**7. Limit of detection (LOD) and limit of quantity (LOQ)**

Validation is crucial in analytical procedures as it assesses the method's sensitivity. The limit of detection (LOD) is the lowest concentration or quantity of a substance that can be detected with sufficient probability using a specific analytical technique. LOD is the mean blank value plus three times the standard deviation, while LOQ is the lowest concentration or quantity that can be quantified with adequate accuracy and precision. There are no studies on the sensitivity of thin-layer chromatography (TLC) in terms of the effect of mobile and stationary phases on LOD and LOQ of fluoxetine and sertraline.

$$\text{LOD} = \frac{\text{3.3}\sigma}{\text{S.}}\tag{1}$$

The limit of quantification (LOQ) is calculated using the following formula:

$$\text{LOQ} = \frac{\mathbf{10}\,\sigma}{\mathbf{S}} \tag{2}$$

Where: σ = standard deviation and S = slope of the calibration curve. Knowing the LOD allows for a rapid calculation of the LOQ value, which is given as follows:

$$\text{LOD} = \mathfrak{Z} \cdot \text{LOQ} \tag{3}$$

Correctly estimated limits of detection must adhere to the following assumptions:

$$\text{10 }\text{-LOD} > \text{C and LOD} < \text{C.} \tag{4}$$

Where C = analyte concentration in the standard samples (**Figure 3**).

Linear calibration curves have well-defined limits, which are essential for sensor measurements. So to determine concentrations from signals, one must discriminate between the signal axis (Y) and the concentration axis (X). The blank and calibration curves represent distribution functions for limit of decision and sample measurements, respectively. They overlap within a statistically defined limit, resulting in the lowest value on the calibration curve with LOD, which is the minimal detectable value by projection to the X-axis in concentration [63–65].

## **Figure 3.**

*Limit of detection (LOD) and limit of quantity (LOQ) graph as a sample, where signal versus concentration are discussed.*
