A spectrum shows ions, not molecules

Lesson 1 · 25 minutes
Prerequisites: Basic atomic structure and arithmetic.
Mathematics: Intuition first; optional working equation.
Audience: Beginner, with practitioner and expert extensions.

A peak is an observation to explain. In this lesson, you will separate that observation from assumptions about the molecule that produced it.

By the end, you will be able to:

• distinguish a neutral analyte, an ion composition, and a charge state;
• read m/z and signal response without treating them as mass and concentration;
• calculate a neutral mass from an explicitly proposed protonated ion;
• use isotope spacing to test a charge assignment.

Predict

A spectrum contains a strong peak at m/z 501.0073. Which statement follows from that position alone?

• A. The neutral molecule has a mass of 501.0073 Da.
• B. The processed spectrum contains signal at m/z 501.0073.
• C. The compound is uniquely identified.
• D. The peak height gives its concentration.

Choose before reading the feedback.

Explain

Follow the measurement

Sample → ion formation → ion transmission and analysis → detection → processed spectrum → interpretation

An analyte is the component you want to investigate. The ion observed in an experiment may contain that analyte plus an attached species, may have lost a proton or an electron, or may contain only a fragment of it. Ion formation is already part of the measurement, rather than a neutral transfer of everything in the sample. Ion-source overview

Detection converts ion behavior into an electrical signal. Some systems detect ion impact; Orbitrap and FT-ICR instruments detect image current induced by moving ions. “Detected” therefore does not always mean “struck a detector.” Exactive operating manual, measuring principle, IUPAC FT-ICR definition

Why charge belongs in the measurement

A neutral molecule has no net electric charge. An ion has an imbalance between positive nuclear charge and electrons. Removing an electron, adding a proton, and attaching a charged species can all make positive ions, but they do not make the same ion. Charge is therefore both a physical property that lets an instrument manipulate an ion and a chemical bookkeeping requirement.

Different analyzers use different motion or frequency measurements. In an idealized time-of-flight experiment, for example, an ion gains kinetic energy proportional to its charge when accelerated through a potential difference. Its flight time consequently depends on mass divided by charge magnitude. You will derive that relationship in lesson 4. Here it explains why a mass spectrometer does not simply weigh every neutral molecule in the vial. Shimadzu TOF derivation

The path also contains losses and selections. An analyte may fail to ionize, fall outside the acquired m/z range, be lost during transfer, or give a response below the reporting threshold. No assigned peak is therefore a description of this experiment; it is not automatically evidence that the sample contains none of the analyte.

Read the axes

m/z describes a coordinate that depends on both ion mass and charge. For the calculations here, take the numerical ion mass in daltons and divide by the magnitude of its integer charge number. Label the spectrum axis m/z, not mass in Da. Negative ions conventionally also have positive displayed m/z values. IUPAC m/z definition and terminology discussion

The vertical axis reports signal response. If a spectrum is normalized to its tallest peak, 100% means the largest displayed response in that spectrum. It does not mean that the corresponding compound makes up 100% of the sample.

Signal is not automatically concentration. A quantitative method must establish how the measured response relates to the amount of analyte, with appropriate standards and checks. EPA Method 6020B, calibration and quality control

Read the display as well as the axes

A profile spectrum retains sampled peak shapes. A centroid spectrum summarizes detected peaks with positions and responses, often drawn as sticks. A thin drawn stick is not evidence of an infinitely narrow experimental peak. Before comparing apparent widths or very close features, find out which representation you have. The precise centroiding algorithm and reported response depend on the data system. Thermo hardware manual: profile and centroid data

A mass spectrum plots response against m/z for a selected acquisition or combination of acquisitions. A chromatogram plots response against time. If software averages spectra across an elution interval, the resulting spectrum may combine ions from more than one coeluting substance. Keep the selected time interval with the spectrum. Thermo data terminology

Which mass are you using?

• Nominal mass sums the integer mass numbers of the most abundant isotopes. For neutral C₆H₁₂O₆ that sum is 180.
• Monoisotopic mass sums isotope masses for the composition using each element's most abundant isotope. For the same formula it is approximately 180.063388 Da.
• Exact mass is a calculated mass for an explicitly specified isotopic composition; monoisotopic mass is one such calculation. “Exact” does not mean a measured value has no uncertainty.
• Average mass averages over an assumed isotope distribution. It need not correspond to any single isotopologue peak.

The distinction matters before any ion calculation: comparing a monoisotopic peak with an average neutral mass introduces the wrong reference. A formula still does not specify a unique structure. IUPAC mass-spectrometry terminology, NIST isotope masses

Name the ion before calculating

For a neutral molecule M, [M+H]⁺ means that one proton has been added. [M+2H]²⁺ means that two protons have been added, giving charge +2. [M−H]⁻ describes loss of a proton, giving charge −1. These are different ion compositions.

