1、General Overview of Detector Systems Daniela Cavagnino2Detectors classificationUniversalThey respond to everything eluting from the column TCD PDD(FID)SelectiveThey may be element selective,structure/functional group selective or selective to other properties FID(very broad selectivity)ECD PID PDDSp
2、ecificThey are so selective to distinguish particular structures or elements NPD FPD3Concentration vs Mass dependent responseCommon conc.dependent:TCD PID PDD ECDCommon mass dependent:FID NPD FPDNon-Destructive vs DestructiveCommon non-destructive:TCD PID PDD ECDCommon destructive:FID NPD FPDDetecto
3、rs classification4Detector Response CharacteristicsSensitivityDetector efficiency to convert the sample in an electrical signalNoiseShort term:high frequency baseline fluctuationLong term:low frequency baseline perturbationDynamic RangeRange of sample concentration for which the detector can provide
4、 a detectable signal variation with analyte amountSelectivityThe ratio of the detector sensitivities of a given compound over a potentially interfering compoundMinimum DetectabilityAmount of sample in which the peak height is 3 times the noise height(S/N=3)5Detector Response Characteristics Sensitiv
5、ity and Minimum DetectabilityFID sensitivity:S=coulomb/g=FPD sensitivity for sulfur:S=*=uV/(gS/s)2MDA=g/sec RF(Response Factor)=MDA=gS/sec peak areasample weightA*sec g peak area S amount PW S amount3N S A*gA*secpeak area amount3N Speak height mass raten-16 Dynamic and Linear Range Dynamic range:ove
6、r which an incremental change in the amount of compounds in the detector volume produces a measurable incremental change in the detector signal Linear range:over which the response deviation is less than 5%Detector Response Characteristics7Flame Ionization Detector Universal response Ionization dete
7、ction Mass detector Destructive8Flame Ionization Detector Hydrogen is mixed with gas stream at bottom of jet and air or oxygen is supplied axially around the jet Hydrogen flame burns at the tip,which also functions as cathode and it is electrically insulated from the body Collector electrode is abov
8、e the burner tip9Flame Ionization DetectorPrinciple of operationCombustion of organic compounds in a oxidizing flameCH+O CHO+e-Electric field betweenthe jet and the collectorelectrodeVoltage-300VCollection of the ions generated into the flameCurrent pAA good combustion step is the prevailing factor
9、to get the best performances10Flame Ionization Detector It responds to all organic compounds except for formic acid Response is greatest with hydrocarbons and decreases with substitution Sensitivity high due to low noise level No response to water,permanent gases,and inorganic compounds simplifies t
10、he resolution of components in analysis of aqueous extracts and in air pollution studies Suitable for fast and ultrafast GC applications 11Flame Ionization DetectorTechnical Specifications Operating temperature limit 450C with ceramic jet Linear rangebetter than 106 Minimum detectable amount 3 x 10-
11、12 gC/s Input range 0 to 10-6 A Input attenuation 4 steps(100-101-102-103)Electrode polarization voltage -300 V Time constant 6 ms 63.2%Acquisition rate up to 300Hz12Flame Ionization DetectorStandard Operating Procedure(SOP)C12C14C16Area Counts 4 000 000C12=6 877 493C14=6 790 762C16=6 988 18113Flame
12、 Ionization DetectorMDL Calculation(C12)C12C14C16V=1.6uL 20ng/uLMass C12=32ng%C=84.7%Mass C=27.1ngC12 A=6877493(0.1uV*s)MDL=3N/S S=Area(uV*s)/mass MDL=48(uV)*27.1(ngC)/687749.3(uV*s)=0.00189 ngC/s=1.89 pgC/sNoise 16 uV14Electron Capture Detector Selective response Non-destructive Ionization detector
13、 principle Concentration-dependent detector Radioactive source 63Ni(10mCi activity)Displaced coaxial-cylinder geometry15Principles of detection Electron Capture Detector-e-e-+-MMMABAB_+*+N2+N2+e-*+Ar +Ar+e-+Ar*Ar*+CH4 Ar+e-+CH4+dissociative-capture mechanismAB+e-A+B-nondissociative mechanismAB+e-AB-
14、side reactionsC+e-C-N2+e-neutralsAB-+N2+neutrals16Electron Capture DetectorPulsed voltagee-V500TTime (s)w=0.1 1 sPULSE voltageDC voltage17 Electron Capture DetectorConstant current methodI=K e-f+-Iff=constI=const18Constant current methodModulation of Pulse Frequencyion current=electrons concentratio
15、n x pulse frequencywith no sample freq.=fwith electronegative sample freq.=fssignal output=fs-f=sample concentration Electron Capture Detector19Electron Capture Detector Radioactive source:Nickel 63 370 MBq(10 mCi)Cell volume 450 L Operating temperature limit:400C Minimum detectable amount:10 fg of
16、lindane Linear dynamic range:104(argon/methane)103(nitrogen)Operation mode:constant current pulse-modulated mode Reference current:0 to 3 nA(0.1 nA steps)Pulse amplitude:5 to 50 V neg.Pulse width:0.1 s(argon/methane),0.5 s,1 s(nitrogen)Technical Specifications20Electron Capture DetectorMolecular fea
