# Shared Flashcard Set

## Details

Instrumental and Analytical
Exam 1 material
67
Chemistry
09/30/2006

Term
 What are the 10 steps of the analytical method?
Definition
 1) Define the problem (quant. vs.qual)2) Choose the method of analysis3) Obtain appropriate sample4) Determine amount of sample5) Get sample into correct form6) Eliminate interfering species7) Run assay8) Data reduction9) Statistical analysis10) Interpret the meaning of results
Term
 What's the general framework for instrumentation?
Definition
 Stimulus --> Sample--> Response
Term
 Difference between accuracy and precision.
Definition
 Accuracy= how close measurements are to the true value.Precision = how close measurements are to each other
Term
 Explain the two types of errors
Definition
 1) random: indeterminate--> equal high and low precision.2) systematic: determinate--> generates from a fixed cause, and is either high or low. affects our accuracy but its generally correctable.
Term
 What's relative standard deviation
Definition
 Comparing two numbers with standard deviations. Relative : s/ avg. x % rel std same thing times one hundred percent
Term
 What's the difference between x, s and Mu, delta in the context of confidence intervals?
Definition
 x, s = experimental sample mean and standard deviation; limited number of results; N < 20Mu, delta = population/true; infinite number of measurements; N >20
Term
 What are two systematic errors that affect accuracy (bias)?
Definition
 1) constant: size of sample doesn't effect this; its the same magnitude (ex: calibration throws off subsequent measurements)2) proportional: magnitude of error is proportional to sample; interferent in sample.
Term
 Corrections for systematic error
Definition
 1) constant --> run a blank. 2) proportional --> run a standard (standard= sample with a known concentration of analyte).
Term
 When would choose a type 1 t-test?
Definition
 When we want to see if experimental data agrees with a known value. tcalc = (Mu - x)(sq. root of N / s)If tcalc < ttable we get statistically valid results.
Term
 When would we use a type 2 t- test?
Definition
 Compare 2 sets of experimental data which are replicates of a single sample:same method w/ 2 diff. analysts or 2 methods with the same analyst.You get 2 sets of data, each w/ Xa Xb and Mu a and Mu b.
Term
 When would you used a case 3 t-test?
Definition
 Compare 2 sets of data where individual measurements are made of multiple samples.tcalc = d/ sd *(sq. root of N)where d = difference between set 1 and 2.
Term
 Why would we do an f-test?
Definition
 Allows us to compare error distributions of 2 sets of experimental data.fcalc = sa^2/ sb^2 (bigger one on top)If fcalc > ftest, they have different error distributions.
Term
 Why would you use the Q-test?
Definition
 Only if there is a particular data point that isn't part of the parent popluation. Qcalc = d (differnece between questionable point and its nearest neighbor) / w (difference between qst. pt. and the farthest point)IF q calc < q table, you must keep the point
Term
 What are the 5 general principles in sample pretreatment?
Definition
 1) Should be done w/o losing analyte2) Should include bringing analyte into the best chemical form.3) should remove interferents from the matrix.4) should be done w/o introducing additional interferents5) should bring the analyte into the proper concentration range
Term
 What are some sources of loss in your sample?
Definition
 1) Adsorption: sticks to the surface (metal on glass)2) Decomposition3) non-quantitative transfer
Term
 How can you maximize your recovery?
Definition
 Optimize the chemical form, Minimize interferences, and optimize concentration
Term
 Standards:2 Types of Instruments
Definition
 1) Direct measure: mass, volume2) Indirect measure: instrument measures signal
Term
 Describe an external standard
Definition
 The sample is different from the standard (in physically different locations).-when matrix is either simple or well defined.- one set of calibration solutions.
Term
 For external standards, what does the calibration curve depend on?
Definition
 The instrument, condition of the instrument at the time of analysis, and other materials in the matrix
Term
 When would you use/apply standard addition?
Definition
 When matrix is very complex, or when it contains proportional interferences.spike varying amts. of standard 1) known conc. of standard 2) const. amt of sample, 3) varying amt of standard 4) const total volume
Term
 For standard addition, on a graph of IR versus volume of standard, what does "Vs0" refer to?
