Modern Analytical Techniques I: Mass Spectrometry and IR Spectroscopy

Flashcards covering the fundamental concepts, terminology, and key data points for Mass Spectrometry and Infrared Spectroscopy as presented in the Modern Analytical Techniques I lecture.

Mass Spectroscopy

An analytical technique used to identify unknown compounds by bombarding a small sample with high energy electrons to cause electron loss and molecule identification.

Molecular ion ($M^+$)

A positively charged ion with one unpaired electron formed when a molecule loses one electron during bombardment in a mass spectrometer.

$[M+1]$ peak

A small peak in a mass spectrum due to the natural abundance of the isotope carbon-13 ($^{13}C$), which is a little over $1\%$, used to help determine the number of carbon atoms in a molecule.

$m/z$ ratio

The mass-to-charge ratio of ion fragments used to separate and detect them in mass spectrometry; it is often referred to as the $m/e$ ratio.

Base peak

The peak in a mass spectrum that corresponds to the most abundant ion.

Isotopes

Different atoms of the same element that contain the same number of protons and electrons but a different number of neutrons, resulting in different mass numbers.

Relative abundance

The proportion of one particular isotope in a mixture of isotopes found in nature, such as chlorine-35 being $75\%$ and chlorine-37 being $25\%$.

Fragmentation

The process where a molecular ion breaks down into smaller ions, molecules, and radicals, often used to determine the specific structure of an unknown compound.

Characteristic fragment at $m/z = 29$

A fragment corresponding to the ethyl group ion, $\text{C}_2\text{H}_5^+$.

Loss of small molecules peaks

Evidence in a mass spectrum showing the loss of specific neutral molecules like $\text{H}_2\text{O}$ ($m/z = 18$), $\text{CO}$ ($m/z = 28$), and $\text{CO}_2$ ($m/z = 44$).

Infrared (IR) spectroscopy

A technique used to identify compounds based on the vibration of atoms (stretching, bending, and twisting) when they absorb infrared radiation of specific frequencies.

Wavenumber

The reciprocal of the wavelength, measured in $cm^{-1}$, used to denote the frequency of IR radiation absorbed by specific chemical bonds.

Resonance frequency

The specific frequency at which molecules will vibrate to stimulate larger vibrations when irradiated during IR spectroscopy.

Carbonyl ($C=O$) absorbance

A strong, sharp absorbance peak typically found around $1710\,cm^{-1}$ in ketones, with ranges varying based on the functional group (e.g., $1740-1720\,cm^{-1}$ for aldehydes).

Alcohol ($O-H$) absorbance

A strong, broad absorbance peak found in the range of $3750-3200\,cm^{-1}$ due to the presence of hydrogen bonding.

Nitrile ($C \equiv N$) absorbance

A characteristic IR stretching absorption range found between $2260-2215\,cm^{-1}$.