TUBE CELL FOR ATOMIC ABSORPTION SPECTROPHOTOMETRY

Abstract: A tube cell (21) for flameless atomic absorption spec­ trophotometry has a transverse cross-section such that the portion (23) above a line (25) extending longitudinally midway between the top and bottom of the tube has a smaller area than the portion (24) below that line. The tube cell has an elongate aperture (26) formed close to the bottom (28) of the tube cell for insertion of a probe (29) or a platform (not shown) which carries a sample to be analysed. The cross-section of the tube is shaped to maximise the size of prove whch can be used, and hence the sample volume, while keeping the total cross-sectional area to a minimum to obtain maximum chemical sensitivity. A number of differently shaped cross-sections are described and illustrated in addition to the triangular cross-­ section.

Description: The invention relates to a tube cell for flameless atomic absorption spectrophotometry comprising a tubular body of electrically conductive material, the transverse cross-section of the tube being substantially constant along its length or reducing continuously from its ends towards its centre, the tubular body being arranged for mounting in a spectrophotometer such that its longitudinal axis is substantially horizontal. Such cells usually comprise a graphite tube which is designed to be mounted between two electrodes for the supply of heating current. The tubes themselves are smooth and normally circularly cylindrical and are usually formed with an opening in the tubular wall in its central region through which a sample to be analysed can be injected. However, tubes having a square cross-section are known from U.K. Patent No. 1,481,124. In the flameless atomic absorption spectrophotometry process the graphite tube is heated to a high temperature by the passage of electric current so that the sample substance is first dried and sometimes ashed before reaching a temperature at which it is atomized, i.e. the temperature within the graphite tube at which an atomic cloud is formed in which the individual elements contained in the sample are present in an atomic condition. A beam of radiation, which preferably includes a resonance line of a specific element to be identified in the sample, is passed through the graphite tube along its length so that the proportion of the specific element in the sample can be deduced from the absorption of the beam. The beam of radiation is normally defined as an image of the monochromator entrance slit focussed at the centre of the tube. The beam therefore diverges horizontally and vertically in both directions along the tube from the central rectangular image. When used in this way, the graphite tube serves several functions. First it serves as a carrier for the samples to be analysed; secondly the heating of the graphite tube causes the sample carrier to be evaporated and the sample then to be atomized and finally the tube serves to hold together the atomic cloud thus produced. The degree of absorption of the beam of radiation and hence the overall sensitivity of the atomic absorption spectrophotometry process is, amongst other things, a function of the number of free atoms in the path of the beam through the tube cell. There are a number of factors which make it desirable for the tube cell to have as small a cross-sectional area as possible. These include the power required to raise the temperature of the tube to the atomisation temperature and the retention of the atomised sample within the radiation beam. It has been proposed to introduce samples into the tube cells by means of probes as disclosed, for example U.K. Patent Applications Nos. 2,071,845A, 2,088,582A, or platforms as disclosed, for example U.K. Patent Application No. 2,052,788A. The probe or platform remains within the tube cell during measurement. Since the probes/platforms are heated to a very high temperature they will give off visible radiation, as will the tube cell. If this radiation falls on the spectrophotometer detector it will tend to swamp the wanted radiation produced by the radiation beam and make the measurement of the absorption produced by the sample more difficult. It is desirable therefore that the detector is arranged so that radiation from the probe/platform and tube cell does not fall on the detector. If the probe/platform partially interrupts the radiation beam the energy falling on the detector will be reduced and consequently the overall signal to noise ratio will be degraded. To prevent these undesirable effects the probe/platform should be situated as close to the bottom of the tube cell as possible so that the maximum area radiation beam can be used. However, with existing tube cells this either means using small probes/platforms which reduces the maximum sample volume to an undesirably low value or necessitates using larger tube cells with a correspondingly increased power requirement for heating and a lower sensitivity due to the increased volume outside the radiation beam. It is an object of the invention to enable the provision of tube cells for flameless atomic absorption spectrophotometry in which the problems outlined hereinbefore can be mitigated. The invention provides