EPA-600/4-86-024
Method 200.6 - Dissolved Calcium, Magnesium, Potassium, and Sodium in
Net Deposition by Flame Atomic Absorption Spectrophotometry
[METHOD ONLY]
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US EPA
Headquarters and Chemical Libraries
EPA West Bldg Room 3340
Mailcode 3404T
1301 Constitution Ave NW
Washington DC 20004
202-566-0556
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US EPA
Headquarters and Chemical Libraries
EPA West Bldg Room 3340
Mailcode 3404T
1301 Constitution Ave NW
Washington DC 20004
202-566-0556
Method 200.6 Dissolved Calcium, Magnesium, Potassium,
and Sodium in Wet Deposition by Flame Atomic
Absorption Spectrophotometry
March 1986
CO
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Performing Laboratory:
Loretta M. Skowron
Carla Jo Brennan
Mark E. Peden
Illinois State Water Survey
Analytical Chemistry Unit
2204 Griffith Drive
Champaign, Illinois 61820
U.S. Environmental Protection Agency
Region 5, UbMfyWiZJ)
H w«st Jackson Boulevard, 12th Roar
Chicago, IL 60604-3590
Sponsoring Agency:
John D. Pfaff, Project Officer
e~}
^~"^ Inorganic Analysis Section
Physical and Chemical Methods Branch
United States Environmental Protection Agency
Office of Research and Development
Environmental Monitoring and Support Laboratory
Cincinnati, Ohio 45268
200.6-1
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INDEX
Section
Number Subject
1 Scope and Application
2 Summary of Method
3 Definitions
4 Interferences
5 Safety
6 Apparatus and Equipment
7 Reagents and Consumable Materials
8 Sample Collection, Preservation, and Storage
9 Calibration and Standardization
10 Quality Control
11 Procedure
12 Calculations
13 Precision and Bias
14 References
TABLES
Method Detection Limits and Concentration Ranges for Flame Atomic
Absorption Spectrophotometric Analysis of Wet Deposition.
Operating Conditions and Suggested Calibration Standard Concentrations
for the Determination of Calcium, Magnesium, Potassium, and Sodium in Wet
Deposition Samples.
Single-Operator Precision and Bias for Calcium, Magnesium, Potassium, and
Sodium Determined from Analyte Spikes of Wet Deposition Samples.
Single-Operator Precision and Bias for Calcium, Magnesium, Potassium, and
Sodium Determined from Quality Control Check Samples.
FIGURES
1. Percentile Concentration Values Obtained from Wet Deposition Samples:
Calcium, Magnesium, Potassium, and Sodium.
200.6-2
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1. SCOPE AND APPLICATION
1.1 This method is appiicaoie to the determination of calcium,
magnesium, potassium, and sodium in wet deposition by flame atomic
absorption spectrophotometry (FAAS).
1.2 The term "wet deposition" is used in this method to designate rain,
snow, dew, sleet, and hail.
1.3 The method detection limits (MDL) for the above analytes determined
from replicate analyses of quality control check solutions containing
0.053 mg/L calcium, 0.018 mg/L magnesium, 0.012 mg/L sodium, and
0.013 mg/L potassium are 0.007, 0.002, 0.003, and 0.003 mg/L,
respectively. The concentration range of this method is outlined in
Table 1.
1.4 Figure 1 represents cumulative frequency percentile concentration
plots of calcium, magnesium, potassium, and sodium obtained from the
analysis of over five thousand wet deposition samples. These data
should be considered during the selection of appropriate calibration
standard concentrations.
2. SUMMARY OF METHOD
2.1 A solution containing the element(s) of interest is aspirated as a
fine mist into a flame where it is converted to an atomic vapor
consisting of ground state atoms. These ground state atoms are
capable of absorbing electromagnetic radiation over a series of
very narrow, sharply defined wavelengths. A distinct line source of
light, usually a hollow cathode lamp specific to the metal of
interest, is used to pass a beam through the flame. Light from the
source beam, less whatever intensity was absorbed by the atoms of the
metal of interest, is isolated by the monochromator and measured by
the photodetector. The amount of light absorbed by the analyte is
quantified by comparing the light transmitted through the flame to
light transmitted by a reference beam. The amount of light absorbed
in the flame is proportional to the concentration of the metal in
solution. The relationship between absorption and concentration is
expressed by Beer's Law:
log(I /I) = abc = A
o
where: I = incident radiant power
1° = transmitted radiant power
a = absorptivity (constant for a given system)
b = sample path length
c = concentration of absorbing species (mg/L)
A = absorbance
The atomic absorption spectrophotometer is calibrated with standard
solutions containing known concentrations of the element(s) of
interest. Calibration curves are constructed from which the
concentration of each analyte in the unknown sample is determined.
200.6-3
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3. DEFINITIONS
3.1 ABSORBANCE (A) the logarithm to the base ten of the reciprocal
of the transmittance, (T):
A = log(l/T)
0.0044 A = the absorption of 1% of
the transmitted light.
The absorbance is related to the analyte concentration by Beer's Law
(Sect. 2.1) where 1/T =1/1
o
3.2 ATOMIC ABSORPTION the absorption of electromagnetic radiation by
an atom resulting in the elevation of electrons from their ground
states to excited states. Atomic absorption spectrophotometry
involves the measurement of light absorbed by atoms of interest as a
function of the concentration of those atoms in a solution.
3.3 SPECTRAL BANDWIDTH ~ the wavelength or frequency interval of
radiation leaving the exit slit of a monochromator between limits set
at a radiant power level half way between the continuous background
and the peak of an emission line or an absorption band of negligible
intrinsic width (14.1).
