Note: Descriptions are shown in the official language in which they were submitted.
This invention relates to method and apparatus
for sampling an inductively generated plasma through an
orifice into a vacuum chamber, and to method and appara
tus for mass analysis using such samplingO The invention
will be described with reference to mass analysis.
Mass analyzers for detecting and analyzing
trace substances require that ions of the substance to
be analyzed be introduced into a vacuum chamber contain-
ing the mass analyzer. It is often desired to pexform
elemental analysis, i.e. to detect and m`easure the rela-
tive quantities of indlvidual elements in the trace sub-
stance. In theory the trace substance can be reduced to
its individual elements by introducing the trace sub-
stance into a high temperature plasma, which produces pre-
dominantly singly charged ions of the elements. The useof a high temperature plasma as an ion source has a number
of well recognized advantages, including the fact that it
produces mostly singly charged ions; interference by other
elements to the element to be detected is reduced, iso-
topic information is obtained, and the ionization ef-
ficierlcy of the source is very high so that numerous
ions are produced for analysis.
However a major difficulty in the past is asso-
ciated with the fact that the plasma is normally operated
at atmospheric pressure; the mass analyzer is located in
a vacuum chamber, and therefore a sample of the plasma
must be extracted from the plasma and directed through a
small orifice into the vacuum chamber. The plasma is at
very high temperature (typically 4,000 degrees K to
lO,000 degrees K)and is arelatively good electrical con-
ductor. It is found that when a portion of the hot
plasma is directed through a small orifice, an arc-like
breakdown occurs between the plasma and the edge of the
orifice, destroying the orifice and producing ultra
violet noise which enters the mass analyzer and inter-
feres with the detection of ions. The effect has been
called the "pinch" effect by other worlcers and it greatly
limits the utility of the plasma ion source approach.
Attempts have been made to solve the pinch ef-
fect problem by boundary layer sampling. In this solu-
tion the orifice through which the plasma is sampled is
located in a flat surface of a plate which is kept rela-
tively cool. As the plasma plays against the cool plate,
it produces a cool boundary layer immediately next to the
plate. Ions are extracted through the boundary layer
rather than from the plasma directly, and since the boun-
dary layer is cool (and therefore is a relatively good elec-
trical insulator), arcing effects are reduced or elimina-
ted. However, a major disadvantage to this approach is
that the ions present in the plasma tend to recombine and
react in the cool boundary layer and to form oxides. The
11 9
recombination and reaction reduce the nurnber of ions
available for analysis, and the oxide formation greatly
complicates the analysis. wherefore the use o-f a cooled
boundary layer for sampling has serious commercial
disadvantages.
Alan L. Gray, at a conference in January, 1982
entitled "19~2 Winter Conference on Plasma Spectro~hem-
istry" at Orlando, Florida, U.S.A. disclosed the use of
a relatively large orifice (which removed the cooled
boundary layer), together with staged vacuum chambers,
which is said to eliminate the prior di,ficulties. How-
ever, the results disclosed appear to be applicable only
in limited special circumstances -The applicant's tests
using a similar sampling arrangement and staged vacuum
pumping have not reproduced these results.
The invention provides method and apparatus for
sampling a plasma through an orifice into a vacuum cham-
ber in which the problem of arcing and generation of ultra-
violet noise at the orifice is greatly reduced, and in
which the problem of recombination and reaction of ions
adjacent the orifice is also reduced. In one aspect the
invention provides apparatus for sampling a plasma into
a vacuum chamber comprising:
(a) means for generating a plasma, including an
2~ electrical induction coil having first and second term-
inals and at least one turn between said first and second
terminals, said turn defining a space within said coil
for generation of said plasma,
(b) a vacuum chamber l~nclu~ing an orifice plate
defining a wall of said vacuurn chamber,
(c) said orifice plate having an orifice therein
located adjacent said space for sampling a portion of
said plasma through said orifice into said vacuum chamber,
(d) and circuit means connected to said coil between
said terminals to reduce the peak to peak voltage swing in
said plasma.
In this description and in the appended claims,
the term "vacuum chamber" is intended to mean a chamber
in which the pressure is substantially less than
atmospheric.
In another aspect the inventlon provides
a method of sampling a plasma into,a vacuum chamber
comprising:
(a) applying a high frequency electrical current
to a coil to generate a plasma within said coil,
(b) reducing the peak to peak voltage variations
in said plasma by limiting the voltage variation
in said coil at a position between the ends thereof, and
(c) directing a portion of said plasma through an
orifice into said vacuum chamber.
