ASTM D6508-00

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An American National Standard
Designation: D 6508 – 00
Standard Test Method for
Determination of Dissolved Inorganic Anions in Aqueous
Matrices Using Capillary Ion Electrophoresis and Chromate
Electrolyte
This standard is issued under the fixed designation D 6508; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This test method cover the determination of the inor-
ganic anions fluoride, bromide, chloride, nitrite, nitrate, ortho-
phosphate, and sulfate in drinking water, wastewater, and other
aqueous matrices using capillary ion electrophoresis (CIE)
with indirect UV detection. See Figs. 1-6.
1.2 The test method uses a chromate-based electrolyte and
indirect UV detection at 254 nm. It is applicable for the
determination or inorganic anions in the range of 0.1 to 50
mg/L except for fluoride whose range is 0.1 to 25 mg/L.
1.3 It is the responsibility of the user to ensure the validity
of this test method for other anion concentrations and untested
aqueous matrices.
NOTE 1—The highest accepted anion concentration submitted for
FIG. 1 Electropherogram of Mixed Anion Working Solution and
precision and bias extend the anion concentration range for the following
Added Common Organic Acids
anions: Chloride to 93 mg/L, Sulfate to 90 mg/L, Nitrate to 72 mg/L, and
ortho-phosphate to 58 mg/L.
D 5810 Guide for Spiking into Aqueous Samples
1.4 This standard does not purport to address all of the
D 5847 Practice for Writing Quality Control Specifications
safety concerns, if any, associated with its use. It is the
for Standard Test Methods for Water Analysis
responsibility of the user of this standard to establish appro-
priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use. For specific hazard
statements, see Section 9.
Annual Book of ASTM Standards, Vol 11.02.
2. Referenced Documents
2.1 ASTM Standards:
D 1066 Practice for Sampling Steam
D 1129 Terminology Relating to Water
D 1193 Specification for Reagent Water
D 2777 Practice for Determination of Precision and Bias of
Applicable Test Methods of Committee D-19 on Water
D 3370 Practices for Sampling Water from Closed Con-
duits
D 3856 Guide for Good Laboratory Practices in Laborato-
ries Engaged in Sampling and Analysis of Water
This test method is under the jurisdiction ofASTM Committee D-19 on Water
and is the direct responsibility of Subcommittee D19.05 on Inorganic Constituents
in Water.
Current edition approved Jan. 10, 2000. Published April 2000. FIG. 2 Electropherogram of 0.2 mg/L Anions Used to Determine
Annual Book of ASTM Standards, Vol 11.01. MDL
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D6508–00
FIG. 3 Electropherogram of Substitute Wastewater
FIG. 6 Electropherogram of Industrial Wastewater
3.2 Definitions of Terms Specific to This Standard:
3.2.1 capillary ion electrophoresis, n—an electrophoretic
techniqueinwhichaUV-absorbingelectrolyteisplacedina50
µmto75µmfusedsilicacapillary.Voltageisappliedacrossthe
capillary causing electrolyte and anions to migrate towards the
anode and through the capillary’s UVdetector window.Anions
are separated based upon the their differential rates of migra-
tion in the electrical field.Anion detection and quantitation are
based upon the principles of indirect UV detection.
3.2.2 electrolyte, n—a combination of a UV-absorbing salt
andanelectroosmoticflowmodifierplacedinsidethecapillary,
used as a carrier for the analytes, and for detection and
quantitation. The UV-absorbing portion of the salt must be
anionic and have an electrophoretic mobility similar to the
FIG. 4 Electropherogram of Drinking Water
analyte anions of interest.
3.2.3 electroosmotic flow (EOF), n—the direction and ve-
locity of electrolyte solution flow within the capillary under an
applied electrical potential (voltage); the velocity and direction
of flow is determined by electrolyte chemistry, capillary wall
chemistry, and applied voltage.
3.2.4 electroosmotic flow modifier (OFM), n—a cationic
quaternary amine in the electrolyte that dynamically coats the
negatively charged silica wall giving it a net positive charge.
This reverses the direction of the electrolyte’s natural elec-
troosmotic flow and directs it towards the anode and detector.
Thismodifieraugmentsanionmigrationandenhancesspeedof
analysis. Its concentration secondarily effects anion selectivity
and resolution, (see Fig. 7).
