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 Joined: Wed Nov 13, 2019 7:07 pm
The formula of the mass error (ppm) is:
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theor_mz  measured_mz / theor_mz x 1e6
However, the formula says the opposite. What am I missing?
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theor_mz  measured_mz / theor_mz x 1e6
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v(10Da) = sqrt(1/10) = 0.3162278 m/s
v(10.1Da) = sqrt(1/10.1) = 0.3146584 m/s
t(10Da)  t(10.1Da) = 0.3162278  0.3146584 = 0.0015694 s
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v(100Da) = sqrt(1/100) = 0.1 s
v(100.1Da) = sqrt(1/100.1) = 0.09995004 s
t(100Da)  t(100.1Da) = 0.1  0.09995004 = 0.00004996 s
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dv/dm=  sqrt(E) / (2 * m * sqrt(m))
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theor_mz  actual_mz / (2*theor_mz * sqrt(theor_mz))
RSD is used to compare sets that can't be compared in absolute values. Like comparing an effect of a weight control drug on elephants and mice. Comparing their absolute SD doesn't make sense, so you normalize by their mean value.Just look at it as the normalized error of a measurment. Similar to the relative standard deviation.
This one I don't get.. They have the same accuracy (or better call it precision). What makes you switch to relative units? In the example with elephants & mice I wanted to compare the effect of the drug on one animal vs the other. But in this example why would you want to compare?Or just assume you have ruler with mm scale. Then you measure two distances 100 m and 1000 m. Each one will be accurate to +/ 1 mm. So how do you express which one is more accurate? > by means of the relative error
Here again  why is it relative to the measured mass? When I look at the mass spectra, I'd like to understand whether this particular measured value is my analyte or not. So I'll look how far the value from the theoretical value, and for this it's much easier to use +/ 0.015. I can see these values with my eyes on the chart, while ppm doesn't tell me anything..So you'll have some residuals and you want to know how big they are, related to the measured mass. And it's easier to say/write 1500 Da +/ 10 ppm than (1500 +/ 0.015) Da
I used length=1m in the calculations (though forgot about 1/2 coefficient).BTW: your formulas for the time are erroneous faulty. The difference of velocity won't give time...
Not only.RSD is used to compare sets that can't be compared in absolute values. Like comparing an effect of a weight control drug on elephants and mice. Comparing their absolute SD doesn't make sense, so you normalize by their mean value.
yeah maybe a missuse of the two terms..This one I don't get.. They have the same accuracy (or better call it precision). What makes you switch to relative units? In the example with elephants & mice I wanted to compare the effect of the drug on one animal vs the other. But in this example why would you want to compare?
Why not?Here again  why is it relative to the measured mass? When I look at the mass spectra, I'd like to understand whether this particular measured value is my analyte or not. So I'll look how far the value from the theoretical value, and for this it's much easier to use +/ 0.015. I can see these values with my eyes on the chart, while ppm doesn't tell me anything..
probably yes, depending on the accuracy. If both have an error of 1000 ppm, the A is significantly different but B is not.To make it concrete, you have 2 analytes A (100Da) and B (1000Da), and the measurements find 101Da and 1001Da. Would you say that A wasn't found, while B might actually be there?
Maybe, but they were not picked willynilly, there are some underlying physical reasons for that. It's not about mere (in)convenience. Even if the error is relative, the relationship isn't always linear! I'm trying to get to these underlying reasons in the Mass Spec (:You can also look at electronics components e.g. resistors and capacitors, they are usually specified by their relative error, not absolute ones.
You can convert absolute to relative and back, yes. But only as long as the values stay correct! It can't be a matter of taste if it's incorrect/misleading.It may also be a matter of taste how to describe the error. But the relative became more popular, because it expresses the precision of the measurement.
A conversion should never change the (base) values. Therefore they will stay correct.You can convert absolute to relative and back, yes. But only as long as the values stay correct!
relative error: https://doi.org/10.1351/goldbook.R05266Result of a measurement minus the true value of the measurand. Since a true value cannot be determined, in practice the conventional true value is used.
percentage relative error: https://doi.org/10.1351/goldbook.P04486Error of measurement divided by the true value of the measurand. Since a true value cannot be determined, in practice the conventional true value is used.
...just that it is not given in % but as ppmThe relative error expressed in percent. It can be calculated from the relative error by multiplying by 100. Comment: The term 'percentage relative error' should always be quoted in full, rather than 'error' or 'percentage error' to avoid confusion.
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