ASTM D3919-99
(Practice)Standard Practice for Measuring Trace Elements in Water by Graphite Furnace Atomic Absorption Spectrophotometry
Standard Practice for Measuring Trace Elements in Water by Graphite Furnace Atomic Absorption Spectrophotometry
SCOPE
1.1 This practice covers the general considerations for the quantitative determination of trace elements in water and waste water by graphite furnace atomic absorption spectrophotometry. Furnace atomizers are a most useful means of extending detection limits; however, the practice should only be used at concentration levels below the optimum range of direct flame aspiration atomic absorption spectrophotometry. Because of differences between various makes and models of satisfactory instruments, no detailed operating instructions can be provided for each instrument. Instead, the analyst should follow the instructions provided by the manufacturer of a particular instrument.
1.2 Wavelengths, estimated detection limits, and optimum concentration ranges are given in the individual methods. Ranges may be increased or decreased by varying the volume of sample injected or the instrumental settings or by the use of a secondary wavelength. Samples containing concentrations higher than those given in the optimum range may be diluted or analyzed by other techniques.
1.3 This technique is generally not applicable to brines and seawater. Special techniques such as separation of the trace elements from the salt, careful temperature control through ramping techniques, or matrix modification by the addition of NH 4 NO 3 may be useful for these samples.
1.4 The analyst is encouraged to consult the literature as provided by the instrument manufacturer as well as various trade journals and scientific publications.
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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An American National Standard
Designation: D 3919 – 99
Standard Practice for
Measuring Trace Elements in Water by Graphite Furnace
Atomic Absorption Spectrophotometry
This standard is issued under the fixed designation D 3919; 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 2. Referenced Documents
1.1 This practice covers the general considerations for the 2.1 ASTM Standards:
quantitative determination of trace elements in water and waste D 1129 Terminology Relating to Water
water by graphite furnace atomic absorption spectrophotom- D 1192 Specification for Equipment for Sampling Water
etry. Furnace atomizers are a most useful means of extending and Steam in Closed Conduits
detection limits; however, the practice should only be used at D 1193 Specification for Reagent Water
concentration levels below the optimum range of direct flame D 3370 Practices for Sampling Water from Closed Con-
aspiration atomic absorption spectrophotometry. Because of duits
differences between various makes and models of satisfactory D 4841 Practice for Estimation of Holding Time for Water
instruments, no detailed operating instructions can be provided Samples Containing Organic and Inorganic Constituents
for each instrument. Instead, the analyst should follow the
3. Terminology
instructions provided by the manufacturer of a particular
3.1 Definitions—For definitions of terms used in this prac-
instrument.
1.2 Wavelengths, estimated detection limits, and optimum tice, refer to Terminology D 1129.
3.2 Definitions of Terms Specific to This Standard:
concentration ranges are given in the individual methods.
Ranges may be increased or decreased by varying the volume 3.2.1 graphite furnace—an electrothermal graphite device
capable of reaching the specified temperatures required by the
of sample injected or the instrumental settings or by the use of
a secondary wavelength. Samples containing concentrations element being determined.
3.2.2 platform or similar device— a flat, grooved or un-
higher than those given in the optimum range may be diluted
grooved piece of pyrolytic graphite inserted in the graphite
or analyzed by other techniques.
1.3 This technique is generally not applicable to brines and tube on which the sample is placed (1).
seawater. Special techniques such as separation of the trace
4. Summary of Practice
elements from the salt, careful temperature control through
4.1 The element is determined by an atomic absorption
ramping techniques, or matrix modification may be useful for
spectrophotometer used in conjunction with a graphite furnace.
these samples.
The principle is essentially the same as with direct flame
1.4 The analyst is encouraged to consult the literature as
aspiration atomic absorption except a furnace, rather than a
provided by the instrument manufacturer as well as various
flame, is used to atomize the sample. The elemental atoms to be
trade journals and scientific publications.
measured are placed in the beam of radiation by increasing the
1.5 This standard does not purport to address all of the
temperature of the furnace, thereby causing the injected speci-
safety concerns, if any, associated with its use. It is the
men to be volatilized. Radiation from a given excited element
responsibility of the user of this standard to establish appro-
is passed through the vapor containing ground-state atoms of
priate safety and health practices and determine the applica-
that element. The decrease in intensity of the transmitted
bility of regulatory limitations prior to use.
radiation is a measure of the amount of the ground-state
This practice is under the jurisdiction of ASTM Committee D-19 on Water and
is the direct responsibility of Subcommittee D19.05 on Inorganic Constituents in
Water. Annual Book of ASTM Standards, Vol 11.01.
