FR2563916A1 - SAMPLE HOLDER FOR FLAME-FREE ABSORPTION AND EMISSION SPECTROSCOPY - Google Patents

Abstract

THE SAMPLE HOLDER IS CONSTITUTED BY MICROPOROUS VITREOUS CARBON. IF NECESSARY, IT IS AT LEAST PARTLY COVERED WITH PYROLITIC GRAPHITE.

Description

PHD 84.064 1

  "Sample holder for absorption and emission spectroscopy

atomic flameless. "

  The invention relates to a sample holder for

  flameless atomic absorption and emission spectroscopy.

  In flameless emission and atomic absorption spectroscopy, the heat treatment of the sample to be analyzed is carried out in an "oven" heated electrically and preferably consisting of graphite. Generally, it is in the form of a small uniform resistance wall tube. This furnace serves simultaneously as a tank for receiving the sample and the envelope and as a concentrator of the clouds of atoms produced for the actual measurement process by electrothermal means. Given this multiple function, the oven is quite often indicated in the literature as "atomizing device". For simplicity, in what follows, the term "SAA cuvette" is used. The basic or standard type of the SAA cuvette is the graphite tube cuvette (according to Massmann). In this basic embodiment, the introduction (brought in) of the sample to be analyzed is carried out in the (cold) cuvette; then it begins a temperature-time treatment, thermal preparation. This latter concept can be explained as follows: generally, for flameless atomic absorption and emission spectroscopy, the sample to be analyzed is first dissolved. From this solution, quantities of the order of magnitude of 10 μl are guided by an opening in the cuvette using a pipette device. Then begins - by switching on the electrical energy - the temperature-time cycle which will be briefly indicated below thermal preparation. In a process, the sample is dried, incinerated and pyrolized (that is to say organic components reduced to carbon) and finally atomized, therefore in this state put in a vapor constituted by atoms, whose existence constitutes the condition (or generally the principle) of atomic absorption or emission spectroscopy. The actual analytical determination of an element sought respectively of a kind of atom sought is effected by the fact that a measuring ray, for example a light ray, conterning the radiation

PHD 84.064 -2-

  characteristic of this element is emitted through the cloud of atoms obtained in the manner mentioned above. The measurement of the absorption spectrum at the beam exit allows an extremely precise qualitative and also quantitative determination concerning the kind and the concentration of the element sought in the analytical sample. The above mentioned steps of the process require temperatures - in chronological order of 100 to 200 C respectively 1000 to 1200 C and depending on the substance - for atomization up to 3000 C. The duration of the total process of the preparation is dependent on the system and inter alia the thermally inert mass of the SAA bowl plays an important role (depending on the system, this process can take up to a minute). An analysis and its result are all the more strongly affected by "secondary processes", as the preparation stage takes longer in an analysis cycle. By "secondary process" is meant processes like the preparation of the sample on the inner surface of the cuvette, the infiltration into the wall of the cuvette (by diffusion), the formation of species molecular "intermediates" and their evaporation, atomic evaporation etc, therefore of the processes causing final effect a depletion and a deconcentration of the substance of the sample. Thus, efforts should be made to go through the process of thermal preparation of the sample in as short a time as possible. According to "Atomic Absorption Spectrochemical Analysis" (London 1970), the concentration of atoms in the "atomic gas cloud) is directly proportional to the rate of temperature increase, and a maximum in sensitivity occurs in the case of a duration of atomization of t C 0.1 sec. A characteristic disadvantage of a known system consists in their high thermal inertia having the effect of a depletion and a deconcentration of the substance to

  analyze and a corresponding negative effect on the result.

  Thus, the new designs aim to cause the sample to heat up as suddenly as possible to the atomization temperature. This goal can also be described so that the sample does not undergo the "thermal preparation process" described above only when the atmosphere, therefore the graphite tube (the SAA cuvette) is already at atomization temperature. . This

PHD 84.064 -3-

  requirement can be fulfilled in principle in two ways, in particular by

  technique called "platform" or by the technique called "probe".

  In the case of the "platform" technique, the sample is introduced into a small container, preferably in the form of a basket, which is in turn introduced into the SAA cuvette. This container, the so-called "platform", is essentially characterized by the following three characteristics: it must be made of a material resistant to high temperatures, its mass must be as low as possible and must be made geometrically so that its contact 0 with the cuvette tube has as small an area as possible,

  so is in point or line form if possible.

  After the introduction of the "platform" with the sample into the SAA tube, the latter is heated and reaches the target temperature more or less quickly depending on the embodiment, since the thermoconductive contact with the "platform" is weak, this ci- and, therefore, the sample - is temporarily heated in a slowed down manner, practically only by means of radiation from the wall of the cuvette. Thus, the above-mentioned aim is achieved because the thermal preparation is only finished when the wall of the bowl has

  reaches atomization temperature.

