US3893769A

US3893769A - Graphite tube furnace

Info

Publication number: US3893769A
Application number: US474398A
Authority: US
Inventors: John Frederick Woolley
Original assignee: International Standard Electric Corp
Current assignee: International Standard Electric Corp
Priority date: 1973-08-21
Filing date: 1974-05-30
Publication date: 1975-07-08
Anticipated expiration: 1992-07-08
Also published as: FR2241756A1; GB1399050A

Abstract

Apparatus for vaporizing samples for analysis by the use of a spectrophotometer. A carbon tube is electrically heated to vaporize the sample. The tube is conveniently held by electrode spring finger contacts. A transparent envelope encompasses the tube and has an inlet arrangement that facilitates flushing with argon.

Description

United States Patent Woolley [451 July 8, 1975 GRAPHITE TUBE FURNACE [56] References Cited [75] Inventors: John Frederick Woolley, UNITED STATES PATENTS sawbridgfiwoflh, England 3,67l,129 6/1972 Wiedeking 356/85 [73] Assignee: International Standard Electric Primary Examiner vincem MCGraw Corpcranon New York Attorney, Agent, or FirmA. Donald Stolzy [22] Filed: May 30, 1974 21 Appl. No.: 474,398 [57] ABSTRACT Apparatus for vaporizing samples for analysis by the use of a spectrophotometer. A carbon tube is electri- [301 Forelgn Apphctlon'Pnonty Data cally heated to vaporize the sample. The tube is con- I973 Umed Kmgdm 39440/73 veniently held by electrode spring finger contacts. A transparent envelope encompasses the tube and has an [52] US. Cl 356/85; 356/244 inlet arrangement that facilitaes flushing with argon. [51] Int. Cl. (201,] 3/30; GOlN 21/16 [58] Field of Search 356/85, 244 30 Chums, 13 Drawing Flgures PATH 7*?" JUL 12 m5 SHEET 1 GRAPHITE TUBE FURNACE BACKGROUND OF THE INVENTION This invention relates to graphite tube furnaces for atomic absorption spectroscopic analysis.

SUMMARY OF THE INVENTION According to the present invention, there is provided a graphite tube furnace for atomic absorption spectroscopy including a graphite tube each end of which is clamped in an electrically conductive heat sink by means of a tapered bushing pressed into a cooperating tapered hole in the heat sink by a ferrule in screwthreaded engagement with the heat sink, the furnace being completely open and having an unobstructed path for the passage of light in the direction of the axis of the tube for spectrophotometric use.

Preferably, the graphite tube is contained in a cylindrical envelope which extends between the heat sinks and which is provided with gas inlet and outlet tubes, the inlet tube being arranged substantially tangentially so that a swirling motion is imparted to the flow of gas within the envelope.

There follows a description of a carbon tube furnace embodying the invention in a preferred form.

BRIEF DESCRIPTION OF THE DRAWINGS The description refers to the accompanying drawings in which FIG. 1 is a perspective view of one embodiment of the present invention;

FIG. 2 is an exploded perspective view of a carbon tube assembly;

FIG. 3 is a longitudinal sectional view through the graphite tube;

FIG. 4 is a diagrammatic view of a spectrophotometer in which the graphite furnace of the present invention may be used;

FIG. 5 is a front elevational view of the graphite furnace of the present invention shown in FIG. 4 taken on the line 55 therein;

FIG. 6 is a left end elevational view of a partial assembly of the graphite furnace shown in FIG. 5',

FIG. 7 is a vertical sectional view through the furnace shown in FIG. 5 taken on the line 7-7 therein;

FIG. 8 is an end elevational view of an envelope for the furnace of the present invention;

FIG. 9 is a front elevational view of the envelope shown in FIG. 8;

FIG. 10 is a vertical sectional view through a heat sink;

FIG. 11 is a horizontal sectional view through the heat sink illustrated in FIG. 10 taken on the line 11-11 therein;

FIG. 12 is a rear elevational view of the heat sink shown in FIG. 10; and

FIG. 13 is a front elevational view of the heat sink shown in FIG. 10.

