CA1314636C - Atmospheric pressure capacitively coupled plasma atomizer for atomic absorption and emission spectroscopy - Google Patents

Abstract

AN ATMOSPHERIC PRESSURE CAPACITIVELY
COUPLED PLASMA ATOMIZER FOR ATOMIC
ABSORPTION AND EMISSION SPECTROSCOPY

ABSTRACT OF THE DISCLOSURE

A novel atmospheric pressure capactively coup-led radio frequency plasma discharge method and apparatus are disclosed. The apparatus is suitable for atomic absorption and emission analysis of small, discrete sample volumes (1-50µl). The plasma can be operated at very low Radio Frequency (RF) input powers (30- 600 W) which allows for optimal conditions for atom resonance line absorption measurements. Sample introduction into the plasma is by an electrically heated tantalum strip vaporizer. The vaporization and dissociation-atomization steps are separately controlled. Analyte absorption takes place in the plasma discharge which is characterized by a long pathlength (10-50 cm) and low support gas flow rate (0.2 to 6 L/m) both of which provide for a relatively long residence time. The optimum RF power for Ag analysis is between 100 and 200 W and the optimum support gas flow rate is about 0.6 L/m. The device exhibits linear calibration plots and provides sensitivities in the range of from 3.5-40 pg.

Description

1 3 1 L~ 6 3 ~

AN ATMOSPHERIC PRESS~RE CAPACITIYELY
COUPLED PLASMA ATOMIZER FOR ATOMIC
ABSORPTION AND EMISSION SPECTROSCOPY

FIELD OF THE INVENTION

This application relate~s to a novel method and apparatus for the generation of an atmospheric pressure plasma and for conducting atomic absorption and emission analysis on small discrete sample volumes.

BACKGROUN~ OF THE INVENTION

During the past two decades inductively coupled plasma optical emission spectroscopy (ICP-OES) has played an important role in elemental analysis.
ICP-OES possesses several distinct advantages over other atomic methods including simultaneous multi-element capability, relative freedom from chemical interferences, low detection limits, and a large linear dynamic range. In recent years the ICP ilas also been used as a source for multi-element atomic fluorescence spectrometry (AFS) ~see A. Montaser and V.A. Fassel, Anal. Chem., 1976, 48, 1490 and D.R. Demers, Spectrochim. Acta, 1985, 40B, 105) and plasma source mass spectrometry (ICP-MS) (see R.S. Houk, V.A. Fassel, G.D. Flesch, H.J. Svec, A.L. Gray, and C.E. Talor, Anal. Chem., 1980, 52, 2283 and A.L. Gray, Spectrochim.
Acta, 1985, 40B, 1525). However, to date, the ICP has not been successfully exploited as an atomizer for atomic absorption spectrometry (AAS).

The properties of the ICP as an atom reservoir for AAS have been investigated by Wendt and Fassel (R.H.
Wendt and V.A. Fassel, Anal. Chem., 1966, 38, 337), Greenfield et. al. (S. Greenfield, P.B. Smith, A.E.

1 3 1 Lr6 ~

Breeze, and N.M.D. Chilton, Anal. Chim. Acta, 1968, 41, 385), and Veillon and Margoshes (C. Veillon and M.
Margos~es, Spectrochim. Acta, 1968, 23B, 503). In addition, Magyae and Aeschbach [s Magyar and F.
Aeschbach, Spectrochim. Acta, 1980, 35s, 839) have studied the theoretical implications of using the ICP
for AAS. They concluded that the ICP provided sensi-tivities a factor of ten poorer than those exhibited by a flame. The relatively low sensitivity of ICP-AAS can be attributed to several factors. A relatively high support gas flow rate is required to operate an ICP and this acts to dilute the sample atoms. The absorption volume in an ICP is not optimum for AAS, in particular, the absorption path length is relatively short and this combined with the high aerosol transport rate means that the residence time of analyte atoms in the absorption volume is short. Moreover, traditional AAS primarily makes use of atomic resonance lines but in the ICP the high temperature favours the production of ionic species-In spite of these factors, a plasma environ-ment does offer several distinctive features which sug-gest that it could offer several advantages over flames 25 and graphite furnaces for atomic absorption measure-ments. The relatively high temperature promotes com-plete vaporization and dissociation and thus aids in the control of chemical interferences. In addition, radio frequency (RF) plasmas are relatively stable and easy to 30 control. The atom reservoir temperature, and hence the characteristics of the absorption volumej can be con-trolled by controlling the input power to the plasma.
Also, since a plasma can be made to operate with a variety of gases (eg. Ar, He, N2, H2, etc.) the gas 35 phase chemistry can be controlled separately from mech-anisms causing energy production. Finally, the shape 1 3 1 ~1-63S

