US3583844A

US3583844A - Atomic absorption spectroanalytical instrument control system

Info

Publication number: US3583844A
Application number: US831615A
Authority: US
Inventors: Stanley B Smith Jr
Original assignee: Instrumentation Laboratory Co
Current assignee: THERMO JARRELL ASH Corp WALTHAM MA A CORP OF; Instrumentation Laboratory Co
Priority date: 1969-06-09
Filing date: 1969-06-09
Publication date: 1971-06-08
Anticipated expiration: 1988-06-08

Abstract

A sensor responsive to hydroxyl ions is provided to monitor the flame at the burner in an atomic absorption spectroanalysis system. The sensor in the disclosed embodiment includes an S4 photodiode and a filter to provide a sensitivity over the wavelength band of 3000-3900 A. The output of the photodiode is applied via an FET amplifier stage and an emitter follower stage to burner control circuitry. The burner control circuitry initiates flame via a pilot line, and if hydroxyl ions are not sensed within a predetermined time, the system is automatically shut down. After flame is established, an interlock permits switching from air to nitrous oxide and on failure to sense hydroxyl ions, the nitrous oxide, the fuel and then the air are turned off in sequence in response to the output of the sensor.

Description

United States Patent [72] Inventor Stanley B. Smith, Jr.

Lexington, Mass. [21] Appl. No 831,615 [22] Filed June 9,1969 [45] Patented June 8,1971 [73] Assignee Instrumentation Laboratory, lnc.

Lexington, Mass.

[54] ATOMIC ABSORPTION SPECTROANALYTICAL INSTRUMENT CONTROL SYSTEM 13 Claims, 3 Drawing Figs.

[52] U.S. Cl. 431/79, 356/87 [51] Int. Cl F23n 5/08 [50] Field of Search 431/4, 79; 356/87, 187

[56] References Cited UNITED STATES PATENTS 3,080,708 3/1963 Carr 431/79X Primary Examiner-Edward G. Favors Alt0rneyWillis M. Ertman ABSTRACT: A sensor responsive to hydroxyl ions is provided to monitor the flame at the burner in an atomic absorption spectroanalysis system. The sensor in the disclosed embodiment includes an S4 photodiode and a filter to provide a sensitivity over the wavelength band of 3000-3900 A. The output of the photodiode is applied via an FET amplifier stage and an emitter follower stage to burner control circuitry. The burner control circuitry initiates flame via a pilot line, and if hydroxyl ions are not sensed within a predetermined time, the system is automatically shut down. After flame is established, an interlock permits switching from air to nitrous oxide and on failure to sense hydroxyl ions, the nitrous oxide, the fuel and then the air are turned off in sequence in response to the output of the sensor.

PATENIEDJUN elsn 3583844 'SHEET '1 OF 2 v FIGI ogre-era? RANGE I F 2 HAND E MISS/0N I I 2000 000 4000 5000 a 6000 l PATENTED JUN a m SHEET 2 UP 2 ATOMIC ABSORPTION SPECTROANALYTICAL INSTRUMENT CONTROL SYSTEM SUMMARY OF INVENTION This invention relates to atomic absorption spectroscopy and more particularly to control arrangements for burners used in such spectroanalytical systems.

In atomic absorption spectroscopy a mixture of combustible fuel and atomized solution containing material to be analyzed are fed into a burner where the material to be analyzed is energized for analysis purposes. Several different fuel oxidant mixtures are used in atomic absorption analysis, including acetylene-air, acetylene-nitrous oxide, hydrogen-air and hydrogen-argon mixtures. Hydrogen-air and hydrogen-argon flames are particularly useful for some elemental determinations, including those elements analyzed in the short ultraviolet ray region near 2000 A, such as lead, iron, arsenic and tin. Loss of flame can create hazardous conditions, therefore it is an object of this invention to provide a comprehensive burner control system which monitors the presence of a nonnal flame at the burner and which will automatically turn off the fuel in the event of abnormal condition such as loss of flame, for example due to abnormal flashback or flashout.

Another object of this invention is to provide a control system which will supervise the proper operation of an atomic absorption burner system that uses a variety of fuels.

Another object of the invention is to provide a novel and improved atomic absorption burner control system that supervises the proper operation of the burner system when using hydrogen as a fuel as well as utilizing acetylene as a fuel.

Another object of the invention is to provide a novel and improved atomic absorption burner control system which will automatically sequence the fuels in desired operation and prevent or terminate ignition if there is insufficient air or fuel pressure or if the burner fails to ignite within a predetermined period of time.

