WO2001065244A1

WO2001065244A1 - Microwave plasma monitoring system for real-time elemental analysis

Info

Publication number: WO2001065244A1
Application number: PCT/US2001/005092
Authority: WO
Inventors: Yixiang Duan
Original assignee: The Regents Of The University Of California
Priority date: 2000-03-02
Filing date: 2001-02-15
Publication date: 2001-09-07
Also published as: AU2001241527A1; WO2001065244A9

Abstract

Ambient air is analyzed in a microwave induced palsma without use of an additional carrier gas. An apparatus is disclosed for analyzing ambient air, other sample gas, or nebulized and desolvated liquids wherein the arrangement of plasma gas and sample gas conduits (100, 106, 110) is used to enhance dependability of the plasma. The plasma torch (118) has a concentric arrangement of plasma gas and sample gas conduits (100, 106, 110) so as to provide a sheath of plasma gas both within and surrounding the flow of analyte into the plasma region (104). The microwave plasma torch (118) can either be contained within a sealed housing or can be operated in ambient air at ambient pressures. The microwave plasma torch (118) is portable and can be operated continuously for real-time analysis of air.

Description

MICROWAVE PLASMA MONITORING SYSTEM

FOR REAL-TIME ELEMENTAL ANALYSIS

This application claims the benefit of U.S. Provisional Application No. 60/186,428,

filed March 2, 2000, and U.S. Provisional Application No. 60/186,458, filed March 2,

2000.

TECHNICAL FIELD

This invention relates to a method and apparatus for elemental analysis. This

invention relates more particularly to a method and apparatus with microwave plasma

source for monitoring of air and liquid streams for elemental contaminants including

transition metals, rare earth elements, actinides, or alkali and alkaline earth elements.

This invention was made with government support under Contract No. W-7405-

ENG-36 awarded by the U.S. Department of Energy. The government has certain rights

in the invention.

BACKGROUND ART

Because of increasing global efforts to protect the environment and increasing

concerns for worker safety, there is increasing need for capability for real-time

monitoring of air and liquid streams for the presence of transition metals, rare earth elements, actinides, or alkali and alkaline earth elements. There is a continuing and

increasing need for sensitive, real-time, portable monitoring devices for detection of the

presence of elements in industrial waste streams.

In one particular example, beryllium is extracted from beryl ore and converted to

beryllium hydroxide for the production of metal alloys, oxides, ceramics, and pure

beryllium for use in various industries and military applications. Because of increased

regulatory activity and awareness of health hazards during extraction and production of

beryllium and during industrial and commercial use of beryllium products, it is important

to be able to accurately evaluate workplace explosures in a timely manner.

Several methods of monitoring air for the presence of beryllium or other trace

elements have been developed.

High volumes of air for 'sampling can be drawn through filters at high flow rates.

After ranges of time from minutes to hours, the filters can be taken off location for

laboratory analysis.

Inductive plasma spectrometry has been used to analyze elements from a plasma

produced by application of an electromagnetic field to a plasma gas; see e.g., U.S. Patent

4,844,612 (Durr and Rozain, July 4, 1989).

Microwave plasmas have been used for trace element monitoring. Microwave

plasmas sustained in a portion of an undiluted furnace exhaust flow have been used for

continuous emission monitoring for trace metals in furnace exhaust by atomic emission spectroscopy. These waveguide devices are constructed of refractory materials

compatible with high-temperature environments and are mountable inside the furnace

exhaust ducts. See, e.g., U.S. Patent 5,671,045 (Woskov, et al., September 23, 1997)

which is a continuation of U.S. Patent 5,479,254.

Fused quartz fiber optics in close proximity to a plasma flame have been used to

transmit UV through visible emissions to three spectrometers for simultaneous

monitoring of several metals. This is disclosed in Woskov, P. P., D. Y. Rhee, P. Thomas

and D. R. Cohn, "Microwave plasma continuous emissions monitor for trace-metals in

furnace exhaust," Rev. Sci. Instrum., 67 (10), October 1996, American Institute of

Physics).

Several methods of monitoring liquid streams have been disclosed in the

literature. These include introduction of vapors generated from liquids to be sampled into

inductively coupled plasma source atomic emission spectrometers for analysis.

However, there is still a need for sensitive, real-time, portable monitoring devices

for detection of the presence of elements in ambient air and liquid streams and a need for

economical, energy-efficient monitoring devices.

Therefore, it is an object of this invention to provide an apparatus and method for

analysis of air or liquid streams for presence of various elements. It is another object of this invention to provide such an apparatus and method

particularly for sensitive, real-time, on-site monitoring of air for the presence of

beryllium.

It is a further object of this invention to provide a reliable, low-energy microwave

plasma monitoring apparatus and method.

