Noise and drift

In HPLC we deal with the time-dependent process. The appearance of the component from the column in the detector represented by the deflection of the recorder pen from the baseline. It is a problem to distinguish between the actual component and artifact caused by the pressure fluctuation, bubble, compositional fluctuation, etc. If the peaks are fairly large, one has no problem in distinguishing them. However, the smaller the peaks, the more important that the baseline be smooth, free of noise, and drift.

Baseline noise is the short time variation of the baseline from a straight line caused by electric signal fluctuations, lamp instability, temperature fluctuations and other factors. Noise usually has much higher frequency than actual chromatographic peak. Noise is normally measured "peak-to-peak": i.e., the distance from the top of one such small peak to the bottom of the next. Sometimes, noise is averaged over a specified period of time. Noise is the factor which limits detector sensitivity. In trace analysis, the operator must be able to distinguish between noise spikes and component peaks. A practical limit for this is a 3 x signal-to-noise ratio, but only for qualitative purposes. Practical quantitative detection limit better be chosen as 10x signal-to-noise ratio. This ensures correct quantification of the trace amounts with less than 2% variance. Figure below illustrates this, indicating the noise level of a baseline(measured at highest detector sensitivity) and the smallest peak which can be unequivocally detected.

Definition of noise, drift, and smallest detectable peak.

Another parameter related to the detector signal fluctuation is drift. Noise is a short-time characteristic of a detector, an additional requirement is that the baseline should deviate as little as possible from a horizontal line. It is usually measured for a specified time, e.g., 1/2 hour or one hour. Drift usually associated to the detector heat-up in the first hour after power-on. Figure also illustrates the meaning of drift.