A qualified flow cell,

must withstand long-term stress,

Hard work without complaint, through hundreds of tests,

More than one screw on a panel,

A detector has only one flow cell.

A qualified flow cell needs to meet the following requirements:

  • Obtain ideal detection limits;
  • Obtain ideal noise, drift and signal;
  • It also lies in the fact that after hundreds of tests, the quality is consistent, stable and reliable.
Flow Cell Schematic

Let’s first take a look at the working principle of the UV detector. The detection principle of the UV detector is based on the Lambert-Beer law. The absorbance of the light-absorbing substance is proportional to the optical path length and concentration of the flow cell.

The mathematical expression of the Beer-Lambert law:

A=lg(1/T)=Kbc

A is the absorbance, T is the transmittance (transmittance), which is the ratio of the intensity of the outgoing light to the intensity of the incoming light.

K is the molar absorption coefficient. It is related to the properties of the absorbing material and the wavelength λ of the incident light.

c is the concentration of the light-absorbing substance, in mol/L.

b is the thickness of the absorber layer (length of the flow cell) in cm.

When a beam of parallel monochromatic light vertically passes through a uniform non-scattering light-absorbing material, its absorbance A is proportional to the concentration c of the light-absorbing material and the thickness of the absorption layer (length of the flow cell) b, but inversely to the transmittance T related.

The longer the detector flow cell length, the longer the optical path, the higher the response and the lower the detection limit. The accuracy of quantitative analysis largely depends on the linear range of concentration detection. Flow cells for analyzing liquid phases typically have a larger path length than those for preparing liquid phases for better response at lower concentrations.

Schematic diagram of the optical path of the UV detector

Next, we use an experiment to verify the differences in the chromatograms formed after analyzing the same sample under the same chromatographic conditions for three flow cells with different optical path lengths of 0.5mm, 1.25mm and 3mm.

Chromatographic conditions

Spectrogram

Chromatogram of the same sample detected at 214nm wavelength
Chromatogram of the same sample detected at 254nm wavelength

From the above figure, we can know that using a flow cell with a longer optical path to detect the same sample, the stronger the signal generated, the higher the peak height, and the better the response. Although increasing the optical path length usually increases the noise, the noise increase is small and the signal-to-noise ratio increases, which is generally suitable for analytical LC applications. Using a flow cell with a small optical path, the peak height is reduced, but some peaks have a certain resolution, and the noise is small. In application, it is generally suitable for preparative liquid chromatography.

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