (CVA) of 0.076 and we assume a pump CVP of 0.05 then the total CVT will be?


Solution for CVT example


Application of the CVT

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


Answer

Quality assurance of sampling and analytical instruments

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