U.S. patent number RE29,304 [Application Number 05/628,071] was granted by the patent office on 1977-07-12 for plasma light source for spectroscopic investigation.
This patent grant is currently assigned to Albright & Wilson (Mfg.) Limited, Raydne Limited. Invention is credited to Christopher Thomas Berry, Stanley Greenfield, Iorwerth Llewelyn William Jones.
United States Patent |
RE29,304 |
Greenfield , et al. |
July 12, 1977 |
Plasma light source for spectroscopic investigation
Abstract
A plasma of annular form is produced by passing a gas stream
along the axis of a coil carrying high frequency-alternating
current and a sample is injected through a low temperature central
region of the plasma annulus into the tail flame and the resulting
spectrum of the plasma is examined.
Inventors: |
Greenfield; Stanley
(Birmingham, EN), Jones; Iorwerth Llewelyn William
(Birmingham, EN), Berry; Christopher Thomas
(Birmingham, EN) |
Assignee: |
Raydne Limited (Workingham,
EN)
Albright & Wilson (Mfg.) Limited (Oldbury,
EN)
|
Family
ID: |
27515889 |
Appl.
No.: |
05/628,071 |
Filed: |
November 3, 1975 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
404625 |
Oct 19, 1964 |
03467471 |
Sep 16, 1969 |
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Foreign Application Priority Data
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Oct 21, 1963 [UK] |
|
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41456/63 |
Mar 6, 1964 [UK] |
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9535/64 |
Oct 5, 1964 [UK] |
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40487/64 |
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Current U.S.
Class: |
356/316; 356/417;
219/121.51; 313/231.31; 219/121.36; 356/36; 219/121.52 |
Current CPC
Class: |
G01N
21/73 (20130101); H01J 65/048 (20130101) |
Current International
Class: |
H01J
65/04 (20060101); G01N 21/71 (20060101); G01N
21/73 (20060101); G01N 021/54 (); G01N 001/04 ();
G01J 003/28 (); G01J 003/50 () |
Field of
Search: |
;356/36,85,87 ;219/121P
;313/231.3,231.5 ;315/111.1,111.2,111.3 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
"Induction-Coupled Plasma Torch"; Reed; Journal of Applied Physics;
vol. 32, No. 5; May 1961; pp. 821-824. .
"Growth of . . . Crystals . . . Plasma Torch"; Reed; Journal of
Applied Physics; vol. 32; No. 12; Dec. 1961; pp. 2534-2535. .
Plasma Torches; Reed; International Science & Technology; June
1962; pp. 42-48..
|
Primary Examiner: McGraw; Vincent P.
Attorney, Agent or Firm: Flynn & Frishauf
Claims
We claim: .[.1. An apparatus for carrying out spectroscopic
investigation of a sample, the combination comprising: a first
tubular member; a coil surrounding a portion of said first tubular
member and thereby defining a plasma-forming region within said
member; a high frequency generator for connection to said coil,
said coil and its connections being wholly external to said first
tubular member; a second tubular member coaxially arranged within
said first tubular member to form a first cylindrical channel
within said first tubular member; an inlet in said first tubular
member for injecting an insulating gas into said first cylindrical
channel; a third tubular member coaxially arranged within said
second tubular member to form a second cylindrical channel within
said second tubular member, said second and third tubular members
terminating substantially at said plasma-forming region; an inlet
in said second tubular member for injecting a plasma gas into said
second cylindrical channel; an inlet in said third tubular member
for injecting a carrier gas for a sample, whereby the insulating
gas, the plasma-forming gas and the carrier gas flow towards the
region of the coil in three coaxial laminar streams and the plasma
formed by the plasma-forming gas is of annular form and has a tail
flame; and means for passing a sample through one third tubular
member to inject the sample into said plasma-forming region,
whereby said carrier gas carries the sample through a low
temperature
central region of said annulus into said tail flame..]. 2.
