Plasma discharge apparatus with temperature sensing

A plasma discharge apparatus has a first electrode, a counter electrode, and a power source for supplying power to the first and counter electrodes for initiating and sustaining plasma discharge therebetween. An isolated heater is provided for heating only the first electrode. An isolated temperature sensor is provided for sensing the temperature of the first electrode, and a temperature controller is provided, responsive to the output of the temperature sensor, for controlling the heater to control the temperature of the first electrode.

BACKGROUND OF THE INVENTION 
The instant invention is related to an atmospheric pressure discharge 
system for generating a plasma discharge for exciting samples to be 
analyzed by optical emission spectrometry. 
Previously, such plasma discharge systems have included microwave-induced 
plasma and inductively coupled plasma. However, these systems have 
drawbacks, set forth below, which the instant invention overcomes. 
In detecting elements in air, water or solid samples, for example, for use 
in environmental research or other types of research, such 
microwave-induced plasma (MIP) systems have been well known for producing 
a plasma discharge to excite samples to be analyzed by known optical 
emission spectrometry systems. Such MIP systems have also been known as 
single electrode plasma torch systems. In using an MIP system, elements 
may be detected, however, such systems require a magnetron tube operating 
at a frequency of 2,450 MHz for example. 
The MIP system can use helium as a carrier gas and thus can detect 
non-metals. However, the employment of the magnetron tube also requires a 
high voltage of from 1000 V to 2000 V to operate the magnatron tube. 
Additionally, it should be noted that the MIP system requires a high flow 
rate for the gas flowing through it. A flow rate of 3 to 4 liters/min. is 
required to properly cool the electrodes. A sufficient exhaust velocity of 
gas from the plasma chamber is necessary in order to prevent leakage of 
surrounding air into the plasma chamber, thus causing inaccuracies in the 
detection of the emission spectra. 
Another system for generating a plasma discharge is the inductively coupled 
plasma (ICP). The ICP system uses only Argon for a carrier gas and thus 
cannot be used for detecting non-metals, because a helium discharge is 
essential for detecting non-metals. The ICP system has a relatively large 
gas consumption and requires extremely large equipment for operation. 
The above systems, single electrode plasma torch (MIP) and ICP, of 
providing a plasma discharge for optical emission spectrometry require 
large amounts of equipment for performing the necessary analysis. Such 
large equipment demands makes the use of the above systems incompatible 
with taking field measurements in environmental research or the like. 
SUMMARY OF THE INVENTION 
Accordingly, a new excitation source for generating a plasma discharge for 
optical emission spectrometry has been developed which overcomes the 
drawbacks of the above described systems. The excitation source of the 
instant invention can be used to detect non-metal elements in a simple and 
relatively small apparatus, and thus allow the identification of organic 
compounds from the elemental detection technique. In the instant 
invention, trace levels of non-metal elements such as oxygen, hydrogen, 
carbon, nitrogen, chlorine and fluorine, as well as other elements, can be 
accurately detected. 
The instant invention includes a heating means to heat one of the 
electrodes in the plasma discharge tube. By heating the electrode in the 
plasma discharge tube, the electron density in the discharge gap is 
increased. This likewise increases the sensitivity of the apparatus of the 
instant invention. Because the electrode of the discharge tube is 
maintained at a high temperature, the discharge tube can also operate with 
a relatively low supply voltage. A further advantage is that the supply of 
gas to the plasma chamber occurs under atmospheric pressure. Therefore, 
the equipment employing the instant invention becomes simple, small, and 
light, and thus is easily adapted to field use. 
The heated electrode emits electrons as a function of the temperature of 
the electrode and of the electrode material. The most common and preferred 
electrode material for such an electrode is platinum, although other 
materials may be used. Platinum is preferred because it is a chemically 
stable material. 
For analytical purposes, it is important for the electrode to have a 
constant temperature. Thus, the invention also includes a controller for 
controlling the electrode temperature. By controlling the temperature of 
the electrode to remain constant, the excitation energy remains constant 
and thus the ratio of the emission spectra intensity and concentration of 
each element to be analyzed remain constant. Accordingly, the accuracy of 
measurements can be improved. 
