question

What are the 10 steps of the analytical method?

answer

1) Define the problem (quant. vs.qual)
2) Choose the method of analysis
3) Obtain appropriate sample
4) Determine amount of sample
5) Get sample into correct form
6) Eliminate interfering species
7) Run assay
8) Data reduction
9) Statistical analysis
10) Interpret the meaning of results

question

What's the general framework for instrumentation?

answer

Stimulus --> Sample--> Response

question

Difference between accuracy and precision.

answer

Accuracy= how close measurements are to the true value.

Precision = how close measurements are to each other

question

Explain the two types of errors

answer

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.

question

What's relative standard deviation

answer

Comparing two numbers with standard deviations.
Relative : s/ avg. x
% rel std same thing times one hundred percent

question

What's the difference between x, s and Mu, delta in the context of confidence intervals?

answer

x, s = experimental sample mean and standard deviation; limited number of results; N < 20
Mu, delta = population/true; infinite number of measurements; N >20

question

What are two systematic errors that affect accuracy (bias)?

answer

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.

question

Corrections for systematic error

answer

1) constant --> run a blank.
2) proportional --> run a standard (standard= sample with a known concentration of analyte).

question

When would choose a type 1 t-test?

answer

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.

question

When would we use a type 2 t- test?

answer

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.

question

When would you used a case 3 t-test?

answer

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.

question

Why would we do an f-test?

answer

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.

question

Why would you use the Q-test?

answer

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

question

What are the 5 general principles in sample pretreatment?

answer

1) Should be done w/o losing analyte
2) Should include bringing analyte into the best chemical form.
3) should remove interferents from the matrix.
4) should be done w/o introducing additional interferents
5) should bring the analyte into the proper concentration range

question

What are some sources of loss in your sample?

answer

1) Adsorption: sticks to the surface (metal on glass)
2) Decomposition
3) non-quantitative transfer

question

How can you maximize your recovery?

answer

Optimize the chemical form, Minimize interferences, and optimize concentration

question

Standards:
2 Types of Instruments

answer

1) Direct measure: mass, volume
2) Indirect measure: instrument measures signal

question

Describe an external standard

answer

The sample is different from the standard (in physically different locations).
-when matrix is either simple or well defined.
- one set of calibration solutions.

question

For external standards, what does the calibration curve depend on?

answer

The instrument, condition of the instrument at the time of analysis, and other materials in the matrix

question

When would you use/apply standard addition?

answer

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

question

For standard addition, on a graph of IR versus volume of standard, what does "Vs0" refer to?

answer

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

question

Describe the Internal Standard method

answer

spike a compound that's similar but different from our analyte
-add a known amt of internal standard (diff from analyte)

question

What are the criteria for a good internal standard

answer

1) similar molecular properties to analyte
2) distinguishable analytical signal

question

What is sensitivity?

answer

How close 2 samples can be in concentration and still produce a measureably different instrumental response

question

Describe calibration sensitivity

answer

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.

question

Describe analytical sensitivity

answer

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.

question

Detection limit

answer

The smallest concentration which produces a signal which can be statistically differentiated form the blank.
Smin = Sblank + 3sblank

cmin = Smin -b/ m

question

Dynamic range

answer

Range of concentration over which the instrumental response/ signal is linearly related to the concentration

question

Selectivity

answer

ability of your method to distinguish signal from analyte versus signal from interferents.

S = maCa + mbCb + Sbl
selectivity coeffic. Kb,a = mb/ma If K is small, method is selective

question

We're talking about light now. A high energy corresponds to what type of wavelength, and frequency?

answer

A short wavelength, and a high frequency

question

From high energy to low energy, list the types of radiation that you can have.

answer

Gamma, x-rays, UV, Visible, Infrared, Microwaves, and Radiowaves

question

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?

answer

In water; the greater the difference in the index of refraction, the more it bends.

question

Name two important facets of the Photoelectric Effect

answer

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

question

Atoms have ____ _____ energy levels, and can only absorb or emit certain frequencies of light.

answer

Discreet, quantized

question

Energy of light must equal _______ between any ___ ___ levels in the atom or molecule.