3.4 SPECTROPHOTOMETER an instrument that provides the ratio, or a
function of the ratio, of the radiant power of two light beams as a
function of spectral wavelength. These two beams may be separated
in time and/or space.
3.5 For definitions of other terms used in this method, refer to the
glossary. For an explanation of the metric system including units,
symbols, and conversion factors see American Society for Testing and
Materials (ASTM) Standard E 380, "Metric Practices" (14.2).
4. INTERFERENCES
4.1 Chemical interference is the most frequently encountered
interference in atomic absorption spectrophotometry. A chemical
interference may prevent, enhance, or suppress the formation of
ground state atoms in the flame. For example, in the case of
calcium determinations, the presence of phosphate or sulfate can
result in the formation of a salt that hinders proper atomization of
the solution when it is aspirated into the flame. This decreases the
number of free, ground state atoms in the flame, resulting in lowered
absorbance values. Aluminum can cause a similar interference when
measuring magnesium. The addition of appropriate complexing agents
to the sample solution reduces or eliminates chemical interferences
and may increase the sensitivity of the method.
200.6-4
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4.2 Alkali metals such as sodium and potassium may undergo ionization in
an air-acetylene flame resulting in a decrease in ground state atoms
available for measurement by atomic absorption. Addition of a large
excess of an easily ionizable element such as cesium will eliminate
this problem, since cesium will be preferentially ionized. The
preferential ionization of the cesium solution results in an enhanced
atomic absorption signal for both potassium and sodium (14.3).
4.3 If a sample containing low concentrations of the metal being
measured is analyzed immediately after a sample having a
concentration exceeding the highest calibration standard, sample
carry-over will result in elevated readings. To prevent this
interference, routinely aspirate water (Sect. 7.2) for about 15
seconds after a high concentration sample. Depending on the
concentration of metal in the last sample analyzed, it may be
necessary to rinse for longer time periods. Complete purging of the
system is ascertained by aspirating water until the absorbance
readout returns to the baseline.
4.4 Wet deposition samples are characterized by low ionic strength and
rarely contain enough salts to cause interferences due to
nonspecific background absorbance. The use of background correction
techniques is not necessary and will decrease the signal to noise
ratio and lessen precision.
5. SAFETY
5.1 The calibration standards, sample types, and most reagents used in
this method pose no hazard to the analyst. Use a fume hood,
protective clothing, and safety glasses when handling concentrated
hydrochloric acid (Sect. 7.5-6).
5.2 Use a fume hood, protective clothing, and safety glasses when
preparing the lanthanum solution. The reaction between the lanthanum
oxide and acid (Sect. 7.7) is extremely exothermic.
5.3 A permanent ventilation system is required to eliminate the large
quantity of hot exhaust gases produced during instrument operation.
Since acetylene is a flammable gas, take precautions when using it.
To avoid explosions, never pass acetylene through copper or
high-copper alloy (brass, bronze) fittings or piping.
5.4 The operator must wear safety glasses to avoid eye damage from the
ultraviolet light emitted by the flame.
5.5 To avoid in-line explosions, do not allow the pressure of acetylene
being delivered to the instrument to exceed 15 psig (10.6 g/m ).
In the event of a flashback, turn off the gas control switch, the
instrument power, and the gas tanks.
5.6 Follow manufacturer's operating guidelines carefully when optimizing
gas flow rates. Too low gas flow rates can result in a combustion
within the gas mixing chamber and therefore a flashback.
200.6-5
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5.7 Check that the drain tube from the gas mixing chamber, fitted with a
safety trap, is filled with water before igniting the flame. Keep
the drain tube filled to prevent explosion in the chamber. The
safety trap may be either looped or valved.
5.8 Avoid any contact with a hot burner head. Serious tissue burns will
result.
5.9 Follow American Chemical Society guidelines regarding safe handling
of chemicals used in this method (14.4).
6. APPARATUS AND EQUIPMENT
6.1 ATOMIC ABSORPTION SPECTROPHOTOMETER Select a double-beam
instrument having a dual grating monochromator, p'hotodetector,
pressure-reducing valves, adjustable spectral bandwith, wavelength
range of 190-800 nm, and provisions for interfacing with a strip
chart recorder or a suitable data system.
6.1.1 Burner ~ Use a long path, single slot air-acetylene burner
head supplied by the manufacturer of the spectrophotometer.
6.1.2 Hollow Cathode Lamps ~ Single element lamps are recommended.
Multi-element lamps are available but are not recommended.
They generally have a shorter lifespan, are less sensitive,
require a higher operating current, and increase the chances
of spectral interferences. When available, electrodeless
discharge lamps (EDL) may also be used.
6.1.3 Monochromator To increase sensitivity of calcium and
potassium determinations, use a monochromator equipped with a
blaze grating in the range of 500-600 nm (14.5). For the
analysis of sodium and magnesium, a blaze grating in the range
of 200-250 nm is adequate.
6.1.4 Photomultiplier Tube A wide spectral range (160-900 nm)
phototube is recommended. Select a red-sensitive phototube to
detect potassium at 766.5 nm and to increase sensitivity to
calcium at 422.7 nm.
6.2 The first time any glassware is used for making stock solutions and
standards, clean with 0.6 N HCl and rinse thoroughly with water
(Sect. 7.2) before use. Maintain a set of Class A volumetric flasks
to be used only when making dilute working standards for the analysis
of wet deposition samples. Store filled with water (Sect. 7.2) and
covered.
