Heating device for an atomic absorption spectrometer

In a heating device for atomic absorption spectrometers with electrothermal excitation of a sample, a tubular body for receiving the sample comprises a radial aperture (15) for introducing the sample at the central part of the tubular body. A pair of contacts (1, 2) can be cooled comprise current supplies (3), each consisting of a detachable upper part (5) and a fixed lower part (6) with the upper part and the lower part being connected together so as to be detachable. Each contact comprises an aperture (9) which is coaxial with the tubular body with the apertures extending partly in the upper part and partly in the lower part of the contacts and holding the ends of the tubular body. Damage and deformations of the contacts are avoided in that the diameters of the apertures (9), with the upper parts (5) laid flush on the lower parts (6), are shorter in the vertical direction than in the horizontal direction. The upper parts (5) of the apertures (9) each have a groove (12) which points upwards and extends parallel to the longitudinal axis of the apertures (9). On each end of the tubular bodies (11) a resilient clamping ring (10) is slid whose inside diameter is smaller than the outside diameter of the tubular body (11) and whose outside diameter corresponds to the apertures of the contacts (1, 2). The clamping ring comprises a vertical slot (13) at the area of the lower part (6). Alternatively instead of the clamping rings the ends of the tubular body (11) optionally comprise flanges (14) at its end and are surrounded by graphite foil.

The invention relates to a heating device for an atomic absorption 
spectrometer with electrothermal excitation of a sample, comprising a 
tubular body for receiving the sample which can be heated by means of 
current passage and which body comprises in its central portion a radial 
aperture for introducing the sample, and a pair of contacts which can be 
cooled comprise current supplies with each pair consisting of a detachable 
upper part and a rigidly connected lower part and with the upper part and 
the lower part being connected together so as to be detachable, each 
contact comprising an aperture which is coaxial with the tubular body with 
the apertures extending partly in the upper part and partly in the lower 
part of the contacts and enclosing ends of the tubular body. 
A heating device having contacts each consisting of an upper part and a 
lower part is shown in FIG. 1 of DE Pat. No. 2,735,467. Details of the 
construction of such a heating device, but with undivided contacts, are 
known from DE Pat. No. 2,413,782. It is known from U.S. Pat. No. 3,893,769 
to provide a conical clamping ring on the ends of the tubular body and to 
arrange the clamping rings with the ends of the tubular body in the 
apertures of undivided contacts. 
In atomic absorption spectrometry (AAS) the sample to be analysed traverses 
a certain temperature-time cycle in the course of which the material of 
the sample is fully decomposed by thermal dissociation and the components 
are transferred into the atomic vapour phase. In this step, which is 
referred to as atomisation, the element to be determined can be detected 
selectively by means of spectrochemical methods and quantitatively with a 
very high accuracy. In accordance with the nature of the sample the 
atomisation step requires temperatures of up to 3000.degree. C. 
Particularly suitable for this sample preparation are thin-walled, i.e. 
approximately 1 mm thick, preferably tubular bodies which can be heated by 
means of current passage, usually of graphite, so-called AAS cuvettes. 
They fulfil a double function in that they serve simultaneously as a 
container for the sample which in most of the cases is a few microliters 
large, and as a resistance element and furnace, respectively, for the 
electric heating. In accordance with the size and the operating system, 
electric powers of up to a few kilowatt are required for the heating. The 
heating currents may reach a few 100 Amperes. The electrical contacts in 
the heating devices of atomic absorption spectrometers are hence exposed 
to particularly high loads. This applies even more so since, in addition 
to high current densities due to the cyclic heating and cooling, during 
operation strong heating currents also have to be overcome and, due to the 
interchangeability of the cuvettes, the contacts must be detachable and of 
the simplest form. The contacts in the heating devices therefore represent 
real vulnerable points. Defects and damage of the contact materials, which 
consist, for example, of nickel-plated copper, in the form of permanent 
plastic deformations at the clamping points of the cuvettes occur in 
particular. This damage does not only adversely influence the progress of 
the analysis, it also leads to expensive maintenance and repair work, for 
example, by replacing parts of the contacts. 
It is the object of the invention to avoid such damage and deformations of 
the contacts and hence to make the heating device more reliable. 
