# Tag Info

### Why does ethylamine have two infra-red spectra? Are both correct?

When looking at IR spectra of hydrogen-bonding groups, always check how IR spectra was recorded. It appears that NIST spectra is recorded using gas phase, while the first one used liquid film. In ...
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### Looking for a dye which emits around 680 nm

There are two dyes commonly used in biochemistry research with $\lambda_\mathrm{Ex}$ of around $\pu{650 nm}$. They are Alexa Fluor 647 from ThermoFisher and Cyanine5 (Cy5) from Lumiprobe. Two example ...
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### Why does tryptophan absorb UV light?

Any molecule can absorb UV light. What the question is probably going for is why those three molecules absorb at longer wavelengths than other amino acids. This has to do with the conjugated pi bonds ...
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### How does molecular structure contribute in the property of fluorescence?

Keep in mind a few things that must happen for an absorption process to result in fluorescence: (1) the initial transition is to an excited electronic state that observes certain rules regarding the ...
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### What is the purpose of UV-VIS Spectroscopy?

The broad absorption bands of the electronic transitions observed in UV/Vis spectroscopy/spectrophotometry, as well as the myriad types of electronic transitions that might lead to absorption at a ...
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### Bohr's model of atom

Since this is pretty well covered in countless number of books and other sources I will try to just emphasise the key points. The emission spectrum of atomic hydrogen was well known at that time, the ...
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### Why is absorbance the negative logarithm of transmittance?

Absorbance is useful because it is additive. That is, it's absorbance which is used in Beer's law: $$A = \epsilon \cdot c \cdot l$$ While you can certainly make a version of this law which uses ...
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### Reference for UV/Vis Spectroscopic Data request

To know the chemical name, molecular structure, or best: the unique registry number provided by the American Chemical Society (CAS-number) is beneficial in your search as trade names may vary ...
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### How to determine the iron content in spinach with a spectrometer?

Here's how I approached the problem. Plot the Data Below is the plot I quickly hacked together, and a fit of the form $g(x) = a\cdot x$. (Note that I do want the line to go through (0,0) because the ...
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### Absorption coefficient for liquid water

What the plot shows is the reciprocal path length needed for a given absorbance to apply, for example slightly less than 100 m in the visible to $10^{-8}$ m in the uv, as you point out, but this is ...
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### Why decarbonate soda for spectroscopy experiment?

The primary reason is that when soda warms up, the $\ce{CO2}$ would escape slowly as bubbles. These bubbles would significantly scatter light affecting the absorption spectra. Usually, the effect of ...
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### When making a calibration curve, do we have to use the wavelength at maximum absorbance?

No it doesn’t. Measurements can in principle be made at any wavelength. If there are two colored specimens in the same solution, it is actually better to measure at the wavelength where the ...
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### Why does ethylamine have two infra-red spectra? Are both correct?

Honestly I have doubt about your textbook version of IR spectrum. However, I can suggest by the experience I have gained that the shapes of the peaks depends on the method you have used to obtain the ...

### The reason for selection of wavelengths in the spectrophotometry of Quinoline Yellow SS

The experiment in your link pg 31 is "Path Length Dependence of Absorbance Values". As you stated, quinoline absorption spectrum has a dip near 337 nm. The reason for choosing the wavelength ...
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### Which is most reliable excitation energy: LUMO-HOMO energy or TD-DFT excitation energies?

When you can, use TDDFT, in part because of the oscillator strengths I like to point out to my group that optical excitations consist of two things consistently (e.g., in a UV/Vis spectra). There are ...
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### Absorption coefficient for liquid water

The plot is from Wikipedia, where important/helpful links to the source information can be found, and is worth a look. A similar and more detailed plot can be found on this page of Martin Chaplin's ...
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Beer's law relates transmitted intensity to concentration $[c]$ of the solution at each wavelength $\lambda$ as $$I_{trans_\lambda}=I_0e^{-\epsilon_\lambda [c]L}$$ where L is the sample path-...