Additional objects, advantages and novel features of the invention will be set forth

in part in the description which follows, and in part will become apparent to those skilled

in the art upon examination of the following or may be learned by practice of the

invention. The objects and advantages of the invention may be realized and attained by

means of the instrumentalities and combinations particularly pointed out in the appended

claims which are intended to cover all changes and modifications within the spirit and

scope thereof. '

DISCLOSURE OF INVENTION

To achieve the foregoing and other objects, and in accordance with the purposes

of the present invention, as embodied and broadly described herein, there has been

invented a process for analyzing ambient air or other gas samples, in a microwave

powered plasma torch without use of an additional carrier gas. There has also been

invented an apparatus for analyzing ambient air, other gas samples, or nebulized and

desolvated liquids wherein a novel arrangement of plasma gas and sample gas conduits is used to enhance dependability of the plasma. This apparatus embodiment of the

invention has a concentric arrangement of plasma gas and sample gas conduits so as to

provide a sheath of plasma gas both within and on the outside of the flow of sample air

into the plasma region.

The multiple conduit apparatus of the invention comprises:

(a) a sample gas conduit positioned to conduct a flow of a sample gas to a

plasma region;

(b) a first plasma gas conduit coaxially positioned within the sample gas

conduit;

(c) a second plasma gas conduit positioned such that the sample gas conduit is

within the second plasma gas conduit;

(d) a microwave energy source sufficient to generate a plasma in the plasma

region;

(e) a microwave transmitter connecting said microwave energy source to the

second plasma gas conduit; and

(f) analytical instrumentation connected to receive signals from the plasma

region.

The microwave plasma torch can either be contained within a sealed housing or

can be operated in ambient air at ambient pressures. The microwave plasma torch of this invention can be operated continuously for

real-time analysis of air.

The apparatuses and methods of the present invention can be used wherever there

is a need for monitoring air for the presence of minor amounts of elements, particularly

transition metals, rare earth elements, actinides, and alkali and alkaline earth elements.

The invention apparatus can also be used to monitor for the presence of halogens, sulfur

and silicon. The invention apparatuses and methods are more particularly useful for

monitoring air for the presence of beryllium.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part of the

specification, illustrate embodiments of the present invention and, together with the

description, serve to explain the principles of the invention. In the drawings:

Figure 1 is a schematic of an example invention device wherein no additional carrier gas is needed for transport of the sample to the plasma region; and

Figure 2 is a schematic of an example of an invention device having three

concentrically arranged conduits to permit the sample gas flow into the plasma region to

be sheathed by two plasma gas flows.

Figure 3 is a flow chart showing process flow for use of the invention to monitor a

liquid stream. BEST MODES FOR CARRYING OUT THE INVENTION

It has been discovered that ambient air can be sampled and analyzed for presence

of transition metals, rare earth elements, actinides, or alkali and alkaline earth elements

without use of additional sample carrier gases. This can be done using a microwave

powered plasma torch with appropriately arranged conduits for the plasma gas and

sample gas (generally ambient air) using equipment small enough to be easily portable.

It is understood that the microwave plasma torch can be operated in either an

upright position with the plasma region above the gas conduits, in a sideways position

with the plasma region laterally positioned with respect to the gas conduits, or in an

upside down position with the plasma region below the gas conduits. An upright position

is generally presently preferred, and for convenience of description, the invention

appratus is shown in the drawings and described as being in the upright position, with

differences in equipment or arrangement of elements for other positions described when a

different position of the microwave plasma torch would make a difference. The terms

"above", "below", "upper", "lower", "at the top" and "at the bottom" will be used with the

understanding that an upright position has been selected merely for convenience of

description, not as a limitation. Figure 1 is a schematic drawing of an apparatus which can be used to analyze air

for the presence of elemental contaminants in a microwave powered plasma torch without

use of additional sample carrier gas. With reference to Figure 1, a plasma torch 10 is

positioned within a sealed housing 12 with a seal housing outlet conduit \Λ. Use of a

sealed housing L2 is useful when the sample gas (the air or other gas containing the

analyte to be monitored) is to be sucked or drawn through the microwave plasma torch K)

from the bottom to the top where the plasma region _16 is located. A pump j_8 is

positioned on a sealed housing outlet conduit 14 so as to provide a negative pressure

within the sealed housing 2 sufficient to draw the sample gas or plasma gas or both

through the device.

A sealed housing 2 can also be useful if the invention apparatus is to be used in a

harsh environment. Generally, *when a positive pressure of sample gas and plasma gas is

to be pumped through the apparatus from below, a sealed housing is not necessary.

Whether the pump is positioned downstream or upstream of the plasma torch, any

pump capable of maintaining the desired flow rates and which does not impede or

interfere with transport of the elements or particles contained within the sample gas can

be used.

The sample gas to be tested is circulated through the sample gas conduit 20 into the plasma torch K). The sample gas conduit 20 has an open upper end 22 where, in the

embodiment of the invention depicted in Figure 1 , the negative pressure in the sealed housing L2 causes the sample gas to circulate upward into the plasma region 16.

The sample gas conduit 20 does not necessarily have to be made of an electrically

conductive material, but it should be relatively inert to the air or other gas to be sampled.

Suitable materials include, but are not limited to, copper, brass, aluminum, alumina,

stainless steel, silver, gold, and mixtures and alloys thereof, and quartz and ceramic

materials.

