Why are fluorescence and phosphorescent spectra measured at a 90-degree angle to the source? (A) Because the sample cell is darkened on two adjacent sides. (B) Because the monochromator directs the light at a 90-degree angle. (C) To make the overall instrument smaller. (D) To ensure that incident, source, photons are not observed. Or (E) because the process of fluorescence and phosphorescence are too intense to observe directly.
To begin with, let’s look at a diagram of the inside of the spectrometer. First, we have the light source. This could be a laser as in an arc lamp or a mercury-vapor lamp, for example. The light then passes through a filter or monochromator. The monochromator might, for example, be a diffraction grating, something which is adjustable and will allow transmission of a specific wavelength of light. This light will then excite the sample, and fluorescence or phosphorescence will occur. The light from these processes is emitted in all directions. However, the detector is placed at a 90-degree angle to the source lamp. Any light traveling towards the detector will pass through another filter or monochromator.
The question we’re being asked is why is the detector placed at a 90-degree angle to the source lamp. Let’s look at each potential answer in turn. First, (A) because the sample cell is darkened on two adjacent sides. While some cuvettes or sample cells do have darkened adjacent sides, this is not the reason that the spectrometer is set up in this fashion. You could argue that you could simply buy cells which did not have adjacent sides darkened. However, if you did that and move the detector in line with the light source, you wouldn’t get good spectra. So let’s rule out (A) and move on to (B).
Answer (B) says that the reason we have this 90-degree angle between the source and detector is because the monochromator directs light at a 90-degree angle. As mentioned before, the monochromator is usually a diffraction grating. This simply allows the adjustable wavelength selection of the light passing through. In this example setup, we have two monochromators, one between the light source and the sample and one between the sample and the detector. Because these are both adjustable, it means that we can determine which wavelengths of light are used to excite the sample and also which wavelengths we’re measuring at the detector. The monochromator doesn’t change the angle of the light, so we can rule out (B) as the answer.
Answer (C) says that this setup makes the overall instrument smaller. If we imagine looking at the spectrometer from the top-down, we can see that bending the beam at 90 degrees really makes the spectrometer look a bit more chunky. This shaded area shows wasted space. It would be much easier and much simpler to build the machine in a linear arrangement, so we can rule out (C).
Now we come to (D) to ensure that incident, source, photons are not observed. In order to answer this part, we should consider fluorescence and phosphorescence as processes. It’s important to remember that the processes of fluorescence and phosphorescence are not necessarily really intense. As radiation is absorbed by molecules in the sample, not all of that radiation is going to cause fluorescence or phosphorescence. Some of that energy may be converted to other forms, such as vibrational or rotational changes. This means that the detector may only be able to see a small amount of fluorescence or phosphorescence at any one time.
Imagine looking directly into a car’s headlights and trying to pick out the light produced by a small pen torch. This is essentially what the detector would be trying to achieve if it were placed in a linear arrangement with the light source. All of the light that’s transmitted through the sample would be picked up by the detector. But then, it would also be trying to pick up the very small amount of fluorescence or phosphorescence. This makes it very difficult.
However, by arranging the detector at 90 degrees to the sample, this reduces the amount of light from the light source which reaches the detector. This makes it much easier to work out which photons come from fluorescence or phosphorescence. This means that answer (D) is correct. We’ve already ruled out answer (E) by discussing how fluorescence and phosphorescence are not necessarily very intense. So this can be ruled out.
So the reason that fluorescence and phosphorescence spectra are measured at a right angle to the source light is to ensure that incident or source photons are not observed, making it easier to observe fluorescence or phosphorescence.