3.2.5 electrophoretic mobility, n—the specific velocity of a
charged analyte in the electrolyte under specific electroosmotic
FIG. 5 Electropherogram of Municipal Wastewater Treatment
flow conditions. The mobility of an analyte is directly related
Plant Discharge
to the analyte’s equivalent ionic conductance and applied
voltage, and is the primary mechanism of separation.
D 5905 Practice for the Preparation of Substitute Wastewa-
3.2.6 electropherogram, n—a graphical presentation of UV-
ter
detector response versus time of analysis; the x axis is
F 488 Test Method for On-Site Screening of Heterotrophic
migration time, which is used to qualitatively identify the
Bacteria in Water
anion, and the y axis is UV response, which can be converted
to time corrected peak area for quantitation.
3. Terminology
3.2.7 hydrostatic sampling, n—a sample introduction tech-
3.1 Definitions—For definitions of terms used in this test nique in which the capillary with electrolyte is immersed in the
method, refer to Terminology D 1129. sample, and both are elevated to a specific height, typically 10
D6508–00
FIG. 7 Pictorial Diagram of Anion Mobility and ElectroOsomotic Flow Modifier
cm, above the receiving electrolyte reservoir for a preset
amount of time, typically less than 60 s. Nanolitres of sample
aresiphonedintothecapillarybydifferentialheadpressureand
gravity.
3.2.8 indirect UV detection, n—a form of UV detection in
which the analyte displaces an equivalent net charge amount of
the highly UV-absorbing component of the electrolyte causing
anetdecreaseinbackgroundabsorbance.Themagnitudeofthe
decreased absorbance is directly proportional to analyte con-
centration. Detector output polarity is reversed in order to
obtain a positive mV response.
3.2.9 midpoint of peak width, n—CIE peaks typically are
asymmetrical with the peak apex shifting with increasing
concentration, and the peak apex may not be indicative of true
FIG. 8 Selectivity Diagram of Anion Mobility Using Capillary Ion
Electrophoresis
analyte migration time. Midpoint of peak width is the midpoint
between the analyte peak’s start and stop integration, or the
peak center of gravity.
3.2.10 migration time, n—the time required for a specific
analyte to migrate through the capillary to the detector. The
migration time in capillary ion electrophoresis is analogous to
retention time in chromatography.
3.2.11 time corrected peak area, n—normalized peak area;
peak area divided by migration time. CE principles state that
peak area is dependent upon migration time, that is, for the
same concentration of analyte, as migration time increases
(decreases) peak area increases (decreases). Time corrected
peak area accounts for these changes.
4. Summary of Test Method
4.1 Capillary ion electrophoresis, see Figs. 7-10, is a free
zone electrophoretic technique optimized for the determination
FIG. 9 Pictorial Diagram of Indirect UV Detection
of anions with molecular weight less than 200. The anions
migrate and are separated according to their mobility in the
electrolyte when an electrical field is applied through the open combination with a negative power supply augments the
tubular fused silica capillary. The electrolyte’s electroosmotic mobility of the analyte anions towards the anode and detector
low modifier dynamically coats the inner wall of the capillary achieving rapid analysis times. Cations migrate in the opposite
changing the surface to a net positive charge. This reversal of directiontowardsthecathodeandareremovedfromthesample
wall charge reverses the natural EOF. The modified EOF in during analysis. Water and other neutral species move toward
D6508–00
FIG. 10 General Hardware Schematic of a Capillary Ion Electrophoresis System
the detector at the same rate as the EOF. The neutral species 5.4 This test method is intended as an alternative to other
migrate slower than the analyte anions and do not interfere multi-analyte methods and various wet chemistries for the
with anion analysis (see Figs. 7 and 8). determination of inorganic anions in water and wastewater.
4.2 The sample is introduced into the capillary using hydro- Compared to other multi-analyte methods the major benefits of
staticsampling.Theinletofthecapillarycontainingelectrolyte CIE are speed of analysis, simplicity, and reduced reagent
is immersed in the sample and the height of the sample raised consumption and operating costs.
