Applications of Zeeman Graphite Furnace Atomic Absorption Spectrometry in the Chemical Laboratory and in Toxicology

655 Pages·1992·20.406 MB·English

by  C. Minoia and S. Caroli (Eds.)

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Applications of Zeeman Graphite Furnace
Atomic Absorption Spectrometry in
the Chemical Laboratory and in Toxicology
Edited by
C. MINOIA
Laboratorio di Igiene Industriale,
Fondazione Clinica del Lavoro, Pavia
and
S. CAROLI
Laboratorio di Tossicologia Applicata,
Istituto Superiore di Sanita, Roma
PERGAMON PRESS
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Copyright © 1992 Perkin-Elmer Italia
All Rights Reserved. No part of (his publication may be
reproduced, stored in a retrieval system or transmitted in
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copyright holder.
First English edition 1992
Library of Congress Cataloging-in-Publication Data
Applications of Zeeman graphite furnace atomic
absorption spectrometry in the chemical laboratory and
in toxicology / edited by C. Minoia and S. Caroli.
p. cm.
Includes bibliographical references.
1. Furnace atomic absorption spectroscopy.
I. Minoia, Carlo. II. Caroli, Sergio.
QD96.A8A65 1992 543\0858«dc20 92-20198
British Library Cataloguing in Publication Data
A catalogue record for this book is available from the
British Library
ISBN 0-08-041019 7
(Italian edition ISBN 88 7784 103 6 Libreria Cortina)
Printed in Great Britain by BPCC Wheatons Ltd, Exeter
PREFACE
The atomization of small aliquots of a sample in an electrically
heated graphite tube has now virtually replaced the flame technique in
atomic absorption analysis.
"Out, out, brief candle" exclaimed Amos in 1972 during a meeting
on analytical spectroscopy, when describing the high detection power
of a number of elements in biological matrices vaporized by means
of a graphite cup. Such detection limits could never be attained
by atomizing samples in air-acetylene flames (which rapidly became
obsolete). In fact, the possibility of performing microanalyses,
along with more effective use of the atomic vapor produced in the
measurement cell have always been and still are the most attractive
features of electrothermal atomization, especially when applied to
biological fluids, such as blood and urine.
Over the years, refinements in method performance have been
paralleled by an increased awareness on the part of operators.
Consequently, pre-existing experimental data have been critically
revised, leading to changes which were so significant as to require
reassessment of reference values for numerous trace elements in
biological fluids. Undoubtedly, technological and instrumental progress
have contributed significantly by increasing the accuracy of data
obtained through characterization and correction of any spurious effects
due to the matrix and not to the element in question.
These techniques, grouped under the name of background correction
systems, include that based on the Zeeman effect which immediately
attracted the attention of researchers and is now widely exploited in
analytical laboratories.
This volume testifies to this widespread diffusion and is the product
of the contribution of various competent researchers in this field. This
work represents an authoritative compendium on the use of the Zeeman
GFAAS technique in both environmental and clinical-medical fields.
í
The editors deserve the warmest congratulations for collecting a
group of complete practical applications (freshwater, seawater, rocks,
soils, food, blood, urine, biological samples, biopsies, etc.) and
in particular for promoting a critical approach to each problem and
not a mere operative manual. Detailed significant chemical and
chemical-physical information is provided in addition to noteworthy
data in the fields of nutrition, the environment and medical pathology.
Furthermore, the reader can often find a useful comparison between
the various analytical procedures applied in resolving the same
problem.
Although the author of these lines does not consider himself to
be a specialist in GFAAS nor in the mechanism of atomization and
background correction in graphite furnaces, he has often exploited this
technique in combination with laser-excited atomic fluorescence to
obtain an analytical detection power of the order of magnitude of
subphemtograms (10-15 g) for selected elernents. It is still necessary to
ascertain whether there is a need for this level of detectability for
environmental and clinical purposes and whether it is useful for
practical diagnostic evaluations. The overwhelming problems of
elemental contamination as well as the lack of suitable standard
reference materials at such concentrations make it all the more
necessary to develop absolute analytical techniques which do not rely on
calibration curves and avoid sample manipulation. The conditions of
STPF atomization, often dealt with in this book, and its combination
with the Zeeman correction technique and direct introduction of solid
samples appear to provide a promising approach to this. This volume
offers a unique survey of the potential problems together with solutions
derived from the carefully chosen use of innovative techniques such as
those based on laser-excited excitation.
In conclusion, I am convinced that this work is of current interest
and represents a necessary reference source for operators in
environmental and/or clinical analysis, from both the applications and
developmental point of view.
