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1、4.4 Basic Operation of Atomic Absorption SpectrophotometerAtomic absorption spectrometry module ofAbility objectivepBe able to use atom absorption spectrophotometer,air compressor,and acetylene steel cylinder correctly.pBe able to troubleshoot simple instrument faults.Problem introductionpFood nutri
2、tion and safety are the two themes most closely related to everyone in modern life.The pollution prevention of trace elements and heavy metals is an important part of food nutrition safety,which can not be separated from the work of analysis and detection.If an existing food sample needs to be deter
3、mined with trace metal elements(such as Zn,Mg,Pb,Cu,etc.),then what method can be used?Please tell us the method you are most familiar.pSolution:Chemical method,potentiometric method and UV-visible spectrophotometric method can be used.These methods have their own characteristics.The sensitivity of
4、chemical method is relatively low,which may be insufficient in the measurement of trace elements.The other two methods are easy to be interfered by the co-existing components in solution.Atomic absorption spectroscopy is a very good method to solve this kind of problem.Analytical processLight source
5、(emission of characteristic spectrum line)atomizer(sample conversion into atomic vapor)spectroscopic system(separation of characteristic spectrum line)detection system(signal conversion,amplification,and display)p Atomic absorption spectroscopy is an analytical method based on the absorption of char
6、acteristic radiation by atomsLight sourceFlame atomizerEffluent outletdetection systemspectroscopic systemTest solutionReading instructionpIn front of the students are the instructions for the use of the atomic absorption spectrophotometer.Please read the instrument introduction and operation sectio
7、n carefully to have a basic understanding of the instrument,and to know the electrical and pneumatic connection of the instrument and the function of each function button by referring to the actual object of the instrument.Turn on the power supply-start the computer-install the hollow cathode lamp-s
8、tart the main engine-open the operation software-initialization-peak-seeking-set the experiment condition-check the drainage safety interlock device-open the air compressor(adjust the outlet pressure 0.25-0.03MPa)-open the acetylene steel cylinder(adjust the outlet pressure 0.05-0.07MPa)-open the ai
9、r outlet-ignite fire-determine sample-save the data-close off the acetylene steel cylinder after determination-close the air compressor after flame extinguished-close the air outlet-withdraw software-close the main engine power supply-turn off the computer-fill in the using record of instrument.Inst
10、rument switch and procedureTurn on the power supplyIn order to ensure the stability of the instruments power supply,a stabilized power supply is provided.A.Press the switch on the rear panel of the stabilized power supply lightlyB.Working light on,voltage display 220VBasic Operation Demonstration 1S
11、tart the computerTAS990 atomic absorption spectrophotometer is equipped with microcomputer system and has special operation software.First start the computer to enter the wendous operation system before starting up.Basic Operation Demonstration 2AAWin V2.1 working softwarep Select the same hollow ca
12、thode lamp as the determination element to align the lamp foot to fit the insertion.Mounted hollow cathode lamp2.Put the HCL lamp into the lamp chamber and remember the position number.1.Insert the projecting portion of the HCL foot into the groove of the lamp holder;3.Tighten the lamp holder fixing
13、 screw;4.Cover the light chamber door.Basic Operation Demonstration 3Turn on the power supplyBasic Operation Demonstration 41.Adjust the rotation adjusting button of the burnerAdjust the burner and align the light path3.Make the spot emitted from the light source directly above the burning slit and
14、parallel to the burning slit.2.Adjust the front and rear adjusting button of the burnerBasic Operation Demonstration 5InitializationThe use of working softwareSelection and preheating of hollow cathode lampSet element conditionMeasurementBasic Operation Demonstration 64.Open the steel cylinder maste
15、r valve counter-clockwise.5.Rotate the pressure reducing valve clockwise.6.Adjust the pressure to 0.05 MPaGas pressure adjusting2.Rotate the pressure adjusting button clockwise.1.Turn on the power source switch 3.Adjust pressure to 0.3 MPaBasic Operation Demonstration 7Branch pipe BCheck the waste l
16、iquid discharge pipe and open the air outletPress the air outlet switch ON to turn on the air outlet;Pour a small amount of water into the drainage safety tank to allow water to flow out of the drain pipe;The air outlet is discharged.Basic Operation Demonstration 8Ignition operationClick the“Ignitio
17、n”buttonLight a flame.Note:Because there will be air in the acetylene pipeline if the instrument is not used for a long time,so the first ignition may not work,but a few more times of trying will do.Basic Operation Demonstration 9Sample absorbance measurement1.Insert the sample capillary into the di
