Note: Descriptions are shown in the official language in which they were submitted.
CA 02231~48 1998-03-10 '~ . ;
FILE, P~IN THIS ~ NF.~E~
W~T TRANSLAl-l~N G
Lonqitudinally or transversely heated tubular atomisinq
furnace
Description
The invention relates to an atomising furnace for
spect:roscopic purposes which consists of carbon
material and is electrically transversely or
longitudinally heated, which furnace is assembled from
a specially produced tube furnace portion having a
sample insertion opening and a specially produced
sample carrier, with the tube furnace portion, in whose
interior chamber the atomization takes place and which
has on the outside contact elements for connection for
elect:rical heating, havin~ a recess for receiving a peg
located on the underside of the sample carrier, the
reces, being in the inside wall at the side lying
approximately opposite to the sample insertion opening,
and with the sample carrier, which serves for the
uptake and delayed vaporisation of the sample to be
analy;,ed, being arranged and held in the inside wall of
the tube furnace essentially outside the path of the
working beam and having, as element for holding it in
the tube furnace portion, a peg extending from its
outside wall downwardly to the wall of the tube furnace
portion and the sample carrier being held by means of
this by insertion in a recess in the wall of the tube
furna~e part corresponding to the shape of the peg.
Atomising furnaces of this type are preferably used for
flameless atomic absorption spectrometry on the basis
of graphite tube technology (GF-AAS) for vaporisation
and atomisation of solid and liquid samples.
In GF-AAS, the aim is to delay the thermal
atomisation of the sample with respect to the heating
of the inner chamber cf the atomising furnace. This is
intended to ensure that the constituents of the sample
vaporise under approximately stabilised temperature
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conditions and are atomised suddenly and cannot
precipitate on comparatively cool parts of the walls of
the inner atomisation chamber. Attempts have been made
to re~lise this aim in a known way by a sample carrier
arranged in the inner furnace chamber. In the ideal
case, the sample carrier should for this purpose be
constructed and fixed in the furnace in such a way that
it is heated neither by heat conduction nor by Joulean
heat but instead exclusively by radiant heat from the
inside wall of the furnace. An arrangement of a
longitudinally heated atomising furnace having a sample
carrier, which arrangement, however, fulfilled the
above-mentioned requirements only approximately, was
suggested for the first time by B. L'vov
("Spectrochimica Acta", Vol. 33B, pp 153 to 193, 1978).
The embodiments of sample carriers for
longitudinally heated atomising furnaces that are
described in the texts of DE-PS 29 24 123, DE-GM 87 14
926.5, DE-OS 37 22 379, DE-GM 88 03 144.6, DE-OS 38 23
346.0 and EP 0 442 009 A1 have likewise in detail
further essential disadvantages with regard to the
above-mentioned requirements.
A further, improved sample carrier for a
longitudinally heated atomising furnace is described in
DD 233 190 A (DE-OS 35 45 635). It is point-fixed by
way of a pin-like support which lies asymmetrically
with respect to the centre of the tube furnace and is
inserted into a hollow located in the inside wall of
the tube furnace. The said sample carrier can,
however, be removed again from the tube furnace at any
time. One of the stated aims of this protective right
appli~ation was that atomising furnace and sample
carrier do not form a non detachable unit in the
operative final state of production, because the sample
carrier itself can be inserted and removed by a
manip-ulator. The result of this is that the position
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of the sample carrier in the atomising furnace is not
positively fixed particularly in the case of shaking or
in the presence of strong magnetic fields. Tests by
the applicant resulted in uncontrolled wall contacts of
the sample carrier and thus current conduction and heat
conduction between the outside edges of the sample
carrier and the inside wall of the furnace and
conse~uently very unreproducible relationships from
measurement to measurement. The sample carrier is
desig-ned to receive only small volumes of substance to
be an~lysed (< 10~1) and is to be produced only from
vitreous carbon or pyrocarbon. Vitreous carbon as well
as solid pyrolytic carbon can be used as materials for
sample carriers to only a limited degree, because the
analytical determination of refractory-carbide-forming
GF-AA,S substances for analysis from surfaces of this
type is not possible, the required material purities
can be realised only with difficulty and the cost-
performance relationship is unfavourable for the user.