Niccold Omenetto
Ispra, October 1989
vi
FROM I/VOVS GRAPHITE FURNACE TO BACKGROUND
CORRECTION BY MEANS OF THE ZEEMAN EFFECT:
GENERAL CONSIDERATIONS
G. Rossi
Institute of the Environment, Chemical Division,
Commission of the European Communities, Ispra, Varese, Italy
Summary A review is made of the development of AAS with a graphite
furnace as the atomizer from the first LVov experiments carried out in 1959 to
the application, by the Zeeman effect, of the splitting of the spectral lines for
the correction of spurious absorption. The Zeeman effect is dealt with in more
detail, including the main aspects of the theory, the practical use of the
instrument and the detection power and dynamic interval of the various
experimental strategies.
INTRODUCTION
The graphite furnace is now 30 years old and can therefore be
considered a mature technique. Indeed, in his first paper, published in
1959 (1), which became known in western countries at a later date (2),
1/Vov had already anticipated most of the problems, and the solutions
to these problems, which would have occupied countless laboratories
all over the world for many years to come. The first results achieved
by LVov, in some respects highly innovative and promising from the
analytical point of view, did not create among scientists the
enthusiastic interest they deserved; on the contrary, the experimental
evidence of the analytical capabilities of atomic absorption
spectrometry using a graphite furnace was considered with a
scepticism only comparable in magnitude to the development and
success of the technique in subsequent years.
3
4 Applications of Zeeman GFAAS
Although this could be considered to be a strange situation, in
practice it is not surprising, as failure to immediately perceive the
potential of a scientific discovery or innovation, or even of an original
application of already known principles, is not unusual in the scientific
community; in the case of atomic absorption spectrometry this has
been a constant situation.
Walsh reports (3) that when he first demonstrated the principle of a
new analytical spectrochemical method based on the application of the
absorption spectra of elements, he could not refrain from excitedly
showing to his friend and colleague Willis the tracings he had got,
saying: "Look, that is the atomic absorption", the chilling answer: "So
what?" was given by a researcher who later became distinguished for
the remarkable contribution he made to the technique of which at that
moment he was unable to perceive the enormous potential.
From Walsh's first measurements in 1952, using a flame as the atom
reservoir, to L/VoVs preliminary experiments, using a graphite furnace
as the absorption cell, only 7 years elapsed. In the course of that time
AAS had overcome the initial mistrust and had become a widely
accepted new instrumental method of analysis. The ingenuity of
L/Vov^s intuition consisted of the fact that, contrary to the generally
accepted idea of atomic absorption measurements on a stationary
system, such as a flame or the King's furnace, he had perceived
the feasibility of measurements on transient phenomena as the
consequence of complete vaporization, in a very short time, of
microsamples introduced into the furnace.
From the first paper published by Walsh (4), it became clear that
AAS was among the most versatile analytical techniques and this has
warranted its widespread use in all analytical laboratories, as a result
of the simplicity, selectivity and specifity inherent in its basic physical
principles. The main requirements, i.e. to obtain in the vapor phase a
cloud of atoms at the fundamental level, is satisfied very simply by
introducing a mist of the solution to be analyzed into a flame having a
temperature from 1500 to 2500°C. However, the limitations set by
the flame as an atomizer are well known to all analysts and these
limitations have prompted researchers to seek more efficient
atomizers.
In a retrospective analysis of the development and progress of AAS,
it can be affirmed that L/VWs studies (5), as well as those of many
researchers afterwards, have provided further evidence of what could
Background correction by Zeeman effect 5
have been deduced from Walsh's early experiments, i.e. that the
technique has the intrinsic potential to lead to absolute analysis.
The graphite furnace and atomization mechanisms
The concept underlying Lvov's experiments was that the
measurement of the atomic density of a cloud produced
instantaneously in some way from a given amount of sample had to be
performed under isothermal conditions. For that reason it was
necessary to split the process into two parts, i.e.-. (i) sample atomization
by means of the fast heating of a convenient sample holder, and (ii)
the measurement of the absorption of the atomic cloud transferred in
a volume of limited size and kept at a convenient predetermined
temperature. This basic concept, thereafter reconsidered by Woodriff et
al. (6), who utilized and described a furnace at constant temperature,
inevitably led to cumbersome instrumentation and to an operational
complexity which, in a sense, justified the low credibility given to the
technique and initially hindered its wide application.
In 1968 Massman (7) described a model of a furnace which can be
considered to be the basis for the subsequent major developments in
that field. For that reason, he can be acknowledged as the founder of
the present family of graphite furnaces. The idea which simplified and
resolved the problem was the cyclic heating of the furnace; this was not
the ideal condition for measurements on real samples, but, on the other
hand, it allowed use of a very simple system which was easy to operate
so that its use for practical applications and consequently its wide
adoption in analytical laboratories was guaranteed.
