Note: Descriptions are shown in the official language in which they were submitted.
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FIELD OE TH _ NvENrrIoN
This in~ention per~ains generally to atomic
absorption spectrometers, and more particularly, to an
automatic gas control system for burners used in atomic
.
~ absorption spectroscopy.
; - BACKGROUND OF THE INVENTION
In atomic absorption spectroscopy (see, for example,
U.S~P.N. 2,847,899), the measurement of the absorption of a
radiation beam at a characteristic resonant spectral line for a
~ ~ .
particular element yields a measure of the concentration of
;~ that element in an original sample solution. Presently, the
'.'t" most common technique for atomizing an element for the purposes
of the absorption measurement, is by introducing a li~uid
sample solution of the element of interest into a gas burner
wherein droplets of the solution are vaporized and the elements
ultimately atomized, so as to form in the path of the apparatus
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radiation beam, a substantial quantity of the element of interst
~- in its atomic state.
In order to effect appropriate burning of the element- -
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~- 20 containing solution, the liquid mus~ be converted into a fine
spray and then mixed with a fuel and oxidant gas before
introduction into the burnerO The fine spray is achieved
through use of a nebulizer, such as described in U.S. Patent
No. 4,125,225, also assigned to the assignee herein.
A nebulizer, generally, employs a venturi-type
restriction which passes rapidly-moving gas (hereinafter
referred to as an oxidant~ past an opening, drawing a portion
of the li~uid sample solution into the gas stream, effecting an
atomizing of the liquid in the process~ The liquid is said to
be aspirated by the venturi effect caused by the rapidly moving
current of gas.
The ~ample laden gas or oxidant, then passes into the
burner chamber where it is mixed with additional oxidant from an
auxiliary inlet/ and fuel such as acetyleneO It i5 then intro-
duced into the burner head where lt is ignited.
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The sensitivity of the absorption mea~urement is de-
pendent on many factoxs~ one of which being the flame condition
of the burner. IPe., the leannes~ or richness of the fuel-
oxidant mixture. Also, the sensitivity of the measuremenk requires
the optimization of the setting of the nebulizer which varies the
amount of liquid sample aspirated by the rapidly flowing gas.
Because of the nature of the mechanism for aspirating more or
less o the sample~ namely varying the flow of oxidant through
the venturi-type restriction, there is lhe obvious side eEfect
on the 1ame condition which has a direct e~fect on the sensiti-
vity of the measurement. In prior systems, the opera~or would
have to go back to the auxlliary inlet to th~ burner and vary the
oxidant flow through it to compensate for the last adjustment to
the nebulizer and the effect thereof on the oxidant flow into the
burner.
The object of an automatic gas control system, would be
to eliminate these readjustmen~ due to the adjustments of the
nebulizer.
Further, an automatic gas control system should allow
for the programmability of optimum analys~s parameters. E.g.,
with respect to the field herein discussed, namely atomic absorp-
tion spectometry, the optimum ~uel-oxidant flow rates, element
wave lengths, uel-oxidant characteristics, and the like, - para
meters which could be optimized by ~he methods analyst in the lab
-- should be maintained constant for each measurement even on
differing instruments~ Desirably, the optimum values for these
~actors can be stored in a memory device, such as on magnetic
cards, which can be used to program different instruments to
insure optimum results.
It is therefore a primaxy object o~ this invention to
provide an apparatus which will respond identically to pre-pro
grammed optimum fuel-oxidant gas flow rates, irregardless of the
instrument system in which employed.
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It is another object of thls invention to pr~vide
operator-free adjustment of the oxidant flow to the auxiliary
inlet of a hurner to offs~t the e~fects of nebuli~er adjust-
ments on th~ oxidant flo~ therethrough.
There is yet another object of the invention to provide
a pneumatic control means for adjusting the oxidant flow to the
auxiliary inlet in response to nebulizer adjustments.
It is still another object of this invention to provide
a pneumatic correction means which both senses a change in the
flow of oxidant to the nebulizer intake and corrects the flow
of the oxidant to the auxiliary inlet in response to the sensed
change to said nebulizer.
SUr~MARY OF THE_INVENTION
In accordance with a broad aspect of the invention,
there is provided a gas flow control system for an atomic
absorption spectrometer comprising a hurner for burning a mix-
ture of fuel, oxidant and sample, a mix:;ng chamber for mixing
the fuel, oxidant and sample r means for supplying fuel to the
mixing chamber, means for supplying a mixture of sample and oxi-
dant to the mixing chamber, and auxiliary means for supplyingoxidant to the mixing chamber. Means are also provided for
varying the supply of sample and oxidant mixture to the mixing
chamber, and means are provided for adjusting the flow of oxi-
dant through the auxiliary supply means in response to a
variation in the supply of oxidant supplied by the sample and
oxidant supply means such that the total oxidant flow supplied
to the burner remains substantially constant.