and extent of a plasma can be controlled through appropriate design of the external electrodes used to couple the RF power into the plasma.

U.S. Patent No. 4,556,318, Barnes et al., dis-closes a spectroanalytical system which includes induc-tion coupled plasma apparatus for exciting sample mater-ial to an atomic state for analysis.

SUMMARY OF THE INVENTION

A novel atmospheric pressure capactively coupled radio frequency plasma discharge apparatus and method are disclosed. They are suitable for atomic absorption and emission analysis of small~ discrete sample volumes 11-50jul)- The plasma can be operated at very low Radio Fre~uency (RF) input powers (30- 600 W) which allows for optimal conditions for atom resonance line absorption measurements. Sample introduction into the plasma may be done by an electrically heated tantalum strip vaporizer. The vaporization and dissociation-atomization steps may be separately controlled. Analyte absorption takes place in the plasma discharge which is characterized by a long path length(10-50 cm) and low support yas flow rate (0.2 to 6 L/m) both of which provide for a relatively long residence time. The optimum RF power for Ag analysis can be between 100 and 200 W and the optimum support gas flow rate can be about 0.6 L/m. The device exhibits linear calibration plots and provides sensitivities in the range of from 3.5-40 pg.

This invention is also directed to a novel low-power RF plasma torch and sample introduction system which is designed primarily for atomic absorption spec-trometry, but which is also useful for emission spectro-1 3 1 '-1 6 ~
metry. This torch operates at atmospheric pressure at very low support gas flow rates and makes use of capacitive power coupling to form the plasma. Sample introduction into the plasma is accomplished by using an electrically heated tantalum strip. In this way the sample vaporization and atomization steps are separated and can each be inde-pendently optimized. The discharge has a long path length tube geometry which is designed for atomic absorption measurements and this feature, in conjunction with the low support gas flow rates, maximizes anolyte residence time.

The plasma is self-initiating, requiring no ignition system. A capilliary tube of an inside diameter of less than 1.5 mm prevents conduction of the plasma to the tantalum strip vaporizer. In the apparatus, the sample introduction means may be through the arc of cathode sputtering, electrical arcs or sparks, a graphite furnace, or hydride generation.

The invention is directed to a method of generat-ing and sustaining an atmospheric pressure plasma compris-ing utilizing an elongated plasma containing volume having capacitively arranged electrodes enclosing at least a portion of the plasma containing volume, said electrodes being electrically insulated from the plasma, and support-ing the plasma with a flowing support gas.

In the method, the plasma may be operated at radio frequency input powers in the range of about 30 to 600 W. The support gas may be selected from the group consisting of Ar, ~Ie, N2, H2, air and mixtures of these gases. The plasma may be supported with a gas flowing at a rate of about 0.1 L/m and lO L/m.

l 6 3 ,~
With the method, atomic absorption or emission analysis may be conducted on a sample by introducing the sample into the plasma containing volume. The sample may be introduced in the plasma by means of a support gas. The sample may be vaporized by a vaporizer. The vaporizer may be an electrically heated tantalum strip vaporizer. The sample size may be between about l and about 50 L/m~

The invention is also directed to an apparatus for generating and sustaining an atmospheric pressure plasma comprising: (a) elongated means for containing a plasma; (b~ two electrode means connected to a radio freguency power supply, electrically insulated from the plasma, and enclosing at least a portion of the elongated means; and (c) a sample supporting means connected to a DC
power supply and housed in a cool plasma support gas conveying chamber communicating with the elongated plasma containing m~ans.

