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-11-005 November 2011 http://water.epa.gov/drink
218.7-1

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
218.7-2

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 CrO4

2- 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. 

Analyte

Chemical Abstracts Services 

Registry Number (CASRN)

Hexavalent chromium (as CrO4

2-

) 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 (μg/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 μg/L (Table 3).

1.3 METHOD FLEXIBILITY – The laboratory is permitted to modify chromatographic 

conditions including IC 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 
218.7-3

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. CrO4

2- is separated from other matrix 

components on an anion exchange column. CrO4

2- 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 CrO4

2-

in the sample is 

calculated using the integrated peak area and the external standard technique. Results are reported 

in units of μg/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 Calibration 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 (LD1 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
218.7-4

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 aliquot 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%.

1 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.
218.7-5

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 LABWARE – The stability of Cr(VI) was demonstrated for this method using high-density 

polyethylene (HDPE) 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 CaCO3 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 
218.7-6

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 HCrO4

- and CrO42-

. The very stable CrO4

2- 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 (HDPE) (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.
218.7-7

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 IC SYSTEM – An analytical system consisting of an autosampler, pump module with 

vacuum degassing option, sample loop, guard column, anion separator column, postcolumn 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 pulsefree 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.
218.7-8

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 IC 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

/HCO3

-

/(NH4)2SO4 (solid) preservative, and IC 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 IC eluent.

7.1.5 SODIUM CARBONATE (Na2CO3, CASRN 497-19-8) – Anhydrous. For preparing 

CO3

-2

/HCO3

-

/(NH4)2SO4 (solid) preservative and IC eluent.

7.1.6 REAGENT WATER – Distilled or deionized water. For preparing calibration standards

and reagents.

7.1.7 1,5-DIPHENYLCARBAZIDE (C13H14N4O, CASRN 140-22-7) – for preparing the postcolumn 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 
218.7-9

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

/HCO3

-

/(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 μg/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. WCHEX-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 μg/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 HDPE or 

polypropylene bottle. The Analyte PDS is used to prepare calibration standards and

to fortify QC samples with Cr(VI).
218.7-10

Stock 

Concentration

Aliquot of Stock

Standard 

Solution (7.2.1, 

1000 μg/mL)

Preservative 

Amounts

Volume 

Reagent 

Water

PDS

Concentration

1000 μg/mL with 

NH4OH/(NH4)2SO4

(liquid) 

preservative

0.10 mL 1 mL preservative

(Sect. 7.1.11) 0.10 L 1000 μg/L

1000 μg/mL with 

solid preservative 0.10 mL

13.3 mg Na2CO3, 

10.5 mg NaHCO3

33 mg (NH4)2SO4 

0.10 L 1000 μg/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.

7.2.2 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

Starting Concentration 

(μg/L) Final Volume (L)

Final Concentration 

(μg/L)

0.50 mL Analyte PDS 1000 0.10 5.0

0.10 mL Analyte PDS 1000 0.10 1.0

10 mL 5.0 μg/L CAL 5.0 0.10 0.50

5.0 mL 5.0 μg/L CAL 5.0 0.10 0.25

2.0 mL 5.0 μg/L CAL 5.0 0.10 0.10

1.0 mL 5.0 μg/L CAL 5.0 0.10 0.050

0.4 mL 5.0 μg/L CAL 5.0 0.10 0.020

Storage stability of the calibration standards was evaluated during method development 

at concentrations between 0.020 μg/L and 1.0 μg/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. 
218.7-11

8.1.2. SOLID FORMULATION - CO3

-2

/HCO3

-

/(NH4)2SO4 – The solid preservative may be 

added to sample bottles prior to shipment. Add 13.3 mg Na2CO3, 10.5 mg NaHCO3 and 

33 mg (NH4)2SO4 to 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

/HCO3

-

/(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
218.7-12

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%.

100

AverageConcentration

Standard Deviation of Measured Concentrations % RSD = ×

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.

100

Fortified Concentration

AverageMeasured Concentration % Recovery = ×

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 = 3.963S

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 HR

FortifiedConcentration

PIR +

× 100 ≤ 150%

The Lower PIR Limit must be ≥50 percent recovery.