According to the invention this object is achieved in that the diameters of 
the apertures in the contacts, with the upper parts laid flush on the 
lower parts, are shorter in the vertical direction than in the horizontal 
direction, that each of the upper parts comprises above the apertures a 
groove which points upwards and extends parallel to the longitudinal axis 
of the apertures, and that on the ends of the tubular body a resilient 
clamping ring is slid having an inside diameter smaller than the outside 
diameter of the tubular body and having an outside diameter corresponding 
to the apertures in the contacts and which at the area of the lower part 
comprises a vertical slot, or that instead of the clamping rings the ends 
of the tubular body optionally comprise flanges at its ends being 
surrounded by graphite foil, or alternatively the inner surfaces of the 
apertures are lined with graphite foil. 
Since, with the upper part lying flush on the lower part, the vertical 
diameter of the aperture formed, i.e. the receiving aperture for the 
cuvette, is slightly smaller than the horizontal diameter, the aperture 
has a shape, i.e. a geometry, which deviates slightly from the circular 
shape. When, after detaching the upper part from the lower part, a 
circular contacting or clamping ring slid on the ends of the tubular body, 
i.e. of the cuvette, is laid in the aperture and when the two contact 
parts are closed, for example by screwing, the upper and lower parts press 
against the clamping ring and this will generally be a point contact. 
The upwardly pointing grooves in the center of the upper parts enable an 
elastic deformation upon contacting in the sense of a "static fulcrum". 
This results in a groove between the upper and lower parts which is 
slightly conical, i.e. widens in the direction towards the aperture. 
The slot of the clamping or contacting ring on the cuvette is adjusted 
vertically and is present in the area of the lower part of the contacts. 
The result of this arrangement of grooves and slots in cooperation with 
the forces provided upon contacting is a tangential grip of clamping ring 
on the one hand and AAS cuvette on the other hand. The result of this is 
that comparatively poorly transmitting point contacts change into readily 
transmitting annular, linear contacts. 
For a perfect operation of the contacting system according to the invention 
it is advantageous to keep the tolerances in the dimensions, in particular 
those of the outside diameters of the clamping rings and the inside 
measures of the receiving apertures in the contact blocks, very small, 
i.e. the two above values should correspond, for example, to the fits 
h6/N6 (according to the ISA-standard). 
In the above-described alternative embodiment such a high accuracy of fit 
may be omitted. For that purpose it is necessary to line the inner 
surfaces of the receiving apertures with graphite foil. 
The diameters of the apertures in the contacts in the vertical direction 
are preferably 0.4 to 0.6% shorter than in the horizontal direction. 
Accordingly the inside diameters of the clamping rings preferably are 0.4 
to 0.5% smaller than the outside diameter of the tubular body. 
An embodiment of the invention is shown in the drawing and will be 
described in detail hereinafter. In the drawing 
FIG. 1 is a perspective view of a heating device and 
FIGS. 2 and 3 are perspective views of two embodiments of cuvettes for a 
heating device.

The heating device (FIG. 1) is characterized by several structural and 
material-bound features which are essential for perfect operation. The 
main components are two solid blocks 1 and 2 preferably of electrically 
readily conductive materials, for example of copper, hereinafter also 
termed contact blocks, which are provided with current supplies 3 and are 
connected to cooling devices, for example, air cooling or water cooling 
(not shown). However, other suitable materials for the contact blocks are 
stainless steel, high-melting-point metals, and graphite. The contact 
blocks may be coated with corrosion-resistant and surface-finishing 
protective layers for example of chromium, chromium-nickel, nickel, gold 
or platinum, and/or with electrically readily conductive, wear-resistant, 
2 to 6 .mu.m thick layers of titanium carbide, titanium nitride, 
carbonitrides or similar materials. The latter layers may be provided at 
temperatures of approximately 800.degree. C. by means of CVD methods, for 
example on copper and steel. The contact blocks are mounted either rigidly 
or movably (for the adjustment of variable distances) on an insulating 
base plate 4, for example of glass ceramic. 
Each contact block consists of a detachable upper part 5 and a rigidly 
fixed lower part 6. The upper part and the lower part are connected 
together so as to be detachable, in the present example by simple 
screw-connections 7. Other possibilities of providing contact-clamping 
forces--instead of the screw connections used in the present example--are, 
for example, those which operate with spring systems, mass forces, 
pneumatic and/or hydraulic systems. The upper part and lower part contact 
each other without inserted cuvette "sealingly" in a common contact face 
between upper and lower part. With inserted cuvette on the contrary the 
contact face is reduced to a contact edge at 8. Both parts of each of the 
two contact blocks are provided with a substantially semicircular aperture 
9 of the same radius. 
