chemistry

Selected reaction monitoring

Selected reaction monitoring is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Selected reaction monitoring rather than just read about it. In short: Selected reaction monitoring (SRM), also called multiple reaction monitoring (MRM), is a method used in tandem mass spectrometry in which an ion of a particular mass is selected in the first stage of a tandem mass spectrometer and an ion product of a fragmentation reaction of the precursor ions is selected in the second mass spectrometer stage for detection. Variants A general case of SRM can be represented by A B C…

Key takeaways

Selected reaction monitoring belongs to chemistry; place it in that map before memorising details.
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Connect Selected reaction monitoring to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Selected reaction monitoring from memory before moving on to harder problems.

Reference excerpt

Selected reaction monitoring (SRM), also called multiple reaction monitoring (MRM), is a method used in tandem mass spectrometry in which an ion of a particular mass is selected in the first stage of a tandem mass spectrometer and an ion product of a fragmentation reaction of the precursor ions is selected in the second mass spectrometer stage for detection.

Variants A general case of SRM can be represented by

A B C D + → A B + C D + {\displaystyle ABCD^{+}\to AB+CD^{+}}

where the precursor ion ABCD+ is selected by the first stage of mass spectrometry (MS1), dissociates into molecule AB and product ion CD+, and the latter is selected by the second stage of mass spectrometry (MS2) and detected. The precursor and product ion pair is called a SRM "transition". Consecutive reaction monitoring (CRM) is the serial application of three or more stages of mass spectrometry to SRM, represented in a simple case by

A B C D + → A B + C D + → C + D + {\displaystyle ABCD^{+}\to AB+CD^{+}\to C+D^{+}}

where ABCD+ is selected by MS1, dissociates into molecule AB and ion CD+. The ion is selected in the second mass spectrometry stage MS2 then undergoes further fragmentation to form ion D+ which is selected in the third mass spectrometry stage MS3 and detected. Multiple reaction monitoring (MRM) is the application of selected reaction monitoring to multiple product ions from one or more precursor ions, for example

A B C D + → A B + C D + {\displaystyle ABCD^{+}\to AB+CD^{+}}

A B C D + → A B + + C D {\displaystyle ABCD^{+}\to AB^{+}+CD}

where ABCD+ is selected by MS1 and dissociates by two pathways, forming either AB+ or CD+. The ions are selected sequentially by MS2 and detected. Parallel reaction monitoring (PRM) is the application of SRM with parallel detection of all transitions in a single analysis using a high resolution mass spectrometer.

Proteomics SRM can be used for targeted quantitative proteomics by mass spectrometry. Following ionization in, for example, an electrospray source, a peptide precursor is first isolated to obtain a substantial ion population of mostly the intended species. This population is then fragmented to yield product ions whose signal abundances are indicative of the abundance of the peptide in the sample. This experiment can be performed on triple quadrupole mass spectrometers, where mass-resolving Q1 isolates the precursor, q2 acts as a collision cell, and mass-resolving Q3 is cycled through the product ions which are detected upon exiting the last quadrupole by an electron multiplier. A precursor/product pair is often referred to as a transition. Much work goes into ensuring that transitions are selected that have maximum specificity. Using isotopic labeling with heavy-labeled (e.g., D, 13C, or 15N) peptides to a complex matrix as concentration standards, SRM can be used to construct a calibration curve that can provide the absolute quantification (i.e., copy number per cell) of the native, light peptide, and by extension, its parent protein. SRM has been used to identify the proteins encoded by wild-type and mutant genes (mutant proteins) and quantify their absolute copy numbers in tumors and biological fluids, thus answering the basic questions about the absolute copy number of proteins in a single cell, which will be essential in digital modelling of mammalian cells and human body, and the relative levels of genetically abnormal proteins in tumors, and proving useful for diagnostic applications. SRM has also been used as a method of triggering full product ion scans of peptides to either a) confirm the specificity of the SRM transition, or b) detect specific post-translational modifications which are below the limit of detection of standard MS analyses. In 2017, SRM has been developed to be a highly sensitive and reproducible mass spectrometry-based protein targeted detection platform (entitled "SAFE-SRM"), and it has been demonstrated that the SRM-based new pipeline has major advantages in clinical proteomics applications over traditional SRM pipelines, and it has demonstrated a dramatically improved diagnostic performance over that from antibody-based protein biomarker diagnostic methods, such as ELISA.

See also Protein mass spectrometry Quantitative proteomics

References

External links SRMatlas; quantify proteins in complex proteome digests by mass spectrometry

Illustrations

Worked examples

Example 1 — a first encounter with Selected reaction monitoring

Start with the simplest possible case. Write down what Selected reaction monitoring claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Selected reaction monitoring before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Selected reaction monitoring ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Selected reaction monitoring

In research: Selected reaction monitoring appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Selected reaction monitoring in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Selected reaction monitoring is common in secondary-school and first-year university syllabi. It links to neighbouring topics Mass spectrometry, Proteomics, so understanding it makes those chapters shorter.
In everyday life: Look for Selected reaction monitoring outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Selected reaction monitoring in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Selected reaction monitoring means in your own words.
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Work through the three examples above with pen and paper.
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Frequently asked questions

What is Selected reaction monitoring in simple terms?

Why does Selected reaction monitoring matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Selected reaction monitoring?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Selected reaction monitoring.