More specifically in accordance with the invention,
there is described herein, an automatic gas control apparatus
for use in an atomic absorption spectrometer instrument system
including a burner for burning a mixture of fuel, o~idant and
an unknown element-containing sample, which comprises a fuel
supply means which provides a predetermined flow of fuel to
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the burner based on a stored electrical signal corresponding to
a previously determined optimum flow rate; a nebulizer, which
introdwces a variable amount of sample, the nebulizer being
adjustable to vary the sample flow so as to optimize the
measured signal of the spectrometer; variable (auxiliary)
oxidant supply means, the flow through which can be varied so
as to make up the difference between the predetermined total
flow of oxidant re~uired for a pres~ribed sensitivi-ty of the
spectrometer and the varying amount supplied by the sarnple
introducing means because of the adjustment feature required to
optimize -the measured signal; means for measuring the flow of
oxidant through both the sample introducing nebulizer and the
variable oxidant supply means; and means for comparing
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the sensed oxidant flows to the previously determined oxidant
flow requixed for a prescribed sensitivity ~ wi~h means for adjus-
ting the flow of oxidant to ~he variable oxidant supply means in
response to the comparison so ~.s to insure that the total flow of
oxidant to the burner is maintained at the predetermined amountO
Pxeferably, the apparatus i5 suhstantially, pneumatiGally control-
led. Particularly, the means for sensing the two oxidant flow
rates and comparing these to the predetermined total flow rate,
is done by a pneumatically operated, computing relay which
further includes means for respondin~ to ~he comparison and ad-
justing the oxidant flow from its inlet to outlet portr the latter
in turn supplyin~ the ~ariable oxidan~ means. The result is that
the total amount of oxidan~ to the burner is maintained at the
predetermined amount.
BR~EF DESCRIPTION OF THE DR~WINGS
Figure 1 is an elevation view of a nebulizer-burner
assembly, typically employed in atomic absorption spectrometer
instrumentation.
; Figure 2 is a block diagram of the gas conkrol system
in accordance with the invention~
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to Figure 1, there is shown a typical
nebulizer-burner assembly used in atomic absorption spectroscopy.
It includes a cham~er 10 for mixing fuel, oxidant and the unknown
element-containing sample. The chamber eeds the burner 12 which
ignites the fuel, oxidant and sample mixture. Feeding the chamber
is a fuel line 14 which supplies a suitable gas, e~g~ acetylene,
from a regulated source.
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A~iallv connected to the chamber 10 i~ a nebulizer 16.
The internal configuration of the nebulizer is not shown, but
is understood to be operational in a manner similar to manv
such devices on the market. A typical configuration can be
seen in U.S. Patent No. 4,125,225, mentioned a~ove. The
nebulizer introduc~s a variable flow of the unknown element-
containin~ sample into the mixing chamber~
The sample solution is contained in a beaker such
as 18. Typically, the sample is an unknown metallic element
in solution. The aspirating action of the venturi-type
restriction in the nebulizer draws solution out of the beaker
through capillary tubin~ 20. For the purpose of the discussion
herein, the term "tube" or "tuhing" is understood to be des-
criptive and includes any appropriate conduit found in the art.
The aspiration of the sample is achieved by rapidly-moving
gas, typically travelling through the venturi restriction,
- which draws the solution into the nebulizer and atomizes it
into a fine spray. The rapidly-moving gas enters the nebulizer
via tubing 22. Generally, this gas is referred to as the
oxidant~ In a typical situation, it might be ni-trous oxide or,
air.
To adjust the nebulizer for most efficient sample
aspiration, the operator of the equipment, for the unit shown,
would turn knob 24. This would alter the flow of sample
into the assembly, but because of the nebuli~er design, there
will be a corresponding effect on the flow of the oxidant
entering the nebulizer through tubin~ 22. The adjustment of
the nebulizer by the operator for an optimum measured signal in the
spectrophotometer will vary from unit to unit, so that differing
effects Oll the oxidant flow rate through tubing 22 will result.
Since the amount of oxidant supplied ~o the burner is altered,
the flame condition and thus the signal measurement would be
altered except for the present invention.
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The nebulizer, typically, is a~ially joined to the
mixin~ chamber by a sealing interface at 26.
Connected to the mixing chamber is a variable (aux-
iliary) supply of oxidant. This is provided through tubing 28.
