United States
Environmental Protection
Agency
METHOD 218.7: DETERMINATION OF HEXAVALENT
CHROMIUM IN DRINKING WATER BY ION
CHROMATOGRAPHY WITH POST-COLUMN
DERIVATIZATION AND UV-VISIBLE SPECTROSCOPIC
DETECTION
Office of Water (MLK 140) EPA Document No. EPA 815-R-l 1-005 November2011 http://water.epa.gov/drink
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METHOD 218.7: DETERMINATION OF HEXAVALENT CHROMIUM IN DRINKING
WATER BY ION CHROMATOGRAPHY WITH POST-COLUMN
DERIVATIZATION AND UV-VISIBLE SPECTROSCOPIC DETECTION
Version 1.0
November 2011
A. Zaffiro and M. Zimmerman (Shaw Environmental, Inc.)
S. Wendelken, G. Smith and D. Munch (U.S. EPA, Office of Ground Water and Drinking Water)
TECHNICAL SUPPORT CENTER
STANDARDS AND RISK MANAGEMENT DIVISION
OFFICE OF GROUND WATER AND DRINKING WATER
U. S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
218.7-1
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METHOD 218.7
DETERMINATION OF HEXAVALENT CHROMIUM IN DRINKING WATER BY ION
CHROMATOGRAPHY WITH POST-COLUMN DERIVATIZATION AND UV-VISIBLE
SPECTROSCOPIC DETECTION
1. SCOPE AND APPLICATION
1.1 METHOD -Method 218.7 provides procedures for the determination of hexavalent
chromium Cr(VI) as the chromate anion CrC>42" in finished drinking water using ion
chromatography. Samples are analyzed by direct injection. This method is intended for use
by analysts skilled in the operation of ion chromatographic instrumentation and in the
interpretation of the associated data.
Chemical Abstracts Services
Analvte Registry Number (CASRN)
Hexavalent chromium (as CrO42") 13907-45-4
1.2 SUPPORTING DATA
1.2.1 Single-laboratory method performance data, presented in Section 17, were collected
using 4-mm i.d. anion exchange chromatographic columns designed for use with
ammonium hydroxide/ammonium sulfate eluent systems and 4-mm i.d. columns
designed for use with carbonate/bicarbonate eluent systems.
1.2.2 Precision and accuracy data have been generated for the analysis of Cr(VI) in reagent
water and finished drinking water from both ground water and surface water sources
(Sect. 17, Tables 4, 5 and 6).
1.2.3 Single laboratory Lowest Concentration Minimum Reporting Levels (LCMRLs) for
Cr(VI) ranged from 0.012 to 0.036 microgram per liter (ng/L) (Section 17, Table 3). The
LCMRL is the lowest spiking concentration such that the probability of spike recovery in
the 50% to 150% range is at least 99%. The procedure used to determine the LCMRL is
described elsewhere.1 Laboratories using this method are not required to determine
LCMRLs, but they must demonstrate that the Minimum Reporting Level (MRL) for
Cr(VI) meets the requirements described in Section 9.2.4.
1.2.4 Determining a detection limit (DL) for Cr(VI) is optional (Sect. 9.2.6). The DL is
defined as the statistically calculated minimum concentration that can be measured with
99% confidence that the reported value is greater than zero.2 DLs for Cr(VI) fortified
into reagent water ranged from 0.0044 to 0.015 |ig/L (Table 3).
1.3 METHOD FLEXIBILITY - The laboratory is permitted to modify chromatographic
conditions including 1C columns and eluent compositions different from those utilized in the
method. Changes may not be made to sample collection and preservation (Sect. 8) or to the
quality control (QC) requirements (Sect. 9). Method modifications should be considered
218.7-2
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only to improve method performance. Modifications that are introduced in the interest of
reducing cost or sample processing time, but result in poorer method performance, may not
be used. In all cases where method modifications are proposed, the analyst must perform the
procedures outlined in the Initial Demonstration of Capability (IDC, Sect. 9.2), and verify
that all on-going QC acceptance criteria in this method (Section 9.3) are met, especially
precision and accuracy in real sample matrixes.
2. SUMMARY OF METHOD3 6
Samples are preserved with a combined buffer/dechlorinating reagent which complexes free
chlorine and increases the pH to a value greater than eight. A measured volume (usually 1 mL) of
the sample is introduced into an ion chromatograph. CrO42" is separated from other matrix
components on an anion exchange column. CrC>42" is derivatized with 1,5-diphenylcarbazide in a
post-column reactor and is detected spectrophotometrically at a wavelength of 530 nm. Cr(VI) is
qualitatively identified via retention time, and the concentration of CrC>42" in the sample is
calculated using the integrated peak area and the external standard technique. Results are reported
in units of |ig/L of Cr(VI).
3. DEFINITIONS
3.1 ANALYSIS BATCH - A set of samples that is analyzed on the same instrument during a
24-hour period that begins and ends with the analysis of the appropriate Continuing Calibra-
tion Check (CCC) standards. Additional CCCs may be required depending on the length of
the Analysis Batch and the number of field samples.
3.2 CALIBRATION STANDARD - A solution of Cr(VI), which includes the method
preservative, prepared from the Primary Dilution Standards. The calibration standards are
used to calibrate the instrument response with respect to analyte concentration.
3.3 CONTINUING CALIBRATION CHECK (CCC) - A calibration standard that is analyzed
periodically to verify the accuracy of the existing calibration.
3.4 DETECTION LIMIT (DL) - The minimum concentration of Cr(VI) that can be identified,
measured, and reported with 99% confidence that the concentration is greater than zero. This
is a statistical determination (Sect. 9.2.6), and accurate quantitation is not expected at this
level.
3.5 FIELD REAGENT BLANK (FRB) - An aliquot of reagent water that is placed in a sample
container in the laboratory and treated as a sample in all respects, including shipment to the
sampling site, exposure to sampling site conditions, storage, preservation, and all analytical
procedures. The purpose of the FRB is to determine if Cr(VI) or other interferences are
introduced into the samples during sampling, transport, and storage.
3.6 LABORATORY DUPLICATES (LDs) - Two sample aliquots (LDi and LD2) taken in the
laboratory from a single sample bottle, and analyzed separately with identical procedures.
By cancelling variation contributed from sample collection, preservation, and storage
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procedures, Laboratory Duplicates provide an estimate of precision associated specifically
with the analytical determination.
3.7 LABORATORY FORTIFIED BLANK (LFB) - An aliquot of reagent water, containing the
method preservative, to which a known quantity of Cr(VI) is added. The LFB is used during
the IDC to verify method performance for precision and accuracy.
3.8 LABORATORY FORTIFIED SAMPLE MATRIX (LFSM) - An aliquot of a field sample to
which a known quantity of Cr(VI) is added. The LFSM is processed and analyzed as a
sample, and its purpose is to determine whether the sample matrix contributes bias to the
analytical results.
3.9 LABORATORY FORTIFIED SAMPLE MATRIX DUPLICATE (LFSMD) - A second ali-
quot of the field sample used to prepare the LFSM which is fortified and analyzed identically
to the LFSM. The LFSMD is used instead of the Laboratory Duplicate to assess method
precision if Cr(VI) is absent from the sample matrix.
3.10 LABORATORY REAGENT BLANK (LRB) - An aliquot of reagent water that contains the
method preservative. The LRB is used to determine if Cr(VI) or other interferences are
introduced from the laboratory environment, the reagents or glassware, and to test for cross
contamination.
3.11 LOWEST CONCENTRATION MINIMUM REPORTING LEVEL (LCMRL) - The single
laboratory LCMRL is the lowest spiking concentration such that the probability of spike
recovery in the 50% to 150% range is at least 99%l LCMRL determinations from multiple
laboratories can be used to develop a statistically derived MRL (Sect. 3.13).
