Collection procedure: A known volume of air is drawn through
a 37-mm diameter filter cassette containing a 0.8 µm mixed cellulose
ester membrane filter (MCEF).
Analytical procedure: Air filter samples are digested with
nitric acid. After digestion, a small amount of hydrochloric acid is added. The
samples are then diluted to volume with deionized water and analyzed by either
flame atomic absorption spectroscopy (AAS) or flameless atomic absorption
spectroscopy using a heated graphite furnace atomizer (AAS-HGA).
Inorganic Service Branch II, OSHA Salt Lake Technical Center,
Salt Lake City, Utah Commercial manufacturers and products mentioned in this
method are for descriptive use only and do not constitute endorsements by
USDOL-OSHA. Similar products from other sources can be substituted.
(1) Introduction.
(a) Scope.
This method describes the collection of airborne elemental
cadmium and cadmium compounds on 0.8 µm mixed cellulose ester membrane
filters and their subsequent analysis by either flame atomic absorption
spectroscopy (AAS) or flameless atomic absorption spectroscopy using a heated
graphite furnace atomizer (AAS-HGA). It is applicable for both TWA and action
level TWA permissible exposure level (PEL) measurements. The two atomic
absorption analytical techniques included in the method do not differentiate
between cadmium fume and cadmium dust samples. They also do not differentiate
between elemental cadmium and its compounds.
(b) Principle.
Airborne elemental cadmium and cadmium compounds are
collected on a 0.8 µm mixed cellulose ester membrane filter (MCEF). The
air filter samples are digested with concentrated nitric acid to destroy the
organic matrix and dissolve the cadmium analytes. After digestion, a small
amount of concentrated hydrochloric acid is added to help dissolve other metals
which may be present. The samples are diluted to volume with deionized water
and then aspirated into the oxidizing air/acetylene flame of an atomic
absorption spectrophotometer for analysis of elemental cadmium. If the
concentration of cadmium in a sample solution is too low for quantitation by
this flame AAS analytical technique, and the sample is to be averaged with
other samples for TWA calculations, aliquots of the sample and a matrix
modifier are later injected onto a L'vov platform in a pyrolytically-coated
graphite tube of a Zeeman atomic absorption spectrophotometer/graphite furnace
assembly for analysis of elemental cadmium. The matrix modifier is added to
stabilize the cadmium metal and minimize sodium chloride as an interference
during the high temperature charring step of the analysis subsection (5)(a) and
(b) of this section.
(c)
History.
Previously, two OSHA sampling and analytical methods for
cadmium were used concurrently WAC
296-62-07449(5)(c) and
(d). Both of these methods also required 0.8
µm mixed cellulose ester membrane filters for the collection of air
samples. These cadmium air filter samples were analyzed by either flame atomic
absorption spectroscopy (subsection (5)(c) of this section) or inductively
coupled plasma/atomic emission spectroscopy (ICP-AES) (subsection (5)(d) of
this section). Neither of these two analytical methods have adequate
sensitivity for measuring workplace exposure to airborne cadmium at the new
lower TWA and action level TWA PEL levels when consecutive samples are taken on
one employee and the sample results need to be averaged with other samples to
determine a single TWA. The inclusion of two atomic absorption analytical
techniques in the new sampling and analysis method for airborne cadmium permits
quantitation of sample results over a broad range of exposure levels and
sampling periods. The flame AAS analytical technique included in this method is
similar to the previous procedure given in the General Metals Method ID-121
(subsection (5)(c) of this section) with some modifications. The sensitivity of
the AAS-HGA analytical technique included in this method is adequate to measure
exposure levels at 1/10 the action level TWA, or lower, when less than
full-shift samples need to be averaged together.
(d) Properties (subsection (5)(e) of this
section).
Elemental cadmium is a silver-white, blue-tinged, lustrous
metal which is easily cut with a knife. It is slowly oxidized by moist air to
form cadmium oxide. It is insoluble in water, but reacts readily with dilute
nitric acid. Some of the physical properties and other descriptive information
of elemental cadmium are given below:
|
CAS No . . . . . . . . . . . .
|
7440-43-9
|
|
Atomic Number . . . . . . . . . . . .
|
48
|
|
Atomic Symbol . . . . . . . . . . . .
|
Cd
|
|
Atomic Weight . . . . . . . . . . . .
|
112.41
|
|
Melting Point . . . . . . . . . . . .
|
321°C
|
|
Boiling Point . . . . . . . . . . . .
|
765°C
|
|
Density . . . . . . . . . . . .
|
8.65 g/mL (25°C)
|
The properties of specific cadmium compounds are described in
reference subsection (5)(e) of this section.
(e) Method performance.
A synopsis of method performance is presented below. Further
information can be found in subsection (4) of this section.
(i) The qualitative and quantitative
detection limits for the flame AAS analytical technique are 0.04 µg
(0.004 µg/mL) and 0.14 µg (0.014 µg/mL) cadmium,
respectively, for a 10 mL solution volume. These correspond, respectively, to
0.2 µg/m3 and 0.70
µg/m3 for a 200 L air volume.
(ii) The qualitative and quantitative
detection limits for the AAS-HGA analytical technique are 0.44 ng (0.044 ng/mL)
and 1.5 ng (0.15 ng/mL) cadmium, respectively, for a 10 mL solution volume.
These correspond, respectively, to 0.007 µg/m3
and 0.025 µg/m3 for a 60 L air
volume.
(iii) The average recovery
by the flame AAS analytical technique of 17 spiked MCEF samples containing
cadmium in the range of 0.5 to 2.0 times the TWA target concentration of 5
µg/m3 (assuming a 400 L air volume) was 104.0%
with a pooled coefficient of variation (CV1) of
0.010. The flame analytical technique exhibited a positive bias of +4.0% for
the validated concentration range. The overall analytical error (OAE) for the
flame AAS analytical technique was ±6.0%.
