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Quality assurance of sampling and analytical instruments

Lecture Notes

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Three basic sources of variability

Workplace

Sampling

Analytical

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Sampling Variability Results fromtwo 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

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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…

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Sampling Train

Random Fluctuations in pump

flow rate Sample stability, Sample loss, etc…

Systematic Improper calibration Sampling train leaks Collection efficiency of

media, etc…

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Analytical

Random Extraction efficiency Instrumentation fluctuation Handling losses, etc…

Systematic Interfering chemical species Calibration solutions Appropriate transfer

materials, etc…

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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

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Flow Calibration

Primary standards - Best bubble tube timer

Secondary standards - OK wet gas meter dry gas meter hot wire anemometer rotameters

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Check sampling train integrity

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

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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

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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.

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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).

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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?

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Solution to problem #1

2

PPM totalSO 6

C VolVol

10

2SO 6PPM

total

VolC 10

Volx

Recall the relationship to determine the volume to add to a volume to create a known PPM concentration

= 0.05L or 50 mL

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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?

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Solution to Problem # 1b

mg 12.8 mg 128 L100

L10 sample L10 on mass

sample L10 on mass Determine3.

L100 in mg 128or g128.0SO of massmole

g21632mole 002.0SO of mass

GMW moles #SO of mass

L100 the tointroduced mass Determine2.

002.0SO of moles #

NTP @ L24.4

moleSO L05.0SO of moles #

mole of fraction Determine1.

2

2

2

2

22

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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

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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

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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?

uL 13.1or mL 0.0131

gmg

1000 mLg

1.323

mg 17.35 ClCH mL

add toClCH of volumeliquid Determine2.

mg 17.35 L

m0.001 L100

m

mg173.5 ClCH of mass

m

mg 173.5or PPM 50 of ionconcentrata at L100 in ClCH of mass Determine1.

22liq

22

3

322

322

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Solution to problem #2 - continued

Determine volume of our known concentration to sample to get 10% or 50% of the PEL

1. Determine sample mass to collect equivalent to 10% or 50 % of PEL assuming a sample volume of 1L

3

3

mg m10% of the PEL equates to a mass of: 25 PPM 3.47 1L 0.001 .1 0.0087 mg

m L

3

3

mg m50% of the PEL equates to a mass of: 25 PPM 3.47 1L 0.001 .5 0.0434 mg

m L

2 22. Determine fraction of 100 L volume containing 17.35 mg of CH Cl for 10% and 50% of PEL

.0087 mg10% PEL sample collect a volume 0.05 L or 50 ml

17.35 mg100 L

.0434 mg50% PEL sample collect a volume 0.25 L or 250 ml

17.35 mg100 L

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In-class problem –spiked samples

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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

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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…

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Managing and minimizing random errors

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

Use coefficients of variability Confidence intervals, etc…

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Cumulative Error or Total Coefficient of Variation

TCV

2n

2

2

21 CVCVCV

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

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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?

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Solution for CVT example

09.0CV

0.05 0.076CV

CV CV CVCV

T

22T

2n

22

21T

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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.

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Answer

3

3

T3

T

3

3

m

ug88972 Conc

972 09.m

ug972 Conc

Conc CVm

ug972 Conc

deviation standard relative thedetermine toCV Usem

ug972 Conc

Lm

001.0minL

15.hr

min60hr 4

ug 35 Conc

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In class problem – reporting relative standard deviation

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