  In the "probe" technique, a kind of probe is used, on the end of which the sample to be analyzed is applied. Then the end of the probe is introduced through an opening in the cuvette tube previously brought to atomization temperature. Thus, said basic requirement that the substance to be analyzed is only heated when the atmosphere has already reached the required temperature is satisfied. Given the above-mentioned requirements for "platforms" and "probes", in particular high resistance to high temperatures for a less thermally inert mass, it is advisable to use solid carbon, in particular electrographite for said sample holder, with or without covering of pyrolitic graphite, even pyrolitic graphite as well as vitreous carbon. This is how it is known, among others from Anal. Chim. Acta 147 (1983) 173-184 to carry out "probes" in pyrolitic graphite. The properties of said sample holders from said kinds of carbon are on the one hand insufficient for said applications, on the other hand the

  realization is complicated and, therefore, expensive.

2S63916

PHD 84.064 -4-

  The invention aims to provide sample holders having a high resistance to high temperatures for a mass less thermally

  inert which lends itself to easy realization.

  In accordance with the invention, this object is achieved by the fact that the sample holders, therefore both the "platforms" and the

  "probes" (probes) are made of vitreous microporous carbon.

  Microporous vitreous carbon is known from DE-OS 26 48 900, 26 54 021 and 27 02 189. It is produced by heating hard tissue in a non-oxidizing atmosphere at a temperature> 800 C. Its particular advantages compared to said solid carbons conventional, are manifold as regards the kind of application according to the invention and should be opposed, point by point, to other carbons, in a classified manner, according to the mechanical, thermal and treatment properties. We need to highlight only a few

particularities.

  The raw material is mechanically stable, although it is porous and its specific weight is only 1.0 g / cm3 (electrographite 1.6 to 1.8; pyrographite 2.0 to 2.25 ; glassy carbon 1.4 to 1.55). The porosity is represented by coherent pore channels with a diameter of 1 µm to 10 µm. Therefore, the raw material has a certain adsorption capacity. This implies that it can contain certain quantities of solution to be analyzed without having to carry out a form of basket for example. This is explained with the help of an example: as "platform" is used a tiny nacelle in electrographite with a length of about 10 mm, a width of 4 mm and a thickness of 1 mm ( with or without a pyrographite layer). In this small block is formed a depression in the shape of a bowl of about 8 mm. 3 mm. 0.5 min. This corresponds to a storage volume of V = 12 mm3. A small block of microporous glassy max carbon of the same dimensions without sinking, on the other hand, has an "internal" storage capacity of V = 20 mm3. The maximum microporosity moreover ensures better wettability by aqueous solutions and therefore implies better "retaining power" for the liquid sample to be analyzed than in the case of

other carbons.

  When the porosity is not desirable, the surface of the "platforms" or "probes" can be completely or only partially sealed by covering with pyrolitic graphite. What stands out

PHD 84.064 -5-

  particularly it is also the very good machinability of raw materials. Thus, from material in the form of a profiled block, probes with a length of 70 mm could be produced for a width of 3 mm and thicknesses up to 0.1 mm. Given their high mechanical stability, these "probes" are very elastic and suitable for handling. Due to their extremely low mass, they have a practically negligible thermal capacity and as a result of the very low thermal conductivity, the heat given off by the probe head carrying the sample to be analyzed ("probe-head")

  can hardly be evacuated by the rod ("probe-handle").

  i0 Some examples of the conforming sample holder

  to the invention are shown in the drawing.

  Figures 1, 2 and 3 show "platforms" in vitreous microporous carbon and Figures 4, 5 and 6 "probes" in vitreous microporous carbon. Figure 1 shows a basic form designated by 1 of a "platform". In FIG. 2, the basic form 1 of a "platform" is provided with an elongated recess 2. FIG. 3 shows a "platform"

  with a circular depression 3.

  Figure 4 shows a basic form designated by 4 of a "probe". Figure 5 shows a "probe" with a rod 4 and a

rectangular head 5.

  In FIG. 6, the head 5 of the "probe" according to FIG. 5 is

  provided with an eyelet 6 for receiving a sample to be analyzed.

PHD 84.064 -6-

Claims (2)

CLAIMS:   1. Sample holder for flameless emission and atomic absorption spectroscopy, characterized in that it consists by microporous vitreous carbon.   2. Sample holder according to claim 1, characterized in that   that it is at least partially covered with pyrolytic graphite.