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Short answer: the van Deemter equation, HETP = A + B/u + C·u, describes how the height equivalent to a theoretical plate (HETP) in a gas chromatography column changes with the carrier gas’s average linear velocity u. The curve has a minimum, the optimum velocity, where the column is most efficient. In capillary GC the A term drops out, and the optimum depends on which carrier gas you run.
Definition and scope
What is gas chromatography?
Gas chromatography (GC), sometimes searched as “chromatography gas” or “gas chromatography meaning”, is a separation technique in which a gaseous mobile phase, the carrier gas, moves a vaporised sample through a column, and the sample’s components separate as they interact differently with the column. Unlike liquid chromatography, the carrier gas is not a chemical lever: Restek’s Jaap de Zeeuw notes that “the carrier gas in itself does not impact the separation in GC”. Its choice is set by availability, price, purity, speed, safety and detector sensitivity. For columns, carrier gases and detectors as a set, see the gas chromatography columns, carrier gases and detectors guide.
The equation and its terms
The relationship was published by van Deemter, Zuiderweg and Klinkenberg (Chemical Engineering Science, 1956, 5(6):271-289, DOI 10.1016/0009-2509(56)80003-1). In its common form:
HETP = A + B/u + C·u
- HETP (plate height, H): column length per theoretical plate. Lower is better. Plate number follows N = (tR/σ)², using retention time and peak standard deviation.
- A, eddy diffusion: band broadening from multiple flow paths through a packed bed. It is zero in open tubular (capillary) columns.
- B/u, longitudinal diffusion: analyte molecules spread along the column axis. It matters most at low velocity, because the sample spends longer in the column.
- C·u, resistance to mass transfer: analyte does not equilibrate instantly between the mobile and stationary phases. It matters most at high velocity. Some texts split C into a mobile-phase and a stationary-phase part.
- u: average linear velocity of the carrier gas, usually in cm/s.
For capillary columns the Golay form applies: HETP = B/u + (Cs + Cm)·u, with no A term. The same equation is used in liquid chromatography; see the HPLC guide for that side.
How it is used in practice
Plotted as HETP against u, B/u falls and C·u rises, so the curve dips to a minimum. The velocity at that minimum is the optimum; it gives the highest plate number for the column. Below the optimum, efficiency drops quickly, while above it the loss is gradual, which is why methods often run faster than the strict optimum to save time. Practical guidance for capillary GC from an LCGC International column (J. Heseltine, 1 January 2010, Column Watch, R. E. Majors) is to aim for an average linear velocity of around 20-25 cm/s.
Because the curve shape depends on the gas, setting velocity rather than a flow rate is the transferable specification. Restek’s worked figure for a 30 m × 0.25 mm column at 30 cm/s is 1.28 mL/min of helium or 1.02 mL/min of hydrogen, so identical velocity needs about 20% less hydrogen flow.
Types and related concepts: carrier gases on the van Deemter curve
| Carrier gas | Reported behaviour | Source |
|---|---|---|
| Nitrogen | Lowest minimum plate height of the three gases, but a narrow velocity range and a steeper curve. Low optimum velocity, so analysis at the optimum takes 2-2.5 times longer than with helium. With a mass spectrometer, sensitivity is usually reduced by up to a factor of 20. | LCGC International (2010); Restek |
| Helium | Optimum linear velocity about 2.5 times higher than nitrogen’s. | Restek |
| Hydrogen | Higher optimum velocity than helium. Reported optima differ by source: about 40-50 cm/s (Restek) and around 60 cm/s (LCGC International). Run at helium’s velocity it sits off its optimum; Restek’s figure shows a loss of about 20% in HETP, though the author calls the practical loss very small. | Restek; LCGC International (2010) |
The point to take from the table is that the optimum is a property of gas and column together, so quote it with its source and confirm it with the column maker’s data rather than treating one number as universal.
Where it matters for labs and supply decisions
The equation is the reasoning behind a carrier gas change. Restek’s pesticide example ran helium and hydrogen at the same 34 cm/s with the same oven programme and showed virtually no loss in separation efficiency; matching average linear velocity also keeps elution temperatures, and so the separation, the same. Two checks before switching, each from the sources above:
- Set velocity, not flow. Convert the method to the same average linear velocity, as in the 1.28 versus 1.02 mL/min example.
- Check the detector. Restek reports that a thermal conductivity detector gives its highest sensitivity with hydrogen, while PDD and HID detectors are restricted to helium, and nitrogen with MS typically costs sensitivity (up to a factor 20).
For the supply context behind many of these changes, see the helium shortage guide; for detectors, the flame ionization detector guide; for choosing between GC and HPLC columns, the GC and HPLC column selection guide; and for how plate-height reasoning carries into smaller-particle LC, the HPLC vs UPLC vs UHPLC comparison.
Common misunderstandings
- “The carrier gas changes selectivity.” Per Restek, the carrier gas in itself does not impact the separation in GC; it changes efficiency and speed.
- “There is one optimum velocity per gas.” The optimum depends on the column and the source’s conditions; hydrogen figures of 40-50 and about 60 cm/s both appear in the literature.
- “Running at the minimum plate height is always best.” The minimum gives the most plates per column length, not the shortest run. Nitrogen has the lowest minimum plate height but the slowest analysis.
- “The A term always applies.” It is zero in open tubular columns, so the capillary curve follows the Golay form.
FAQ
What is the van Deemter equation in gas chromatography?
It is HETP = A + B/u + C·u, linking plate height to carrier gas linear velocity. The minimum of the curve marks the most efficient velocity.
What is the definition of gas chromatography?
A separation method in which a carrier gas moves a vaporised sample through a column; components are resolved by their differing interaction with the column.
Does the A term matter in capillary GC?
No. Eddy diffusion is zero in open tubular columns, leaving B/u and the mass-transfer terms.
Can hydrogen replace helium without losing separation?
Restek reports that at matched linear velocity the separation was virtually unchanged in a pesticide example, with about 20% HETP loss shown at helium’s optimum velocity. Check detector compatibility first.
Sources
Links checked 2026-10-29.
- J. J. van Deemter, F. J. Zuiderweg, A. Klinkenberg, “Longitudinal diffusion and resistance to mass transfer as causes of nonideality in chromatography”, Chemical Engineering Science 5(6):271-289 (1956), https://doi.org/10.1016/0009-2509(56)80003-1 (origin of the equation).
- Jaap de Zeeuw, “Impact of GC Parameters on The Separation, Part 6: Choice of Carrier Gas and Linear Velocity”, Restek / Separation Science, https://discover.restek.com/wp-content/uploads/GNOT5248.pdf
- James Heseltine, “Hydrogen as a Carrier Gas for GC and GC-MS”, LCGC International, 1 January 2010, https://chromatographyonline.com/view/hydrogen-carrier-gas-gc-and-gc-ms
- Background on the equation’s terms: Wikipedia, https://en.wikipedia.org/wiki/Van_Deemter_equation