As will ~e seen hereafter in the discussion of Figure 2, the
amount of oxidant supplied at this point will be equal to the
difference between the total flow of oxidant predetermined,
for example, by a methods analYst as necesary to insure a
prescribed sensitivity, and the varying amount supplied to
the chamber by the nebulizer. As clearly shown in Figure 1,
in this embodiment the oxyyen is supplied through discrete
first and second conduits 22 and 28 respectively.
In order to sense the flow of oxidant supplied to the
nebulizer, in-line means, such as a restrictor is inserted in
the oxidant supply tubing 22. The restrictor is shown at 30.
The flow rate is sensed by monitoring the pressure on
either side o~ the restrictor. This is accomplished by pressure
monitoring parts 32 and 34, connected into the tubing 22 and 82
; respectively. For the indicated 1OW direction for the oxidant,
the pressure at termlnal 34 would be higher than the pressure at
32.
Ports 32 and 34 are shown as being directed to a so-
called 'icomputing relay" whose operation as it concerns the gas
flow control system of the invention will be discussed with re-
spect to Figure 2. Generally, its function is to compare the
pressure differential across the restrictor 30 to a predetermined
command pressure based on a prescribed sensitivitv of the spectro-
meter and to adjust the oxidant flow between the auxiliary inlet
[hereinabove referred to as inlet 28) and the nebulizer inlet
[hereinabove referred to as inlet 22) to compensate for the var-
iations of oxidant flow in 22 due to nebulizer adjustments at
knob 24.
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Referrin~ now to Figure 2, there is shown in block
diagram form, an arrangement of the various pneumatic components
which effect the purposes of the invention. The few situations
where reference numerals are identical to those employed in/
~ 10
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Fi~ure 1, those are done to identify the same identical compon
ents or tubing even though in this la~ter figure ~hey are a block
dia~ram equivalent.
Considsring the fuel supply section initially, the
burner fuel, ac~tylene, is supplied to the system ~ia tubing 36.
Acetylene is employed becaus~ it i5 readily available and inex-
pensive, Tubing 36 is connected to a pres~ure switch 38 which
se~ses a safe level before closing. Typically, the acetylene at
the input might bP on the order o lS psi~, and the threshhold
pressure of the switch 38 set at 7 p~igO ~ha switch directs the
fuel to a pair vf solenoids via tubing 40 and 42~ The irst such
~olenoid 44 is energixed at start up and direct~ the fuel through
tubing 46 to the igniter section of the burner. Once ignited,
solenoid 44 is opened and the fuel is blocked from that pa~sageway.
During subsequent hurner operation, solenoid 48 is
closed and the fuel directed therethrough to tubing 50 and pres-
sure regulator 52~ The output of ~he regulator 54 ~ typically,
would have a fuel gas pressura at 12 psig~ This pressure level
is the maximum that can be employed in the burner because of the
ins~ability of acetylene above that pressure.
The regulator is connected to a volume booster 56. A
typical unit would be a model 20, manufactured by the Industrial
Products Division of the Fairchild Company. It responds to a
command pressure on line 58, in order to further reduce the pres-
sur~ of the acetylene from 12 psig down to a value determined
previously to be optimum for spectrometer sensitivity. For
example, the pressure of the gas in tubing 57, typically, will be
a~ 6 psigO The tubing 57 i~ connected to the fuel inlet duct 14
- previou~ly referred o in Figure 1.
The command pressure input to the volume booster 56 is
supplied by a voltage to pressure transducer 60~ The latter
rec~ives an analog signal on input line 62 from a digital ko
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analo~ converter 6~. The D/A converter is supplied, via line 66,
with a digital word permanently stored on a typical memory device
such as a magnetic card or disc~ The digital word xepresents
the optLmum flow of the fuel as pr~viously d~termined by a
methods analyst in arriving at opt~mum parameters ~or the ~ystem,
The volume booster, in a situation where acetylene is
~mployed, for example, is a non-relieving type, i.e., it would
bleed off the necessary amount of acetylene into the burner to
achieve the commanded pressure diferential and not into the air
as migh~ be ~he case with relievi~y~type boosters.
The analog signal appearing on line 62 to the trans-
ducer 6Q, typically, is on the order of 0 to 9 v~olts, with the
corresponding pressure out of t~e ~ran~ducer in tubing 58, be-
tween 3 and 15 psig, ~ ty~ica1 transducer is model T5109, again
manufactured by the Industrial Produc~s Div.ision of the Fairchild
Company.
Thus, there has been described means for supplyinq a
predetermined flow rate of fuel for the burner in response to a
pre-existing command. Thus optLmi~ation of a critical parameter
is assured.
