System for changing oxidants in a flame atomic absorption spectrophotometer

A flame atomic absorption spectrophotometer burner apparatus for burning a fuel such as acetylene as one essential gas in the oxidant such as nitrous oxide as another essential gas includes a system for safely changing oxidants between air and nitrous oxide during start up. The system has a pressure sensor for sensing the pressure of at least one of the essential gasses, and a valve selectively operable in response to the pressure sensor for effecting the supplying of air as the oxidant and continuing to supply air as the oxidant when the pressure of the essential gas is low and for selectively switching to nitrous oxide as the oxidant only when the pressure of the essential gas is high.

BACKGROUND OF THE INVENTION 
The present invention relates to a flame atomic absorption 
spectrophotometer of the type which requires air as a start-up and 
shut-down oxidant and uses nitrous oxide as a high energy oxidant for the 
burner flame. The present invention particularly relates to an improved 
apparatus for safely changing from air as the oxidant to nitrous oxide as 
the oxidant, and back again. 
In atomic absorption spectroscopy, 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, one of the most common techniques for 
atomizing an element for purposes of the absorption measurement is by 
introducing a liquid 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 radiation 
beam, a substantial quantity of the element of interest in its atomic 
state. A sample light beam, which originates from a line-emitting light 
source, and which includes a resonance line of the element to be measured, 
is directed through the flame. The desired element in the sample absorbs 
the resonance lines characteristic of the element and the emerging light 
beam is directed to a monochromator and thence to a detector which 
measures the degree to which the desired element absorbs the resonance 
lines of the sample beam. This absorption degree represents the amount of 
desired element in the sample substance. 
In such spectrophotometers, in order to produce a flame which has a high 
enough temperature for the best measurement results for certain elements, 
it is preferred to use acetylene gas as a fuel and to use nitrous oxide 
(N.sub.2 O) as the source of oxygen for the combustion of the acetylene 
gas. In order to initiate combustion in a safe manner, it is necessary to 
begin combustion of the acetylene gas using air as the oxygen source, and 
to then switch over to the nitrous oxide after the acetylene gas flame is 
ignited and stable. 
The essential gases for the steady-state high temperature operation of the 
burner of a flame atomic absorption spectrophotometer, as described above, 
are the fuel acetylene and the oxidant nitrous oxide. It is important that 
both of these essential gases be available in adequate quantities for the 
system to successfully shift from operation on air to operation on nitrous 
oxide during start-up, and that those essential gases continue in adequate 
supply in order to sustain operation on nitrous oxide. 
It is also important that there must be a flame at the time of switch-over 
from air to nitrous oxide, and that the flame be maintained in order to 
support operation on nitrous oxide. Furthermore, since the entire control 
system of the spectrophotometer is usually operated by electrical power, 
including the control of fuel flow, it is important that electric power 
should continue to be available during operation under nitrous oxide. 
Still further, it is important that a sufficient volume of oxidant, under 
sufficient pressure, should be available at the burner of the flame atomic 
absorption spectrophotometer (not having been reduced too much by the 
manual flow control adjustment valve) in order to provide for a successful 
change-over from air to nitrous oxide. 
In prior burner control systems, the operator has been relied upon for 
assuring that some or all of the above mentioned conditions were met in 
switching from air as the start up oxidant to nitrous oxide as the running 
oxidant and for assuring that the conditions are maintained. Such an 
arrangement involves risks because of possible operator error or 
inattention, and the result can often be an explosion, or improper 
combustion. Furthermore, some of the above conditions are not immediately 
apparent to the operator. 
Accordingly, it is an important object of the present invention to provide 
an improved burner control system for a flame atomic absorption 
spectrophotometer apparatus in which one or more of the above listed 
requirements for safe switching from air to nitrous oxide as the oxidant 
are automatically assured, and in which the switch over to nitrous oxide 
cannot be made without the fulfillment of the condition or conditions. 