In the apparatus, the elongated means may be a high melting point electrically insulating material. In the apparatus the elongated means may be a quartz kube.
The two electrodes may be a pair of elongated electrodes positioned on either side of the quartz tube.
The sample supporting means may be a tantalum strip connected to a pair of supports which are positioned within a water cooled jacket. The conveying chamber may have a support gas inlet.
In the apparatus, a sample introduction inlet may be formed in the quartz tube. The quartz tube and the conveying chamber may be connected together in a "T"
configuration, the chamber forming the stem of the "T".

1 3 1 '1 6;`7~() The elongated means for containing the plasma may have thereon fins which extend between the two electrode means to insulate against electrical discharge taking place between the two electrode means.

DRAWINGS

In the drawings, which illustrate a specific embodiment of the invention, but which should not be regarded as limiting or restricting the scope of the invention in any way:

Figure l depicts a schematic diagram of the capacitively coupled plasma and sampling system;
Figure la depicts an end view of the capaci-tively coupled plasma discharge tube;

Figure 2 depicts a schematic block diagram of the experimental arrangement;

Figure 3 depicts a plot of time resolved absorption and emission signals acquired from the capa-citively coupled plasma;
Figure ~ depicts a background emission spec-trum of the capacitively coupled plasma using argon as a support gas;

Figure 5 depicts a background emission spec-trum of the capacitively coupled plasma using helium as a support gas;

Figure 6 depicts the transmittance of argon -capacitively coupled plasma and helium - capacitively coupled plasma over a specified wavelength range;

Figure 7 deplcts the eEfects oE chan~es in radio frequency input power on absorbance of the AgI
328.1 nlr~ line;

Figure 8 depicts the effect of support gas flow rate on AgI 328.1 nm absorbance; and Figure 9 depicts a calibration plot for AgI
328.1 nm.
Figure 10 depicts a perspective view of a configuration of the capacitively coupled plasma and sampling system suitable for mounting in a conventlonal atomic absorption spectrophotometer.
Figures lla and llb depict side and end views respectively of an electrode and tube design wherein the tube has insulating fins between the electrodes to prevent electrical discharge between the electrodes.
DETAILED DESCRIPTION OF A SPECIFIC
EMBODIMENT OF THE INVENTION
.

Descriptlon of the Plasma Torch A schematic diagram of the device 2 is pro-vided in Figure 1. Functionally, the device 2 consists of two par-ts, the capacitively coupled plasma (CCP) discharge tube 4 and the tantalurn strip electrothermal vaporization sample introduction system 6. The main body of both parts is constructed of quartz glass and the two parts are joined through a narrow neck 8 to form a T-shaped device. The plasma 9 is contained in a quartz tube 10 20.0 cm in length and 0.4 cm inside diameter and 0.6 cm outside diameter. Power is coupled into the plasma using two stainless steel strips 12, 18.0 cm long and 0.5 cm wide which are placed on either side of, and in contact with, the outside of this quartz tube 10 (see Figure la for an end view). These stain-1 3 1 '1 6 s~', less steel elect~odes 12 are connected to the RF powersuppl~ (not shown in Figure 1 but see Figure 2). The plasma 9 has been run using a fixed frequency 27.18 MHz RF supp:Ly and also with a 125 - 375 KHz variable frequency RF supply. It has been found that a stable plasma 9 can be sustained at RF powers ranging from 30 -600 w. Plasma support gas ls introduced using an inlet 14 on the side of the main body 6 of the quartz container~ It has been discovered that the discharge will operate at gas flow rates ranging from 0.2 to 6 L/m. The plasma 9 has been sustained using a variety of support gases including ~r, He, and mixtures of these gases with N2, H2, and air. Sample vaporization is accomplished using a tantalum strip 16 which is fastened to two copper rod conductors 18 which are connected to an electrothermal atomizer power supply (not shown).
These electrodes 18 are surrounded by a water cooled jacket 20 with water inlet 22 and water outlet 24.
Samples are placed on the tantalum strip vaporizer 16 through one of two ground glass tapered inlets 26 using a micro-pipette. Sample sizes vary from 1 to 50 ul.