Mean HR

FortifiedConcentration

PIR − × 100 ≥ 50%
218.7-13

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 11. 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:

DL = s x t(n-1,1-α = 0.99)

where

t(n-1,1-α = 0.99) = Student's t value for the 99% confidence level with n-1 degrees of 

freedom (for seven replicate determinations, the Student’s t value 

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 
218.7-14

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:

( ) 100

C

A - B %R = ×

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.”
218.7-15

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:

( ) 100

LD LD / 2

LD LD

RPD

1 2

1 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:

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.

( ) 100

LFSM LFSMD /2

LFSM LFSMD

RPD ×

+

− =
218.7-16

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 – IC 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., IC 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 IC 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 IC 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 

≤MRL, the result should be within +50% of the true value. Cr(VI) concentrations in

other calibration levels should calculate to be within +15% of the true value. If these 

criteria cannot be met, the analyst may have difficulty meeting ongoing QC criteria. 

Corrective action is recommended, such as reanalyzing the calibration standards, 

restricting the range of calibration, or performing instrument maintenance.
218.7-17

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 ≤MRL must calculate to be within +50% of the true value. The calculated 

concentration in CCCs fortified at all other levels must be within +15%. If these limits 

are exceeded, then all samples analyzed since the last acceptable CCC are invalid. Reanalyze these samples, providing they are still within holding time, after an acceptable 

calibration has been restored.

10.4 REMEDIAL ACTION – Failure to meet CCC QC performance criteria requires remedial 

action. Acceptable method performance may be restored simply by preparing fresh postcolumn reagent and eluent followed by flushing the column and reaction coil for an extended 

period. Following this and other minor remedial action, check the calibration with a midlevel CCC and a CCC at the MRL, or alternatively recalibrate according to Section 10.2. If 

calibration failures persist, maintenance may be required such as replacing the guard column, 

analytical column or reaction coil. These latter measures constitute major maintenance, and 

the analyst must return to the initial calibration step (Sect. 10.2) and verify sensitivity by 

analyzing a CCC at or below the MRL. 

11. PROCEDURE

This section describes the procedures for sample preparation and analysis. Important aspects of 

this analytical procedure include proper sample collection and storage (Sect. 8), ensuring that the 

instrument is properly calibrated (Sect. 10), and that all required QC elements are included (Sect.

9). All field, QC, and calibration samples including the LRB must be preserved as described in 

Section 8. 

11.1 SAMPLE ANALYSIS

11.1.1 Establish IC operating and post-column reagent delivery conditions per the guidance in 

Section 10.1.

11.1.2 Flush the column, start the post-column reagent delivery pump, warm up the UV–Vis 

detector, and take other steps necessary to stabilize the IC system prior to beginning each 

analysis sequence. This step is especially important if the system has been idle for an 

extended period.

11.1.3 Establish a valid initial calibration following the procedures in Section 10.2 or confirm 

that the existing calibration is still valid by analyzing a low-level CCC (Sect. 10.3). 

Analyze field and QC samples in a properly sequenced Analysis Batch as described in 

Section 11.2.
218.7-18

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.
218.7-19

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

/HCO3

-

/(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 μg/L Cr(VI), samples fortified with 1.0 μg/L Cr(III), and unfortified 

samples (~0.060 μg/L Cr(VI)). Unfortified samples (to study the storage stability of lowlevel 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
218.7-20

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. J.; 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. IC 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.
218.7-21

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 Conditions

Ion chromatograph Dionex ICS 5000 with AS Autosampler and PC10 Post-column Pneumatic 

Delivery Package

Guard Column Dionex NG1 (4 x 35 mm)

Anion Separator Column Dionex IonPac® AS7 (4 x 250 mm)

Column compartment 

temperature 30 oC Autosampler tray temperature Ambient

Eluent Isocratic: 250 mM ammonium sulfate, 100 mM ammonium hydroxide

Eluent flow rate 1.0 mL/min

Post-column flow rate 0.33 mL/min

Sample volume 1000μL

Post-column reagent 2 mM 1,5-diphenylcarbazide, 10% methanol, 1 N sulfuric acid

Reaction coil, temperature 750-μL knitted polytetrafluoroethylene reaction coil, 30 oC