The flow rate of the sample gas can be controlled by any suitable means such as a

flowmeter in the sealed housing outlet conduit j4 or, in the case of a positive pressure of

sample gas being introduced at the bottom of the plasma torch, a flowmeter in the conduit

20 through which the sample gas is being transported to the plasma torch 10. A flow rate

in the range from greater than 0 to about 10 liters per minute can be used, depending upon

the shape of the plasma desired, the kind of plasma gas employed, the size of the

conduits, the sensitivity desired, and the particle density in the sample gas. A sample gas

flow rate in the range from about 0.05 liter per minute to about 4.0 liters per minute is

generally presently preferred. The sample gas and the plasma gas can be introduced

simultaneously at different rates according to which flow rate ratios maximize stability of

the plasma and provide the amount of sample gas needed for accurate measurement of

elements therein. If desired, the sample gas to be monitored or tested can be pretreated as needed.

The example in Figure 1 depicts a dessicating system which can be incorporated in the

sample gas conduit 20 to dry the sample gas before introduction of the sample gas into

the plasma region |6. As shown in Figure 1, a membraneous conduit 24 is put into the

sample gas conduit 20 so that the sample gas to be tested is circulated through the

membraneous conduit 24. The membraneous conduit 24 is encased in a larger conduit 26

or molecular sieve which is filled with particles of a dessicant 28 such as calcium

carbonate, copper sulfate, or any suitable commercially available dessicant such as

Dryrite™.

Other methods of drying the sample gas, such as placing two or more commercial

dryers in series in the sample gas conduit, can be employed as needed.

If there is a very low concentration of the element to be monitored in the sample

gas, then it may be desired to concentrate the sample gas by using impact, electrostatic or

ultrasonic concentrators to obtain more accurate detection and measurement of the

element in the sample gas.

Any suitable means for delivering the plasma gas into the plasma formation area

can be used. Pressurized plasma gas can be released into the plasma gas conduit, or

plasma gas can be pumped into the plasma gas conduit.

The flow rate of the plasma gas can be controlled by any suitable means such as a

flowmeter in the conduit transporting the plasma gas into the plasma torch. A flow rate in the range from about 0.1 liter per minute to about 10 liters per minute can be used,

depending upon the shape of the plasma desired, the kind of plasma gas employed, the

size of the conduits, the sensitivity desired, and the analyte density in the sample gas. A

plasma gas flow rate in the range from about 0.05 liter per minute to about 4.0 liters per

minute is generally presently preferred. The sample gas and the plasma gas can be

introduced simultaneously at different rates according to which flow rate adjustments

maximize stability of the plasma and provide the amount of sample gas needed for

accurate measurement of elements therein.

In the embodiment of the invention shown in Figure 1, the plasma gas is

introduced into a plasma gas conduit 30 which is larger than and coaxially positioned

with respect to the portion of the sample gas conduit 20 in the plasma torch 10 so as to

provide a plasma gas cavity or anulus 34 between the outer wall of the sample gas

conduit 20 and the inner wall of the plasma gas conduit 30. The plasma gas conduit 30

has an inlet end 36 into which the plasma gas is introduced and an open upper outlet end

38 through which the plasma gas exits the plasma gas conduit 30 and enters the plasma

region 16.

The sample gas and plasma gas conduits can be any convenient length from only

2 cm long to as much as 200 cm long, depending upon space within which the plasma

torch is to be operated and the extent of portability desired. Generally, for operating the

plasma torch at 1/4 or 3/4 wavelength, a length from about 5 cm to about 15 cm is useful. Alternatively, in another embodiment of the invention, a conduit transporting the

plasma gas has a smaller diameter than the sample gas conduit and coaxially positioned

with respect to the sample gas conduit so as to form a sample gas cavity in the annulus

between the outer wall of the plasma gas conduit and the inner wall of the sample gas conduit.

The outermost conduit (which is the plasma gas conduit 30 in Figure 1 or the

sample gas conduit in the alternative in which the conduit transporting the plasma gas is

the one with the smaller diameter) must be made of an electrically conductive material

which is compatible with microwave energy transmission. Suitable materials include, but

are not limited to, copper, brass, alumina, stainless steel, silver, gold, and mixtures and

alloys thereof.

The innermost conduit (the sample gas conduit 20 in Figure 1 or the plasma gas

conduit in the alternative in which the plasma gas conduit is inside the sample gas

conduit) can be made of any suitable conducting or nonconducting material, including,

but not limited to, copper, brass, aluminum, alumina, stainless steel, silver, gold, and

mixtures or alloys thereof, or quartz or ceramic materials. Ceramic materials are useful

for withstanding erosion by sample gases.

Any gas which will form a plasma when excited by microwave energy, ignited,

then sustained by microwave energy, can be used as the plasma gas. Useful plasma gases

include, but are not limited to, argon, helium, nitrogen, or air, depending upon the sensitivity of monitoring needed and upon which analytes are being monitored in the

sample gas. Depending upon what kinds of analyzing or monitoring instruments are

used, it is desirable to select a plasma gas which will not produce background signals that

will make the analyte signals difficult to detect or read. For example, argon is generally

most useful when the sample gas is air which is being monitored for trace amounts of

beryllium because of the plasma temperatures and helium is generally most useful as a

plasma gas when the sample gas is air which is being monitored for trace amounts of

silicon because helium provides very high excitation energies.