Descri_tion of Experimental Facilities (a) Equipment and Setup. The experimental setup is schematically outlined in Figure 2 and details of the equipment used are provided in Table I below. The CCP
discharge was mounted inside a model PT-2500 torch box.
Two systems were used. With System 1, power was coupled to the CCP 2 by inserting a secondary coil 29 into the normal ICP load coil. The leads from this secondary coil 29 were attached to the two stainless steel strip electrodes 12. With System 2, the stainless steel strip electrodes 12 were connected directly to the output of the RF generator 30. The CCP was run using both of the power supplies outlined in Table I. However, all of the results disclosed herein were collected using System 1.
A plasma ignition system (tesla coil) was not re~uired 1 3 1 ~r 6 ) t) since it was found that the CCP automatically ignites upon application of approx. 100 W RF power.

A 25 cm focal length fused silica lens 31 was used to focus the hollow cathode lamp (HCL~ 32 at the middle o~ the CCP tube (50 cm object distance) and a 10 cm focal length fused silica lens 34 was used to image the HCL and CCP onto the entrance slit of the monochromator 36 with object and image distances of 27 and 17 cm respectively. A stainless steel plate with a 0.4 cm hole (not shown) to cut down on the amount of un-absorbed HCL
radiation and to reduce the plasma background reaching the entrance slit was placed at each end of the CCP dis-charge. Both absorption by, and emission from, analytein the CCP discharge could be simultaneously measured, monitoring the output from both the preamplifier 38 and the lock-in-amplifier 40.

(b) Analytical Procedure. All absorption measurements were carried out using the following procedure. A 2 -5 ul aqueous sample was placed on the tantalum strip 16 through the inlet port on the side of the quartz body.
The plasma was off at this stage. The sample was dried and ashed. The plasma was then ignited at the end of the ash stage. The sample was then atomized. Data was collected through the atomize cycle. After each atomize cycle the atomizer was tested for a memory effect.

(c) Standard Solutions. All analytical standards were prepared using Fisher 1000 ppm atomic absorption stan-dards. The solutions were diluted to volume using 1%
HNO3. A 1% HNO2 solution was used as the reagent blank.

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TABLE I.

Experimental Facilities and Operatinq Conditions Plasma Power Supply System 1. Perkin-Elmer ICP 5500 System consisting of a Plasma~
Therm (Kreeson, N.J.), HFP-2500F
RF generator, AMN-2500E automa-tic matching network, APCS-3 automatic power control system and PT - 2500 torch box.
System 2. ENI Power Systems Inc.
(Rochester, N.Y.) Model HPG-2 RF
Power Supply. Frequency: 125 K~lz - 375KHz., Output Power: 0 - 200 W.

Sample Vaporization Tantalum strip 1.5 cm by 0.5 cm with a depression at the center.
Power: Varian Model C~A-61. Nor-mal operting cycle: Dry - 105C
for 60s, Ash - 300 - 600C for 15s, Atomize - 2000 - 3700C for 2s.
Spectrometer Schoeffel-McPherson ¦Acton, MA) Model 27G, 0.35 m Czerny-Turner mount scanning monochromator with 600 rulings/mm holographic gra-ting. Reciprocal linear disper-sion of 40 A/mm in the first order.

Slits Entrance and exit slits set to 50 um.

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Hollow Cathode Lamps Hollow cathode lamps (HCL) were powered using a home-built, elec-tronically modulated power supply.
Modulation frequency - 250 Hz and a duty cycle of 50%. ~ormal oper-ating currents were used for the lamps.

