Hi there, I'm Jamie Dyer. If I want to measure my height, I'll use a measuring stick. But this cool device, it measures things using a light. It's called a spectrophotometer. It measures the intensity of light of different wavelengths in the visible spectrum. Spectrophotometers can be used to do lots of different things. Let me give you one example. In this 2017 paper, scientists characterize the strain of bacteria that can do bio-remediation, which is when living organisms are used to break down or remove toxic chemicals in the environment. It's a way to use living things to clean up pollution. In this paper, the scientists identified a type of bacteria that can do bio-remediation of a toxic weed-killing uricide called quincloric. It's used to kill weeds in rice fields and lawn, but it's toxic to many types of crops, and it's slow to break down in the environment. The scientists isolated a strain of bacteria from a field that was contaminated with high levels of quincloric, but they needed to show that as the bacteria grow, the bacteria actually degrade the toxic chemical. To do that, the scientists showed that in the lab, over several days, the bacteria grew in number, and higher numbers of bacteria correlated with higher degradation of quincloric. How they measured bacterial growth was with the spectrophotometer. I have here two glasses of coolade. One of them is more concentrated than the other. Can you tell which one? This one. It's darker. What you just did with your eyes is essentially what the spectrophotometer does. When you looked at the glasses, the coolade particles in the darker glass absorb more light. That's why it looked darker. Spectrophotometers do a similar thing. They shine light through a sample and measure how much light goes through. When a sample has more light absorbing particles, less light goes through the sample, which the spectrophotometer reads out as absorbance. But there's one thing that makes the spectrophotometer a little more complicated than just you looking through a glass of coolade. Spectrophotometers don't use plain old light. They use specific wavelengths of light. White light is composed of all the colors of the rainbow, which can be separated by a prism. Each color is a specific wavelength of light. By using specific wavelengths of light, the spectrophotometer is more accurate when measuring the absorbance of a sample. The scientists were able to measure the growth of the bacteria using a spectrophotometer, because more bacteria cells in the liquid meant higher absorbance. So using the spectrophotometer with a quick and easy way to measure how many bacteria were in the culture. You need to use a specific wavelength of light that depends on what your sample is. Make sure the spectrophotometer is set to that wavelength. Feeling to set the spectrophotometer to the correct wavelength will give you an absorbance value that is totally meaningless. Uh, absorbance is never negative. If you get a negative number, something went wrong, so you should start over. Almost everything absorbs a little bit of light, so if what you want to measure is the absorbance of bacteria, then you don't want to include in your measurement the absorbance of the media that are growing it. To avoid doing that, before you measure your sample, you must set the blank. Your blank is a Q-Vet, but the same kind of liquid that your sample is in, but without the sample in it. If you forget to set the blank, or if your blank is a different kind of liquid than what your sample is in, you will get the absorbance value that's totally meaningless. Make sure you set the blank using the same kind of liquid that the sample is in. So let's say you have multiple samples, or in the same kind of liquid that you're measuring with the same wavelength, you only have to set the blank once in the beginning. Measureing the samples pretty easy, piped it into the Q-Vet, insert the Q-Vet into the spectrophotometer, and close the lid. Note that the Q-Vet is designed for light waves to pass through it in a specific direction. Make sure you orient it in the spectrophotometer so that the light beam goes through it the right way. Don't touch the part of the Q-Vet that the light goes through, because your fingerprints can interfere with the absorbance reading. In fact, it's good practice to wipe the Q-Vet, just to make sure you remove any potential fingerprints or dust before putting it into the spectrophotometer. On many spectrophotometers, once you put the sample in, it will automatically display the absorbance, without even needing to push a button. That's it! Now you know how to use a spectrophotometer, and maybe one day you'll be the one using a spectrophotometer to do research on bioremediation or something else really cool.