In accordance with the invention there is provided an atomic absorption burner control system which includes a sensor responsive to hydroxyl ions (free radicals) in the flame for controlling the operation of the burner system. In particular embodiments the flame sensor includes a photosensor that responds to radiation including the range of emission bands of hydroxyl free radicals. Those emission bands are in the order of 2600 A, 2800 A, 3200 A and 3500 A and the photosensor has an upper limit of response below 4500 A. In a particular embodiment a photocell and filter combination provide a photosensor that has a band width response that includes the wavelengths 30003900 A.

In a particular embodiment the atomic absorption burner control system includes a nebulizer into which the sample to be analyzed is aspirated by air which is used as an oxidant, a premixing chamber in which the nebulized sample and fuel are mixed and a burner head structure to which the oxidant-fuelsample mixture is supplied. This burner head structure is aligned with the optical path of the atomic absorption equipment and an igniter structure that includes a pilot fuel line and igniter electrode is arranged to automatically ignite the main flame under operator control. The sensor is mounted in alignment with the optical path for sensing the presence of flame at the main burner. The control system sequences ignition and transfer between oxidants in a coordinated manner. Shutdown of the burner system occurs automatically in the event of flame failure or loss of fuel or oxidant pressure.

The invention provides a comprehensive supervision system for a burner system in atomic absorption spectroanalytical apparatus which permits supervised use of a variety of fuels. Other objects, features and advantages of the invention will be seen as the following description of a particular embodiment progresses, in conjunction with the drawings, in which:

FIG. I is a diagrammatic view of an atomic absorption burner control system constructed in accordance with the invention;

FIG. 2 is a Colthup chart representation of the band emission of different ions in flames employed in atomic absorption spectroscopy; and

FIG. 3 is a schematic diagram of control circuitry employed in the apparatus shown in FIG. 1.

DESCRIPTION OF PARTICULAR EMBODIMENT With reference to FIG. 1, the atomic absorption burner system includes a premix chamber 10 on which is mounted a burner head 12 that is connected to the premix chamber 10 by means of conduit 14. The configuration of the burner head changes as a function of the fuel employed, for example a particular burner head for use with air as an oxidant employs an exit orifice 10 centimeters in length having one or more slots; while a burner head for use with nitrous oxide as an oxidant has a single slot 5 centimeters in length. A switch 16 on conduit I4 is closed when the nitrous oxide burner head is in proper position on conduit 14. At the end of the premix chamber remote from the burner head 12 is secured an end cap that supports a nebulizer structure generally indicated at 18. A capillary tube 20 extends from nebulizer structure to the source of liquid sample to be analyzed. This sample is aspirated through nebulizer 18 into premix chamber 10 by oxidant, for example air being supplied over line 22 as controlled by solenoid operated valve 24 and nitrous oxide being supplied over line 26 as controlled by solenoid operated valve 28.

Switches

30, 32, respectively, are closed if pressures in the supply lines are within preestablished limits. A fuel input line 34 controlled by solenoid operated valve 36 applies fuel through metering orifice 38 and manifold 40 to chamber 10. Switch 42 is closed if fuel pressure is within preestablished limits. Oxidant is also supplied to manifold 40 via metering orifice 44.

Extending from manifold 40 is a pilot line 46 that has interposed in it a control valve 48 and has its outlet 50 (0.040 inch l.D.) spaced several inches from burner head 12. An igniter rod electrode 52, disposed in proximity to the outlet 50 of the pilot fuel line, is connected to a blocking oscillator-induction coil circuit 54 which is controlled by circuit 56.

Supported above the burner head is a detector unit 58 which includes a type TS-433E photodiode 60 and a filter element 61 (Coming Filter No. 7-54). The photodiode 60 has a sensitivity over the range 3000-5500 A while the filter 61 passes wavelengths in the range of 2400-3900 A. Thus this filter and photodiode combination define a system wavelength sensitivity in the range of 3000-3900 A. The detector 58 is mounted at an angle of 45 to the optical axis 62 of the atomic absorption system, the detector being diagrammatically shown in FIG. 1 1, from its actual system position in the preferred embodiment for clarity. The output of diode 60 is applied through two amplifier stages, the first stage including a field effect transistor 63 and the second stage including an NPN transistor 64, which produces an output signal for application to control circuitry shown in FIG. 3.