Detector Electronics The photocurrent from a Hammatsu R955 photomultiplier tube was am-plified by a home-built preampli-fier and fed to a Princeton Applied Research Model 121 Lock-In-Amplifier. The photomultiplier tube was powered using a McPherson Model EU 42A PMT powe supplyO

Data Acquisition Digital data acquisition was car-ried out using a Tulsa Computers (Owasso, OK~ Telex Model 1280 IBM-At compatible computer equipped with an RC Electronics (Santa Barbara, CA) Model ISC-16 analog-digital converter running the RC
Computerscope software package.
Analog data was acquired using a Servocorder 210 chart recorder.

Results We have noted that Ar-CCP can be generated as soon as the RF power is applied to the discharge tube.
The plasma is light blue in color and fills the discharge tube 10 but does not enter the vaporization chamber nor is any arcing observed between the plasma - and the tantalum strip 16. ~he plasma appears stable without any observable flicker or modulation and fills the discharge tube unlformly. At support gas flow rates less than 4 L/m the plasma is contained inside the discharge tube 10, but at support gas flow rates exceeding 4 L/m a small plasma jet can be seen emanating from each end of the discharge tube. When a sample containing a relatively high concentration of Li is vaporized, a red band of Li emission can be observed to move from the junction of the T down the length of each branch of the discharge tube 10. The device 2 has also been found to operate with no difficulty on pure He and on Ar-H2, Ar-N2, Ar-air, He-H2, He-N2, and He-air mixtures. The addition of H2, N2, and air to the argon support gas allows for the adjustment of excitation conditions in the plasma and permits the discharge to provide either an inert, oxidizing, or reducing environment. It is anticipated that this feature will prove to be very useful for the future application of the device to different sample types.
For example, a reducing environment can be created by using an Ar-H2 mixture. This should help to control the formation of refractory oxides in the discharge.

Typical time resolved absorption and emission ~5 signals acquired from the CCP are provided in Figure 3.
To record these signals, 0.1 ng of Ag was introduced on-to the tantalum strip 16, vaporized into the Ar plasma discharge and emission and absorption Eor the AgI 328.1 nm line measured. The power used was 125 W and the sup-port gas Flow rate was 0.6 L/m. The origin on the timeaxis, which is marked in units of ms, corresponds to the beginning of the atomization cycle. The vertical (sig-nal) axis is in arbitrary units. The apparent noise on the emission signal is from the modulated hollow cathode lamp. The signals start to appear after about 0.6 s and persist for about 0.6 s following first appearance. The 1 3 1 ~ 6 -)fi un~erlying backgrond is relatively flat ~or both absor-ption and emission and is not appreciably affected by the vaporlzation step.

Background emission from the Ar-CCP and He-CCP
recorded over the wavelength range 200-450 nm at an RF
power of 200 W are provided in Figures 4 and 5 respec-tively. In both of these plasmas, the main spectral features are OH emission in the 280~285 nm and 302-317 10 nm regions and NO emission in the 215-272 nm region.
The transmittance of the Ar-CCP and He-CCP over the wavelength range 200-380 nm at an RF power of 200W ls recorded in Figure 6. This was recorded using a D2 lamp and measuring the broadband ~ T at 10 nm intervals.
The transmittance decreases with an increase in wave-length for both plasmas. For the He-CCP the transmit-tance is greater than 95% and for the Ar-CCP it is greater than 85% over this wavelength range.

