US 3819279 A
A known atomizing device, especially useful for atomizing the sample in a flameless atomic absorption spectrometer, includes a hollow body accommodating the sample which is heated electrically to a high temperature; this body is typically a graphite tubular sample cell which is heated up by applying a large current to its opposite ends. It is proposed to surround this body by at least one radiation absorbing (and therefore re-emitting) protective jacket, for example, a tubular member also made of graphite. This protective jacket reduces the amount of electric energy necessary to heat the body to a particular temperature since a substantial part of the radiation emitted from the body is absorbed and re-emitted back to the body by the protective jacket. In an alternative construction the protective jacket comprises a relatively deep layer of a porous material such as porous coal, so that the non-conducting pores cause the jacket to act substantially like a plurality of separate portions of concentric jackets.
Description (OCR text may contain errors)
United States Patent a 8 Braun et al.
[111 3,819,279 [4 June 25, 1974  Assignee: Bodenseewerk Perkin-Elmer & Co.,
GmbH, Bodensee, Germany  Filed: Apr. 20, 1973  Appl. No.: 353,088
 Foreign Application Priority Data.
Apr.2l,l972 Germany 2219594 52 US. Cl. 356/244, 356/85"  Int. Cl G0ln 21/16, GOlj 3/30  Field of Search 356/85, 87, 244
 References Cited UNITED STATES PATENTS 6/1972 Wiedeking 356/244 11/1972 Braun et al. 356/244 Primary ExaminerR0nald L. Wibert Assistant Examiner-V. P. McGraw Attorney, Agent, or Firm-Daniel R. Levinson [5 7] ABSTRACT A known atomizing device, especially useful for atomizing the sample in a flameless atomic absorption spectrometer, includes a hollow body accommodating the sample which is heated electrically to a high temperature; this body is typically a graphite tubular sample cell which is heated up by applying a large current to its opposite ends. It is proposed to surround this body by at least one radiation absorbing (and therefore reemitting) protective jacket, for example, a tubular member also made of graphite. This protective jacket reduces the amount of electric energy necessary to heat the body to a particular temperature since a substantial part of the radiation emitted from the body is absorbed and re-emitted back to the body by the protective jacket. In an alternative construction the protective jacket comprises a relatively deep layer of a porous material such as porous coal, so that the nonconducting pores cause the jacket to act substantially like a plurality of separate portions of concentric jack- MS.
7 Claims, 2 Drawing Figures is maintained.
, 1 SAMPLE ATOMIZING DEVICE HAVING A RADIATION ABSORBING PROTECTIVE JACKET FOR FLAMELESS ATOMIC ABSORPTION SPECTROSCOPY This invention relates to a device for atomizing a sample for flameless atomic absorption spectroscopy, in which a body accommodating the sample being atomized'can be heated electrically to a high temperature. Such a device is, for instance, already known in the form of a graphite tube which is heated to glow temperature by passage of electric current, a shortcoming being the high radiation losses particularly at high atomizing temperatures which require a correspondingly high electric power input. Especially at high temperatures, the radiation losses increase very quickly with increasing temperature so that with a considerable increase in the power only a relatively small increase in temperature can be achieved.
Moreover, it is already prior art to reflect at least partly the radiated power back again onto the graphite tube. For this purpose, the chamber containing the graphite tube is designed to be specularly reflecting (i.e. metallic) or diffusely reflecting. The reflection, however, is not perfect. As compared with blackened chambers, at the most a temperature increase by about 100 C can be achieved by this reflecting technique at high atomizing temperatures. Besides, the chamber very quickly blackens during use, as the highly heated graphite tube emits graphite particles which deposit on the chamber walls.
It is an object of this invention to improve the relationship betweenv the achieved temperature of the atomizing device and power spent to reach this temperature.
According to the invention this object is attained by providing that the surfaces of the atomizing body turned away from the sample, and therefore away from the resulting atomic cloud, are surrounded by at least one radiation-absorbing protective jacket having low heat conductivity.