Although it would appear to be redundant, it is necessary to recall
the working principles of the system in support of what has been said.
In practice, a graphite tube, 10 to 30 mm in length and 4 to 8 mm in
internal diameter, made of pyrolytic graphite, is heated by the Joule
effect through water-cooled contacts in an inert gas atmosphere to
avoid oxidation. As the electrical resistivity of graphite is quite low, it
is necessary to use high current supplies (some hundreds of ampères)
at low voltages (10-12 V). The sample to be analyzed (a few microliters
of solution) is placed by means of a micropipette at the center of the
graphite tube. The atomization is achieved by heating the graphite
tube in three temperature steps corresponding to solvent evaporation
(110 °C), charring of the residue (350-1200 °C) and vaporization of the
residue, respectively.
6 Applications of Zeeman GFAAS
So as to be effective from an analytical point of view, the system has
to meet the need for extremely fast production of atoms in order to
produce a high atomic density and enable their transfer into the
measurement region as quickly as possible. From the instrumental
point of view this requires a heating system allowing a fast rise in
temperature and very fast electronics to measure transient phenomena
in the range of tenths of a second.
It was a complex problem to obtain a high heating rate with the first
instrumentation available as the electrical power applied to the
furnace could only correspond to that required to maintain the final
temperature chosen for the atomization. With modern instrumentation
great flexibility in the program of the heating cycle is obtained by
means of a microprocessor-controlled power supply. In addition, optical
pyrometers are used to measure the temperature of the external wall of
the furnace with feed-back to the applied electric power when the
programmed temperature has been obtained. In other words, the steps
of temperature rise and constancy at preselected values are separated
and controlled with a high degree of precision and repeatability.
The initial mistrust with regard to this atomization procedure was
also justified by the fact that the optical and temporal coincidence of
the vaporization, atomization and atomic density measurement
processes very often gave rise to serious interference problems due to
spurious absorption phenomena and matrix effects. In a sense, this
could be considered a setback in the development of spectrochemical
analysis. Indeed, these were the problems which had long plagued
optical emission methods based on electrical arcs and sparks as the
excitation sources. It will be shown later how a solution has been found
to the above problems.
The first generation of graphite furnaces consisted of graphite tubes
which, owing to the porosity of the material, could lead to partial loss of
the sample by diffusion processes through the tube wall.
This problem has been overcome by use of tubes made of different
materials. Among the materials used are pyrolytic graphite, vitreous
carbon and carbon, fibers and composite materials, both covered with
pyrolytic graphite.
This last material is the one adopted in most commercially available
instruments. Tubes made entirely of pyrolytic graphite have a high
conductivity, both electrical and thermal, high mechanical strength
and anisotropy. However, their generalized use is hindered by their
very high cost.
Background correction by Zeeman effect 7
Vitreous carbon is made of graphite chains wound in a stochastic
manner so that a preferred orientation of the crystalline planes does
not exist. This material is isotropic and is characterized by low density
and conductivity, both thermal and electrical. Tubes made of this
material have excellent wettability. However, the reactivity and the
resistance to oxidation of the material have not yet been fully
investigated.
Carbon fibers are made of graphite foils or strips, strongly oriented
along the fiber axis and characterized by a much stronger thermal and
electrical conductivity along this axis. This makes it possible, during
manufacturing, to obtain a wide range of variable properties by careful
control of the fiber orientation with respect to the optical axis of the
graphite furnace.
Tubes made of a composite material covered with pyrolytic graphite
exhibit electrical and thermal properties determined by the substrate;
as these can be varied over a wide range, tubes made of this material
represent the best compromise in satisfying the furnace requirements
for AAS operations.
It must be recalled here that the requirements to be met in a furnace
are very often opposite in sign: on the one hand it is necessary to have
a large reactive surface area in order to obtain kinetically limited
processes of oxygen reduction; on the other hand the reduced reactivity
accompanying a limited surface favors a decrease in absorption and
desorption processes. Among the main requirements are the thermal
shock factor (which has to be very high), porosity and permeability to
gases (which have to be very low), chemical purity (which obviously has
to be as high as possible) and constancy of the chemical and physical
properties.
Pyrolytic graphite employed to line the tubes creates a surface which
is inert toward most chemical reactions.
However, the surface has a number of active sites where oxygen
reduction processes take place. Migration of metals or of their
compounds towards these active sites can occur. When saturation of
the sites is achieved, coalescence phenomena can lead to the formation
of droplets which can freely move on the surface, their mobility being
independent of the area and of the temperature of the surface. These
processes can occur at a temperature corresponding to half the value of
the melting point of the material present in the furnace.