21_Atomic_spectroscopy

21_Atomic_spectroscopy - = “ g ~‘ . w‘s- . : .'- .t-s...

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Unformatted text preview: = “ g ~‘ . w‘s- . : .'- .t-s . 302;. w: .c. ,- 4... 3...: »“ amm- I " f u Atomic SPBI‘OScopy f These methods deal with the absorption ' and emission of radiation by atoms. .' " General concerns and considerations. Atomic spectra have narrow lines. Anything that causes broadening of the lines Line spectra are observed The two largest problems are Doppler broadening Pressure broadening Specific spectral lines can be used for elemental analysis - both qualitative and quantitative. - = “ ~‘ ‘TA- _ m~ .‘il'mf *3 x .v... ma 3*...“- I. I: _ Doppler broadening . ' During atomization/ionization, our species may The effect arises from the collision move towards or away from the detector. of our sample atoms with other species causing some energy to This will result in a Doppler shift in the resulting . be exchanged. line. Spectral lines that should be about 1-5nm can end up being 100 times wider. The efiQCt is greater as the L temperature increases. ji There is not much you can do about it except recognize that is occurs. ’w = n ‘71- _ Nu .m..»:a:‘ .t-,_ ,- 4... m .__-.n...\:' I ' -- } Effect of temperature " .1"? Atomic emission A group of techniques that are typically classified based on the excitation source. _7 _ Our source must have a stable temperature as this can . dramatically affect the number of atoms that are ionized. Flame photometry Atomic fluorescence Electrical excitation - arc and spark Plasma emission minimum energy of ionization Note: this is a measure of the kinetic energy. We can conduct both qualitative and quantitative analysis with these methods. ~ ' - if} Atomic emission " Qualitative analysis ’ Quantitative analysis ‘ Methods rely on the presence of specific " .' 3359‘? 0" measuring the Iiitens'ty Of an emission lines. emissmn line. Element Major emission line, it i . intensity a K 0 A9 3231 _ ;-.-_ Best for metals. sensitivity 3 0.001% Cu 3243 - £:___ Large relative error. i 1 - 5% Hg 2537 .. K 3447 - 3' :5_ Sensitivity and error are highly dependent on Zn 3345 the element and line being used. Excitation Sources ; Excitation Sources The arrangement of the electrodes is based on the state of the sample. Relies on a pair of high purity carbon electrodes. Arc - continuous electrical excitation. Spark - short burst of excitation. "as: (Haas U solids liquids Temperature 4000 - 800000 for a spark Voltage 15,000 - 40,000 V ” I Excitation sources "' T Excitation Sources Flame excitation ‘ ' This method of excitation is of relative low temperature: This approach works best for Group IA and HA elements because they l are easier to ionize. Ail"’Hz 2100 0C 0er2 2700 0c N20ICZH2 3050 0c Samples are introduced via aspiration into the flame so must be liquids or gases. This results in only a very small percentage of the atoms being ionized (<1%). One option is to go to higher T - Plasma emission. f '- Excitation Sources 7?] Similar to flame photometry. An RF field is used to excite an inert gas _ _. _ (typically argon) which in turn ionizes our 5: Inductively coupled ’; sample. . plasma source 3 ' ' (ICP) Higher temperatures (310,000K) are achieved so we obtain better sensitivity than with a flame. . " " if} Excitation Sources 7;. "' 3."? Excitation Sources 73. Microwave plasma source reagent as cathode -' _ _ viewing Tuned to He vibrations Three / area -‘:'_: waterinl .7; electrode DC plasma anode source amde 5" ‘/ I3 microwave -. GaVitY water out 1' ' Because emission lines are very narrow (< .01 nm ), high resolution grating are required. One common approach is to use a curved grating. This approach disperses the lines with .. ._ ; focal points based on the curvature of the I; ; Entrance grating - Rowland's circle. slit “we ‘1‘ u _ r_ .