The plasma gas can be pumped through the plasma gas conduit inlet 36 into the

plasma gas anulus 34 with a pump pushing it into or by simply relying upon the negative

pressure created in the sealed housing 2 to pull the plasma gas into the plasma region _16.

However, generally the plasma 'gas is introduced into the plasma gas conduit 30 from a

pressurized cylinder (not shown) and thus is under positive pressure as it enters the

plasma gas conduit 30 through the plasma gas conduit inlet 36.

In an alternative to the embodiment of the invention shown in Figure 1, at least

one hole (not shown) near the base of the sample gas conduit 20 can be used to allow a

portion of the plasma gas from the plasma gas conduit 30 to be admitted to the sample

gas conduit 20 so as to mix with the sample gas for transport up the sample gas conduit

20 to the plasma region. The positive pressure of the plasma gas from a pressurized

container, if greater than the sample gas pressure, can be used to accomplish intermingling of the plasma gas with the sample gas. Otherwise, pressure of the sample

gas flowing upward in the sample gas conduit 20 can pull plasma gas into the sample gas

conduit 20 by a Venturi effect.

Proximate to the upper end 22 of the sample gas conduit 20 and the upper end 38

of the plasma gas conduit 30, a collar 40 of electrically conductive material is positioned

around the plasma gas conduit 30. One or more refractory spacers are positioned between

the collar 40 and plasma gas conduit 30. The collar 40 needs to be close enough to the

upper end 22 of the plasma gas conduit 30 to permit effective delivery of microwave

energy to the plasma region 16 and to avoid setting up interferences in the microwaves

traveling longitudinally in the plasma torch 10.

In the embodiment of the invention shown in Figure 1, the plasma gas conduit 30

serves as the conductance tube for the microwave energy. A moveable microwave

reflector 42 is moveably, or more particularly, slidably, connected to the plasma gas

conduit 30, proximate to the lower end of the plasma gas conduit 30 so that the length of

the conductance tube between the collar 40 and the microwave reflector 42 can be

adjusted to a length equal to an odd number of quarter fractions of the wavelength. For

example, the torch can be operated successfully with a conductance tube length equal to

one fourth, three fourths, one and a fourth, one and three fourths, . . . of the wavelength of

microwave energy being used. It is generally presently preferred to use conductance tube

lengths precisely at the selected wave-length increments for better performance. One- quarter wavelength conductance tube lengths are presently preferred for enabling smaller

instrument size.

Any suitable device or method can be used to move the microwave reflector 42

along the length of the plasma gas conduit 30; for example, as shown in Figure 1, a screw

44 mounted on a base 46 can be used.

A microwave transmitter 48 through which microwave energy can be transmitted

connects a microwave source 50 to the conductive collar 40. The transmitter passes

through an opening 52 in the housing if housing is used.

The microwave transmitter 48 can be either a coaxial cable, waveguide, or other

suitable equipment for transmitting the microwaves from the microwave source 50 to the

collar 40.

Any suitable microwave¹ source which can provide microwave energy in the

appropriate range can be used in the invention apparatuses. Generally useful microwave

sources include, but are not limited to, microwave oscillators, magnetrons, or klystron

generators. Presently preferred as a microwave source is a magnetron because of the

wide range of amounts of power which can be produced.

The invention apparatus can be operated using low levels of microwave energy,

depending upon what degree of sensitivity is needed, selected flow rates of plasma and

sample gases, choice of plasma gas and kind of analyte being monitored. The energy

source can be operated in pulsed or continuous wave mode. Energy levels in the range from about 20 to about 300 Watts can be used. Generally presently preferred are energy

levels in the range from about 100 to about 300 Watts when air is being monitored for

beryllium and argon is used as the plasma gas. Monitoring of samples from liquid

streams which have been nebulized and entrained in carrier gas has been successfully

conducted operating the plasma torch in the range from about 50W to about 200 W.

Appropriate adjustment of power to the torch can minimize reflected power, which is

typically less than 10W during operation.

An electrical semiconductor cooling chip or other suitable cooling device can be

used to take heat away from the plasma torch, thereby enabling operation at either high or

low power levels.

Microwaves in the range from about 1000 MHz to about 10,000 MHz can be used

to activate and sustain the plasma. Presently preferred are microwaves in the range from

about 2000 MHz to about 3000 MHz, and presently most preferred is a 2450 MHz

frequency.

The plasma is initiated by contact of the plasma gas in the plasma region 16 with

a sufficient amount of ignition energy to provide a seed electron. The initiation of the

plasma can be accomplished by any suitable means such as contacting the plasma gas in

the plasma region \6 with energy from a tesla coil, a laser, UV radiation, or an electrical

spark. In Figure 1 , the plasma initiator 54 has connected thereto an initiator energy

conductor 56 which extends through the wall of the sealed housing 2 into the plasma

region 16. Depending upon the type of plasma initiator used, the initiator energy

conductor can be simply an electrically conductive wire, or any other suitable means of

getting energy into the plasma region.

