DE29616475U1 - Plasma torch tip - Google Patents

Description

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Plasma torch tip
Description

The invention relates to a plasma torch tip for a capacitively coupled microwave plasma (CMP).

A plasma torch tip enables the operation of a capacitively coupled microwave plasma (CMP), as described for example by Disam, Tschöpel & Tölg (1982) in Fresenius Z. Anal. Chem. 310; pp. 131 -143; Springer Verlag.

In the tip shapes of plasma torches that were previously used, the plasma gas was guided through several symmetrically arranged holes past a central metal tip so that the gas flowed concentrically around it. To operate a stable argon plasma, volume flows of 2 to 6 standard liters of argon per minute at a microwave power of 200 to 400 watts were required.

The plasma burns on the central metal tip. However, since a plasma is more viscous than the surroundings, there is a risk that a large part of the aerosol to be measured will pass by the plasma and not reach the favorable excitation zone. This problem was countered by a sheath gas in order to increase the stability of the plasma in particular. However, this has the disadvantage of complex designs of the resonator and the burner tip.

The present invention is based on the object of developing a plasma burner tip for which a significantly smaller amount of plasma gas is required in order to ensure stable operation of a plasma, in particular an argon plasma.

This task is solved by the fact that the plasma torch tip is designed in the form of an elongated ellipsoid halved transversely to the main axis, which has a bore centrally in the direction of the main axis and which is firmly and tightly connected to a resonator in the area of the cutting surface of the ellipsoid.

When using such a plasma torch tip, only 5 to 10% of the usual amount of plasma gas is required for stable operation of an argon plasma, for example. In addition, the previously necessary sheath or support gas, which surrounds the plasma to support it, is no longer required.

The diameter of the hole in the plasma torch tip is between 0.1 and 5 mm, the length of the hole is between 3 and 30 mm, in particular 10 mm.

The entire gas flow is guided through a single central hole in the plasma torch tip. This means that very high gas velocities are achieved even with small volume flows. The plasma, which is ignited in argon, inclines into the gas flow that flows vertically upwards. A very narrow discharge thread is formed that widens towards the top.

For example, the plasma is stable at gas flows of 70 to 300 standard milliliters per minute and a microwave power of 40 to 450 watts.

The plasma torch tip is made of a material that has high electrical conductivity and high thermal conductivity and a low tendency to oxidize. The plasma torch tip is made primarily of gold or silver, but copper is also used. This reduces oxidation of the material to a minimum, and largely prevents wear and tear or melting of the plasma torch tip, as well as contamination of the plasma stream and blockage of the hole in the plasma torch tip.

An adapter is arranged between the plasma torch tip and the resonator. This serves to make the plasma torch tip usable for commercially available resonators.

The adapter has a central channel that is conical in the direction of the plasma torch tip and opens into the hole in the plasma torch tip. This effectively reduces the risk of the hole in the plasma torch tip becoming blocked by condensation droplets, for example moist aerosol components. Any condensate that may occur is mainly deposited in the conical area of the channel in the adapter.

An embodiment of the invention is explained in more detail with reference to the drawings:

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Figure 1 shows a cross-section through a plasma torch tip and an adapter attached to it.

The plasma torch tip 1 is made of a material that is resistant to oxidation, electrically conductive and has excellent heat conductivity. Silver and gold have proven to be particularly suitable for this purpose.

The tip 1 has the shape of half an elongated ellipsoid with a length of 10 mm. A thread 4 with a length of 5 mm is attached to the flat side. The maximum cutting surface of the ellipsoid is 4 mm. A longitudinal hole 2 with a diameter of 0.8 mm leads through the tip 1.

&iacgr;&ogr; The tip is screwed into an adapter 5 made of brass or copper. This serves to close the waveguide of the resonator in which the plasma is ignited.

Figure 2 shows a resonator of a capacitively coupled microwave plasma with a plasma torch tip according to the state of the art in cross section.

The resonator consists of a lower part with a tuning element, an outer metal cylinder 19 and an inner metal cylinder 20, on the end of which there is a tip 22 according to the state of the art. This tip is characterized according to the state of the art by a tip 22 and a further cylinder 23, into which a sample aerosol is guided, which exits around the tip 22.

A sheath gas is passed between this cylinder 23 and the inner metal cylinder 20.

When using the plasma torch tip 1 according to the invention, the cylinder 23 with the tip 22 and the supply 25 of the sheath gas are omitted. Instead, the plasma torch tip 1 is inserted directly on the inner metal cylinder 20.

In a further advantageous variant, the plasma torch tip 1 can be inserted, for example, into the existing inner metal cylinder 20 using an adapter 5.

