X axis: diffferent concentrations of DNA
y AXİS: current
I have avarage standart deviation value: 0.29
My equation: y=0,95 x+0.4467 R2=0,9928
Also, what is the difference between sensitivity and detection limit?
This is true, the LOD is three times the noise and LOQ 10. However, your intercept (0.4467) is to high to be considered zero because this means that for zero DNA concentration you will get a current of 0.4467. An explication for this can be the concentration range used, which probably is to high. From your equation LOD=3x0.29/0.95=0.91 (units of concentration). Probably, a concentration of 0.8 can be detected, but the current value will be bellow the calibration slope. On the another hand, LOQ~3LOD, so if you r first concentration of DNA will be 1, the next one should be 3, or a bit more. If the next one will be 25, and the last concentration 500, the intercept value will be very high
The sensitivity is the slope of your linear (or other) concentration dependence (so if its not linear it might be different for different concentrations). The detection limit is the lowest concentration you can distinguish from noise. So one simple way to determine it is taking 3 times the standard deviation of the blank.
You can use the common formula of LOD=3 SD/Slope and LOQ=10 SD/Slope, where SD is the standard deviation from the blank measurement. and Slope from the calibration curve of the standard.
This is true, the LOD is three times the noise and LOQ 10. However, your intercept (0.4467) is to high to be considered zero because this means that for zero DNA concentration you will get a current of 0.4467. An explication for this can be the concentration range used, which probably is to high. From your equation LOD=3x0.29/0.95=0.91 (units of concentration). Probably, a concentration of 0.8 can be detected, but the current value will be bellow the calibration slope. On the another hand, LOQ~3LOD, so if you r first concentration of DNA will be 1, the next one should be 3, or a bit more. If the next one will be 25, and the last concentration 500, the intercept value will be very high
Agree with Enache. Actually, the calculation of LOD with the equation LOD=3xSD/slope is work only when the intercept is zero.If the intercept is zero, the linear equation for analysis is expressed as Y=aX. Thus the calculation of LOD means when the signal is 3xSD, what is the concentration of target. Based on this concept, the LOD for any method can be obtained with a equation derevied from the analysis equation. Therefore, the LOD of your method is calculated from LOD=(3x0.29-0.4467)/0.95. The LOQ is obtained similarly.
I am also confused about the defference between sensitivity and LOD. The sensitivity is the signal change corresponding to the change of target in textbood. But we also say one method is sensitive when the LOD is low. So, is the later comment wrong?
It is not clear, which SD you mention. If we calculate the regression line from x & y data, there will be at least two SD, SD of the slope and SD of the intercept. Such as mentions above, general formula to get the LOD using the linear regression is LOD = 3 SD / slope (in another paper they write 3.3 SD / slope)
the SD can be defined in two ways:
1. SD of the blank measurement (usually 20 blank measurement), or
2. Standard deviation of y-intercepts of regression lines
A good pair of textbooks to get insights about your problem are "Statistics and Chemometrics for Analytical Chemistry" by Miller and Miller (Chapter 5 in the 6th Edition) and Fundamentals of Analytical Chemistry by Skoog (Section 8D in the 9th Edition). I think those books are pretty comprehensive in the topics you are looking for.
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