An iron atom excitation temperature was measured using a method previously described in the literature ~M.W. Blades and B.L. Caughlinr Spectrochim.
Acta, 1985, 40B, 579). A section oE iron wire was introduced into the plasma at the junction of the "T" to provide a source of iron atoms. The collection optics were set up to image the center of the discharge onto the entrance slit of the monochromator. Emission ~rom a set of seven FeI lines in the region 370-385 nm covering an energy range from 27000 to 35000 cm~l were used for this measurement. The lines used were the same as those which were outlined in the M.W. Blades et al.
reference above. A Schoeffel-McPherson (Acton, MA) Model 2061 l-meter monochromator equipped with a linear photodiode array was used to carry out the measurement.
The complete system has been described elsewhere (see Z.H. Walker and M.W. Blades, Spectrochim. Acta, 1986, 1 3 I ~ 6 "!j 41B, 761). The temperature was measured at an RF input power o~ 400W and a support gas flow rate of 0.6L/m~ A
linear regression slope temperature indicated a temperature of 3960 ~ 300K at this power. One of the co- inventors, with another, has previously measured FeI
excitation temperatures for a low-flow, low-power ICP
system and found a temperature of 4000K at an RF power of 400W (see L.L. Burton and M.W. Blades, Appl.
Spectrosc., 1986, 40, 265). Also, for the ICP, the temperature was found to have a roughly linear relation-ship with power. An extrapolation to 100 W suggests that the temperature at this power should be on the order oE 3000-3500K, in the same range as that found in N2O acetylene flames.
The effect of changes in RF input power on absorbence of the AgI 328.1 nm line was studied at a support gas flow rate of 0.6 L/m. The results over the power range 50-500 W are provided in Figure 7. The optimal RF power Eor this line was found to be between 100 and 200 W. At a power of 50 W the absorbence drops to 0 and at powers higher than 200 W the absorbence decreases steadily until at 400 W the absorbence is near 0. At the low end of the power scale, it is suspected that the formation of undissociated gas phase molecules reduces the sensitivity, and at the high end, the formation of Ag ions reduces the sensitivity.

The effect of variation in support gas flow rate on the absorbence signal for the AgI 328.1 nm line at an RF power of 150 W is depicted in Figure 8. The support gas has a dual role to play in the operation of the CCP discharge. First, it acts to carry the analyte vapor into the discharge tube and second, it supports the discharge itself. Therefore the response of analyte absorption is affected both by changes in transport Ll 6 ~ ,) rate, and hence residence time, and also by changes in the nature oE the plasma as the support gas flow rate changes. From Figure 8 it can be seen that the maximum absorbence is obtained at a support gas flow rate of 0.
L/m. At higher and lower support gas flow rates, the absorbence decreases.

To check on analytical performance, a support gas flow rate of 0.6 L/m and an RF input power of 150 W
were chosen as the working conditions. An absorbence calibration plot from 0 to 1 ng for the AgI 328.1 nm line is provided in Figure 9. The CCP device exhibits good linearity over the concentration range 0 - 10 ng total analyte. A listing of 0.0044 absorbence unit sensitivities is provided in Table II below Eor Ag, Cd, Cu, Li, and Sb for the plasma system described in this disclosure and for conventional graphite furnace AAS
(see C.W. Fuller, "Electrothermal Atomization for Atomic Absorption Spectrometry", Analytical Sciences Monograph, the Chemical Society, London (1977)). It can be seen that the sensitivities for the CCP are in the range of from 3.5 to 40 pg depending on the element involved and are comparable to those obtained with a graphite furnace. The foregoing are preliminary results run under the compromise conditions outlined above, but nevertheless demonstrate the viability and effectiveness of the invention. It is expected that these values can be improved substantially by conducting a full optimiza-tion study.
The inventors believe that the atmospheric pressure capacitively coupled plasma described herein is a new atom reservoir and source for carrying out ele-mental analysis using atomic absorption and emission spectroscopy. It has been designed for the analysis oE
small sample volumes of a size typically analyzed using 1 3 1 ''1`63~i furnace atomic absorption. ~owever, it is also possible to introduce dried aerosol or hydrides through the sup-port gas inlet when continuous sample introduction is desired. The plasma discharge tube and sample intro-duction device allows for the separate control of thevaporization and atomization environments. This new spectrochemical source has a long absorption path length which provides extended analyte residence times when compared with a graphite furnace. As demonstrated, the plasma can be operated at very low support gas flow rates which further enhances the analyte residence time.
Also, since the analyte is embedded in a plasma environ-ment, vapor phase condensation is not a problem. Pre-liminary results of temperature measurement yield a value of around 4000K. At this temperature, potential chemical interferences should be minimized. The ability to operate on a variety of pure support gases and gas mixtures permits the atomization environment to be made inert, reducing, or oxidizing as the analysis situation demands.