Apparatus in accordance with claim .[.1,.]. .Iadd.13, .Iaddend.in
which a capillary tube extends through that end of the third
tubular member which is remote from the coil, said capillary tube
extending substantially to the end of said third tubular member
adjacent the plasma-forming region, and in which the end of the
third tubular member adjacent the plasma-forming region has
a bore of restrictive cross-section to form a jet. 3. Apparatus in
accordance with claim .[.1,.]. .Iadd.13, .Iaddend.in which said
inlets in said first and second tubular members are tangentially
arranged, whereby a rotary motion is imparted to said laminar
streams of insulating gas and
plasma-forming gas. 4. Apparatus in accordance with claim .[.1,.].
.Iadd.13, .Iaddend.in which the coil has at least two turns and the
high frequency generator has an operating frequency lying between 5
and 3000
mc./s. 5. Apparatus in accordance with .[.1,.]. .Iadd.13,
.Iaddend.in which the insulating gas is the same as the gas used
for the plasma
formation. 6. Apparatus in accordance with claim .[.1,.]. .Iadd.13,
.Iaddend.additionally comprising a photoelectric device mounted so
as to be exposed to said plasma-forming region; means for imposing
a cyclic modulation on the amplitude of said high frequency output
of said high frequency generator whereby the output of said
photoelectric device is
correspondingly modulated. 7. Apparatus in accordance with claim 6,
in which the frequency of modulation of the high frequency waveform
is
between 100 and 360 c./s. 8. Apparatus in accordance with claim 6,
additionally comprising means for examining radiation from the gas
plasma downstream of the point at which the sample is introduced,
together with an interrupter for intermittently interrupting the
passage of the said radiation from the gas plasma to the examining
means; and means synchronizing the operation of said interrupter
with the modulation of the
high frequency waveform. 9. Apparatus in accordance with claim 6,
including a limiter circuit for limiting the amplitude of the
modulation
peaks of the high frequency waveform applied to the coil. 10. A
method for the spectroscopic investigation of a sample, comprising:
passing a high frequency alternating current through a coil
surrounding a tubular member to define a plasma-forming region in
said tubular member; passing along the axis of said coil within
said tubular member three coaxial laminar gas flows, an outer
insulating gas flow, an intermediate gas flow of a plasma-forming
gas and an inner gas flow for forming a passage through the plasma
formed by the intermediate gas flow, whereby the plasma is in the
form of an annulus coaxial with the coil and has a tail flame;
maintaining the three coaxial laminar gas flows separate from one
another until they reach the plasma-forming region; introducing
into the plasma-forming region means for initiating the formation
of a plasma therein when the high frequency alternating current is
flowing in said coil; withdrawing the plasma initiating means from
said plasma region once the plasma has been started; introducing a
sample into said inner gas flow and thereby injecting said sample
through a low temperature central region of said annulus into said
tail flame; and examining the spectrum of the plasma downstream of
the point at which said sample was introduced. .Iadd. 11. A method
for the spectroscropic investigation of a sample, comprising:
passing a high frequency alternating current through a coil
surrounding a tubular member to define a plasma-forming region in
said tubular member; forming an annular plasma surrounding a low
temperature substantially central region and a tail flame by:
passing along the axis of said coil within said tubular member
three coaxial laminar gas flows which include an outer insulating
gas flow, an intermediate gas flow of a plasma-forming gas and an
inner gas flow for forming a passage through the plasma formed by
the intermediate gas flow, whereby the plasma is in the form of an
annulus coaxial with the coil and has a tail flame; maintaining the
three coaxial laminar gas flows separate from one another until
they reach the plasma-forming region; introducing into the
plasma-forming region means for initiating the formation of a
plasma in the plasma-forming region when the high frequency
alternate current is flowing in said coil; withdrawing the plasma
initiating means from said plasma region once the plasma has been
started; introducing a sample into said inner gas flow and thereby
injecting said sample through the low temperature central region
surrounded by said annulus into said tail flame; and examining the
spectrum of the plasma downstream of the point at which said sample
was introduced. .Iaddend. .Iadd. 12. A method in accordance with