With reference to FIG. 2, wavelengths emitted by various molecules in flames are indicated. Thus C radicals radiate in a band 65 at about 3400 A and in the so-called Swan bands 66 in the green section of the spectrum; CI-l radicals radiate in a band 67 at about 4300 A; and OH radicals radiate in a series of narrow bands 68 near 3000 A. While acetylene produces C radicals, these radicals are not present where hydrogen is used as the fuel. The OH radicals are produced by acetylene-air, acetylene-nitrous oxide, fuel mixtures as well as hydrogen-air and hydrogen-argon mixtures and other hydro-carbon fuels such as propane. The photosensor 58 has a response in range 69 and is constrained to sense wavelengths produced by the OH radicals and to exclude sensing the C radicals. It will be obvious that other photosensor configurations, such as those employing interference filters, for example, could also be employed.

A schematic diagram of the control circuitry employed in this system is shown in FIG. 3. That circuitry controls pilot solenoid 48, fuel solenoid 36, air solenoid 24, nitrous oxide solenoid 28 and igniter circuitry 56 as a function of signals from sensor 58 and

switches

16, 30, 32 and 42 and controls which include a FLAME ON control button 70 which applies a negative 12 volt signal on line 72 when the button 70 is depressed; a FLAME OFF button 74; an air flush control 76; nitrous oxide ON control switch 78 and nitrous oxide OFF control switch 80.

The circuitry further includes a main control flip-flop 82 that includes

transistors

84 and 86; an oxidant control flip-flop 90 that includes

transistors

92 and 94;

transistor switch circuits

96, 98, 100 and 102 that control

solenoids

24, 28, 36 and 48 respectively and which are controlled in turn by

transistor circuits

104, 106, 108 and 110 respectively; transistor 112 which responds to the output of sensor 58; and

transistors

114, 116 and 1 18 which provide sequencing control.

When the system is turned on and no flame is present at burner 12,

transistors

86 and 94 are in nonconductive condition. If the fuel and oxidant pressures are above the minimum, the minimum for switch 42 being p.s.i., for

switches

30 and 32 being 8 p.s.i., 12 volt signals are applied through those switches to the circuitry. Upon depression of FLAME ON button 70, a negative transition is applied via diode 120, the voltage network consisting of

resistors

122 and 124 and diode 126 to the base electrode of transistor 86 to turn that transistor on. The resulting change in condition of transistor 86 turns output transistor 128 on and a negative transition is applied via resistor 130 and diode 132 to place transistor 118 in conducting condition. The resulting transition applied via resistor 134 to gating transistor 106 which is conditioned by the conducting output transistor 136 to turn on transistor 106 and transistor 98 to energize solenoid coil 24, providing an air flow through the mixing chamber and the burner head 12.

The turn on of transistor 118 also applies a transition via diode 138 to the voltage divider network of

resistors

140 and 142 to turn on transistor 108 which in turn turns on transistor 100 and operates the fuel control solenoid 36 to supply fuel to the mixing chamber 10.

The transition resulting from transistor 112 being placed in conduction is also applied to the voltage divider network consisting of

resistors

144 and 146 to turn off transistor 114. J unction 148 then goes to +12 volts potential and timing capacitor 150 commences to charge. The transition at junction 148 is applied through resistor 152 to diode 154 to turn on transistor 116. A positive going output transition is applied via resistor and diode 138 to transistor 84. That transition has no effect, however, as transistor 84 is in nonconducting condition.

Control transistor 112 is in conducting condition when there is no flame present. In that condition the cathode of diode 156 is at ground. At the same time that driver transistor 128 is turned on, its complementary driver transistor 158 on the other side of the control flip-flop 82 is turned off and the cathode of diode 160 also goes to ground allowing junction 162 to rise to ground potential. The resulting transition is applied via voltage divider

network including resistors

164 and 166 to turn on transistor 110 which in turn causes transistor 102 to conduct and energize pilot solenoid 48. The transition at junction 162 is also applied via voltage divider network of

resistors

168 and 170 and time delay circuit including resistor 172 and capacitor 174 to turn on transistor 176 of control circuit 56 after a short time delay to energize the igniter control 54 which includes a blocking oscillator and an ignition coil to apply a spark periodically between igniter electrode 52 and the tip of pilot line 50 (which is the negative electrode) to ignite fuel oxidant mixture flowing through the pilot line orifice 50.