DESCRIPTION OF THE PREFERRED EMBODIMENT The ends of a graphite tube 1' are clamped in a pair of electrically conductive heat sinks 2' and 3' electrically insulated from a supporting platform 4 in FIG. 1. The heat sinks 2' and 3' are respectively provided with electrical terminal posts 5' and 5" to which electrical connection may be made, and with inlet and outlet tubes 6' for the circulation of cooling water.

The wall of the tube 1 is thinned over a central region 7' so as to tend to localize the heating zone. Less power is then required to reach a given temperature. and the temperature gradient at the ends of the hot zone are sharper. Within this central region is a small hole 8' through which a pipette may be inserted and through which a specimen for spectroscopic analysis may be inserted into tube 1'. Both of the ends of the tube 1' are clamped in respective heat sinks 2', 3' by means of respective split tapered bushings 9', 10' urged into cooperating tapered holes in the heat sinks by knurled ferrules 11' and 12', respectively. The ferrule l2 has a threaded tube 13' into which is fitted the larger end of its associated tapered bushing 10' so that it bears against an inwardly directed flange 14'. The assembly of ferrule ll may be identical to that of ferrule 12'. The outside of the tube 13 is threaded at 15' and engages a complementary thread formed in the heat sink. Both tapered bushings 9', 10 are provided with small channels 16' to provide a point of purchase to facilitate their removal from the respective heat sinks when it becomes desirable to replace a graphite tube.

The graphite tube 1' is contained in a tubular silica or transparent pyrex glass envelope 1') which extends between the two heat sinks 2', 3' and which is provided with a gas inlet tube 18' and an outlet-pipette tube l9. These two tubes 18', 19' have axes that lie in a common plane normal to the axis of the tube 1 and the axis of envelope 17'. The outlet tube 19' extends radially from and has an axis that is normal to that of tube 1'. The axis of tube I9 is also the same as that of 8' so that a sample for spectroscopic analysis can be dispensed inside the tube 1' from the tip of a pipette (not shown) introduced through the outlet tube 19 into hole 8'. Preferably, the pipette is chosen to seat in the end of the pipe in such a way that the positioning of the sample within the tube 1' is automatically reproducable.

To make it convenient to use the pipette, the tube I9 is angled at about 45 to the vertical. The inlet I8 is arranged so that the gas issues from it in a substantially tangential direction relative to the cylindrical envelope 17' so as to impart a swirling motion to the gas flow within the envelope around the tube 1'. In order to provide a large distance for the gas to flow between inlet and outlet tubes I8 and I9, the inlet tube 18' is situated at the bottom of the envelope 17 so that there is an arc of approximately five-eights of a circle between the point of entry of the gas and its point of egress.

The furnace is placed so that radiation from the light source of a spectrophotometer passes through the furnace along the axis XY (X'Y in FIG. 2) to be detected by the analyzer of the photometer. The envelope 17' is flushed with an inert gas, usually argon, and then a measured quantity of liquid to be analyzed is pipetted into the interior of the tube. At this stage, the outlet tube 19' is substantially sealed by the pipette with the result that argon escapes via the hole 8' and/or other leakage locations (the arrangement is not gas tight). In this way, oxygen is flushed also from the interior of the tube 1. The power from a power supply is caused to flow through the graphite tube 1' heating it so that the sample is atomized. The power supply is programmed to provide two preheating periods prior to atomization as is conventional. The first preheat is designed to drive off water or any other solvent, while the second pyrolizes any carbonaceous material.

Note will be taken in FIG. I that one insulator is provided between heat sink 2 and platform 4. Similarly, an insulator 21 is provided between platform 4' and heat sink 3.

In FIG. 3, graphite tube 1' is shown again. Note that in FIG. 3, graphite tube I has one end tapered at 22 and another end tapered at 23. The tapers at 22 and 23 make it possible to assemble the bushings 9' and 10' over the ends of graphite tube 1' and make it possible to spring the fingers thereof between the slots thereof outwardly so that they can rest at a convenient point along the external surface of graphite tube I at the respective ends thereof.