The plasma region L6 is generally in a naturally occurring flame shape or toroidal

shape situated above the upper aperture of the plasma gas conduit 30 because the

microwave energy is highly focused there.

A chimney 58 may be used to shield the plasma region J_6 from air currents or

outside gas and to contain and stabilize the plasma. The chimney 58 may be made of any

material which can withstand the plasma temperature and is transparent to the spectral

wavelength of the analytes being monitored. For example, the chimney may be made of

quartz, glass or gallium. Alternatively, if desired, a metal screen can be used to shield the

plasma.

The light in the plasma region \6 is focused and collected by any suitable

system, depending upon the system set up, what type of signal transmitter is used, and

what analysis equipment is used. Various combinations and configurations of optical

focusing lenses, optical filters, waveguides, and/or optical fibers can be employed. For

example, a collimating lens can be used to direct light from a side-on view of the plasma region into an optical fiber. In a presently preferred arrangement, a double lens focusing

is used to enlarge the beam collection angle.

Any of several types of transmitter can be used to transmit signals from the

plasma region to the analysis instrument. What is the most suitable transmitting

equipment will depend upon the choice of analytical instrumentation and the type,

wavelengths and intensity of the signal to be transmitted.

Generally useful for transmitting signals from the plasma region to an atomic

emission spectrometer, monochronometer with a photomultiplier detector or other

analytical equipment are focusing lens and fiber optic lines. Alternatively, a light pipe

can be used for directing light into a spectrometer. Electrically conductive wires

connected to electrodes on either side of the plasma region can be used for transmitting a

signal from the plasma region to a potentiometer.

A single signal transmitting line is usually sufficient, particularly when sampling

for only a single element or when sequential sampling capability is desired. However, in

some particular cases, a plurality of transmission lines or other devices for transmission

of the signals from the plasma region to the analyzing equipment can be used if needed

for a particular monitoring application. For example, atomic and ionic spectral lines have

different optimum observation heights and need to be monitored at different positions.

Also, elements with significant differences in chemical and physical properties may need

to be monitored using different equipment. The light from the plasma region \6 is analyzed by any suitable means, depending

upon the spectral wavelengths of the analytes and the desired speed of analysis. An

atomic emission spectrometer with a photomultiplier tube or a charge-coupled detector

(CCD) or mass spectrometer can be used. Generally presently preferred when analyzing

air for presence of trace amounts of beryllium is an integrated small spectrometer with a

CCD detector or a monochronometer with a photomultiplier tube detector. In another

generally presently preferred embodiment, when using argon plasma gas to monitor a

liquid stream for presence of beryllium, a dual channel spectrometer is used for optical

beam dispersal and signal measurement and a linear CCD array detector with 2048

element pixels is used for instantaneous measurements of analyte signals.

When it is already known what the analytes are and it is desired simply to

measure concentrations on a production basis, a potentiometer which measures electrical

resistance also could be used for signal analysis.

In the example embodiment of the invention shown in Figure 1, three analytical

instruments 60, 62, and 64 receive signals from the plasma region 16 through three fiber

optic lines 66, 68, and 70 which each have one end proximate to the quartz glass chimney

58 and are each connected by a distal end to one of the instruments 60, 62, and 64. Or, if

no chimney 58 is used, each of the signal transmitting lines are positioned proximate to

the plasma region _16. Any suitable computer control systems can be used for flow meter controls and

for processing data from the analytical instruments. Portable computing devices are

particularly useful when the invention apparatus is to be used for in-situ sampling or to be

transported to multiple sites.

Commercially available software can be used. For example, an OOIBAS 32-bit

software package can be used for all data acquisition and control and to provide a real¬

time interface to the signal processing function. A notebook computer with a 100 kHz

sampling frequency DAQ-700 card can be used to show a real time spectrum on its screen

and to store data. A CBL-2-NI interface cable can be used to make a connection between

the spectrometer and the computer.

The invention device can be operated continuously for as long a period of time as

desired with continuous readouts or recording of elements and amounts thereof in the

sample gas being circulated through the plasma torch.

It is generally sometimes preferred to not use a carrier gas for the sample to be

tested because use of the carrier gas would dilute the concentration of the analytes in the

air or other sample gas. However, a carrier gas can be used. This is sometimes done in

the embodiment of the invention in which the sample gas is sheathed by a plasma

envelope in order to maintain the energy density in the plasma region and to help stabilize

the plasma. Any suitable carrier gas which is stable in monoatomic form and has potential for

high energy in a plasma such as argon, helium, neon, xenon and krypton can be used.

Also, diatomic nitrogen can also be used as the carrier gas. A mixture of different carrier

gases can be used.

Generally it is better to use the same gas used as the plasma gas as a carrier gas

because the spectrum is simpler than that obtained with multiple gases.

When a carrier gas is used, it is mixed with the sample gas prior to or during

introduction of the sample gas into the sample gas conduit. The ratio of sample gas to

carrier gas selected will depend upon the plasma gas used, analyte concentration, and the

sensitivity required.

In another embodiment of the invention, a flow of the gas (usually air) to be tested

or monitored is conducted through the apparatus in such a manner as to be shielded or

enveloped by an active plasma on two sides of the sample gas entering the plasma region.

