1. Why Beer's law is expressed in terms of absorbance instead of transmittance?
2. What is the significance of spectral scanning?
3. What is the importance of the baseline?
1. Because it's a log relationship. A = log 1/T
2. I don't know unless it's to see where the material absorbs most and least and which absorbance peak should be used.
3. Don't you need to know where the absorbance starts (how much) as well as where it ends (how much).?
what is apectral scanning? I really don't have an idea on this. As for the baseline, we indicated a range of 350 nm to 600 nm in the spectrophotometer and found that lambda max is at 510.4 nm. What then is the significance of the baseline?
You don't give much information for a detailed analysis; however, when a sample is scanned, typiclly the spectrophotomter is set at say 400 nm, the scan is turned on and the instrument measures the absorbance and a pen marks the absorbance on the paper (usually a preprinted sheet of paper) as the unit moves from 400 nm to say 700 nm. (I see you started at 350 nm and not 400 nm but its the same thing. Actually, you may start anywhere and end anywhere as long as the interest is within the range you scan.) Thus a graph is presented to you that shows what the absorbance is at all wavelengths between 400 and 700 nm. That allows you to know where the material absorbs and you can pick out the wavelength(s) at which the sample has maximum absorbance as well as those wavelengths where the absorbance is a minimum. When you are ready to do quantitative analytical work with a spectrophotomter you will need to know what the absorbance is at the wavelength chosen for the analysis. Typically, again, the wavelength chosen is lambda max, in yuor case at 510.4 nm. But what is the absorbance just before and just after lambda max? To give an example, suppose the A at lamda max is 0.4 but the A just before and just after the peak at lamda max is 0.1. Therefore, the A due to the sample is only 0.3 (0.4-0.1=0.3) for the sample was absorbing at 0.1 BEFORE and AFTER the peak. The increase is what you want and not the absolute absorbance. Actually, the absorbance just before and just after is not always the points to pick AND those points are not always the absorbance as I have indicated in this simple example (also I'm trying to describe a graph in writing instead of being able to draw and show you). You will learn how to draw a baseline and determine the absorbance of the baseline at lambda max in order to be able to subtract that from the absorbance at lambda max. I hope this addresses your concerns. If haven't done one of these in the lab most of this will be shown to you when you do the actual experiment.
By the way, you MAY subract absorbance from absorbance. Some preprinted graph papers as well as some spectrophotometers are calibrated in percent transmittance and you may NOT subtract percent T values. They must first be converted to A valures through log 1/T, then may be subtracted.
I think I have answered this elsewhere.
1. Beer's law, also known as the Beer-Lambert law, relates the absorbance of a sample to its concentration. Absorbance is a measure of how much light is absorbed by a substance, while transmittance is a measure of how much light passes through a substance.
The reason Beer's law is expressed in terms of absorbance instead of transmittance is that absorbance values are more directly proportional to concentration. According to Beer's law, absorbance is directly proportional to the concentration of the sample, whereas transmittance is inversely proportional to the concentration. This means that as the concentration of a sample increases, the amount of light transmitted decreases, making transmittance values less practical for measuring concentration changes.
In addition, absorbance values are easier to work with mathematically because they follow a linear relationship with concentration. By measuring the absorbance of a sample at a specific wavelength, it is possible to determine the concentration of the substance in the sample using Beer's law.
2. Spectral scanning refers to the process of measuring the absorbance or transmittance of a substance across a range of wavelengths. It involves collecting a spectrum of data, which provides information about the absorption characteristics of a substance at different wavelengths.
The significance of spectral scanning lies in its ability to provide detailed information about the interaction between light and matter. By analyzing the absorption spectrum, scientists can identify specific wavelengths where a substance absorbs light most strongly. This information can be used to determine the chemical composition of a sample, detect impurities, or study the electronic structure of molecules.
Spectral scanning is commonly used in various fields, such as chemistry, biochemistry, pharmacology, and environmental science. It allows researchers to understand the behavior of substances in terms of their interaction with light, providing valuable insights for quantitative analysis and characterization.
3. The baseline is an essential component in spectroscopy, particularly in measuring absorbance or transmittance. It refers to the reference or starting point from which the measurement is made.
The importance of the baseline lies in its ability to provide a reference for comparison. In spectroscopy, the baseline is typically established by measuring the absorbance or transmittance of a blank or reference sample that does not contain the analyte of interest. This baseline measurement acts as a "zero point" and provides a relative measurement scale.
The baseline is crucial because it allows for accurate determination of the absorption or transmission of light by the analyte. By subtracting the baseline value from the measured absorbance or transmittance of the sample, the true absorption or transmission caused by the analyte can be obtained. This helps to eliminate any contributions from other factors, such as scattering or impurities, which may interfere with the analysis.
Furthermore, the baseline can also help identify and correct for instrumental drift or background noise. By monitoring the stability of the baseline, any variations or fluctuations can be accounted for, ensuring accurate and reliable measurements.