In FIG. 4, the graphic furnace of the present invention is illustrated generally at 24 in a spectrophotometer which includes a light source 25 to beam light through the furnace 24 including through the gas vaporized inside tube I. The conventional light source 25 directs light along a path in the direction of an arrow 26 which is, after the light passes through furnace 24 and is received by a conventional spectral analyzer 27 in a direction as indicated by an arrow 28.

The pipette is indicated generally at 29. The argon gas source is illustrated generally at 30 and may be conventional. A conventional source of potential or power supply is illustrated at 31. The cooling system may be conventional and is illustrated at 32.

In FIG. 5, platform 4' may be integral with or fixed to a post 33 for mounting. Cap

screws

34 and 35 hold the heat sinks on the platform 4. A screw 36 holds a bracket 37 to platform 4'. A screw 38 holds a spring clip 39 to bracket 37. As shown, spring clip 39 extends around and secures in a fixed position the end of inlet tube 18'. As will be evident later, post 33 has a recess 40 for mounting.

As can be seen in FIGS. 6 and 11, both heat sinks have an annular groove 41 into which the annular edges of envelope 17' are slidable.

A portion of the assembly has been omitted in FIG. 6 illustrating that heat sink 3' has a bore 42. As indicated previously, both of the heat sinks 2' and 3 may be substantially identical except that they are mirror images of one another.

As shown in FIG. 7, platform 4 may be secured to post 33 by one, two, three or four or more countersunk screws 43.

A view ofenvelope 17' with inlet and outlet tubes 18 and I9, respectively, are shown completely in elevation in FIG. 8. The view in FIG. 8 is a side elevational view.

Envelope I7 with

tubes

18 and 19, respectively, are also shown in a front elevational view in FIG. 9. The view of FIG. 9 is completely in elevation.

In FIG. 10, connecting

passageways

45, 46 and 47 complete the circuit for circulating a cooling fluid through heat sink 2'. A plug 48 is hard soldered at 49 to heat sink 2'. The same is true of post 5' at 50, and tubes 6' at 51. Tapped holes 52 are provided in heat sink 2' into which cap screws 34 may be threaded. A bore 53 of heat sink 2' is also shown in FIG. 10. The bore 53 is tapered, as shown in FIG. 11.

As shown in FIG. 11, heat sink 2' has the threaded counterbore at 54 to receive the thread corresponding to thread IS in FIG. 2.

FIG. 12 is a rear end elevational view of the heat sink 2.

FIG. 13 is a front elevational view of the heat sink 2'.

Although such a construction has not been shown and is not absolutely necessary, the extreme ends ofthe spring fingers of bushings 9' and 10' may be slightly flared outwardly so that the spring fingers may even more easily slide over the tapered ends 22 and 23 of the graphite tube 1' shown in FIG. 3.

All of the structures shown in FIG. 4 may be entirely conventional except for the furnace 24 of the present invention. Notwithstanding this statement, graphite furnaces are broadly old in the art, and it is only the new and novel features of the graphite furnace 24 claimed herein for which a patent is sought.

The apparatus of FIG. 4 is employed in many ways for many purposes. One such purpose will be related hereinafter. At any rate, the purpose of the spectrophotometer of FIG. 4 is to determine the content or concentration of a substance in the vapor created by heating a sample in the graphite tube 1'.

One purpose of the invention is to provide a graphite furnace for a spectrophotometer, the use of which can be made as a laboratory instrument to determine the concentration of the constituents of the vapor inside graphite tube I typically within the region 10"g to l0 g in a top plate sample equivalent to 10 microliters. Such an ultra-trace investigation of small samples is required, for instance, in forensic analysis such as in the examination of a blood sample for arsenic.

Channels 16 in bushings 9' and [0 shown in FIG. 2 may be useful in that a tool, not shown, can be inserted into the hollow interior of tube 13' of ferrule 12' to remove a bushing from a corresponding ferrule.

When the pipette is inserted into outlet tube 19', preferably the pipette protrudes through hole 8 in graphite tube 1' to terminate preferably at about the axis of the graphite tube 1'.