18、stilling water.After the absorbance is stable,click zero calibration,and the absorbance is shown to be zero.2.Lift the capillary and wipe off the moisture with the filter paper.3.Insert the sample solution,read and record the absorbance after the absorbance is stable.Basic Operation Demonstration 10
19、Shutdown operation:1.Inhale distilled water for 5 min2.the acetylene main valve should be closed3.Fire shut-out and close the pressure reducing valve5.Close the air outlet4.Turn off the air compressor6.Turn off the mainframe powerBasic Operation Demonstration 11 Summary of instrument operation metho
20、dpSwitch on the power and turn on the computer;pInstall the hollow cathode lamp；pTurn on the power supplypOpen operation software,and initialize;pSet experimental conditions,peak seeking;pCheck the drainage device;pOpen air compressor,adjust outlet pressure is 0.3 MPa;pOpen acetylene steel cylinder,
21、adjust outlet pressure to 0.05 MPa;pIgnition;Sample determination;pAfter finishing work,turn off in reverse order,and fill in the using record of the instrument.pWhen igniting,open the air first,then open the acetylene.Turn off the acetylene before closing the air.pWhen peak seeking is completed,the
22、 position of the burner is sometimes needed adjustment before ignition,so that the light emitted by the hollow cathode lamp is parallel to and directly above the burning slit.pUnlike nitrogen,air and oxygen steel cylinders,acetylene steel cylinders are filled with activated carbon and acetone,with a
23、cetylene dissolved in acetone,can not be completely used,and must be set aside 0.5 MPa,otherwise acetylene will volatilize into the flame with the background increased and burning instability.pThe instrument should have good grounding when connected to the power supply.pAtom absorption analysis ofte
24、n contacts with electrical equipment,high-pressure steel cylinders,and the use of open fire,so we should always pay attention to safety,master the necessary electrical knowledge,first aid knowledge,and fire extinguisher using method.Cautions for operation 1pAfter installing the hollow cathode lamp,t
25、he lamp chamber door should be closed.You should not put your hands into the lamp chamber while the lamp is turning.When pressing the ignition button,make sure that other personnels hands and face are not above the burning chamber,and the combustion chamber shield is better closed.pDo not place anyt
26、hing on a flame or use it for other purposes.pDo not touch the burner by hand during burning.It is best to close the chamber shield during the determination.High-temperature flame may produce ultraviolet light and burn peoples eyes.pAfter the flame is extinguished,the burner is still hot,so you shou
27、ld not touch it within 20 minutes.Cautions for operation 2pIn the process of test,if emergencies such as gas leakage and power failure happen,you should first press the emergency fire stop switch,shut the acetylene steel cylinder general valve,open the laboratory window,check the cause of the accide
28、nt,and restart after the cause elimination of emergency.pA complete safety protection device is arranged inside the instrument,which includes a gas leakage alarm(the alarm will be sounded when the gas is leaking),air pressure detection(when the air pressure is less than 2.0 bar,the instrument will a
29、larm),liquid level detection of waste liquid,and flame condition monitoring.Only when all the safety devices are in normal condition can the instrument be ignited for measurement.Any abnormal situation may cause the flame to extinguish and provide an alarm signal.Emergency settlement proceduresp Pra
30、ctice the operation of the atomic absorption spectrophotometer according to the above demonstration.pThe experimental conditions are set according to the following parameters and the absorbance of the solution of（Mg)0.3g/mL is measuredLight source:magnesium hollow cathode lamp;Analysis line:285.2 nm
31、;Lamp current:2 mA;Burner height:6 mm;Spectral passband:0.4 nm;Gas flow rate:2000 mL/min.Student practical training operationFailure phenomenonPossible ReasonSolutionNo power supply of the instrument 1.There is no power supply in the room;2.The power plug falls off and loosens;3.The instrument fuse
32、has blown.Check every link in the circuit and replace the fuse.Medium-wavelength motor appears in the process of initialization1.Check whether the hollow cathode lamp is installed and lit;2.Objects in the light path block the light;3.The host and computer communication system are disconnected.1.Rein
33、stall the lamp;2.Remove the light blocking object in the light path;3.Restart the instrument.Element light is not on1.Check whether the lamp power line is off-welding;2.The lamp socket is loose;3.Hollow cathode lamp is damaged.1.Reinstall the hollow cathode lamp;2.Replace the lamp position and reins