- we :5. A. we in e tion 3' ’ Several approaches can be used. Eve May be used with simpler arc emission sources. You move the ocular around the Rowland's circle to look for lines. A calibrated strip indicates the wavelength you are looking at (and possibly the element). Used mostly for qualitative work. x ' “. sure-mile 13"; 23:;- :"Mflifi".2";flt.'i['flr$.-|Ei‘v3my? ' NW3“- :;t‘:{»‘t. =_:M..x.-.\:' ; " ' 3}" Detection A Film can be used as a detector .' " Photomultiplier Tube Qualitative - position of lines .__ Quantitative - degree of exposur I.” "a. 1; m. aura-v :5, .‘ai‘ .9- K§-.."::z\.'.'i;'.. 1.:r:.‘-.t-: 3...}. *‘.-..- can; cures-ya- _ I. ' D I ' . ' . -. etectlon - _ 4 ' . A PM tube provides a means of obtaining quantitative data. Typically, it is easier to move the tube. Moving the grating will alter the position of the Rowland’s circle. \Film strip / '2; _ 7 Multielemental work requires several PM tubes : '"‘ or recalibration after tube movement. ometer :t‘mm E» ' The monochromator/detector system for a ' flame photometer is much simpler than with the other types of systems. We have fewer species to look at. Either a grating or a simple filter These systems are similar in layout to molecular filter photometers or single beam spectrophotometers. v “war r5 7-- W " w: m s2 _ x”: "avers 'F’ W ? Absorption methods Atomic absorption spectroscopy (AA) A quantitative method of analysis based on the absorption of light by atoms in the free atomic state. The method relies on the Beer-Lambert relationship - calculations are the same as with molecular absorption methods. As the temperature increas _ more atoms are excited. Most are still in the atomic state. Minimum energy for ionization energy 1142:” cm skew“. .( My rum tion equipment ? Atomic abs .3 In its simplest form, an AA resembles a single beam ' spectrophotometer. ' source samp'e - -:| l detector monochromator chopper Atomic absorption ' Basis of method With electrical or flame excitation, most atoms remain in the unexcited state. Even with plasma emission, this is still true but not as large a problem. if we can look at the free atoms, we can potentially develop a more sensitive method of analysis. 1 Atomic absorption 2' Advantages over emission lZlFewer interferences ; lZlLess dependent on temperature _:_ ElMost elements exhibit better sensitivity and accuracy - ppb range with 12% accuracy. " 3_-._ Disadvantages over emission [ZlMetals only - most other elements form oxides to rapidly. ElQuantitative analysis only. A molecular spectrophotometer relies on a broad band light source. With atomic absorption, a line source is required to reduce interferences from other elements and background. Two basic types Hollow cathode lamp - HC Electrodeless discharge lamp - EDL ? Hollow cathode lamp I. This source produces emission lines specific for the element used to construct the cathode. The cathode must be capable of conducting a current. Repeated bombardment of the metal atom by the gas causes it to be excited. lt ultimately relaxes, producing specific atomic emission lines. This source produces emission lines specific for the element used to construct the cathode. The cathode must be capable of conducting a current for it to work. The lamp is filled with an inert gas like argon or neon. When a potential is applied, it causes the gas to become excited and it is driven towards the cathode. Metal atoms are then sputtered off the surface of the cathode. Hollow cathode lamp An HC lamp will only produce the emission lines for the cathode element. Multi-element HC lamps are available but are limited. Not all metals will make suitable cathodes Metal is too volatile A good cathode can't be produced The metal may not be good conductors I ’ An alternative to the hollow cathode lamp. A salt containing the metal of interest is sealed in a quartz tube along with an inert gas. An RF field is used to excite the gas which in turn causes the metal be be ionized. ‘ ' Light intensity is about 10-100 times greater but are not as stable as HC lamps. A chopper is used to provide signal modulation - in conjunction with a lock-in amplifier. its