(iv) The average recovery by the AAS-HGA
analytical technique of 18 spiked MCEF samples containing cadmium in the range
of 0.5 to 2.0 times the action level TWA target concentration of 2.5
µg/m3 (assuming a 60 L air volume) was 94.2%
with a pooled coefficient of variation (CV1) of
0.043. The AAS-HGA analytical technique exhibited a negative bias of -5.8% for
the validated concentration range. The overall analytical error (OAE) for the
AAS-HGA analytical technique was ±14.2%.
(v) Sensitivity in flame atomic absorption is
defined as the characteristic concentration of an element required to produce a
signal of 1% absorbance (0.0044 absorbance units). Sensitivity values are
listed for each element by the atomic absorption spectrophotometer manufacturer
and have proved to be a very valuable diagnostic tool to determine if
instrumental parameters are optimized and if the instrument is performing up to
specification. The sensitivity of the spectrophotometer used in the validation
of the flame AAS analytical technique agreed with the manufacturer
specifications (subsection (5)(f) of this section); the 2 µg/mL cadmium
standard gave an absorbance reading of 0.350 abs. units.
(vi) Sensitivity in graphite furnace atomic
absorption is defined in terms of the characteristic mass, the number of
picograms required to give an integrated absorbance value of 0.0044
absorbance-second (subsection (5)(g) of this section). Data suggests that under
stabilized temperature platform furnace (STPF) conditions (see (f)(ii) of this
subsection), characteristic mass values are transferable between properly
functioning instruments to an accuracy of about twenty percent (subsection
(5)(b) of this section). The characteristic mass for STPF analysis of cadmium
with Zeeman background correction listed by the manufacturer of the instrument
used in the validation of the AAS-HGA analytical technique was 0.35 pg. The
experimental characteristic mass value observed during the determination of the
working range and detection limits of the AAS-HGA analytical technique was 0.41
pg.
(f) Interferences.
(i) High concentrations of silicate interfere
in determining cadmium by flame AAS (subsection (5)(f) of this section).
However, silicates are not significantly soluble in the acid matrix used to
prepare the samples.
(ii)
Interferences, such as background absorption, are reduced to a minimum in the
AAS-HGA analytical technique by taking full advantage of the stabilized
temperature platform furnace (STPF) concept. STPF includes all of the following
parameters (subsection (5)(b) of this section):
(A) Integrated absorbance;
(B) Fast instrument electronics and sampling
frequency;
(C) Background
correction;
(D) Maximum power
heating;
(E) Atomization off the
L'vov platform in a pyrolytically coated graphite tube;
(F) Gas stop during atomization;
(G) Use of matrix modifiers.
(g) Toxicology
(subsection (5)(n) of this section).
Information listed within this section is synopsis of current
knowledge of the physiological effects of cadmium and is not intended to be
used as the basis for WISHA policy. IARC classifies cadmium and certain of its
compounds as Group 2A carcinogens (probably carcinogenic to humans). Cadmium
fume is intensely irritating to the respiratory tract. Workplace exposure to
cadmium can cause both chronic and acute effects. Acute effects include
tracheobronchitis, pneumonitis, and pulmonary edema. Chronic effects include
anemia, rhinitis/anosmia, pulmonary emphysema, proteinuria and lung cancer. The
primary target organs for chronic disease are the kidneys (noncarcinogenic) and
the lungs (carcinogenic).
(3) Analysis.
(a) Safety precautions.
(i) Wear safety glasses, protective clothing
and gloves at all times.
(ii)
Handle acid solutions with care. Handle all cadmium samples and solutions with
extra care (see subsection (1)(g) of this section). Avoid their direct contact
with work area surfaces, eyes, skin and clothes. Flush acid solutions which
contact the skin or eyes with copious amounts of water.
(iii) Perform all acid digestions and acid
dilutions in an exhaust hood while wearing a face shield. To avoid exposure to
acid vapors, do not remove beakers containing concentrated acid solutions from
the exhaust hood until they have returned to room temperature and have been
diluted or emptied.
(iv) Exercise
care when using laboratory glassware. Do not use chipped pipets, volumetric
flasks, beakers or any glassware with sharp edges exposed in order to avoid the
possibility of cuts or abrasions.
(v) Never pipet by mouth.
(vi) Refer to the instrument instruction
manuals and SOPs (subsection (5)(h) and (i) of this section) for proper and
safe operation of the atomic absorption spectrophotometer, graphite furnace
atomizer and associated equipment.
(vii) Because metallic elements and other
toxic substances are vaporized during AAS flame or graphite furnace atomizer
operation, it is imperative that an exhaust vent be used. Always ensure that
the exhaust system is operating properly during instrument use.
(b) Apparatus for sample and
standard preparation.
(i) Hot plate, capable
of reaching 150°C, installed in an exhaust hood.
(ii) Phillips beakers, 125 mL.
(iii) Bottles, narrow-mouth, polyethylene or
glass with leakproof caps: used for storage of standards and matrix
modifier.
(iv) Volumetric flasks,
volumetric pipets, beakers and other associated general laboratory
glassware.
(v) Forceps and other
associated general laboratory equipment.
(c) Apparatus for flame AAS analysis.
(i) Atomic absorption spectrophotometer
consisting of a(an):
Nebulizer and burner head; pressure regulating devices
capable of maintaining constant oxidant and fuel pressures; optical system
capable of isolating the desired wavelength of radiation (228.8 nm); adjustable
slit; light measuring and amplifying device; display, strip chart, or computer
interface for indicating the amount of absorbed radiation; cadmium hollow
cathode lamp or electrodeless discharge lamp (EDL) and power supply.
(ii) Oxidant: Compressed air,
filtered to remove water, oil and other foreign substances.
(iii) Fuel: Standard commercially available
tanks of acetylene dissolved in acetone; tanks should be equipped with flash
arresters.
|
Caution:
|
Do not use grades of acetylene containing solvents
other than acetone because they may damage the PVC tubing used in some
instruments.
|
(iv)
Pressure-reducing valves: Two gauge, two-stage pressure regulators to maintain
fuel and oxidant pressures somewhat higher than the controlled operating
pressures of the instrument.