Current edition approved June 10, 1999. Published August 1999. Originally The boldface numbers in parentheses refer to the list of references at the end of
published as D 3919 – 80. Last previous edition D 3919 – 94a (1998). this standard.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D 3919 – 99
element in the vapor. A monochromator isolates the character- ence and the composition of the sample matrix can have a
istic radiation from the hollow-cathode lamp and a photosen- major effect on the analysis. Therefore, for each different
sitive device measures the attenuated transmitted radiation. matrix encountered, the possibility of these interferences
4.2 Dissolved elements are determined on a filtered sample should be considered. The tests as outlined in 6.2.1-6.2.5 are
with no pretreatment. See 9.5.
recommended prior to reporting analytical data. These tests
4.3 Total recoverable elements are determined following will provide indication whether positive or negative interfer-
acid digestion and filtration. If suspended material is not
ence effects are operative in any way on the analyte elements
present, this digestion and filtration may be omitted. thereby distorting the accuracy of the reported values.
6.2.1 Spiking Verification—When the sample absorbance is
5. Significance and Use
40 % or less of the absorbance of the highest standard on the
5.1 Elemental constituents in potable water, receiving water,
standard curve, the amount of spike added to the sample should
and wastewater need to be identified for support of effective
result in a net increase equal to 50 % of the highest standard
pollution control programs. Currently, one of the most sensitive
concentration. The purpose of adding a large spike is to
and practical means for measuring low concentrations of trace
differentiate between matrix interferences and random errors.
elements is by graphite furnace atomic absorption spectropho-
The recovery of the spike must be between 90 and 110 % for
tometry.
verification of the original determination. If the result of the
original determination is above 40 % on the curve, two aliquots
6. Interferences
should be withdrawn and diluted at least 1 + 1. One of the
6.1 Background absorption is caused by the formation of
aliquots should be spiked before dilution with an amount
molecular species from the sample matrix that absorb or scatter
resulting in a net increase over the unspiked aliquot equivalent
the light emitted by the hollow cathode or electrodeless
to 50 % of the highest standard concentration. The reported
discharge line source. Without correction, this will cause the
result should be based on the analysis of the diluted aliquot.
analytical results to be erroneously high. Three approaches
For verification of this result, the spike recovery must be
exist for simultaneous background correction: continuum
between 90 and 110 %. For spiking verification to be valid in
source, Zeeman, and Smith-Hieftje.
either situation in the presence of nonspecific absorbance,
6.1.1 Continuum Source— The continuum source procedure
simultaneous background correction must be used during
involves the use of a deuterium arc source for the ultraviolet or
analysis. If the result of the determination cannot be verified,
a tungsten halide lamp for the visible region of the spectrum.
the sample should be treated in one or more of the following
Light from the primary spectral source and the appropriate
ways:
continuum source are alternately passed through the graphite
6.2.2 Serial Dilution— Successively dilute and reanalyze
furnace. Narrow-band emission of the primary source is
the sample using spiking verifications to determine if the
affected by the scatter and background absorption from the
interference can be eliminated. This assumes that the analyte
matrix as well as the absorption of light by analyte atoms. The
occurs at a sufficiently high concentration.
broad-band emission of the continuum source is affected only
6.2.3 Matrix Modification—Matrix modifiers are frequently
by the background absorption. The effect of the background is
removed by taking a ratio of the energy of the two sources. used to stabilize volatile or moderately volatile analyte metals
such as lead, cadmium, chromium, and nickel. Metals such as
6.1.2 Zeeman Correction—The Zeeman correction system
involves the use of an external magnetic field to split the these begin to volatilize at very low temperatures and require
atomic spectral line. When the magnetic field is off, both that the charring/ashing temperature be lowered. Lower
sample and background are measured. When the magnetic field charring/ashing temperatures reduce the chance of removing
is applied, the absorption line is shifted and only the back- potential interferents from the matrix during the charring/
ground absorption is measured. Background correction is ashing step. Adding certain chemical compounds or combina-
performed by electronically comparing the field-off and tions of chemical compounds will reduce the volatility of
field-on measurements, yielding an analyte-only absorption selected metals by the formation of less volatile compounds
during the charring/ashing process. The use of ammonium
response.
