Two student teams carry out UV-Vis absorption measurements of caffeine in a samp
ID: 901493 • Letter: T
Question
Two student teams carry out UV-Vis absorption measurements of caffeine in a sample of 'energy drink' such that Team 1 did the measurements at 274 nm, whereas Team 2 was using the 284 nm wavelength. The optical absorbance values and related measurement errors (st.dev.) for the same examined sample were 0.422 plusminus 0.003 (n = 5) and 0.188 plusminus 0.005 (n= 6) for Team 1 and Team 2, respectively. Calibration data obtained by the teams for the same series of standard solutions are shown below: What is the caffeine concentration in the examined sample as measured by Team 1 and Team 2? Which student team (Team 1 using lambda_abs=274 nm or Team 2 using lambda_abs=284 nm) could carry out the caffeine analysis with a higher sensitivity? Explain. Which student team obtained a lower limit of detection? Show your calculations. Did the results obtained by the two teams show a statistically significant difference at the 95% confidence level?Explanation / Answer
a.
Team 1 result:
A = 0.04708 C + 0.09036
0.422 = 0.04708 C + 0.09036
C = 7.044 ppm
Relative error was 100x 0.003/0.422 = 0.7%
(0.7/100)x7.044 ppm = 0.050
Report the result as
7.044 ppm s= 0.050
Team B result:
A = 0.02257 C + 0.02936
0.188 =0.02257 C + 0.02936
C = 7.028
Relative error was 100x 0.005/0.188 = 2.7%
(2.7/100)x7.028 ppm = 0.19
Report the result as
7.03 ppm s= 0.19
b. Team 1 . At 274 nm the coefficient of molar absorptivity is higher.
c.
Team 1
Looking to the calibration graph I will assume that the concentration error 0.003 is rather constant.
The detection limit is 0.009 ppm (3 sigma rule).
Team 2
Looking to the calibration graph I will assume that the concentration error 0.005 is rather constant.
The detection limit is 0.015 ppm (3 sigma rule).
Answer =Team 1
d.
The two results don’t show a significant difference even at 68 % confidence level.
The interval 7.03 ± 0.19 ppm includes 7.044
and the interval 7.044 ± 0.050 ppm includes 7.03
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