3.12 MATERIAL SAFETY DATA SHEETS (MSDS) - Written information provided by vendors
concerning a chemical's toxicity, health hazards, physical properties, fire and reactivity data,
storage instructions, spill response procedures, and handling precautions.
3.13 MINIMUM REPORTING LEVEL (MRL) - The minimum concentration that can be
reported by a laboratory as a quantified value for Cr(VI). This concentration must meet the
criteria defined in Section 9.2.4 and must be no lower than the concentration of the lowest
calibration standard. A laboratory may be required to demonstrate a specific MRL by a
regulatory body if this method is being performed for compliance purposes.
3.14 POST-COLUMN REACTOR - For this method, the post-column reactor consists of a
reagent delivery pump, a mixing tee, and a reaction coil.
3.15 PRIMARY DILUTION STANDARD (PDS) - An aqueous solution containing Cr(VI),
which is prepared from a Stock Standard Solution. The PDS solution is diluted to prepare
calibration standards and sample fortification solutions.
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3.16 PROCEDURAL CALIBRATION - A calibration technique in which calibration standards
are processed through the entire method, including sample preparation and addition of
preservative.
3.17 QUALITY CONTROL SAMPLE (QCS) - A solution containing Cr(VI) at a known
concentration that is obtained from a source external to the laboratory and different from the
source of calibration standards. The purpose of the QCS is to verify the accuracy of the
primary calibration standards.
3.18 REAGENT WATER - Purified water that does not contain any measurable quantity of
Cr(VI) or interfering compounds at or above one-third the MRL.
3.19 STOCK STANDARD SOLUTION - A concentrated standard solution that is prepared in the
laboratory using assayed reference materials, or that is purchased from a commercial source
with a certificate of analysis.
4. INTERFERENCES
4.1 LAB WARE - The stability of Cr(VI) was demonstrated for this method using high-density
polyethylene (HOPE) sample bottles. Polypropylene copolymer bottles are also acceptable.
Aliquots of the PDS and sample fortification solutions were transferred using polypropylene
pipette tips. Other types of sample bottles may be used; however, the laboratory must
confirm the stability of Cr(VI) in these materials over 14 days by formal experiment.
4.2 REAGENTS AND EQUIPMENT - Method interferences may be caused by contaminants in
reagents (including the method preservative) and in the ion chromatographic system. All
laboratory reagents and instruments must be routinely demonstrated to be free from
interferences, and to contribute less than one-third the MRL for Cr(VI), under the conditions
of the analysis. This may be accomplished by analyzing LRBs, as described in Section 9.3.1.
4.3 MATRIX INTERFERENCES - Matrix interferences are caused by contaminants that are
present in the sample. The extent of matrix interferences will vary considerably from source
to source, depending upon the nature of the water. Matrix components may directly interfere
by producing a signal at or near the retention time of the Cr(VI) peak; however, the method is
extremely selective due to the chromatographic separation of the analyte from matrix
components, coupled with the discrimination of the post-column reagent for the chromate
anion. Sample ionic strength may enhance or suppress Cr(VI) response; however, the 4-mm
column systems used during method development tolerate typical concentrations of common
anions in drinking water in combination with method preservative. Acceptable method
performance has been demonstrated for samples with hardness up to 350 mg/L as CaCOs and
total organic carbon content of 3 mg/L. The analysis of Laboratory Fortified Sample Matrix
(Sect. 9.3.4) provides the user of the method with evidence for the presence (or absence) of
matrix effects.
4.4 OXIDATION-REDUCTION (REDOX) CONCERNS - To ensure sample integrity, Cr(VI)
must be protected from reduction, and Cr(III), if present, must not oxidize to Cr(VI) during
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sample storage. Within the normal pH range in drinking water, Cr(VI), present as a result of
pollution or oxidation of Cr(III) in source water during treatment, forms oxyanions, which
are typically represented as HCrO/f and CrC>42". The very stable CrC>42" anion dominates
above pH 78; therefore, the method preservative is designed to buffer samples to at least pH
8. Chromate compounds are quite soluble, mobile and stable, particularly in an oxidizing
environment.8 In contrast, soluble Cr(III) species oxidize to Cr(VI) in the presence of free
chlorine9, although natural organic matter in surface water sources may complex Cr(III),
slowing its oxidation even in a highly oxidizing environment.7 The rate of Cr(III) oxidation
increases with chlorine concentration and is pH-dependent.7 For these reasons, both
preservation options prescribed in this method include ammonium ions to complex free
chlorine. The resulting formation of chloramines minimizes, but does not completely
prevent, the oxidation of Cr(III).7 During method development, experiments were conducted
that demonstrated the ability of the method preservative to minimize the oxidation of Cr(III)
and to prevent the reduction of Cr(VI) for at least 14 days in drinking water from ground and
surface water sources. Representative study results for a surface water source are presented
in Section 17, Table 7.
5. SAFETY
5.1 Each chemical should be treated as a potential health hazard and exposure to these chemicals
should be minimized. Each laboratory is responsible for maintaining an awareness of OSHA
regulations regarding safe handling of chemicals used in this method. A reference file of
MSDSs should be made available to all personnel involved in the chemical analysis.
Hexavalent chromium in solid form presents an inhalation hazard, is toxic and a suspected
carcinogen. All forms of hexavalent chromium should be handled with appropriate
precautions. A fact sheet on the health effects of hexavalent chromium in the workplace is
available on the OSHA website @ www.osha.gov.
5.2 Preparation of the post-column reagent and the ammonium hydroxide preservative require
the use of concentrated acid and concentrated base. These reagents should be prepared in a
hood, adding acid to water, and wearing splash goggles with chemical resistant gloves.
Gloves and splash goggles should be worn when transferring the post-column reagent to the
instrument reservoir.
6. EQUIPMENT AND SUPPLIES
References to specific brands or catalog numbers are included as examples only and do not imply
endorsement of the product. Such reference does not preclude the use of other vendors or
suppliers.
6.1 SAMPLE CONTAINERS - 125-ml, wide-mouth, high-density polyethylene (HOPE) (Fisher
Scientific Cat. No. 02-911-958 or equivalent); 125-mL polypropylene copolymer (Fisher
Scientific Cat. No. 02-893-A or equivalent).
6.2 AUTOSAMPLER VIALS - Size and material meeting vendor specification for the ion
chromatograph. Polypropylene and polystyrene are commonly used for ion chromatography.
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6.3 AUTOMATIC PIPETTE - (Eppendorf Research Pro or equivalent). An automatic pipette
with polypropylene tips is recommended for preparing all standard solutions and for
fortifying QC samples.
6.4 ANALYTICAL BALANCE - Capable of weighing to the nearest 0.0001 gram (g).
6.5 ION CHROMATOGRAPHY SYSTEM WITH POST-COLUMN REACTOR
6.5.1 1C SYSTEM - An analytical system consisting of an autosampler, pump module with
vacuum degassing option, sample loop, guard column, anion separator column, post-
column reagent addition capability, post-column reaction coil, UV-Vis absorbance
detector set to monitor a wavelength of 530 nm, and a data acquisition and management
system. The system must not contain any metal parts in the sample, eluent and reagent
flow paths.
6.5.2 SAMPLE LOOP - Polyetheretherketone (PEEK) construction and sized for the column
system. One- and 1.25-mL sample loops were used to generate the performance data
presented in this method. Smaller or larger injection volumes may be used as long as the
Initial Demonstration of Capability (Sect. 9.2), calibration, and sample analyses are
performed using the same injection volume. The laboratory must be able to meet the
MRL verification criteria (Section 9.2.4) using the selected injection volume.
6.5.3 GUARD COLUMN - Size and resin per vendor specification; capable of removing
strongly adsorbing organic compounds and particles that could damage the analytical
column.
6.5.4 ANALYTICAL COLUMN - Anion exchange column capable of resolving the chromate
anion from matrix components. Any column that provides adequate resolution, peak
shape, capacity, accuracy and precision (Sect. 9), and does not result in suppression or
enhancement of analyte response (Sect. 4.3) may be used.
