The baseline noise may appear to be greater after a few hours, because the column may be becoming more ‘worn’ as more injections are being made. Also, if a column heater/chiller is not being used, there may be an increase in ambient temperature during the day, which would give rise to greater column/sample reactions.
Baseline noise is the short term deviation of the baseline from a straight line, caused by electric signal fluctuations, lamp instability, temperature fluctuations and other factors. Noise normally has a much higher frequency than an actual chromatographic analyte peak. Noise is normally measured "peak-to-peak", this is the distance from the top noise 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 analyst has to distinguish between noise and analyte peaks. A practical limit for this is around 3 x signal-to-noise ratio for qualitative purposes. A practical quantitative detection limit may be 10x signal-to-noise ratio. This should ensure accurate and precise quantification of the trace analytes to a variation of less than 2%. The following figure indicates the noise level of a baseline (measured at highest detector sensitivity) and the smallest analyte peak which can be absolutely detected.
Mathematically, Peak to Peak Noise is defined to be the algebraic difference of the maximum and minimum residuals between each data point and the least-square line. The “residual” is determined by subtracting the y value of the data point predicted by the line from the y value of the data point.
The formula for Peak to Peak Noise is:
Peak to Peak Noise = max residual – min residual
where:
residual = (yi – ypi)
yi is the y value of the data point
ypi is the y value of the data point predicted by the line
Peak to Peak Noise is expressed in detector units. For example, the peak to peak noise
Signal to Noise Ratio- Divide the peak height of a substance of known concentration by the surrounding noise peak levels.
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