Definition
 The negative of this value gives you the amount of standard you would've had to add to give you the same signal as the standard. Cx = -(Vs)0 Cs/ Vx
Term
 Describe the Internal Standard method
Definition
 spike a compound that's similar but different from our analyte-add a known amt of internal standard (diff from analyte)
Term
 What are the criteria for a good internal standard
Definition
 1) similar molecular properties to analyte2) distinguishable analytical signal
Term
 What is sensitivity?
Definition
 How close 2 samples can be in concentration and still produce a measureably different instrumental response
Term
 Describe calibration sensitivity
Definition
 Sensitivity can be determined by slope of the calibration curve. -We want 2 samples close together, but produce a large difference in IR to be able to distinguish and measure them.
Term
 Describe analytical sensitivity
Definition
 Gamma = m/ s, where s= std. dev. of sample IR.-If you have another sample that falls within the same std. deviation of one sample, you won't be able to distinguish them.
Term
 Detection limit
Definition
 The smallest concentration which produces a signal which can be statistically differentiated form the blank. Smin = Sblank + 3sblankcmin = Smin -b/ m
Term
 Dynamic range
Definition
 Range of concentration over which the instrumental response/ signal is linearly related to the concentration
Term
 Selectivity
Definition
 ability of your method to distinguish signal from analyte versus signal from interferents. S = maCa + mbCb + Sblselectivity coeffic. Kb,a = mb/ma If K is small, method is selective
Term
 We're talking about light now. A high energy corresponds to what type of wavelength, and frequency?
Definition
 A short wavelength, and a high frequency
Term
 From high energy to low energy, list the types of radiation that you can have.
Definition
 Gamma, x-rays, UV, Visible, Infrared, Microwaves, and Radiowaves
Term
 For the index of refraction, if there are two mediums that light can pass through (air and water for example), in what medium will the angle between the normal be smallest?
Definition
 In water; the greater the difference in the index of refraction, the more it bends.
Term
 Name two important facets of the Photoelectric Effect
Definition
 1) Light must have a critical/threshold frequency (doesn't matter what intensity the light is, the wavelength has to be shorter than the medium)2) Light having greater than critical frequency causes ejected electrons to be ejected w/ increased KE
Term
 Atoms have ____ _____ energy levels, and can only absorb or emit certain frequencies of light.
Definition
 Discreet, quantized
Term
 Energy of light must equal _______ between any ___ ___ levels in the atom or molecule.
Definition
 the difference; 2 energy levels
Term
 Describe the three types of emission that can occur
Definition
 1) Line spectra: arise from atomic transitions (vphoton = E1-E0/ h)2) Band spectra: arise from molecules (they are broader)3) Contiuum Spectra aka blackbody radiation (wavelength max = TK)
Term
 Name two types of absorbtion
Definition
 1) Atomic: no vibrational levels, only electronic 2) Molecular: electronic, vibrational, and rotational
Term
 When we measure absorbance, what are we actually measuring?
Definition
 The power in and power out. T = P/PoA= -log T
Term
 What are the practical quantitative aspects of UV-Vis?
Definition
 1) Selection of lambda max2) Prepare solns which have constant temp, electrolyte concentration, interfering species, pH3) Cells or cuvettes (ideally usd matched ones)
Term
 What do we run a user baseline?
Definition
 To account or correct for differences in the cuvette
Term
 Important characteristics of UV-Vis
Definition
 1) wide applicability to both inorganic and organic 2) detection limit (E-4, E-5)3) moderate selectivity4) good accuracy5) easy data acquisition
Term
 How about qualitative aspects of UV-Vis?
Definition
 Not done frequently because you get broad, few peaks- can get some functional groups-must choose solvent carefully (must be transparent at wavelengths of interest)- polar solvents blur the spectrum
Term
 What's the difference between single vs. double beam UV-Vis?
Definition
 single beam= measure the reference than sample.double beam = measure reference and sample at the same time
Term
 Photometer versus Spectrophotometer
Definition
 Spectro: measure spectrum, photo: can't; measures absorbance at one wavelength.