6.5.5 COLUMN COMPARTMENT - Temperature controlled recommended.
6.5.6 POST-COLUMN REACTOR- Pneumatic or mechanical reagent pump capable of pulse-
free operation, mixing tee, and reaction coil (sized and configured per vendor
specifications).
6.5.7 DATA SYSTEM - An interfaced data system is required to acquire, store, and output
data. The computer software must have the capability of processing stored data by
recognizing and integrating a chromatographic peak within a given retention time
window. The software must be able to construct a linear regression or quadratic
regression calibration curve and calculate the Cr(VI) concentration using the external
standard technique.
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7. REAGENTS AND STANDARDS
7.1 REAGENTS AND SOLVENTS - Reagent grade or better chemicals must be used. Unless
otherwise indicated, it is intended that all reagents will conform to the specifications of the
Committee on Analytical Reagents of the American Chemical Society (ACS), where such
specifications are available. Other grades may be used if the reagent is demonstrated to be
free of Cr(VI) and other interferences, and all requirements of the IDC are met when using
these reagents.
7.1.1 AMMONIUM HYDROXIDE (NH4OH, CASRN 1336-21-6) - 28-30% NH3 w/w (Fisher
Cat. No. AC42330 or equivalent). For preparing NH4OH/(NH4)2SO4 (liquid)
preservative and 1C eluent.
7.1.2 AMMONIUM SULFATE [(NH4)2SO4, CASRN 7783-20-2] (Sigma-Aldrich Cat. No.
A4915 or equivalent). For preparing NH4OH/(NH4)2SO4 (liquid) preservative,
CO3"2/HCO37(NH4)2SO4 (solid) preservative, and 1C eluent.
7.1.3 METHANOL (CH3OH, CASRN 67-56-1) - (Fisher Optima® LC/MS grade or
equivalent). For preparing the post-column reagent.
7.1.4 SODIUM BICARBONATE (NaHCO3, CASRN 144-55-8) - for preparing CO3'2/HCO3'
/(NH4)2SO4 (solid) preservative and 1C eluent.
7.1.5 SODIUM CARBONATE (Na2CO3, CASRN 497-19-8) - Anhydrous. For preparing
CO3"2/HCO37(NH4)2SO4 (solid) preservative and 1C eluent.
7.1.6 REAGENT WATER - Distilled or deionized water. For preparing calibration standards
and reagents.
7.1.7 1,5-DIPHENYLCARBAZIDE (Ci3Hi4N4O, CASRN 140-22-7) - for preparing the post-
column reagent.
7.1.8 SULFURIC ACID (H2SO4, CASRN 7664-93-9) - 93 to 98 percent, trace metal grade
(Fisher Cat. No. A510 or equivalent). For preparing the post-column reagent.
7.1.9 PREPARATION OF POST-COLUMN REAGENT - 2-mM 1,5-diphenylcarbazide, 10%
methanol and 0.5 M (1 N) sulfuric acid. Add 28 mL of sulfuric acid to approximately
500 mL of reagent water in a 1-liter volumetric flask or suitably sized bottle. Mix and
cool to room temperature in a water bath. While this solution is cooling, weigh 0.50
gram of 1,5-diphenylcarbazide into a 100-mL beaker, add 75 mL of methanol, and
sonicate for five minutes to dissolve the solid. Transfer this solution to a 100-mL
volumetric flask; bring to volume with methanol and mix. Add the entire contents of the
volumetric flask to the sulfuric acid solution and dilute to 1.0 L with reagent water. Mix
and transfer the solution to the post-column reagent reservoir. Method 218.7 was
developed adhering to a schedule for replacing post-column reagent five days after the
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original date of preparation. Users of this method must determine when this reagent
should be replaced based on the recommendations of the instrument manufacturer, and on
the ability to meet the QC requirements in Section 9.
7.1.10 PREPARATION OF CO3'2/HCO37(NH4)2SO4 (SOLID) PRESERVATIVE - Nominally
13.3 mg Na2CO3, 10.5 mg NaHCO3 and 33 mg (NH4)2SO4. These are the appropriate
amounts for a 100-mL sample. Weigh 1-1.25 times these values and transfer to a dry,
125-mL sample bottle.
7.1.11 PREPARATION OF NH4OH/(NH4)2SO4 (LIQUID) PRESERVATIVE -Dissolve 3.3 g
(NH4)2SO4 in 75 ml of reagent water. Add 6.5 mL ammonium hydroxide and dilute to
100-mL final volume. During method development, the stability of the concentrated
preservative was verified for one month when stored at ambient temperature.
Laboratories using this method are required to determine when this reagent should be
replaced.
7.1.12 PREPARATION OF COLUMN ELUENTS - Representative column eluent systems are
listed in Section 17, Tables 1 and 2. Eluent solutions of these types should be prepared
on a weekly basis or at intervals recommended by the instrument manufacturer.
7.2 STANDARD SOLUTIONS - Solution concentrations listed in this section were used to
develop this method and are included only as examples. Guidance on the storage stability of
Primary Dilution Standards and calibration standards is provided in the applicable sections
below. Although estimated stability times for standard solutions are given, laboratories
should use standard QC practices to determine appropriate storage conditions and when
standards need to be replaced.
7.2.1 ANALYTE STOCK STANDARD SOLUTION (1000 jig/mL) - Prepare from neat
material (ACS reagent grade, >99% purity) in reagent water or obtain Cr(VI) as a
certified solution in water (e.g., Ultra Cat. No. ICP-024A, AccuStandard Cat. No. WC-
HEX-10X-1 or equivalent). For K2Cr2O7 starting material, dry the salt at 100 °C to a
constant weight; weigh 0.283 g, dissolve in reagent water, and dilute to 100 mL. Store
stock standards at room temperature.
7.2.1.1 ANALYTE PRIMARY DILUTION STANDARD (Analyte PDS) (1000 |ig/L) -
Prepare the Analyte PDS by diluting the Analyte Stock Standard solution (1:1000)
into reagent water. Include the same preservative used for field samples. An
example preparation of Analyte PDS solutions (those used to collect data presented in
Section 17) is provided in the table below. Store the PDS in a 125-mL HOPE or
polypropylene bottle. The Analyte PDS is used to prepare calibration standards and
to fortify QC samples with Cr(VI).
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Stock
Concentration
1000 ug/mL with
NH4OH/(NH4)2SO4
(liquid)
preservative
1000 ug/mL with
solid preservative
Aliquot of Stock
Standard
Solution (7.2.1,
1000 jig/mL)
0.10 mL
0.10 mL
Preservative
Amounts
1 mL preservative
(Sect. 7.1.11)
13.3mgNa2CO3,
10.5mgNaHCO3
33 mg (NH4)2SO4
Volume
Reagent
Water
0.10 L
0.10 L
PDS
Concentration
1000 ug/L
1000 ug/L
Storage stability of the Analyte PDS was verified during method development. The
Analyte PDS was stable for at least 14 days when stored at room temperature.
CALIBRATION STANDARDS (CALs) - Prepare a series of calibration standards (at
least six levels) by diluting the Analyte PDS into reagent water. Include the same
preservative used for field samples. The lowest calibration standard must be at or below
the concentration of the MRL (Sect. 9.2.4). The calibration standards may also be used
as CCCs. An example preparation of calibration standards (starting with the Analyte
PDS) used to collect method performance data is provided in the table below.
Dilution Aliquot
0.50 mL Analyte PDS
0. 10 mL Analyte PDS
10 mL 5.0 ug/L CAL
5.0 mL 5.0 ug/L CAL
2.0 mL 5.0 ug/L CAL
1.0 mL 5.0 ug/L CAL
0.4 mL 5.0 ug/L CAL
Starting Concentration
(Ug/L)
1000
1000
5.0
5.0
5.0
5.0
5.0
Final Volume (L)
0.10
0.10
0.10
0.10
0.10
0.10
0.10
Final Concentration
(HS/L)
5.0
1.0
0.50
0.25
0.10
0.050
0.020
Storage stability of the calibration standards was evaluated during method development
at concentrations between 0.020 |ig/L and 1.0 |ig/L. The calibration solutions were stable
for at least 14 days when stored at 4 °C.