6.1.3 Smith-Hieftje System—This system involves cycling dihydrogen phosphate or phosphoric acid results in higher
the atomic line source at high currents for brief intervals. These volatilization temperatures for many elements, thus permitting
intervals cause nonexcited atoms of the source element to the use of higher charring/ashing temperatures to remove or
undergo the process of self-reversal by emitting light at reduce matrix interferences. Nickel nitrate has been shown to
wavelengths other than those of the analyte. This light is perform the same role for arsenic and selenium by forming
absorbed only by the background, so that interspersing periods high temperature arsenides and selenides. An alternate ap-
of high and low source current permit correction of the proach to the same problem is to reduce the temperature at
background. which the matrix volatilizes, permitting it to be removed at a
6.2 Some types of interference problems encountered in lower charring/ashing temperature. Sodium chloride in seawa-
direct aspiration atomic absorption spectrophotometry can be ter can be volatilized by adding ammonium nitrate as a matrix
observed with the furnace technique. Although quite rare, modifier. The sodium nitrate and ammonium chloride formed
spectral interference may be encountered. When this occurs, are more volatile than the sodium chloride and can be
the use of another wavelength is suggested. Additionally, the volatilized at much lower charring/ashing temperatures. Other
furnace technique is subject to chemical and matrix interfer- matrix modifiers include various organic acids such as citric
D 3919 – 99
and ascorbic acid. These acids are believed to reduce matrix 6.8 Ionization interferences have to date not been reported
interferences by preventing the formation of large salt crystals with furnace techniques.
which can occlude the analyte. A table of additional matrix 6.9 Contamination of the sample can be a major source of
modifiers is given in Appendix X1. See also the literature error because of the extreme sensitivities achieved with the
(2–18). furnace. Keep the sample preparation work area scrupulously
6.2.4 Platform Furnaces—The pseudo-constant tempera- clean (see 9.1). Clean all glassware with dilute HNO (1 + 1).
ture furnace design suggested by L’Vov (1) has minimized Pipet tips have been known to be a source of contamination. If
matrix and gas phase interference problems. L’Vov placed a suspected, acid soak them with HNO (1 + 1) and rinse thor-
graphite platform inside the graphite tube furnace to approxi- oughly with water. The use of only high-quality pipet tips
mate a constant temperature design. Since the platform is greatly reduces this problem. It is very important that special
heated by radiation, it lags behind the tube walls in tempera- attention be given to reagent blanks in both the analysis and the
ture, and delays the atomization of the analyte until the tube correction of analytical results. Lastly, pyrolytic graphite,
atmosphere is at a higher, more constant temperature. This because of the production process and handling, can become
results in reduced vapor-phase condensation and reduces the contaminated. As many as five, to possibly ten, high-
effect of the sample matrix on the analyte signal. The integrated temperature burns may be required to clean the tube before use.
absorbance signal is proportional to the number of atoms in the 6.10 Oxide formation is greatly reduced because atomiza-
sample, independent of the rate at which atomization occurs. tion occurs in an inert atmosphere.
This type of furnace is commercially available or the modifi- 6.11 Several investigators who have studied interferences in
cation can be made by the user (19). the graphite furnace have concluded that nitrate is the preferred
6.2.5 Standard Additions—Analyze the sample by method anion of the matrix. Therefore, nitric acid is preferable for any
of standard additions while noting the precautions and limita- digestion or solubilization step. If the situation absolutely
tions of its use. See 12.4. requires the use of another acid in addition to HNO ,orin
6.3 Gases generated in the furnace during the atomization place of HNO (for example, tin), use the minimum amount of
may have molecular absorption bands encompassing the ana- acid. This applies particularly to hydrochloric and perchloric
lytical wavelength. When this occurs, either using background acids, but also to sulfuric and phosphoric acids to a lesser
correction or choosing an alternative wavelength outside the extent.
absorption band should eliminate this interference. Nonspecific
7. Apparatus
broad band absorption interference can also be compensated by
background correction. 7.1 Atomic Absorption Spectrophotometer—Single- or dual-
6.4 Memory effects occur if, during atomization, all the channel, single- or double-beam instrument having a grating
analyte is not volatilized and removed from the furnace. This monochromator, photomultiplier detector, adjustable slits,
condition is dependent on several factors, such as the volatility wavelength range from 190 to 800 nm, and simultaneous
of the element and its chemical form, whether pyrolytic background correction.
graphite is used, the rate of atomization, and furnace design. If 7.2 Hollow-Cathode Lamps—Single-element lamps are pre-
this situation is detected throu
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