Term
 Single versus Multichannel
Definition
 Single channel: looks at one wavelenght at a timeMultichannel: all wavelengths at once
Term
Definition
 Direct: directly measures P and PoNull: put optical wedge in until you get same intensity of P and Po.
Term
 What are some general characterisitics of the light source you need for UV-Vis?
Definition
 1) stable intensity--> fairly intense2) cover all wavelengths of interest (can't do)3) easily replaced and realigned (can do)
Term
 Deuterium versus Tungsten light source
Definition
 Deuterium: UV-light sourceTungsten: pass current through filament, resistive heating --> blackbody radiation (350-2500 nm)
Term
 Describe two slit experiment
Definition
 Interference pattern spacing: smaller spacing of slits yields larger spaces of diffraction pattern. - the place where diffraction spots fall is lambda dependent
Term
 What does n*(lambda) = d(sin i + sin r) tell you?
Definition
 It tells you where in space you get positive interference for a particular wavelength
Term
 Performance characteristics for monochromator
Definition
 1) Inverse linear dispersion2) Resolution3) Effective bandwidth4) Scattered light
Term
 Inverse linear dispersion. Describe it.
Definition
 A small inverse linear dispersion is good (bad is large teheh)IF you want a low D-1, you need a large space or large spectrum
Term
 What's resolution
Definition
 How close 2 spectral peaks can be together and still be distinguished R = lambda/ delta lambda for 1 peak (detla lambda = Full Width Half MaxFor 2 peaks, R = lambda ave/ delta lambda
Term
 What are factors that effect the resolution?
Definition
 Distance between the blazes, diffraction order (n), size of the grating R= nN
Term
 Describe effective bandwidth
Definition
 Range of wavelengths exiting the monochromator. delta lambda eff = D-1 * w, where D-1 = inverse linear dispersion, and w = width of the slit
Term
 What size bandwidth will give you a greater sensitivity?
Definition
 A larger bandwidth; more light, more sensitive. BUT, if you have an analyte and interferent, you need a small bandwidth in order to distinguish the two (sensitivity vs. selectivity)
Term
 What's the effect of scattered light on beer's law?
Definition
 You get non-linearity, and an erroneous low absorbance.
Term
 Colored glass filter vs. Interference filter
Definition
 1) colored glass: every thing else is absorbed except for colored wavelength2) interference: destructive interference for all other wavelenghts except for wavelength of interest.
Term
 Sample and Reference cellsMeasure P and Po simultaneously vs. in series
Definition
 1) Simultaneously: minimize time between P and Po, difference in detector response between cause errors2) series: chopper! mirror = sample, holes = reference
Term
 Purpose of detectors
Definition
 Transduce our signal from a non-electronic domain to an electronic domain.
Term
 Photomultiplier Tube
Definition
 Taking a small optical signal, and transducing it (a multiplying it) to a large electronic signal.- each dinode multiplies the number of electrons, 1 photon = 10^7 electrons
Term
 Performance characteristics in PMT
Definition
 1) Good sensitivity (small number of photons gives a reasonable current)2) Consistent response regardless of lambda3) High gain (1 photon --> lots of electrons)
Term
 Multichannel DetectorsPMT vs. Array
Definition
 PMT: exit slit determines bandwidth; scanArray: all multichannel is single beam,gives us spectrum chunk at one time
Term
 For array of detectors, what must you change about the monochromator?
Definition
 You must remove the exit slit, so the light is open to all detectors (the width of each detector is the band width or the pixel)
Term
 Semiconductors
Definition
 - solid w/ dissociated electrons. Electrons can move w/o being attached to a particular atom or molecule. Holes or electrons moves about = both can be an electrical conductor.- electricity conducted by electrons (n-type) or holes (p-type)
Term
 An n-type semiconductor is doped w/ group __.An p-type semiconductor is doped w/ group __.
Definition
 n-type: group 5.p-type: group 3.
Term
 What are the 4 limitations of Beer's law?
Definition
 1. Only works for dilute solutions.2. Equilibrium pushes system to chemical deviations3. Polychromatic deviations (this is why we measure absorbance at flat region).4. Stray radiation (light which reaches the detector but hasn't originated from the light source and through the sample).
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