I have a function of type 

$$C(t)=0.31 \left(e^{-0.267t}-e^{-1.72t}\right)$$

Where $t$ is in hours

The function represents the evolution in a window of 24 hours of a drug blood concentration, after a dose of a drug is given at $t = 0$ to a patient. I would like to use Fourier transform, to identify th best sampling step to reconstruct the concentration curve. Because it is a human whose blood we are sampling here I would like to reduce the sampling as much as possible, with a reconstruction error of around 20%. 

I am using Matlab FFT function to compute the function's Fourier transform and plot the spectrum.

However I am not sure if my frequency are in Hertz or Millihertz. Because I am sampling in hours, I am expecting lower frequencies (millihertz), but I am not sure of the logic.

Here is my Matlab Code

```
clear all; clc;close all;
%% Time-domain signal
Ts=0.25; % time discretization step
t=0:Ts:24 % in hours
C=0.31* (exp(-0.267*(t))-exp(-1.72*(t)));

figure,plot(t,C);

title('Time domain signal');
xlabel('Time in hours ')

%Frequency domain signal Via Matlab FFT function
F2=fft(C);
L=length(F2);
figure,plot((0:L-1)*(1/(L*Ts)),abs(F2));
title('Frequency domain signal via FFT of C(t)');
xlabel('Frequency in Hz or Millihertz?')

```