Back To CourseAnalytical Chemistry: Help & Review
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Nicky has taught a variety of chemistry courses at college level. Nicky has a PhD in Physical Chemistry.
Imagine you are standing in a concert hall. How do you detect the music? You might feel the vibrations from the instruments, but you are most likely going to hear the music with your ears. Ears are detectors of sound. Your ears pick up the sound waves and your brain makes sense of the information you receive.
Unfortunately, you are a long way back in the hall so you can barely detect the music. What could you do to hear the music better? You could move closer or turn your head towards the music; this will help a little. Good! Amplification of the sound would also help your ears detect the music.
Now the people around you are starting to talk. This is interfering with you hearing the music. It is getting in the way; the noise is bothering you. Again, you would try and move away from the noise or at least filter it out so it moves into the background. The most important thing you want to do is to improve how well your ears can detect the music. And this is what we are going to learn about in this lesson. Not how to improve your hearing, but how to improve the detection of signals in spectroscopic techniques.
You may recall that spectroscopy is the study of how light interacts with matter. It allows us to figure out the structure of unknown materials and is commonly used in chemistry labs around the world. For us to do this, we must be able to detect light.
A spectroscopic detector is a device that produces an electric signal when it is struck by photons. And a photon is a particle of light that contains a finite amount of energy, depending on its frequency. This ability to convert photons into a measurable signal is very important. Just how photons are converted varies, but detectors can be divided into two main types. The first type exploits the photoelectric effect and the second type is based on the properties of semiconductors.
The photoelectric effect can be simply described as the observation that many metals emit electrons when light of high energy hits them. These ejected electrons, or photoelectrons, are collected and measured as an electric current.
A semiconductor, simply put, is a material that can conduct electricity in a controlled way. Detectors based on semiconductors do not eject photoelectrons from the surface of a metal. Instead, absorbed photons excite and promote electrons from the valence band into the conduction band. This excitation results in holes left behind by the electrons. This allows current to flow through the material, which is collected and measured.
Now a perfect detector would be able to convert the photons of energy into an electrical signal with 100% efficiency. In other words, the size of the current produced is directly proportional to the number of photons striking the detector. Unfortunately, this is not the case. Limitations include the range of detection, detector response time, and noise. Just like us listening to music, outside noise is a real problem. All detectors generate small signals that do not correspond to absorption of a photon. Noise does not contain any useful information and just like our talking concert mates, it can seriously get in the way of what we want to detect. It is important to know that detectors can be modified to reduce noise to a minimum.
Now that we understand basically how detectors work, let's take a look at two ways detectors can be adapted to improve performance.
The first type of detector we will consider is the photocathode detector. This uses the photoelectric effect to generate a signal and works by accelerating ejected photons towards an anode. Photocathodes are effective detectors but their sensitivity can be limited. To increase sensitivity, a device called a photomultiplier tube (or a PMT) is used.
The key advantage of a photomultiplier tube is that photoelectrons are amplified using a chain of dynodes. A dynode can be thought of as a metal surface that easily lets go of several electrons each time it is struck by an accelerated electron. Accelerated electrons strike a positive dynode, which knocks off a number of electrons from the dynode. These new electrons are accelerated towards the next dynode and even more electrons are knocked off. This process is repeated several times and more than 1,000,000 electrons are collected for each electron striking the first surface.
You can think of this like a snowball rolling down the mountain. Here we have a small snowball at the top of the mountain and then it begins to roll downhill. As it goes, it gains momentum and mass until you have a massive snowball at the bottom!
PMTs dramatically increase sensitivity without increasing unwanted noise. Extremely low-light intensities can be detected and measured. PMTs work well with a wide variety of spectrometers. Interestingly, as sensitive as the photomultiplier is, the human eye is 10 times more sensitive!
Detectors containing PMTs slowly scan through a spectrum one wavelength at a time. But in a photodiode array, all wavelengths can be rapidly measured at once. A photodiode array is found in detectors that use semiconductors to collect a signal. Now, I'm not going to go into too much detail here because semiconductors can get very complicated. But we do need to know the basics to see how these arrays work.
A semiconductor consists of two regions: n-type, which has excess negatively-charged electrons, and p-type, which has excess positively-charged holes. A diode is the junction between the two regions called the p-n junction. Electrons near the diode drift towards the p-type region and holes drift the other way. This drift results in a depletion zone that has an electric field, allowing current to flow in one direction only.
It is possible to manipulate the drift of the electrons and holes by applying an external potential, or bias, to the diode. We can also control the size and direction of the potential barrier. If something called a reverse bias is applied, the holes are drawn towards the negative charge and the electrons are drawn towards the positive charge. This has the effect of widening the depletion zone, increasing the potential barrier, and prevents currents from flowing. Instead, the charge is stored on either side of the junction.
We can use this to our advantage. When light hits the semiconductor, free electrons and holes are made and they migrate to their opposite charge. As these electron-hole pairs are separated and accelerated in opposite directions by the electric field, a current is produced; it is this current we measure.
Multiple photodiode detectors can be arranged in an array, known as a photodiode array (or PDA). These arrays work in parallel, allowing for the collection of much more data with the same response time. PDAs can be tuned to different wavelengths and are useful in a wide variety of applications. One big disadvantage of PDAs is that they are not so effective in detecting very small numbers of photons compared to photomultiplier tubes.
In this lesson, you have learned that in spectroscopy, a detector produces electric signals when light shines on it.
Detectors can be divided into two main types. One uses the photoelectric effect, which is the observation that many metals emit electrons when light of high energy hits them. And the second type uses a semiconductor, a material that can conduct electricity in a controlled way.
It is important to maximize the signal detected and we discussed two different ways we can do that. The first is to use a photomultiplier tube, where photoelectrons are amplified using a chain of dynodes. PMTs dramatically increase sensitivity without increasing unwanted noise. They work well in low-light intensities and have a wide variety of uses.
The second way discussed was the photodiode array, found in semiconductor spectrometers. PDAs measure all wavelengths at once, giving us faster acquisition times and a good signal-to-noise ratio. One disadvantage is they are not so effective in detecting very small numbers of photons compared to photomultiplier tubes.
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Back To CourseAnalytical Chemistry: Help & Review
8 chapters | 92 lessons