not practical to have two separate cells, so the light is simply split, with half being sent around the atomization source. This reduces some noise from the atomization source and accounts for instrumental variations. We need to be able to convert our sample to free atoms. Two approaches are used. Flame atomization liquids and gases Flameless atomization graphite furnace liquids and solids ‘salt’ containing bulb _ E Signal modulation -- is} Flame atomization A flame atomizer will usually have a long, narrow burner head that serves as a sample path (b). Sample is introduced via aspiration. mman . chamber The nebullzer controls sample flow, producing a mist. The mixing chamber assures that the sample mixes with the oxidant and fuel prior to entry into the flame. nebulizer Flame atomization tends to produce stable signals in the ppm range for most metals. It is a dynamic method Sample is constantly being consumed. Large sample size (>1 ml). Your sample must be a fluid. The detection limits are relatively high since only a small portion of your sample is present in the flame at any given time. water cooling The most common fuel to use is acetylene. ‘ Either air or nitrous oxide are used as oxidants, with N20 producing a hotter flame. Temperature, °C CszlAir 2100 - 2400 CZHZINZO 2600 - 2800 N20 also tends to produce a noisier flame. " Samples are placed in a carbon tube which is heated electrically - graphite furnace Sample residence time is greater so you have improved detection limits and sensitivity. Solid samples can also be assayed. You can’t simply heat your sample to atomization temperatures or the sample will splatter. We use a temperature program to ensure £1.25?" reproducible atomization. "' A three stage program is the most common. Argon is often used as a purge gas to: A fixed temperature and time used to remove “Remove excess materiaI during the dry and N your SOIVent (50'200°C)- .f:___ _; char phases and after atomization _}_', Char '3. __ ._ c/ Reduce oxidation of the tube. ' A second temperatureltime used to decompose _ _ _ your matrix (200_8oooc)_ _ 3: f" V Provrdes a protective blanket during ' atomization srnce high temperature carbon wrll Atomization _ react with nitrogen to produce cyanogen - you A rapid increase to 2000-300000 for just a few :5- Shou'd a'WaYS Vent t" 3 “00d anyway- seconds - when you collect your data. ‘ ' A high resolution, holographic grating is used to Modulation of your signal (using a chopper) is an resolve your lines. it is not designed to be easy way to account for instrumental variations used in ‘scan’ mode. and ‘flame flicker’. The tYPical detector is a photomultiplier tube. ii' It is not Very 900d at accounting for baCkground I. . absorption or emission. An additional component that is very common is . ‘ a method of background correction. Two approaches that are commonly used are " ' 5 ': D2 or Zeeman correction ‘ 3 Background correction These methods rely on D2 background correction Atomic lines being very narrow. A continuous-source correction method. Background is typically molecular so is broad Light from both the AA source lamp and a D2 band. -. lamp alternately pass through the sample. Because the spectral slit width is significantly larger than the AA source line, the D2 lamp sends a much broader band of light through the sample. molecular } Background correction B ckground correction With the HC, we measure Hc absorption of our element and background over a very narrow bandwidth. With the D2 lamp, we measur absorption over a much large . Dz bandwidth. Because the '- elemental line is so narrow, we mostly measure background. 