(v)
Exhaust vent installed directly above the spectrophotometer burner
head.
(d) Apparatus for
AAS-HGA analysis.
(i) Atomic absorption
spectrophotometer consisting of a(an):
Heated graphite furnace atomizer (HGA) with argon purge
system pressure-regulating devices capable of maintaining constant argon purge
pressure; optical system capable of isolating the desired wavelength of
radiation (228.8 nm); adjustable slit; light measuring and amplifying device;
display, strip chart, or computer interface for indicating the amount of
absorbed radiation (as integrated absorbance, peak area); background corrector:
Zeeman or deuterium arc. The Zeeman background corrector is recommended;
cadmium hollow cathode lamp or electrodeless discharge lamp (EDL) and power
supply; autosampler capable of accurately injecting 5 to 20 µL sample
aliquots onto the L'vov Platform in a graphite tube.
(ii) Pyrolytically coated graphite tubes
containing solid, pyrolytic L'vov platforms.
(iii) Polyethylene sample cups, 2.0 to 2.5
mL, for use with the autosampler.
(iv) Inert purge gas for graphite furnace
atomizer: Compressed gas cylinder of purified argon.
(v) Two gauge, two-stage pressure regulator
for the argon gas cylinder.
(vi)
Cooling water supply for graphite furnace atomizer.
(vii) Exhaust vent installed directly above
the graphite furnace atomizer.
(e) Reagents. All reagents should be ACS
analytical reagent grade or better.
(i)
Deionized water with a specific conductance of less than 10 µS.
(ii) Concentrated nitric acid,
HNO3.
(iii)
Concentrated hydrochloric acid, HCl.
(iv) Ammonium phosphate, monobasic,
NH4H2PO4.
(v) Magnesium nitrate,
Mg(NO3)2
6H2O.
(vi)
Diluting solution (4% HNO3, 0.4% HCl): Add 40 mL
HNO3 and 4 mL HCl carefully to approximately 500 mL
deionized water and dilute to 1 L with deionized water.
(vii) Cadmium standard stock solution, 1,000
µg/mL: Use a commercially available certified 1,000 µg/mL cadmium
standard or, alternatively, dissolve 1.0000 g of cadmium metal in a minimum
volume of 1:1 HCl and dilute to 1 L with 4% HNO3.
Observe expiration dates of commercial standards. Properly dispose of
commercial standards with no expiration dates or prepared standards one year
after their receipt or preparation date.
(viii) Matrix modifier for AAS-HGA analysis:
Dissolve 1.0 g
NH4H2PO4
and 0.15 g Mg(NO3)2
6H2O in approximately 200 mL deionized water. Add 1 mL
HNO3 and dilute to 500 mL with deionized
water.
(ix) Nitric Acid, 1:1
HNO3/DI H2O mixture: Carefully
add a measured volume of concentrated HNO3 to an equal
volume of DI H2O.
(x) Nitric acid, 10% v/v: Carefully add 100
mL of concentrated HNO3 to 500 mL of DI
H2O and dilute to 1 L.
(f) Glassware preparation.
(i) Clean Phillips beakers by refluxing with
1:1 nitric acid on a hot plate in a fume hood. Thoroughly rinse with deionized
water and invert the beakers to allow them to drain dry.
(ii) Rinse volumetric flasks and all other
glassware with 10% nitric acid and deionized water prior to use.
(g) Standard preparation for flame
AAS analysis.
(i) Dilute stock solutions:
Prepare 1, 5, 10 and 100 µg/mL cadmium standard stock solutions by making
appropriate serial dilutions of 1,000 µg/mL cadmium standard stock
solution with the diluting solution described in (e)(vi) of this
subsection.
(ii) Working standards:
Prepare cadmium working standards in the range of 0.02 to 2.0 µg/mL by
making appropriate serial dilutions of the dilute stock solutions with the same
diluting solution. A suggested method of preparation of the working standards
is given below.
|
Working
standard
(MICROg/mL)
|
Std
solution
(MICROg/mL)
|
Aliquot
(mL)
|
Final vol.
(mL)
|
|
0.02
|
1
|
10
|
500
|
|
0.05
|
5
|
5
|
500
|
|
0.1
|
10
|
5
|
500
|
|
0.2
|
10
|
10
|
500
|
|
0.5
|
10
|
25
|
500
|
|
1
|
100
|
5
|
500
|
|
2
|
100
|
10
|
500
|
Store the working standards in 500-mL, narrow-mouth
polyethylene or glass bottles with leak proof caps. Prepare every twelve
months.
(h)
Standard preparation for AAS-HGA analysis.
(i)
Dilute stock solutions: Prepare 10, 100 and 1,000 ng/mL cadmium standard stock
solutions by making appropriate ten-fold serial dilutions of the 1,000
µg/mL cadmium standard stock solution with the diluting solution
described in (e)(vi) of this subsection.
(ii) Working standards: Prepare cadmium
working standards in the range of 0.2 to 20 ng/mL by making appropriate serial
dilutions of the dilute stock solutions with the same diluting solution. A
suggested method of preparation of the working standards is given below.
|
Working
standard
(ng/mL)
|
Std
solution
(ng/mL)
|
Aliquot
(mL)
|
Final vol.
(mL)
|
|
0.2
|
10
|
2
|
100
|
|
0.5
|
10
|
5
|
100
|
|
1
|
10
|
10
|
100
|
|
2
|
100
|
2
|
100
|
|
5
|
100
|
5
|
100
|
|
10
|
100
|
10
|
100
|
|
20
|
1,000
|
2
|
100
|
Store the working standards in narrow-mouth polyethylene or
glass bottles with leakproof caps. Prepare monthly.
(i) Sample preparation.
(i) Carefully transfer each sample filter
with forceps from its filter cassette unit to a clean, separate 125-mL Phillips
beaker along with any loose dust found in the cassette. Label each Phillips
beaker with the appropriate sample number.
