Do many of you often encounter such distress? Maybe your instrument is good for making samples, but when it runs for a long time or you do not know which needle to start from, the baseline noise suddenly becomes larger, and even baseline drift occurs in more serious cases. So how does this problem arise? And how to solve it? Here we are to give you a detailed introduction about it

Noise and drift

Noise and drift are the main manifestations of detector stability.
Noise, is defined as the change of the signal output by the detector when no solute passes through the detector, expressed in Nd. Noise refers to the random disturbance change of the detector output signal independent of the measured sample. Noise can be divided into short noise and long noise (see Figure 1).

Short noise, commonly known as burr, makes the baseline fluff, which is caused by the fluctuation of signal frequency. It is a baseline disturbance with higher frequency than the effective value of chromatographic peak. The existence of short noise does not affect the resolution of chromatographic peaks, but has a certain impact on the detection limit. Short noise usually comes from the pulsation of the electronic system and pump of the instrument, which can be eliminated by appropriate filter.

Long noise is the random and low-frequency variation of the output signal, which is composed of baseline disturbance with a frequency similar to the chromatographic peak. The long noise may be regular fluctuation, the baseline is wavy, or it may be irregular fluctuation, which makes it difficult to distinguish chromatographic peaks. For different types of detectors, the main sources of long noise may be different. Some are due to unstable components of the detector itself, some are due to bubbles or contamination in the mobile phase, and long noise may be caused by temperature and flow rate. For the differential refractive index detector, the fluctuation of temperature and pressure caused by the change of surrounding environment and mobile phase flow rate changes the refractive index of the liquid in the detection tank, which is the main cause of long noise. Reducing long noise can be achieved by improving the design of the detector.

Drift refers to the deviation of the baseline in a single direction with the increase of time. It is an output disturbance with a lower frequency than the effective value of the chromatographic peak, which will not blur the chromatographic peak, but the baseline needs to be adjusted frequently in order to work effectively. The reason for the drift is that the power supply voltage is unstable; Slow change of temperature and mobile phase velocity; The stationary phase washes down from the column; The replaced new solvent has not reached equilibrium in the column.

Figure 1. Noise and drift
(a) Short noise (b) long noise (c) drift

Noise and drift directly affect the error and detection ability of analysis work, and corresponding measures should be taken to eliminate them according to different conditions.

Noise sources and solutions in UV detectors

The noise of UV detector mainly comes from detector and separation system. The most commonly used method to determine the source of noise is to systematically change the flow rate of mobile phase. If the noise is positively correlated with the change of flow rate, the noise may come from the separation system; When the noise is strictly proportional to the change of flow velocity, it can be determined that the noise must come from the separation system.

1. Noise from the detector

For the optical absorption detector, when there is no sample absorption, the detection signal is a function of the light intensity related to wavelength, the propagation efficiency of the optical system and the photoelectric conversion efficiency. If the photoelectric conversion efficiency is low, the output signal is small, close to the natural noise of the photoelectric conversion element.

The light intensity and SNR can be increased by using strong light source or wide spectral band. If only the amplification factor of the amplifier is increased, the noise will be amplified simultaneously, and the SIGNal-to-noise ratio will not be improved.

Many optical absorption detectors use deuterium lamp as light source, with the increase of time, deuterium lamp intensity decreases, increasing noise, need to be replaced in time.

In addition, due to electrostatic action, the detector is easy to absorb dust from the surrounding environment in the use process, and the dust covered on the optical elements reduces the transmission efficiency of light and improves the scattering of light, so it is not good for detection. The irradiation of strong ultraviolet light will degrade the creep layer of some optical materials and slowly increase the noise. The detector’s SNR can be reduced by a quarter or more over a year.

2. Noise from the separation system

Early UV-vis detectors were very sensitive to changes in mobile phase flow rates, which led to the use and development of constant-current pumps. The change of refractive index of mobile phase caused by temperature change is the main reason of velocity sensitivity of UV-visible detector. Incident light must pass through the air-light window and the mobile phase before entering the detection cell. Therefore, incident light has reflection or scattering loss, which has the same effect as compound absorption (about 10^(-4)). When the refractive index difference between media is large, more light will be reflected and scattered to lose. Since the refractive index is very sensitive to changes in temperature (most solvents have refractive index temperature coefficients between 10^(-4)-10^(-3)), it is necessary to control the temperature of the mobile phase in the detection cell and reduce light loss using thermal equilibrium. A heat exchanger is a typical device for heat balance.

Besides temperature, the change of refractive index of mobile phase is also related to pressure. The pulse of the pump causes a change in the pressure of the mobile phase, which also causes a change in the refractive index of the mobile phase, which affects the propagation of ultraviolet light through the mobile phase and leads to an increase in baseline noise. Pressure baseline noise can be reduced by adding pulse dampers.

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