Presentation on theme: "Quality assurance of sampling and analytical instruments"— Presentation transcript:
1 Quality assurance of sampling and analytical instruments Lecture Notes
2 Three basic sources of variability SamplingThree basic sources of variabilityAnalyticalWorkplace
3 Sampling Variability Results from two types of error Random or Statistical Errorscan’t be eliminated - try to minimizecan be accounted for by statistical analysisSystematic Errorscan be eliminated - reduce chance of occurringcan’t be accounted for by statistical analysisRandom errors and fluctuations are sometimes called statistical errors since they can be accounted for statistically even thought they can’t be eliminatedSystematic 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 currentsProcess rate changes, etc...SystematicUnexpected process upsetWinter “close-up” or summer “open-up”,Work practices, etc…
5 Sampling Train Random Systematic Fluctuations in pump flow rate Sample stability,Sample loss, etc…SystematicImproper calibrationSampling train leaksCollection efficiency of media, etc…
6 Analytical Random Systematic Extraction efficiency Instrumentation fluctuationHandling losses, etc…SystematicInterfering chemical speciesCalibration solutionsAppropriate transfer materials, etc…
7 Managing and minimizing systematic error Most important for IH to controlCalibratetimers - flows - etc.Check sampling train integrityUse blanks and control samplesPeriodic employee samplingSample different conditions
8 Flow Calibration Primary standards - Best Secondary standards - OK bubble tubetimerSecondary standards - OKwet gas meterdry gas meterhot wire anemometerrotametersPrimary 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 lengthSecondary standard is one which can trace its calibration to a primary standardThey 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 cassettesTight connectionsTubing with no leaksPump diaphragms intact, etc…
10 Blanks and control samples Field blankHandled exactly the same as the field samples, except no air is drawn through itUsed to estimate contamination in preparation for sampling, shipment and storage prior to measurementPut right on workerField blank: A sampler handled exactly the same as the field samples, except no airis 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 blankAn 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 blankReagent(s), without analyte or sampling media added, which are analyzed to determine their contribution to the total blank readingSpikesA 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 #1Recall 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?
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 #2Determine the volume and concentration of methylene chloride we wantSelect a volume of 100 LSelect a concentration of 2 x the PELBased 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
22 Periodic employee sampling Regular intervals e.g. every 6 monthsRandomly select employees of the same SEGSample as many workers as the budget allows – not 1 or 2 unless your budget restricts you to thatSample 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 weekSample different times of the yearSample 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 uncertaintyUse coefficients of variabilityConfidence 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 CVTThe 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?