An essential feature of the invention is that the diameter of the aperture 
9, with flush upper parts, is slightly shorter in the vertical direction 
than in the horizontal direction. This difference is produced in that the 
aperture which has been drilled circularly symmetrically, with the center 
in the center of the interface of upper and lower parts, is provided with 
the interposition of a thin spacer (for example a foil). After removing 
the spacing member, for example with the thickness .delta.=50 .mu.m, the 
difference in diameter .DELTA.D upon combining upper and lower parts is 
obtained according to the equation 
EQU .DELTA.D=D.sub.2 -D.sub.1 =.delta.(for D.sub.2 +D.sub.1), 
wherein D.sub.1 is the diameter in the vertical direction, for example 
9.950 mm, and D.sub.2 is the diameter in the horizontal direction, for 
example 10.000 mm. 
This difference in diameter, i.e. outside diameter of the clamping ring 10 
minus inside diameter of the apertures 9 of the contact blocks, must 
correspond approximately to the tolerances h6/N6 usual in the art. This 
"uncircularity" of the receiving apertures for the cuvettes 11 enables the 
close contact of two concave surfaces by means of the grooves 12 provided 
in the upper parts 5, i.e. the lowest possible hertzian surface pressure, 
i.e. the lowest possible tensile stresses in materials sensitive to 
tensile stresses, for example, graphite. This results in a uniform contact 
pressure which is distributed over the surface in three points. This 
effect of the uniform distribution of the contact pressure is achieved by 
the following further characteristic feature: 
Two resilient clamping rings 10 are used which are provided with slots 13 
at one point of the circumference which after mounting is situated in the 
area of the lower part 6. The slots 13 should be provided as vertically as 
possible, so pointing downwards perpendicularly. The inside diameter of 
the clamping rings is slightly smaller than the outside diameter of the 
cuvette tube; their outside diameters correspond to the receiving 
apertures of the contact blocks with tolerances of, for example, h6/N6. 
The clamping rings 10 preferably consist of graphite, for example 
electrographite. Other useful materials for the rings are pyrolytic 
graphite, vitreous carbon and high-melting-point metals, for example, 
molybdenum, tungsten, tantalum or the carbides thereof. When the 
above-given prescriptions are maintained the clamping rings can be slid on 
the tubular body, i.e. the cuvette, with a slight force fit. Such an 
arrangement is shown in FIG. 2. The operation of the clamping rings may be 
described as follows: first they operate in the sense of the already 
mentioned force distribution by tangential slide. They can absorb or 
compensate for small expansions or shrinkages dependent on thermal or 
mechanical stresses. Moreover, the immediate contact of the cuvette which 
is heated at a high temperature and the comparatively cold contact block 
is avoided. By suitable choice of the material--for example pyrolytic 
graphite--and proportioning of the ring, electrical and thermal 
resistances of the clamping rings can be varied in wide limits and hence 
the temperature distribution and the thermal balance on the cuvette can be 
influenced positively. Moreover, the clamping ring construction above all 
also facilitates axial expansions and contractions of approximately 0.05 
to 0.1 mm, as they occur in every analysis cycle as the result of the 
strong temperature change. The coefficient of thermal expansion of 
pyrolytic graphite (a known material for AAS curvettes) in the 
crystallographic ab direction namely is approximately 1.times.10.sup.-6 
/.degree.C. With a cuvette length of 30 mm a longitudinal expansion of 
approximately 30.times.3000.times.10.sup.-6 mm equal to approximately 0.1 
mm is obtained for a maximum temperature of 3000.degree. C. 
A further characteristic detail of the heating device is the groove 12 in 
each of the upper parts 5 of the contact blocks shown in FIG. 1. The 
groove permits a hinge-like deformation of the upper part and only thereby 
permits the above-mentioned close 3-point-contact pressure and an elastic 
and resilient expansion compensation, respectively, and thereby again 
permits an optimum clamping force distribution over the circumference of 
the cuvette. A groove starting at 8 is formed in that the facing surfaces 
of the upper part and the lower part with closed contacts enclose an angle 
of approximately 0.14.degree. with the apex of the angle forming the line 
in which the upper part and the lower part of each contact block contact 
each other on the outside. 
In a modified manner the heating device was also tested on AAS cuvettes 
with flanges 14 (FIG. 3). Instead of the above-described clamping ring a 
piece of graphite foil (not shown; trade name, for example, Sigraflex, 
Papyex, Grafoil) of 0.2 mm thickness, as a contact means between the rigid 
cuvette ends and the contact members, was laid in the contact recesses in 
such manner that in the operating condition they firmly enclose the 
cuvette ends and cuvette flanges, respectively, under the action of the 
compression pressure of the contacts. The thickness of the foil may vary 
between 0.1 and 0.5 mm. 
