ASTM E863-98
(Practice)Standard Practice for Describing Flame Atomic Absorption Spectroscopy Equipment
Standard Practice for Describing Flame Atomic Absorption Spectroscopy Equipment
SCOPE
1.1 This practice covers those features that are important for evaluating atomic absorption spectroscopy equipment. It also discusses performance characteristics and identifies parameters that should be recorded in analytical procedures.
1.2 This standard does not purport to address all of the safety problems, 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. Specific warning statements are given in the Note in 5.3 and specific precautionary statements are given in Section 7.
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Designation: E 863 – 98
Standard Practice for
Describing Atomic Absorption Spectrometric Equipment
This standard is issued under the fixed designation E 863; 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 5. Description of Equipment
1.1 This practice covers those features that are important for 5.1 Optical System—Both single-beam and double-beam
evaluating atomic absorption spectroscopy equipment. It also optical systems are in use. A single-beam system employs one
discusses performance characteristics and identifies parameters optical path to measure both incident and transmitted radiation.
that should be recorded in analytical procedures. The two measurements are separated in time. A double-beam
1.2 This standard does not purport to address all of the system separates the incident radiation into two portions. One
safety concerns, if any, associated with its use. It is the of these portions (often designated I) is measured after travers-
responsibility of the user of this standard to establish appro- ing the analytical cell; the other portion (often designated I )
o
priate safety and health practices and determine the applica- represents the radiation incident on the analytical cell. Signals
bility of regulatory limitations prior to use. Specific warning from the two portions are used to compensate for baseline
statements are given in the Note in 5.3 and specific precau- variability. In single-beam systems, instability of either the
tionary statements are given in Section 7. incident intensity or the detector-amplifier sensitivity-gain
contributes to the variability of the signal. Double-beam
2. Referenced Documents
systems can correct completely for either type of instability.
2.1 ASTM Standards:
Neither single-beam or double-beam systems can correct for
E 135 Terminology Relating to Analytical Chemistry for fluctuations occurring in the flame.
Metals, Ores, and Related Materials
5.2 Radiation Sources—The most widely used source of
E 416 Practice for Planning and Safe Operation of a Spec- absorbing lines is the hollow cathode lamp. It emits narrow
trochemical Laboratory
spectral lines, has low background, and is applicable to
E 520 Practice for Describing Detectors in Emission and virtually all elements amenable to atomic absorption analysis.
Absorption Spectroscopy
The high-frequency electrodeless discharge lamp (EDL) is also
E 1770 Practice for Optimization of Electrothermal Atomic
an excellent source of sharp line spectra. It provides more
Absorption Spectrometric Equipment intensity than the hollow cathode lamp, but is not as univer-
E 1812 Practice for Optimization of Flame Atomic Absorp-
sally applicable. EDLs require a special power supply.
tion Spectrometric Equipment 5.2.1 The hollow cathode lamp shall emit stable, low-drift
radiation of usable intensity, free of interfering spectral lines
3. Terminology
due to filler gas or cathode impurities.
3.1 For definitions of terms used in this practice, refer to
5.2.2 Stability—Intensity drift shall not exceed 3 % per 15
Terminology E 135.
min after a 30-min warm up. Many lamps available now are
better than 1 % per 15 min after warm up.
4. Significance and Use
5.2.3 Lamp Life—The hollow cathode lamp or EDL shall
4.1 This practice provides criteria for instrument selection
maintain the above characteristics over the manufacturer’s
and should be useful for setting up an atomic absorption
warranted life. The expected life of hollow cathode lamps is
facility.
approximately 5 Ah.
5.3 Flames—Flames are classified as diffusion or premixed.
The characteristics of the diffusion flame are determined by the
rate at which the fuel diffuses into the ambient oxygen or the
This practice is under the jurisdiction of ASTM Committee E-1 on Analytical
oxidant supplied through a separate orifice. In the premixed
Chemistry for Metals, Ores and Related Materials and is the direct responsibility of
Subcommittee E01.20 on Fundamental Practices and Measurement Traceability.
