The chemistry of the analytical method should be explained.
Quenching is the process of nonradiative energy transfer from an excited species to another molecule. This requires a contact between the excited species and a quenching agent. The rate of quenching is higher when the quencher concentration is high, because of higher number of collisions during the lifetime of the excited state.
A plot of data should be constructed.
Standard addition method is used to analyze complex samples which matrix effect is significant. One common approach is standard addition technique is spiking of sample. In this method, one or more increments of a standard solution is added to the several aliquots of sample containing same volume. Each solution is then diluted to a fixed volume and experiment is proceeded.
A relationship for multiple standard additions should be derived. An equation for the unknown concentration should be obtained in terms of the slope and the intercept of the standard addition plot.
Several aliquots with volume Vx of an unknown solution which have a concentration Cx are measured into volumetric flasks with volume Vt and then a standard solution with concentration Csis added in variable volumes of Vs. Then other reagents that aid for the detection are added and each volumetric flask is topped up to the maximum volume. Then instrumental measurements are made on each solution. If the instrumental response is proportional to the concentration of analyte, following equation can be derived.
k is a proportionality constant. According to above equation the instrumental signal versus Vs is a plot in the form of , where,
Concentration of unknown then can be obtained by following equation.
The equation for the line representing the decrease in fluorescence relative to volume of standard should be determined.
In the least squares method, in order to find the slope and the intercept the three quantities Syy, Sxx and Sxy should be determined.
The slope of the line,
- mean of y values
- mean of x values
The standard deviation of the slope and the intercept should be determined.
Standard deviation about regression,
N − number of points used.
The standard deviation of the slope,
The standard deviation of the intercept,
The concentration of F- in the sample in parts per billion should be determine.
Cx − concentration of the unknown solution
Cs − concentration of standard solution
Vx − volume of unknown solution
m − slope of the instrumental signal versus volume of standard added curve
b − intercept of the instrumental signal versus volume of standard added curve
The standard deviation of the result in (f) should be determined
Sv − standard deviation in volume
Sy − standard deviation in signal
N − number of points
Sc − standard deviation in concentration
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