Further objects and advantages of the invention
ill appear from the following description, taken
together with the accompanying drawings in which:
Fig. 1 is a diagrammatic view (not to scale)
showing prior art apparatus for mass analysis and with
which the invention may be used,
~l~L8~
Fig. 2 is a schematic drawing of an impedance
matching circuit and induction coil used with the appara-
tus of Fig. l;
Fig. 3 is a schematic drawing similar to that
of Fig 2 but showing an impedance matching clrcuit and
induction coil modified according to the invention,
Fig. 4 is a graph showing the absolute value
of the plasma RF voltage plotted against the position of
the tap taken axially along the coil,
Fig. 5 is a graph showing the energy and
energy spread of ions transmitted into the mass spectro-
meter from the plasma for two positions of the ground
tap of Fig. 3; -
Fig. 6 is a further graph showing the energy
and energy spread of ions transmitted from the plasma
into the mass analyzer for two positions of the g.round
tap of Fig. 3;
Fig. 7 is a mass spectrum for strontium taken
with the ground tap of Fig. 3 at a first position; I'
Fig. 8 is a mass spectrum for strontium taken
with the ground tap of Fig. 3 at a second position;
Fig. 9 is a cross sectional view of an orifice
plate having a blunt orifice structure;
Fig. 10 is a cross sectional view of an orifice
plate having a sharp edge orifice structure;
Fig. 11 is a mass spectrum for cerium taken
using the blunt orifice structure of Fig. 9;
Fig. 12 is a mass spectrum for cerium taken
using the sharp edge orifice structure of Fig. 10;
FigO 13 is a graph showing relative numbers of
ions versus their ionization potential; and
Fig. 14 shows an alternative electrical circuit
for use with the invention.
Reference is first made to Fig. 1, which shows
a plasma tube 10 around which is wrapped an electrical
induction coil 12~ A carrier gas (e.g~ argon) used to
form the plasma is supplied from a source 13 and is dir-
ected via conduit 14 into the plasma tube lOo A further
stream of the carrier gas is directed from the source 13
through an inner tube 15 within the plasma tube .10 and
exists via a flared end 16 just upstream of the coil 12.
A sample gas containing the trace substance to be analyzed
is supplied in argon from source 17 and is fed into the
plasma tube 10 through a thin tube 18 within and coaxial
with the tube 15. Thus thy sample gas is released into
the centre of the plasma to be formed.
The coil 12 normally has only a small number of
turns (four turns in the embodiment tested) and is sup-
plied with electrical power from an RF power source 20
fed through an impedance matching network 22. The power
fed to the coil 12 varies dependi.ng on the nature of the
plasma required and may range between 200 and 10,000
. .. . .. . .
watts. The energy supplied is at high frequency, typical-
ly 27 MHz. The voltage across the coil 12 is believed to
be up to several thousand volts, depending on operating
conditions. The plasma genera-ted by this arrangement
is indicated at 24 and is at atmospheric pressure
The plasma tube 10 is located adjacent a first
orifice plate 26 which defines one end wall of a vacuum
chamber 28. Plate 26 is water cooled, by means not shown.
Gases from the plasma 24 are sampled through an orifice
30 in the plate 26 into a first vacuum chamber section
32 which is evacuated through duct 34 by a pump 36. The
remaining gases from the plasma exit through the space
38 between the plasma tube 10 and the plate 26.
The first vacuum chamber section 32 is separated
lS from a second vacuum chamber section 40 by a second ori-
fice plate 42 containing a second orifice 44. The second
vacuum chamber section 40 is evacuated by a vacuum pump
46. Located in the second vacuum chamber secticn 40 is
a mass analyzer indicated at 48. The mass analyzer may
be a quadrupole mass spectrometer having rods 50. For
purposes of clarity the plasma tube 10 in Fig. 1 has been
shown greatly enlarged with respect to the vacuum chamber
In use, the first vacuum chamber section 32 is
typically maintainer at a pressure of about 1 torr, and
a second vacuum chamber section 40 is 'cypically maintained
a-t a pressure of 10 torr. A portion of the plasma 24
is sampled through the first orifice 30 in-to the first
.
vacuum chamber section 32. Ions in the plasma are
drawn through the first orifice 30 into the first
vacuum chamber section 32 by the gas flow through the
first orifice 30. The ions are then drawn through the
second orifice 44 again by the gas flow through the
second orifice 44. -
As discussed, it is found that when the systemshown in Fig. 1 is used, the plasma 24 tehds to arc
through or to the first orifice 30 and sometimes may
even arc through or from the first orifice 30 to the
second orifice 44. The arcing destroys the oriices
and also generates ultraviolet noise which interferes
with the analysis of any ions which may enter the mass
analyzer 48. In addition, ions characteristic of the
orifice material may appear in the mass spectrum and
interfere with the analysis.