The pilot jet flame in turn ignites the main burner flame and that flame is sensed by the detector 58 so that the photocell 60 produces an output which is amplified by an FET amplifier stage 178 and an emitter follower transistor stage 180 to produce a positive transition which turns off transistor 112. With the turn off of transistor 112, the voltage at junction 162 falls, transistors 110 and 176 cease conducting, terminating ignition and pilot fuel flow. A transition is also applied via the voltage network including resistors 182 and 184 and diode 186 to hold transistor 114 in nonconducting condition, thus overriding the effect of the charging of capacitor 150. A similar transition is applied through the voltage divider network of

resistors

188 and 190, diode 192 to hold transistor 116 in conducting condition. Should there be an intermittent flame condition so that transistor 112 returns to conduction, a transition is immediately applied via diode 186 to turn transistor 114 on and transistor 116 off and that transition through resistor 194 and diode 196 turns transistor 84 on and resets the control flip-flop 82.

Assuming that flame is established and remains established, the system is now operating on a fuel air mixture. If it is desired to transfer from air as the oxidant to nitrous oxide,

switch 78 is depressed which completes a circuit from the junction of

diodes

198 and 200 through switch 16 (closed by the proper burner head) to the oxidant control flip-flop 90 to turn on transistor 94 and its output transistor 202. The change in state of flip-flop 90 causes transistor 106 to cease conducting and

transistors

104 and 96 to conduct and energize the nitrous oxide control solenoid 28. It will be noted that diode 200 is connected to nitrous oxide pressure switch 32 and diode 202 is connected to the collector of flame detector control transistor 112. The l2 volt signal required to operate control flip-flop 90 to turn on transistor 94 will not be available unless the nitrous oxide pressure switch 32 is closed and flame is being detected at the burner head by sensor 58. When those conditions are present, the system may be switched from air to nitrous oxide by depression of switch 76. Depression of switch applies a 1 2 volt signal to the other input of control flip-flop to cause transistor 98 to conduct and transistor 96 to cease conducting, immediately switching from nitrous oxide to air.

Should either switch 30 or 42 open, due to air pressure or acetylene pressure drop, respectively, the voltage divider network of

resistors

204 and 206 will produce a transition atjunction 108 which will be coupled by diode 210 to turn transistor 86 off and by diode 212 to force control flip-flop 90 to the "air" mode, if it was not there already. Should the nitrous oxide pressure sensing switch 32 open, a voltage transition will be coupled by the divider network of

resistors

214 and 216 and diode 218 to switch the control flip-flop 90 to the air" mode and terminate the flow of nitrous oxide if the system had been in that mode. The control flip-flop 82 is also reset on depression of the FLAME OFF button 74.

When control flip-flop 82 is reset,

transistors

84 and 158 are turned on.

Transistors

86 and 128 are turned off and capacitor 220 connected between the base and collector electrodes of transistor 118 starts to charge. Transistor 108 is turned off after a 7 second delay terminating fuel flow and transistor 106 is turned off after a 10 second delay, thus terminating the flow of air to the burner system and completing a shut down of the burner in safe condition.

While a particular embodiment of the invention has been shown and described, various modifications thereof will be apparent to those skilled in the art and therefore it is not intended that the invention be limited to the disclosed embodiment or to details thereof and departures may be made therefrom within the spirit and scope of the invention as defined in the claims.

What 1 claim is:

1. In an atomic absorption spectroanalysis system comprising means defining an optical axis, a burner disposed below said optical axis for producing a flame to energize a sample to be analyzed so that said energized sample passes across said opticalaxis to modify radiation in said optical axis in an atomic absorption analysis, and means to supply a fuel oxidant mixture to said burner to produce said flame, the improvement of a sensor responsive to hydroxyl ions for monitoring the presence of flame at said burner.

2. The apparatus as claimed in claim 1 wherein said sensor is a photosensor responsive to emission from hydroxyl ions in the flame.

3. The apparatus as claimed in claim 2 wherein said photosensor has a band width of response, the upper limit of which is below 4500 A.

4. The apparatus as claimed in claim 3 wherein said photosensor has a bandwidth of response that includes the wavelength of 3000-3900 A.

5. An atomic absorption spectroanalysis system comprising a burner for producing a flame to energize a sample to be analyzed, a chamber for supplying a fuel-oxidant-sample mixture to said burner, a sensor responsive to hydroxyl ions for monitoring the presence of flame at said burner,

a first control for controlling the supply of fuel to said chamber, a second control for controlling the supply of oxidant to said chamber and circuitry responsive to said sensor for controlling said first and second controls 6. The apparatus as claimed in claim 5 wherein said circuitry further includes an igniter for igniting a flame, a flame initiating control, circuitry responsive to said flame initiating control for operating said first and second controls and said igniter in sequence and circuitry responsive to said sensor for terminating flow of oxidant and fuel to said chamber unless said sensor produces an output signal indicative of flame within a predetermined interval.