As indicated previously, when the pipette is in position in tube 19', the pipette seals tube 19' so that the argon introduced into the envelope 17' via tube 18' is obliged to escape elsewhere. Some of the gas escapes around the ends of the envelope 17' where they rest in respective grooves similar to or the same as grooves 41 shown in FIGS. 6 and II. The annular edges of the envelope 17' are only loosely located in the grooves corresponding to groove 41. The remainder of the gas may escape through hole 8 into the interior of the graphite or carbon tube 1' and out through both of the open ends, the same ends thereof being in free and open communication with the ambient atmosphere.

The sample is not driven off by flushing the furnace with argon. The sample merely drops into graphite tube I and is soon vaporized or atomized or changed into a gaseous state.

Immediately after the sample has been dispensed into the graphite tube 1', the pipette is removed, and the heating cycle is initiated. The whole cycle takes less than a minute. In FIG. 2, the internal cylinder surface of tube 13' is truly cylindrical and not tapered. The same is true of the corresponding surface in tube 13' of ferrule 11. Thus, bushing I0 is easily slidable inside tube 13' and bushing 9' is easily slidable inside the tube of ferrule 11 corresponding to tube 13.

If it is not apparent from the foregoing, the concentration of elements, compounds or mixtures in the vapor or gas produced by the vaporization or atomization of the sample in the graphite tube 1' is determined by detecting the absorption spectra of those selfsame elements, compounds or mixtures.

What is claimed is:

1. A graphite tube furnace for atomic absorption spectroscopy, said furnace comprising: a support; a pair of electrically conductive heat sinks fixed to said support; a hollow ferrule threaded into each of said heat sinks; a hollow tapered bushing mounted inside each respective ferrule; a graphite tube having both ends open and having the said ends thereof mounted inside respective ferrules, each of said bushings being slotted and having spring fingers clamping respective graphite tube ends, each heat sink having a tapered hole cooperating with the taper of respective bushings, said ferrules, bushings and graphite tube forming an assembly defining a hollow. unobstructed path extending completely therethrough through which light can pass in the direction of the axis of the tube.

2. A graphite tube furnace as claimed in claim 1, wherein a tubular envelope is provided and mounted between said heat sinks around said graphite tube, said envelope being provided in its side wall with an inlet tube which is substantially tangential to said envelope, said envelope being provided with an outlet tube which is substantially radial of the envelope.

3. A graphite tube furnace as claimed in claim 2, wherein the inlet and outlet tubes have axes that lie in a common plane normal to the axis of said envelope.

4. A graphite tube furnace as claimed in claim 3, wherein the inlet and outlet tubes are located relative to each other in a manner such that the path of a gas entering said inlet tube is circumferential inside said envelope and extends around inside said envelope over approximately five-eighths of a circle.

5. A graphite tube furnace as claimed in claim wherein said envelope is made of silica.

6. A graphite tube furnace as claimed in claim wherein said graphite tube contains a central region reduced wall thickness.

7. A graphite tube furnace as claimed in claim wherein said heat sinks have means to permit them be water cooled.

8. A graphite tube furnace as claimed in claim wherein said envelope is made of silica.

9. A graphite tube furnace as claimed in claim wherein said graphite tube contains a central region reduced wall thickness.

10. A graphite tube furnace as claimed in claim wherein said heat sinks have means to permit them be water cooled.

11. A graphite tube furnace as claimed in claim wherein said envelope is made of silica.

12. A graphite tube furnace as claimed in claim 11,

wherein said graphite tube contains a central region of reduced wall thickness.

13. A graphite tube furnace as claimed in claim ll, wherein said heat sinks have means to permit them to be water cooled.

14. A graphite tube furnace as claimed in claim 2, wherein said envelope is made of transparent glass.

15. A graphite tube furnace as claimed in claim 14, wherein said graphite tube contains a central region of reduced wall thickness.

16. A graphite tube furnace as claimed in claim 14, wherein said heat sinks have means to permit them to be water cooled.

17. A graphite tube furnace as claimed in claim 3, wherein said envelope is made of transparent glass.

18. A graphite tube furnace as claimed in claim 17, wherein said graphite tube contains a central region of reduced wall thickness.