The undesired arcing is aggravated when (as
in the present case) there is a vacuum chamber 28 on
the side of the first orifice plate 26 remote from the
plasma 24. The increased arcing occurs because the
increased flow of gas through the orifice 30 caused
by the vacuum tends to remove the cooled layer of gas
which would otherwise tend to collect against the
outside of the orifice plate 26 and which would provide
some electrical insulation against arcing. If the first
orifice 30 is made sufficiently small, then the cooled
-- 10 --
layer 51 of gas overlying the first orifice plate 26 at
the first orifice will tend to exist even with vacuum
pumping, but with a very small orifice 30, only a small
sample of the plasma 24 can be drawn into the first
vacuum chamber section 32, reducing the ion signal. In
addition if the first orifice 30 i9 made very small it
more readily tends to melt or clog. If the first orifice
30 is made l.arger, then the cooled layer 51 of gas over-
lying the orifice plate 26 becomes thin or vani.shes and
arcing occurs as indicated
The applicant has discovered after extensive
research that the arcing appears to be caused by large
peak to peak voltage swings in the plasma itself. Al-
though it is difficult to measure voltages in the plasma
generated by a high frequency electrical field (because
the probe used for measurement tends to be melted by the
plasma and because of undesirable RF pick-up produced by
the generating field), a determination has been made that
the peak to peak voltage swing in the plasma with the ar-
rangement shown is very large (e.g. of the order of upto 1,000 volts). Having made this determination, the
next problem was to determine how this voltage swing was
being produced.
Tests were then conducted to determine the
origin of the large voltage swings in the plasma, and
these tests will be explained with reference to Fist 2.
Fig. 2 shows a circuit for the typical tuning and impedance
matching device 22 used to supply RF power to the plasma.
The impedance matching device 22 consists of two variable
capacitors Cl, C2 connected in series at terminal 52
with the power source 20 connected across capacitor Cl
at terminals 52, 54. A terminal 56 at the free end of
capacitor C2 is connected to terminal 58 a.t the upstream
end of the coil 12 while the other end 60 of coil 12 is
connected to terminal 54. The directionJof gas flow
through the coil 12 is indicated by arrow 62. The
arrangement as shown in Fig. 2 produced the very large
voltage swings discovered in the plasma 24.
The first test was to connect a ground to term-
inal 60 immediately at the downstream end of the coil,on the theory that the long lead used from 60 to 54 had
inductance kick was generating a voltase swing at term-
inal 60 and that this was contributing to the voltage
swing in the plasma. This additional ground reduced the
voltage swing to less than half of that originally detect-
ed, but a large voltage swing in the plasma remained
and still produced arcingO
Next, the impedance matchins circuit was modi-
fied as shown in Fig. 3, so that the former connection
between ground and terminal 54 was removed. Instead
- 12 -
the coil 12 was tapped at 64 and the tap 64 was grounded.
The tap 64 was then moved back and forth along the coil
and the peak to peak voltage swings in the plasma 24 were
measured for different positions of the tap 64 along the
coil 12. The measurements are plotted to form curve 66
in Fig. 4, where the absolute value of the plasma peak
to peak voltage swing is shown on the vertical axis
and the position of the tap 64 is shown on the horizontal
axis. On the horizontal axis the number "0" indicates
the terminal 60 at the downstream or extend o the coil
12, and the number "4" denotes the terminal 58 at the
entrance or upstream end of the coil 12. The numbers
"1", "2" and "3" indicate turns 1, 2 and 3 respectively
of the coil 12. The center of the coil is located at
"2" in FigO 4O
In the Fig. 4 curve it will be seen that at
point 68, the tap 64 is located downstream of terminal
60, between terminals 54 and 60. It will be seen in
Fig. 4 that the absolute value of the peak to peak
voltage swing 66 in the plasma decreases as the tap 64
is moved from the downstream end "0" of the coil
toward the center "2" of the coil, reaching a minimum
at the two turn location. The voltage swiny then
increases as the tap 64 is moved toward the upstream
25 end "4" of the coil. The voltage at the null point 70 .
is indicated as being about 13 volts, but it is difficult
to measure the voltage accurately to within less than
- 13 -
five volts absolute value because of RF pick-up difficul-
ties. In addition, a small voltage (of the order of 10
volts) is generated in the plasma by heating currents
flowing through the plasma and this voltage is apparently
not eliminated by moving the tap 64. It will be noted
that the voltage measurements shown were of the absolute
value of the voltage swing in the plasma, because it is
difficult to measure the polarity of such voltage. How-
ever, in theory it is expected that the voltage swings
being measured would reverse in phase as the tap 64 is
moved past the center ~2~ of coil 12~
When the tap 64 was located near the center
of the coil (e.g. within about one-quarter turn from the
center of the coil for a four turn coil, it was found
that arcing at the orifices 30, 44 was eliminated and
in addition both the energy and the energy spread of
the ions travelliny through the orifices were much
reduced. Specifically, reference is next made to Fig.