Current edition approved May 10, 1998. Published July 1998. Originally
published as E 863 – 82. Last Previous edition E 863 — 82 (1996). “Guidelines for the Purity and Handling of Gases Used in Atomic Absorption
Annual Book of ASTM Standards, Vol 03.05. Spectroscopy”, AI2.1a, 1978, Scientific Glass Makers Assoication, 1140 Connecti-
Annual Book of ASTM Standards, Vol 03.06. cut Ave., N.W., Washington, DC 20036.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E 863
flame, the characteristics are determined by the kinetics of the aspirated solution that actually reaches the flame, shall be
chemical combustion producing the flame. In general, diffusion specified for pre-mix nebulizer burner systems. The efficiency
flames are more turbulent than pre-mixed flames and the flame is a function of the rates of solution uptake and of the discharge
reactions are not spatially resolved. Premixed acetylene and air through the drain tube.
or acetylene and nitrous oxide flames are the most commonly
5.4.2 Gas Flow Controls—Stable flame operation depends
used atomic absorption flames. Acetylene is the most common
on reproducible and stable flow of fuel and oxidant gases. A
fuel. Hydrogen, natural gas, propane, and similar fuels are
minimal system shall have a two-stage regulating device at the
sometimes used to produce cooler flames. Air and nitrous oxide
fuel source and at the oxidant source. There shall be a pressure
are the normal oxidants. Oxygen has sometimes been used, but
meter for the nebulizing gas, a flow meter for each gas, and a
due to higher flame propagation velocities, it is not used
flow control system for each gas line at the burner.
anymore.
5.4.2.1 The manufacturer shall specify suitable gas flows.
Flow meters should be calibrated in litres per minute or some
NOTE 1—Warning: Never use copper tubing for acetylene because of
comparable unit. If the burner system is to be used with a
the possible formation of explosive copper acetylide.
nitrous oxide acetylene flame, special precautions to avoid
5.4 Burners—Historically two types of burners were used in
flashback when the flame is ignited and extinguished, shall be
atomic absorption spectroscopy, the direct injection or total
provided. The gas control system shall allow the oxidant to be
consumption burner and the pre-mix nebulizer burner. The
changed from air to nitrous oxide and back again without
total consumption burner ensures freedom from flashbacks and
interruption of flow to the burner. The flame shall be ignited
was once popular for flame spectroscopy. However, it has
using air and acetylene, then switched to nitrous oxide. The
serious disadvantages such as high noise levels, lack of distinct
reverse procedure shall be followed when the flame is extin-
flame zones, high background emission, and difficulties in
5 guished. Some instruments have systems which automatically
obtaining flame shape suitable for absorption measurement,
provide the startup-shutdown sequence, monitor gas flows, and
and, therefore is no longer being used. The most widely used
shut down automatically if the gas flows are outside a
burner now is the pre-mix nebulizer burner. In this burner a
prescribed range.
small fraction of the aspirated solution reaches the flame. Only
5.5 Electrothermal Atomizers—Most flame atomic absorp-
the very finest droplets are used, which makes this an energy-
tion spectrometers manufactured currently can be easily
efficient system. The burner produces a high-temperature flame
adapted for electrothermal analysis, while some atomic absorp-
with relatively low emission background or noise. The flame
tion spectrometers are dedicated to electrothermal analysis.
can be shaped to provide for a long light path through it. A
5.5.1 The most commonly used electrothermal atomizer is
serious disadvantage is that the pre-mix chamber contains an
the graphite tube furnace. This atomizer consists of a graphite
explosive gas mixture during operation. An explosive flash-
tube positioned in a water-cooled unit designed to be placed in
back is possible when the burning velocity exceeds the velocity
the optical path of the spectrophotometer so that the light from
of the burning gases passing through the burner orifice.
the hollow cathode lamp passes through the center of the tube.
Equipment shall be designed to minimize the probability of a
The tubes vary in size depending upon a particular instrument
flashback, protect the instrument operator and anyone nearby
manufacturer’s furnace design. These tubes are available with
from injury if a flashback does occur, and minimize damage to
or without pyrolytic graphic coating. However, because of
the burner, chamber, and instrument.
increased tube life, tubes coated with pyrolytic graphite are
5.4.1 Nebulizers—The nebulizer is used to aspirate the
commonly used. The water-cooled unit or atomizer head which
sample and convert it to a fine mist. The most common
holds the graphite tube is constructed in such a way that an
nebulizers employ oxidant gas as the source of energy for
inert gas, usually argon or nitrogen, is passed over, around, or
nebulization. The mist consists of a wide distribution of
through the graphite tube to protect it from atmospheric
particle sizes but only the smallest (less than 1 μm in diameter)
oxidation. The heating of all of these atomizers is controlled by
reach the flame and contribute to the atomic absorption signal.