(ii) Digest the sample by adding 5 mL of
concentrated nitric acid (HNO3) to each Phillips beaker
containing an air filter sample. Place the Phillips beakers on a hot plate in
an exhaust hood and heat the samples until approximately 0.5 mL remains. The
sample solution in each Phillips beaker should become clear. If it is not
clear, digest the sample with another portion of concentrated nitric
acid.
(iii) After completing the
HNO3 digestion and cooling the samples, add 40 µL
(2 drops) of concentrated HCl to each air sample solution and then swirl the
contents. Carefully add about 5 mL of deionized water by pouring it down the
inside of each beaker.
(iv)
Quantitatively transfer each cooled air sample solution from each Phillips
beaker to a clean 10-mL volumetric flask. Dilute each flask to volume with
deionized water and mix well.
(j) Flame AAS analysis.
Analyze all of the air samples for their cadmium content by
flame atomic absorption spectroscopy (AAS) according to the instructions given
below.
(i) Set up the atomic
absorption spectrophotometer for the air/acetylene flame analysis of cadmium
according to the SOP (subsection (5)(h) of this section) or the manufacturer's
operational instructions. For the source lamp, use the cadmium hollow cathode
or electrodeless discharge lamp operated at the manufacturer's recommended
rating for continuous operation. Allow the lamp to warm up ten to twenty
minutes or until the energy output stabilizes. Optimize conditions such as lamp
position, burner head alignment, fuel and oxidant flow rates, etc. See the SOP
or specific instrument manuals for details. Instrumental parameters for the
Perkin-Elmer Model 603 used in the validation of this method are given in
subsection (6) of this section.
(ii) Aspirate and measure the absorbance of a
standard solution of cadmium. The standard concentration should be within the
linear range. For the instrumentation used in the validation of this method a 2
µg/mL cadmium standard gives a net absorbance reading of about 0.350 abs.
units (see subsection (1)(e)(v) of this section) when the instrument and the
source lamp are performing to manufacturer specifications.
(iii) To increase instrument response, scale
expand the absorbance reading of the aspirated 2 µg/mL working standard
approximately four times. Increase the integration time to at least three
seconds to reduce signal noise.
(iv) Autozero the instrument while aspirating
a deionized water blank. Monitor the variation in the baseline absorbance
reading (baseline noise) for a few minutes to insure that the instrument,
source lamp and associated equipment are in good operating condition.
(v) Aspirate the working standards and
samples directly into the flame and record their absorbance readings. Aspirate
the deionized water blank immediately after every standard or sample to correct
for and monitor any baseline drift and noise. Record the baseline absorbance
reading of each deionized water blank. Label each standard and sample reading
and its accompanying baseline reading.
(vi) It is recommended that the entire series
of working standards be analyzed at the beginning and end of the analysis of a
set of samples to establish a concentration-response curve, ensure that the
standard readings agree with each other and are reproducible. Also, analyze a
working standard after every five or six samples to monitor the performance of
the spectrophotometer. Standard readings should agree within ±10 to 15%
of the readings obtained at the beginning of the analysis.
(vii) Bracket the sample readings with
standards during the analysis. If the absorbance reading of a sample is above
the absorbance reading of the highest working standard, dilute the sample with
diluting solution and reanalyze. Use the appropriate dilution factor in the
calculations.
(viii) Repeat the
analysis of approximately ten percent of the samples for a check of
precision.
(ix) If possible,
analyze quality control samples from an independent source as a check on
analytical recovery and precision.
(x) Record the final instrument settings at
the end of the analysis. Date and label the output.
(k) AAS-HGA analysis.
Initially analyze all of the air samples for their cadmium
content by flame atomic absorption spectroscopy (AAS) according to the
instructions given in (j) of this subsection. If the concentration of cadmium
in a sample solution is less than three times the quantitative detection limit
(0.04 µg/mL (40 ng/mL) for the instrumentation used in the validation)
and the sample results are to be averaged with other samples for TWA
calculations, proceed with the AAS-HGA analysis of the sample as described
below.
(i) Set up the atomic
absorption spectrophotometer and HGA for flameless atomic absorption analysis
of cadmium according to the SOP (subsection (5)(i) of this section) or the
manufacturer's operational instructions and allow the instrument to stabilize.
The graphite furnace atomizer is equipped with a pyrolytically coated graphite
tube containing a pyrolytic platform. For the source lamp, use a cadmium hollow
cathode or electrodeless discharge lamp operated at the manufacturer's
recommended setting for graphite furnace operation. The Zeeman background
corrector and EDL are recommended for use with the L'vov platform. Instrumental
parameters for the Perkin-Elmer Model 5100 spectrophotometer and Zeeman HGA-600
graphite furnace used in the validation of this method are given in subsection
(7) of this section.
(ii) Optimize
the energy reading of the spectrophotometer at 228.8 nm by adjusting the lamp
position and the wavelength according to the manufacturer's
instructions.
(iii) Set up the
autosampler to inject a 5-µL aliquot of the working standard, sample or
reagent blank solution onto the L'vov platform along with a 10-µL overlay
of the matrix modifier.
(iv)
Analyze the reagent blank (diluting solution, (e)(vi) of this subsection) and
then autozero the instrument before starting the analysis of a set of samples.
It is recommended that the reagent blank be analyzed several times during the
analysis to assure the integrated absorbance (peak area) reading remains at or
near zero.
(v) Analyze a working
standard approximately midway in the linear portion of the working standard
range two or three times to check for reproducibility and sensitivity (see
subsection (1)(e)(v) and (vi) of this section) before starting the analysis of
samples. Calculate the experimental characteristic mass value from the average
integrated absorbance reading and injection volume of the analyzed working
standard. Compare this value to the manufacturer's suggested value as a check
of proper instrument operation.