The known foil is a product which is manufactured by cold rolling from 
pretreated natural graphite. Since it relates to pure graphite without 
additions, the product is free from impurities. Similar to highly oriented 
pyrolytic graphite, its particularity is an excellent anisotropy of its 
physical properties which is very favourable in the present application. 
Moreover said material is plastically deformable. In this respect it 
behaves as a foil of a ductile metal, for example, lead. The result of 
this is that in the play of forces the cuvette ends during operation are 
truly impressed in the foil material and as regards their position thus 
fix themselves. This is of great advantage for the usually automated 
injection of the sample solution through the filling holes of the 
cuvettes. 
Exchanging cuvettes is carried out as follows: after removing the upper 
parts of the two contact blocks--this is done by fully unscrewing the four 
screws 7--the used cuvette is taken out of its seating in the lower part 
of the contact block and a fresh cuvette is placed in the semicircular 
apertures of the lower parts in such manner that the annular slots 13 each 
point downwards and the filling aperture 15 for the analysis sample 
obtains the correct position for the usually automated filling operation. 
This necessary adjustment can always be achieved in the cuvette system 
described which consists of a cuvette tube 11 and clamped contact rings 
(with or without a slot) since it is exactly the clamping seat which 
enables a corresponding matching, albeit by turning about or sliding along 
the cuvette axis (and cuvette surface, respectively). 
It is to be noted that the screw connection mentioned in the example of the 
upper part and the lower part of the contact blocks was chosen only for an 
experimental construction. For an AAS apparatus which can readily be used 
in practice a different closing mechanism in which, as already mentioned, 
resilient forces, mass forces, pneumatically or hydraulically moved 
contacts or the like are used, is to be preferred since the latter permits 
a very much more protected and certainly also more rapid exchange of the 
cuvettes. 
Various shapes of AAS cuvettes were tested upon testing the device 
according to the invention. The electrical energy required for the heating 
EQU E.sub.e1 =N.multidot.t.sub.E 
wherein 
N=power in Watt 
t.sub.E =switch-on period per pulse 
was adjusted--for comparison--in all cases at approximately 7.0 to 7.2 kWs. 
The tests were carried out in pulse operation with t.sub.E =8 s 
switching-on time (=heating phase) and t.sub.A =120 s off-time (=cooling 
phase) over each time differently long pulse sequence times. The voltages 
at the cuvettes were approximately 3.5 to 5.5 Volts, the corresponding 
electric currents were 250 to 160 A. From this it results, in agreement 
with the above-mentioned equation, that 
EQU N.t.sub.E =3.5 V.250 A.8 s=7,000 W.s or 7.0 kW.s 
EQU N.t.sub.E =5.5 V.160 A.8 s=7,040 W.s or 7.04 kWs 
The cuvette was always in a vacuum (recipient) at a pressure of 1.33 to 
2.66.times.10.sup.-5 mBar. Test durations (pulse repetition times) of 10 
hours (approximately 280 load pulses) to 200 hours (approximately 5600 
load pulses) were carried out. The highest temperatures measured and 
corrected by means of optical pyrometers were approximately 2300.degree. 
C. in the inner space of a cuvette (center), approximately 2000.degree. C. 
on a cuvette surface (center), approximately 1870.degree. C. on the inner 
surface of a cuvette surrounded by a contact (cuvette end, inside), and 
approximately 300.degree. C. in the expansion groove of the contact block. 
The expression "corrected" means that the measured temperatures were 
converted to the real temperatures. The influence of the losses resulting 
from reflection at and absorption in the glass recipient as well as the 
emission coefficient of the cuvette were taken into account. 
No corrosion or mechanical damage or deformation was observable at the 
contact faces of the copper blocks used. No disturbance occurred. All test 
were ended at will. 
The self-fixing, i.e. the adjustment of the cuvettes in a defined position, 
operated perfectly when the graphite foil method was used. When using 
foils in immediate contact with the metal parts of the device, these 
parts, after long-term tests, showed a certain "plating-in" of graphite 
which apparently has a favourable effect as regards corrosion-resistance. 
An evaluated calculation of the energy balance in the system described 
leads to the result that, in accordance with the type of cuvette and 
arrangement, the delivery of the supplied energy occurs up to 70 to 75% by 
radiation and up to 25 to 30% by thermal conductivity. In the tests a fan 
cooling of the solid copper lead-throughs (cross-section approximately 100 
mm.sup.2) outside the recipient was sufficient to produce a sufficiently 
low temperature of the metal contacts.