4 - 1: ‘ be? as “raw ~ "‘ L- to _- -- j- Background correction _' f B ckground correction Limitations of D2 correction I '- Either undercorrection or overcorrection can occur based on sample. background bkg + sample The difference between the two l signals gives us _- our background __ corrected sampl _ absorbance Background may vary around line Composition of background can differ based on position in flame - requires good alignment of 02 output HC and D2 lamps. ._HC output D2 output IS not very good at > 350 nm ~ _ 3 Background correction Example showing our elemental line occurring near the maximum of some sort of molecular absorption from the background ' An alternate approach is to make use of the Zeeman effect. When an atomic vapor is exposed to a strong magnetic field (1 - 10kG), there is a splitting of the atoms electronic energy levels. in this example, D2 . correction would This essentially moves our absorption away tend to overcorrect. :.. from the emission lines. The magnetic field is applied to the sample. - {__j- Background correction ~ . . .~ .- For each element, you must consider: .. _ Usmg this method of _g , _ _ I _ correction, we can measure at Wthh 7k arid Silt Wldth to use a fixed arid "arrow 1:. "f Determines sensitivity and linear range. absorption ' ; : :__ For flame AA wavelength. line At regular intervals, we simply _ Ftame type move our sample component I I" I I . . out of the way. _ '- Method of sample mixmg split lines ‘ _ _ in presence of a strong magnetic field This allows us to directly For flametess AA measure background. :5: '_ f 2' Optimum temperatures to use ._ " 3 Flame AA example - Mn Relative Sensitivity Linear range I Noise (mgi'l) (mgll) .;_ 279.5 1.0 0.052 2.0 " '- 279.8 0.77 0.06? 3.0 '. 1. 280.1 0.88 0.11 5.0 Fortunately, AA is a reasonably well worked out technique. Standard conditions for all elements that can be . measured by AA are available _ ”' Other conditions Air/02H2 flame — lean, blue Slit width of 0.2 nm Flow spoiler '3' 4. 0.2% CaCI2 can be added to overcome interference from presence of Si. If you have a computer based system, it will even help set up the proper conditions . . Aqueous A modification of the normal AA setup will permit _ A 2795 nm atomic fluorescense to be done. If Slit width 0.2 nm ' " ' The HC lamp is placed at a right angle to the '. Temperatures detector Maximum Char 1100°C Optimum Atomization 270000 Sensitivity 4 pg I 0.0044A _ ___ ' Linear range 200 pg _f Not as common as AA or plasma emission. Any atoms that fluoresce can be measured. Fluorescence detector I l—C\ EC] sample Absorption detector /Grating source chopper ' We can also use the high energy end of the EM spectrum to conduct elemental analysis. X-rays can be used for measurement of Absorption, Diffraction and Emission/Fluorescence The principles of the methods are basically the ' same but the components vary quite a bit. Electrons or other X—rays can be used to ‘knock-out’ an inner electron. electron Source electron Ejected electron 3 Atomic fluorescense (0.1 - 10 nm) Most AA instruments permit the measurement of emission. Any element that can be sufficiently excited by the flame will produce spectral lines when they relax - flame photometer. If no HC is present (or turned off) and emission mode is selected, normal flame emission can be conducted. Production of X-rays If an inner electron is “knocked out’ of an atom, an outer electron will fall into the hole that is produced. The energy difference is emitted as an X-ray. Since this difference is a function of the type of atom, the energy of the X-ray is a function of the atom’s atomic number. Emission of X-rays is independe I. of chemical form — only depends on Z. The | of the emission can be used to identify the element for Z 3 20 .. 1;.- ._.. . ‘ Jenn Harm v:.:".t-.. “ ,3 m. "Ia ‘5» - 3 mm: equipment ..-" -. 13$ 7. . ‘ census. 3% 11K» .J-a “\ .-. j X y abso tion ’ Atoms can also absorb X-rays where: 24 atomic weight K edge The source of the X-rays must L edges have a higher '_ atomic number than the sample Wavelength The arrangement of the various components in an X-ray system is dependent on the type of method to be used. Absorption a Let's start by reviewing some of these components since they are different than what we’ve dealt with so far. absorption An electron beam is used to produce X-rays from a target metal. Common metals are Cu, W and Mo. 2 must be greater than the elements you plan on working with. ';' ' Collimeter Source . r r: m» «.m' ' = '.u ‘ u: .1 I _:. ‘ ‘ma'fi . -.g‘.;t..2". ..