a tube cell for flameless atomic absorption spectrophotometry as set forth in the opening paragraph characterised in that the cell is shaped such that when the tubular body is so mounted in a spectrophotometer the transverse cross-sectional area adjacent to the centre of the tube is such that the portion of said area above a horizontal line midway between the top and bottom of the tube is smaller than the portion of said area below that line. In U.K Patent No. 1564165, the provision of the recess in the tube cell described is for the purpose of reducing the cell temperature in this region. The tube cell according to the invention enables a large sample volume to be used while limiting the overall size of the cell to give maximum chemical sensitivity. A dosing aperture may be formed in the upper surface of the tube cell at a point midway between its ends. An elongate aperture may be formed to-wards the bottom of the tube cell midway between its ends. The elongate aperture allows a probe carrying a sample to be analysed to be inserted into the tube cell and is located near to the bottom of the tube cell so that the probe lies below the detected radiation beam. Since the temperature of the tube cell and probe is such that, during atomisation and measurement they emit visible radiation it is desirable that they are out of the detected radiation beam to avoid interference. The cells described and claimed herein may be used in the method and apparatus disclosed in U.K. Patent Application No. 8305745 (PHB 32962) provided that the dosing aperture and elongate aperture are provided. It is also possible to dispense with the dosing aperture if the sample is placed on the probe outside the tube cell and inserted into the tube cell on the probe. A platform for containing the sample may be inserted into the tube cell either longitudinally or through the elongate aperture instead of a probe. The transverse cross-sectional shape of the tube cell may be trapezoidal with the parallel sides extending horizontally. Alternatively the transverse cross-sectional shape may be triangular, one side of the triangle being substantially horizontal. These shapes maximise the size of probe and hence the sample volume possible while making the cross-sectional area of the tube conform closely to the radiation beam. The tubular body may flared from a point midway between its ends towards both ends. This enables the tube cell to conform closely to the radiation beam along the whole of its length. The tube cell may be formed totally of pyrolytic graphite by means of chemical vapour deposition on a mandrel. This enables relatively complex shapes to be produced without requiring machining operations since the shape is defined by that of a mandrel on which the pyrolytic graphite is deposited. Such a process is disclosed in U.K. Patent No. 2,064,500 the contents of which are incorporated herein by reference. Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:- Figure 1 shows a first embodiment of a tube cell according to the invention, Figure 2 is a cross-sectional view on line A-A of the tube cell shown in Figure 1, Figure 3 shows a second embodiment of a tube cell according to the invention, Figure 4 is a cross-sectional view on line B-B of the tube cell shown in Figure 3, Figure 5 shows a third embodiment of a tube cell according to the invention, Figure 6 is a cross-sectional view on line C-C of the tube cell shown in Figure 5, Figure 7 shows a longitudinal cross-sectional view of a fourth embodiment of a tube cell according to the invention, As shown in Figures 1 and 2 the tube cell comprises a tubular body 1 having a dosing aperture 2 midway between its ends. As can be seen from Figure 2 the cross-section in a plane transverse to the longitudinal axis of the tube comprises a semicircular upper portion 3 and a rectangular lower portion 4. Thus the cross-sectional area of the upper portion 3 is less than that of the lower portion 4. The boundary between the portions 3 and 4 is defined by an imaginary line 5 located midway between the top and bottom of the tube as viewed in Figure 2. Figures 3 and 4 show a tube cell similar to that shown in Figures 1 and 2 but modified for use with probe or platform sampling. The tube cell comprises a tubular body 11 having a dosing aperture 12 midway between its ends. An elongate aperture 16 is formed in a side wall 17 of the tubular body 11 close to the bottom wall 18. The purpose of the aperture 16 is to allow access to the interior of the tube cell for a probe 19 or for a platform (not shown). In a spectrophotometer the detector is masked so that it sees a radiation beam 20 which is rectangular in section. It is important to ensure that the detector does not see the walls of the tube or the probe since they will emit radiation at the atomising temperature. Consequently the probe 19 should be below the detected radiation beam. With a conventional circular cross-section tube this severely limits the size of sample which may be carried since as the position of the probe within the tube is lowered its depth must be correspondingly reduced since the chord length reduces. This makes it difficult to use probe sampling with small diameter tubes. Increasing the