Transversely heated atomising furnaces have been
known since 1987 (DE-GM 87 14 670). EP 0 321 879 A2
describes an atomising furnace having a sample carrier
in a longitudinally heated embodiment and in a
transversely heated embodiment, which sample carrier is
conne~ted to the inside wall of the furnace in a non
detachable manner by way of a web which lies
symmetrically with respect to the centre of the
furna~e. Sample carrier and furnace form a material
structural unit, which is produced from one crude body.
The sample-holder portion extends only over a central
region of the furnace portion. Consequently, there is
only ~ small receiving volume for the substance to be
analysed. The connecting web itself has a plurality of
transverse bores as a material-reducing measure. An
atomising furnace of this type, consisting of a solid
graphite blank can be produced only with high technical
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expencliture. This has a negative effect on the price
to the user for this wearing portion.
';ample carriers having supporting rings for
transversely heated atomising furnaces in accordance
with DE 42 43 767 C2 can likewise be produced only
cost-i.ntensively and with high technical expense,
although both sample carrier and furnace can each be
produced as a component part.
l'he underlying ob-ject of the invention has been to
so construct sample carriers and adapt them to the
condit:ions in the atomising furnaces surrounding them
that t:he above-mentioned technical and analysis
defici.encies of the known prior art no longer occur
with t:hem in practice. In particular, the sample
carrier is to be constructed and accommodated in the
tube f.urnace in such a way that during the analysis,
there are obtained with this arrangement measured
signa]s which are formed more sharply in comparison
with t:he prior art, an(1 a fast decay of these signals
to the noise level of the measuring arrangement takes
place, i.e. so that more precise analysis results are
achieved than hitherto and a plurality of such precise
analysing processes can be carried out one after the
other.
A further object was to develop in connection with
the above-mentioned features of the object a
combination of tube fu:rnace - sample carrier made of a
material which permits determinations of the content of
all e]ements which can typically be analysed by GF-AAS,
namely 59, to be carried out.
l'he object is ach:ieved in accordance with the
invention by the featu:res of claim 1.
Preferred developments of the solution in
accordance with the invention are the subject matter of
the dependent claims.
The text of the claims is herewith incorporated
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into lhe description.
The object is met by the following technical
featu-res:
'rhe furnace body and the sample carrier consist of
elect-rographite with the same or similar physical and
chemical properties. ~hey are each produced separately
and only then joined together. After the joining
together, the gas-accessible surfaces of the
combination sample carrier - furnace body are coated
with a pyrocarbon layer. As a result of this, the
porous surfaces of the electrographite are sealed in a
fluid-tight manner. Not until then is the arrangement
ready for use. The sarnple carrier is constructed as a
shell and has on its underside a peg, which is arranged
centrally with regard to the longitudinal extent and
the transverse extent of said sample carrier and faces
the lower portion of the inside wall of the furnace.
This peg is inserted into a depression, which
complements the shape of the peg and is located in the
centre of the longitudinal extent of the inside of the
furnace portion, in the furnace wall approximately
oppos:ite the sample insertion opening. As a result of
the shape of the peg, which is preferably not round,
and the depression in the inside wall of the furnace
that :is complementary thereto, the sample carrier is
fixed in the furnace in a form-locking manner, and as a
resull of the pyrocarbon coating is additionally fixed
in a rnaterial-locking, well-defined and reproducible
manne-r. The sample carrier is of minimal mass and its
trough-like or shell-like portion preferably extends
over as large as possible a portion of the inner
furnace chamber available to it. Where it suffices for
achieving the working tasks set, the sample carrier can
even have a smaller longitudinal extent. With
longiLudinally heated furnaces, the sample carrier
prefe:rably extends over a region of 50 to 85~ of the
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longitudinal extent of the inner furnace chamber. In
transversely heated atomising furnaces, the length of
the sample carrier preferably amounts to 75~ of the
length of the inner furnace chamber and more, and
parti(~ularly preferably to at least 80~. The walls of
the shell-like portion of the sample carrier preferably
have a wall thickness of less than 0.5 mm, particularly
prefe:rably of less than 0.3 mm. The trough-like or
shell-like portion of the sample carrier is able to
receive up to 50 ~l of solution to be analysed in the
case of transversely heated furnaces, and up to 40 ~l
of solution to be analysed in the case of
longitudinally heated furnaces. All parts are
const:ructed in such a way that their production
requi:res as little expenditure as possible.
