Which packet arrives first — 100 Da or 400 Da, both at +1?
Demo 13 of 17 · Part four · Chemistry · Stage 1
Same charge, different masses
Both ions carry the same single positive charge. Which one reaches the detector first, and by how much?
Before you start. An ion is an electrically charged atom or molecule. A dalton (Da) is the mass unit for atoms and molecules. This instrument gives every ion a push with the same voltage, then times how long it takes to coast down a tube in vacuum — so the only thing that differs between these two ions is mass.
Commit to a prediction, then press Run detector and watch both packets cross the tube.
Flight time grows with the square root of mass, not with mass: four times the mass takes twice the time.
1 · BoostVoltage adds kinetic energy before the clock starts.
2 · CoastIons cross a 1.50 metre field-free tube in vacuum.
3 · DetectArrival time becomes the measured evidence.
Flight tube1.50 metre (m)
Voltage boost20 kilovolts (kV)
Da means dalton, a mass unit for atoms and molecules. +1e means one positive elementary charge; +2e means two. The charge number z is 1 or 2 here.
Arrival values and the trace stay hidden until you run the detector.
Live computed on this device
1 µs = one millionth s · 160 ms on screen
Linear time-of-flight instrument
Detector clockWaiting for Run
Detector eventNo arrivals recorded
Detector trace
Fixed time axis for all three investigations: 0–18 microsecondsA peak represents a group of ions arriving over a range of times. The navy trace is the total detector signal; coloured ticks mark the known packets' centre times. Moving markers show group centres, not exact individual paths.
Ion B has four times the mass of Ion A. How much longer did it take to arrive?
Demo 13 of 17 · Part four · Chemistry · Stage 2
Same mass, different positive charges
Both ions weigh 200 Da. If one carries twice the charge, what does the same voltage do to it?
Before you start. Stage 1 held charge fixed and changed mass. Now the reverse. The charge number z counts how many elementary charges an ion carries — +1e is one, +2e is two. The voltage does work on charge, so a doubly charged ion is pushed twice as hard out of the same voltage.
Two ions of 200 Da, one at +1 and one at +2. Which arrives first?
Commit, then press Run detector and compare the two arrival times.
The same voltage gives a doubly charged ion twice the energy, so it crosses the tube sooner even at identical mass.
Ion B carried +2 and arrived sooner than Ion A at +1. Why?
Demo 13 of 17 · Part four · Chemistry · Stage 3
Different values, equal mass-to-charge ratio
Ion A is 200 Da at +1 and Ion B is 400 Da at +2. What does the detector record?
Before you start. Stages 1 and 2 changed mass and charge one at a time. This stage changes both together. m/z is the conventional mass-to-charge label: divide the mass value by the charge number, so 200 at +1 and 400 at +2 both give 200. Watch what that does to two ions that are, by any other measure, different substances.
Equal m/z, different mass and charge. One peak or two?
Commit, then press Run detector and look at where the two centre ticks land on the trace.
Flight time fixes m/z and nothing more, so one peak does not establish one kind of ion.
A real sample produces one detector peak at 10.80 µs. Which conclusion does flight time alone justify?
Demo 13 of 17 · Part four · Chemistry · Stage 4
When do two peaks count as separate?
A 1.00 m tube cannot tell 200 Da from 205 Da. Does making the tube longer fix that?
Before you start. Real ion packets do not all arrive at one exact instant, so every peak has a timing width. Sigma (σ) is the Gaussian spread of arrival times and FWHM is the full width at half maximum, which is 2.355 × σ. This stage compares the same ion pair in a 1.00 m and a 2.00 m tube under two assumed width rules — the widths are stated, not measured.
Doubling the tube doubles the gap between two arrivals. Is that enough to separate the peaks?
Press Run paired comparison on the fixed-width assumption, then switch to Proportional width and run it again.
Whether a longer tube separates two peaks depends entirely on how the peak width behaves — the length alone does not decide it.
Liveunder an assumed width rule you choose computed on this device
Two tubes, one ion pair
200 Da and 205 Da, both at +1, in a 1.00 m tube and a 2.00 m tube. The arrival times and the summed detector signal are computed here from the same model the core stages use. The timing width is an assumption, and you pick which one.
Navy line: total detector signal. Dashed sky and gold lines: the two known simulated packet components. Both rows use the same vertical signal scale.
| Tube | Peak separation | Mean peak width (FWHM) | Criterion |
|---|
Symbols and units used on this page
t = L √[ m / (2qV) ]
- t is field-free flight time in seconds, shown here in microseconds (µs). One µs is one millionth of a second.
- L is tube length in metres (m). m is ion mass, converted from daltons (Da) to kilograms.
- q = z·e is positive charge in coulombs. z is the dimensionless charge number; e is one elementary charge.
- V is acceleration voltage in volts, shown in kilovolts (kV); one kV is 1,000 volts. Acceleration happens before this clock starts.
- m/z is the conventional mass-to-charge label. For example, 400 Da with z = 2 gives m/z = 400 ÷ 2 = 200.
- σ (sigma) is the assumed Gaussian timing spread. FWHM, the full width at half maximum, equals 2.355 σ.
In this stage, peaks count as resolved when their centre separation is at least their mean FWHM. That is a declared classroom criterion, and the timing widths are assumptions. A longer tube helps under the fixed-width assumption here; it is not a universal instrument promise. Full sources are in Details.