‘.,'.- 10:: 'u‘a‘.‘ : ‘ at.“ .-.,- -- X-ray source .- The filter is a metal with an atomic number one greater than the element of interest. The collimeter is a series of metal plates with holes in the middle. filter absorption it is designed to absorb all X—rays not traveling in the proper direction source emission .' that...” .. 13¢ 7,. - ‘ 'A‘Lhrti‘flf“ " : Wt at ,9 “\ .- *2“ - a» ' " {if Mono h mator -- {.f } Detector I ’ Normal gratings or prisms can not be used with X-radiation. It would either pass through or be absorbed - no dispersion. Three types - two classes I: Non-discriminating --.': If one is required, it is typically G33 filled ' GM tUbe ' constructed of NaCl and works by diffraction of the X_rays by the ions_ 1: {j} Discriminating - can resolve the energy. . Scintillation - Nal(Tl) Solid State - Si(Li) il } Scintillation Detectors 5 air") Very common material used for y and X-ray detection. I ’ An argon filled tube with a potential difference of 1000V or more. i: "j At this voltage, you get the a maximum number of ionizations per radiation event When an X-ray interacts with a Nal(Tl) crystal, light is produced. . Each event causes a total " avalanche of ions resulting in a single, large pulse. You get one photon for each 200 eV. The number of photons is proportional to the '5 it does not discriminate. energy of the X-ray ‘.' sew" -. I w-j:.'§-If ‘ smut Eartha“ an ‘9 l.‘ " . '4 i. Imus ‘th _:. fig ‘ I x-.4';:t..;.'figg&- L- a... i '. -.. : .. 'D.“.‘.,' lush; g: {:9- I Nal(T I) detector h j- s intination Detec ors ’ When exposed to X-rays, the crystal will produce a burst of photons. The intensity of the light is proportional to the energy. Nal(Tl) crystal "" ~ 5* t Z" I ’ Silicon doped with lithium Nam-I) An aluminum ‘shield’covers . crystal the crystal to Prevent A solid state device contamination and keep water _’ _' th t d t t X Photo_ out- can destroy the crystal. ';_ : j a can e 90 'rays multi Iier " 7 wife f The inner surface is reflective to direct light towards the photomultiplier tube. ’ When a small voltage difference is applied to the ' crystal, you create a positive, negative and a depleted zone. 00000000 " .. '3 “0 “9t ' movement of charge. : This is similar to a transistor (np type). ‘ 7; negative - electrons o o c o c o c o 32' positive - + holes I- . ’ This approach is used to find the dimensions and When ionizing radiation shape of a crystal unit. passes through the . ' ' detector, it leaves behind .gg ._ a trail of + and - charges --_ :.~ " it can be used to: in the depletion zone. _ __ Deduce the structure of a new material Detection is based on the ' I Ident'fy a SUbSta nce current required to re- ' Tell structure of a polymer establish the depletion Elemental anarysis zone. . «rant-eck’aah'nj-Hiher:--.r:aa.i-iu<r~r-"=Rt.w'tvtamasw" “"r- -'.'.v-'-‘.-r.1::~'-..-.-‘ m3 is F‘xM-u‘b-nn‘a- -.-;= .141" :"..a-nmwhtaec: Tenors)...» Kiesa'q'nj-Hih afi--'.'t':<r~r-":"~‘..w-vtarcher? “"e- -'.'.m‘.~-‘~'.a:i-9..‘-': we're-muse.e-s;;=r;;-h»t.a-tm~w=.-s;: ' X-ray diffraction i_ Film can be used for ' X-ray absorption i_ ' This is a common approach for measuring the detection of the patterns I I_ I I Of casts and It is now more common to rotate the crystal and detect the x-rays with a fixed position -- . detector. ' y ' Its major use is in This way, you have i i ' medicine and data that can be ' dentistry. processed by a - computer When a sample is exposed to X-rays, ' Nam.) secondary x-rays are produced. _ ' ' detector Both qualitative and quantitative data can be obtained (2 311). In addition, this is a multi-elemental, non- destructive form of testing. ...
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21_Atomic_spectroscopy - = “ g ~‘ . w‘s- . : .'- .t-s...

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