Gas Density Detector
Definition and meaning of Gas Density Detector in chemistry.
A gas density detector is a universal, non-destructive detector used in gas chromatography that identifies analytes by measuring changes in the density (and, secondarily, viscosity) of the carrier gas stream as sample components elute from the column.
In more detail
The detector operates by comparing two pressure signals derived from the effluent gas: one, produced across an orifice, responds to gas density and flow velocity; the other, produced across a capillary, responds to gas viscosity and flow velocity. Subtracting these two signals cancels out the effect of flow-rate fluctuations, leaving a signal proportional to analyte concentration. Because eluting analytes almost always differ in molecular weight and physical properties from the carrier gas (commonly helium, nitrogen, or argon), their presence produces a measurable density change. This detector is universal, responding to virtually any compound, organic or inorganic, whose density differs from that of the carrier gas, and it is non-destructive to the sample, allowing it to be recovered downstream. It is also known as the gas density balance (GDB).
Key facts
| Field | Analytical Chemistry |
|---|---|
| Synonym | Gas density balance (GDB) |
| Detector type | Universal (responds to virtually all compounds, organic or inorganic) |
| Detection principle | Pressure signals responsive to carrier-gas density and viscosity |
| Common carrier gases | Helium, nitrogen, argon |
In gas chromatographic analysis of a mixture of alkanes (pentane, hexane, heptane) using a gas density detector with helium carrier gas, each alkane produces a distinct peak proportional to its concentration, with the peak retention time identifying the specific compound and the peak area quantifying it.
Frequently asked questions
Why is a gas density detector called universal?
Because it responds to any analyte whose density differs from that of the carrier gas, making it applicable to a wide range of organic and inorganic compounds rather than only one chemical class.
How does a gas density detector differ from a flame ionization detector?
A gas density detector is non-destructive and universal but comparatively insensitive; a flame ionization detector (FID) ionizes carbon-containing compounds in a hydrogen flame, giving much higher sensitivity, but it destroys the sample and does not respond to compounds lacking ionizable carbon.