One prior arrangement for inexpensive burner systems involves the use of a 
separate valve for each oxidant source, the change over being accomplished 
by simply closing the valve for one oxidant while opening the valve for 
the other oxidant. Such a procedure involves the risk that the change over 
may be slow, resulting in undesirable fuel-oxidant mixtures, or that the 
oxidant from one source may be completely shut off before the oxidant from 
the other source is turned on. This may result in extinguishment of the 
flame together with the risk of possible later explosion if the change is 
being made to nitrous oxide. 
Another object of the invention is to provide an improved burner system 
which includes an improved means for switching over from one oxidant to 
the other in a very rapid manner so as to avoid the possibility of 
improper fuel-oxidant mixtures. 
Another object of the invention is to provide an improved burner system for 
a flame atomic absorption spectrophotometer which is operable to switch 
from one oxidant to another while avoiding any risk of an interval with 
both oxidants shut off. 
It is another object of the invention to provide an improved burner system 
for a flame atomic absorption spectrophotometer apparatus which 
automatically fulfills one or more of the above mentioned requirements for 
safe operation and which is very efficient and cost effective. 
Other objects and advantages of the invention will be apparent from the 
following description and the accompanying drawings. 
SUMMARY OF THE INVENTION 
In carrying out the invention there is provided a flame atomic absorption 
spectrophotometer burner apparatus for burning a fuel such as acetylene as 
one essential gas in an oxidant such as nitrous oxide as another essential 
gas, a system for safely changing oxidants between air and nitrous oxide 
during start up comprising means for sensing the pressure of at least one 
of said essential gasses, and means selectively operable in response to 
said pressure sensing means for effecting the supplying of air as the 
oxidant and continuing to supply air as the oxidant when the pressure of 
said essential gas is low and for selectively switching to nitrous oxide 
as the oxidant only when the essential gas pressure is high.

DETAILED DESCRIPTION 
Referring more particularly to the drawing, the burner for a flame atomic 
absorption spectrophotometer is schematically shown at 10, which normally 
has a flame 12 when in operation. The burner contains a nebulizer into 
which a liquid sample for analysis is introduced through an inlet 14. 
Acetylene fuel is supplied to the burner through a conduit 16, and oxidant 
through a conduit 18. 
The oxidant for conduit 18 is supplied either as air from a compressed air 
source such as air compressor 20, or as nitrous oxide from a source such 
as a pressure canister schematically indicated at 22. The fuel for the 
fuel line 16 is supplied from a fuel source such as a canister indicated 
at 24. 
The nitrous oxide from canister 22 and the fuel from canister 24 are 
sometimes referred to in this specification as "essential" gases because 
they are both essential for maintaining a flame in the selected mode, 
different from the default mode of operation which involves an air and 
acetylene mixture. 
Briefly and broadly described, the system includes a pressure actuated 
switching valve 26 which is controlled by one or more pilot valves 28, 30, 
and 32. The switching valve 26 is operable to switch the supply of oxidant 
from the air compressor 20 through an air conduit 34 to the nitrous oxide 
supplied from canister 22 through a conduit 36. A manual control valve 38 
is provided in the oxidant line to adjust the flow of either oxidant to 
the burner 10. 
Assuming all of the pilot valves 28, 30, and 32 are open, the switching 
valve 26 receives actuating pressure through the sensing line conduit 
sections 36A, 36B, 36C, 36D, and 36E from the nitrous oxide source 22. The 
switching valve 26 is preferably spring biased, as schematically 
illustrated by a spring 40, with a spring force which is overcome by 
nitrous oxide pressure in the chamber 42 when that pressure is at, or 
above, an adequate nitrous oxide operating pressure for the system. Thus, 
the switching valve itself senses the adequacy of the nitrous oxide 
pressure, and thus measures the adequacy of the nitrous oxide supply as a 
condition of operation. 
All of the valves are shown in a simplified schematic section form to 
illustrate the principle of operation of each valve in the system. It will 
be understood that the valves may be constructed in various ways and may 
incorporate various refinements which are not illustrated. 
