Unit 5: Antioxidant Activity - Practice Quiz

1 What is the main purpose of measuring antioxidant activity in a food sample?

2 Which instrument is commonly used to measure absorbance in an antioxidant assay?

3 What does a reagent blank usually contain?

4 Why is a standard solution used in a total antioxidant activity assay?

5 What is an antioxidant?

6 What is usually prepared from a food sample before testing its antioxidant activity?

7 What may happen to the absorbance when more colored product is formed in an assay?

8 Why should the same wavelength be used for comparable assay readings?

9 What is a common reason for testing a sample in duplicate or triplicate?

10 Which type of water is generally preferred for preparing assay solutions?

11 What does TLC stand for?

12 What is the stationary phase in a common TLC plate?

13 What is the mobile phase in TLC?

14 Where should the sample be applied on a TLC plate?

15 Why is pencil commonly used to mark a TLC plate?

16 What is the solvent front in TLC?

17 What does an value compare?

18 Which formula represents the TLC retention factor?

19 What does one spot on a TLC plate commonly suggest about a sample?

20 How can colorless compounds on a TLC plate often be detected?

21 In a DPPH assay, a food extract causes the absorbance of the DPPH solution to decrease. What does this decrease primarily indicate?

22 A sample has an initial DPPH absorbance of 0.800 and a final absorbance of 0.320. What is the percentage of radical-scavenging activity?

23 Why is a reagent blank included when determining antioxidant activity spectrophotometrically?

24 A calibration curve for a standard antioxidant is used mainly to:

25 If the absorbance of a sample is higher than the reagent control in a colorimetric antioxidant assay, the most appropriate first action is to:

26 Why should replicate measurements be performed for a food antioxidant assay?

27 An antioxidant assay is performed on 2.0 g of food, and the extract is made up to 25 mL. Which change would most directly affect the reported activity per gram?

28 Why is a fixed incubation time important in a DPPH antioxidant assay?

29 A diluted extract gives an antioxidant activity within the calibration range, while the undiluted extract exceeds it. Which result is preferable for quantification?

30 If two extracts show 80% and 40% DPPH inhibition at the same concentration, what is the most reasonable conclusion?

31 In thin layer chromatography, the stationary phase is usually:

32 A compound travels 3.0 cm while the solvent front travels 6.0 cm. What is the compound's value?

33 Why must the sample spot be placed above the solvent level in the developing chamber?

34 A mixture produces three separate spots on a TLC plate. This observation most directly suggests that the mixture:

35 For a silica TLC plate, which compound would generally have the lower in the same solvent system?

36 Why is a pencil preferred over ink for marking the origin and solvent front on a TLC plate?

37 If the solvent front is not marked immediately after removing the TLC plate, the calculated value may be inaccurate because:

38 Two compounds have identical values in one solvent system. What is the best interpretation?

39 A TLC spot remains near the origin even after development. Which adjustment would most likely improve its movement?

40 Why should a TLC sample be applied as a small, concentrated spot rather than a large wet spot?

41 A food extract gives an absorbance of 0.620 at 695 nm. The ascorbic-acid calibration equation is , where is in . If the extract was diluted 20-fold before measurement and prepared from 0.50 g of food to a final volume of 25 mL, what is the antioxidant activity in mg ascorbic-acid equivalents per gram of food?

42 In a phosphomolybdenum total antioxidant assay, the sample absorbance is 0.410, the reagent blank is 0.080, and the sample matrix blank without molybdate reagent is 0.050. The standard curve is . Which concentration should be used to calculate the sample antioxidant equivalent?

43 A calibration curve is linear from 10 to 80 . A diluted sample produces an absorbance corresponding to 135 . Which action gives the most defensible result?

44 Two food extracts have identical total antioxidant-equivalent values by the phosphomolybdenum assay, but one has a much higher ferric-reducing power. What is the best interpretation?

45 A sample is tested at two dilutions. The 10-fold dilution gives 0.88 absorbance, whereas the 20-fold dilution gives 0.47. The calibration range is 0.10–0.80 absorbance. Which result should be accepted for quantification?

46 A food sample is extracted twice. The first extraction recovers 82% of the final measured antioxidant activity, and the second recovers the remaining 18%. If only the first extract is assayed, what is the principal consequence?

47 A sample extract is intensely colored at the assay wavelength. The reaction mixture has an absorbance of 0.900, while a sample blank containing extract and solvent but no assay reagent has an absorbance of 0.300. What is the primary correction?

48 For a standard curve, concentrations are 0, 20, 40, 60, and 80 , with absorbances 0.02, 0.19, 0.37, 0.55, and 0.73. The unknown gives 0.46. Which concentration is the most appropriate estimate by linear interpolation?

49 A standard curve has slope 0.015 absorbance units per and an intercept of 0.025. Replicate sample readings are 0.410, 0.414, and 0.620. Which statistical decision is most appropriate before calculating antioxidant activity?

50 A result is reported as 12.6 mg Trolox equivalents per gram using a 0.200 g sample extracted to 10.0 mL. If the analyst accidentally records the sample mass as 0.0200 g during back-calculation, how will the reported value change?

51 A TLC plate has a solvent-front distance of 8.0 cm. Compound X travels 2.4 cm and compound Y travels 5.6 cm. Which conclusion is justified?

52 On a silica TLC plate developed with a relatively nonpolar solvent, an antioxidant spot remains near the origin while a second spot travels close to the solvent front. Which interpretation is most likely?

53 A sample spot is applied below the solvent level in the developing chamber. What chromatographic artifact is most expected?

54 A TLC plate develops with a curved solvent front, and compounds near the edges show different values from identical compounds in the center. Which procedural change best addresses the problem?

55 A sample produces one visible spot in solvent system A but three well-separated spots in solvent system B. What is the strongest conclusion?

56 The solvent front reaches the top edge of a TLC plate before the plate is removed. Why are the resulting values less reliable?

57 A compound gives in a solvent system. Which adjustment is most likely to improve separation from the solvent front?

58 A TLC spot appears as a broad streak rather than a compact band. Which combination of causes is most plausible?

59 Two antioxidant standards have values of 0.42 and 0.44, while an unknown gives one spot at 0.43 under the same conditions. What is the strongest defensible claim?

60 A TLC plate is viewed under UV light, but the antioxidant compound is not visible. The solvent front and a reference standard are visible. Which explanation is most likely?