Proton addition uses the proton mass: approximately 1.0072764665789 Da. Neutral hydrogen-1 has a different mass: 1.00782503223 Da. Use neutral atomic masses to sum a neutral elemental formula; use the proton mass when adding or removing H⁺. NIST CODATA 2022, NIST hydrogen isotope data

Optional working equation

Let M₀ be the numerical neutral monoisotopic mass in Da, h the numerical proton mass in Da, n a positive integer, and x the numerical m/z value. For the proposed ion [M+nH]ⁿ⁺:

x = (M₀ + n × h) / n

Therefore:

M₀ = n × x − n × h

The resulting neutral mass is reported in Da. The same formula does not describe a sodium adduct or an ion formed by electron removal; those require their own composition bookkeeping. These equations are arithmetic consequences of the stated ion assignment, not a test that the assignment is correct.

Negative charge changes the bookkeeping, not the axis direction

For proton removal, [M−nH]ⁿ⁻, use x = (M₀ − n × h) / n. Rearranging gives M₀ = n × x + n × h. The positive integer n is the magnitude of the charge; the negative sign is already represented by the ion composition.

For example, a neutral teaching mass of 180.06338810418 Da gives [M−H]⁻ at m/z 179.0561. Recovering the neutral mass requires adding back a proton. Do not put a minus sign in front of the displayed m/z, and do not subtract another proton. This is a mass-balance calculation, not a claim that every molecule can form this ion.

Manipulate

Make a prediction, then change one input in this model. The equations and limitations travel with the plot.

Use a hypothetical peptide P with neutral monoisotopic mass 1000.0000 Da. This is a rounded teaching input, not an identified peptide sequence or measured spectrum.

Prediction: If P forms a doubly protonated ion instead of a singly protonated ion, will its peak move to higher or lower m/z?

Check the third row with your own calculation. Each calculation starts with the same neutral mass. These positions do not predict the relative signals or guarantee that all three charge states form.

Apply

Worked example using isotope spacing

You observe neighboring peaks at m/z 501.0073 and 501.5090 in a resolved isotope envelope. Assume they are related isotopologues rather than unrelated overlapping signals. An isotopologue has the same elemental composition but a different isotope composition.

A single substitution of carbon-13 for carbon-12 increases mass by approximately 1.00335483507 Da. At charge magnitude 2, the calculated m/z shift is 0.501677417535. NIST carbon isotope data

1. Subtract the observed positions: 501.5090 − 501.0073 = 0.5017.
2. Compare with the one-carbon substitution model: 1.00335483507 / 2 ≈ 0.5017. The spacing supports charge magnitude 2.
3. Now additionally assume that the first peak is the monoisotopic [P+2H]²⁺ peak. Calculate M₀ = 2 × 501.0073 − 2 × 1.0072764665789.
4. The inferred neutral mass is 1000.0000 Da, rounded to four decimal places.

The spacing tests charge. It does not prove protonation, identify which peak is monoisotopic, or establish the peptide sequence. The calculation uses rounded observed positions, so its digits should not be treated as a measurement-uncertainty statement.

What can you claim

Observation: There is signal at the reported m/z values.

Supported interpretation under the stated model: The spacing is consistent with a doubly charged isotope envelope.

Conditional calculation: If the first peak is monoisotopic [P+2H]²⁺, the neutral mass is approximately 1000.0000 Da.

Still unestablished: A unique compound identity or concentration.

Practitioner extension

Annotate a feature with three separate fields: observed m/z, proposed ion, and supporting evidence. If the charge assignment changes, revisit the neutral-mass calculation rather than keeping the previous compound label.

Expert extension

Challenge the premise that the two peaks belong to one envelope. Consider overlapping ions, missing low-intensity isotope peaks, and isotope contributions beyond carbon. Decide which additional observation would most directly test your preferred explanation.

Check your understanding

1 A peak is at m/z 400

Can you report a neutral mass of 400 Da?

2 Isotope peaks are about 0.3345 apart

Under the one-carbon substitution model, which charge magnitude fits?

3 A peak is displayed at 100% relative intensity

What does that mean in a spectrum normalized to its tallest peak?

4 The peak position fits your candidate exactly

Which report is defensible from that match alone?

5 Transfer challenge: write the report

A synthetic negative-ion spectrum has features at m/z 298.9927, 299.4944, and 299.9961. A colleague proposes that the first is monoisotopic [Q−2H]²⁻. Write the observed spacing, calculate the conditional neutral mass, and name two assumptions still needing evidence.

6 Transfer challenge: did the amount halve?

Two spectra are separately normalized to their base peaks. A target is 50% in run A and 25% in run B. In arbitrary unnormalized response units, the base peaks are 2000 and 8000, respectively. Calculate the target responses. Can you infer the change in concentration?

Primary references

The worked data are synthetic. The linked standards and manuals support the definitions and physical models; they do not validate the hypothetical assignments.

• IUPAC Recommendations 2013, entries reproduced on Mass Spec Terms: m/z and FT-ICR. Use the labeled 2013 material, not the site's separate draft terminology.
• NIST: CODATA 2022 constants, hydrogen, and carbon. Numerical inputs for the original teaching calculations above.
• Thermo Fisher Scientific: Ionization source overview and Exactive operating manual, measuring principle, printed page 1-23.
• US EPA: Method 6020B, sections 9–11, as a concrete example of method-specific quantitative calibration and QC.

Next step

Explain why P can give three different peak positions while retaining one underlying neutral mass. Then learn to distinguish charge states from isotopes, adducts, and fragments.

Continue to lesson 2 — One molecule can make many peaks
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