17、tures governing the response of ECD Low response for alcohols,amines,phenols,aromatics and vinyl type fluorinated hydrocarbons High response for halocarbon compounds,nitroaromatics,and conjugated compounds containing two groups which individually are not strongly electron attracting but become so wh
18、en connected by specific bridges Response towards the halogens decreases in the order IBrClF 21Electron Capture DetectorMolecular features governing the response of ECD Multiple substitution with simple electron attracting groups or atoms may increase the molecular absorption by an amount much great
19、er than expected of a simple additive effect The absorption conferred by a simple electrophoric group is also sensitive to the position in the molecule Some inorganic compounds are electron absorbers,as carbon disulfide,ozone and the oxides of nitrogen22Electron Capture DetectorRelative sensitivityE
20、thaneBenzene 1ButanolAcetoneChlorobutane1-102Chlorobenzene1,2-dichlorobenzeneAntracene102-104ChloroformNitrobenzene104-105Carbon tetrachlorideDinitrophenolDiethyl oxalate105-106DihydropyridineInfluence of detector temperatureDetection limit(x10-9 g)CCl4 0.01 0.01 0.01 CHCl3 1.0 0.1 0.05CH2Cl2 1000 4
21、0 8CH2ClCH2Cl 1000 20 1 80C 227C 350C23Electron Capture DetectorRelative response of halocarbonsCF3CF2CF31.0CF3Cl3.3CF2=CFCl100CF3CF2Cl170CF2=CCl2670CF2Cl23 x 104CHCl33.3 x 104CHCl=CCl26.7 x 104CF3Br8.7 x 104CF2ClCFCl21.6 x 105CF3CHClBr4.0 x 105CF3CF2CF2I6.0 x 105CF2BrCF2Br7.7 x 105CFCl31.2 x 10624g
22、 -LINDANEMDA=10 fg with S/N=3ALDRIN(15pg)HEPTACHLOR(10 pg)g-LINDANE(10 pg)x 2x 64ECD performanceMinimum Detectable Amount25Best conditions for ECD sensitivity ECD sensitivity is affected by the following factors:Reference current The higher is the reference current,the greater is the signal response
23、 but also the baseline noise.The S/N ratio needs to be determined for sensitivity evaluation Ionizing gas(makeup)The ECD is a concentration-dependent detector.The lower is the makeup flow rate(up to 15-20mL/min),the higher is the response.Argon/methane as makeup gas allows to operate at lower freque
24、ncies while using higher reference current setting Detector temperature For some compounds the sensitivity will increase with the cell temperature (dissociative mechanism of reaction)26ECD performanceLinearity27Best conditions for ECD linearity ECD linearity is very dependant upon several factors:EC
25、D conditions Pulse Voltage:lowest is better(up to 15V)according to the output freq.Base frequency:must be around 1KHz Reference Current:it can be reduced to 0.7-0.8nA if necessary Ionizing gas(makeup)Argon/methane:assure the widest linearity range Nitrogen:should be used under clean conditions with
26、0.5us of Pulse WidthIn both cases the flow rate can be increased to 40-45mL/min for maintaining a low base frequency Carrier gas Hydrogen:linear range up to about 100pg Helium:sligthly better than hydrogen Nitrogen:linear range shifted up to about 200pgThe complete system including the gas supply li
27、nes and gases have to be very clean to achieve a low base frequency28Electron Capture DetectorStandard Operating Procedure SOP LindaneAldrinNoise 73uV(10V FS)S/N 4000Lindane=4820Aldrin=443129Electron Capture DetectorLindaneAldrinNoise 73uV(10V FS)V=1.6uL 30pg/uLMass Lindane=48pgLindane A=6122928(0.1
28、uV*s)MDL=3N/S S=Area(uV*s)/mass MDL=219(uV)*48(pg)/612292.8(uV*s)=0.0172 pg/s=17 fg Lindane/sMDL Calculation(Lindane)30Electron Capture DetectorMultiple simultaneous detectionFID,NPD or FPD stacked on ECD31AirColumn effluent+Hydrogen/Makeup Heated source(Rb ceramic matrix)CollectingelectrodeNitrogen
29、 Phosphorous Detector Specific response vs N and P organic compounds Ionization-type detector Mass detector Destructive Rb ceramic bead as thermionic ionization source 32 Sample decompositionElectronegative products(e.g.NO2,CN,PO2)2+Hot source NegativeionsElectronegativespeciesNPD:detection mechanis
30、mTID-2(Black Source)33Sample Electronegativedecomposition products+Hot source NegativeionsElectronegativespecies2ENS mode:detection mechanismTID-1(White Source)34NPD mode:new glass bead Can replace TID-2 source Higher response for Phosphorous compounds (but tailing peaks can be observed)Lower operat
31、ing current:extended lifetime Same flow rates setting as TID-2 Blos-Source (Glass Bead)35 The thermionic source element is a consumable component that must be replaced periodically Easily interchangeable thermionic sources Thermionic source lifetime is strictly dependent on the operative conditions.
