Transcript Nature of Air Contaminants

Quality assurance of sampling and
analytical instruments
Lecture Notes
1
Sampling
Three basic sources
of variability
Analytical
Workplace
2
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

3
Workplace

Random




Systematic


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4
Varying emission rates
Routine air currents
Process rate changes, etc...
Unexpected process upset
Winter “close-up” or
summer “open-up”,
Work practices, etc…
Sampling Train

Random




Systematic


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Fluctuations in pump
flow rate
Sample stability,
Sample loss, etc…
Improper calibration
Sampling train leaks
Collection efficiency of
media, etc…
Analytical

Random




Systematic



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Extraction efficiency
Instrumentation fluctuation
Handling losses, etc…
Interfering chemical species
Calibration solutions
Appropriate transfer
materials, etc…
Managing and minimizing systematic error

Most important for IH to control

Calibrate

7
timers - flows - etc.

Check sampling train integrity

Use blanks and control samples

Periodic employee sampling

Sample different conditions
Flow Calibration

Primary standards - Best
bubble tube
 timer


Secondary standards - OK
wet gas meter
 dry gas meter
 hot wire anemometer


8
rotameters
Check sampling train integrity
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
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
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10
Blanks and control samples

Media blank

11
An unexposed filter, sampling tube etc. not taken
to the field, used for background correction of
sample readings or for recovery studies.
Blanks and control samples

Reagent blank


Spikes

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Reagent(s), without analyte or sampling media
added, which are analyzed to determine their
contribution to the total blank reading
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

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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 gassampling 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
CPPM 
VolSO2 
14
VolSO2
Vol total
x 106
C PPM  Vol total
106
= 0.05L or 50 mL
Preparing spikes – Problem # 1b

15
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
1. Det erminefract ionof mole
# molesof SO 2  0.05 L SO 2 
mole
24.4 L
@ NT P
# molesof SO 2  0.002
2. Det erminemass int roducedt o t he100 L
mass of SO 2  # moles GMW
mass of SO 2  0.002 mole 32  16  2 
g
mole
mass of SO 2  0.128g or 128 mg in 100 L
3. Det erminemass on 10 L sample
mass on 10 L sample 
16
10 L
128 mg  12.8mg
100 L
Preparing spikes – Problem #2
17

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
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
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?
mg
1. Determinemass of CH 2Cl 2 in 100 L at a concentration of 50 P P Mor 173.5 3
m
mg
m3
mass of CH 2Cl 2  173.5 3 100 L  0.001  17.35mg
m
L
2. Determineliquid volumeof CH 2Cl 2 to add
17.35mg
mLliq CH 2Cl 2 
 0.0131mL or 13.1uL
g
mg
1.323
 1000
mL
g
19
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
mg
m3
10% of the PEL equates to a mass of: 25 PPM  3.47 3 1L  0.001
 .1  0.0087 mg
m
L
mg
m3
50% of the PEL equates to a mass of: 25 PPM  3.47 3 1L  0.001
 .5  0.0434 mg
m
L
2. Determine fraction of 100 L volume containing 17.35 mg of CH2Cl2 for 10% and 50% of PEL
20
10% PEL sample collect a volume 
.0087 mg
 0.05 L or 50 ml
17.35 mg
100 L
50% PEL sample collect a volume 
.0434 mg
 0.25 L or 250 ml
17.35 mg
100 L
In-class problem –spiked samples
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Periodic employee sampling
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
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
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
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
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
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 
T
2
CV1
2
2
 CV  CVn
2
Typical CV for a sampling pump is assumed to be .05
25
Example of using CVT

26
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
CVT  CV  CV  CV
2
1
CVT 
2
n
0.076  0.05
CVT  0.09
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2
2
2
2
Application of the CVT

28
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
Conc 
35 ug
min
L
m3
4 hr  60
 .15
 0.001
hr
min
L
ug
Conc  972 3
m
Use CVT to determinetherelativestandarddeviation
ug
 CVT  Conc
3
m
ug
Conc  972 3  .09  972
m
ug
Conc  972 88 3
m
Conc  972
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In class problem – reporting relative
standard deviation
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