For elucidation of the inventive idea it shall be assumed that a glowing graphite tube of the temperature T is disposed in a chamber whose internal walls are blackened and are maintained at the temperature T by cooling water. Then, the radiation losses S are S K 3 o) 1 wherein K is a constant which, inter alia, includes the effect of particular radiating surface of the graphite tube.
Now, let a protective jacket of graphite be slipped over the graphite tube concentrically therewith. The protective jacket absorbs the impinging radiation, is thereby heated itself, emits part of the heat energy in the form of radiation back onto the graphite tube, while emitting another part outwardly towards the chamber walls. The temperature of the protective jacket T, will become balanced. Then, from the graphite tube to the protective jacket a radiation flux of the magnitude Between the protective tube and the chamber walls aradiation flux of the'magnitude is obtained. v
The radiating surface of the protective jacket is greater only marginally than that of the graphite tube, so that without substantial errors K 1 K. When furthermore neglecting the heat conduction through the protective tube, then in a state of equilibrium, both radiation fluxes must be equally great:
wherefrom it can be concluded:
T14 T To 5 When substituting this value in equation (2), then for the radiation of the graphite tube S it is found:
i K 11 0 r wherein the subscript l characterizes the use of a single protective tube. Thus the radiation losses of the graphite tube are reduced to half.
If more than one protective tube are arranged at spaced distances concentrically with respect to each other, then the radiation losses will be reduced even more. Here, too, when neglecting the heat conduction through the protective tube, the radiation fluxes from each one of the protective tubes to the next one must be equal to each other. When numbering the protective tubes from the outside inwardly, thus, when referencing the temperature of the outer protective tube T the next inner one T and so on, then the following applies:
Tn T, T T T T04 7 The radiation flux without protective tube is proportional to T, T of those with n protective tubes proportional to T T Simply stated mathematically, it is found that When-considering the equation (7), it results:
R" o"=( nn) 9 or, if S, represents the radiation losses with n protective tubes and S the radiation losses without any protective tubes:
Thus, by the use of several protective tubes, the radiation losses can be reduced quite substantially. However, it would be unwise to select the nember of protective tubes excessively great, for on the one hand, in this manner also the mass being heated is increased, which has a disadvantageous effect on the rate of temperature rise, while, on the other hand, by heat conduction in the inner protective tubes a certain temperature differ- 3. ence is already maintained which cannot be practically reduced further by additional protective tubes farther outwardly.
The radiation losses of the atomizing device, for instance, of the graphite tube, are reduced in that opposite to the outwardly radiating surface of this atomizing device another absorbing surface is arranged whose temperature is only slightly below the temperature of the atomizing device and which returns part of the absorbed radiation back onto the atomizing device as by emission. The greater the number of protective jackets is, the less is the temperature difference between the atomizing device and the first protective jacket surface, thus the less are the radiation losses.
The device according to the invention is distinguished from the previous attempts of improving the relationship of temperature and electric power, in that the heat which is radiated by the graphite tube is not reflected back onto the graphite tube by the cooled housing. Rather the protective jacket absorbs the radiated heat and becomes itself heated, so that it emits thermal radiation towards the grahpite tube or the like.
Experimentally it has shown that the maximum temperature of a graphite tube can be increased by 300 C at the same electric power by the use of a tubular protective jacket surrounding the graphite tube.
At least one solid protective jacket can surround the body at a spaced distance therefrom. In a further modiflcation, the protective jacket can consist of a porous material such as porous coal. Such a porous protective jacket corresponds in its effect to a plurality of protective jackets.
Heat conduction is strongly reduced by the porous structure. A great part of the heat transmission is effected internally of the pores by thermal radiation. But the heat transfer by radiation is very small since internally of each pore only small temperature differences occur. In order to reduce the heat radiation of the protective jacket outwardly, the protective jacket can have its outside rendered reflecting.