(vi) Analyze the reagent blank, working
standard, and sample solutions. Record and label the peak area (abs-sec)
readings and the peak and background peak profiles on the
printer/plotter.
(vii) It is
recommended the entire series of working standards be analyzed at the beginning
and end of the analysis of a set of samples. Establish a concentration-response
curve and ensure standard readings agree with each other and are reproducible.
Also, analyze a working standard after every five or six samples to monitor the
performance of the system. Standard readings should agree within ±15% of
the readings obtained at the beginning of the analysis.
(viii) Bracket the sample readings with
standards during the analysis. If the peak area reading of a sample is above
the peak area reading of the highest working standard, dilute the sample with
the diluting solution and reanalyze. Use the appropriate dilution factor in the
calculations.
(ix) Repeat the
analysis of approximately ten percent of the samples for a check of
precision.
(x) If possible, analyze
quality control samples from an independent source as a check of analytical
recovery and precision.
(xi) Record
the final instrument settings at the end of the analysis. Date and label the
output.
(l)
Calculations.
|
Note:
|
Standards used for HGA analysis are in ng/mL. Total
amounts of cadmium from calculations will be in ng (not MICROg) unless a prior
conversion is made.
|
(i) Correct for
baseline drift and noise in flame AAS analysis by subtracting each baseline
absorbance reading from its corresponding working standard or sample absorbance
reading to obtain the net absorbance reading for each standard and
sample.
(ii) Use a least squares
regression program to plot a concentration-response curve of net absorbance
reading (or peak area for HGA analysis) versus concentration (µg/mL or
ng/mL) of cadmium in each working standard.
(iii) Determine the concentration
(µg/mL or ng/mL) of cadmium in each sample from the resulting
concentration-response curve. If the concentration of cadmium in a sample
solution is less than three times the quantitative detection limit (0.04
µg/mL (40 ng/mL) for the instrumentation used in the validation of the
method) and if consecutive samples were taken on one employee and the sample
results are to be averaged with other samples to determine a single TWA,
reanalyze the sample by AAS-HGA as described in (k) of this subsection and
report the AAS-HGA analytical results.
(iv) Calculate the total amount (µg or
ng) of cadmium in each sample from the sample solution volume (mL):
|
W=(C)(sample vol, mL)(DF)
|
|
Where:
|
W=Total cadmium in sample
|
|
C=Calculated concentration of cadmium
|
|
DF=Dilution Factor (if applicable)
|
(v)
Make a blank correction for each air sample by subtracting the total amount of
cadmium in the corresponding blank sample from the total amount of cadmium in
the sample.
(vi) Calculate the
concentration of cadmium in an air sample (mg/m
3 or
µg/m
3) by using one of the following
equations:
|
mg/m3=Wbc/(Air vol sampled,
L)
|
|
or
|
|
|
MICROg/m3=(Wbc)(1,000
ng/MICROg)/(Air vol sampled, L)
|
|
Where:
|
Wbc=blank corrected total
MICROg cadmium in the sample. (1MICROg=1,000 ng)
|
(4) Backup data.
(a) Introduction.
(i) The purpose of this evaluation is to
determine the analytical method recovery, working standard range, and
qualitative and quantitative detection limits of the two atomic absorption
analytical techniques included in this method. The evaluation consisted of the
following experiments:
(A) An analysis of
twenty-four samples (six samples each at 0.1, 0.5, 1 and 2 times the TWA-PEL)
for the analytical method recovery study of the flame AAS analytical
technique.
(B) An analysis of
eighteen samples (six samples each at 0.5, 1 and 2 times the action level
TWA-PEL) for the analytical method recovery study of the AAS-HGA analytical
technique.
(C) Multiple analyses of
the reagent blank and a series of standard solutions to determine the working
standard range and the qualitative and quantitative detection limits for both
atomic absorption analytical techniques.
(ii) The analytical method recovery results
at all test levels were calculated from concentration-response curves and
statistically examined for outliers at the ninety-nine percent confidence
level. Possible outliers were determined using the Treatment of Outliers test
(subsection (5)(j) of this section). In addition, the sample results of the two
analytical techniques, at 0.5, 1.0 and 2.0 times their target concentrations,
were tested for homogeneity of variances also at the ninety-nine percent
confidence level. Homogeneity of the coefficients of variation was determined
using the Bartlett's test (subsection (5)(k) of this section). The overall
analytical error (OAE) at the ninety-five percent confidence level was
calculated using the equation (subsection (5)(l) of this section):
OAE=±[|Bias|+(1.96)(CV1(pooled))(100%)]
(iii) A derivation of the International Union
of Pure and Applied Chemistry (IUPAC) detection limit equation (subsection
(5)(m) of this section) was used to determine the qualitative and quantitative
detection limits for both atomic absorption analytical techniques:
|
Cld=k(sd)/m
|
(Equation 1)
|
|
|
Where:
|
Cld=the smallest reliable
detectable concentration an analytical instrument can determine at a given
confidence level.
|
|
k=3 for the Qualitative Detection Limit at the
99.86% Confidence Level
|
|
=10 for the Quantitative Detection Limit at the
99.99% Confidence Level.
|
|
sd=standard deviation of the reagent blank (Rbl)
readings.
|
|
m=analytical sensitivity or slope as calculated by
linear regression.
|
(iv)
Collection efficiencies of metallic fume and dust atmospheres on 0.8-µm
mixed cellulose ester membrane filters are well documented and have been shown
to be excellent (subsection (5)(k) of this section). Since elemental cadmium
and the cadmium component of cadmium compounds are nonvolatile, stability
studies of cadmium spiked MCEF samples were not performed.
(b) Equipment.
(i) A Perkin-Elmer (PE) Model 603
spectrophotometer equipped with a manual gas control system, a stainless steel
nebulizer, a burner mixing chamber, a flow spoiler and a 10 cm (one-slot)
burner head was used in the experimental validation of the flame AAS analytical
technique. A PE cadmium hollow cathode lamp, operated at the manufacturer's
recommended current setting for continuous operation (4 mA), was used as the
source lamp. Instrument parameters are listed in subsection (6) of this
section.