The body of the sample carrier is essentially
const:ructed from the two function-determining portions
peg and sample shell and has a minimum mass, typically,
and unlike known solutions, less than 100 mg. The
special connection of the sample carrier to the tube
furnace by way of a centrally arranged peg, the mass of
which has been minimised, in combination with the small
mass of the sample shell, means a considerable
reduc_ion in heat conduction. Electrical heating by
Joulean heat is anyway excluded in the case of this
arrangement. As a result of this, after the desired
time-delayed heating of the inside wall of the furnace,
a sample to be analysed that is located in the sample
shell is heated to atomisation temperature extremely
quickly by radiant heat alone. As the absence of
memor-y effects working with the arrangement in
accordance with the invention shows (see in this
respe~t Figure 9), the substance to be analysed that is
put in is completely vaporised, and after the
measurement process is also removed completely from the
atomisation zone of the tube furnace.
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The shell-like portion of the sample carrier,
which is preferably designed to receive volumes of
substance to be analysed of up to 50 ~l, preferably has
along its shell base an additional groove having
preferably perpendicular walls. This groove serves as
an additional obstacle to prevent solutions to be
analysed from running off.
The arrangement, in accordance with the invention,
of sa-mple carrier and tube furnace can be used both for
transversely heated and for longitudinally heated
atomising furnaces, and for working with both liquid
and solid substances to be analysed without structural
alterations to the sample carrier.
As a result of the unalterable fixing of the
sample carrier in the tube furnace, which fixing is
already carried out by the manufacturer, there are
considerable advantages during handling and during
working with the analysing arrangement in accordance
with the invention, because, for example, damage to or
incorrect alignments of the sensitive sample carrier
are ruled out. When working analytically with the
arrangement in accordance with the invention,
practically-no more memory effects are established. It
is consequently possikle to carry out a large number of
analysis procedures one after the other. This results
in cost advantages for the user.
By reducing the contact surfaces between the
furnace and the sample carrier to a minimum which is
technically only just controllable, a considerable
improvement in comparison with the known analysis
arrangements (DE-PS 25~ 24 123; DE-GM 87 14 926.5; DE-OS
37 22 379; DE-GM 88 03 144.6; DE-OS 38 23 346.0; EP 0
442 009 A1) with their comparatively large contact
surfaces has been achieved.
The type of fastening, in accordance with the
invention, of the sample carrier in the tube furnace
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addit.ionally ensures a maximum time delay in heating
the sample carrier in direct comparison to the heating
of the inside wall of the atomising furnace.
The arrangement in accordance with the invention
permi_s the charging the atomisation of a very large
amoun'_ of substance to be analysed of up to 50 ~l, in
connection with a maximum heating rate of equal to or
greater than 2000 K/s. In this connection, the heating
occurs after a desired delay with respect to the
heating of the inside wall of the furnace.
]3y using polycrystalline electrographite of
unifo:rm technical quality in order to form the furnace
and sample carrier, and as a result of the uniform
pyrolytic coating which takes place after mechanical
fixing, it is possible to analyse all 59 of the
elements of the periodic system that can be analysed
with (JF-AAS. Only with the analysis of refractory
elements such as V, Ti, Si, for example, do small
memory effects occur, which can be controlled by known
measu:res.