answer

the difference; 2 energy levels

question

Describe the three types of emission that can occur

answer

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)

question

Name two types of absorbtion

answer

1) Atomic: no vibrational levels, only electronic
2) Molecular: electronic, vibrational, and rotational

question

When we measure absorbance, what are we actually measuring?

answer

The power in and power out. T = P/Po
A= -log T

question

What are the practical quantitative aspects of UV-Vis?

answer

1) Selection of lambda max
2) Prepare solns which have constant temp, electrolyte concentration, interfering species, pH
3) Cells or cuvettes (ideally usd matched ones)

question

What do we run a user baseline?

answer

To account or correct for differences in the cuvette

question

Important characteristics of UV-Vis

answer

1) wide applicability to both inorganic and organic
2) detection limit (E-4, E-5)
3) moderate selectivity
4) good accuracy
5) easy data acquisition

question

How about qualitative aspects of UV-Vis?

answer

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

question

What's the difference between single vs. double beam UV-Vis?

answer

single beam= measure the reference than sample.
double beam = measure reference and sample at the same time

question

Photometer versus Spectrophotometer

answer

Spectro: measure spectrum,
photo: can't; measures absorbance at one wavelength.

question

Single versus Multichannel

answer

Single channel: looks at one wavelenght at a time
Multichannel: all wavelengths at once

question

Null versus Direct read out

answer

Direct: directly measures P and Po
Null: put optical wedge in until you get same intensity of P and Po.

question

What are some general characterisitics of the light source you need for UV-Vis?

answer

1) stable intensity--> fairly intense
2) cover all wavelengths of interest (can't do)
3) easily replaced and realigned (can do)

question

Deuterium versus Tungsten light source

answer

Deuterium: UV-light source
Tungsten: pass current through filament, resistive heating --> blackbody radiation (350-2500 nm)

question

Describe two slit experiment

answer

Interference pattern spacing: smaller spacing of slits yields larger spaces of diffraction pattern.
- the place where diffraction spots fall is lambda dependent

question

What does n*(lambda) = d(sin i + sin r) tell you?

answer

It tells you where in space you get positive interference for a particular wavelength

question

Performance characteristics for monochromator

answer

1) Inverse linear dispersion
2) Resolution
3) Effective bandwidth
4) Scattered light

question

Inverse linear dispersion. Describe it.

answer

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

question

What's resolution

answer

How close 2 spectral peaks can be together and still be distinguished
R = lambda/ delta lambda for 1 peak (detla lambda = Full Width Half Max

For 2 peaks, R = lambda ave/ delta lambda

question

What are factors that effect the resolution?

answer

Distance between the blazes, diffraction order (n), size of the grating
R= nN

question

Describe effective bandwidth

answer

Range of wavelengths exiting the monochromator. delta lambda eff = D-1 * w, where D-1 = inverse linear dispersion, and w = width of the slit

question

What size bandwidth will give you a greater sensitivity?

answer

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)

question

What's the effect of scattered light on beer's law?

answer

You get non-linearity, and an erroneous low absorbance.

question

Colored glass filter vs. Interference filter

answer

1) colored glass: every thing else is absorbed except for colored wavelength
2) interference: destructive interference for all other wavelenghts except for wavelength of interest.

question

Sample and Reference cells
Measure P and Po simultaneously vs. in series

answer

1) Simultaneously: minimize time between P and Po, difference in detector response between cause errors
2) series: chopper! mirror = sample, holes = reference

question

Purpose of detectors

answer

Transduce our signal from a non-electronic domain to an electronic domain.

question

Photomultiplier Tube

answer

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

question

Performance characteristics in PMT

answer

1) Good sensitivity (small number of photons gives a reasonable current)
2) Consistent response regardless of lambda
3) High gain (1 photon --> lots of electrons)

question

Multichannel Detectors
PMT vs. Array

answer

PMT: exit slit determines bandwidth; scan
Array: all multichannel is single beam,gives us spectrum chunk at one time

question

For array of detectors, what must you change about the monochromator?

answer

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)

question

Semiconductors

answer

- 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)

question

An n-type semiconductor is doped w/ group __.
An p-type semiconductor is doped w/ group __.

answer

n-type: group 5.
p-type: group 3.

question

What are the 4 limitations of Beer's law?

answer

1. Only works for dilute solutions.
2. Equilibrium pushes system to chemical deviations
3. 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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