5, which shows on the vertical axis the number of ions
travelling through the orifices 30, 44 into the mass
analyzer 4B, and on the horizontal axis the energies
of such ions in electron volts. Curve 72 shown in
solid lines and with solid measurement points was pro-
duced when the tap 64 was located one-~uarter turn from
the end "0" of the coll, and curve 74 shown in dotted
lines and with outline measurement points resulted when
the tap 64 was located at one and thre-~quarter turns
14 -
from the end "0" of the coil (i.e. nearly at the center
of the coil). For curve 72 considerable arcing occurred
through the orifice and there was considerable scatter
of the observed points, as shown, so a smoothed line was
drawn through the points. It will be seen that the
energy spread of the ions at 10~ height was about 44
electron volts and at 50~ height was about 17 electron
volts. In addition the maximum energy of a substantial
number of the ions exceeded 30 electron volts. The high
energies and energy spread of the ions greatly reduce
the ability of the quadrupole mass analyzer 48 to analyze
the trace substance being examined. In contrast, it will
be seen from curve 74 that the energy spread of thy ions
passing through the orifices was much less, namely about 11
,
electron volts at 10% height and about 5 electron volts
at half height. The improvement was dramatic and
leads to a corresponding improvement in detection and
analysis, as will be explained.
Fig. 6 is similar to Fig. 5 but shows
curve 76 produced when the tap 64 was located at three-
quarters of a turn from the end "0" of the coil and
curve 78 produced when the tap 64 was again located one
and three-quarter turns from the end "0" of-the coil.
The results are similar to those described previously,
i.e. for the tap 64 near the center of the coil, both
the energy spread of the ions and the average energy of
the ions are much reduced.
The effect of the reduced ion energy and
ion energy spread will be explained with reference to
Figs. 7 and 8, which are mass spectra for a ten parts
per million solution of the elernent strontium. The
number of ion counts detected is shown on the vertical
axis and the mass in atomic mass units (amu) is shown on
the horizontal axis. jig. 7 shows the mass spectrum
obtained with the tap 64 located three-quarters of a turn
from the downstream end "0" of the coil was shown for
curve 75 in Fig. 6)~ jig. 8 shows the mas`s spectrum
obtained when the tap 64 is located one and three-quarter
turns from the downstream end "0" of the coil (as shown
for curve 78 in Fig. 6). In both cases the full
scale value on the vertical axis was 3 X 104 counts
per second. It will be seen that in Fig. 8 the three
strontium peaks indicated at 80a, 82a and 84a (cor-
responding to 86, 87 and 88 atomic mass units) have
been clearly resolved whereas in Fig. 7 the same peaks
80b, 82b, 84b have been poorly resolved and the maximum
level of peak 84b is lower than that oF peak 84a. As
expected, the reduced ion energies and energy spread
have produced substantially greater resolution and
increased ion signal for analysis.
A further advantage of the invention is that
because there is no need to sample from a cool boundary
layer used to protect the orifice, the orifice sampling
plate may be arranged to reduce or eliminate any such
- 16 -
V~
cool boundary layer. This aspect of the invention is ex-
plained with reference to Figs. 9 and 10. Fig 9 shows a
first orifice plate 26a having a blunt cbnical orifice
structure 88 defined by a conical side wall 89, a flat
S (i.e. blunt) top wall 90, and an orifice 30a in the top
wall 90. In use the blunt top wall 90 tends to produce
a cool boundary layer (as shown at 51 in Fig 1) of gas
over the orifice 30a, which boundary layer insulates the
orifice from the plasma in order to reduce arcing. Un-
fortunately since the plasma is at atmospheric pressure,rapid recombination and reaction of the ions with oxygen
occurs at the cool boundary layer (the recombination rate
varies with the third power of the pressure and the
reaction rate varies with the second power of the pres-
sure). This results not Dnly in loss of ion signal avail-
able for analysis but also in the entrance of oxides into
the mass analyzer, complicating the analysis.
Fig.10 shows an alternative first orifice plate
26b having a sharp edge orifice structure 92 defined by
a conical side wall 94 teminating at a sharpe edge 96.