Figure lO depicts a perspective view of a configuration of the capacitively coupled plasma and sampling system suitable for mounting in a conventional atomic absorption spectrophotometer.

Figures lla and llb depict side and end views respectively of an electrode and tube design wherein the tube has insulating fins between the electrodes to pre-vent electrical discharge between the electrodes. Thefins 50 extend radially from the tube 52 between the electrodes 54 so that they effectively block any elec-trical discharge between the opposing electrodes 54.

1 3 1 '1 6 ") TABLE II.

Sensitivies and Wavelenqths for Aq, Cd, Cu, Li, and Sb.

5 Element Wav~length (nm) Sensitivity (pg) CCP-AAS GF~AAS*
Ag 328.1 10 5 Cd 228.8 3.5 Cu 324.8 40 30 10 Li 670.8 23 10 Sb 217.6 24 20 .

* C.W. Fuller, "Electrothermal Atomization for Atomic Absorption Spectrometry", Analytical Sciences Monograph, The Chemical Society~ London (1977).

As will be apparent to those skilled in the art, in the light of the foregoing disclosure, many al-terations and modifications are possible in the practiceof this invention without departing from the spirit or scope thereof. Accordingly, the scope of the invention is to be construed in accordance with the substance de-fined by the following claims.

Claims (22)

1. A method of conducting atomic absorption or emission analysis on a sample by vaporizing the sample in a manner so that it is introduced into a gas-supported atmospheric pressure plasma comprising utilizing a chamber as a plasma-containing container, an elongated quartz first chamber of hollow circular cross-section, the chamber having on opposed radial sides thereof, with a space therebetween, a pair of capacitively arranged elongated curved electrodes enclosing at least a portion of the length of the hollow cylindrical plasma containing chamber, the curves of said electrodes being concentrically oriented with the curve of the exterior of the elongated quartz first chamber, said electrodes being connected to a radio frequency generator and electrically insulated from the plasma by the chamber, vaporizing a sample in a second chamber and introducing the sample and a plasma support gas into the central internal region of the first chamber so that the support gas flows from each end of the first chamber, and conducting atomic absorption or emission analysis on the vaporized sample in the plasma.

2. A method as defined in claim 1 wherein the plasma is operated at radio frequency input powers in the range of 30 to 600 W.

3. A method as defined in claim 1 wherein the support gas is selected from the group consisting of Ar, He, N2, H2, air and mixtures of these gases.

4. A method as defined in claim 3 wherein the plasma is supported with a gas flowing at a rate of about 0.1 L/m and 10 L/m.

5. A method as defined in claim 4 wherein the vaporized sample is conveyed by the support gas into the plasma in the first chamber.

6. A method as defined in claim 5 wherein the sample is vaporized by a vaporizer.

7. A method as defined in claim 6 wherein the vaporizer is an electrically heated tantalum strip vapor-izer .

8. A method as defined in claim 7 wherein the sample size is between about 1 and about 50 µ1.

9. A method of generating free and excited atoms for atomic spectroscopy comprising:
(a) generating a capacitively coupled atmospheric pressure plasma in an elongated electrically insulated first chamber having separated electrically conducting electrodes on opposed sides of the exterior of the chamber, the electrodes being connected to a source of RF power;
(b) atomizing a sample in a second chamber connected to the first chamber;
(c) introducing the atomized sample into the central internal region of the first chamber by introducing a plasma support gas into the second chamber, which sub-sequently flows with the atomized sample into the first chamber, and (d) maintaining the atmospheric pressure plasma in the first chamber so that it escapes from each end of the first chamber thereby resulting in a long residence time of atomized atoms in the first chamber.