7. The apparatus as claimed in claim 6 wherein said sensor responsive circuitry operates said first control to terminate flow of fuel to said chamber prior to operation of said second control to terminate flow of oxidant to said chamber in the absence of said output signal from said sensor.

8. The apparatus as claimed in claim 5 and further including means for supplying first and second oxidants to said chamber,

each said oxidant employing a different burner unit, said second control controlling the supply of one of said oxidants and a third control controlling the supply of said second oxidant and an interlock responsive to a burner unit for controlling the operation of said third control.

9. The apparatus as claimed in claim 8 and further including pressure responsive control for sensing the supply pressure of said second oxidant and circuitry responsive to a low supply pressure of said second oxidant for operating said third control to terminate the flow of said second oxidant to said chamber and for automatically operating said second control to initiate the flow of said first oxidant to said chamber.

10. The apparatus as claimed in claim 9 wherein said circuitry includes a first bistate device for controlling burner operation and a second bistate device for controlling the supply ofoxidant to said chamber.

11. The apparatus as claimed in claim 10 wherein said sensor is a photosensor responsive to emission from hydroxyl ions in the flame.

12. The apparatus as claimed in claim 11 wherein said photosensor includes a photocell and a filter arranged to have a response bandwidth which includes the wavelengths of 3000-3900 A.

13. The apparatus as claimed in claim 11 wherein said photosensor is a photodiode and has an output terminal connected to an amplifier circuit that includes a field effect transistor for generating an output signal for control of said first, second and third controls as a function of the flame condition at said burner.

Claims

2. The apparatus as claimed in claim 1 wherein said sensor is a photosensor responsive to emission from hydroxyl ions in the flame.
3. The apparatus as claimed in claim 2 wherein said photosensor has a band width of response, the upper limit of which is below 4500 A.
4. The apparatus as claimed in claim 3 wherein said photosensor has a bandwidth of response that includes the wavelength of 3000-3900 A.
5. An atomic absorption spectroanalysis system comprising a burner for producing a flame to energize a sample to be analyzed, a chamber for supplying a fuel-oxidant-sample mixture to said burner, a sensor responsive to hydroxyl ions for monitoring the presence of flame at said burner, a first control for controlling the supply of fuel to said chamber, a second control for controlling the supply of oxidant to said chamber and circuitry responsive to said sensor for controlling said first and second controls.
6. The apparatus as claimed in claim 5 wherein said circuitry further includes an igniter for igniting a flame, a flame initiating control, circuitry responsive to said flame initiating control for operating said first and second controls and said igniter in sequence and circuitry responsive to said sensor for terminating flow of oxidant and fuel to said chamber unless said sensor produces an output signal indicative of flame within a predetermined interval.
7. The apparatus as claimed in claim 6 wherein said sensor responsive circuitry operates said first control to terminate flow of fuel to said chamber prior to operation of said second control to terminate flow of oxidant to said chamber in the absence of said output signal from said sensor.
8. The apparatus as claimed in claim 5 and further including means for supplying first and second oxidants to said chamBer, each said oxidant employing a different burner unit, said second control controlling the supply of one of said oxidants and a third control controlling the supply of said second oxidant and an interlock responsive to a burner unit for controlling the operation of said third control.
9. The apparatus as claimed in claim 8 and further including pressure responsive control for sensing the supply pressure of said second oxidant and circuitry responsive to a low supply pressure of said second oxidant for operating said third control to terminate the flow of said second oxidant to said chamber and for automatically operating said second control to initiate the flow of said first oxidant to said chamber.
10. The apparatus as claimed in claim 9 wherein said circuitry includes a first bistate device for controlling burner operation and a second bistate device for controlling the supply of oxidant to said chamber.
11. The apparatus as claimed in claim 10 wherein said sensor is a photosensor responsive to emission from hydroxyl ions in the flame.
12. The apparatus as claimed in claim 11 wherein said photosensor includes a photocell and a filter arranged to have a response bandwidth which includes the wavelengths of 3000-3900 A.
13. The apparatus as claimed in claim 11 wherein said photosensor is a photodiode and has an output terminal connected to an amplifier circuit that includes a field effect transistor for generating an output signal for control of said first, second and third controls as a function of the flame condition at said burner.