Two illustrative embodiments of this invention will now be described more fully with reference to the accompanying drawing in which:
FIG. 1 is a longitudinal section through a graphite tube cell having a protective jacket of graphite.
FIG. 2 is a longitudinal section through a graphite tube cell having a porous protective jacket.
FIG. 1 illustrates schematically a graphite tube cell in longitudinal section. To the graphite tube 1 the electric current is supplied via contact cones 2 of graphite which are mounted in cooling chambers 3. The left contact cone has an annular recess 4 into which the protective tube 5 is inserted, which is also made of graphite. To allow sample introduction, the protective tube 5 is provided with a lateral bore 6 which is disposed above the corresponding bore 7 of the graphite tube 1.
A modification of this embodiment consists in that the protective tube 5 is not made of graphite, but of a reflecting material, such as metal. Then, the protective tube will blacken on the inside by emitted graphite particles during use, but will remain bright (reflecting) on the outside. Thereby, radiation of the protective tube outwardly will be reduced. The temperature of the protective tube rises, and the power radiated towards the graphite tube increases so that the resultant radiation transmission from the graphite tube to the protective tube is reduced. Here too, the essential inventive idea consists in that the radiation emitted by the graphite tube is not reflected back, but that the protective jacket is heated so strongly by radiation absorption that as much radiation as possible is emitted inwardly and is again absorbed by the graphite atomizing tube.
FIG. 2 illustrates another embodiment. Herein, the protective tube 5 is replaced by a hollow-cylindrical body 10 of porous coal. The body 10 has a lateral opening 11 for sample introduction and for the supply of protective (i.e., inert) gas. To avoid an electric shortcircuit, there is provided an insulating layer 12 on the right side on the outside, and also an annular insulating body 13.
Between the porous coal body 10 and the graphite tube 1 a narrow gas space 14 is provided. If the porous body 10 is made so loose that only a negligibly small electric conduction occurs, then the coal body 10 may also be slipped directly over the graphite tube 1, i.e. in contact with the same. This also applies to the protective jacket 5, if the same consists of an electrically insulating material.
Although the invention has been elucidated relative to two graphite tube cells, however, the measures according to the invention are also applicable to other devices for atomizing a sample or the like.
1. In a device for atomizing a sample for flameless atomic absorption spectroscopy, in which a body accommodating the sample being atomized is heated electrically to a high temperature, the improvement comprising:
the surfaces of the body (1) facing away from the sample, and therefore ultimately the atomic cloud, are surrounded by at least one radiation absorbing protective jacket (5;l0) having low heat conduction.
2. A device as claimed in the claim 1, in which:
at least one solid protective jacket (5) surrounds the body (1) at a spaced distance therefrom.
3. A device as claimed in the claim 1, in which:
said protective jacket (10) consists of a porous material, such as porous coal.
4. A device as claimed in the claim 1, in which:
said protective jacket (5) is designed to be reflecting on the outside.
5. A device as claimed in the claim 1, in which:
said body is a graphite tube (1) which has a contact cone (2) at each end and is mounted with these contact cones (2) in correspondingly conical surfaces of housing portions (3) through which coolant is flowing and through which the heating current is supplied, and which surround the graphite tube (1) like a jacket from both sides,
' and said tubular protective jacket (5) surrounds the graphite tube (1) and is arranged between the same and the housing portions (3).
6. A device as claimed in the claim 5, in which:
said protective jacket is a solid tube (5) which surrounds the graphite tube at a spaced distance therefrom said is mounted in a contact cone (2) on one side.
7. A device as claimed in the claim 5, in which:
said protective jacket is a thick porous tube as compared with the graphite tube (10) and surrounds the graphite tube at a spaced distance therefrom and is mounted in one of the housing portions (2) through which a coolant is flowing, while being insulated relative to the other housing portion by an insulation (12; 13).