During the analysis, some people may encounter the problem that the noise of baseline suddenly gets louder and even baseline drift occurs after long time running. Why and how does problem like this occur?
Noise and Drift
Noise and drift are the main performance of detector’s stability.
Noise is defined as the signal change output by detector when no solute passes through the detector, denoted by Nd. Noise refers to the random disturbance change of detector output signal which is independent of the tested sample.Noise can be divided into short noise and long noise (Fig 1).
Short noise, commonly known as burr, makes the baseline villous. It is caused by the fluctuation of signal frequency and is a higher baseline disturbance than the effective value frequency of chromatographic peak. The presence of short noise does not affect the resolution of chromatographic peaks, but it does affect the detection limit to a certain degree. The short noise usually comes from the electronic system of the instrument or the pulsation of the pump and it can be eliminated with an appropriate filter.
Long noise is the random and low frequency variation of the output signal, which is composed of the baseline disturbance of the frequency that is similar to the chromatographic peak frequency. Long noise may fluctuate regularly with wavy baseline or fluctuate irregularly, causing difficulties in chromatographic peak resolution. For different types of detectors, the main source of long noise may differ. Some noise is due to the instability of the detector itself, some are due to the presence of bubbles in the mobile phase or pollution while other long noise may be caused by the fluctuation of temperature and flow rate. For the RID detector, the fluctuation of temperature and pressure caused by the change of ambient environment and flow rate of mobile phase makes the refractive index of the liquid in the detection pool differ, which is the main reason of the long noise. Thus, improving the design of the detector can achieve the purpose of reducing long noise.
Noise and drift directly affect the error and detection ability of the analysis work, thus corresponding measures should be taken to eliminate them according to different situations.
Source and Solution of Noise in Ultraviolet Detector
The noise of ultraviolet detector mainly comes from detector and separation system. The common method to determine the source of noise is to systematically change the flow rate of mobile phase. When there is a positive correlation between the noise and the change of flow rate, it can be determined that the noise must come from the separation system.
01 Noise from Detector
For the optical absorption detector, when there is no sample absorption, the detection signal is a function of the light intensity related to the wavelength, the transmission efficiency of the optical system and the photoelectric conversion efficiency. If the photoelectric conversion efficiency is low, the output signal will be weak and close to the natural noise of the photoelectric conversion element.
We can increase the light intensity and improve the SNR by using the method of strong light source or wide spectral band. If only the amplifier magnification is increased, the noise will be amplified at the same time while the SNR can’t get improved.
Many optical absorption detectors use deuterium lamp as the light source. With the increase of using time, the deuterium lamp intensity decreases while the noise increases, thus the deuterium lamp needs to be replaced in time.
In addition, due to the electrostatic effect, the detector is easy to absorb dust from the surrounding environment during use. The dust covered on the optical element reduces the transmission efficiency of light and improves the light scattering, thus this is not good for detection. The irradiation of strong ultraviolet light will also cause some coatings of optical materials degrade, which will slowly increase the noise. The detector’s SNK can be reduced by a quarter or more a year.
02 Noise from Separation System
Early uv-visible detectors were very sensitive to the change of flow rate of mobile phase, which led to the use and development of constant-current pumps. The main reason of flow rate sensitivity of uv-visible detector is the change of refractive index of mobile phase caused by temperature change. The incident light must pass through the air-light window and light window-mobile phase interfaces before entering the detection pool. The incident light therefore produces reflection or scattering loss, which has the same effect as the absorption of light by the compound (about 10^(-4)). When the refractive index difference between media is large, more light will be reflected, scattering loss. Since the refractive index is very sensitive to temperature changes (most solvents have refractive index temperature coefficients between 10^(-4)-10^(-3), it is necessary to control the temperature of the mobile phase of the detection pool and reduce light loss by using heat balance. Heat exchangers are typical equipment for heat balance.
Except for temperature, the change of refractive index of mobile phase is also related to its pressure. The pump pulse causes a change in mobile phase pressure, which also causes a change in the refractive index of the mobile phase. This will further affect the transmission of ultraviolet light through the mobile phase, resulting in the increase of baseline noise. The pressure baseline noise can be improved by adding a pulse damper.
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