The analysis arrangement in accordance with the
invention effects with analysis operations a good long-
ter~ stability of sensitivity and reproducibility
(relative standard deviation (RSD) less than 2~ for
diluted acidic standard solutions) and an extended
linea:r concentration working region with regard to the
time-integrated extinction (surface integral of the
signal variation) necessitated by the method.
:By using electrographite as basic material for the
produ_tion of sample carrier and tube furnace and the
construction of the portions in a manner suited to
ratio:nal production, expenditure on production that is
lower than that of the prior art is achieved. From
this results a further cost advantage for the user.
The combination cf tube furnace portion and sample
carrier is constructed. in such a way that the sample
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carrier is arranged inside the tubular portion so as to
lie substantially outside the optical beam path and
have only one place of attachment, which is located on
the common central axis of the two portions.
In this way, geometrical symmetry of the two
components with respect to each other is maximized for
transversely heated and for longitudinally heated tube
furnaces and current conduction through the sample
carrier is completely avoided. By means of the hollow-
shell-like construction of the sample carrier over the
greater part of the wh~le length of the tube furnace,
the introduction and reliable protection of a maximum
volume of substance to be analysed is rendered
possible.
The peg serving to fix the sample carrier in the
tube Eurnace portion advantageously has a non circular
cross-section in order to position the sample carrier
in a complementary depression in the tube furnace in a
manne:r such that it is protected against torsion.
Apart from this, the peg is constructed so as to have
at least two steps and only the portion thereof that
faces the inside wall of the furnace is located in the
depression in the insi,~e wall of the furnace. The
broader portion of the peg rests on the inside wall of
the furnace and holds the shell-like portion of the
sample carrier at a distance from the inside wall of
the furnace. The insi,~e of the peg can have, starting
from :its lower face, a hollow space, preferably in the
form of a circular or oval countersinking. The size of
the hollow space and consequently the effectively
active cross-sectional area of the peg permit an
adjuslment of the heat conduction with regard to an
optimal delay with respect to time and a minimisation
of the total mass of the sample carrier. The place of
attachment for this connecting web in the case of
longiludinally and transversely heated furnaces is also
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-10 -
at the same time in what is relatively speaking the
coldest region of the inside wall of the atomising
furnace during the heating process, as published tests
by Falk and colleagues ("Spectrochimica Acta", Vol.
40B, pp 533 to 542, 1985) for longitudinally heated
furnaces and our own measurements for transversely
heated furnaces (see Figure 4) cover.
The chosen arrangement principle consequently
ensures as well that the desired time-delayed heating
of the sample takes place almost exclusively by
radiation energy, which is radiated only from the
inside wall of the respective tubular atomising furnace
portion.
The invention is explained further in the
following by way of example, with the aid of the
following drawings, in which:
:Figure la shows a longitudinal section through an
atomising furnace in accordance with the invention;
:Figure lb shows a cross-section through an
atomising furnace in accordance with Figure la, along
the sectional plane A-A;
:Figure 2 shows an opened-up representation of a
trans-versely heated atomising furnace in accordance
with the invention;
:Figure 3 shows a plan view of a sample carrier in
accordance with the invention;
:Figure 4 shows photographs and graphs of the
tempe:rature-time curve when heating a transversely
heated atomising furnace in accordance with the
invention;
:Figure 5a shows a longitudinal section through an
atomising furnace for longitudinal heating or for
transverse heating, with an additional longitudinal
groov~ on the base of the sample carrier;
Figure 5b shows a cross-section through an
atomising furnace in accordance with Figure 5a for the
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longitudinally heated embodiment;
Figure Sc shows a cross-section through an
atomising furnace in accordance with Figure 5a for the
trans-versely heated embodiment;
Figures 6a and 6b show three-dimensional
representations of the sample carrier of an arrangement
according to Figure 5a in oblique views from above and
from :oelow;
Figures 7a and 7b show three-dimensional, cut open
representations of atomising furnaces in accordance
with Figure 5b, with views onto the shell-like platform
of the sample carrier (Figure 7a) and onto the peg and
the base of the sample carrier (Figure 7b);
Figures 8a and 8b show the representations
corresponding to Figures 7a and 7b, but for
transversely heated atomising furnaces;
Figure 9 shows measurement graphs of test
analyses, which were obtained with various types of
sample carriers in atomising furnaces.
