chemistry

Label-free quantification

Label-free quantification 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 Label-free quantification rather than just read about it. In short: Label-free quantification is a method in mass spectrometry that aims to determine the relative amount of proteins in two or more biological samples. Unlike other methods for protein quantification, label-free quantification does not use a stable isotope containing compound to chemically bind to and thus label the protein.

Key takeaways

Label-free quantification belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Label-free quantification to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Label-free quantification from memory before moving on to harder problems.

Reference excerpt

Label-free quantification is a method in mass spectrometry that aims to determine the relative amount of proteins in two or more biological samples. Unlike other methods for protein quantification, label-free quantification does not use a stable isotope containing compound to chemically bind to and thus label the protein.

Implementation

Label-free quantification may be based on precursor signal intensity or on spectral counting. The first method is useful when applied to high precision mass spectra, such as those obtained using the new generation of time-of-flight (ToF), fourier transform ion cyclotron resonance (FTICR), or Orbitrap mass analyzers. The high-resolution power facilitates the extraction of peptide signals on the MS1 level and thus uncouples the quantification from the identification process. In contrast, spectral counting simply counts the number of spectra identified for a given peptide in different biological samples and then integrates the results for all measured peptides of the protein(s) that are quantified. The computational framework of label free approach includes detecting peptides, matching the corresponding peptides across multiple LC-MS data, selecting discriminatory peptides. Intact protein expression spectrometry (IPEx) is a label-free quantification approach in mass spectrometry under development by the analytical chemistry group at the United States Food and Drug Administration Center for Food Safety and Applied Nutrition and elsewhere. Intact proteins are analyzed by an LCMS instrument, usually a quadrupole time-of-flight in profile mode, and the full protein profile is determined and quantified using data reduction software. Early results are very encouraging. In one study, two groups of treatment replicates from mammalian samples (different organisms with similar treatment histories, but not technical replicates) show dozens of low CV protein biomarkers, suggesting that IPEx is a viable technology for studying protein expression.

Detecting peptides Typically, peptide signals are detected at the MS1 level and distinguished from chemical noise through their characteristic isotopic pattern. These patterns are then tracked across the retention time dimension and used to reconstruct a chromatographic elution profile of the mono-isotopic peptide mass. The total ion current of the peptide signal is then integrated and used as a quantitative measurement of the original peptide concentration. For each detected peptide, all isotopic peaks are first found and the charge state is then assigned. Label-free quantification may be based on precursor signal intensity and has problems due to isolation interference: in high-throughput studies, the identity of the peptide precursor ion being measured could easily be a completely different peptide with a similar m/z ratio and which elutes in a time frame overlapping with that of the former peptide. Spectral counting has problems due to the fact that the peptides are identified, thus making it necessary to run an additional MS/MS scan which takes time and therefore reduces the resolution of the experiment.

Matching corresponding peptides In contrast to differential labelling, every biological specimen needs to be measured separately in a label-free experiment. The extracted peptide signals are then mapped across few or multiple LC-MS measurements using their coordinates on the mass-to-charge and retention-time dimensions. Data from high mass precision instruments greatly facilitate this process and increase the certainty of matching correct peptide signals across runs. Clearly, differential processing of biological samples makes it necessary to have a standard which can be used to adjust the results. Peptides that are not expected to change in their expression levels in different biological samples may be used for this purpose. However, not all peptides ionize well and therefore the choice of candidates should be done after an initial study which should only characterize the protein content of the biological samples that will be investigated.

Selecting discriminatory peptides Finally, sophisticated normalization methods are used to remove systematic artefacts in the peptide intensity values between LC-MS measurements. Then, discriminatory peptides are identified by selecting the peptides whose normalized intensities are different (e.g., p-value < 0.05) among multiple groups of samples. In addition, newer hybrid mass spectrometers like LTQ OrbiTrap offer the possibility to acquire MS/MS peptide identifications in parallel to the high mass precision measurement of peptides on the MS1 level. This raises the computational challenge for the processing and integration of these two sources of information and has led to the development of novel promising quantification strategies.

References

Worked examples

Example 1 — a first encounter with Label-free quantification

Start with the simplest possible case. Write down what Label-free quantification 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 Label-free quantification 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 Label-free quantification 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 Label-free quantification

In research: Label-free quantification 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 Label-free quantification 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: Label-free quantification is common in secondary-school and first-year university syllabi. It links to neighbouring topics Biochemistry detection methods, Mass spectrometry, so understanding it makes those chapters shorter.
In everyday life: Look for Label-free quantification 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 Label-free quantification in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Label-free quantification means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Label-free quantification out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Label-free quantification in simple terms?

Why does Label-free quantification 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 Label-free quantification?

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 Label-free quantification.