200.6-6
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6.3 LABORATORY FACILITIES Laboratories used for the analysis of
wet deposition samples should be free from external sources of
contamination. The use of laminar flow clean air workstations is
recommended for sample processing and preparation to avoid the
introduction of airborne contaminants. If a clean air bench is
unavailable, samples must be capped or covered prior to analysis. A
positive pressure environment within the laboratory is also
recommended to minimize the introduction of external sources of
contaminant gases and participates. Windows within the laboratory
should be kept closed at all times and sealed if air leaks are
apparent. The use of disposable tacky floor mats at the entrance to
the laboratory is helpful in reducing the particulate loading within
the room.
7. REAGENTS AND CONSUMABLE MATERIALS
7.1 PURITY OF REAGENTS Use chemicals of reagent grade or better for
all solutions. All reagents shall conform to the specifications of
the Committee on Analytical Reagents of the American Chemical Society
(ACS) where such specifications are available.
7.2 PURITY OF WATER ~ Use water conforming to ASTM Specification D
1193, Type II (14.6). Point of use 0.2 micrometer filters are
recommended for all faucets supplying water to prevent the
introduction of bacteria and/or ion exchange resins into reagents,
standard solutions, and internally formulated quality control check
solutions.
7.3 ACETYLENE (C H ) Fuel Minimum acceptable acetylene purity
is 99.5% (v/v). Change the cylinder when the pressure reaches
75 psig (53 g/m } if the acetylene is packed in acetone.
Pre-purified grades that contain a proprietary solvent can be used to
30 psig (21 g/m ) before replacement. Avoid introducing these
solvents into the instrument. Damage to the instrument's plumbing
system can result. Solvent in the system is indicated by abnormally
high pulsating background noise. To prevent solvent carryover, allow
acetylene cylinders to stand for at least 24 hours before use.
CAUTION: Acetylene is a highly flammable gas. Follow the
precautions in Sect. 5.3-6 regarding safe operating pressures,
suitable plumbing, and operator safety.
7.4 CESIUM SOLUTION (1.0 mL = 100.0 mg Cs) lonization Supgressant
Dissolve 126.7 g of cesium chloride (CsCl), dried at 105 C for one
hour, in water (Sect. 7.2) and dilute to 1 L. Store at room
temperature in a high density polyethylene or polypropylene
container. Add to samples and standards as directed in Sect. 9.4 and
11.4 for the determination of potassium and sodium.
7.5 HYDROCHLORIC ACID (6.0 N) Carefully add 1 volume of concentrated
hydrochloric acid (HC1, sp gr 1.19) to an equal volume of water
(Sect. 7.2).
200.6-7
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7.6 HYDROCHLORIC ACID {0.6 N) Add 50 mL of concentrated hydrochloric
acici (HC1, sp gr 1.195 to 900 mL of water (Sect. "7.21 and dilute to
1 L.
7.7 LANTHANUM SOLUTION (1.0 mL = 100.0 mg La) Releasing Agent In a
glass 1 L volumetric flask, place 117.0 g of lanthanum oxide
(La O ), dried at 105°C for one hour. Add 6 N HCl very
carefully to the solid in increments of about 0.5 mL. Cool the
solution between additions. Continue adding the acid solution to the
flask in increasing increments until a total of 500 mL of 6 N HCl has
been added. Dilute to 1 L with water (Sect. 7.2). Store at room
temperature in a high density polyethylene or polypropylene
container. Add to samples and standards as directed in Sect. 9.4.3
and 11.4 for the determination of calcium and magnesium.
CAUTION: Dissolving lanthanum oxide in hydrochloric acid is a
violently exothermic reaction; use extreme caution when dissolving
the reagent. Refer to Sect. 5.2 for proper safety precautions when
preparing this solution.
7.8 OXIDANT (air) The air may be provided by a compressor or
commercially bottled gas supply. Remove oil, water, and other
foreign matter from the air using a filter recommended by the
manufacturer. Refer to the manufacturer's guidelines for recommended
delivery pressure.
7.9 STOCK STANDARD SOLUTIONS Stock standard solutions may be
purchased as certified solutions or prepared from ACS reagent grade
materials as detailed below. Store the solutions at room temperature
in high density polyethylene or polypropylene containers.
7.9.1 Calcium Solution, Stock (1.0 mL = 1.0 mg Ca) Add 2.497 g
of calcium carbonate (CaCO ), dried at 180 C for one
hour, to approximately 600 mL of water (Sect. 7.2). Add
concentrated hydrochloric acid
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8. SAMPLE COLLECTION, PRESERVATION, AND STORAGE
8.1 Collect samples in high density polyethylene (HOPE) containers that
have been thoroughly riiibed with ASTM Type II water (7.2). Do not
use strong mineral acids or alkaline detergent solutions for cleaning
collection vessels. Residual acids may remain in the polyethylene
matrix and slowly leach back into the sample. Alkaline detergents
may also leave residues that may affect the sample chemistry. Cap
collection bottles after cleaning to prevent contamination from
airborne contaminants; air dry collection buckets in a laminar flow
clean air workstation and wrap in polyethylene bags prior to use. If
a laminar flow workstation is not available, pour out any residual
rinse water and bag the buckets immediately. Do not dry the bucket
interior by any method other than air drying in a laminar flow clean
air workstation.
