Quality assurance of sampling and analytical instruments Lecture Notes

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

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).

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…

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…

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

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

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

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

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).

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.

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).

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 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

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 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

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/m3?

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

In-class problem –spiked samples

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

Example of using CVT The NIOSH method 1005 for methylene chloride reports a method overall precision (CVA) of 0.076 and if 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 .15 LPM and have a reported mass of 35ug. Report your concentration and its relative standard deviation.

Answer

In class problem – reporting relative standard deviation

The End