claim 11, in which the step of forming the annular plasma includes
tangentially introducing the plasma-forming gas and the insulating
gas relative to the axis of said coil and tubular member, to impart
a rotary motion to the laminar streams of plasma-forming gas and
insulating gas. .Iaddend. .Iadd. 13. Apparatus for carrying out
spectroscopic investigation of a sample, comprising:
a first tubular member;
a coil which surrounds a portion of said first tubular member and
defines a plasma-forming region within said first tubular
member;
a high frequency generator coupled to said coil for supplying
electrical energy to said coil at a predetermined frequency and
power, said coil and its connections being wholly external to said
first tubular member;
a second tubular member coaxially arranged within said first
tubular member to form a first cylindrical channel within said
first tubular member;
an inlet in said first tubular member for injecting an insulating
gas at a given rate into said first cylindrical channel;
a third member coaxially arranged within said second tubular member
to form a second cylindrical channel within said second tubular
member, said second and third tubular members terminating
substantially at said plasma-forming region;
means for producing an annular plasma having a low temperature
substantially central region and a tail flame, including:
an inlet in a side wall portion of said second tubular member
remote from said plasma-forming region for injecting a plasma gas
at a given rate into said second cylindrical channel such that said
injected plasma gas flows along said second cylindrical channel
toward said plasma-forming region;
an inlet in a side wall portion of said third tubular member remote
from said plasma-forming region for injecting a carrier gas for a
sample;
said given rates of injection of said insulating gas and plasma
gas, and said predetermined frequency and power of said high
frequency generator being such that: (i) the insulating gas, the
plasma gas and the carrier gas flow in their respective channels
towards the region of the coil in three coaxial laminar streams,
and (ii) the plasma formed by the plasma gas at the plasma-forming
region is of annular form and has a tail flame; and
means for passing a sample through said third tubular member with
said carrier gas to inject the sample into the central region of
said annular plasma in said plasma-forming region, whereby said
carrier gas carries the sample through said low temperature
substantially central region surrounded by said annular plasma and
then into said tail flame. .Iaddend.
.Iadd. 14. Apparatus in accordance with claim 13 further comprising
spectroscopic examination means positioned to determine the
spectrum of said sample in the region of the tail flame. .Iaddend.
Description
This invention relates to the production of a gas plasma and to the
spectroscopic examination of the radiation emitted therefrom, and
may be used for the control of manufacturing processes which
require the analysis of raw material or a product. An example of
such a process is the electric reduction process for making
phosphorus.
It is known to carry out the spectroscopic analysis of materials by
introducing a sample of the material into the hot plasma gas of an
electric arc. This method necessitates the presence of arc-emitting
electrodes which may themselves emit intense characteristic
radiation. In an attempt to separate the plasma from the
confinement of the electrodes and so separate the radiation emitted
by the sample from that emitted by the electrodes, liquid samples
have been atomised, and a jet of the resulting aerosol directed
into the plasma column of a D.C. arc plasma jet. The plasma column
is nevertheless not entirely free from contamination by material
from the electrodes. A further disadvantage of this type of plasma
source is that the electrodes are fairly readily corroded; this
limits the kinds of gas that may be used to form the plasma and
necessitates frequent replacement of the electrodes. The aerosol
injection system, which is only applicable to liquids, including
solutions, admits a very small proportion of the total sample to
the plasma, and in low concentrations, thus reducing the
sensitivity of the apparatus.
We have now discovered a method of avoiding the disadvantages
imposed by an electrode-maintained arc.
According to our invention, for the spectroscopic investigation of
a sample a stream of gas is passed along the axis of an
electrically conductive coil and a high frequency alternating
current is passed through the coil so that the heating of the gas
by the resulting high frequency alternating electric field serves
to maintain a plasma within the gas stream; a sample is introduced
into the gas plasma and the spectrum of the plasma is examined
downstream of the point of injection.