Then the plasma gas 11 flows through the inner cylinder 20, which serves as a waveguide, into the resonator 20 (not shown). In this example, the inner cylinder 20 is water-cooled with the cooling water inlet 12 and outlet 13. Furthermore, the resonator has a removable upper part 21, which has a water-cooled chimney

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To couple in the microwave energy, the core of the coaxial cable coming from the magnetron is connected to the inner conductor. The shielding of the cable and the outer cylinder 19 are also connected.

The argon plasma produced with this type of plasma torch tip shape is very stable both spatially and with respect to the introduction of moist and dry aerosols. For this reason, this plasma is ideally suited for use in optical emission spectrometry.

As an example, the detection limits of chromium, nickel, copper, zinc and lead in moist aerosols were determined according to the 3a criterion.

&iacgr;&ogr; The plasma torch with tip (resonator) is located on an optical bench. From here, the emitted radiation is guided through a lens and focused. In a monochromator, it is finally spectrally broken down. The intensity of the spectral line required for detection is quantified using a secondary electron multiplier. The samples were converted into aerosol form using a pneumatic atomizer and introduced into the plasma.

The following equipment and chemicals were used:

Water: All solutions were prepared with deionized water from

a water treatment plant Miili-Q Water of the Miilipore family

Top material: silver

Argon: Purity grade 4.8 (Messer-Griesheim)

Lead acetate: p.a. (Merck)

Nickel chloride: p.a. (Merck)

Potassium dichromate p.a. (Merck)

Copper (II) chloride: p.a. (Merck)

Zinc sulfate: p.a. (Merck)

Peristatic pump: Perimax 12 (Spectec)

Plasma torch CMP: 16/30 coaxial type according to Kessler/ARL,

(Analysentechnik Feuerbacher, Tübingen)

Microworld generator:

2.45 GHz, 450 watts adjustable (Erbe Elektromedizin KG, Tübingen)

Atomizer: V-gap, Legere type, (homemade)

Photomultiplier: 1P28, (Hamamatsu) Data acquisition: 24 bit TTL I/O card UT 8032 (!sei) and IBM PC-AT,

80286 CPU and coprocessor monochromator: ARL, 1 m Czerny-Turner setup

Set operating parameters:
Plasma gas: Argon

Gas flow: 250 ml/min

Power: 350 watts

Observation height above the tip: 3 to 5 mm

The following results were achieved:
Detection limits of the individual elements

Element/Line [nmj Detection limit [mg/L] CrI425.44 0.021 CrI357.87 0.011 No! 352,45 0.12 Nile 341.48 0.066 PbI405.78 1.0 CuI324.75 0.41 ZnI213.86 0.30

It turns out that with the described arrangement, the elements mentioned can be detected in the sub ppm range. The analytical performance is therefore comparable to capacitively coupled argon plasmas operated with conventional tip shapes.

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However, there are clear advantages due to the low gas consumption. In addition, the investigations showed that matrix effects, e.g. in the determination of phosphorus, are significantly lower with the tip shape presented here than with conventional tip shapes.

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List of reference symbols

1 plasma torch tip

2 Hole

3 Exit of the plasma stream from the plasma torch tip

4 threads in plasma torch tip

5 adapters

6 thread in adapter 5

7 Channel in Adapter 5

&iacgr;&ogr; 11 plasma gas;

12 Cooling water inlet;

13 Cooling water drain;

14 tuning element;

15 Coaxial cable connection 16 Ignition pin

17 quartz windows

18 Chimney cooling jacket

19 outer metal cylinder

20 inner metal cylinder 21 upper part of the resonator

22 State-of-the-art plasma torch tip

23 Cylinder with holder for the plasma torch tip according to the state of the art 22

24 Sample aerosol supply 25 Mantle gas supply

Claims (7)

: \*..1 ·:..: :··. 228OGM Protection claims 1. Plasma burner tip in the form of an elongated ellipsoid cut in half across the main axis, which has a central bore in the direction of the main axis and which is firmly and tightly connected to a resonator in the area of the cutting surface of the ellipsoid. 2. Plasma burner tip according to claim 1, characterized in that the diameter of the bore in the plasma burner tip is between 0.1 and 5 mm &iacgr;&ogr; is. 3. Plasma burner tip according to claim 1 or 2, characterized in that the bore in the plasma burner tip has a length between 3 and 30 mm, in particular 10 mm. 4. Plasma torch tip according to one of claims 1 to 3, characterized in that the plasma torch tip consists of a material that has a high electrical conductivity and a high thermal conductivity and a low tendency to oxidation. 5. Plasma torch tip according to claim 4, characterized in that the plasma torch tip consists of gold or silver. 6. Plasma torch tip according to one of claims 1 to 5, characterized in that between the plasma torch tip and the resonator a Adapter is arranged. ': .: :8th..·. ^: ..:&ggr;·. 228OGM 7. Plasma torch tip according to claim 6, characterized in that the adapter has a central channel which tapers conically in the direction of the plasma torch tip and opens into the bore of the plasma torch tip.