Detector, wavelength UV–Vis absorbance, 530 nm

TABLE 2. ION CHROMATOGRAPHIC CONDITIONS USED TO COLLECT METHOD 

PERFORMANCE DATA: CARBONATE/BICARBONATE ELUENT

Parameter Conditions

Ion chromatograph Metrohm IC Professional 850, Model 887 UV–Vis Detector, Model 858 

Autosampler, Model 800 Dosino post-column reagent delivery system

Guard Column Metrohm RP2 Guard/3.5

Anion Separator Column Metrohm Metrosep A SUPP 5-150/4

Column compartment 

temperature 45 oC Autosampler tray temperature Ambient

Eluent Isocratic: 12.8 mM sodium carbonate, 4.0 mM sodium bicarbonate

Eluent flow rate 0.70 mL/min

Post-column flow rate 0.22 mL/min

Sample volume 1250μL

Post-column reagent 2 mM 1,5-diphenylcarbazide, 10% methanol, 1 N sulfuric acid

Reaction coil, temperature 375-μL knitted polytetrafluoroethylene reaction coil, ambient

Detector, wavelength UV–Vis absorbance, 530 nm
218.7-22

TABLE 3. LOWEST CONCENTRATION MINIMUM REPORTING LEVELS (LCMRL) and 

DETECTION LIMITS (DL) for Cr(VI)

Conditions Preservative

Calculated 

LCMRL, μg/L DL Fortification Level, μg/L DL

Ammonium hydroxide/ammonium 

sulfate eluent system

Liquid a 0.012 0.0125 0.0054

Solid b 0.012 0.0125 0.0044

Carbonate/bicarbonate eluent 

system

Liquid a 0.036 0.050 0.010

Solid b 0.023 0.020 0.015 a NH4OH/(NH4)2SO4 (liquid) preservative. b CO3

-2

/HCO3

-

/(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

Mean % 

Recovery

% Relative 

Standard 

Deviation

Mean % 

Recovery

% Relative 

Standard 

Deviation

Mean % 

Recovery

% Relative 

Standard 

Deviation

Fortification: 0.020 μg/L 0.20 μg/L 1.0 μg/L

Ammonium 

hydroxide/ammonium 

sulfate eluent system

90.9 4.2 93.5 1.5 94.4 1.8

Fortification: 0.0625 μg/L 0.20 μg/L 1.0 μg/L

Carbonate/bicarbonate eluent 

system 98.5 8.0 96.9 4.9 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)

Native 

matrix μg/L

Mean % 

Recoverye

% Relative 

Standard 

Deviation

Mean % 

Recoverye

% Relative 

Standard 

Deviation

Fortification: - 0.060 μg/L 1.0 μg/L

Well water treated only by 

chlorination a

None 

detected 87.1 3.5 96.6 0.82

Fortification: - 0.050 μg/L 1.0 μg/L

Finished groundwater b 0.023 96.2 1.3 100 1.0

Finished surface water c 0.060 (n=3) 95.5 2.9 99.8 1.2

Finished surface water 

with GAC filtration d 0.048 (n=3) 94.4 2.4 98.3 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. 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 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)

Native 

matrix, μg/L

Mean % 

Recoverye

% Relative 

Standard 

Deviation

Mean % 

Recoverye

% Relative 

Standard 

Deviation

Fortification: - 0.060 μg/L 1.0 μg/L

Well water treated only by 

chlorination a

None 

detected 103 16 93.5 0.46

Fortification: - 0.050 μg/L 1.0 μg/L

Finished groundwater b 0.031 76.3 35 97.5 1.2

Finished surface water c 0.053 (n=3) 92.7 9.4 97.5 1.1

Finished surface water 

with GAC filtration d 0.033 98.6 4.4 96.1 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. 
218.7-24

TABLE 7. SAMPLE HOLDING TIME DATA FOR Cr(VI) IN CHLORINATED SURFACE WATERa PRESERVED AND STORED 

ACCORDING TO METHOD SECTION 8 (n = 2 for each experimental condition)

Experimental Condition Fortification, 

μg/L

Day 0 Day 1 Day 2 Day 7 Day 14 Day 21

Mean

Cr(VI) 

μg/L

RPD Result. 