We have also discovered that, when a heated gas plasma is
maintained by the action of a high frequency electric field, which
is modulated by a waveform of such frequency and amplitude that the
plasma does not deionise while the field passes through the
modulation troughs, when the temperature of the plasma will undergo
a cyclic variation at the modulation frequency. This cyclic
variation causes a corresponding variation in the total intensity
of the emitted radiation, so that when the radiation falls upon a
photoelectric cell, an alternating current is produced. The output
of a photoelectric call requires amplification to be of practical
use. It is very much simpler to amplify an alternating current than
the direct current produced by a photoelectric cell under constant
illumination. It has hitherto been the general practice to
interrupt at regular intervals a beam of the radiation being
analysed, for example by means of a rotating shutter, so that the
resulting intermittent radiation falling upon the photometer
produces an alternating current. The use of a modulated high
frequency signal removes the need for a mechanical interrupter.
A further advantage of using a modulated high frequency signal is
that it enables the problem of temperature selection to be solved.
With prior methods and apparatus, in order to conduct analyses at
widely differing temperatures it has been necessary to employ
different kinds of source, each appropriate to a particular
temperature range. For example a coal gas/air burner may be used
for lower temperatures, while electric arcs are frequently employed
for high temperatures. Finer temperature control has only been
possible either by viewing only the radiation emitted from a
certain region of the source, or by making complicated alterations
in operating conditions, such as power supply and rate of delivery
of gas. If a modulated high frequency field is employed and the
emission from the pulsating plasma source is intermittently
interrupted, for example by a mechanical beam interrupter, suitably
synchronised with the modulation of the HF current to produce a
stroboscopic effect, then only the radiation emitted at a
particular temperature is allowed to fall upon any analysing or
measuring device.
Alternatively, the maximum temperature of the plasma may be
adjusted so that it equals the desired temperature by passing the
modulated high frequency signal through an amplitude limiter. This
has the effect of extending the period in each cycle for which the
plasma is at the desired temperature while preserving the
alternating character of the signal from the photocell. If desired,
the radiation resulting from the plasma maintained by this
amplitude limited modulated high frequency signal may be subjected
to stroboscopic interruptions as described above, to remove the low
temperature components of radiation. In this case the periods
available for examination of the radiation by the photometer may be
greatly extended. However, this is not always necessary as the
temperature is at the desired value for a considerable proportion
of each cycle and the part of the cycle at lower temperatures may
be ignored as a first approximation.
Where stroboscopic temperature selection is desired the
interrupting mechanism may comprise a rotating disc provided with a
radial slit, or more conveniently a rotating barrel with a narrowly
spaced, multi-shuttered, axially placed slit. Such a disc or barrel
may be rotated in a beam of radiation from the plasma, the phasing
of the rotation being adjusted so that only the radiation emitted
during a particular phase of the plasma temperature cycle is
allowed to pass through the slit. The particular phase desired may
be readily selected by rotating the motor about its shaft when it
is revolving at its correct speed.
A preferred form of interrupter is a form of the Kerr or Karolus
cell, through which light may only pass when the plates of a
capacitor are charged. If a variable voltage of the same frequency
as the modulation on the high frequency current used to maintain
the plasma is applied to the plates of such a cell, then a
particular temperature for examination may be selected by adjusting
the phase of the voltage applied to the cell with respect to the
phase of the temperature cycle of the plasma.
In order to examine the radiation any conventional spectrometer or
photometer may be used, but where a plasma maintained by a
modulated high frequency current is employed the need for a
mechanical "chopper" to interrupt the beam of radiation is
eliminated.
The high frequency alternating current may have a frequency of 5 to
3000 mc./s. and is preferably between 5 and 200 mc./s. while the
modulation may, for example, have a frequency of from 100 to 360
c./s. This may be conveniently achieved by supplying the
high-frequency oscillator valve with a full-wave rectified mains
voltage which is not fully smoothed. It is preferred to include
means for controlling the amplitude of the modulation superimposed
on the H.T. output voltage, for example by the use of a variable
D.C. smoothing circuit between the oscillator and the source of
rectified mains voltage. Either a capacitance or an inductance in
the smoothing circuit may be adjusted.
