EPA-600/4-86-024
Method 200.6 - Dissolved Calcium, Magnesium, Potassium, and Sodium in
Wet Deposition by Flame Atomic Absorption Spectrophotometry

[METHOD ONLY]

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Method 200.6 — Dissolved Calcium, Magnesium, Potassium,
                and Sodium in Wet Deposition by Flame Atomic
                Absorption Spectrophotometry
                    March 1986
               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, library {PH2.J)
                             77 West Jackson Boulevard. 12th Fto*
                             Chicago, It  60604-3590
                 Sponsoring Agency:

           John D. Pfaff, Project Officer

             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 applicable 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

         where:  I  = incident radiant power
                 I  = 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.

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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
    5.8
    5.9
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.
Avoid any contact with a hot burner head.
result.
Serious tissue burns will
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 particulates.  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 Suppressant  —
         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
     acid (HC1, sp gr 1.19) to 900 mL of water (Sect. 7.2) and dilute to
     1 L.
     (La O ), dried at 105 C for one hour.
     carefully to the solid in increments of about 0.5 mL.
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
                                            Add 6 N HCl very
                                                            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 (HCl, sp gr  1.19) slowly  until
             all  the solid has dissolved.   Dilute to 1 L with water  (Sect.
             7.2) .

      7.9.2  Magnesium Solution, Stock  (1.0 mL = 1.0 mg  Mg) —  Dissolve
             1.000 g of magnesium ribbon  in a minimal volume  of 6 N  HCl and
             dilute to 1 L with water  (Sect. 7.2).

      7.9.3  Potassium Solution, Stock  (1.0 mL = 1.0 mg  K) — Dissolve
             1.907 g of potassium chloride  (KC1), dried  at 105  C  for one
             hour,  in water  (Sect.  7.2)  and dilute to 1  L.

      7.9.4  Sodium Solution, Stock  (1.0  mL =  1.0 mg Na) —  Dissolve
              2.542 g of sodium chloride  (NaCl), dried at 105  C  for  one
             hour, in water  (Sect.  7.2)  and dilute to 1  L.

7.10  SAMPLE CONTAINERS — Use polyolefin sample cups that have been
      thoroughly rinsed with water  (Sect. 7.2) before use.
                               200.6-8

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 8-  SAMPLE COLLECTION,  PRESERVATION,  AND  STORAGE
     8.1  Collect  samples  in  high  density polyethylene  (HOPE)  containers that
          have been thoroughly rinsed with ASTM Type  II  water  (72)    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
          rinTnal fl°Y°rkStati°n ^ not available, pour out  any residual
          rinse water and bag the buckets immediately.  Do not dry the bucket
                                 °ther
     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.
         8.3.1
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 a 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 run and 589.5 run.  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 "f-q&v
       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/rn  ).
                          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 unciog  the  nebulizer.
9.1.8
                                                                 may
            Flame Conditions — If the flame temperature is too low,
            compounds containing the analyte will not be completely
            dissociated.   Alternatively,  too high a flame temperature
            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 MDLs 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. 9.4).
     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
     a 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 (Sect. 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 time begins  to 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 stable 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 mg/L Cs.   Mix well.  Use the same stock of ionization
            suppressant or  releasing  agent  for  the samples and the
            calibration standards.

            Note:  The final volume of each working  standard solution
            exceeds the nominal volume by 1%.   This  adjustment is
            necessary to  maintain consistency when the appropriate volume
            of  suppressor solution is added to the wet deposition samples.
\
jj

J
                               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 time 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 ail 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
           guide.-lines 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 H^ 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 the 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 +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 or
              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
10.6
10.7
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%.

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 
-------

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 may 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

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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 LarSample = 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:
                      y =.BQ + BIX

where:  y  = standard concentration in mg/L
        x  = absorbance measured
        B  = y-intercept calculated from:
        B  = slope calculated from:
                                                      y - B x
- x)
                       - y)/
                                              - x) 2
                         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

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     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 _>°-9995.  The second degree
           polynomial  equation is expressed as follows:
V
                                                     B
                                                      0
           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
           of 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 fay a
           competent operator.  These data are presented in Table 4.

14.  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 ASTM 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

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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

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fees'
                          Table 1.  Method Detection Limits and Concentration  Ranges  for
                                    Flame Atomic Absorption Spectrophotometric Analysis
                                    of Wet Deposition.
                     Analyte
Method Detection
     Limit,
      mg/L
                                                                   Concentration
                                                                        Range,
                                                                         mg/L
Calcium
Magnesium
Potassium
Sodium
0.007
0.002
0.003
0.003a
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

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     Table  2.   Operating Conditions and Suggested Calibration
               Standard 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
    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

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          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
0.087
0.221
0.018
0.045
0.021
0.052
0.099
0.249
a
n
20
20
20
20
18
13
19
20
Mean
Percent
Recovery
101.5
98.3
97.2
96.6
145.2
108.1
107.1
100.2
Mean
Bias,
mg/L
0.001
-0.003
-0.001
-0.002
0.010
0.004
0.007
0.000
Standard
Deviation,
mg/L
0.010
0.011
0.001
0.002
0.006
0.002
0.011
0.008
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

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           Table  4.   Single-Operator  Precision  and  Bias  for  Calcium,
                     Magnesium,  Potassium,  and  Sodium Determined
                     from Quality Control Check Samples.
in'aoreticaj. Measured
Concentration, Concentration,
Analyte mg/L mg/L na
Calcium

Magnesium

Potassium

Sodium

0.
0.
0.
0.
0.
0.
0.
0.
053
406
018
084
021
098
082
465
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,
mg/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
0
0
0
0
0
0
0
0
Precision,
s, RSD,
mg/L %
.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

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O
o
                6-S
                a
                o-
                                    Figure 1
Percentile Concentration Values  Obtained from

Wet Deposition Samples:  Calcium,  Magnesium,

Potassium, and Sodium.
                                                                                                  magnesium
                                  1.00
                                             2.00
                                                         3.00
                                                                                  0.20
                                                                                              0.40
                                                                                                         0.60
                U
                                                potassium
                                                                     20
                                                                     10
                            0.10
                                    0.20
                                           0.30
                                                 0.40
                                                        0.50
                                                                           0.50
                                                                                      1.50
                                                                                               2.50
                                                                                                      sodium
                                                                                                          3.50
                                                       CONCENTRATION (mg/L)

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