Limit of Detection

The Limit of Detection (LOD) for this assay using Photopette® is determined by factoring in the standard-deviation for blank measurements as well as experimental data using the equation given below:

LOD = 3 x SDblank / Slope(Standard Curve)

Standard Deviation for blank measurements with 50 repeats using same CuveTip was found to be 0.003 AU and 0.001 AU for 570 nm and 850 nm respectively.

For 570 nm: LOD = 3 x (0.003 OD) / (0.002 OD/NTU)

≈ 4.5 NTU

For 850 nm: LOD = 3 x (0.001 OD) / (0.001 OD/NTU)

≈ 3 NTU

DETERMINING UNKNOWN TURBIDITY OF SAMPLES

For 570 nm, a water sample was collected and the average of triplets resulted in an OD of 0.591. The linear regression of the turbidity standards of Figure 3 is:

y= 0.002 x + 0.0221

Re-arranging the “x” in the equation results in:

X = (y -0.021)/0.002

Inserting the OD of the unknown sample results in

X = (0.591 -0.021)/0.002

X = 285 NTU.

For 850 nm, a water sample was collected and the average of triplets resulted in an OD of 0.257. The linear regression of the turbidity standards of Figure 4 is:

y= 0.001x + 0.001

Re-arranging the “x” in the equation results in:

X = (y -0.001)/0.001

Inserting the OD of the unknown sample results in

X = (0.257 -0.001)/0.001

X = 256 NTU.

LIMITATIONS

Suspended particles in a turbid solution may tend to settle down. Therefore, it is very important the solution is well-mixed and the readings are taken as soon external agitation is removed.