Food Analysis Previous Exam I

University of Kentucky, Department of Animal & Food Sciences, FSC 535

1. True (T) or False (F) (30 pts.)

Titration of a 1 molar solution of an acid (with 1 equivalents per mole) will require more standard base than titration of an equal volume of a 1 molar solution of acid with 2 equivalents per mole.

Food acids are generally weak acids.

Weak acids generally make poor buffers.

Electromagnetic radiation in the UV region has less energy than electromagnetic radiation in the visible range.

The concentration of an analyte in solution will effect the wavelength absorption maximum for that compound.

A diode-array detector measures the entire UV-visible spectrum of a solution almost instantaneously.

A anion exchange resin posses a negative charge.

Hexane is a "strong"solvent for reverse-phase separation.

In gas chromatography, the detector is maintained several degrees cooler than the column temperature.

A refractive index detector is susceptible to temperature effects from the atmosphere.

Column temperature has only a minor effect on GLC separation.

Water activity is a good measure of the moisture content in foods.

Dry ashing is a good method for the more volatile elements.

Gravimetric methods are an accurate means of determining the mineral contents of food samples.

In gel filtration chromatography, the small compounds elute first.


2. Citric acid has a molecular weight of 192.12 and three hydrogen ions capable of dissociation (pks of 3.06, 4.74 and 5.4). If you were titrating a solution or grapefruit juice from pH 2 to 7, what equivalent weight of citric acid would you use in your calculations? (5 pts.)


3. A 1 liter solution contains 2 moles of lactic acid and 1 mole of lactate ion. The pK of lactic acid is 3.86. What is the pH of this solution? How much lactic acid would you have to add to make the pH to 3.4? (8 pts.)


4. Why does a compound absorb electromagnetic radiation in the ultraviolet/visible region? What happens to the energy after it is absorbed? (6 pts.)


5. Describe how a standard curve is prepared and used in conjunction with Beer’s Law. (6 pts.)


6. Describe the difference between a conventional spectrophotometer and a diode-array spectrophotometer. What is one application where a diode-array would be particularly beneficial ? (6 pts.)


7. Describe three types of sample degradation that can occur during moisture analysis that can effect the outcome of the assay. Briefly describe how or what method can be used to inhibit these reactions. (9 pts.)


8. Briefly describe the primary mode of analyte separation for the following chromatography media. (8 pts.)

A. Reverse-Phase

B. Gel-Permeation

C. Cation Exchange

D. Affinity


9. Describe the advantages, characteristics and application of a good internal standard used in either gas chromatography of HPLC separations. (6 pts.)


10. List and briefly describe the major components of a Gas Chromatography System. (12 pts.)


11. List three detectors used for HPLC analysis and briefly describe their mode of operation and a situation where each would have a particular advantage over other types of detectors. (9 pts.)


13. Briefly write down the differences between the following items. (20 pts.)

a. Normality vs. Molarity

b. Weak acid vs. Strong acid

c. Fluorescence excitation vs. Fluorescence emission

d. Absorptivity vs. Absorbance

e. Wet ashing vs. Dry ashing

f. Karl Fisher titration vs. Distillation Moisture assay

g. Gravimetric vs. Colorimetric Mineral Assays

h. Internal Standard vs. External Standard

I. Refractive Index Detector vs. Evaporative Light Scattering Detector

j. Packed GC columns vs. Capillary columns


14. You are given a mixture of 2-hydroxy-nonanoic acid (pK 4.9) and 2-hydroxy-3-decanone that need to be separated. Unfortunately, these two compounds have very similar boiling points, molecular weights, and polarities.

Describe a method that should provide a baseline separation of these two compounds and explain why it should work. Give details of the type of column, detector, and any other major equipment or procedures necessary to complete your separation. Using your proposed method, which compound should elute first and why? (15 pts.)