34、tall;3.Try another lamp.The energy is too low or the energy is above the upper limit in peak-seeking1.Element light is not on;2.The element lamp position is not correct;3.Error in line selection;4.There is a light blocking object in the light path;5.Lamp aging,low emission intensity1.Reinstall the h
35、ollow cathode lamp;2.Re-set the lamp position;3.Choose the most sensitive line;4.Remove the light shield;5.Replace new lamp.Common fault and troubleshootingClick the ignition button,the igniter has no high-voltage discharge ignition1.There is no pressure or insufficient pressure in the air;2.Acetyle
36、ne is unopened or under pressure.3.The liquid level of liquid waste is too low;4.The emergency fire-extinguishing switch is lit;5.Acetylene leakage,alarm.6.There is strong light shining on the flame probe.1.Check the outlet pressure of the air compressor;2.Check the outlet pressure of acetylene;3.Po
37、ur the distilled water into the waste liquid discharge safety interlock device;4.Extinguish the fire by pressing an emergency fire-extinguishing switch;5.Close acetylene,check piping,open doors and windows.6.Remove the light shield;Click the ignition button,the igniter has a high-voltage discharge i
38、gnition,but the burner flame cannot be ignited1.Acetylene is unopened or under pressure.2.If the pipe is too long,the acetylene has not entered the instrument;3.There is strong light shining on the flame probe4.Unsuitable fuel gas flow;1.2.Check and adjust acetylene pressure to normal value;repeat i
39、gnition for multiple times.3.Block the strong light on the flame probe;4.Adjust the fuel gas flow.Test Baseline Unstable,Big Noise1.The instrument energy is low and the negative high voltage of the photomultiplier is too high;2.Inaccuracy of wavelength;3.Unstable emission of element lamp;4.Unstable
40、external voltage and vibration of the table.1.Check whether the lamp current is suitable,if not,reset;2.Check whether the peak seeking is normal,if not,restart peak seeking;3.Try to replace the known lamps;4.Check the stabilized power supply to ensure its normal operation,remove the vibration source
41、.Common fault and troubleshootingLow or no absorbance during testing1.The burning slit is not aligned with the light path;2.The height of burner is not appropriate;3.The flow rate of acetylene is not suitable;4.The analysis wavelength is not correct;5.Energy values are low or saturated;6.The suction
42、 capillary is blocked,and the atomizer does not spray;7.The sample content is too low.1.Adjust the burner;2.Raise the height of the burner;3.Adjust the flow rate of acetylene;4.Check and adjust the analysis wavelength;5.Carry out energy balance;6.Remove and clean the capillary tube;7.Re-process the
43、sample.Flame instability during testing1.The outlet pressure of air compressor is unstable;2.The acetylene pressure is very low and the flow rate is unstable.3.There are salt crystals in the burning slit,and the flame is serrated;4.There is wind around the instrument.1.Check the pressure gauge of th
44、e air compressor;2 Replace acetylene steel cylinders;3.Clean the burner;4.Open the air outlet and close the doors and windows.When clicking the computer function key,the instrument does not execute the command1.The computer and the host are in the state of offline working;2.The host is not finished
45、executing other commands;3.The communication cable is loose.4.Computer is crashed,and there is virus attack.1.Restart;2.Close other commands or wait;3.Reconnect the communication cable;4.Restart the computer.Common fault and troubleshootingStructure of instrumentLight sourceAtomization systemCheckin
46、g systemSpectroscopic SystemLight sourcep Function:produce the characteristic radiation needed for atomic absorption.p Requirements:sharp line light source;strong light;stable;small background;long service life;cheap price.p Type:hollow cathode lamp,non-polar discharge lamp,vapor discharge lamp,lase
47、r source lamp.Hollow cathode lampElectrodeless discharge lampRadio-frequency coilCeramic frameLampp Appearance:lamp foot,glass tube,quartz light window;p Cathode:Metals or alloys to be measured;Hollow cathode lamp structurep Anode:Titanium wire or tantalum sheet on tungsten rod;p The tube is filled
48、with several hundred Pa of low-pressure inert gases.(neon or argon)Quartz light through windowCathodeFast filling gasGlass pipeAnodeLamp footVoltage adding between two polesGas glow dischargeGas ionizationCationic impinging cathodeFree atom sputteringAtomic excitationEmission of characteristic lineT
49、he working principle of hollow cathode lampPicture exampleLight quantumMetal-copper ground state atomExcited state copper atomElectronResonance emission line of copperp HCL operation parameters:lamp current.p In order to obtain stable and high-intensity narrow-line spectrum,the hollow cathode lamp u
50、sually adopts pulse power supply mode,and the lamp current is in the average value of pulse signal.p The lamp current is usually set at 120 mA.Different elements lamps will choose the appropriate lamp current according to needs.If the electric current of the lamp is too small,the discharge will be n