Figure la shows a longitudinal section through a
tubular atomising furnace 1 consisting of
electrographite coated with pyrocarbon. Located in the
tube furnace portion 17 is a sample carrier 2, which is
mounted in a recess in the tube furnace portion 17,
opposite a sample insertion opening 3 in the tube
furnace portion 17, by means of a supporting foot or
peg 4. Like the tube furnace portion 17, the sample
carrier 2 consists of electrographite and, after it was
placed into the tube furnace portion 17, was coated,
together with the latter, with pyrocarbon.
The sample carrier 2 has, in its platform 16, a
shell-like recess 5 for receiving a sample. The ends
10 of the recess 5 are not so deeply worked, so that
edges result which form run-off obstacles for the
sample liquid. The peg 4 is constructed in a stepped
manner so that an intermediate stage 6 ensures that the
CA 0223l~48 l998-03-lO
-12-
required constant distance from the inside wall of the
tube Eurnace portion 17 is guaranteed.
Figure lb shows a cross-section through the
atomi,,ing furnace 1 shown in Figure la, along a section
line A-A, with contact pieces 7 and 8 for transverse
heating being shown to some extent. It can be seen
here l_hat, with the exception of the recess 5, the
sample carrier 2 has straight side faces 9, which are
techn:ically easy to produce.
Figure 2 shows an opened-up representation of a
complete transversely heated atomising furnace 1 with
the sample carrier 2.
Figure 3 shows a three-dimensional representation
of an embodiment of the sample carrier 2 of Figure 2.
The peg or supporting foot 4 has a cross-section
which deviates from the circular, in order to avoid
mutua:L torsion when mounting the sample carrier 2 in
the tube furnace portion 17.
Figure 4 shows the temperature distribution T (t)
of a lransversely heated atomising furnace in the
embod:iment in accordance with the invention as a
funct:ion of the time (t) during a fast heating process
to a I?redetermined atomisation temperature in the
stages tl, t2 and t3. It can be seen that the central
zone of the tube furnace is advantageously last to be
heated to the desired final temperature.
Figure 5a shows a longitudinal section through a
tube :Eurnace portion 17 - sample carrier 2 -
arrangement for longitudinally and transversely heated
furnaces, with a further embodiment of a sample carrier
2 in accordance with the invention. In order that the
sample to be analysed is received in a secure manner,
there is located in the base of the shell-like sample
carrier 2 an additional groove 11 which extends over
the g:reater part of the length of the sample carrier 2,
is preferably sunk in and has substantially
- CA 02231~48 1998-03-10
perpendicular side walls 12. The base of the groove 11
is preferably formed so as to be level for reasons of
ease of production. The peg 4 of the sample carrier 2
has a recess 13, which is advantageously bored or sunk
in and preferably extends in the axial direction 18, in
order to reduce its thermal conduction further and to
minimise the mass of the sample carrier 2. It is
particularly advantageous that all of the wall
thicknesses 14 do not exceed a dimension of 0.5 mm, as
a result of which the total mass of the sample carrier
2 is kept very small.
Figure 5b reproduces a cross-section through the
centre of the furnace arrangement according to Figure
5a for the case of a longitudinally heated furnace,
while Figure 5c is a corresponding cross-sectional
representation for a transversely heated atomising
furna,-e.
Figures 6a and 6b each show a three-dimensional
representation of the sample carrier 2 of Figures 5a to
5c in an oblique view from above and an oblique view
from below.