The instant invention is therefore directed to a plasma discharge apparatus 
having a first electrode, a counter electrode, and a power source for 
supplying power to the first and counter electrodes for initiating and 
sustaining plasma discharge therebetween. An isolated is provided for 
heating only the first electrode. An isolated temperature sensor is 
provided for sensing the temperature of only the first electrode, and a 
temperature controller is provided, responsive to the output of the 
temperature sensor, for controlling the heating means to control the 
temperature of the first electrode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The plasma discharge tube of this invention is illustrated in 
cross-sectional form in FIG. 1. The plasma discharge tube is comprised of 
a heated electrode 12 and a spherical counter electrode 14. Each electrode 
is connected by leads to the power source, as illustrated in FIG. 4, and 
as will be discussed below. Heated electrode 12 is preferably a platinum 
filament electrode having leads 16 extending through and supported by 
tubing 20. The tubing 20 is an insulating tubing and preferably formed of 
alumina tubing or other similar type of tubing. A recess 22 is formed in 
the upper portion of tubing 20 in order to accommodate the heated 
electrode 12 therein. A thermocouple 24 (for example, a 
platinum-platinum/rhodium thermocouple) is attached directly to the 
filament of the heated electrode 12. The leads 26 of the thermocouple 24 
also extend through the tubing 20 and are connected to a temperature 
controlling circuit 64 (described below). The counter electrode 14 is 
preferably a platinum spherical electrode (for example 3 mm in diameter) 
which is supported by tube 28 (preferably an alumina tubing or the like) 
having lead 18 extending therethrough and connecting with a power source. 
The filament of the heated electrode, as positioned in recess 22 might have 
a preferable outside diameter of 0.4 mm and a length of 3.5 mm. In order 
to accommodate the electrode, recess 22 might be provided with a depth of 
5 mm and an inside diameter of 4.5 mm. 
The electrodes are encased within a quartz envelope 30 having rubber 
stoppers or the like at each end thereof. An intake tube 34 is provided 
for admitting the sample gas and helium mixture into the interior of the 
plasma discharge tube 10. Exhaust tube 36 is provided for exhausting the 
sample gas and helium (or other type of carrier gas) from the plasma 
discharge tube 10. In the illustration in FIG. 1, end plates 38 are 
provided to support the rubber stoppers 32 and electrodes therebetween. 
Mounting bolts 40 are provided to secure end plates 38 to be fixed with 
respect to one another. 
FIG. 2 is a perspective view of the heated electrode 12 positioned inside 
of the recess 22 of tubing 20. As can also be seen in FIG. 2, the 
thermocouple 24 is attached directly to the filament of heated electrode 
12. Leads 16 and 26 extend through the tubing 20. 
Accordingly, FIG. 3 is a perspective view of the spherical counter 
electrode 14 supported by tubing 28. While a spherical electrode is the 
preferable form of electrode to be used in this application, other shapes 
of electrodes may be used. Lead 18 extends from counter electrode 14 
through tubing 28 to the power source described in detail below. 
FIG. 4 discloses the plasma discharge tube connected in a discharge system 
including a power supply. The plasma discharge tube 10 is illustrated with 
heated electrode 12 and counter electrode 14 inside of the quartz envelope 
30 (or quartz window). Thermocouple 24 is attached to the heated electrode 
12, and an intake tube 34 is provided into the quartz envelope 30, while 
an exhaust tube 36 exhausts the sample gas and helium mixture from the 
quartz envelope 30. 
Power for this system may be provided from a standard 117 volt AC (60 Hz) 
system. Of course, power from systems with different voltage and frequency 
may also be used. The 117 volt AC power is supplied to input leads 42 and 
44 which are each attached to an opposite side of variable transformers 50 
and 52. Step-up transformer 48, the primary coil of which is connected to 
variable transformer 52, provides supply voltage to the electrodes to 
initiate and sustain the plasma discharge therebetween. For example, in 
FIG. 4, step-up transformer 48 has terminals 54 and 56 on the outputs of 
the secondary winding. A voltage of 117 volts is provided to the primary 
winding, and for example, a voltage of a maximum of 600 volts is output 
from the secondary winding, with a current of about 50 mA. Of course, 
other voltages may be provided as necessitated by the specific 
requirements of the plasma discharge tube used. 
The output from terminal 56 of step-up transformer 48 is connected to a 
choke coil 58 and thereafter to a resistor 60. The choke coil 58 may 
typically have a value of 3 Henry, while resistor 60 may typically have a 
value of 2 K.OMEGA.. Resistor 60 is then connected to lead 18 of counter 
electrode 14. The other side of the secondary winding of step-up 
transformer 48 is connected from terminal 54 to terminal 62 of the 
secondary winding of step-down transformer 46. Terminals 62 and 64 of 
step-down transformer 46 are connected to leads 16 of heated electrode 12. 