The plasma may be maintained by means of an apparatus which
comprises: a tubular vessel, closed at one end and encircled
externally and coaxially by a coil for carrying the high frequency
alternating current a plasma gas supply tube projecting coaxially
through the closed end of the vessel and provided with an inlet for
the entry of the plasma gas; a sample injector for injecting sample
material into a plasma, formed during operation within the coil and
beyond the injector and plasma gas supply tube: and an inlet near
the closed end of the vessel for the entry thereto of an insulating
fluid. For maximum stability the plasma should be annular in form
having a low temperature central region through which a jet of the
sample is directed into the region of the plasma tail flame. To
obtain a plasma with a stable annular form, alternating currents
having frequencies of from 5 to 3000 mc./s. are required, depending
on the desired diameter of the plasma and the coil should have at
least two turns. The smaller the diameter of plasma desired, the
greater is the frequency required to generate a stable annular
plasma. For generating an annular plasma in a vessel of from 25 to
35 mm. diameter, current frequencies of from 5 to 200 mc./s. give
good results. The optimum power output for the high frequency
source depends on the design and efficiency of the coil and on the
dimensions of the apparatus. The power requirements are also
dependent to a large extent upon the type of inert atmosphere
employed. The temperature and intensity of the plasma may be
controlled by varying the power supplied to the coil. Normally one
end of the coil is earthed and the other end connected to a high
frequency source. In these circumstances the voltage potential and
hence the intensity of the resulting plasma is greater the closer
that part is to the high potential end of the coil. A plasma will
be most readily formed at the live end of the coil, and for a long
coil may only be formed near that end. The coil should be
positioned with respect to the apparatus so that the part within
which the plasma is generated lies beyond the end of the plasma gas
supply tube and the end of the injector. The plane of the first
live half-turn of the coil is preferably perpendicular to the axis
of the coil.
Stable plasmas are most readily obtained when a rotary motion is
imparted to the plasma gas stream; this may be conveniently
achieved by admitting the gas tangentially into the plasma gas
supply tube. By adjusting the pressure uner which the plasma gas,
insulating gas and sample injection gas are delivered, suitable
rates of flow may be obtained. These will depend on the dimensions
of the apparatus, but may be determined in any particular instance
by simple experimental variation of the three flow rates. In
general too fast a delivery of plasma gas and/or sample injection
gas will cause the plasma to be "blown out". The insulating gas may
conveniently be delivered at a rate of from 3 to 5 times that of
the plasma gas, being too rapid a rate to permit a plasma to form
in the insulating stream.
Any gas may be used to form the plasma, but argon, helium, nitrogen
and oxygen are preferred. It is most convenient to use the same gas
for the plasma gas and the insulating fluid, as well as any gas
used for injecting the sample. A mixture of two gases is often
desirable in order to achieve a stable plasma and reduce background
interference. A mixture of 80% nitrogen and 20% argon gives less
background interference than pure argon. In general, monatomic
gases are preferred to polyatomic gases, since the latter give rise
to more complex background spectra. The excitation atmosphere may
be neutra, reducing or oxidising. The insulating fluid may be a
liquid, such as water.
In order that the invention may be better understood one example of
apparatus embodying the invention will now be described with
reference to the accompanying drawings, in which:
FIG. 1 is a diagrammatic representation of the plasma containing
tube encircled by the coil;
FIG. 2 is a circuit diagram of the high frequency generator which
supplies the coil of FIG. 1; and
FIG. 3 illustrates diagrammatically apparatus for selecting a
particular temperature range of the plasma temperature variation
when the output of the high frequency generator is modulated.