8. SAMPLE COLLECTION, PRESERVATION AND STORAGE
8.1 ADDITION OF PRESERVATIVE -The samples are preserved with a combined
buffer/dechlorinating reagent. Either the liquid formulation or the solid formulation of the
preservative described in the following sections may be used. Only one preservative
formulation should be used (liquid or solid) to prepare the sample bottles.
8.1.1. LIQUID FORMULATION - NH4OH/(NH4)2SO4 - The liquid preservative may be added
to sample bottles prior to shipment. Apply the concentrated preservative (Sect. 7.1.11) at
the rate of 1 mL per 100 mL of sample. Sample bottles prepared in advance may be
stored for one month prior to use.
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8.1.2. SOLID FORMULATION - CO3"2/HCO37(NH4)2SO4 - The solid preservative may be
added to sample bottles prior to shipment. Add 13.3 mg Na2CO3, 10.5 mg NaHCOs and
33 mg (NH4)2SO4to each bottle following the instructions in Section 7.1.10.
8.2 SAMPLE COLLECTION - Open the tap and allow the system to flush for approximately 5
minutes. Fill sample bottles with 100-mL of sample, taking care not to flush out the
preservative. Invert the bottle several times to mix the sample with the preservative.
8.3 SAMPLE SHIPMENT AND STORAGE - Storage stability studies have demonstrated that
samples are stable for at least 14 days at both ambient temperature (25 °C) and chilled
temperature (6 °C). If the anticipated shipping conditions would expose the samples to
temperature extremes, samples may be chilled during shipment. Standard quality control
practices should be put in place to confirm that the shipping conditions do not adversely
affect sample stability. A laboratory fortified sample that is shipped with the sample kit can
aid in making this determination. Upon sample receipt, measure the free chlorine and sample
pH. The free chlorine concentration must be less than 0.1 mg/L and the pH must be >8 for
the sample to be valid. In the laboratory, it is recommended that the samples are stored at or
below 6 °C until analysis.
8.4 SAMPLE HOLDING TIMES - Results of the sample storage stability study (Table 7)
indicate that Cr(VI) is stable for at least 14 days when collected, preserved, shipped and
stored as described in Sections 8.1 to 8.3. Samples should be analyzed as soon as possible,
but must be analyzed within 14 days.
9. QUALITY CONTROL
9.1 QC requirements include the IDC and ongoing QC requirements. This section describes each
QC parameter, its required frequency, and the performance criteria that must be met. The
QC criteria discussed in the following sections are summarized in Section 17, Tables 8 and 9.
These QC requirements are considered the minimum acceptable QC program. Laboratories
are encouraged to institute additional QC practices to meet their specific needs.
9.2 INITIAL DEMONSTRATION OF CAPABILITY (IDC) - The IDC must be successfully
performed prior to analyzing any field samples. The IDC must be repeated if changes are
made to analytical parameters not previously validated during the IDC, for example,
changing from NH4OH/(NH4)2SO4 preservative to the CO3"2/HCO37(NH4)2SO4 preservative
or translating the method to a 2-mm column system. Prior to conducting the IDC, the analyst
must meet the calibration requirements outlined in Section 10.2.
9.2.1 DEMONSTRATION OF LOW SYSTEM BACKGROUND - Analyze an LRB after the
analysis of the highest concentration calibration standard. Confirm that the blank is free
of contamination as defined in Section 9.3.1.
9.2.2 DEMONSTRATION OF PRECISION - Prepare and analyze seven replicate LFBs.
Fortify these samples near the midrange of the initial calibration curve. The method
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preservative must be added to the LFBs as described in Section 8. The percent relative
standard deviation (%RSD) of the concentrations of the replicate analyses must be <15%.
n, _ „_ Standard Deviation of Measured Concentrations ,
% RSD = x 100
Average Concentration
9.2.3 DEMONSTRATION OF ACCURACY - Using the same set of replicate data generated
for Section 9.2.2, calculate the average percent recovery. The average percent recovery
for Cr(VI) must be within +15% of the true value.
.. _, Average Measured Concentration , ^
% Recovery = x 100
Fortified Concentration
9.2.4 MINIMUM REPORTING LEVEL (MRL) CONFIRMATION - Establish a target
concentration for the MRL based on the intended use of the method. Analyze an initial
calibration following the procedures in Section 10. The lowest calibration standard used
to establish the initial calibration (as well as the low-level CCC) must be at or below the
concentration of the MRL. Establishing the MRL concentration too low may cause
repeated failure of ongoing QC requirements. Confirm the MRL following the procedure
outlined below.
9.2.4.1 Fortify and analyze seven replicate LFBs at or below the proposed MRL
concentration. The LFBs must contain the method preservative as specified in
Section 8. Calculate the mean (Mean) and standard deviation for these replicates.
Determine the Half Range for the Prediction Interval of Results (HRpiR) using the
equation
HRPIR = 3.9638
where S is the standard deviation and 3.963 is a constant value for seven replicates.1
9.2.4.2 Confirm that the Upper and Lower limits for the Prediction Interval of Results (PIR =
Mean ± HRpiR) meet the upper and lower recovery limits as shown below.
The Upper PIR Limit must be < 150 percent recovery.
Mean + HRprR
^ ... . _ . x 100< 150%
Fortified C oncentration
The Lower PIR Limit must be >50 percent recovery.
Mean - HRprK
-xlOO>50%
Fortified Concentration
218.7-12
-------
9.2.4.3 The MRL is valid if both the Upper and Lower PIR Limits meet the criteria described
above. If these criteria are not met, the MRL has been set too low and must be
confirmed again at a higher concentration.
NOTE: These equations are only valid for seven replicate samples.
9.2.5 QUALITY CONTROL SAMPLE (QCS) - Analyze a mid-level Quality Control Sample
(Sect. 9.3.6) to confirm the accuracy of the primary calibration standards.
9.2.6 DETECTION LIMIT DETERMINATION (optional) - While DL determination is not a
specific requirement of this method, it may be required by various regulatory bodies
associated with compliance monitoring. It is the responsibility of the laboratory to
ascertain whether DL determination is required based upon the intended use of the data.
The DL, as defined for this method, is an MDL2 with the additional requirement that the
analyses for the procedure must be performed over at least three days. Prepare at least
seven replicate LFBs at a concentration estimated to be near the DL. This concentration
may be estimated by selecting a concentration at two to five times the noise level. The
method preservative must be added to the samples as described in Section 8. Process the
seven replicates through all steps of Section 1 1 . Do not subtract blank values when
performing DL calculations.
NOTE: If an MRL confirmation data set meets these requirements, a DL may be
calculated from the MRL confirmation data, and no additional analyses are
necessary.
Calculate the DL using the following equation:
where
*(n-i,i-a = 0.99) = Student's t value for the 99% confidence level with n-1 degrees of
freedom (for seven replicate determinations, the Student's lvalue
is 3.143 at a 99% confidence level),
n = number of replicates, and
s = standard deviation of replicate analyses.
9.3 ONGOING QC REQUIREMENTS - This section describes the ongoing QC elements that
must be included when processing and analyzing field samples.
9.3.1 LABORATORY REAGENT BLANK (LRB) - Analyze an LRB during the IDC and
with each Analysis Batch. Prepare the LRB by adding reagent water to a sample bottle
representative of those used to collect the samples, preferably from the same sample kit.
The LRB must contain the sample preservative. Cr(VI), or contaminants that produce a
signal overlapping with the Cr(VI) peak, must be less than one-third the MRL. If Cr(VI)
is detected in the LRB at concentrations equal to or greater than this level, then all
218.7-13
-------
samples analyzed in the corresponding Analysis Batch are invalid. Subtracting blank
values from sample results is not permitted.
