CHEM3440Lec4F06 - CHEM*3440 Chemical Instrumentation Topic...

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Unformatted text preview: CHEM*3440 Chemical Instrumentation Topic 4 Signal vs. Noise Signal In All experiments there is: Sample Response: the instrument's response when the analyte is present. Blank Response: the instrument ! s response when the analyte is absent. The Signal is the difference between the sample and the blank response. Output Response Time Blank Blank Sample Sample Signal Background or Baseline Ideally, the blank response would be exactly zero. Signal would then be equal to the sample response. This is never the case, but it can often be adjusted to be close to zero. There is always a residual signal associated with the instrument ! s blank response. This is called the background or the baseline . Output Response Time Blank Blank Sample Sample Signal Baseline Drift Ideally the baseline response is constant in time. However, baseline changes slowly with time. This is called drift . Sometimes the drift is linear in time, but often it is more complex and difFcult to predict. Output Response Time Blank Blank Sample Sample Signal Baseline Noise Noise is a random(or almost random) time-dependent change in the instrument ! s output signal that is unrelated to the analyte response. These variations will tend to make the accurate measurement of sample, blank, and baseline response less certain. Noise can arise from many sources (to be discussed soon). The frequency of the noise response can span the entire spectrum. We can treat noise as if it were a sine wave, or the sum of many (initie?) sine waves. Measuring the intensity of the noise and comparing it to the signal is the key to determining the accuracy of a measurement and in specifying the smallest signal level one is able to measure (detection limit). Peak-to-Peak Noise One measure of the amplitude of a sine wave is the peak-to-peak amplitude ( twice the amplitude which appears in the deFning equation for a sine wave). Noise is usually speciFed by measuring the peak-to-peak noise maximum found over a reasonable length of time (reasonable depends upon length of time needed to make a measurement).-4-3-2-1 1 2 3 4 2 4 6 8 10 12 14 16 18 20 V(peak-to-peak) or V p-p p-p Noise 2 Even though the noise is clearly not a perfect sine wave, we know it can be decomposed into a sum of many sine waves and we can treat it mathematically as a sine wave. V p-p Average Noise? Another way of measuring the intensity of noise might be the average noise. If noise were truly random, then N average = 0. (Excursions above zero should balance excursions below zero over time). If noise were not 0, then another signal must be present and we would need to account for it. Therefore, average noise is NOT a useful measure. Root-Mean-Square Noise For average noise, it was the cancelling of the positive and negative excursions that made N average not a useful measure....
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CHEM3440Lec4F06 - CHEM*3440 Chemical Instrumentation Topic...

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