Question

Questions 1-3 please. The correct answers are circled. Please just explain the theory proving why the answer is correct and the others are wrong.

Thanks!

1. Why the detector and the light source in a fluorescence spectrophotometer is perpendicular to each other (90°)? (a). to prevent power fluctuation (b). to eliminate fluorescence (c). to protect the detector from damaging. (d). to protect light source from damaging. (e to protect the detector from being interfered. 2. A hydrocarbon compound is to be analyzed using GC. Which one of the following detectors would give the best result? (a). UV-visible (b). Fluorescence (c). TCD ((d) FID e). Photomultiplier tube 3. In a lab, a mixture of Co2+ and Ni2+ was analyzed using Spectronic 20 spectrometer. This is based on which of the following principles (b). Co2+ and Ni2+ have a similar absorption spectrum a absorption is additive c). Co2+ and Ni2+ have similar chemical properties (d). They are non-volatile (e). None

Answer 1

1).In a Fluorescence spectrophotometer detector and light source is at 9 degree because although light source use in this process is a monochromatic source but no monochromatic source is a perfect source and it will transmit some stray light, that is, light with other wavelengths than the targeted. An ideal monochromator would only transmit light in the specified range and have a high wavelength-independent transmission. When measuring at a 90° angle, only the light scattered by the sample causes stray light. This results in a better signal-to-noise ratio, and lowers the detection limit by approximately a factor 10000,  when compared to the 180° geometry when put detector at front.

2). FID detector stands for Flame Ionization Detector. To analyze a hydrocarbon using Gas Chromatography FID detector is good.  In an FID the sample undergoes a combustion in a hydrogen/synthetic air fl ame. Ions and free electrons are formed in the fl ame. The charged particles produce a measurable current flow in the gap between two electrodes in the detector. The resulting current flow is of greater strength than the signal produced by the pure carrier gas and the fuel gas flame alone. This signal differential provides information about the sample. The current is proportional to the information which depends on the composition of the separated sample.

The Flame lonisation Detector Collector electrode Flame ignition Polarising voltage Hydrogen Air Column

3).This is one of the example when the presence of one species like Co⁺² in a sample does not influence the
measurement of another species like Ni⁺² in the same sample; i.e., they do not interfere. the absorption of light by the components of the sample solution is additive; that is, the total absorption of light at any given wavelength is just the sum of the absorbances of the two substances would show if measured individually under the same conditions. Spectronic 20 spectrometer is use to measures the absorption of visible light through a solution.