The switching valve 26 is illustrated as including two valve lands 44 and 
46. The valve is shown in the "air" position as biased by the spring 40 in 
the absence of substantial pressure in pressure chamber 42. In this 
position, the air ports 48 and 50 are open to provide for delivery of air 
as the oxidant. Upon the presence of sufficient nitrous oxide pressure in 
chamber 42, the valve piston moves downwardly and the ports 48 and 50 are 
closed by land 44. Substantially concurrently, land 46 opens up ports 52 
and 54 to provide for delivery of nitrous oxide instead of air through the 
conduit 36. 
The valve 26 may be characterized as a means for sensing the available 
pressure of at least one of the essential gases (nitrous oxide). The valve 
26 may also be characterized as a means selectively operable in response 
to the sensing of the available pressure of the essential gas for 
effecting the supplying of air as the oxidant and continuing to supply air 
as the oxidant when the pressure of the essential gas is low and for 
selectively switching to nitrous oxide as the oxidant only when the 
essential gas pressure is high. The biasing spring 40 is operable to 
switch off the nitrous oxide, and to switch on the air if the nitrous 
oxide supply is depleted sufficiently to cause the pressure to drop below 
the bias force as sensed in the switching valve 26. 
Pilot valve 32 is a pressure actuated valve which responds to the oxidant 
pressure in conduit 18 as supplied to the burner 10. The pilot valve 30 is 
a pressure actuated valve which responds to the pressure of the fuel 
supply from canister 24 through a conduit 56. 
The pilot valve 28 is a solenoid actuated pilot valve with a piston having 
lands 58 and 60 which is movable by means of a solenoid 20 plunger 62 
under the electromagnetic force from a winding 64. The structure is 
illustrated in an idealized schematic form. The winding 64 is energized 
from the main power source as indicated by the terminals 66 and 68 through 
25 switches 70 and 72. Switch 70 is closed manually when nitrous oxide 
operation is to be initiated by the operator. Switch 72 is a schematic 
representation of a switch (or relay contact) which is actuated by a flame 
sensing device such as a photo cell 74 in response to illumination from 
the flame 12. Thus, the switch 72 is closed if the flame is present, and 
open if the flame is absent. 
Therefore, if there is no flame, the solenoid valve 28 is not actuated, and 
the system cannot be shifted over to nitrous oxide operation even if the 
manual switch 70 is closed. Furthermore, if the flame goes out, the flame 
sensor 74 control of switch 72 releases the solenoid valve so that it 
closes under the force of a biasing spring 76, which causes the system to 
switch back to air as the oxidant. When the solenoid is not energized, the 
piston of the solenoid valve is biased closed by spring 76 as shown, with 
the piston land 60 closing the ports associated with conduits 36A and 36B. 
At the same time, conduit 36B is vented through a branch conduit 36E and 
an exhaust port 78. When the solenoid is energized, piston land 60 moves 
downwardly, uncovering the ports associated with conduits 36A and 36B so 
as to provide an interconnection between the two, and piston land 58 
covers the ports at conduit branch 36E and 78 to close off that exhaust 
path. 
Whenever the solenoid is de-energized, such as by the loss of the flame, 
the piston returns to the raised position illustrated in the drawing, 
closing off the ports associated with conduits 36A and 36B, and opening 
the exhaust path to assure the immediate reduction of pressure in conduit 
36B so as to cause the switching valve 26 to immediately switch the system 
back to air as the oxidant. 
It will be apparent that the system is also automatically switched back to 
air as the oxidant if system power fails, or is shut off, or if the 
selector switch 70 is opened to de-energize the solenoid 
The pressure actuated pilot valve 30 includes a biasing spring 80 which 
provides an appropriate bias to determine the minimum fuel pressure at 
which conversion to operation with nitrous oxide as the oxidant is to be 
permitted. Whenever that pressure is equalled or exceeded, the piston of 
that pilot valve is forced 15 upwardly, opening the ports associated with 
the conduits 36B and 36C to transmit the control pressure through to the 
switching valve 26. At the same time, the lower land of the valve piston 
closes off an exhaust which otherwise exhausts the pressure in conduit 36C 
through a branch 36F. 