This can be done, for example, using three co-axially arranged tubes or conduits as shown

in Figure 2.

A centermost first conduit 100 transports plasma gas to the center 102 of a plasma

region 104 where it is in intimate contact with the air or gas to be sampled. The air or

other gas to be sampled is transported to the plasma region 104 through a second conduit 106 having a larger diameter than the centermost first conduit 100 and coaxially

positioned with respect to the centermost first conduit 100 so as to form an anulus 108 between the first conduit 100 and the second (sample gas) conduit 106. The sample gas

conduit 106 is generally longer than the centermost, first conduit 100, with the end of the

sample gas conduit 106 extending upward beyond the end of the first conduit 100. When

the sample gas conduit 106 longer than the first plasma gas conduit 100, there is some

mixing of the plasma gas from the first conduit 100 with the sample gas from the sample

gas conduit 106 prior to entry of the plasma gas and sample gas into the plasma region

104.

In an alternative embodiment, if desired, the innermost central conduit 100

carrying plasma gas can have a plurality of openings 109 arranged about the

circumference near the lower end of the conduit 100 for the purpose of allowing plasma

gas from the conduit 100 to be mixed with the sample gas in the sample gas conduit 106.

The second (sample gas) conduit 106 is in turn surrounded by a third (plasma)

conduit 110, which is larger in diameter than the second conduit 106 and coaxially

arranged with respect to the second conduit 106 so as to form an anulus 112 between the

outer wall of the second conduit 106 and the inner wall of the third conduit 110. The

third conduit 110 is generally the same length as the second conduit 106 so that the upper

ends of both are at approximately the same level. A second portion of plasma gas is

transported through the annulus 112 to the plasma region 104 where it forms an outer

sheath 114 about the air or other sample gas flow 116 into the plasma region 104. The triple-conduit plasma torch T8 of this embodiment of the invention can be

otherwise set up and operated in a manner similar to that in which the above-described

embodiments are set up and operated. Any suitable conventional microwave power

source 120 is connected to a conductive collar 122 using a microwave transmitter 124 or

any other suitable connector. The conductive collar 122 is positioned on the outer wall of

the outermost third conduit 110 near enough the upper ends of the three conduits to

permit excitation of the plasma proximate to the end where the plasma region 104 is to

form. There is a refractory spacer between the conductive collar 122 and the outer wall

of the third conduit 110. As in the other embodiments of this invention, it is generally

advantageous to have the upper ends of the plasma gas conduits 106 and 110 aligned

(extending to the same height) with each other. The sample gas conduit 106 is not

necessarily aligned with the plasma gas conduits 106 and 110, and is generally somewhat

longer than the plasma gas conduits 106 and 1]0 to allow some mixing of the sample gas

and plasma gas. Microwaves from the microwave power source 120 are reflected by a

microwave reflector 126 located near the lower end of the three conduits at a distance

from the conductive collar 122 equal to an odd number of quarters of the wavelength

being used for effective microwave energy resonance. The microwave reflector 126 is

mounted upon a base 128, such as, for example, a screw 130, in such a manner as to

permit adjustment of the distance between the collar 122 and the microwave reflector 126

by moving the microwave reflector 126 longitudinally on the three conduits. The plasma is initiated as described for other embodiments of the invention using

an initiator energy source 132 and an initiator energy conductor 134 which extends into

the plasma region 104.

Instrumentation 136 for analyzing signals from the plasma region 104 is

positioned so as to receive signals through, in this particular example, a fiber optic cable

138.

Choices of microwave power and wavelengths, plasma gas, plasma and sample

gas flow rates, and analytical instrumentation of the types described for the first

embodiments of the invention are used with this multiple-plasma-gas conduit

embodiment of the invention.

When it is desired to use any of the invention embodiments for monitoring and

analysis of liquid streams, a sample portion of the liquid is nebulized and/or desolvated to

obtain an aerosol or gas sample. For monitoring and testing, the aerosol or gas sample is

then processed in the manner described above.

Figure 3 is a flow chart for one particular example of use of the invention to

monitor and analyze a liquid stream. Samplings from standard solutions for calibration

or from streams are transported into a nebulizer using a peristaltic or other suitable pump

or a flow injection valve. The liquid sample is first nebulized in any convenient nebulizer

such as an ultrasonic nebulizer to produce an aerosol. The ultrasonic nebulizer can be

cooled using semiconductor chips rather than traditional water chiller methods. Then, in a second subsequent step, the aerosol is desolvated in a two-step desolvation process with

a membrane desolvator as a first stage and a sieve sorbent device as the second to further

dry the sample. The effectiveness of desolvation steps can be enhanced by ^"pre-heating

the aerosal. The sample is then transported to the plasma torch which is configured and

operated in accordance with any of the above-described invention embodiments.

Alternatively, the nebulized sample can be transported with a carrier gas directly to the

plasma torch without further drying.

In the particular example depicted in the flow chart of Figure 3, a portion of

plasma gas is combined with the nebulized sample as a carrier gas prior to further drying

of the sample gas. That provides for an intimate mixture of sample gas and plasma gas in

the plasma region.