32、Nitrogen Phosphorous Detector36Technical Specifications Operating temperature limit 450C Linear range better than 104 Minimum detectable amount 5 x 10-2 pgN/s 2 x 10-2 pgP/s Selectivity N/C=105:1 P/C=2 x105:1 Input range 0 to 10-6 A Input attenuation 4 steps(100-101-102-103)Heating current setting 1
33、.00 to 3.50 A in 0.01 step Polarization voltage setting 1.0 to 9.9 V in 0.1 stepNitrogen Phosphorous Detector37Nitrogen Phosphorous DetectorStandard Operating Procedure SOP AzobenzeneMethylparathionArea Counts 1500000(Azobenzene)3000000(Methylparathion)Azobenzene=1598004Methylparathion=468863538Nitr
34、ogen Phosphorous DetectorAzobenzeneMethylparathionMDL Calculation(Azobenzene)V=1.6uL 1ng/uLMass Azobenzene=1.6ng%N=15.3%Mass N=0.244ngazobenzene A=1598004(0.1uV*s)MDL=3N/S S=Area(uV*s)/mass MDL=54(uV)*0.244(ngN)/159800.4(uV*s)=8.24E-5 ngN/s=8.2E-2 pgN/sNoise 18uV39Flame Photometric Detector specific
35、 response vs S or P compounds destructive light-emission detection mass detector single-flame design dual parallel configuration40Flame Photometric DetectorPrinciple of operationSulphur compoundsH2S+H HS+H2HS+H S+H2S+S S2*S+S+M S2*S2*S2+h Quadratic response for sulfur compounds!Phosphorous compounds
36、PO+H+M HPO*+MPO+OH+H2 HPO*+H2OHPO*HPO+h 41Flame Photometric DetectorChemiluminescent emission spectra of sulfur and phosphorous compounds in hydrogen-rich flameEmission signalWavelength(nm)Wavelength(nm)Sulfur(S2*)Phosphorous(HPO*)42Flame Photometric Detector The emission of excited molecules is mea
37、sured against some flame background by means of narrow bandpass interference optical filters:S 394 nmP 526 nmSn 610 nm43Flame Photometric DetectorNon-linear response in sulfur-selective detection Intensity of the sulfur emission:I Sn log S 1/n log I the exponential factor n is theoretically equal to
38、 2 it is dependent upon the FPD operating conditions it is strongly compound dependent it is experimentally determined44Flame Photometric DetectorQuenching effect in sulfur-selective detection collisional quenching of S2*by CO2,CH4,and other combustion products reduces the sulfur response hydrocarbo
39、ns are particularly effective in quenching the sulfur response in case of coelution at high concentrations,quenching effect may be observed,leading to a curvature of the calibration curve45Flame Photometric Detector The optimum air flowrate should be experimentally determined after correct setting o
40、f the hydrogen flow rate Variations in the air/hydrogen ratio lead to deviations from the sulfur quadratic response When operating in phosphorous mode,variations in the air/hydrogen ratio can strongly affect the response for certain phosphorous compounds while tiophosphates are unaffected The positi
41、on of the column end is especially critical,since most compounds containing sulfur and phosphorous are very activeThe photomultiplier tube noise increases with increasing detector temperature By increasing the H2/air ratio,the negative response of HC decrease and also the S compounds tailing decreas
42、esPractical hints46Flame Photometric DetectorMaintaining and troubleshooting if a high noise and standing current is observed,it can be due to:-column bleeding-optical system not light-tight-too high temperature near the photomultiplier tube if low sensitivity is observed,it can be due to:-hydrogen
43、flow rate too low-air flow rate too high-reduced optical clarity on optical windows to prevent damaging the photomultiplier tube,avoid any exposure to light,even for short period,when powered on.47Flame Photometric DetectorTechnical Specifications FPD temperature limit 350C Sulfur filter 394 nm Phos
44、phorous filter 526 nm Detection limit Sulfur 5 x 10-12 gS/s(Parathion)Phosphorous 1 x 10-13 gP/s(Parathion)Selectivity S/hydrocarbon 105 P/hydrocarbon 106 Linear range 103 for Sulfur after linearization 104 for Phosphorous Photomultiplier Tube Voltage selectable to 800 V(low)and 900 V(high)48Flame P
45、hotometric DetectorStandard Operating Procedure SOP(S filter 394nm)S/N 40 S/N=73MethylparathionNoise 213uV49Flame Photometric DetectorMethylparathionNoise 213uVMDL Calculation(S filter 394nm)V=1.6uL 1ng/uLMass Methylparathion=1.6ng%S=12.1%Mass S=0.194ngMethylparathion A=257001(0.1uV*s)MDL=(3N/S)S=(Area(uV*s)/mass)*(PW/mass)S=25700(uV*s)/194(pgS)*1.34(s)/194(pgS)=0.91 uV/(pgS/s)2 MDL=(639(uV)/0.91)=26.5 pgS/s