8.2 The frequency of sample collection and the choice of sampler design
are dependent on the monitoring objectives. In general, the use of
wet-only samplers is recommended to exclude dry deposition
contributions, minimize sample contamination, retard evaporation,
and enhance sample stability. Sample collection frequency may vary
from subevent to monthly sampling periods. Collection periods of
more than one week are not recommended since sample integrity may be
compromised by longer exposure periods.
8.3 The dissolution of particulate materials can affect the stability of
calcium, magnesium, sodium, and potassium in wet deposition samples
(14.7). This instability generally results in a concentration
increase for these constituents. Measurements should be made
immediately after sample collection to obtain representative data.
Refrigeration of samples at 4°C will minimize but not eliminate
concentration changes.
8.1.1 Filtration of samples through a 0.45 micrometer membrane leached
with water (Sect. 7.2) is effective at stabilizing samples that
are influenced by the dissolution of alkaline particulate matter
(14.7). Monitoring of the filtration procedure is necessary to
ensure that samples are not contaminated by the membrane or
filtration apparatus. Filtered samples are stable for six weeks
when stored at room temperature.
9. CALIBRATION AND STANDARDIZATION
9.1 SETTING INSTRUMENT PARAMETERS
9.1.1 Lamp Current Refer to manufacturer's guidelines for
optimization of this parameter. The use of excessively high
currents will shorten lamp life. High currents also cause
line broadening, resulting in a reduction in sensitivity and
calibration curve linearity, especially in the determination of
magnesium. The use of currents that are too low will cause lamp
instability and insufficient throughput of energy through the
instrument's optical system. The result is increased signal
noise due to excess electrical gain applied to the photodetector.
200.6-9
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9.1.2 Light Beam Position a small card over the burner slot to
intercept the light beam from the hollow cathode lamp. Check
that the beam is focused midway along the slot and, if
necessary, focus according to the manufacturer's guidelines.
Rotate the lamp within its holder for maximum energy output
readings.
9.1.3 Burner/Beam Alignment Position & small card over the burner
slot to intercept the light beam from the hollow cathode lamp.
For optimal sensitivity when analyzing calcium, magnesium,
potassium, and sodium, adjust the burner height so that the
center of the light beam is approximately 6 mm above the
surface of the burner slot. By adjusting the burner alignment
and rotation, set the light beam to coincide with the burner
slot. While observing from above, move the card along the
full length of the burner slot to ensure that the beam is
centered over the slot for the entire length of the burner.
Optimize this parameter for maximum instrumental sensitivity
as directed in Sect. 9.2.
9.1.4 Wavelength Set the wavelength of the spectrophotometer for
each analyte according to Table 2 by following the
manufacturer's operating guidelines. After the instrument has
warmed up with the flame burning (about 30 minutes), check the
wavelength and readjust if necessary.
Note: The sodium spectrum is characterized by a doublet at
589.0 nm and 589.5 nm. The wavelength chosen for sodium
determinations depends on the degree of analytical sensitivity
desired by the operator. A setting of 589.0 nm will provide
maximum sensitivity in the concentration range of most wet
deposition samples. For those samples with higher sodium
concentrations, a less sensitive setting of 589.5 nm is more
appropriate. Refer to Tables 1 and 2 for information
regarding working ranges, standards, and detection limits for
sodium at each wavelength setting.
9.1.5 Spectral Bandwidth The selection of optimum bandwidth
depends upon the spectrum of the particular element being
analyzed. For the determination of calcium, magnesium, and
potassium, a relatively wide (1.0 nm) bandwidth is
appropriate. Because the sodium spectrum is characterized by
a doublet, use a smaller bandwidth of 0.5 nm.
9.1.6 External Gas Settings Follow manufacturer's recommended
delivery pressures for air and acetylene. Never allow
acetylene pressure to exceed 15 psig (10.6 g/m ).
200.6-10
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9.1.7 Nebulization Rate Set the acetylene and air flow rates as
recommended by the manufacturer. Adjust the nebulizer sample
uptake rate to approximately 5 mL/min. If an adjustable glass
bead nebulizer is used, adjust it according to manufacturer's
guidelines. Exact placement of the glass bead is critical to
ensure that a uniform vapor of the smallest size particles is
introduced into the flame. Improper spacing of the bead from
the nebulizer end will result in poor precision and
sensitivity. Optimize the sample uptake rate for maximum
sensitivity as directed in Sect. 9.2.
Note: The nebulizer can clog easily if particulates are
present in the samples. Symptoms of this are decreased
sensitivity and/or dramatically increased signal noise,
especially noticeable at the higher concentration levels.
A thorough cleaning with a small diameter wire is usually
sufficient to unclog the nebulizer.
9.1.8 Flame Conditions If the flame temperature is too low,
compounds containing the analyte will not be completely
dissociated. Alternatively, too high a flame temperature may
result in ionization. In both cases, a decrease in the
apparent concentration of the analyte will result. In
general, calcium exhibits maximum sensitivity at higher fuel
and oxidant flow rates. Maximum sensitivity for potassium is
obtained with minimal gas flow rates, resulting in lower flame
temperature and allowing longer residence time of the atomic
vapor in the flame. The MOLs stated in Sect. 1.3 for
magnesium and sodium are obtained over a wide range of flame
conditions. Optimize this parameter for maximum instrumental
sensitivity as directed in Sect. 9.2.
CAUTION: Follow manufacturer's operating guidelines
carefully when setting gas flow rates since combustion within
the gas mixing chamber can occur if caution is not exercised.
9.2 Optimization Allow the instrument to warm up for 30 minutes before
beginning the optimization. Set the instrument readout to absorbance
units and set the integration time to <0.5 seconds. Use either a
strip chart recorder or set the display in a continuous read mode to
monitor absorbance readings. Aspirate a calibration standard at a
concentration near the midpoint of the working range (Sect. 9.43.