The invention device is portable and can be operated in a continuous operation

mode, with a microwave plasma which can be sustained at approximately atmospheric

pressure. Carrier gases, although not necessary for air monitoring, can be used as needed

to deliver the samples, which can be air or gas samples, including aerosols, and airborne

particulates, through the sample gas conduit into the microwave plasma where they can

be analyzed in any of a variety of ways. Low power microwave plasmas can be used.

The invention instrument is less complex than other presently used air monitoring

systems, and can be more compact and convenient for on-site, real-time analysis. While the apparatuses and methods of this invention have been described in detail

for the purpose of illustration, the inventive apparatuses and methods are not to be

construed as limited thereby. This patent is intended to cover all changes and

modifications within the spirit and scope thereof.

INDUSTRIAL APPLICABILITY

The apparatus and method of this invention can be used wherever there is a need

for monitoring air for presence of minor amounts of elements, particularly transition

metals, rare earth elements, actinides, and alkali and alkaline earth elements.

Claims

WHAT IS CLAIMED IS:

1. An apparatus for elemental analysis comprising:

(a) a sample gas conduit positioned to conduct a flow of a sample gas

to a plasma region;

(b) a first plasma gas conduit coaxially positioned within said sample

gas conduit;

(c) a second plasma gas conduit positioned such that said sample gas

conduit is within said second plasma gas conduit;

(d) a microwave energy source sufficient to generate a plasma in the

plasma region;

(e) a micro Wave transmitter connecting said microwave energy source

to said second plasma gas conduit; and

(f) analytical instrumentation connected to receive signals from the

plasma region.

2. The apparatus recited in Claim 1 further comprising a sealed housing.

3. The apparatus recited in Claim 2 wherein said sealed housing has an outlet

conduit with a pump positioned thereon so as to provide a negative pressure within said

sealed housing.

4. The apparatus recited in Claim 1 wherein said sample gas conduit is made

from a material selected from the group of copper, brass, aluminum, alumina, stainless

steel, and mixtures and alloys thereof, and quartz and ceramic materials.

5. The apparatus recited in Claim 1 further comprising a flowmeter in said

sample gas conduit.

6. The apparatus recited in Claim 1 further comprising a flowmeter in said

first plasma gas conduit and a flowmeter in said second plasma gas conduit.

7. The apparatus recited in Claim 1 wherein each of said sample gas conduit,

said first plasma gas conduit and said second plasma gas conduit are in the range from

about 2 cm to 200 cm in length.

8. The apparatus recited in Claim 7 wherein said sample gas conduit is longer

than said first plasma gas conduit.

9. The apparatus recited in Claim 7 wherein said sample gas conduit is

substantially the same length as said second plasma gas conduit.

10. The apparatus recited in Claim 1 wherein said first plasma gas conduit is

made from a material selected from the group of copper, brass, aluminum, alumina,

materials.

11. The apparatus recited in Claim 1 wherein said second plasma gas conduit is

stainless steel, silver, gold, and inixtures and alloys thereof, and quartz and ceramic

materials.

12. The apparatus recited in Claim 1 wherein said sample gas conduit has at

least one hole on its circumference near the base of said sample gas conduit connecting

said sample gas conduit with the annulus between said sample gas conduit and said

second plasma gas conduit.

13. The apparatus recited in Claim 1 wherein said first plasma gas conduit has

at least one hole on its circumference near the base of said first plasma gas conduit

connecting said first plasma gas conduit with the annulus between said first plasma gas

conduit and said sample gas conduit.

14. The apparatus recited in Claim 1 further comprising an electrically

conductive collar surrounding the outer surface of a first end of said second plasma gas

conduit, said collar connected to said microwave transmitter so as to direct microwaves

longitudinally in said second plasma gas conduit.

15. The apparatus recited in Claim 1 further comprising a moveable microwave

reflector moveably connected to said second plasma gas conduit proximate a second end

of said second plasma gas conduit.

16. The apparatus recited in Claim 1 wherein said microwave transmitter is a

coaxial cable.

17. The apparatus recited in Claim 1 wherein said microwave source is a

magnetron.

18. The apparatus recited in Claim 1 wherein said microwave source outputs

microwave energy in the range from about 20 to about 300 Watts.

19. The apparatus recited in Claim 1 wherein said microwave source outputs

microwave energy in the range from about 1000 MHz to about 10,000 MHz.

20. The apparatus recited in Claim 1 further comprising a plasma initiator and

an initiator energy conductor positioned so as to initiate a plasma in said plasma region.

21. The apparatus recited in Claim 1 further comprising a cooling device

positioned and connected so as to take heat away from said apparatus.

22. The apparatus recited in Claim 21 wherein said cooling device is an

electrical semiconductor cooling chip.

23. The apparatus recited in Claim 1 further comprising a chimney positioned

to shield said plasma region.

24. The apparatus recited in Claim 1 further comprising a metal screen

positioned to shield the plasma region.

25. The apparatus recited in Claim 1 further comprising a light collection

system positioned to receive light from the plasma region.