(ii) A PE Model 5100
spectrophotometer, Zeeman HGA-600 graphite furnace atomizer and AS-60 HGA
autosampler were used in the experimental validation of the AAS-HGA analytical
technique. The spectrophotometer was equipped with a PE Series 7700
professional computer and Model PR-310 printer. A PE System 2 cadmium
electrodeless discharge lamp, operated at the manufacturer's recommended
current setting for modulated operation (170 mA), was used as the source lamp.
Instrument parameters are listed in subsection (7) of this section.
(c) Reagents.
(i) J.T. Baker Chem. Co. (Analyzed grade)
concentrated nitric acid, 69.0-71.0%, and concentrated hydrochloric acid,
36.5-38.0%, were used to prepare the samples and standards.
(ii) Ammonium phosphate, monobasic,
NH4H2PO4
and magnesium nitrate hexahydrate,
Mg(NO3)2.6
H2O both manufactured by the Mallinckrodt Chem. Co.,
were used to prepare the matrix modifier for AAS-HGA analysis.
(d) Standard preparation for flame
AAS analysis.
(i) Dilute stock solutions:
Prepared 0.01, 0.1, 1, 10 and 100 µg/mL cadmium standard stock solutions
by making appropriate serial dilutions of a commercially available 1,000
µg/mL cadmium standard stock solution (RICCA Chemical Co., Lot# A102)
with the diluting solution (4% HNO3, 0.4%
HCl).
(ii) Analyzed standards:
Prepared cadmium standards in the range of 0.001 to 2.0 µg/mL by
pipetting 2 to 10 mL of the appropriate dilute cadmium stock solution into a
100-mL volumetric flask and diluting to volume with the diluting solution. (See
subsection (3)(g)(ii) of this section).
(e) Standard preparation for AAS-HGA
analysis.
(i) Dilute stock solutions: Prepared
1, 10, 100 and 1,000 ng/mL cadmium standard stock solutions by making
appropriate serial dilutions of a commercially available 1,000 µg/mL
cadmium standard stock solution (J.T. Baker Chemical Co., Instra-analyzed, Lot#
D22642) with the diluting solution (4% HNO3, 0.4%
HCl).
(ii) Analyzed standards:
Prepared cadmium standards in the range of 0.1 to 40 ng/mL by pipetting 2 to 10
mL of the appropriate dilute cadmium stock solution into a 100-mL volumetric
flask and diluting to volume with the diluting solution. (See subsection
(3)(h)(ii) of this section).
(f) Detection limits and standard working
range for flame AAS analysis.
(i) Analyzed
the reagent blank solution and the entire series of cadmium standards in the
range of 0.001 to 2.0 µg/mL three to six times according to the
instructions given in subsection (3)(j) of this section. The diluting solution
(4% HNO3, 0.4% HCl) was used as the reagent blank. The
integration time on the PE 603 spectrophotometer was set to 3.0 seconds and a
four-fold expansion of the absorbance reading of the 2.0 µg/mL cadmium
standard was made prior to analysis. The 2.0 µg/mL standard gave a net
absorbance reading of 0.350 abs. units prior to expansion in agreement with the
manufacturer's specifications (subsection (5)(f) of this section).
(ii) The net absorbance readings of the
reagent blank and the low concentration Cd standards from 0.001 to 0.1
µg/mL and the statistical analysis of the results are shown in Table 1.
The standard deviation, sd, of the six net absorbance readings of the reagent
blank is 1.05 abs. units. The slope, m, as calculated by a linear regression
plot of the net absorbance readings (shown in Table 2) of the 0.02 to 1.0
µg/mL cadmium standards versus their concentration is 772.7 abs.
units/(µg/mL).
(iii) If these
values for sd and the slope, m, are used in Eqn. 1 ((a)(ii) of this
subsection), the qualitative and quantitative detection limits as determined by
the IUPAC Method are:
|
Cld=
|
(3)(1.05 abs. units)/(772.7 abs.
units/(MICROg/mL))= 0.0041 MICROg/mL for the qualitative detection
limit.
|
|
Cld=
|
(10)(1.05 abs. units)/(772.7 abs. units/MICROg/mL))
=0.014 MICROg/mL for the quantitative detection limit.
|
The qualitative and quantitative detection limits for the
flame AAS analytical technique are 0.041 µg and 0.14 µg cadmium,
respectively, for a 10 mL solution volume. These correspond, respectively, to
0.2 µg/m3 and 0.70
µg/m3 for a 200 L air volume.
(iv) The recommended Cd standard
working range for flame AAS analysis is 0.02 to 2.0 µg/mL. The net
absorbance readings of the reagent blank and the recommended working range
standards and the statistical analysis of the results are shown in Table 2. The
standard of lowest concentration in the working range, 0.02 µg/mL, is
slightly greater than the calculated quantitative detection limit, 0.014
µg/mL. The standard of highest concentration in the working range, 2.0
µg/mL, is at the upper end of the linear working range suggested by the
manufacturer (subsection (5)(f) of this section). Although the standard net
absorbance readings are not strictly linear at concentrations above 0.5
µg/mL, the deviation from linearity is only about ten percent at the
upper end of the recommended standard working range. The deviation from
linearity is probably caused by the four-fold expansion of the signal suggested
in the method. As shown in Table 2, the precision of the standard net
absorbance readings are excellent throughout the recommended working range; the
relative standard deviations of the readings range from 0.009 to
0.064.
(g) Detection
limits and standard working range for AAS-HGA analysis.
(i) Analyzed the reagent blank solution and
the entire series of cadmium standards in the range of 0.1 to 40 ng/mL
according to the instructions given in subsection (3)(k) of this section. The
diluting solution (4% HNO3, 0.4% HCl) was used as the
reagent blank. A fresh aliquot of the reagent blank and of each standard was
used for every analysis. The experimental characteristic mass value was 0.41
pg, calculated from the average peak area (abs-sec) reading of the 5 ng/mL
standard which is approximately midway in the linear portion of the working
standard range. This agreed within twenty percent with the characteristic mass
value, 0.35 pg, listed by the manufacturer of the instrument (subsection (5)(b)
of this section).