The edge 96 defines the first orifice 30b. The Fig. 9
orifice structure results in the reduction or elimination
of a cool boundary layer over orifice 30b (even thollgh -the
plate 26b itself may be cooled), because there is not -flat
surface adjacent the orifice over which a cooled boundary
- 17 -
~8~
layer can readily form. Thus the plasma being sampled
through orifice 30b is not greatly cooled until after it
enters vacuum chamber section 32. Since the pressure in
vacuum chamber section 32 is only about one torr (as com-
pared with 760 torr on the outside of orifice plate 26b),the recombination raze is reduced by about 76G3 and the
reaction rate by about 7602.
The improvement produced by the use of the
sharp edge orifice structure 92 (which can be used
without arcing because of the tap 64 located near the
center of the coil) is shown in Figs. 11 and 12, which
show mass spectra obtained for a ten parts per million
solution of cerium. Fig. ll shows the mass spectrum
98 obtained using the blunt orifice structure 88 of
lS Fig. 9 and Fig. 12 shows the mass spectrum lOO obtained
using the sharp edge orifice structure 92 of Fig. lO.
Here full scale on the vertical axis was 10 counts
per second. It will be seen that in Fig. ll the peak
at 140 amu (which is the mass of cerium) is extremely
small, while a large peak is located at mass 156 (cexium
oxide) and a smaller peak (but still larger than the
cerium peak) is located at mass 158 (the oxide of an
isotope of cerium).
In contrast Fig. 12 shows a large peak at mass
25 140 (cerium) and a substantial peak at mast 142 (an
isotope of cerium). Only a small peak now appears at
. .
- 18 -
mass 156 (cerium oxide), and virtually no peak appears
at mass 158. The enormous increase in ion signal for
the elemental ions and the corresponding reduction in
the quantity of oxides produced greatly improve the
ability to decipher the complex spectrum obtained when
many elements are mixed together. (For Fig 4 12 the
resolution was deliberately reduced to ensure that there
would be no mass discrimation against the higher mass
oxides.)
A further advantage of the i~`vention is that
it improves the response to elements of high ionization
potential. Formerly it was common practice to place an
extra water cooled orifice plate between the first ori-
fice 30 and the plasma 24. 'rhus a reduced scale, rapidly
cooling plasma was sampled through the first orifice 30.
Air mixed rapidly into this plasma and reacted thereon
to produce nitric oxide NO). The ionization potential
of NO is 9.25 electron volts. Metal ions of higher
ionization potential in the plasma tended to undergo
change transfer reactions with the NO to produce NO and
neutral metal atoms. The metal atoms, having become
neutral, could not be detected by the mass analyzer.
When the invention is used, sampling may be
carried out much closer to the ho-t plasma (since arcing
has been essentially eliminated) and air has less oppor-
tunity to mix into the plasma sample. Therefore nitro-
gen oxides are less likely to form. Thus ions of higher
- 19 -
ionization potential do not lose their charge and hence
can be seen by the mass analyzer. This is illustrated in
Fig. 13, which shows relative numbers of ions on the ver-
tical axis on a log scale, and the ionization potential
of the elements in electron volts (various elements are
marked on the graph) on the horizontal axis.-- The curve
for the prior art method without the use of the invention
is shown at 110 and the curve with the invention used is
shown at 120. For higher ionization potential elements
such as zinc, the improvement in ion signal can be by a
factor of fifty. For mercury the improvement i5 even
greater.
It will be realized that although the tap 64
is shown as grounded, it may instead be clamped to a
different fixed poten-tial, depending on the circuit ar-
rangements provided. Alternatively a variable voltage
may be applied to tap 64, so long as the effect is to
reduce sufficiently the peak to peak voltage swing in
the plasma.
As a further alternative the tap 64 may be
eliminated entirely and a circuit such as that shown
in Fig. 14 may be used. In the Fig. 14 circuit the power
supply 20 is connected to terminals 54, 56, i.e. across
the two capacitors now indicated as Cl', C2', and the
terminal 52 between capacitors Cl', C2' is groundedO
Terminals 56, 58 are connected together as are terminals
54, 60, as before. Provided that the circuit is carefully
- 20 -
balanced so that the capacitance of Cl' and its leads
is equal to the capacitance of C2' and its leads, the
circuit will be symmetrical and will be equivalent
electrically to ha~Jing a ground centre tap in coil 12.
Thus the RF voltage at the centre of coil 12 will re-
main at or near zero as be~oreO
Impedance matching, if needed for the Fig. 14
circuit, may be effected by a transformer or other ma
located between the RF power source 20 and the location
in the circuit no shown -for the source 20.
Although a four turn coil has been shown, more
or fewer turns may be used as appropriate fur the applica-
tion in question.