26. The apparatus recited in Claim 25 wherein said light collection system

comprises a collimating lens.

27. The apparatus recited in Claim 25 wherein said light collection system

comprises a double lens focusing apparatus.

28. The apparatus recited in Claim 1 wherein said analytical instrumentation is

selected from the group of: atomic emission spectrometers, monochronometers with

photomultiplier detectors and mass spectrometers.

29. The apparatus recited in Claim 1 wherein said analytical instrumentation

comprises a plurality of analytical instruments.

30. The apparatus recited in Claim 1 further comprising a nebulizer positioned

to process liquid samples before entry of the sample into said sample gas conduit.

31. The apparatus recited in Claim 30 further comprising a desolvation device

positioned to process sample aerosol output from said nebulizer and before entry of the

sample gas into said sample gas conduit.

32. A method of monitoring for presence of elements in a sample gas, said

method comprising:

(a) conducting a portion of said sample gas without an additional carrier gas

to a plasma region;

(b) conducting a portion of a plasma gas to said plasma region;

(c) energizing said plasma region to form a plasma; and

(d) analyzing spectral images emanating from said plasma.

33. The method recitedin Claim 32 wherein said plasma region is energized

using microwave energy.

34. A method of monitoring for presence of elements in a sample gas, said

method comprising:

(a) conducting a first portion of a plasma gas to a plasma region;

(b) conducting a portion of a sample gas to a location in said plasma

region outside said first portion of plasma gas; (c) conducting a second portion of a plasma gas to a location in said

plasma region outside said portion of said sample gas;

(d) energizing said plasma region to form a plasma; and

(e) analyzing spectral images emanating from said plasma.

35. The method recited in Claim 34 wherein said first portion of plasma gas

and said second portion of plasma gas are the same kind of gas.

36. The method recited in Claim 34 wherein said first portion of plasma gas

and said second portion of plasma gas are different kinds of gas.

37. The method recited in Claim 34 wherein said first portion of plasma gas

and said second portion of plasma gas are selected from the group of: argon, helium,

nitrogen and air.

38. The method recited in Claim 34 wherein said sample gas contains a

nebulized aerosol from a liquid solution.

39. The method recited in Claim 34 wherein said sample gas contains a

desolvated aerosol.

40. The method recited in Claim 34 wherein said sample gas is monitored for

an element or mixture of elements from the group of: transition metals, rare earth

elements, actinides, alkali eath elements, alkaline earth elements, sulfur, halogens,

silicon.

41. The method recited in Claim 40 wherein said sample gas is monitored for

beryllium.

42. The method recited in Claim 34 carried out in ambient air at ambient

pressures.

43. The method recited in Claim 34 carried out at a negative pressure.

44. The method recited in Claim 34 wherein said sample gas is conducted to

said plasma region at a rate in the range from greater than 0 to about 10 liters per minute.

45. The method recited in Claim 34 wherein said sample gas is conducted to

said plasma region at a rate in the range from about 0.005 liter per minute to about 4.0

liters per minute.

46. The method recited in Claim 34 wherein said first portion of said plasma

gas is conducted to said plasma region at a rate in the range from greater than 0 to about

10 liters per minute.

47. The method recited in Claim 34 wherein said first portion of said plasma

gas is conducted to said plasma region at a rate in the range from about 0.05 liter per

minute to about 4.0 liters per minute.

48. The method recited in Claim 44 wherein said sample gas is conducted to

said plasma region at a rate of flow different from the rate of flow of said first portion of

said plasma gas.

49. The method recited in Claim 44 wherein said sample gas is conducted to

said plasma region at a rate of flow different from the rate of flow of said second portion

of said plasma gas.

50. The method recited in Claim 46 wherein the rate of flow of said first

portion of said plasma gas is different from the rate of flow of said second portion of said

plasma gas.

51. The method recited in Claim 34 wherein said sample gas is mixed with a

portion of said first portion of said plasma gas prior to being conducted to said plasma

region.

52. The method recited in Claim 34 wherein said sample gas is mixed with a

portion of said second portion of said plasma gas prior to being conducted to said plasma

region.

53. The method recited in Claim 34 wherein said sample gas flow into said

plasma region is assisted with a carrier gas.

54. The method recited in Claim 53 wherein said carrier gas is one selected

from the group of: argon, helium, neon, xenon, krypton, nitrogen, and mixtures thereof.

55. The method recited in Claim 34 further comprising pretreating said sample

gas.

56. The method recited in Claim 55 wherein said sample gas is pretreated by

drying.

57. The method recited in Claim 56 wherein said sample gas is dried by a

series of more than one drying steps.

58. The method recited in Claim 55 wherein said sample gas is pretreated by

concentration of said sample gas.

59. The method recited in Claim 34 wherein said plasma region is energized

using microwave energy.

60. The method recited in Claim 59 wherein said microwave energy is in the

range from about 20 to about 300 Watts.

61. The method recited in Claim 59 wherein said microwave energy is in the

range from about 1000 MHz to about 10,000 MHz.

62. The method as recited in Claim 34 wherein said spectral images are

analyzed continuously in real-time contemporaneously with burning of sample gas in

plasma.