Chem. 133 – 4/13 Lecture

Announcements I Exam 2 Results: Ave = 55% (10% below last year’s) Lab: Should finish Set 2 Period 4 lab today Thursday is make-up day Can start on term projects this week if desired Due Dates: Set2Per3 April 28 th ; last lab due May 5 th ? RangeN s2 60s1 50s4 <505

Announcements II HW Set 3: Part will be posted soon; first assignment due 4/23 Today’s Lecture Atomic Spectroscopy Instrumentation/Interferences NMR Overview and Theory

Atomic Spectroscopy Absorption Spectrometers The lamp is a hollow cathode lamp containing the element(s) of interest in cathode The lamp is operated under relatively cool conditions at lower pressures to reduce Doppler and pressure broadening of atomic emission lines A very narrow band of light emitted from hollow cathode lamps is needed so that absorption by atoms in flame mostly follows Beer ’ s law The monochromator serves as a coarse filter to remove other wavelength bands from light and light emitted from flames Lamp source Flame or graphite tube monochromatorLight detector

Atomic Spectroscopy Absorption Spectrometers A narrower emission spectrum from hollow cathode lamp (vs. flame absorption) results in better Beer’s law behavior wavelength Intensity or absorbance hollow cathode lamp emission Atomic absorption spectrum in flame Additional broadening in flame from temperature (Doppler) or pressure

Atomic Spectroscopy Interference in Absorption Measurements Spectral Interference –Very few atom – atom interferences –Interference from flame (or graphite tube) emissions are reduced by modulating lamp no lamp: signal from flame vs. with lamp then with lamp: signal from lamp + flame – absorption by atoms –Interference from molecular species absorbing lamp photons (mostly at shorter wavelengths and light scattering in EA-AA) –This interference can be removed by periodically using a deuterium lamp (broad band light source)

Atomic Spectroscopy Interference in Absorption Measurements Chemical Interference –Arises from compounds in sample matrix or atomization conditions that affects element atomization –Some examples of specific problems (mentioned previously) and solutions: Poor volatility due to PO 4 3- – add Ca because it binds strongly to PO 4 3- allowing analyte metal to volatilize better or use hotter flames Formation of metal oxides and hydroxides – use fuel rich flame Ionization of analyte atoms – add more readily ionizable metal (e.g Cs) –Another approach is to use a standard addition calibration procedure (this won ’ t improve atomization but it accounts for it so that results are reliable)

Atomic Spectroscopy Interference in Absorption Measurements Standard Addition –Used when sample matrix affects response to analytes –Commonly needed for AAS with complicated samples –Standard is added to sample (usually in multiple increments) –Needed if slope is affected by matrix –Concentration is determined by extrapolation (= |X-intercept|) Area Concentration Added Analyte Concentration standards in water

Atomic Spectroscopy Emission Spectrometers In emission measurements, the plasma (or flame) is the light source Flame sources are generally limited to a few elements (only hot enough for low E – visible light emissions) A monochromator or polychromator is the means of wavelength discrimination Sensitive detectors are needed ICP-AES is faster than AAS because switching monochromator settings can be done faster than switching lamp plus flame conditions Plasma (light source + sample) Monochromator or Polychromator Light detector or detector array Liquid sample, nebulizer, Ar source

Atomic Spectroscopy Emission Spectrometers Sequential vs. Simultaneous Instruments Sequential Instruments use: –A standard monochromator –Select for elements by rotating the monochromator grating to specific wavelengths Simultaneous Instruments use: –A 1D or 2D polychromator (Harris Color Plate 24/25) –1D instruments typically use photomultiplier detectors behind multiple exit slits –2D instrument shown in 4/1 lecture slide 13 –Selected elements (1D instruments) or all elements can be analyzed simultaneously resulting in faster analysis and less sample consumption.