The primary coil of step-down transformer has an input of 117 volts, the 
secondary coil has an output of substantially 3-6 volts, with a maximum 
current of 10 amperes. Of course, as with step-up transformer 48, the 
output values of step-down transformer 46 may be adjusted to meet the 
particular requirements of the plasma discharge tube used. 
Step-up transformer 48 is therefore provided to initiate and sustain the 
plasma discharge. Step-down transformer 46 is provided to heat the heated 
electrode 12 to the proper temperature for emitting electrons to increase 
the electron density needed to sustain the plasma discharge. Because the 
current from the step-up transformer 48 also flows through heated 
electrode 12, affecting the temperature thereof, it is necessary to 
provide some means in which to keep the temperature of the heated 
electrode 12 at a constant level. It should be noted that the temperature 
of the heated electrode can also be affected by the amount of gas flowing 
past it. Accordingly, thermocouple 24 is attached directly to the filament 
of heated electrode 12, and the leads of thermocouple 24 are further 
connected to a temperature controlling means which in turn controls the 
variable transformer 50 in order to control step-down transformer 46 and 
thus regulate the current to heated electrode 12, and therefore to control 
the temperature thereof. 
In FIG. 4, the temperature controlling circuit is generally indicated 
within the dotted line 63. The two leads of thermocouple 24 are connected 
to the inputs of amplifier 66. One output of amplifier 66 is input 
directly to power amplifier 68. A temperature setting circuit 70 includes 
a power source 72 and a potentiometer 74 connected in parallel. The tap of 
the potentiometer 74 is connected to an input of power amplifier 68. The 
outputs of power amplifier 68 control motor 76, which in turn controls the 
tap of variable transformer 50 to control the amount of current flowing 
through, and therefore the temperature of, heated electrode 12. 
Temperature controlling circuit 63 is only one possible method of 
controlling the temperature of the heated electrode 12. It is contemplated 
that a microcomputer or other electronic circuits may be used to control 
the temperature of the heated electrode 12. 
FIG. 5 is a graph illustrating the effect of electrode temperature upon the 
break-down voltage of the plasma discharge. Of course, the break-down 
voltage is the voltage required to initiate the plasma discharge itself. 
The graph of FIG. 5 illustrates the break-down required for gaps between 
the heated electrode 12 and the counter electrode 14 of 3, 4 and 5 mm. As 
clearly seen, in each case, as the electrode temperature increases, the 
break-down voltage decreases. Thus, if temperature is allowed to 
fluctuate, the breakdown voltage will also fluctuate. This changes the 
electron density which is important for excitation and providing a 
constant emission intensity. 
FIG. 6 is a graph illustrating the influence of heated electrode 
temperature on the impedance of discharge. As set forth above, it is very 
important to maintain the proper temperature of the electrode for proper 
analysis of the sample. FIG. 6 illustrates how, as the temperature of the 
heated electrode 12 increases, the impedance of discharge decreases. 
Additionally, the lower impedance of discharge means the higher electron 
density of discharge and the higher electron density of plasma have more 
energy to excite the element to be analyzed. Thus, it is important to 
maintain the temperature of the heated electrode in order to promote a 
stable discharge to have reproducible results. 
FIG. 7 shows a graph illustrating the effect of supply voltage on the 
emission intensity of hydrogen, for example. For detecting hydrogen in a 
sample, it is important for the relative spectral emission intensity from 
the plasma discharge to be as large as possible, in order to promote 
accurate readings. FIG. 7 illustrates that as the supply voltage to the 
electrodes increases so does the relative spectral emission intensity. 
FIG. 7 also illustrates that as the electrode temperature of the heated 
electrode is increased, a significant increase in the relative spectral 
emission intensity can be found. Therefore, it is important to note that 
since the emission intensity is also a function of the electrode 
temperature, the electrode temperature must be maintained at a constant 
value in order to maintain the relative spectral emission intensity at a 
constant value, in order to insure accuracy of results. 
