Mass Spectrometry
Mass spectrometry is a powerful analytical technique that is widely used in clinical diagnostics and bioinformatics for the identification and quantification of biomolecules, including proteins, peptides, and small molecules.
In this guide, we will cover some basic technical aspects of mass spectrometry, including its principles of operation, common instrumentation and experimental approaches, and applications in clinical diagnostics and bioinformatics.
Principles of Mass Spectrometry
Mass spectrometry is based on the principle that charged particles (ions) can be separated based on their mass-to-charge ratio (m/z).
In a mass spectrometer:
A sample is first introduced into the instrument, where it is ionized and vaporized into a gas-phase plasma.
The resulting ions are then accelerated and passed through a mass analyzer, which separates the ions based on their m/z ratio.
The separated ions are then detected and their relative abundance is measured, providing a mass spectrum that reflects the composition of the sample.
Types of Mass Analyzers
There are several different types of mass analyzers that can be used in mass spectrometry, each with its own unique set of advantages and disadvantages in terms of speed, sensitivity, and resolution. The most common types of mass analyzers include:
Quadrupole mass analyzers
- Quadrupole mass analyzers are simple, low-cost mass analyzers that use four parallel rods to filter ions based on their m/z ratio. They are relatively fast and efficient, but have a limited mass range and are not well suited for high-resolution analysis.
Ion trap mass analyzers
- Ion trap mass analyzers use an electric field to trap ions in a three-dimensional well, where they can be selectively ejected based on their m/z ratio. They are also relatively fast and efficient, and can be used for high-resolution analysis, but are limited in their mass range and sensitivity.
Time-of-flight mass analyzers
- Time-of-flight mass analyzers use an electric field to accelerate ions, which are then detected based on the time it takes for them to travel a fixed distance. They have a large mass range and can be used for high-resolution analysis, but are relatively slow and have limited sensitivity.
Orbitrap mass analyzers
Orbitrap mass analyzers use a combination of an electric field and a magnetic field to trap ions in an orbit around a central electrode. The ions are then detected based on the frequency of their oscillations, which is proportional to their m/z ratio.
Orbitrap mass analyzers are capable of high-resolution, high-mass accuracy analysis, but are relatively slow and expensive.
Experimental Approaches
There are many different types of mass spectrometers available on the market, ranging from small, benchtop instruments to large, floor-standing systems.
The optimal choice of instrument depends on the specific needs of the application, including:
Type of sample
Mass range of interest
Required sensitivity and resolution.
There are also many different experimental approaches that can be used in mass spectrometry, depending on the nature of the sample and the specific questions being asked.
Some common approaches include:
Direct infusion
- In direct infusion, the sample is introduced directly into the mass spectrometer using a syringe or a flow-through device. This is a fast and simple method that is well suited for small molecules and high-throughput applications.
Chromatography
- In chromatography, the sample is separated using chromatographic techniques (e.g., liquid chromatography or gas chromatography) before being introduced into the mass spectrometer. This allows for the separation of complex mixtures and can improve the sensitivity and resolution of the analysis.
Tandem (AKA MS/MS)
In tandem mass spectrometry (AKA MS/MS), the sample is first separated by mass in a mass spectrometer, and then the resulting ions are further fragmented and analyzed by a second mass spectrometer. This allows for the identification of specific molecules within a sample and can be used to determine the sequence of amino acids in a protein or the structure of a small molecule.
One of the key advantages of tandem mass spectrometry is its ability to provide high-resolution, high-mass accuracy information about the structure and composition of molecules. It is particularly useful for the identification and characterization of complex mixtures, such as those found in biological samples.
MS/MS Types
There are several different types of tandem mass spectrometers available, each with its own unique set of advantages and disadvantages.
The most common types include:
TQMS
TQMS = Triple quadrupole mass spectrometers
- TQMS instruments use two quadrupole mass analyzers in tandem, allowing for the selective fragmentation and analysis of specific ions. They are widely used for the identification and quantification of small molecules, such as drugs and metabolites.
QTOF
QTOF = Quadrupole time-of-flight mass spectrometers
- QTOF instruments use a quadrupole mass analyzer for the initial separation of ions, followed by a time-of-flight mass analyzer for the fragmentation and analysis of the resulting ions. They are capable of high-resolution, high-mass accuracy analysis and are widely used for the identification and characterization of proteins and peptides.
FTICR
FTICR = Fourier transform ion cyclotron resonance mass spectrometers
- FTICR instruments use a combination of an electric field and a magnetic field to trap ions in an orbit around a central electrode. The ions are then detected based on the frequency of their oscillations, which is proportional to their mass-to-charge ratio. FTICR mass spectrometers are capable of extremely high-resolution, high-mass accuracy analysis and are widely used for the study of complex biomolecules.
Clinical Diagnostics
Mass spectrometry is widely used in clinical diagnostics for the identification and quantification of biomolecules in various types of samples, including blood, urine, and tissues.
Some common applications include:
Microbiology
- In microbiology, mass spectrometry is used to identify and quantify the presence of microorganisms in a sample, providing important information for the diagnosis and treatment of infections.
Metabolic profiling
- For metabolic profiling, mass spectrometry can be used to analyze the levels of various metabolites in a sample, providing insights into the health and function of an individual. This can be useful for the diagnosis of various diseases, including diabetes, cancer, and neurological disorders.
Toxicology
- In toxicology, mass spectrometry is commonly used to detect and quantify the presence of drugs and other toxins in a sample, providing important information for the treatment of patients who have been exposed to these substances.
Bioinformatics
In the field of bioinformatics, mass spectrometry is widely used for the identification and quantification of proteins and peptides in various types of samples, including cells, tissues, and body fluids. Some common applications include:
Proteomics
- In proteomics, mass spectrometry is also to used identify and quantify the levels of proteins in a sample, providing insights into the function and regulation of biological systems. This can be useful for the study of diseases, including cancer and neurological disorders, as well as for the development of new drugs and therapies.
Peptidomics
- In Peptidomics, mass spectrometry can be used to identify and quantify the levels of peptides in a sample, providing insights into the processing and turnover of proteins in the body. This can be useful for the study of various diseases, including diabetes and cardiovascular disease.
Overall, mass spectrometry is a powerful analytical technique that plays a crucial role in both clinical diagnostics and bioinformatics. Its ability to accurately identify and quantify biomolecules at very low levels makes it an invaluable tool for the study of biology and medicine.