Mass Spectrometry for Dumbies

Mass spectrometry is a bemusing pair of words to the common eye. It is so specialized and niche that even chemists and biologists only get a paragraphs worth of explanation in a textbook they read in undergrad. I was fortunate enough to be mentored for a summer by an intelligent gentleman with a PhD in the subject. While I will never come close to his vast knowledge of the specifics, I have a decent enough understanding to realize what an amazing technique it is- and how it is a shame that so many people have never even heard of it, or if they do, know little more than its definition. So I’ll start with that.
According to MedicineNet.com, mass spectrometry is, “A process used to identify chemicals in a substance by their mass and charge. Mass spectrometers are instruments that measure mass and charge of molecules. A mass spectrometer also can determine how much of a compound is present in a mixture.” Honestly, I couldn’t have said it better myself. But I can say it simpler: mass spectrometry takes a sample, ionizes it, then sorts those ions by their mass to charge ratio. The applications of this are practically endless, as it can be used to identify unknown substances, and range from research to clinical use and even forensics! Its handy skill of being able to identify bacteria in a blood sample is much faster and more accurate than traditional means and is fast growing in the clinical diagnostics field. Scientists utilize mass spectrometry in everything from medical uses such as proteomics (the study of proteomes, proteins, and peptides), cancer and disease screening, drug discovery and absorption, and measuring bacteria antibiotics resistance to testing the quality of drinking water for a city to analyzing trace evidence at a crime scene!
Of course nothing in science is ever as simple as it seems. Mass spectrometry is a broad term for the technique; there are many different ways of accomplishing this process, all customized for their specific purposes. All mass spectrometers are composed of at least 3 things: an ion source, a mass analyzer, and a detector. But within these 3 categories, the types of variations are numerous. For starters- the ion source. For all their complicated powers, mass spectrometers are blind if the molecules have not been charged to become ions, and there are many different ways to do this. For example, some mass spectrometers utilize “soft” ionization and others utilize “hard.” Soft leaves the molecules themselves in tact, instead of fragmenting them like hard would. Soft ionization is perfect when analyzing biological substances that contain larger molecular masses, like analyzing the unique protein makeup of a bacteria to identify what it is.

The way the sample is ionized can be done in a number of ways, but I’ll mention two particular approaches that I have actually had experience with. The first is the Matrix Assisted Laser Desorption/Ionization method. For obvious reasons, it is commonly referred to as MALDI. As the name implies, the ionization is accomplished by hitting the sample with a laser beam repeatedly. It is a soft ionization technique used for larger molecules such as proteins. A liquid matrix is laid over the sample that crystallizes upon drying, so the sample itself is not directly struck, preserving its integrity and amplifying the results. Electrospray Ionization (ESI) works by introducing a voltage to a liquid sample to ionize them into an aerosol, and the components of that are analyzed. Both are soft ionization techniques, with some key differences. MALDI can be ran in minutes, while ESI can take up to days. However, ESI can produce multiple-charged ions, while MALDI only does +1, which can accommodate many magnitudes smaller and larger of ions as well as better handle any fragmentation. Additionally, before samples can go through ESI, many of them have to go through an extra step, the chemistry technique of liquid chromatography, which separates components in a mixture using pressurized solvents and an absorbent material. Basically, MALDI is better for larger scale, like identifying what bacteria is present, and gives quicker results, while ESI is better for studying in depth the amount and kinds of each molecule, such as protein, present, but you’ll have to be more patient.
After ionization, no matter how it is done, the mass of the ions have to be analyzed. A technique usually paired with the MALDI is Time-of-Flight (TOF). Here, an electrostatic field accelerates the ions through an area under vacuum and the period of time it takes for the ions to go through it is precisely measured and recorded. ESI is commonly paired with the quadrupol mass analyzer, which is composed of 4 cylindrical poles. The ions are separated by how stable their trajectories are in the electric fields applied to the rods. The electric field can be manipulated to where only ions of a specific mass to charge ratio will have a stable trajectory, or can scan for a range by using a variety of voltages. By combining different ion sources and mass analyzers there are many useful variations to mass spectrometry.
To learn more about the fascinating topic of mass spec, I would highly recommend the American Society of Mass Spectrometry (ASMS) as a starting place. I hope this post was informative and not too terribly boring, as I continue with my passion of educating others and myself about the biomedical and biotechnology industry.
