16

Question 1

State the difference between transmittance, absorption and molar absorption

Answer 1

• Transmittance: fraction of incident light absorbed by the sample P/Po
• Absorbance: -logT
• Molar absorbance: constant of proportionality between absorbance+path length

Question 2

State the wavelenghts and the colors absorbed

Answer 2

• Violet: 380-435
• Blue: 435-500
• Cyan: 500-520
• Green: 520-565
• Yellow: 565-590
• Orange: 590-625
• Red: 625-740

Question 3

What is an absorbance spectrum?

Answer 3

A graph of absorbance/molar absorptivity versus wavelength

Question 4

What are the differences between a single-beam and double-beam spectrophotometer?

Answer 4

• One has one sample holder, the other two
• Single requires removal/replacement of cuvette between reference and sample measurement
• Double has mirrors and choppers to divert light and allow several Po/P measurements/minute

Question 5

What is the best wavelength for a tungsten and a deuterium lamp?

Answer 5

• Deuterium: 110-400 nm
• Tungsten: 320-2500 nm

Question 6

What are the advantages and disadvantages of decreasing monochromatic slit width?

Answer 6

• Higher resolution
• Worse SNR ratio

Question 7

Why is a photomultiplier such a sensitive photodetector?

Answer 7

• Photomultiplier have multiple dynodes that amplify and propagate the signal
• Each electron can produce more than one electron from a dynode
• In a vacuum, no residual gas to collide with electrons

Question 8

What is the difference between molecular mass and nominal mass?

Answer 8

• Molecular mass: average weighted mass based on natural abundance of all isotopes
• Nominal mass: integer mass based on most abundant isotope

Question 9

What is the difference between eluent and eluate?

Answer 9

• Eluent into the column
• Eluate is the solvent leaving the column

Question 10

Why would a band be broader with time?

Answer 10

• At long retention times, longitudinal diffusion causes peak broadening

Question 11

Explain why chromatography separations normally has optimal flow rates that gives the best separations?

Answer 11

• Flow rate too low: analytes stay in column too long, longitudinal diffusion broadens the peaks
• Flow rate too high: analytes don’t have time to interact with the SP and separate

Question 12

Suggest a reason what optimal linear flow rates is much higher in GC than in LC

Answer 12

Longitudinal diffusion is much faster in GC than LC, therefore the flow rate of GC must be much higher to compensate

Question 13

What kind of info does a MS detector give in GC that is useful for qualitative analysis?

Answer 13

• Isotopic information
• Molecular mass of the analyte
• Structural information

Question 14

What kind of info does a MS detector give in GC that is useful for quantitative analysis?

Answer 14

Area under the curve proportional to concentration. A calibration curve is needed to determine the detector’s response for a given analyte

Question 15

Define

Absolute uncertainty

Answer 15

(Number * percent uncertainty)/100

Question 16

Define

Relative uncertainty

Answer 16

Absolute uncertainty/number

Question 17

How to add uncertainties?

Answer 17

(x^2 + y^2)^(1/2)

Question 18

How to multiply uncertainties?

Answer 18

1. Calculate relative uncertainty
2. (x^2 + y^2)^(1/2)

Question 19

What source of error in a single-beam
instrument is absent in the double-beam instrument?

Answer 19

The single-beam spectrophotometer is more susceptible to random error, due to misplacement of the cuvette and fingerprint contamination