9.3 .2 CONTINUING CALIBRATION CHECK (CCC) - Analyze CCC standards at the
beginning of each Analysis Batch, after every ten field samples, and at the end of the
Analysis Batch. See Section 10.3 for concentration requirements and acceptance criteria
for CCCs. Additional guidance on sequencing proper Analysis Batches is provided in
Section 11.2.
9.3.3 LABORATORY FORTIFIED BLANK (LFB) - Because this method utilizes procedural
calibration standards, which are fortified reagent waters, there is no difference between
the LFB and the Continuing Calibration Check standard. Consequently, the analysis of a
separate LFB is not required as part of the ongoing QC; however, the term "LFB" is used
for clarity in the IDC.
9.3 .4 LABORATORY FORTIFIED SAMPLE MATRIX (LFSM) - Within each Analysis
Batch, analyze a minimum of one LFSM. The background concentration of Cr(VI) in the
sample matrix must be determined in a separate aliquot and subtracted from the measured
value in the LFSM. If various sample matrixes are analyzed regularly, for example,
drinking water processed from ground water and surface water sources, performance data
should be collected for each source.
9.3.4.1 Prepare the LFSM by fortifying a sample with an appropriate amount of the Analyte
PDS (Sect. 7.2. 1.1). Generally, select a spiking concentration that is greater than or
equal to the native concentration of Cr(VI). If the native concentration does not allow
this criterion to be met without exceeding the calibration range, dilution with reagent
water containing the method preservative is permitted. Selecting a duplicate aliquot
of a sample that has already been analyzed aids in the selection of an appropriate
spiking level. If this is not possible, use historical data when selecting a fortifying
concentration.
9.3.4.2 Calculate the percent recovery (%R) using the equation:
C
where
A = measured concentration in the fortified sample,
B = measured concentration in the unfortified sample, and
C = fortification concentration.
9.3.4.3 Cr(VI) recovery for samples fortified at concentrations near or at the MRL (within a
factor of two times the MRL concentration) must be within +50% of the true value.
Recovery for samples fortified at all other concentrations must be within +15% of the
true value. If the accuracy falls outside the designated range, and the laboratory
performance is shown to be in control in the CCCs, the recovery is judged matrix
biased. Report the result in the unfortified sample as "suspect/matrix."
218.7-14
-------
NOTE: In order to obtain meaningful percent recovery results, correct the measured
value in the LFSM and LFSMD for the native level in the unfortified samples, even if
the native value is less than the MRL. This is the only time that values below the
MRL may be used for calculations.
9.3.5 LABORATORY DUPLICATE OR LABORATORY FORTIFIED SAMPLE MATRIX
DUPLICATE (LD or LFSMD) - Within each Analysis Batch, analyze a minimum of one
Laboratory Duplicate or one Laboratory Fortified Sample Matrix Duplicate. If Cr(VI) is
not routinely observed in field samples, analyze an LFSMD rather than an LD.
9.3.5.1 Calculate the relative percent difference (RPD) for duplicate measurements (LD1 and
LD2) using the equation:
LD,-LD,
l
(LDj+LDj/2
9.3.5.2 RPDs for Laboratory Duplicates must be <15%. Greater variability may be observed
when Laboratory Duplicates have Cr(VI) concentrations that are near or at the MRL
(within a factor of two times the MRL concentration). At these concentrations,
Laboratory Duplicates must have RPDs that are <50%. If the RPD falls outside the
designated range, and the laboratory performance is shown to be in control in the
CCC, the precision is judged matrix influenced. Report the result in the unfortified
sample as "suspect/matrix."
9.3.5.3 If an LFSMD is analyzed instead of a Laboratory Duplicate, calculate the RPD for the
LFSM and LFSMD using the equation:
LFSM-LFSMD
(LFSM + LFSMD)/2
9.3.5.4 RPDs for duplicate LFSMs must be <15%. Greater variability may be observed when
the matrix is fortified near or at the MRL (within a factor of two times the MRL
concentration). LFSMs at these concentrations must have RPDs that are <50%. If
the RPD falls outside the designated range, and the laboratory performance is shown
to be in control in the CCC, the precision is judged matrix influenced. Report the
result in the unfortified sample as "suspect/matrix."
9.3.6 QUALITY CONTROL SAMPLE (QCS) - A QCS must be analyzed during the IDC, and
then each time new calibration standards are prepared. Prepare the QCS near the
midpoint of the calibration range. The acceptance criterion for the QCS is 85 to 115% of
the true value. If the accuracy for Cr(VI) fails the recovery criterion, prepare fresh
standard dilutions and repeat the QCS evaluation.
218.7-15
-------
10. CALIBRATION AND STANDARDIZATION
Demonstration and documentation of initial analyte calibration are required before performing the
IDC and prior to analyzing field samples. The initial calibration must be repeated each time a
major instrument modification or maintenance is performed.
10.1 OPTIMIZATION
10.1.1 ION CHROMATOGRAPHY INSTRUMENT CONDITIONS - 1C operating conditions
and columns used to collect method performance data are given in Section 17, Tables 1
and 2. Conditions different from these (e.g., 1C columns and eluent systems) may be
used if the QC criteria in Sections 9.2 and 9.3 are met, and chromatographic precision
and accuracy is demonstrated within each Analysis Batch and for representative sample
matrixes.
10.1.2 POST-COLUMN REAGENT DELIVERY CONDITIONS - Representative conditions
for reaction coil volume and post-column reagent flow rates are provided in Section 17,
Tables 1 and 2. Select conditions that provide the best signal-to-noise values for Cr(VI)
at concentrations near the MRL.
10.1.3 UV-Vis DETECTOR - Monitor wavelength 530 nm.
10.2 INITIAL CALIBRATION
10.2.1 CALIBRATION STANDARDS - Prepare a set of calibration standards (at least six
levels) as described in Section 7.2.2. The Cr(VI) concentration in the lowest calibration
standard must be at or below the MRL. Field samples must be quantified using a
calibration curve that spans the same concentration range used to collect the IDC data
(Sect. 9.2); i.e., analysts are not permitted to use a restricted calibration range to meet the
IDC criteria and then use a larger dynamic range during analysis of field samples.
10.2.2 CALIBRATION - Calibrate the 1C system using the Cr(VI) peak area and the external
standard technique. Fit the calibration points with either a linear regression or quadratic
regression (response vs. concentration). Weighting may be used. Forcing the calibration
curve through the origin is not recommended. The 1C instruments used during method
development were calibrated using inverse concentration-weighted linear curves.
10.2.3 CALIBRATION ACCEPTANCE CRITERIA - Validate the initial calibration by
calculating the concentration of Cr(VI) using the regression equations for each of the
standard runs used to generate the calibration curve. For calibration levels that are
-------
10.3 CONTINUING CALIBRATION CHECKS (CCCs) - Analyze a CCC to verify the initial
calibration at the beginning of each Analysis Batch, after every tenth field sample, and at the
end of each Analysis Batch. The beginning CCC for each Analysis Batch must be at or
below the MRL. This CCC verifies instrument sensitivity prior to the analysis of samples.
Alternate subsequent CCCs between the remaining calibration levels.
10.3.1 Calculate the concentration of Cr(VI) in the CCC. Calibration standards fortified at a
level
-------
11.2 THE ANALYSIS BATCH - An Analysis Batch is a sequence of samples, analyzed within a
24-hour period of no more than 20 field samples that includes all required QC samples (LRB,
CCCs, LFSMs and LFSMDs or LDs). The required QC samples are not included in counting
the maximum field sample total of 20. Dilutions are counted as samples. The purpose of the
20-sample limit is to ensure that a low-level CCC and an LRB are repeated on a regular and
frequent basis. Analytical conditions for the Analysis Batch must be the same as those
applied during calibration.