Figures 7a and 7b show the sample carrier
embodiments 2 in accordance with Figures 5a and 5b as
well as Figures 6a, b, in a longitudinally heated tube
furnace portion 17 as a three-dimensional, cut open
repre,entation.
:Figures 8a and 8b show partially cut open, three-
dimensional representations of a transversely heated
atomising furnace 1 with sample carriers 2 in
accordance with Figures 5a, 5c, 6a and 6b.
Figure 9 shows absorption signals obtained by
means of a furnace arrangement in accordance with the
invention in comparison with absorption signals which
were obtained with furnace arrangements having sample
carriers according to the prior art. The measurements
were carried out with a test solution containing 0.1
CA 0223l~48 l998-03-lO
-14-
,ul/ml vanadium in 0 .5% HNO (sic). Different absorption
signals or absorption curves, as well as the
temperature progress in the furnace chamber interior,
are shown over the time axis for sample carriers of
vario-us embodiments in longitudinally heated atomising
furna_es.
~ urve 1 resulted from the use of an atomising
furnace in accordance with the invention.
_urve 2 was obtained in the case of a measurement
using a sample carrier of the "fork platform" type made
of solid pyrographite material, in accordance with EP 0
442 009 A.
Curve 3 resulted from the use of a sample carrier
in ac-cordance with DD 233 190 A (DE-OS 35 45 635), i.e.
using a sample carrier made of glassy carbon, which is
detachably held in a bore in the wall of the tube
furnase by means of a peg located on its underside.
_urve 4 resulted from the use of a sample carrier
of the "fork platform" type, which was coated with
pyrocarbon.
All curves admittedly have the desired temperature
delay with respect to time, often also called "platform
effect", in comparison with the heating of the inside
wall of the furnace, to the extent that the atomisation
signals ("peaks") do not result until after the final
temperature state has been reached, but they differ
clearly in the construction and decay performance of
their signals.
Curve 1 has clearly visible the most sensitive
signal and decays as desired to the zero line again
within the measuring period of 10 seconds. There are
therefore no residues remaining in the furnace. The
ratio of signal level to noise level is very high and
consequently extraordinarily favourable. As a result
of this, a high reproducibility of the measurements is
ensured.
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Curves 2 and 3 show that the atomisation signal
does not decay in practice. Large amounts of the
substance to be analysed remain in the analysis
arrangement comprising sample carrier and tube furnace,
which amounts are gradllally vaporised and atomised only
after the time availabLe for the analysis. Both the
struct:ural form of the sample carrier and also the
material from which the analysis arrangement is made
are responsible for such a performance. Measurement
result:s of this type cannot be evaluated for analysis
purposes, because the analysing process lasts for too
long and the result of the subsequent measurement cycle
is fa]sified by residues of substance to be analysed
that have not been completely vaporised ("memory
effect:").
C'urve 4 was obtained with an analysis arrangement
in which both the sample carrier and the atomising
furnace were coated with pyrocarbon. Nevertheless, the
atomisation signal which is obtained is much smaller
than in Figure 1 and does not decay completely. The
reason for this is that the sample carrier is mounted
at several points in the furnace and consequently
experiences heating which-does not come only from the
radiat:ion of the inside wall of the furnace. It is
namely also heated by undesired electrical transverse
heating and increased heat conduction from the inside
wall of the furnace. rrhis equally has a damping effect
both on signal level and signal area. The signal does
not decay completely. Here, as in the case of curves 2
and 3, it can be recognised that not all of the atoms
of the substance to be analysed that were inserted into
the furnace are completely released in an atomisation
cycle and deliver a signal contribution. Therefore, in
this case as well, quantitative determinations of
elements forming residues, particularly of refractory
elements, are not poss:ible with sufficient accuracy.
CA 0223l548 l998-03-lO
-16-
The measurement graphs of the curves 1 to 4 of
Figure 9 show in an impressive way the technical
progress achieved by the solution in accordance with
the invention.
