Matthew T - achemchem



Matthew T. Reher

6 December 2011

Chem 221-802

Lab Partners: Shoa Xiong, Larry B and Caroline W

Cory Hawkins

Lab 9 Fluorimetric Determination of Quinine in Tonic Water

  Principles:

Much like in absorbance spectroscopy, a signal intensity is emitted when electron’s fall down energy levels. As seen in the picture below:

[pic]

1

Whereas in absorbance spectroscopy, the electrons start at the ground state, are brought to the excited state and finally emit a signal intensity, fluorescence starts at the excited state and then emits (actually slower than absorbance). However, a fluorophore (a carrier of fluorescence) will eventually no longer fluoresce because all of the electrons have returned to the ground state. Fluorescence signals have a longer wavelength than absorbance because of its decreased energy.

It is important to note that the energy levels, within the excited states of the fluorescence, are very close together. So close that they do not have time to fluoresce in the time that it goes from the higher energy levels to the lower energy levels. When this happens it is called internal conversion. However, when the electron relaxes to the ground state it has time to emit fluorescence because there is a relatively large gap between this excited state and now the ground the state.

Quinine is used as a malaria remedy but is also prevalent in tonic water for its bitter taste. It is also used as a standard for fluorescence ()

[pic][pic]



The important part of the quinine molecule is the aromatic rings that include the nitrogen atom. These electrons that are a part of the pi bond are the electrons that give off fluorescence.

Methods:

The concentration of a fluorophore is proportional to the signal intensity, therefore if we take a series of different concentrations and match them to their signal intensity we should be able to find the coefficient to make a ratio into an equation. This is the goal of the plot done in this experiment.

This experiment utilizes a spectrofluorimeter. This uses xenon as a fluorescence source. A schematic is shown below:

[pic]



The detector is at a 90 degree angle, which is a foundational element of spectrofluorimetry. This 90 degree angle is plausible because of the basic fact that the fluorophore fluoresce in all directions, 360 degrees. The great advantage so being at 90 degrees is that the detector picks up a negligible intensity from the UV lamp (which will continue to travel in a straight line).

The fluorescence from the fluorophore will be picked up by the detector, which will then be processed by a computer.

Calculations and Results:

C1V1=C2V2

2500ppb x 0mL/25mL=0ppb

2500ppb x 1mL/25mL=100ppb

2500ppb x 3mL/25mL=300ppb

2500ppb x 5mL/25mL=500ppb

2500ppb x 10mL/25mL=1000ppb

[pic]

LOD (limit of dection)= 3*standard deviation

3*1.049979592=3.14993877

Concentration=

Y=.2537x+99.516

X=(y-99.516)/.2537

X=(207.333333-99.516)/.2537=>424.979633ppb *(100mL/1mL)* (1microgram/1L)*(1mg/1000microgram)=42.498mg of Quinine/L

|Slope |0.2537 |

|Intercept |99.516 |

|Standard error of the estimate |9.034909619 |

|Lower limit of detection |3.14993877 |

|Concentration |42.498mg of Quinine/L |

Data Treatment:

|Ppb Quinine |Mean |Standard Deviation |%RSD |

|0 |104.752857 |1.049979592 |1.002339814 |

|100 |119.9175 |3.402688888 |2.837524871 |

|300 |166.956667 |1.149101098 |0.68826308 |

|500 |236.986667 |1.255082999 |0.529600679 |

|1000 |351.02 |1.415768343 |0.403329822 |

|35.50347864 |207.3333333 |1.670219546 |0.805572129 |

Discussion

According to “21 CFR 172.575 Quinine” states that tonic is not to exceed 83mg of Quinine/L, our end value ended up being 42.498mg of Quinine/L so this is consistent with this regulation. If 83 is the true value then the percent error would be 48.80%, which is very statistically poor. However, the precision of this experiment was excellent, as the highest percent relative standard deviation was 2.83%.

This indicates that the weakest link of this experiment was the accuracy because the precision was nearly immaculate, by default, not knowing what the true mean really is concludes that it is more likely that the accuracy was the weaker quality.

................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download