11.2.1 After a valid calibration curve is established, begin every Analysis Batch by analyzing an
initial low-level CCC at or below the MRL. This initial CCC must be within ±50% of the
true value. Continue the Analysis Batch by analyzing an LRB, followed by field and QC
samples at appropriate frequencies (Section 9.3). Analyze and rotate between a mid- and
a high-level CCC after every ten field samples and at the end each Analysis Batch. Do
not count QC samples (LRBs, LDs, LFSMs, LFSMDs) when calculating the required
frequency of CCCs. After 20 field samples or 24 hours, the low-level CCC and LRB
must be repeated to begin a new Analysis Batch.
11.2.2 The close-out CCC completes the Analysis Batch. The acquisition start time of the
closeout CCC must be within 24 hours of the acquisition start time of the low-level CCC
at the beginning of the Analysis Batch. More than one Analysis Batch within a 24-hour
period is permitted.
12. DATA ANALYSIS AND CALCULATIONS
12.1 For each Analysis Batch, establish an appropriate retention time window to identify Cr(VI).
Base this assignment on measurements of actual retention time variation for Cr(VI) in
standard solutions over the course of time. The suggested variation is plus or minus three
times the standard deviation of the retention time for a series of injections. The injections
from the initial calibration and from the IDC (Sect. 9.2) may be used to calculate the
retention time window. However, the experience of the analyst should weigh heavily on the
determination of an appropriate range.
12.2 At the conclusion of data acquisition, use the same software settings established during the
calibration procedure to identify Cr(VI) in the predetermined retention time window.
Confirm the identity by comparison of the retention time with that of the corresponding
Cr(VI) peak in an initial calibration standard or CCC.
12.3 Calculate the Cr(VI) concentration using the multipoint calibration established in Section
10.2. Report only those values that fall between the MRL and the highest calibration
standard.
12.4 Calculations must use all available digits of precision, but final reported concentrations
should be rounded to an appropriate number of significant figures (one digit of uncertainty),
typically two, and not more than three significant figures.
218.7-18
-------
12.5 Prior to reporting the data, the chromatograms must be reviewed for incorrect peak
identification or improper integration. The laboratory is responsible for ensuring that QC
requirements have been met and that any appropriate qualifier is assigned.
12.6 The analyst must not extrapolate beyond the established calibration range. If the Cr(VI)
result exceeds the range of the initial calibration curve, the sample may be diluted using
reagent water containing the method preservative. Re-inject the diluted sample. Incorporate
the dilution factor into final concentration calculations. The resulting data must be annotated
as a dilution, and the reported MRL must reflect the dilution factor.
13. METHOD PERFORMANCE
13.1 PRECISION, ACCURACY AND DETECTION LIMITS - Single laboratory method
performance data are presented in Section 17. LCMRLs and DLs for both
NH4OH/(NH4)2SO4 preservative and the CO3"2/HCO37(NH4)2SO4 preservative are presented
in Table 3. Precision and accuracy data are presented for Cr(VI) fortified into reagent water
and preserved with NH4OH/(NH4)2SO4 (Table 4). These data were collected using columns
designed for use with an ammonium hydroxide/ammonium sulfate eluent system and
columns designed for use with a carbonate/bicarbonate eluent system. Precision and
accuracy data are presented for Cr(VI) fortified into two sources of chlorinated ground water,
a chlorinated surface water finished with granular activated carbon (GAC) filtration, and a
chlorinated surface water finished without GAC filtration. These data were collected using
columns designed for an ammonium hydroxide/ammonium sulfate eluent system (Table 5)
and columns designed for a carbonate/ bicarbonate eluent system (Table 6). All precision
and accuracy data were collected for samples preserved with NH4OH/(NH4)2SO4. Figures 1
through 4 are chromatograms of Cr(VI) in reagent water and drinking water obtained under
the conditions employed during method development.
13.2 SECOND LABORATORY EVALUATION - Four independent laboratories demonstrated
acceptable method performance using 2- and 4-mm column systems with ammonium
hydroxide/ammonium sulfate eluent, and one independent laboratory demonstrated
acceptable method performance using a 4-mm column system with carbonate/bicarbonate
eluent. The authors wish to acknowledge the Utah Water Research Laboratory (Logan, UT),
Aqua Pennsylvania, Inc. (Bryn Mawr, PA), Metrohm USA, Inc. (Riverview, FL), Thermo
Fisher Scientific/Dionex (Sunnyvale, CA), and MWH Laboratories (Monrovia, CA) for their
contribution to the method development effort.
13.3 STORAGE STABILITY STUDY - Chlorinated surface water samples were preserved as
required in Section 8 and stored over a 21-day period. Experimental conditions included
samples fortified at 1.0 |ig/L Cr(VI), samples fortified with 1.0 |ig/L Cr(III), and unfortified
samples (-0.060 jig/L Cr(VI)). Unfortified samples (to study the storage stability of low-
level Cr(VI) concentrations) were held under refrigerated and ambient storage temperatures.
Both preservative systems were studied for each of the preceding conditions. Samples
fortified with Cr(III), and samples fortified with Cr(III) plus an additional 3-mg/L chlorine
were studied to confirm the ability of the method preservative to prevent oxidation of
dissolved Cr(III). The percent recovery (based on the mean concentration at day zero) and
precision of two replicate analyses for each condition conducted after 0, 1, 2, 7, 14, and 21
218.7-19
-------
days of storage are presented in Section 17, Table 7. For samples fortified with Cr(III),
results are expressed as the percent conversion of Cr(III) to Cr(VI) using the mean
concentration of Cr(VI) at day zero as the baseline.
14. POLLUTION PREVENTION
14.1 For information about pollution prevention applicable to laboratory operations described in
this method, consult: Less is Better, Guide to Minimizing Waste in Laboratories., a web-based
resource available from the American Chemical Society at http://www.acs.org.
15. WASTE MANAGEMENT
15.1 The Agency requires that laboratory waste management practices be consistent with all
applicable rules and regulations, and that laboratories protect the air, water, and land by
minimizing and controlling all releases from fume hoods and bench operations. In addition,
compliance is required with any sewage discharge permits and regulations, particularly the
hazardous waste identification rules and land disposal restrictions.
16. REFERENCES
1. Winslow, S.D.; Pepich, B.V.; Martin, J. I; Hallberg, G.R.; Munch D.J.; Frebis, C.P.; Hedrick,
E.J.; Krop, R.A. Statistical Procedures for Determination and Verification of Minimum
Reporting Levels for Drinking Water Methods. Environ. Sci. Technol. 2006; 40, 281-288.
2. Glaser, J.A.; Foerst, D.L.; McKee, G.D.; Quave, S.A.; Budde, W.L. Trace Analyses for
Wastewaters. Environ. Sci. Technol. 1981; 15, 1426-1435.
3. Arar, Elizabeth J.; Praff, John D. Determination of Dissolved Hexavalent Chromium in
Industrial Wastewater Effluents by Ion Chromatography and Post-column Derivatization with
Diphenylcarbazide. Journal of Chromatography, 1991; 546, 335-340.
4. Technical Note 26, Dionex Corp., Sunnyvale, CA, 1998.
5. Application Update 144, Dionex Corp., Sunnyvale, CA, 2003.
6. 1C Application Work AW US6-0152-012011, Metrohm USA, Inc., 2011.
7. Clifford, D.; Man Chau, J. The Fate of Chromium (III) in Chlorinated Water; EPA/600/S2-
87/100; U.S. EPA, Water Engineering Research Laboratory: Cincinnati, OH, January 1988.
8. Kotas, J.; Stasicka, Z. Chromium Occurrence in the Environment and Methods of its Speciation.
Environmental Pollution. 2000; 107, 263-283.
9. Ulmer, Nancy S.; Effect of Chlorine on Chromium Speciation in Tap Water; EPA/600/M-86/015;
U.S. EPA Environmental Research Brief: Cincinnati, OH June 1986.
