Question

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Low concentrations of thallium near the detection limit gave the following dimensionless instrument readings: 206.9,173.7.163.9,175.9, 220.9.161.7, 224.7, 203.3,136.3, and 207.1. Ten blanks had a mean reading of 42.1. The slope of the calibration curve is 1.76 times 10^9M^-1. Estimate the signal and concentration detection limits and the lower limit of quantitation for thallium. Signal Detection Limit: Concentration Detection Limit: Lower Limit of Quantitation:

Answer 1

1)Determine absolute limit of detection. Reduce the concentration or intensity of the standard. Input a smaller signal or concentration until the analyte peak is about three times the height of your average noise peak. This intensity or concentration is the absolute limit of detection.

2)Determine the quantification limit of detection. Increase your input intensity or concentration to the point the peak height is 10 times the average noise peak. This is the lowest concentration for which you can reasonably state the concentration or intensity of the analyte.

In analytical chemistry, the detection limit, lower limit of detection, or LOD (limit of detection), is the lowest quantity of a substance that can be distinguished from the absence of that substance (a blank value) within a stated confidence limit (generally 1%).

The detection limit is estimated from the mean of the blank, the standard deviation of the blank and some confidence factor. Another consideration that affects the detection limit is the accuracy of the model used to predict concentration from the raw analytical signal.

LOD (limit of detection)
The detection limit of an individual analytical procedure is the lowest amount of analyte in a sample which can be detected but not necessarily quantitated as an exact value. Several approaches for determining the detection limit are possible.
Based on visual evaluation:

The detection limit is determined by the analysis of samples with known concentrations of analyte and by establishing the minimum level at which the analyte can be reliably detected.

Based on Signal-to-Noise Approach: Determination of the signal-to-noise ratio is performed by comparing measured signals from samples with known low concentrations of analyte with those of blank samples and by establishing the minimum concentration at which the analyte can be reliably detected.

A signal-to-noise ratio between 3 or 2:1 is generally considered acceptable for estimating the detection limit.
Based on the Standard Deviation of the Response and the Slope: The detection limit (DL) may be expressed as:=3X STANDRD DEVIATION OF LOW CONCEN/ SLOPE OF THE CALIBRATION LINE