(ii) The peak
area (abs-sec) readings of the reagent blank and the low concentration Cd
standards from 0.1 to 2.0 ng/mL and statistical analysis of the results are
shown in Table 3. Five of the reagent blank peak area readings were zero and
the sixth reading was 1 and was an outlier. The near lack of a blank signal
does not satisfy a strict interpretation of the IUPAC method for determining
the detection limits. Therefore, the standard deviation of the six peak area
readings of the 0.2 ng/mL cadmium standard, 0.75 abs-sec, was used to calculate
the detection limits by the IUPAC method. The slope, m, as calculated by a
linear regression plot of the peak area (abs-sec) readings (shown in Table 4)
of the 0.2 to 10 ng/mL cadmium standards versus their concentration is 51.5
abs-sec/(ng/mL).
(iii) If 0.75
abs-sec (sd) and 51.5 abs-sec/(ng/mL) (m) are used in Eqn. 1 ((a)(iii) of this
subsection), the qualitative and quantitative detection limits as determined by
the IUPAC method are:
|
Cld=
|
(3)(0.75 abs-sec)/(51.5 abs-sec/(ng/mL)= 0.044
ng/mL for the qualitative detection limit.
|
|
Cld=
|
(10)(0.75 abs-sec)/(51.5 abs-sec/(ng/mL)= 0.15
ng/mL for the quantitative detection limit. The qualitative and quantitative
detection limits for the AAS-HGA analytical technique are 0.44 ng and 1.5 ng
cadmium, respectively, for a 10 mL solution volume. These correspond,
respectively, to 0.007 MICROg/m3 and 0.025
MICROg/m3 for a 60 L air volume.
|
(iv)
The peak area (abs-sec) readings of the Cd standards from 0.2 to 40 ng/mL and
the statistical analysis of the results are given in Table 4. The recommended
standard working range for AAS-HGA analysis is 0.2 to 20 ng/mL. The standard of
lowest concentration in the recommended working range is slightly greater than
the calculated quantitative detection limit, 0.15 ng/mL. The deviation from
linearity of the peak area readings of the 20 ng/mL standard, the highest
concentration standard in the recommended working range, is approximately ten
percent. The deviations from linearity of the peak area readings of the thirty
and forty ng/mL standards are significantly greater than ten percent. As shown
in Table 4, the precision of the peak area readings are satisfactory throughout
the recommended working range; the relative standard deviations of the readings
range from 0.025 to 0.083.
(h) Analytical method recovery for flame AAS
analysis.
(i) Four sets of spiked MCEF
samples were prepared by injecting 20 µL of 10, 50, 100 and 200
µg/mL dilute cadmium stock solutions on 37 mm diameter filters (part No.
AAWP 037 00, Millipore Corp., Bedford, MA) with a calibrated micropipet. The
dilute stock solutions were prepared by making appropriate serial dilutions of
a commercially available 1,000 µg/mL cadmium standard stock solution
(RICCA Chemical Co., Lot # A102) with the diluting solution (4%
HNO3, 0.4% HCl). Each set contained six samples and a
sample blank. The amount of cadmium in the prepared sets were equivalent to
0.1, 0.5, 1.0 and 2.0 times the TWA PEL target concentration of 5
µg/m3 for a 400 L air volume.
(ii) The air-dried spiked filters were
digested and analyzed for their cadmium content by flame atomic absorption
spectroscopy (AAS) following the procedure described in subsection (3) of this
section. The 0.02 to 2.0 µg/mL cadmium standards (the suggested working
range) were used in the analysis of the spiked filters.
(iii) The results of the analysis are given
in Table 5. One result at 0.5 times the TWA PEL target concentration was an
outlier and was excluded from statistical analysis. Experimental justification
for rejecting it is that the outlier value was probably due to a spiking error.
The coefficients of variation for the three test levels at 0.5 to 2.0 times the
TWA PEL target concentration passed the Bartlett's test and were
pooled.
(iv) The average recovery
of the six spiked filter samples at 0.1 times the TWA PEL target concentration
was 118.2% with a coefficient of variation (CV1) of 0.128. The average recovery
of the spiked filter samples in the range of 0.5 to 2.0 times the TWA target
concentration was 104.0% with a pooled coefficient of variation (CV1) of 0.010.
Consequently, the analytical bias found in these spiked sample results over the
tested concentration range was +4.0% and the OAE was ±6.0%.
(i) Analytical method recovery for
AAS-HGA analysis.
(i) Three sets of spiked
MCEF samples were prepared by injecting 15 µL of 5, 10 and 20 µg/mL
dilute cadmium stock solutions on 37 mm diameter filters (part no. AAWP 037 00,
Millipore Corp., Bedford, MA) with a calibrated micropipet. The dilute stock
solutions were prepared by making appropriate serial dilutions of a
commercially available certified 1,000 µg/mL cadmium standard stock
solution (Fisher Chemical Co., Lot# 913438-24) with the diluting solution (4%
HNO3, 0.4% HCl). Each set contained six samples and a
sample blank. The amount of cadmium in the prepared sets were equivalent to
0.5, 1 and 2 times the action level TWA target concentration of 2.5
µg/m3 for a 60 L air volume.
(ii) The air-dried spiked filters were
digested and analyzed for their cadmium content by flameless atomic absorption
spectroscopy using a heated graphite furnace atomizer following the procedure
described in subsection (3) of this section. A five-fold dilution of the spiked
filter samples at 2 times the action level TWA was made prior to their
analysis. The 0.05 to 20 ng/mL cadmium standards were used in the analysis of
the spiked filters.