While watching the absorbance readings, adjust the instrument
parameters with small, discrete changes until maximum values are
obtained. Parameters such as flame conditions, nebulization rate,
and the region of maximum atom concentration in the flame are
interrelated. Adjustment of any of these three parameters usually
requires adjustment of the other two.
200.6-11
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9.3 Instrument Response Time Determine the minimum sample uptake time
before taking a reading on a sample or standard solution. Use either
ft strip chart recorder or set the display in a continuous read mode
to monitor absorbance readings. After purging the system with water
(Sect. 7.2), aspirate the highest calibration standard (Sect. 9.4)
and measure the length of time necessary to obtain a stable reading.
Aspirate water fSect. 7.2) and measure the time it takes for the
baseline to return to zero.
Note: If the time necessary for the baseline to return to zero is
longer than 15 seconds, a clogged nebulizer may be suspect. If
purging tine begins co increase during sample analysis, this may also
be an indication of nebulizer clogging.
9.4 CALIBRATION SOLUTIONS
9.4.1 Five calibration solutions and one zero standard are needed to
generate a suitable calibration curve. The lowest calibration
solution should contain the analyte of interest at a
concentration greater than or equal to the method detection
limit. The highest solution should approach the expected
upper limit of concentration of the analyte in wet deposition.
Prepare the remaining solutions such that they are evenly
distributed throughout the concentration range. Suggested
calibration standards for each analyte are listed in Table 2.
9.4.2 Prepare all calibration standards by diluting the stock
standards (Sect. 7.9) with water (Sect. 7.2). Use glass
(Class A) or plastic pipettes that are within the bias and
precision tolerances specified by the manufacturer. The
calibration standards are stdble for three months if stored at
room temperature in high density polyethylene or polypropylene
containers.
9.4.3 After preparing the calibration standards to volume, add the
lanthanum solution (Sect. 7.7) to the calcium and magnesium
standards to yield 1000 mg/L La. Add the cesium solution
(Sect. 7.4) to the potassium and sodium standards for
1000 rag/L Cs. Mix well. Use the same stock of xonization
suppressant or releasing agent for the samples and the
calibration standards.
Note: The final volume of each working standard solution
exceeds the nominal volume toy 1%. This adjustment is
necessary to maintain consistency when the appropriate volume
of suppressor solution is added to the wet deposition samples.
200.6-12
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9.5 CALIBRATION
9.5.1 To establish a baseline, aspirate the zero standard and set
the absorbance readout to 0.000. Aspirate the calibration
standards, allowing tints for each standard to equilibrate in
the flame and gas mixing chamber before measuring the
absorbance (Sect. 9.3). Construct calibration curves for each
of the four analytes according to Sect. 12.
9.5.2 Analyze all the calibration standard solutions. The apparent
concentration values must agree with the nominal
concentrations within the predetermined control limits (Sect.
10.2.1) of three times the standard deviation (±3s) . If
results fall outside of these limits, recalibrate the
instrument. If there is a consistent bias greater than
x _* 2s and less than x +_ 3s, for all of the concentration
values measured, reestablish the baseline with the zero
standard and reanalyze the calibration standards.
9.5.3 Verify the calibration curve after every ten samples and at
the end cf each day's analyses according to Sect. 10.7.
10. QUALITY CONTROL
10.1 Each laboratory using this method should develop formalized quality
control protocols to continually monitor the bias and precision of
all measurements. These protocols are required to ensure that the
measurement system is in a state of statistical control. Estimates
of bias and precision for wet deposition analyses cannot be
made unless these control procedures are followed. Detailed
guidelines for the development of quality assurance and quality
control protocols for precipitation measurement systems are
published in a manual available from the United States
Environmental Protection Agency, Research Triangle Park, NC 27711
(14.8). Included in this manual are procedures for the development
of statistical control charts for use in monitoring bias and
precision as well as recommendations for the introduction of
reagent blanks, laboratory duplicates, field duplicates, spike
samples, and performance evaluation samples. These guidelines are
to be used by all laboratories involved with wet deposition
measurements.
10.2 ESTABLISHMENT OF WARNING AND CONTROL LIMITS ~ Warning and control
limits are used to monitor drift in the calibration curve, analyses
of quality control check samples (QCS), and measured recoveries
from laboratory spikes.
10.2.1 Calibration Curve After a calibration curve has been
constructed according to Sect. 12, reanalyze additional
aliquots of all the standards. Calculate the
concentrations using the previously derived calibration
curve. Repeat this procedure until at least ten
200.6-13
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determinations at each concentration level have been made.
These data should be collected on ten different days to
provide a realistic estimate of the method variability.
Calculate a standard deviation (s) at each concentration
level. Use the nominal standard concentration as the mean
value (x) for determining the control limits. A warning
limit of x ^ 2s and a control limit of x ^ 3s should be
used. Reestablish these limits whenever instrumental
operating conditions change.
10.2.2 Quality Control Check Samples {QCS} Calculate warning
and control limits for QCS solutions from a minimum of ten
analyses performed on ten days. Use the calculated
standard deviation (s) at each QCS concentration level to
develop the limits as described in Sect. 10.2.1. Use the
certified or NBS traceable concentration as the mean
(target) value. Constant positive or negative measurements
with respect to the true value are indicative of a method
or procedural bias. Utilize the data obtained from QCS
measurements as in Sect. 10.6 to determine when the
measurement system is out of statistical control. The
standard deviations used to generate the QCS control limits
should be comparable to the single operator precision
reported in Table 4. Reestablish new warning and control
limits whenever instrumental operating conditions are
varied or QCS concentrations are changed.