power supplies which make it possible to heat the graphite tube
Some pre-mix chambers employ an impingement bead to
to 3000°C in less than 1 s. Temperatures and drying, pyrolysis,
enhance the generation of smaller particles, and mix the gases
and atomization times are controlled by these power supplies.
and vapors before passing through the burner slots. Other
pre-mix chambers enhance particle size separation by baffles 5.6 Automatic Background Correction—Automatic back-
that allow only the finest particles to reach the flame. The result ground correction is recommended for all atomic absorption
is a smaller but more stable absorption signal. Ultrasonic units.
nebulizers have been used experimentally for fog generation.
5.6.1 Automatic background correction is a necessity for all
They provide an efficient generation of a uniform mist. In
spectrophotometers used with electrothermal devices. When
practice, however, such problems as feeding the sample uni-
electrothermal atomizers, especially graphite furnaces, are
formly, conducting the vapor to the burner, and sample
heated to high temperatures, background from absorption is
memory have prevented this nebulization system from becom-
produced within the graphite tube. Also, small amounts of
ing popular. The nebulizer efficiency, or the fraction of the
particulate matter in the furnace contribute to the background
signal. Therefore, it is essential to correct or compensate for
this background.
5.6.1.1 Magnetic (Zeeman type) background correction
Kirkbright, G. F., and Sargent, M., Atomic Absorption and Fluorescence
Spectroscopy, Academic Press, New York, NY, 1974, p. 201. gives wider range of correction than hydrogen or deuterium
E 863
background correctors and is a preferred type for spectrometers 5.7.5 Wavelength Range of the Spectrometer—The effective
using electrothermal atomization. wavelength range of the spectrometer is a function of the
5.7 Spectrometer—The wavelength where absorption mea- monochromator, the detector, and the medium through which
surements are made is isolated from the total spectrum of the the optical path passes. The minimum and maximum wave-
primary radiation source by means of a monochromator, or by lengths at which the monochromator optics can be set shall be
nondispersive devices.
specified. If this wavelength range can be varied by inter-
changeable gratings or prisms, it shall be so stated, and the
5.7.1 Monochromator Types—Monochromators are prima-
rily classified as to whether the wavelength-discriminating minimum and maximum values given. The usable wavelength
function is performed by a grating, a prism, optical filters of range of the overall system, including the detector, will usually
various types ranging from single glass filters to multilayer be limited at the lower wavelength by the spectral absorption of
interference filters (transmission or reflection types), or a the ambient atmosphere and optics, and at the higher wave-
combination of grating and prism such as may be used in some
length by spectral sensitivity of the detector. Descriptions of
double monochromators. systems shall differentiate between the wavelength range of the
5.7.1.1 Optical Design of Monochromators—Grating and monochromator and the usable wavelength range of the overall
prism monochromators are further classified by the optical
system.
configuration of the incident and dispersed beams. Such
5.7.6 Wavelength Scanning—If the monochromator pro-
configurations include the Czerny-Turner, Ebert, Eagle, Lit-
vides for scanning the wavelength range at a predetermined
trow, and Littrow-Echelle mounts, used singly or in combina-
rate, it shall be so specified. The following parameters describe
tion. The various types differ in the application or configuration
the scanning functions: single speed, multiple speeds, or
of the collimating and dispersing elements, or both. The
continuously variable scanning speeds. The minimum and
different configurations take into account optical speed, disper-
maximum speeds expressed in nanometres per second or
sion, stability, optical defects, and astigmatism. There is no
nanometres per minute should be stated, as should the specific
overwhelmingly superior configuration.
values of scanning speeds.
5.7.1.2 Description of the Grating shall include the follow-
5.7.7 Wavelength Accuracy—The maximum distance in na-
ing information: (a) Type: plane or concave; (b) Size: width
nometres between an indicated wavelength and the mean
and height of ruled area expressed in millimetres; (c) Groove
wavelength passed by the monochromator at that setting shall
spacing: grooves per millimetre; and (d) Blaze wavelength:
be specified along with the wavelength range over which the
expressed in nanometres.
specification applies. An example might be 60.1 nm between
5.7.1.3 Description of the Prism shall include the following
200 and 700 nm. In the case of monochromators having
information: (a) Type: Cornu, Littrow or other type; (b)
nonlinear dispersion, the accuracy s
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