Atomic Spectroscopy Interference in Emission Measurements Interferences –Atom – atom interferences more common than in atomic absorption because monochromators offer less selectivity than hollow cathode lamps –Interference from molecular emissions are reduced by scanning to the sides of the atomic peaks –Chemical interferences are less prevalent due to greater atomization efficiency Emission Spectrum Atomic peak background

Atomic Mass Spectrometry Most common arrangement consists of ICP torch placed to MS interface The Ar + ions (and electrons) collide with metals leading to ionization The MS interface consists of skimmer cones to allow ions in, and to drop the pressure in stages, and ion optics ICP-MS typically is the most sensitive elemental analysis method Interference can arise from metals (e.g. 138 Ba 2+ vs. 69 Ga + ) or from ICP species (e.g. 40 Ar + and 40 Ca + ) Use of secondary isotopic masses and collision cell reactions can reduce these interferences Plasma (atomizer + ion source) Liquid sample, nebulizer, Ar source Mass spectrometer (e.g. quadrupole) collision cell

Atomic Spectroscopy Comparison of Instruments InstrumentCostSpeedSensitivity Flame-AA Low (~$10-15K) SlowModerate (~0.01 ppm) GF-AA Moderate (~$40K) SlowestVery Good Sequential ICP- AES Moderate MediumModerate Simultaneous ICP-AES High FastGood ICP-MS Highest (~$200K) FastExcellent

Atomic Spectroscopy Some Questions 1.Why is AES with a plasma normally more sensitive than AES with a flame? 2.List two ways in which a process in a flame can lead to reduced sensitivity and a way to deal with each process so its effect on the analysis is minimized. 3.Why can a simultaneous ICP-AES be more sensitive than an sequential ICP-AES if use for analysis of 12 metals? 4.If a sample matrix produces molecular emissions that interfere with atomic emissions, how would this be observed and how can this be accounted for? 5.What can cause interferences in ICP-MS?

Nuclear Magnetic Resonance (NMR) Spectrometry Major Uses Identification of Pure Compounds (Qualitative Analysis) Structural Determination (e.g. protein shape) Quantitative Analysis Characterization of Compounds in Mixtures Imaging (MRI) – not covered

NMR Spectrometry Theory Spin –a magnetic property that sub atomic particles have (electrons, some nuclei) –some combinations do not result in observable spin (paired electrons have no observable spin; many nuclei have no observable spin) –Electron spin transitions occur at higher energies and are the basis of electron paramagnetic spectroscopy (EPR) –Nuclear spin given by Nuclear Spin Quantum Number (I)

NMR Spectrometry Theory Nuclear Spin (continued) –I = 0 nuclei → no spin (not useful in NMR) – e.g. 12 C –I = ½ nuclei → most commonly used nuclei ( 1 H, 13 C, 19 F, many others) –I > 1 nuclei → used occasionally, important for spin- spin coupling –number of different spin states (m) = 2I + 1 –examples: 1 H (I = ½), 2 states 2 H (I = 1), 3 states up state (m = +1/2) down state (m = -1/2)up state (m = 1) middle state (m = 0) down state (m = -1)

NMR Spectrometry Theory Effect of External Magnetic Field on Nuclei States –aligned nuclei (m = +1/2) have slightly lower energy (are more stable) than anti- aligned states (m = -1/2) –the greater the magnetic field (B 0 ), the greater the energy difference between the states Applied Magnetic Field B 0 * “up” state – m = +1/2 “down” state – m = -1/2 *Note: technically B 0 is the magnetic field at the nucleus which is not quite the same as the applied magnetic field Note: arrows drawn at angles because spin vectors precess about B 0 path made by vector tips

NMR Spectrometry Theory Energy depends on nucleus, spin state (m), and magnetic field  (gamma) = magnetogyric ratio (constant for given nuclei) and h = Planck ’ s constant Energy difference Energy B0B0 ΔEΔE

NMR Spectrometry Theory Transitions between the ground and excited state can occur through absorption of light Lowest Resolution Spectroscopy CH 3 CF 2 OH or signal H 1H1H 19 F 13 C (small because most C is 12 C) H is traditionally used for x-axis because older instruments involved changing H (most newer instrument don’t). A frequency plot at constant field would be reversed ( 1 H at highest frequency). H scanned at fixed