Hey guys! This installment of #Macademics is going to focus on ionization. I thought it was appropriate since I am now in the mass spec world and my project focuses on creating a new ionization technique. So here we go…
First things first, I’ll be talking about protonated ions, or ions that have an extra hydrogen(s) in their structure. This is called positive ion mode. There is a negative ion mode, where the analytes have negative charges, but they abide by different mechanisms for ionization and I won’t go there. But in general, there are two types of ionization, hard and soft ionization. Hard ionization methods fragment every possible piece of the molecule of interest, while soft ionization methods are at a lower energy and fragment in baby steps, if you will. In my research, we take advantage of soft ionization methods so we can understand the connectivity of our analytes and it helps for identification purposes since analytes have characteristic fragment ions. A characteristic fragment ion is an ion that always shows up in your spectrum and it inherent to the analyte you’re studying. Below is some of the characteristic fragments for methamphetamine.
But anyway, back to soft ionization. There are tons of methods out there that ionize analytes in different ways – there’s no one way to charge a species. I’ll give you two examples.
Atmospheric Pressure Chemical Ionization (APCI) uses a similar setup to ESI (see previous ESI post here) but instead of applying the voltage to the actual solvent, the voltage is applied to a needle at atmospheric pressure. At the tip of this needle, there is a discharge that creates ions. A flow of liquid combined with a heated gas volatilizes (converts liquid to gas phase) the sample which gets injected into the discharge. The reaction of the sample spray and the discharge ions makes positively charged analyte ions that we use for analysis.
Matrix Assisted Laser Desorption Ionization (MALDI) is a common method of ionization via a laser, which makes it perfect for imaging applications. The sample is mixed with a matrix that can absorb the radiation produced by the laser beam. These matrices characteristically contain a benzene ring since they are ideal for absorbing light. As the matrix is absorbing the light, it transfers a proton to the sample, creating the ions we need for analysis, and those ions travel through the instrument for detection.
I hope you guys learned something from this month’s installment of #Macademics!
See you next week.