Prerequisites: Reading m/z and signal axes; addition and percentages. Start with a spectrum shows ions, not molecules if those are unfamiliar.
Your task
Turn a chromatogram screenshot into a reproducible observation. You will calculate three traces from identical scans, state an extraction window unambiguously, and investigate a peak that might contain more than one component. All numerical data below are synthetic teaching examples, not operating settings or validation results.
1. Spectrum or time trace?
A mass spectrum places signal against m/z for one acquisition or a defined combination of acquisitions. A chromatogram places a selected signal against chromatographic time. An apex at 4.20 minutes is a time coordinate, not an ion mass. A signal at m/z 300.1234 is an ion coordinate, not a retention time.
Software can display a spectrum averaged across the time interval you selected. That makes the interval part of the result. Selecting a wide interval may combine a peak, its neighbors and background. Start with this question: “Which scans contributed to the picture I am reading?”
Three common time traces summarize those scans differently:
- TIC: the total included ion signal at each time. IUPAC calls the chromatogram a total ion current chromatogram, or TICC; software commonly labels it TIC.
- BPC: the greatest included intensity at each time. The m/z supplying that maximum can change.
- XIC or EIC: signal extracted for specified m/z values or windows. Here we explicitly sum the included centroid intensities.
See IUPAC Recommendations 2013 for the formal terminology. None of these labels makes the vertical axis a concentration scale.
2. Calculate three traces from the same scans
Assume these are positive-ion MS1 centroid scans containing only the three listed channels. All values are arbitrary signal units. No smoothing, baseline subtraction or normalization has been applied. For this first XIC, select only m/z 100.
| Time (min) | m/z 100 | m/z 200 | m/z 300 |
|---|---|---|---|
| 1.0 | 10 | 80 | 10 |
| 1.1 | 60 | 30 | 10 |
| 1.2 | 10 | 10 | 80 |
Calculate the three TIC values, the three BPC values and the three XIC values. Which summary hides the changing distribution most completely?
Reveal the arithmetic and interpretation
TIC = 100, 100, 100: at 1.1 minutes, for example, 60 + 30 + 10 = 100. BPC = 80, 60, 80: its contributing m/z switches from 200 to 100 to 300. XIC = 10, 60, 10: only the selected channel contributes.
The flat TIC hides the redistribution. The BPC is not a continuous record of one ion, although it is drawn as one line. The XIC highlights the selected signal but still does not identify its chemical source.
Now exclude m/z 300 from the TIC calculation. You obtain 90, 90, 20. Nothing about the underlying scans changed; the selection did. This is why two screenshots labeled “TIC” can disagree if their scan filters or mass ranges differ.
These three times are enough to practice arithmetic, not to establish peak shape or a sampling requirement. A real integration needs adequate time sampling for the method and an explicitly defined baseline and integration interval. Counting three points in this table does not justify a three-point laboratory peak.
3. Declare the extraction window
“Extract m/z 300.1234” leaves a decision unstated: how close must a measured centroid be? Declare the center, tolerance units and whether the tolerance is a half-width. In this lesson, ±5 ppm means five parts per million on either side of the target.
Half-width = 300.1234 × 5 / 1,000,000 = 0.001500617
The resulting interval is 300.121899383–300.124900617. Its full width is about 0.0030012 in m/z. An alternative absolute half-width of ±0.0100 gives 300.1134–300.1334, a much wider interval. Neither choice is automatically correct for an unknown instrument and method.
For a synthetic scan, suppose centroids occur at 300.1240 with response 400 and 300.1300 with response 250. Which response enters each extraction?
Check both extraction choices
The ±5 ppm interval includes 300.1240 but excludes 300.1300, giving 400. The ±0.0100 interval includes both, giving 650. Calling the larger number “more analyte” would assume that the second ion belongs to the analyte. The calculation itself does not establish that.
If a later target centroid shifts outside the narrow interval, that extraction can lose signal. Inspect the measured masses and method performance before changing the window; do not adjust it merely to obtain the preferred peak.