10 cm for 30 s where low nanolitre volumes are siphoned into
6. Interferences
thecapillary.Aftersampleloading,thecapillaryisimmediately
immersed back into the electrolyte. The voltage is applied
6.1 Analyte identification, quantitation, and possible comi-
initiating the separation process.
gration occur when one anion is in significant excess to other
4.3 Anion detection is based upon the principles of indirect
anions in the sample matrix. For two adjacent peaks, reliable
UV detection. The UV-absorbing electrolyte anion is displaced
quantitation can be achieved when the concentration differen-
charge-for-charge by the separated analyte anion. The analyte
tial is less than 100:1. As the resolution between two anion
anion zone has a net decrease in background absorbance. This
peaks increase so does the tolerated concentration differential.
decrease in UV absorbance in quantitatively proportional to
In samples containing 1000 mg/L Cl, 1 mg/L SO can be
analyte anion concentration (see Fig. 9). Detector output
resolved and quantitated, however, the high Cl will interfere
polarity is reversed to provide positive mVresponse to the data
with Br and NO quantitation.
-1
system, and to make the negative absorbance peaks appear
6.2 Dissolved carbonate, detected as HCO , is an anion
positive.
present in all aqueous samples, especially alkaline samples.
4.4 The analysis is complete once the last anion of interest
Carbonate concentrations greater than 500 mg/L will interfere
is detected. The capillary is vacuum purged automatically by
with PO quantitation.
the system of any remaining sample and replenished with fresh
6.3 Monovalent organic acids, except for formate, and
electrolyte. The system now is ready for the next analysis.
neutral organics commonly found in wastewater migrate later
in the electropherogram, after carbonate, and do not interfere.
5. Significance and Use
Formate, a common organic acid found in environmental
5.1 Capillary ion electrophoresis provides a simultaneous samples, migrates shortly after fluoride but before phosphate.
separation and determination of several inorganic anions using Formate concentrations greater than 5 mg/Lwill interfere with
nanolitres of sample in a single injection.All anions present in fluoride identification and quantitation. Inclusion of 2 mg/L
the sample matrix will be visualized yielding an anionic profile formate into the mixed anion working solution aids in fluoride
of the sample. and formate identification and quantitation.
5.2 Analysis time is less than 5 minutes with sufficient 6.4 Divalent organic acids usually found in wastewater
sensitivity for drinking water and wastewater applications. migrate after phosphate. At high concentrations, greater than
Time between samplings is less than seven minutes allowing 10 mg/L, they may interfere with phosphate identification and
for high sample throughput. quantitation.
5.3 Minimal sample preparation is necessary for drinking 6.5 Chlorate also migrates after phosphate and at concen-
water and wastewater matrices. Typically, only a dilution with trations greater than 10 mg/L will interfere with phosphate
water is needed. identification and quantitation. Inclusion of 5 mg/L chlorate
D6508–00
into the mixed anion working solution aids in phosphate and Chemical Society, where such specifications are available.
chlorate identification and quantitation. Other grades may be used, provided it is first ascertained that
6.6 As analyte concentration increases, analyte peak shape the reagent is of sufficient high purity to permit its use without
becomes asymmetrical. If adjacent analyte peaks are not lessening the performance or accuracy of the determination.
baseline resolved, the data system will drop a perpendicular Reagent chemicals shall be used for all tests.
between them to the baseline. This causes a decrease in peak
NOTE 3—Calibration and detection limits of this test method are biased
area for both analyte peaks and a low bias for analyte amounts.
by the purity of the reagents.
For optimal quantitation, insure that adjacent peaks are fully
8.2 Purity of Water—Unless otherwise indicated, references
resolved, if they are not, dilute the sample 1:1 with water.
to water shall be understood to mean Type I reagent water
conforming or exceeding specification D 1193. Freshly drawn
7. Apparatus
water should be used for preparation of all stock and working
7.1 Capillary Ion Electrophoresis System—the system con-
standards, electrolytes, and solutions. Performance and detec-
sists of the following components, as shown in Fig. 10 or
4 tion limits of this test method are limited by the purity of
equivalent:
reagent water, especially TOC.
7.1.1 High Voltage Power Supply, capable of generating
8.3 Reagent Blank—Reagent water, or any other solution,
voltage (potential) between 0 and minus 30 kV relative to
used to preserve or dilute the sample.
ground with the capability working in a constant current mode.
8.4 Individual Anion Solution, Stock
7.1.2 Covered Sample Carousel, to prevent environmental
contamination of the samples and electrolytes during a multi-
NOTE 4—It is suggested that certified individual 1000 mg/L anion
standards be purchased for use with this test method.
sample batch analysis.
NOTE 5—All weights given are for anhydrous or dried salts. Reagent
7.1.3 Sample Introduction Mechanism, capable of hydro-
puritymustbeaccountedforinordertocalculatetruevalueconcentration.
static sampling technique, using gr
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