Chap. 1 (Introduction), Chap. 2 (Components and Circuits)

Transcription

1 CHEM 455 The class describes the principles and applications of modern analytical instruments. Emphasis is placed upon the theoretical basis of each type of instrument, its optimal area of application, its sensitivity, its precision, and its limitations. Elementary integrated circuitry and treatment of analytical data are introduced. Outcomes Understanding the fundamental chemical and physical properties that are used in instrumentation Ability to evaluate and utilize data from instruments Limitation of experimental instrumentation Investigation of modern topics in analytical chemistry 1-1

2 Chap. 1 (Introduction), Chap. 2 (Components and Circuits) Methods of Analytical Chemistry Classical methods Separation of components * Precipitation, extraction, distillation Identification by properties Color, melting point, solubilities Amount determined by mass or titration Instrumental methods interaction with electrons or photons * Electronic or nuclear properties are identified Some properties based on bulk properties * Thermal analysis 1-2

3 Methods 1-3

4 Instruments Converts information in the sample to information expressed in an instrument Results are given in differing data domains Electrical and non-electrical 1-4

5 Components 1-5

6 Interdomain Conversions 1-6

7 Analog domains Electrical Domains Magnitude of electrical quantity Voltage, currant, power, charge * Correlation of two signals NMR, IR, DTA * Susceptible to noise 1-7

8 Time domains Time component of signals Duration of signal above a threshold Electrical domains * HI above and LO below Frequency and period 1-8

9 Electrical Domains Digital Domain 2 level scheme On, off; high, low * Each selection is a bit Bit is termed count digital data Numbers can be represented in binary 1 or 0 * 101 = 1x2 2 +0x2 1 +1x2 0 = 5 1-9

10 Detectors, Transducers, and Sensors Terms often interchanged Detector Device that identifies, records, or indicates change in variable Pressure, temperature, concentration, charge, radiation Chemical, mechanical, or electrical device Transducer Converts information from non-electrical to electrical domain Photomultipliers, photodiodes 1-10

11 Sensor Sensors monitor a specific chemical species Ion sensitive electrode Piezoelectric crystal Functionalization of surface Crystal oscillates at constant frequency in an electric circuit Change in mass can be detected 2 CF M F = A C is a constant, F is frequency, M is mass, and A is surface area * Also works in spectroscopy (photoacoustic) 1-11

12 Selection of Method Accuracy Sample size Concentration range Interference Matrix Number of samples 1-12

13 Criteria Precision Degree of mutual agreement Can be quantified Bias Difference in true sample measurement Bias=m-x t m is mean, x t is true measurement 1-13

14 Criteria Sensitivity Ability to distinguish analyte differences Based on linear calibration curve * S=mc+S bl S = signal, m = slope, c= concentration, S bl =blank signal Slope is calibration sensitivity Analytical senstivity g=m/s, s s s =standard deviation Detection limit Minimum analyte concentration that can be detected Approaches blank signal Generally blank plus 3 times blank standard deviation 1-14

15 Detection limit Criteria Equation used to convert S m to detection limit (c m ) Dynamic range c m = S m S m bl 1-15

16 Criteria Selectivity Impact of matrix on sample measurement Saturation of detector by matrix Similar chemical behavior * Group chemistry Physical properties * Mass similarity Can be defined by selectivity coefficient 0 and above * Can be greater than unity 1-16

17 Calibration Calibration curve Standard are measured and response recorded Need to consider matrix Can be nonlinear 1-17

18 Calibration Standard Addition Addition of standard directly to sample Multiple samples used to determine unknown concentration S1c svs cx = ( S2 S1) Vx c x =unknown concentration, S is response for sample (1) and sample plus standard (2), c s is concentration of added standard, V x is volume of unknown, V s is volume of added sample Internal Standard Standard added to all samples, blanks, and calibration 1-18

19 Chap 2 Electric components and circuits Instruments rely upon measurements in a circuit Based on electricity laws Ohms law * V=IR (V= potential difference in volts, R is resistance in ohms, and I is current in amperes Kirchhoff s law * Sum of current around a point is 0 Power Law * P=IV (P in watts) 1-19

20 Current Circuits Series Circuit R and V related based on fundamental laws V x =V(R x /R) Apply Kirchhoff s law and Ohm s law to find voltage at any point 1-20

21 Voltage divider In many circuits, it is necessary to obtain a voltage not available from the main power source can derive other voltages from the main power source voltage is less than the voltage from the main source use resistors in an appropriate configuration to reduce the voltage from the power source Potentiometers 1-21

22 A collection of series circuits Kirchhoff s and Ohm s law still apply More electrons flow through resistor with lower resistance Voltage for each section Parallel Circuit V=I x R x I t =V/R t 1 1 = R t R x x 1-22

23 Measurements Voltmeter Integrated circuit Power supply Display Uses analog to digital converter 1-23

24 AC circuits Current or potential fluctuates with time F=1/t (Hz) Cycles per second Waves can differ Sinusoidal, square wave, ramp, sawtooth Equations can describe fundamental properties Angular velocity ω = 2πf 1-24

