Difference between Sensitivity and Limit of Detection in Analytical Chemistry? Are they same word? Let us know here.

Sensitivity and Limit of Detection (Detection Limit)

Many times the sensitivity and limit of detection are regarded as synonymous and used interchangeably, let us find out is it correct?

Sensitivity indicates the capacity of the method to differentiate between two very

close concentrations of analyte. Like, can a given method distinguish between

0.11 ppb and 0.12 ppb or not. 

Whereas detection limit (or Limit of Detection) is a measure of the

smallest concentration which can be determined with a specified confidence level. 

Let us see the following graph (Fig.1). Slope of Chloride ion calibration graph is higher

compared to phosphate ion. So the analytical instrument used in this case

(Ion Chromatography with conductivity detector) is more sensitive to Chloride ion

than phosphate ion.  

Fig.1. Calibration graph for Chloride and Phosphate ion with Ion Chromatography

with conductivity detector

Table 1 lists the slope and error in the intercepts from the fitted calibration graphs

of Fig.1.

Ion	Slope	Error in Intercept
Chloride	6x10-3	9x10-3
Phosphate	6x10-4	3x10-3

Sensitivity can be calculated from the slope of the calibration graph using Table 1 and Eqn.1. 

Sensitivity = Slope = Δ(Peak Area)/ Δ(Conc.) (Eqn.1)

Assuming 99.9% Confidence level, Limit of Detection (LOD) can be calculated using

Table 1 and Eqn.2

LOD = 3.3 x Error in Intercept/ slope (Eqn.2)

Table 2: Sensitivity and LOD values calculated for Chloride and Phosphate ions

from Table 1.

Ion	Sensitivity	LOD (ppb)
Chloride	6x10-3	4.95
Phosphate	6x10-4	16.5

So the instrument is more sensitive to chloride ion compared to phosphate ion (Table 2).

Let us see what sensitivity actually means for each ion using Eqn.3. Suppose we can

measure 0.001 units of peak area for both the peaks. 

Δ(Conc.) (ppb) = Δ(Peak Area)/ Sensitivity (Eqn.3)

Table 3: Δ(Conc.) (ppb) values calculated for Chloride and Phosphate ions for the

smallest measurable signal of 0.001 unit peak area.

Ion	Sensitivity	Δ(Conc.) (ppb)
Chloride	6x10-3	0.2
Phosphate	6x10-4	1.7

                        

So it is proven here (Table 3) that for chloride ions we can distinguish between two concentrations with a difference of 0.2 ppb i.e. 20.0 and 20.2 ppb can be differentiated. 

But for Phosphate ion 20 and 20.2 ppb can not be differentiated but 20 and 21.7 ppb only can be differentiated. 

The values for sensitivity are also quite different from LOD values for each ion. 

Conclusion: 

• Two terms Sensitivity and Detection Limit are not synonymous and should not be used interchangeably.

• Methods for estimating both Sensitivity and Detection Limit is shown in detail. 

Reference

[1]Skoog, Douglas A., et al. Fundamentals of analytical chemistry. Brooks Cole Cengage

Learning, 9th Ed, 2014.

[2] Valcárcel, Miguel. Principles of analytical chemistry: a textbook. Springer Science &

BusinessMedia, 2012.

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