The apparatus of FIG. 1 comprises a tubular vessel 1 (which may
have a diameter of 25 mm.) open at one end and provided near the
closed end with a tangential inlet 2 for an insulating gas. The
vessel 1 is encircled externally and coaxially by a coil 3, which
may conveniently have an internal diameter of 30 mm. and comprises,
in this example, three and a half turns of copper tubing. The plane
of that half turn of the coil which lies nearest the open end of
the vessel is perpendicular to the axis of the coil. 6 mm. diameter
copper tubing may conveniently be used to form the coil and the
spacing between the turns may, for example, be 5 mm. Projecting
coaxially through the closed end of the vessel 1 is a plasma gas
supply tube 4 which is closed at the end lying outside the vessel 1
and is provided with a tangential inlet 5 for plasma gas. The
diameter of the plasma gas supply tube increases near the open end
to form a tubular skirt 6 (which in this example is 20.5 mm. in
diameter and 22 mm. in length and may terminate from 20 to 40 mm.
from the open end of the vessel 1 and adjacent to the first turn of
the high frequency coil. A sample injector projects coaxially
through the closed end of the plasma gas supply tube 4. The said
sample injector comprises two coaxial tubes, of which the outer
tube 7 is provided with an inlet 8 and terminates, at the end lying
within the plasma gas supply tube 4, in a jet 9, the other end
being sealed about the inner, capillary tube 10. One extremity of
the tube 10 lies within the jet 9, the other extremity projecting
into a reservoir formed by an inclined drum 11 which may be rotated
about a shaft 12 by a motor 13.
The vessel 1, the tube 4 and the jet 9 are constructed of a
heat-resistant material such as silica or quartz. Those parts of
the tube 7 and the capillary 10 which are remote from the plasma
may, however, be constructed of plastic or metal. If desired,
screws and/or perforated discs may be incorporated in the barrel 7
of the injector to enable the capillary tube 10 to be accurately
centered.
A typical example of the operation of the apparatus is as follows:
argon gas is fed into the plasma gas inlet 5 at a rate of five
liters per minute and into the insulating gas inlet 2 at a rate of
seventeen liters per minute. The end of the coil 3 nearest the open
end of the vessel 1 is connected to a source of alternating current
of frequency 36 mc./s., the said source having an output power of
1.5 kw., and the other end of the coil 3 is earthed as close as
possible to the turn itself in order to reduce the potential
difference existing between the bottom end of the work coil and the
gas injection point. The efficiency of the coil is improved by
pumping a cooling fluid such as water through it. A carbon rod is
introduced into the open end of the vessel 1 as far as the jet 9.
The rod is heated by high frequency currents produced therein and
heats the surrounding gas which is thus ionised sufficiently to
initiate a stable plasma which forms an annulus owing to the "skin
effect." The rod is withdrawn and the plasma is now maintained by
the operation of high frequency eddy currents and capacitively
induced currents resulting from the flow of current in the coil.
Argon gas is fed into the injector inlet 8 at a rate of five liters
per minute. The powder sample is placed in the drum 11, which is
rotated by the motor 13 at a rate of about 500 r.p.m. The powder is
drawn up the capillary tube 10 and sprayed through the low
temperature central region of the plasma to form a tail flame at
the open end of the tubular vessel 1. The spectrum of this flame
may be analysed using a suitable spectrometer.
The production of the plasma in annular form instead of a plasma
ball and the directing of the substance along the axis of the
plasma annulus provides a convenient way of bringing the substance
into the tail flame region. The use of the tail flame of the plasma
to burn the substance is advantageous in that the plasma temperture
is frequently higher than is desirable for analysis and in that the
temperature of the tail flame can be adjusted by varying the
voltage applied across the coil. This flexibility is desirable in
view of the fact that the optimum temperature for analysis varies
from substance to substance, and the temperature variation is much
greater than can be obtained with conventional flame sources, for
example air/acetylene burners, and whilst D.C. or A.C. are systems
will give higher temperatures than such burners they also produce
sample contamination.