diameter of the tubes allows larger samples to be used but reduces the instrument sensitivity as a larger proportion of the excited atoms is outside the detected radiation beam. The construction shown in Figures 3 and 4 overcomes the problem of smaller chordal distances limiting the sample size which can be carried by the probe while not greatly increasing the cross-sectional area of the tube cell. Figures 5 and 6 show a tube cell comprising a tube cell 21 having a triangular cross-section. A dosing aperture 22 is provided midway between its ends at the apex of the triangle. A longitudinal elongate aperture 26 is provided in one side 27 of the triangle close to the base 28. As can be seen from Figure 6 the portion 23 of the cross-sectional area above an imaginary centre line 25 is smaller than the portion 24 of the cross-sectional area below that line. This arrangement enables a relatively large sample volume to be placed on the probe within the tube cell while minimising the cross-sectional area of the cell. Thus a high sensitivity may be achieved with a tube cell as shown in Figures 5 and 6. Various other cross-sectional shapes may be produced which fulfill the condition that the portion of the cross-sectional area above half height is less than that below half height. For example the cell shown in Figures 5 and 6 may take a more definitely trapezoidal shape or the side walls may be made concave to more nearly fit the beam shape, care being taken to ensure that this does not unduly restrict the sample size. It should be noted that the sample should not be allowed to come into contact with the walls of the tube cell before atomisation when probe sampling is used otherwise the benefits of probe sampling will be lost. The tube cell shown in Figure 7 comprises a tubular body 131 with the upper portion 133 flared to-wards both ends of the tube from a point midway between its ends so that the upper wall of the tube cell follows the outline of the radiation beam 140. The lower portion 134 is defined by a straight wall 138 and has the same cross-sectional shape and area along the length of the tubular body. This modification may be made to any of the tube cells shown in Figures 1 to 6 and to tube cells of any other cross-sectional shape. An elongate aperture 136 is provided at a point close to the bottom wall 138 of the tubular body and midway between the ends of the tubular body. The radiation beam 140 is normally brought to a focus at a point midway along the tubular body and by making the bottom wall 138 straight space is created below the radiation beam 140 for the positioning of a probe through the aperture 136. The tube cells described may be formed from graphite which is machined to shape as is well known for circularly cylindrical tube cells but since the shapes proposed are difficult and expensive to produce by machining it is advantageous to produce the tube cells by a chemical vapour deposition technique, for example as described in U.K. Patent Application No. 2,064,500. The tube cells described with reference to Figures 1 to 6 can be produced particularly simply by this method but the remaining tube cells require the use of split mandrels to enable the deposited tube cells to be removed from the mandrels. Subsequent machining steps may be necessary to produce the dosing and probe insertion apertures.

Claims: A tube cell for an atomic absorption spectrophotometer comprising a tubular body of electrically conductive material, the transverse cross-section of the tube being substantially constant along its length or reducing continuously from its ends towards its centre, the tubular body being arranged for mounting in a spectrophotometer such that its longitudinal axis is substantially horizontal characterised in that when the tubular body is so mounted in a spectrophotometer the transverse cross-sectional area adjacent to the centre of the tube is such that the portion of said area above a horizontal line midway between the top and bottom of the tube is smaller than the portion of said area below that line. A tube cell as claimed in Claim 1 characterised in that a dosing aperture is formed in the upper surface of the tube cell at a point midway between its ends. A tube cell as claimed in Claim 1 or Claim 2 in which an elongate aperture is formed towards the bottom of the tube cell midway between its ends. A tube cell as claimed in any preceding claim characterised in that the transverse cross-sectional shape is trapezoidal with the parallel sides extending horizontally. A tube cell as claimed in any of Claims 1 to 3 characterised in that the transverse cross sectional shape is triangular, one side of the triangle being substantially horizontal. A tube cell as claimed in any preceding claim characterised in that the tubular body is flared from a point midway between its ends towards both ends. A tube cell as claimed in any preceding claim formed totally of pyrolytic graphite by means of chemical vapour deposition on a mandrel.

Inventors: ANDREWS, ARTHUR STANLEY C/O PATENT DEPARTMENT, WIDMER, DAVID SYDNEY
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN, PHILIPS ELECTRONICS UK LIMITED
Application year: 1984
Main classification: G01N21
IPC codes: G01N21