% RPD Result, % RPD Result, % RPD Result, % RPD Result, % RPD

NH4OH/(NH4)2SO4 (liquid) preservative, 6 oC b Native 0.059 1.5 103 1.2 98.5 2.6 106 2.9 110 5.3 113 4.7

CO3

-2

/HCO3

-

/(NH4)2SO4 (solid) preservative,

6 o

Cb Native 0.055 6.5 97.8 0.0 99.8 3.3 99.2 0.18 101 3.4 101 1.3

NH4OH/(NH4)2SO4 (liquid) preservative, 6 oC b 1.0 Cr(VI) 1.04 0.33 100 1.8 100 0.46 104 1.4 102 0.12 102 1.1

CO3

-2

/HCO3

-

/(NH4)2SO4 (solid) preservative,

6 oC b 1.0 Cr(VI) 1.03 1.2 100 1.0 100 0.70 103 1.5 100 2.1 102 0.24

NH4OH/(NH4)2SO4 (liquid) preservative, 6 oC c 1.0 Cr(III) 0.072 3.2 0.43 7.7 0.67 4.8 1.6 6.0 2.7 8.2 3.2 5.0

CO3

-2

/HCO3

-

/(NH4)2SO4 (solid) preservative,

6 o

Cc 1.0 Cr(III) 0.068 0.74 0.45 5.1 0.24 2.1 0.60 5.1 0.73 6.6 0.93 10

NH4OH/(NH4)2SO4 (liquid) preservative,

6 oC (fortified w/additional 3 mg/L Cl2) c 1.0 Cr(III) 0.077 4.7 -0.025 4.5 -0.16 8.6 2.4 7.5 3.4 4.2 4.7 0.88

CO3

-2

/HCO3

-

/(NH4)2SO4 (solid) preservative,

6 o

C (fortified w/additional 3 mg/L Cl2) c 1.0 Cr(III) 0.064 7.8 -0.53 10 -0.76 2.3 0.17 15 -0.12 3.5 0.53 0.57

NH4OH/(NH4)2SO4 (liquid) preservative,

ambient storage b Native 0.060 1.7 100 8.7 100 6.7 110 3.4 117 15 118 2.0

CO3

-2

/HCO3

-

/(NH4)2SO4 (solid) preservative, 

ambient storage b Native 0.056 2.5 99.0 4.2 94.9 3.6 98.7 3.6 105 4.1 101 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. c 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 μg/L Cr(III) * 100.
218.7-25

TABLE 8. INITIAL DEMONSTRATION OF CAPABILITY (IDC) QUALITY CONTROL REQUIREMENTS

Method 

Reference Requirement Specification and Frequency Acceptance Criteria

Section 9.2.1 Demonstration of low 

system background Analyze an LRB after the high calibration standard during the IDC calibration. Cr(VI) concentration is <1/3 of the MRL.

Section 9.2.2 Demonstration of 

precision

Analyze seven replicate Laboratory Fortified Blanks (LFBs) fortified near the 

midrange of the calibration curve.

Percent relative standard 

deviation must be ≤15%.

Section 9.2.3 Demonstration of 

accuracy Calculate average recovery for replicates used in Section 9.2.2. Mean recovery within 

+15% of the true value.

Section 9.2.4 MRL confirmation

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.

Upper PIR ≤ 150%

Lower PIR ≥ 50%

Section 9.2.5 Quality Control Sample 

(QCS) Analyze mid-level QCS. Cr(VI) must be within 

+15% of the true value.
218.7-26

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 1⁄3 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.
218.7-27

Figure 1. Calibration standards, NH4OH/(NH4)2SO4 (liquid) preservative: 0.020 μg/L analyzed using ammonium 

hydroxide/ammonium sulfate eluent system (top) and 0.050 μg/L analyzed using carbonate/bicarbonate eluent system (bottom).
218.7-28

Figure 2. Field sample, tap water from surface water source analyzed using ammonium hydroxide/ammonium sulfate eluent system: 

NH4OH/(NH4)2SO4 (liquid) preservative (top) and CO3

-2

/HCO3

-

/(NH4)2SO4 (solid) preservative (bottom); native concentration is 

~0.060 μg/L.
218.7-29

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 μg/L.
218.7-30

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 μg/L.