:~ The total oxidant supply to the system appears in tubing
68 and wses as its source ei~her a supply of nitrous oxide entering
on line ~9, ~hrough pres~ure switch 70 and solenoid 72, or air on
line 73 through pressure switch 74 an~ solenoid 76. The pressure
~ switches 70 and 74, typically, have a setting at 25 psig~ De~
: pending on the oxidant to be u~ed, either solenoid 72 or 76 would
be selected by appropriate control.
The oxidant in tu~ing 68 is supplied to a pressuxe re-
gulator 78 which maintains a prassure level in tubing 80 at, typi
cally, 32 psig. Tubing Y0 i5 connected by a T-connectionat
tubing 82 and 84. Tubin~ 82 ~pre~iou~ly referred to with respect
~o Figllre 1) is connected to restrictor 300 As discussed earlier~,
g
the down stream side of the restrictor is supplied to the
oxidant inlet on the nebulizer via tubing 22.
86 refers to a pneumatic computing means, known typic-
ally as a computing rela~,7. A standard unit is a model 22
computing relay as manufactured by the Industrial Products
Division of Fairchild Company. It likewise generally, would be
a non-relieving type. The computing relay includes an oxidant
inlet port~ S, and outlet port, P~ These are connected,
respectivel.y~to tubing 84, the variable oxidant flow supply,
and the auxiliary inlet 28, to the mixing chamber~
~ Further, the relay includesports C and A which are
~ connected respectively to the pressure monitoring ports on
either side of the in-line restrictor 30.
Also, the computing relav includes a comrnand pressure
port B which is connected to a command pressure supply in line
88 which emanates from a voltage to pressure transducer 90.
The latter provides a command pressure on its output from,
typically, 3 to 15 psig in response to an analoy signal of
0 to 9 volts, as received on input line 92. The analog signal
is produced by a digital to analog converter 94 and is pro-
portional to a predetermined digital word received on input
electrical line 96. The digital word appearing on line 96
would be stored, much like the signal representing the pre-
determined fuel rate on a memory device such as a magnetic
card or disc. Its value, again, would be previousl~7 determined
by a methods analyst in arriving at optimum values for the
various parameters necessary to be considerd in optimizing the
sensitivity of the instrument.
The input pressure supply for the voltage transducer-
90, and the previously described transducer 60, is developedfrom an air supply line and is inputted to the transducer 90
on line 98. Line 98 is connected to a pressure regulator 100
which is connected by line 102 to the previously discussed
pressure switch 74 on the
air input line. The r~gulator 100 maintains the pressure in lines
98 and 104 ~ the supply lines for the transducers, at an adequate
pressure necessary for the command function performed by each.
Typically, the pressure in those lines might be on the order o
2 0 psig .
The computing relay is a well known device which employs
chambers and diaphragms ~o solve the equation P=A~B-C~K,
Where P is the pressure in the oxidant outlet port, A
and C are the pressures on either side of the line restrictor,
and B is the command pressure out of transducer 90.
K i~ an ofset which is effected by a mechanical adjus-
tment on the computing relay unit~ It is set initially so as to
assure a~ port P~ a sufficiant pressure to provide the lowest flow
~ rate of oxidant in response to the lowest digital command on line
: 96.
The compllting relay is thus seen to perorm the function
of sensing and comparing the flow of oxidant to the nebulizer and
and the auxiliary oxidant supply means ~o a pxedetermined com
mand flow rate for the oxidant as represented by the pressure on
line 88. The relay adjusts the ~low of oxidant to the auxiliary
inlet in response to this comparison and does so and continues to
readjust the 10w thereto as it sen~es variations in the flow to
the nebulizer across the restrictor 30.
Other variation~ of the above embodiment would be ap
parent to those skilled in the art in light of the above. For
example, inskeacl of employing ~a computing relay~ means for sen
~ing ~he flow of oxidan~ ~o the sample introducing means (nebu-
lizer), in oxidant supply line, 22, and as well as means sensing
the flow in line 28 could be employed. The~e might, typically,
produce elec~rical s.~gnals which would then be compared with the
command electrical signalO Valves in ~ach of the supply lines
- could be provided which would be operated upon by the compared
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electrical signals 50 as to vary the amount of oxidant f lowin~
into ~he auxiliary based on th~ compared readingsO
The advantage of the present technique, is that the
computing relay can both sense the variations in oxidant f low to
the nebulizer and effect an adjustment in accordance therewith to
the oxidant flow to the auxiliary inlet.
The above described embc~diment is not to be construed
as limitin~ the extent and breadth o th~3 invesltion which ls
defined in the appended Claims,
