Detectors equipped with the flow-through cell was a major breakthrough in the development of modern liquid chromatography. Such detection was first applied by the group of Tiselius, in Sweden in 1940, by continuously measuring the refractive index of the column effluent. Current LC detectors have wide dynamic range normally allowing both analytical and preparative scale runs on the same instrument. They have high sensitivities often allowing the detection of nanograms of material, and the better models are very flexible, allowing rapid conversion from one mobile phase to another and from one mode to another.
Almost all LC detectors are the on-stream monitors. The only relatively successful off-line detector is FTIR spiral disk monitor, which require sample transfer on the germanium disk and the following scanning in FTIR instrument. HPLC detectors always used under continuous flow conditions and the sample is always dissolved in the eluent during detection. Actual sample is only present in a ng quantity in the detector, but in trace analysis, this quantity could be fg and even the single molecule! The mobile phase is a factor which must always be considered.
In the last decade there is a significant progress in the development of LC/MS interfacing systems. MS as an on-line HPLC detector is said to be the most sensitive, selective and in the same time the most universal detector. But it is still the most expensive one.
Actually, now we could not recommend any universal detector, so the liquid chromatographer must expect to eventually use more than one type of detector.
Here we will discuss the most common HPLC detectors:
and the less common, but important
|Evaporative light scattering|