10. A method of generating free and excited atoms for atomic spectroscopy in an atmospheric pressure capacitively coupled plasma comprising:

(a) generating a capacitively coupled atmospheric pressure plasma in the top hollow bar of a "T" shaped hollow container, the hollow bar of the "T" having capaci-tively opposed separated electrodes on either side thereof, the electrodes being connected to an RF power source;
(b) analyte atomizing in the hollow stem of the "T" shaped hollow container by an electrothermal vaporizer a sample containing atoms for atomic spectroscopy investi-gation, controlling the evaporation of the atomic analyte, and passing the atomized analyte into the plasma in the top hollow bar; and (c) supporting the atmospheric pressure generated plasma in the hollow bar of the "T" shaped torch with a low plasma support gas flowing through the hollow bar of the "T" shaped torch.

11. An apparatus for conducting atomic absorption or emission analysis on a vaporized sample in an atmospheric pressure capacitively generated plasma comprising:
(a) elongated hollow means constructed of an electrically insulating material for containing a plasma discharge;
(b) two elongated separated electrodes connected to a radio frequency power supply, the elongated separated electrodes being electrically insulated from the plasma, the two elongated separated electrodes being positioned to be in axial alignment with the elongated hollow means and generally opposite one another on opposite sides of the exterior of the elongated hollow means;
(c) means for supporting and vaporizing a sample to be analyzed;
(d) means for conveying a plasma support gas and a vaporized sample into the central region of the interior of the elongated hollow means; and (e) analysis means for conducting atomic absorp-tion or emission analysis on the vaporized sample.

12. An apparatus as defined in claim 11 wherein the elongated means is a high melting point electrically insulating material.

13. An apparatus as defined in claim 11 wherein the elongated means is a quartz tube.

14. An apparatus as defined in claim 13 wherein the two electrodes are a pair of curved elongated electrodes positioned on either side of the quartz tube.

15. An apparatus as defined in claim 14 wherein the sample supporting means is a tantalum strip connected to a pair of supports which are positioned within a water cooled jacket.

16. An apparatus as defined in claim 15 wherein a second chamber is connected to the hollow means and the second chamber has a plasma support gas inlet.

17. An apparatus as defined in claim 16 wherein a sample introduction inlet is formed in the quartz tube, and is connected to the second chamber.

18. An apparatus as defined in claim 17 wherein the quartz tube and the second chamber are connected together in a "T" configuration, the tube forming the top bar of the "T" and the second chamber forming the stem of the "T".

19. An apparatus as defined in claim 11, 12 or 13 wherein the elongated means for containing the plasma has thereon fins which extend between the two electrode means to insulate against electrical discharge taking place between the two electrode means around the outside of the elongated means.

20. An apparatus for enabling atomic absorption or emission analysis to be conducted on a vaporized sample in an atmospheric pressure capacitively generated plasma which comprises:
(a) an elongated open-ended hollow quartz tube having curved sides and a generally circular cross-section adapted for containing the plasma;
(b) a pair of curved stainless steel elongated electrodes connected to a radio frequency power supply, the electrodes being separated from one another and being positioned in alignment with and on opposed elongated sides of the quartz tube;
(c) a tantalum sample support means for enabling a sample to be vaporized by applying electrical current from an electrothermal atomizer power supply to the tantalum sample support means;
(d) means for extracting heat from the tantalum sample support;
(e) means for enabling the vaporized sample to enter and commingle with the plasma contained in the quartz tube at a mid-point in the plasma;
(f) an opening in the quartz tube proximate to means (e) for enabling plasma support gas to be conveyed to and support the plasma, and to exit from each side of the tube;
(g) a furnace power supply connected through an rf filter to the tantalum sample support means; and (h) means for conducting atomic absorption or emission analysis on the vaporized sample in the plasma.

21. An apparatus as defined in claim 20 wherein the quartz tube is closed at each end with transparent quartz windows and openings are located proximate to each end of the quartz tube for enabling plasma support gas to be introduced through one opening and withdrawn through the other opening.

22