Understanding Analytical Chemistry

[clıppy]

I am hopelessly lost in a sea of all the different types of analytical chemistry, can someone please explain to me the different types there are and how we use them?

Huge thanks in advance

[kaiipoo_]

ill attempt to help ya, pull it all out of my head haha ! but shits official, i think.

Volumetric Analysis:
eg. Titration; The purpose of a titration is to determine the concentration of a certain unknown compound in solution using a solution of known volume (known as the standard), as well as stoichiometry. Back titrations are done in a similar way, however instead of 2 you have 3 steps.

Gravimetric Analysis:
As the name implies, Gravimetric Analysis has to do with masses. One example of Gravimetric Analysis is the heating of a hydrated salt to constant mass. In doing this, the chemist basically evaporises all the water in the sample to obtain a 'pure' sample. The reason i place pure in inverted commas is that there is a large margin of error when undergoing this type of Analysis.

Chromatography:
Chromatography is in a nutshell, used to seperate mixtures of compounds into their components, based on their molecular polarity and other factors. The components in a mixture usually dont behave the same, thus they 'adsorb' and 'desorb' onto the stationary phase based on how strong the attractions are. Adsorption is the formation of bonds with the stationary phase, which i will explain about later.

In chromatography, you have 2 phases. A mobile phase, and a stationary phase. The Mobile Phase in Thin-Layer Chromatography, High-Performance Liquid Chromatography and Paper Chromatography is generally a liquid. Whereas, the mobile phase in Gas Chromatography is a gas.

Spectroscopy:

Spectroscopy is what you will find most difficult in my opinion, as there are so many forms and they are all used for different purposes.

Atomic Absorption Spectroscopy (AAS): Is used with metallic ions, such as copper, sodium etc. Basically what happens is you have a hollow cathode lamp, in which the filament of the lamp is composed of the element of interest. Electrical Energy is supplied to this lamp, causing it to emit light of a SPECIFIC frequency, which so happens to be the frequency the element of interest would absorb. This is ultimately used to find concentration, and also just for general qualitative analysis.

Atomic Emission Spectroscopy : Light of a specific wavelength is chosen, and is shot at a sample, in which the electrons absorb the energy and jump to higher energy lvls, something you would've learnt in 1/2 chem. The emitted light is shot at a detector, which measures its emission. This is used for qualitative and quantitative analysis.

UV-VIS: As we know, most elements do not absorb in the visible region and thus AES and AAS is unsuitable. We then turn to UV Vis, as this type of electromagnetic radiation has the frequency and wavelengths we want. Works in a similar way to AES and AAS, the electrons absorb energy, jump up and emit energy in THE UV REGION THUS WE CANNOT SEE IT. thats what the detectors for

Infrared Spectroscopy: Infrared Spectroscopy is used to determine the FUNCTIONAL GROUPS present in a sample, and thus can give us information about its structure. Infrared Spectroscopy plays on not the electrons, as infrared has insufficient energy to excite them. But rather, it plays on the covalent bonds between molecules, and their characteristic scissoring, bending etc.

NMR: Nuclear Magnetic Resonance uses energy in the RADIO REGION = the least energetic. NMR plays on the characteristic spins of hydrogen nuclei, and 13-carbon nuclei. There are the main isotopes used, though isotopes of fluorine and nitrogen can also be used. The reason why they are used is because there is no neutrality in the nucleus, thus the nucleus undergoes spin. The main thing you need to know is that Nuclear magnetic resonance is used to determine neighbouring atoms, and hydrogen/13-c environments. Thus, the chemist can determine the shape of the overall molecule n shit.


didnt go into too much detail, so just hit me up if you're unclear.

[clıppy]

Thanks kai, that really helps clear up a lot of the confusion.
If we were talking about colorimetry would that fall under the same category as AAS/UV-Vis because we still choose specific wavelengths (colours) and shoot them at a sample?

[kaiipoo_]

np =]

Well, the techniques are similar. However, i wouldn't 'categorise' them. They all differ in their own ways and the information they give about a certain sample, and thus they all have different names. I'll explain what they are for

The process of Colorimetry is most similar to AAS/AES. Basically what happens is, as the electrons of the atoms in the compound absorb a certain quantum or 'packet' of energy, they become excited and jump to various energy levels. The extent or level of the jump is ultimately based on the amount of energy provided, thus in an emission spectrum, an element, lets say Calcium, has more than 1 emission line. Anyway, as the electrons fall back down to their ground state, they emit energy proportional to that absorbed, so we can think of it and energy in= energy out. In Colorimetry/AAS/AES, this energy has the frequency and wavelength characteristic of 'VISIBLE LIGHT'. The amount of energy emitted is put through a detector, and this detector works to tell you the absorbance reading. After attaining the absorbance reading, what you would do is create a series of standards of KNOWN concentration, and put them through the machine. After getting the absorbance reading for all of those, you would plot them on an absorbance vs concentration graph, and this is known as a calibration curve. Finally, fit a regression line and read it straight off, ez.