25 Capacitors and Capacitance Conductors separated by dielectric substance Can store an electrical charge Capacitor accepts electrons and stores the charge Releases electrons Equipment can have charge when unplugged v r c 1-25

26 Band Gap and Semiconductor Distinguish between conductor and insulators based on band gap Thermal and photoexcitation Thermal promotes electron and creates hole Chemical excitation Inject impurities Dopants * i.e., P for Si, extra electron goes into conductive band Can apply Bohr model to describe single electron Replace e o with e (dielectric constant) and m e with effective mass 4 E = k m = 8ε e 0 e h

27 Chemical doping Dieletric constant for Si = 11.7, effective mass is 0.2 m e = 0.02 ev, measurement is ev Calculate Bohr radius r 1 =ae/m= 30 A Greater orbit Donor electron promoted to conduction band by thermal energy Ionized electron goes into conduction band * Level below conduction band is donor level Most electrons from P Electrons in band from P and governed by P 1-27

28 Dopants The value of semiconductors for solid state device fabrication lies in the fact that the number and type of conducting electric charge carriers [electrons are n-type (negative), holes are p-type (positive)] can be controlled through incorporation of appropriate dopant elements. Thus the substitutional incorporation of Group V elements (Sb, As, P) provides for shallow donor levels in the band gap at about 0.01 ev from the conduction band. The substitutional incorporation of Group III elements (B, Al) generates acceptor levels in the band gap at about 0.01 ev from the valence band. The two types of impurities are almost completely ionized at room temperature and give rise to extrinsic n-type and p-type conductivity the basis for the formation of junction devices such as diodes and transistors (fig. 8). Of increasing importance are compound III V ( adamantine) semiconductors, such as GaAs, InSb, InP and GaP (compounds of Group III and Group V elements). Together these compounds provide eight valence electrons and, by sp 3 hybridization, are able to form a diamond-like, covalent crystal structure with semiconductor properties. These compounds (GaAs, for example) exhibit electron mobilities which are higher than those of silicon and, therefore, are of considerable interest for advanced device technology. 1-28

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30 N-type (negative) Doping Electron extrinsic semiconductor (extra e - ) Dope with subvalent impurity B in Si Removes electron from valance band * Creates hole that is mobile Apply Bohr model is hole Energy level are slightly about valence band Acceptor level (0.01 ev about valence band) * P- type 1-30

31 Semiconductor Make a p-n junction 1-31

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33 Solids Ordered solids crystalline Irregular solids No long range order Amorphous (glass) Band gaps > 3 ev are transparent Shapes are based on packing 1-33

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36 Digital Electronics and Microcomputers Analog and digital signals Use of binary numbers ADC and DAC Transducers to data Controlled by computer 1-36

37 Lecture Extra Bohr Atom Models of atoms Plum pudding Bohr atom Inclusion of quantum states Based on Rutherford atom Bohr atom for 1 electron system E total =1/2m e v 2 +q 1 q 2 /4pe o r q 2 =-e * Include proton and electron 1/2m e v 2 -Ze 2 /4pe o r 1-37

38 Bohr Atom Net force on the electron is zero 0=F dynamic +F coulombic 1/2m e v 2 /r+q 1 q 2 /4pe o r 2 Force is 1/r 2 Energy 1/r E 1/2m e v 2 /r-ze 2 /4pe o r 2 Z is charge on nucleus Quantize energy through angular momentum mvr=nh/2p, n=1,2,3. Can solve for r, E, v = Fdr 1-38

39 Bohr radius R=(e o h 2 /pm e e 2 )(n 2 /Z) Radius is quantized and goes at n 2 R=0.529 Å for Z=1, n=1 A o (Bohr radius) 1-39

40 Atomic Spectra Quantum numbers n=1,2,3,4 r=a o n 2 /Z for gases with 1 electron Energy E=-(m e e 4 /8e o2 h 2 )Z 2 /n 2 For ground state H E=2.18E-18 J/atom=k * Can determine J/mole 1312 kj/mole Energy goes as k/n 2 * System converges to limit 1-40

41 Energy n=infinity, r=infinity, E=0, unbound e - Ionization energy k is ionization energy Velocity v=nh/2pm e r Ionization energy Minimum energy required to remove electron from atom in gas phase Multiple ionization energies 1-41

42 Balmer states Gas H in tube Four lines in visible region Fit lines 1/l=(1/2 2-1/n 2 )R, R=1.1E-7 m -1 1/l=n (wavenumber) E=1/2m e v 2 =ev (V=Volts) At 1 V = 1.6E-19 J =ev K=13.6 ev 1-42

43 Matter energy interaction E incident =1/2mv 2 =qv E scattered DE =E incident -E scattered DE=kZ 2 (1/n 2 final-1/n 2 in ) =hn=hc/l De-excitation of electron results in photon emission Corresponds to line emission 1-43