Thermo's Create Mass Trace documentation explicitly defines absolute and ppm tolerances as ± values and separately selects MS order and polarity. Other software may present different controls. Save the actual settings, rather than copying a number without its meaning.
Selection is more than an m/z interval
- Scan type: an MS1 ion trace and an MS/MS product-ion trace answer different questions. Record precursor selection and fragmentation conditions for the latter.
- Polarity: do not merge positive and negative acquisitions as though their identical displayed m/z values represented identical ions.
- Acquired range and time window: extraction cannot retrieve an ion from scans that never measured its region. A scheduled acquisition may not cover the whole run.
- Representation: this lesson uses centroids. Profile-data extraction, centroiding, smoothing and intensity thresholds can change the result; consult the data system's processing definition.
A selected-reaction-monitoring transition is an acquisition channel, not simply a full-scan XIC with a narrow window. Keep that distinction when comparing targeted and full-scan data. The IUPAC definitions distinguish these acquisition and extraction terms.
4. Review the front, apex and tail
One attractive peak can hide coelution. Compare individual scans or matched short intervals from its rising front, apex and falling tail. Keep their acquisition conditions comparable. Examine the full spectra as well as the chosen trace, then ask whether candidate companion ions follow the same time profile.
| Position | Ion A | Ion B | B/A |
|---|---|---|---|
| Front, 4.16 min | 20 | 4 | 0.20 |
| Apex, 4.20 min | 100 | 20 | 0.20 |
| Tail, 4.24 min | 20 | 16 | 0.80 |
If B is proposed as a companion ion of A, what should the last row make you investigate?
Interpret the changing ratio
B is relatively stronger on the tail. An additional later-eluting contributor is one hypothesis. Noise, processing or changing acquisition behavior must also be considered. This table warrants inspection; it does not prove a particular interference. Conversely, matching ratios would support a shared time profile but would not prove one compound: unresolved components can track together.
Compare sample and relevant blank traces with the same extraction and integration rules. Inspect the unnormalized signal scale; separately autoscaled traces can make a tiny blank feature look as tall as a large sample peak. Keep peak height separate from integrated area. If the baseline or integration limits move, the area may change without a corresponding change in the sample.
Retention time, mass agreement, isotope relationships and suitable fragmentation evidence can each constrain an assignment. Their adequacy depends on the question and method. A single clean XIC is compatible with several contributors inside one window, so it cannot establish purity or unique identity on its own.
5. Write an annotation someone can reproduce
Practice with this case: file S17, positive MS1 centroid data, target m/z 300.1234, ±5 ppm summed extraction, no smoothing, apex at 4.20 minutes. A solvent blank acquired before S17 has a smaller feature in that interval. No reference standard or MS/MS comparison is available.
Write two sentences: first the observation with its settings, then the strongest supported conclusion and one unresolved alternative.
Compare a defensible annotation
“S17 contains a feature with an apex at 4.20 minutes in a positive-ion MS1 centroid XIC centered on m/z 300.1234, summed within ±5 ppm without smoothing. A smaller feature occurs in the preceding solvent blank under the same processing settings; the sample feature's identity and possible background contribution remain unresolved.”
This preserves the evidence without calling the blank clean, subtracting it automatically or claiming a confirmed compound. Add integration boundaries and baseline rules if you report an area. Retain the raw file, processing version and reference comparisons with the record.
Before you move on
Can you explain why a flat TIC can conceal a changing sample, why a BPC can switch ions, and why narrowing an XIC can both help and hurt? If so, you can read the trace as a measurement with explicit choices.
Primary references
- IUPAC, Definitions of terms relating to mass spectrometry (2013): chromatograms and acquisition terminology. The 2013 terminology transcription provides an alternative reading route.
- Thermo Fisher, Compound Discoverer 3.4: Create Mass Trace: a concrete example of polarity, MS-order and tolerance controls, not a universal processing prescription.
- Thermo Fisher, FreeStyle: chromatogram trace ranges: trace-type and extraction-setting examples.
Next step
Use blanks, controls and carryover to design the comparison your chromatogram needs. Revisit resolution and mass accuracy before choosing tolerances, or one molecule, many peaks when testing companion-ion assignments.