19. A graphite tube furnace as claimed in claim 17, wherein said heat sinks have means to permit them to be water cooled.

20. A graphite tube furnace as claimed in claim 4, wherein said envelope is made of transparent glass.

21. A graphite tube furnace as claimed in claim 20, wherein said graphite tube contains a central region of reduced wall thickness.

22. A graphite tube furnace as claimed in claim 20, wherein said heat sinks have means to permit them to be water cooled.

23. A graphite tube furnace as claimed in claim wherein said graphite tube contains a central region reduced wall thickness.

24. A graphite tube furnace as claimed in claim wherein said heat sinks have means to permit them be water cooled.

25. A graphite tube furnace as claimed in claim wherein said graphite tube contains a central region reduced wall thickness.

26. A graphite tube furnace as claimed in claim wherein said heat sinks have means to permit them be water cooled.

27. A graphite tube furnace as claimed in claim wherein said graphite tube contains a central region reduced wall thickness.

28. A graphite tube furnace as claimed in claim wherein said heat sinks have means to permit them be water cooled.

29. A graphite tube furnace as claimed in claim wherein said graphite tube contains a central region reduced wall thickness.

30. A graphite tube furnace as claimed in claim wherein said heat sinks have means to permit them be water cooled.

Claims

1. A graphite tube furnace for atomic absorption spectroscopy, said furnace comprising: a support; a pair of electrically conductive heat sinks fixed to said support; a hollow ferrule threaded into each of said heat sinks; a hollow tapered bushing mounted inside each respective ferrule; a graphite tube having both ends open and having the said ends thereof mounted inside respective ferrules, each of said bushings being slotted and having spring fingers clamping respective graphite tube ends, each heat sink having a tapered hole cooperating with the taper of respective bushings, said ferrules, bushings and graphite tube forming an assembly defining a hollow, unobstructed path extending completely therethrough through which light can pass in the direction of the axis of the tube.

3. A graphite tube furnace as claimed in claim 2, wherein the inlet and outlet tubes have axes that lie in a common plane normal to the axis of said envelope. 4. A graphite tube furnace as claimed in claim 3, wherein the inlet and outlet tubes are located relative to each other in a manner such that the path of a gas entering said inlet tube is circumferential inside said envelope and extends around inside said envelope over approximately five-eighths of a circle.

5. A graphite tube furnace as claimed in claim 4, wherein said envelope is made of silica.

6. A graphite tube furnace as claimed in claim 5, wherein said graphite tube contains a central region of reduced wall thickness.

7. A graphite tube furnace as claimed in claim 5, wherein said heat sinks have means to permit them to be water cooled.

8. A graphite tube furnace as claimed in claim 2, wherein said envelope is made of silica.

9. A graphite tube furnace as claimed in claim 8, wherein said graphite tube contains a central region of reduced wall thickness.

10. A graphite tube furnace as claimed in claim 8, wherein said heat sinks have means to permit them to be water cooled.

11. A graphite tube furnace as claimed in claim 3, wherein said envelope is made of silica.

12. A graphite tube furnace as claimed in claim 11, wherein said graphite tube contains a central region of reduced wall thickness.

13. A graphite tube furnace as claimed in claim 11, wherein said heat sinks have means to permit them to be water cooled.

23. A graphite tube furnace as claimed in claim 1, wherein said graphite tube contains a central region of reduced wall thickness.

24. A graphite tube furnace as claimed in claim 1, wherein said heat sinks have means to permit them to be water cooled.

25. A graphite tube furnace as claimed in claim 2, wherein said graphite tube contains a central region of reduced wall thickness.

26. A graphite tube furnace as claimed in claim 2, wherein said heat sinks have means to permit them to be water cooled.

27. A graphite tube furnace as claimed in claim 3, wherein said graphite tube contains a central region of reduced wall thickness.

28. A graphite tube furnace as claimed in claim 3, wherein said heat sinks have means to permit them to be water cooled.

29. A graphite tube furnace as claimed in claim 4, wherein said graphite tube contains a central region of reduced wall thickness.

30. A graphite tube furnace as claimed in claim 4, wherein said heat sinks have means to permit them to be water cooled.