10.2.3 Laboratory Spike Solutions A minimum of ten analyte
spikes of wet deposition samples is required to develop a
preliminary data base for the calculation of warning and
control limits for spike recovery data. Select the spike
concentration such that the working range of che method
will not be exceeded. Samples selected for the initial
spike recovery study should represent the concentration
range common to wet deposition samples in order to reliably
estimate the method accuracy. Calculate a mean and
standard deviation of the percent recovery data using the
formulas provided in the glossary. Determine warning and
control limits using H-2s and +3s, respectively. If
the data indicate that no significant method bias exists
(14.9), the 100 percent recovery is used as the mean
percent recovery. Where a significant bias is determined
at the 95% confidence level, the control limits are
centered around the bias estimate. Routine spiked sample
analyses that yield percent recovery data outside of the
control limits are an indication of matrix interferences
that should be resolved before routine analyses are
continued.
200.6-14
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10.2.4 All warning and control limits should be reevaluated on a
continual basis as additional data are collected during
routine analyses. The limits should be broadened c~
narrowed if a recalculated standard deviation under similar
operating conditions provides a different estimate of the
procedure variability.
10.3 Monitor the cleaning procedure by pouring a volume of water (Sect.
7.2} that approximates the median sample size into the collection
vessel. Allow the water to remain in the sealed or capped
collection container for at least 24 hours and determine the
concentration of the analytes of interest. If any of the measured
concentrations exceed the MDL, a contamination problem is indicated
in the cleaning procedure. Take corrective action before the
sampling containers are used for the collection of wet deposition.
10.4 Keep daily records of calibration data and the instrument operating
parameters used at the time of data acquisition. Use these
historical data as general performance indicators. Gross changes
in sensitivity, curve linearity, or photomultiplier tube voltage
are indicative of a problem. Possibilities include instrument
malfunction, clogged nebulizer, incomplete optimization, bad hollow
cathode lamp, contamination, and inaccurate standard solutions.
10.5 Precision will vary over the analyte concentration range. Standard
deviation (s) increases as concentration increases while relative
standard deviation (RSD) decreases. At approximately 100 times the
MDL, the RSD should remain less than 1%.
10.6 Analyze a quality control check sample (QCS) after a calibration
curve has been established. This sample may be formulated in the
laboratory or obtained from the National Bureau of Standards (NBS
Standard Reference Material 2694, Simulated Rainwater). The check
sample(s) selected must be within the range of the calibration
standards. Prepare according to Sect. 11.4. If the measured value
for the QCS falls outside of the +3s limits (Sect. 10.2.2), or if
two successive QCS checks are outside of the +2s limits, a
problem is indicated with the spectrophotometer or calibration
curve. Reestablish the baseline with the zero standard and/or
recalibrate. If the QCS analysis is still beyond control limits,
inaccurate working standards might be the problem. Prepare new
standards. Plot the data obtained from the QCS checks on a control
chart for routine assessments of bias and precision.
10.7 Verify the calibration curve after a maximum of ten samples and at
the end of each day's analyses. Analyze a zero standard and
calibration standards at the low and high ends of the working
range. If the routine calibration checks do not meet the criteria
described in Sect. 10.6, recalibrate the system and reanalyze all
samples from the last time the system was in control. Verify the
new calibration curve with the QCS according to Sect. 10.6 and
reanalyze all samples from the last time the measurement system was
in control.
200.6-15
-------
10.8 Submit a Field Blank (FB) to the laboratory for every 20 samples.
The FB may consist of a water sample (Sect. 7.2) or a known
reference solution that approximates the concentration levels
characteristic of wet deposition. The FB is poured into the
sampling vessel at the field site and undergoes identical
processing and analytical protocols as the wet deposition
sample(s). Use the analytical data obtained from the FB to
determine any contamination introduced in the field and laboratory
handling procedures. The data from the known reference solution
can be used to calculate a system precision and bias.
10.9 Prepare and analyze a laboratory spike of a wet deposition sample
according to the guidelines provided in "Quality Assurance Manual
for Precipitation Measurement Systems" (14.8). Compare the
results obtained from the spiked samples to those obtained from
identical samples to which no spikes were added. Use these data
to monitor the method percent recovery as described in Sect.
10.2.3.
10.10 Participation in performance evaluation studies is recommended for
precipitation chemistry laboratories. The samples used for these
performance audits should contain the analytes of interest at
concentrations within the normal working range of the method. -The
true values are unknown to the analyst. Performance evaluation
studies for precipitation chemistry laboratories are conducted
semiannually by the USEPA Performance Evaluation Branch, Quality
Assurance Division, Research Triangle Park, NC 27711.
10.11 INSTRUMENT MAINTENANCE Strictly adhere to manufacturer's
maintenance schedule.
10.11.1 Exposed optical mirrors should be replaced yearly to
maintain optimal sensitivity and precision.
10.11.2 If the instrument is used for other sample types that
have high analyte concentrations it nay be necessary to
disassemble the entire burner-nebulizer system for
cleaning before analyzing wet deposition samples. This
is best accomplished by placing the components in a water
(Sect. 7.2) bath in an ultrasonic cleaner for a half
hour. Rinse with water (Sect. 7.2) after cleaning and
allow to air dry in a dust-free environment before
reassembly. Check o-rings for wear and replace if
necessary.