In operation, a sample is mixed with helium, or other type of carrier gas 
and input to the quartz envelope 30 through intake tube 34. Such sampling 
techniques are well known, and may be used for taking solid samples, 
liquid samples, or gas samples, such as air. Solid samples are often 
heated and the elements to be analyzed are therefore vaporized and mixed 
with a carrier gas, such as helium. In a liquid sample, the water may be 
removed by freeze drying or other process, and then the resulting powder 
can be heated as the solid sample above. In analyzing a gas sample, such 
as air, the gas can simply be introduced into the quartz envelope 30. If 
required, known methods of pre-concentration can be used. 
The sample gas with helium mixture may be input to intake tube 34 under 
atmospheric pressure. There is no requirement to have high pressure gas, 
or to have low pressure gas flowing through this system. Further, the flow 
rates of the gas itself may be determined by the sizes of intake tube 34 
and exhaust tube 36. The instant invention only requires a flow rate of 
around 0.8 liters/min. It has been found that when the exiting gas has a 
velocity of about 5 m/sec. or greater there is no problem of leakage of 
the atmosphere back into the plasma chamber. Additionally, it should be 
noted that separate elements present in a solid sample (for example) can 
be separated off by changing the level of heating of the sample. Since 
different elements to be analyzed have different boiling points, different 
elements may be detected by heating the sample to the boiling point 
corresponding to the specific element. The apparatus of the instant 
invention can be used not only for element detection, but also for 
detecting the relative ratio of constituent elements. 
Power is supplied to step-down transformer 46 in the form of 117 volts to 
the primary coil, and the voltage output from the secondary coil to the 
heated electrode 12 is generally from 3-6 volts, with a maximum current of 
about 10 amperes. The electric discharge in the atmospheric pressure 
helium carrier gas is created by a 60 Hz, 550-600 volt, 40-50 mA power 
supply. The discharge current is provided to electrodes 12 and 14 through 
the step-up transformer 48, variable transformer 52, and a discharge 
current stabilizing circuit which includes choke coil 58 and resistor 60. 
It should be noted that a known "neon transformer" may be used in place of 
the discharge current stabilizing circuit. "Neon transformers" are well 
known and provide a high voltage until the discharge begins, and then the 
voltage declines according to the needs of the circuit. 
An important feature of the excitation source of the instant plasma 
discharge apparatus is the use of the heated filament electrode and 
monitoring its temperature. The heated platinum electrode 12 emits 
thermionic electrons which conform to the Richardson-Dushman's expression: 
EQU I=AT.sup.2 e.sup.-w/kT 
where, I is the thermionic current density (A/Cm.sup.2), w is the work 
function of the emitter and T is the temperature (.degree.K.) of the 
emitter. A is a constant equal to 4.pi.mek.sup.2 /h.sup.3, where k is 
Boltzmann's constant, h is Planck's constant, m is the mass of an 
electron, and e is the charge of an electron. The number of emitted 
electrons in the plasma depends on the electrode temperature and the work 
function of the emitter. In this device, the electron density of the 
plasma can be controlled by changes in the platinum filament temperature 
and the electrode supply voltage. 
The high electron density plasma of the heated electrode of this invention 
has many advantages: (1) low break-down-voltage; (2) stable high current 
arc discharge; (3) helium is easily ionized and can readily form a plasma; 
(4) the helium plasma is able to excite non-metal elements by the Penning 
effect. 
As illustrated in FIG. 4, the spectral emission from the plasma discharge 
of the plasma discharge tube 10 is detected by an optical emission 
spectrometer 61. Such optical emission spectrometers are well known in the 
art. For example, the following table illustrates detection limits for 
several elements detectable by the instant invention in a 60 Hz discharge 
system: 
______________________________________ 
DETECTION DETECTED 
LIMIT STANDARD WAVE- 
ELEMENT (.times.10.sup.-9 g/sec.) 
COMPOUND LENGTH 
______________________________________ 
Oxygen 65 H.sub.2 O 777.2 nm 
Hydrogen 4.9 H.sub.2 O 656.3 nm 
Carbon 0.27 CCl.sub.2 F.sub.2 
247.5 nm 
Chlorine 1.4 CCl.sub.2 F.sub.2 
725.7 nm 
Fluorine 0.97 CCl.sub.2 F.sub.2 
685.6 nm 
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Although a specific form of embodiment of the instant invention has been 
described above and illustrated in the accompanying drawings in order to 
be more clearly understood, the above description is made by way of 
example and not as a limitation to the scope of the instant invention. It 
is contemplated that various modifications apparent to one of ordinary 
skill in the art could be made without departing from the scope of the 
invention which is to be determined by the following claims.