218.7-20
-------
17. REFERENCES, TABLES, DIAGRAMS, FLOWCHARTS, AND VALIDATION DATA
TABLE 1. ION CHROMATOGRAPHIC CONDITIONS USED TO COLLECT METHOD
PERFORMANCE DATA: AMMONIUM HYDROXIDE/AMMONIUM SULFATE
ELUENT
Parameter
Ion chromatograph
Guard Column
Anion Separator Column
Column compartment
temperature
Eluent
Eluent flow rate
Post-column flow rate
Sample volume
Post-column reagent
Reaction coil, temperature
Detector, wavelength
Conditions
Dionex ICS 5000 with AS Autosampler and PC10 Post-column Pneumatic
Delivery Package
Dionex NG1 (4 x 35 mm)
Dionex lonPac® AS7 (4 x 250 mm)
,,„„„ Autosampler tray
temperature
Ambient
Isocratic: 250 mM ammonium sulfate, 100 mM ammonium hydroxide
1.0 mL/min
0.33 mL/min
lOOO^L
2 mM 1,5-diphenylcarbazide, 10% methanol, 1 N sulfuric acid
750-(iL knitted polytetrafluoroethylene reaction coil, 30 °C
UV-Vis absorbance, 530 nm
TABLE 2. ION CHROMATOGRAPHIC CONDITIONS USED TO COLLECT METHOD
PERFORMANCE DATA: CARBONATE/BICARBONATE ELUENT
Parameter
Ion chromatograph
Guard Column
Anion Separator Column
Column compartment
temperature
Eluent
Eluent flow rate
Post-column flow rate
Sample volume
Post-column reagent
Reaction coil, temperature
Detector, wavelength
Conditions
Metrohm 1C Professional 850, Model 887 UV-Vis Detector, Model 858
Autosampler, Model 800 Dosino post-column reagent delivery system
Metrohm RP2 Guard/3.5
Metrohm Metrosep A SUPP 5-150/4
45 °C Autosampler tray ^^
temperature
Isocratic: 12.8 mM sodium carbonate, 4.0 mM sodium bicarbonate
0.70 mL/min
0.22 mL/min
1250(iL
2 mM 1,5-diphenylcarbazide, 10% methanol, 1 N sulfuric acid
375-(iL knitted polytetrafluoroethylene reaction coil, ambient
UV-Vis absorbance, 530 nm
218.7-21
-------
TABLE 3. LOWEST CONCENTRATION MINIMUM REPORTING LEVELS (LCMRL) and
DETECTION LIMITS (PL) for Cr(VI)
Conditions
Ammonium hydroxide/ammonium
sulfate eluent system
Carbonate/bicarbonate eluent
system
Preservative
Liquid a
Solid b
Liquid3
Solid b
Calculated
LCMRL, jig/L
0.012
0.012
0.036
0.023
DL Fortification Level, jig/L
0.0125
0.0125
0.050
0.020
DL
0.0054
0.0044
0.010
0.015
1 NH4OH/(NH4)2SO4 (liquid) preservative.
b CO3-2/HCO3Y(NH4)2SO4 (solid) preservative.
TABLE 4. SINGLE LABORATORY PRECISION AND ACCURACY RESULTS FOR Cr(VI) IN
REAGENT WATER (n=7); NH4OH/(NH4)2SO4 (LIQUID) PRESERVATIVE
Fortification:
Ammonium
hydroxide/ammonium
sulfate eluent system
Fortification:
Carbonate/bicarbonate eluent
system
Mean %
Recovery
% Relative
Standard
Deviation
0.020 jig/L
90.9
4.2
0.0625 jig/L
98.5
8.0
Mean %
Recovery
% Relative
Standard
Deviation
0.20 jig/L
93.5
1.5
0.20 jig/L
96.9
4.9
Mean %
Recovery
% Relative
Standard
Deviation
l.Ofig/L
94.4
1.8
1.0 fig/L
96.2
0.93
TABLE 5. SINGLE LABORATORY PRECISION AND ACCURACY RESULTS FOR Cr(VI) IN
DRINKING WATER MATRIXES USING AMMONIUM SULFATE/AMMONIUM
HYDROXIDE ELUENT SYSTEM AND NH4OH/(NH4)2SO4 (LIQUID) PRESERVATIVE
(n=7 unless noted)
Fortification:
Well water treated only by
chlorination a
Fortification:
Finished groundwaterb
Finished surface water °
Finished surface water
with GAC filtration d
Native
matrix jig/L
-
None
detected
-
0.023
0.060 (n=3)
0.048 (n=3)
Mean %
Recovery"
% Relative
Standard
Deviation
0.060 jig/L
87.1
3.5
0.050 jig/L
96.2
95.5
94.4
1.3
2.9
2.4
Mean %
Recovery"
% Relative
Standard
Deviation
1.0 fig/L
96.6
0.82
1.0 fig/L
100
99.8
98.3
1.0
1.2
0.75
a Well water parameters: pH = 7.94; total hardness = 252 mg/L as CaCO3; free chlorine = 0.03 mg/L; total chlorine = 0.64
mg/L.
b Ground water parameters: pH = 7.66; total hardness = 322 mg/L as CaCO3; free chlorine = 0.84 mg/L; total chlorine =
0.84 mg/L.
0 Surface water parameters: TOC = 3.1 mg/L C; pH = 6.77; total hardness = 120 mg/L as CaCO3; free chlorine = 1.2 mg/L;
total chlorine = 1.52 mg/L.
d Surface water parameters: total hardness = 96 mg/L as CaCO3; free chlorine = 1.12 mg/L; total chlorine = 1.22 mg/L.
GAC = granular activated carbon.
e Recoveries corrected for native levels in the unfortified matrix.
218.7-22
-------
TABLE 6. SINGLE LABORATORY PRECISION AND ACCURACY RESULTS FOR Cr(VI) IN
DRINKING WATER MATRIXES USING CARBONATE/BICARBONATE ELUENT
SYSTEM AND NH4OH/(NH4)2SO4 (LIQUID) PRESERVATIVE (n=7 unless noted)
Fortification:
Well water treated only by
chlorination a
Fortification:
Finished groundwaterb
Finished surface water °
Finished surface water
with GAC filtration d
Native
matrix, jig/L
-
None
detected
-
0.031
0.053 (n=3)
0.033
Mean %
Recovery"
% Relative
Standard
Deviation
0.060 jig/L
103
16
0.050 jig/L
76.3
92.7
98.6
35
9.4
4.4
Mean %
Recovery"
% Relative
Standard
Deviation
1.0 ug/L
93.5
0.46
1.0 ug/L
97.5
97.5
96.1
1.2
1.1
1.6
a Well water parameters: pH = 7.94; total hardness = 252 mg/L as CaCO3; free chlorine = 0.03 mg/L; total chlorine = 0.64
mg/L.
b Ground water parameters: pH = 7.66; total hardness = 322 mg/L as CaCO3; free chlorine = 0.84 mg/L; total chlorine =
0.84 mg/L.
c Surface water parameters: TOC = 3.1 mg/L C; pH = 6.77; total hardness = 120 mg/L as CaCO3; free chlorine = 1.2 mg/L;
total chlorine = 1.52 mg/L.
d Surface water parameters: total hardness = 96 mg/L as CaCO3; free chlorine = 1.12 mg/L; total chlorine = 1.22 mg/L.
GAC = granular activated carbon.
e Recoveries corrected for native levels in the unfortified matrix.