The pressure actuated pilot valve 32 may be substantially identical to the 
pilot valve 30, and operates in a similar manner, except that the biasing 
spring 82 will typically have a different spring constant in order to set 
a different minimum pressure for the oxidant output supply which permits 
the switching to nitrous oxide. 
In accordance with another preferred feature of the invention, a second 
switching valve 84 is provided for increasing the flow of fuel through the 
fuel conduit 16 when the oxidant supply is switched from air to nitrous 
oxide. The switching valve 84 is substantially identical to the switching 
valve 26, except for the omission of ports 48 and 50. The spring bias of 
switching valve 84 is selected to be slightly less than the spring bias of 
spring 40 of switching valve 26 so that the fuel switching valve 84 will 
be sure to operate whenever the switching valve 26 operates so that there 
will always be an additional acetylene supply whenever nitrous oxide is 
switched in as the oxidant. 
As shown in the drawing, the fuel supply system includes two parallel 
paths, a master path represented by conduit 86, and a slave path 
represented by conduit 88. When operation is carried out with air as the 
oxidant, fuel is supplied only through the master path 86, which includes 
a manual fuel flow adjusting valve 90. When the slave path 88 is opened by 
the switching valve 84, the flow through the slave path 88 is controlled 
to be proportional to, and preferably substantially equal to the flow 
through the master path 86. This is accomplished by means of a pressure 
responsive flow control valve 92 which detects and responds to the 
respective flows in the master and slave paths by the detection of 
pressure drops through orifice devices 94 and 96. This fluid flow control 
apparatus, including the valve 92, and the orifices 94 and 96, forms at 
least a part of the subject matter of a co-pending patent application Ser. 
No. 670,714 filed concurrently with the present application for a FLUID 
FLOW CONTROL SYSTEM and assigned to the same assignee as the present 
invention. 
While the switching valve 26 (and the switching valve 84) are shown as 
controlled by a combination of three separate series connected pilot 
valves 28, 30, and 32. It will be appreciated that if all of the recited 
control functions are not desired, only one or two of these pilot valves 
may be employed. When the pressure sensing conduit line 36A-36E is 
pressurized by the operation of all of the pilot valves the switch over 
from air to nitrous oxide is rapidly accomplished, while avoiding any risk 
that both oxidants are shut off at any time. While not necessarily evident 
from the drawing, the spacing of the lands 44 and 46 in valve 26 is 
preferably such that as the ports 48 and 50 for the air are cut off, the 
ports 52 and 54 are being opened up. 
While the switching valves 26 and 84 are illustrated in the drawing as 
substantially the same size as the pilot valves 28, 30, and 32, it will be 
understood that, since the pilot valves need not provide for a substantial 
fluid flow, and are only basically pressure gating devices, the pilot 
valves may be much smaller than the switching valves 26 and 84. Thus, much 
less expensive structures are required for the pilot valves. This is an 
important economic advantage in the present invention. Thus, for instance, 
if a solenoid valve is employed in place of the switching valve 26, a much 
larger, and more expensive solenoid valve is required. Also, a large 
switching valve 26 which is simply pressure actuated, as in the present 
system, is considerably less expensive than a solenoid valve of the same 
capacity. Accordingly, there is an over-all saving in the combination of 
the solenoid valve 28 and the switching valve 26, in addition to the 
attributes of response to various conditions, including a minimum nitrous 
oxide pressure. 
As previously mentioned, the switching valves 26 and 84, and the pilot 
valves 28, 30, and 32, are illustrated schematically to promote the 
understanding of the system. It will be understood that various pneumatic 
valve structures may be used which accomplish the same functions. Such 
valve structures are commercially available from vendors such as Clippard 
Instrument Laboratory, Inc., 7390 Colerain Road, Cincinnati, Ohio 45239. 
While this invention has been shown and described in connection with 
particular preferred embodiments, various alterations and modifications 
will occur to those skilled in the art. Accordingly, the following claims 
are intended to define the valid scope of this invention over the prior 
art, and to cover all changes and modifications falling within the true 
spirit and valid scope of this invention.