FIG. 2 is a circuit diagram of a typical high frequency generator
for use with the plasma-forming apparatus. The oscillator valve 20
has a "tank" circuit including a tank coil 21 in the form of a
conductor of wide strip material. The capacitor of the tank circuit
is formed primarily by the inter-electrode capacitance of the valve
20. An output coil 22 of side strip is inductively coupled to the
tank coil and has one end earthed and the other end connected
through a variable capacitance 23 to one end of the work coil
surrounding the tube 1. The value of the voltage across the work
coil can be adjusted, in accordance with the plasma temperature
requirements, by means of the variable capacitor.
If, for the reasons set out above, a modulated high-frequency field
is required, the power supply 24 may be constructed with inadequate
smoothing. This will provide a modulation of the high-frequency
field at twice the frequency of the mains supply.
An apparatus for selecting a particular plasma temperature, in each
cycle of temperature variation produced by the modulated high
frequency alternating field, is shown diagrammatically in FIG. 3.
In the arrangement of FIG. 3, the coil 3 surrounding the tube 1 is
connected to the high frequency generator 28 which is supplied from
a power supply 24 which is unsmoothed. The plasma formed within the
tube 1 is examined through a lens 29 by means of a stroboscope 30.
The stroboscope 30 includes a cylinder 31 which is rotated by a
synchronous electric motor 32 and which contains an opening 33 cut
through the cylinder 31 parallel to the axis of revolution. The
opening is divided by equally spaced narrow slats arranged
longitudinally within the opening, the effect of these slats being
to provide an extremely rapid cut-off between the point at which
the opening 33 is directed at the plasma in the tube 1 and the
point at which the opening no longer passes light from the plasma.
This permits maximum use to be made of the radiation from the
plasma while restricting the time during which radiation is allowed
to pass as much as possible in order to achieve greater resolution
of the temperature cycle.
The phasing of the movement of the opening 33 with respect to the
temperature cycle is shifted by rotation of the motor 32. A scale
34 is provided to facilitate adjustment of the phasing and a
locknut 35 enables the motor to be locked into its desired angular
position.
Radiation which passes through the stroboscope reaches an entrance
slit 37 and thereafter passes through a broad-band optical filter
38 followed by a narrow-band interference filter 39. It is then
brought by a lens 40 to a focus at the photo-transistor 41, the
output of which is amplified and rectified in the electronic
circuit 42 and is then used to energise a galvanometer 43.
The temperature of the sample in the tail flame, as estimated from
the degree of ionisation indicated by the relative intensities of
the 4226.728 A. and the 3933.666 A. lines of the calcium spectrum,
is between 7,000.degree. K. and 7,500.degree. K. approximately.
Statistical analysis of a series of 30 sec. exposures of Ilford N
30 plates with a 0.01 mm. slit, the source being focused with
quartz optics, and using a solution containing 20 p.p.m. calcium,
showed a relative deviation in the Ca. 3933.666/Ca. 4226.728 ratio
of .+-.2.9% in the tail flame. This corresponds to a very high
stability in the radiation emitted by the injected sample.
An alternative to tilted rotating drum of FIG. 1 is a reservoir
consisting of a rotating disc with a V-shaped concentric groove
into which the solid is poured from a hopper. In a modification
particularly suitable for the continuous monitoring of powdered raw
materials and/or products of a manufacturing process, the injector
draws the solid directly from the moving belt, preferably V-shaped
in cross section, by which it is being transported to or from the
manufacturing plant.
The sample may also be injected as an aerosol, generally formed by
forcing the liquid through an atomiser into a stream of gas, which
is passed through a separator, such as a cyclone separator, to
remove gross drops of liquid. The aerosol is then injected into the
plasma through an injector in the form of a tube. Since the
majority of the liquid passing through the atomiser is subsequently
removed by the separator, the sensitivity possible using this
method is seriously limited and the size of the sample required is
fairly large.
Alternatively, the sample may be introduced into the plasma in the
form of a wire or a rod of compacted powder. Our invention enables
simpler photometers to be constructed than hitherto, and enables a
single apparatus to be used for the investigation of spectra
emitted at a wide range of different temperatures. It is even
possible by means of our invention to use a single source to
examine simultaneously the spectra emitted at different
temperatures by a given substance.
* * * * *