218.7-23
-------
TABLE 7. SAMPLE HOLDING TIME DATA FOR Cr(VI) IN CHLORINATED SURFACE WATER3 PRESERVED AND STORED
ACCORDING TO METHOD SECTION 8 (n = 2 for each experimental condition)
Experimental Condition
NH4OH/(NH4)2SO4 (liquid) preservative, 6 °C b
CO3"2/HCO37(NH4)2SO4 (solid) preservative,
6°Cb
NH4OH/(NH4)2SO4 (liquid) preservative, 6 °C b
CO3"2/HCO37(NH4)2SO4 (solid) preservative,
6°Cb
NH4OH/(NH4)2SO4 (liquid) preservative, 6 °C c
CO3"2/HCO37(NH4)2SO4 (solid) preservative,
6°CC
NH4OH/(NH4)2SO4 (liquid) preservative,
6 °C (fortified w/additional 3 mg/L C12) c
CO3"2/HCO37(NH4)2SO4 (solid) preservative,
6 °C (fortified w/additional 3 mg/L C12) °
NH4OH/(NH4)2SO4 (liquid) preservative,
ambient storage b
CO3"2/HCO37(NH4)2SO4 (solid) preservative,
ambient storage b
Fortification,
Mg/L
Native
Native
1.0Cr(VI)
1.0Cr(VI)
1.0Cr(III)
1.0Cr(III)
1.0Cr(III)
1.0Cr(III)
Native
Native
DayO
Mean
Cr(VI)
Hg/L
0.059
0.055
1.04
1.03
0.072
0.068
0.077
0.064
0.060
0.056
RPD
1.5
6.5
0.33
1.2
3.2
0.74
4.7
7.8
1.7
2.5
Day 1
Result.
%
103
97.8
100
100
0.43
0.45
-0.025
-0.53
100
99.0
RPD
1.2
0.0
1.8
1.0
7.7
5.1
4.5
10
8.7
4.2
Day 2
Result,
%
98.5
99.8
100
100
0.67
0.24
-0.16
-0.76
100
94.9
RPD
2.6
3.3
0.46
0.70
4.8
2.1
8.6
2.3
6.7
3.6
Day?
Result,
%
106
99.2
104
103
1.6
0.60
2.4
0.17
110
98.7
RPD
2.9
0.18
1.4
1.5
6.0
5.1
7.5
15
3.4
3.6
Day 14
Result,
%
110
101
102
100
2.7
0.73
3.4
-0.12
117
105
RPD
5.3
3.4
0.12
2.1
8.2
6.6
4.2
3.5
15
4.1
Day 21
Result,
%
113
101
102
102
3.2
0.93
4.7
0.53
118
101
RPD
4.7
1.3
1.1
0.24
5.0
10
0.88
0.57
2.0
2.3
a Surface water parameters: TOC = 3.1 mg/L C; pH = 6.77; total hardness = 120 mg/L as CaCO3; free chlorine = 1.2 mg/L; total chlorine = 1.52 mg/L.
b Result expressed as the percent recovery of Cr(VI) relative to the mean concentration at Day 0, e.g. at Day 1:
(Mean [Cr(VI)] / Mean [Cr(VI)] Day 0) * 100.
0 Result expressed as the percent conversion of Cr(III) to Cr(VI) using the mean concentrations at Day 0 as the baseline, e.g. at Day 1:
(Mean [Cr(VI)] - Mean [Cr(VI)] Day 0) / 1.0 ng/L Cr(III) * 100.
218.7-24
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TABLE 8. INITIAL DEMONSTRATION OF CAPABILITY (IDC) QUALITY CONTROL REQUIREMENTS
Method
Reference
Section 9.2.1
Section 9.2.2
Section 9.2.3
Section 9.2.4
Section 9.2.5
Requirement
Demonstration of low
system background
Demonstration of
precision
Demonstration of
accuracy
MRL confirmation
Quality Control Sample
(QCS)
Specification and Frequency
Analyze an LRB after the high calibration standard during the IDC calibration.
Analyze seven replicate Laboratory Fortified Blanks (LFBs) fortified near the
midrange of the calibration curve.
Calculate average recovery for replicates used in Section 9.2.2.
Fortify and analyze seven replicate LFBs at the chosen MRL concentration.
Confirm that the Upper Prediction Interval of Results (PIR) and Lower PIR
(Sect. 9.2.4.2) meet the recovery criteria.
Analyze mid-level QCS.
Acceptance Criteria
Cr(VI) concentration is
50%
Cr(VI) must be within
+15% of the true value.
218.7-25
-------
TABLE 9. ONGOING QUALITY CONTROL REQUIREMENTS
Method
Reference
Requirement
Specification and Frequency
Acceptance Criteria
Section
10.2
Initial calibration
Use the external standard calibration technique to
generate a linear or quadratic calibration curve.
Use at least six standard concentrations. Validate
the calibration curve as described in Section 10.2.3.
When each calibration standard is calculated as an
unknown using the regression equations, the lowest
level standard should be within +50% of the true
value. All other points should be within +15% of the
true value.
Section
9.3.1
Laboratory Reagent Blank
(LRB)
Analyze one LRB with each Analysis Batch.
Demonstrate that Cr(VI) is below Vs the Minimum
Reporting Level (MRL), and that other sources of
interference do not prevent identification and
quantitation.
Section
10.3
Continuing Calibration
Check (CCC)
Verify initial calibration by analyzing a low-level
CCC at the beginning of each Analysis Batch.
Subsequent CCCs are required after every 10 field
samples and after the last field sample in a batch.
The lowest level CCC must be within +50% of the
true value. All other points must be within +15% of
the true value.
Results for field samples that are not bracketed by
acceptable CCCs are invalid.
Section
9.3.4
Laboratory Fortified Sample
Matrix (LFSM)
Analyze one LFSM per Analysis Batch. Fortify the
LFSM with Cr(VI) at a concentration greater than
the native concentrations. Calculate LFSM
recovery.
For LFSMs fortified at concentrations <2 x MRL, the
result must be within +50% of the true value. At
concentrations greater than the 2 x MRL, the result
must be within +15% of the true value.
Section
9.3.5
Laboratory Fortified Sample
Matrix Duplicate (LFSMD)
or Laboratory Duplicate
(LD)
Analyze at least one LFSMD or LD with each
Analysis Batch.
For LFSMDs or LDs, relative percent differences
must be <15%. (<50% if concentration <2 x MRL.)
Section
9.3.6
Quality Control Sample
(QCS)
Analyze mid-level QCS with each new calibration
curve.
Cr(VI) must be +15% of the true value.
218.7-26
-------
2011 0602*3
•0.100-
00
10
3.0
4X1
SO
60
70
SO
90
10.0
11.0
12.0
Cr(VI)
Figure 1. Calibration standards, NH4OH/(NH4)2SO4 (liquid) preservative: 0.020 ug/L analyzed using ammonium
hydroxide/ammonium sulfate eluent system (top) and 0.050 ug/L analyzed using carbonate/bicarbonate eluent system (bottom).
218.7-27
-------
20110603*12
0.500-
0375-
0250-
20110603*16
-0.200-
10.0
Figure 2. Field sample, tap water from surface water source analyzed using ammonium hydroxide/ammonium sulfate eluent system:
~2
(liquid) preservative (top) and CO3~/HCO37(NH4)2SO4 (solid) preservative (bottom); native concentration is
-0.060 ug/L.
218.7-28
-------
0.0
\IWL:530 nm
30
8.0
SO
110
12.0
'\
A
J* IS 3-0 SS
SJ S.S
Figure 3. Field sample, tap water from a ground water source, NH4OH/(NH4)2SO4 (liquid) preservative: analyzed using ammonium
hydroxide/ammonium sulfate eluent system (top) and using carbonate/bicarbonate eluent system (bottom); native concentration is
-0.020 ug/L.
218.7-29
-------
2011 06 03 #24
Cr(VI)
Cr(VI)
90 0.1
iS 46
I* U
** IS
1*4 W.i M
Figure 4. Field sample, tap water from a surface water source, NH4OH/(NH4)2SO4 (liquid) preservative: analyzed using ammonium
hydroxide/ammonium sulfate eluent system (top) and using carbonate/bicarbonate eluent system (bottom); LFSM at 1.0 ug/L.
218.7-30
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