(iii) The
results of the analysis are given in Table 6. There were no outliers. The
coefficients of variation for the three test levels at 0.5 to 2.0 times the
action level TWA PEL passed the Bartlett's test and were pooled. The average
recovery of the spiked filter samples was 94.2% with a pooled coefficient of
variation (CV1) of 0.043. Consequently, the analytical bias was -5.8% and the
OAE was ±14.2%.
(j) Conclusions.
The experiments performed in this evaluation show the two
atomic absorption analytical techniques included in this method to be precise
and accurate and have sufficient sensitivity to measure airborne cadmium over a
broad range of exposure levels and sampling periods.
(5) References.
(a)
Slavin, W. Graphite Furnace AAS
-- A Source Book; Perkin-Elmer Corp., Spectroscopy Div.: Ridgefield, CT, 1984;
p. 18 and pp. 83-90.
(b)
Grosser, Z., Ed.; Techniques in Graphite Furnace Atomic Absorption
Spectrophotometry; Perkin-Elmer Corp., Spectroscopy Div.: Ridgefield, CT,
1985.
(c)
Occupational Safety and Health Administration Salt Lake Technical
Center: Metal and Metalloid Particulate in Workplace Atmospheres (Atomic
Absorption) (USDOL/OSHA Method No. ID-121). In OSHA Analytical Methods Manual
2nd ed. Cincinnati, OH: American Conference of Governmental Industrial
Hygienists, 1991.
(d)
Occupational Safety and Health Administration Salt Lake Technical
Center: Metal and Metalloid Particulate in Workplace Atmospheres (ICP)
(USDOL/OSHA Method No. ID-125G). In OSHA Analytical Methods Manual 2nd ed.
Cincinnati, OH: American Conference of Governmental Industrial Hygienists,
1991.
(e)
Windholz, M., Ed.; The Merck Index, 10th ed.; Merck & Co.: Rahway,
NJ, 1983.
(f)
Analytical Methods for Atomic Absorption Spectrophotometry, The
Perkin-Elmer Corporation: Norwalk, CT, 1982.
(g)
Slavin, W., D.C. Manning, G.
Carnrick, and E. Pruszkowska: Properties of the Cadmium Determination with the
Platform Furnace and Zeeman Background Correction. Spectrochim. Acta
38B:1157-1170 (1983).
(h)
Occupational Safety and Health Administration Salt Lake Technical
Center: Standard Operating Procedure for Atomic Absorption. Salt Lake City, UT:
USDOL/OSHA-SLTC, In progress.
(i)
Occupational Safety and Health
Administration Salt Lake Technical Center: AAS-HGA Standard Operating
Procedure. Salt Lake City, UT: USDOL/OSHA- SLTC, In
progress.
(j)
Mandel, J.: Accuracy and Precision, Evaluation and Interpretation of
Analytical Results, The Treatment of Outliers. In Treatise On Analytical
Chemistry, 2nd ed., Vol.1, edited by I. M. Kolthoff and P. J. Elving. New York:
John Wiley and Sons, 1978. pp. 282-285.
(k)
National Institute for
Occupational Safety and Health: Documentation of the NIOSH Validation Tests by
D. Taylor, R. Kupel, and J. Bryant (DHEW/NIOSH Pub. No. 77-185). Cincinnati,
OH: National Institute for Occupational Safety and Health,
1977.
(l)
Occupational Safety and Health Administration Analytical Laboratory:
Precision and Accuracy Data Protocol for Laboratory Validations. In OSHA
Analytical Methods Manual 1st ed. Cincinnati, OH: American Conference of
Governmental Industrial Hygienists (Pub. No. ISBN: 0-936712-66-X),
1985.
(m)
Long,
G.L. and J.D. Winefordner: Limit of Detection -- A Closer Look at the IUPAC
Definition. Anal.Chem. 55:712A-724A (1983).
(n)
American Conference of
Governmental Industrial Hygienists: Documentation of Threshold Limit Values and
Biological Exposure Indices. 5th ed. Cincinnati, OH: American Conference of
Governmental Industrial Hygienists, 1986.
Table 1 -- Cd Detection Limit Study
[Flame AAS Analysis]
|
STD (MICROg/mL)
|
Absorbance
reading at
228.8 nm
|
Statistical
analysis
|
|
Reagent blank
|
5
|
2
|
n=6.
|
|
4
|
3
|
mean=3.50.
|
|
4
|
3
|
std dev=1.05.
|
|
|
|
CV=0.30.
|
|
0.001
|
6
|
6
|
n=6.
|
|
2
|
4
|
mean=5.00.
|
|
6
|
6
|
std dev=1.67.
|
|
|
|
CV=0.335.
|
|
0.002
|
5
|
7
|
n=6.
|
|
7
|
3
|
mean=5.50.
|
|
7
|
4
|
std dev=1.76.
|
|
|
|
CV=0.320.
|
|
0.005
|
7
|
7
|
n=6.
|
|
8
|
8
|
mean=7.33.
|
|
8
|
6
|
std dev=0.817.
|
|
|
|
CV=0.111.
|
|
0.010
|
10
|
9
|
n=6.
|
|
10
|
13
|
mean=10.3.
|
|
10
|
10
|
std dev=1.37.
|
|
|
|
CV=0.133.
|
|
0.020
|
20
|
23
|
n=6.
|
|
20
|
22
|
mean=20.8.
|
|
20
|
20
|
std dev=1.33.
|
|
|
|
CV=0.064.
|
|
0.050
|
42
|
42
|
n=6.
|
|
42
|
42
|
mean=42.5.
|
|
42
|
45
|
std dev=1.22.
|
|
|
|
CV=0.029.
|
|
0.10
|
|
84
|
n=3.
|
|
|
80
|
mean=82.3.
|
|
|
83
|
std dev=2.08.
|
|
|
|
CV=0.025.
|
Table 2 -- Cd Standard Working Range
*Rejected as an outlier -- this value did not pass the
outlier T-test at the 99% confidence level.
Correction. Zeeman Graphite Furnace