200.6-16
-------
11. PROCEDURE
11.1 Set instrument parameters and optimize the instrument each day
according to Sect. 9.1-2.
11.2 Prepare all standards and construct calibration curves according
to Sect. 9.4-5.
11.3 After the calibration curve is established, analyze the QCS. If
the measured value for the QCS is not within the specified limits
(Sect. 10.2.2), refer to Sect. 10.7.
11.4 Pipette the appropriate cesium or lanthanum solution into the
empty sample cup (Cs or La:Sample = 1:100). For the determination
of calcium and magnesium, use the lanthanum solution described in
Sect. 7.7. For potassium and sodium determinations, add cesium
solution (Sect. 7.4). Pour the sample into the sample cup
containing Cs or La; 3 mL of sample for 30 uL of Cs or La is
suggested. Mix well, aspirate, wait for equilibration in the flame
(Sect. 9.3), and record the measured absorbance (or concentration).
11.5 If the absorbance (or concentration) for a given sample exceeds
the working range of the system, dilute a separate sample with
water (Sect. 7.2). Prepare and analyze according to Sect. 11.4.
11.6 When analysis is complete, rinse the system by aspirating water
(Sect. 7.2) for ten minutes. Follow the manufacturer's guidelines
for instrument shut-down.
12. CALCULATIONS
12.1 For each analyte of interest, calculate a linear least squares fit
of the standard concentration as a function of the measured
absorbance. The linear least squares equation is expressed as
follows :
where: y = standard concentration in mg/L
x = absorbance measured
BQ = y-intercept calculated from: ? - B 5c
B. = slope calculated from:
n n
£ (x - x) (y - y-)/ £ (x - x}
i-l 1=1
where: x = mean of absorbances measured
y = mean of standard concentrations
n = number of samples
The correlation coefficient should be 0.9995 or greater. Determine
the concentration of analyte of interest from the calibration
curve.
200.6-17
-------
12.2 If the relationship between concentration and absorbance is
nonlinear, use a second degree polynomial least squares equation to
derive a curve with a correlation X).9995. The second degree
polynomial equation is expressed as follows:
y = B2x + Bj^x + BQ
A computer is necessary for the derivation of this function.
Determine the concentration of analyte of interest from the
calibration curve.
12.3 An integration system or internal calibration software may also
be used to provide a direct readout of the concentration of the
analyte of interest.
12.4 Report concentrations in mg/L as Ca+ , Mg+ , Na , and K+.
Do not report data lower than the lowest calibration standard.
13. PRECISION AND BIAS
13.1 The mean percent recovery and mean bias of this method were
determined from the analysis of spiked wet deposition samples
according to ASTM Standard Practice D4210, Annex A4 (14.9). The-
results are summarized in Table 3. No statistically significant
biases were found for any of the metal cations.
13.2 Single-operator precision and bias were obtained from the analysis
o£ quality control check samples that approximated the levels
common to wet deposition samples. These results reflect the
accuracy that can be expected when the method is used by a
competent operator. These data are presented in Table 4.
1^. REFERENCES
14.1 Annual Book of ASTM Standards, Part 42, "Standard Definitions of
Terms and Symbols Relating to Molecular Spectroscopy," Standard E
131-81, 1981, p. 66.
14.2 Annual Book of RSTM Standards. Section 11, Vol. 11.01 (1),
"Excerpts from Standard for Metric Practice," Standard E 380-79,
1983, pp. 679-694.
14.3 Van Loon, J. C., Analytical Atomic Absorption Spectroscopy,
Selected Methods Academic Press, Inc., New York, N. Y., 1980,
p. 42.
14.4 "Safety in Academic Chemistry Laboratories," American Chemical
Society Publication, Committee on Chemical Safety, 3rd Edition,
1979.
200.6-18
-------
14.5 Instrumentation Laboratory, Inc., Operator's Manual Model IL951,
AA/AE Spectrophotometer, Instrumentation Laboratory, Inc.,
Wilmington, Massachusetts, 1982, pp. 3-4.
14.6 Annual Book of ASTM Standards, Section 11, Vol. 11.01 (1),
"Standard Specification for Reagent Water," Standard D 1193-77,
1983, pp. 39-41.
14.7 Peden, M. E. and Skowron, L. M., 'Ionic Stability of Precipitation
Samples," Atmos. Environ. 12, 1978, pp. 2343-2349.
14.8 Topol, L. E., Lev-On, M., Flanagan, J., Schwall, R. J., Jackson, A.
E., Quality Assurance Manual for Precipitation Measurement
Systems, 1985, U.S. Environmental Protection Agency,
Environmental Monitoring Systems Laboratory, Research Triangle
Park, NC 27711.
14.9 Annual Book of ASTM Standards, Section 11, Vol. 11.01 (1),
"Practice for Intralaboratory Quality Control Procedures and a
Discussion of Reporting Low-Level Data," Standard D4210 Annex A4,
1983, pp. 15-16.
200.6-19
-------
Table 1. Method Detection Limits and Concentration Ranges for
Flame Atomic Absorption Spectrophotometnc Analysis
of Wet Deposition.
Analyte
Method Detection
Limit/
mg/L
Concentration
Range,
mg/L
Calcium
Magnesium
Potassium
Sodium
0.007
0.002
0.003
0.003d
0.030 -
0.010 -
0.010 -
0.010 -
3.00
1.00
1.00
i.ooa
0.007
0.020 - 2.00
a. 589.0 nm wavelength setting
b. 589.5 nm wavelength setting
200.6-20
-------
Table 2. Operating Conditions and Suggested Calibration
Concentrations for the Determination of
Calcium, Magnesium, Potassium, and Sodium in Wet
Deposition Samples.
Analyte
Wavelength
Setting,
nm
Spectral
Bandwidth,
nm
Working
Standards,
mg/L
Calcium
422.7
1.0
zero
0.03
0.75
1.50
2.25
3.00
Magnesium
285.2
1.0
zero
0.01
0.25
0.50
0.75
1.00
Potassium
766.5
1.0
zero
0.01
0.25
0.50
0.75
1.00
Sodium
589.0
0.5
zero
0.01
0.25
0.50
0.75
1.00
589.5
0.5
zero
0.02
0.50
1.00
1.50
2.00
a. Based on the MDL and 95th percentile concentration of each analyte
obtained from analyses of over five thousand wet deposition samples
from the NADP/NTN precipitation network.
b. Refer to Sect. 9.1.2 for details on wavelength selection
200.6-21
-------
Table 3. Single-Operator Precision and Bias for Calcium,
Magnesium, Potassium, and Sodium Determined from Analyte
Spikes of Wet Deposition Samples.
Analyte
Calcium
Magnesium
Potassium
Sodium
Amount
Added,
mg/L n
0
0
0
0
0
0
0
0
.087
.221
.018
.045
.021
.052
.099
.249
20
20
20
20
18
13
19
20
Mean
Percent
Recovery
101
98
97
96
145
108
107
100
.5
.3
.2
.6
.2
.1
.1
.2
Mean
Bias,
mg/L
0.
-0.
-0.
-0.
0.
0.
0.
0.
001
003
001
002
010
004
007
000
Standard
Deviation,
mg/L
0
0
0
0
0
0
0
0
.010
.011
.001
.002
.006
.002
.011
.008
Statistically
Significant
Bias?
No
No
No
No
No
No
No
No
a. Number of replicates
b. 95% Confidence Level
c. 589.0 nm wavelength
200.6-22
-------
Table 4. Single-Operator Precision and Bias for Calcium,
Magnesium, Potassium, and Sodium Determined
from Quality Control Check Samples.
Theoretical
Concentration,
Analyte mg/L
Calcium
Magnesium
Potassium
Sodium
0
0
0
0
0
0
0
0
.053
.406
.018
.084
.021
.098
.082
.465
Measured
Concentration,
mg/L n
0
0
0
0
0
0
0
0
.051
.413
.017
.083
.020
.095
.084
.479
145
145
145
145
127
122
123
122
Bias,
mq/L %
-0
0
-0
-0
-0
-0
0
0
.002
.007
.001
.001
.001
.003
.002
.014
-3.8
1.7
-5.6
-1.2
-4.8
-3.1
2.4
3.0
Precision,
s , RSO ,
mg/L %
0
0
0
0
0
0
0
0
.002
.003
.001
.001
.001
.001
.001
.003
3.9
0.7
5.9
1.2
5.0
1.0
1.2
0.6
The above data were obtained from records of measurements made under the
direction of the NADP quality assurance program.
a. Number of replicates
b. 589.0 nm wavelength
200.6-23
-------
NJ
§
T
Figure ! Percentile Concentration Values Obtained from
Wet Deposition Samples: Calcium, Magnesium,
Potassium, and Sodium.
calcium
i.oo
2.00
3.00
potassium
o.io
0.20
20
10
magnesium
0.40
0.60
sodium
0.40 0.50
CONCENTRATION (mg/L)
0.50
1.50
2.50
3.50
-------
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RECORD NUMBER: 2
Main Title
Author
CORP Author
Year Published
Report Number
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Owner Libraries
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Control Number
Cataloging Source
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PUB Date Free Form
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Primary Description
Abstract
t
Development of Standard Methods for the Collection and Analysis of
Precipitation
Peden, M E. ; Bachman, S R ; Brennan, C J , Demir, B. , James, K O. ;
Illinois State Water Survey Div , Champaign .Environmental Monitoring and
Support Lab.-Cmcinnati, OH.
1986
EPA-R-810780; EPA/600/4-86/024;
LIBRARY CALL NUMBER
MTIC MOST EPA LIBRARIES HAVE FICHE COPY. CHECK WITH
N ' lb INDIVIDUAL LIBRARY ABOUT PAPER COPY.
- National Technical Information Service
PB86-201365
NT
619921977
NTIS/MT
Rept. for May 83-Apr 86.
May 86
278p |
Sponsored by Environmental Monitoring and Support Lab.-Cincinnati, OH.
Standards, Precipitation(Meteorology); Water pollution. Evaluation; Collecting
methods; Manuals; Preserving, Processing; Accuracy, Flame photometry,
Colonmetric analysis; Spectrophotometry, Chromatographic analysis;
Volumetric analysis; Laboratories, Comparison; Acid precipitation
1 3B; 4B; 7D. 68A; 68D; 55C; 99A j
PCA13/MFA01 |
600/06
To provide the scientific community with a set of standardized procedures for '
the collection and analysis of wet precipitation samples, an analytical methods
manual for use in acid deposition studies has been developed The manual
includes detailed methods documentation for the major inorganic constituents
of interest in wet deposition as well as guidelines for the collection,
preservation, and processing of samples. The analytical methodologies include
Iof2
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ii preservation, and processing of samples. The analytical methodologies include
flame atomic absorption spectrophotometry, ion selective electrode, automated
colonmetry, ion chromatography, and titrimetnc procedures.
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