A process for operating a polymerization reactor system, the process comprising: contacting a catalyst system comprising a first transition metal compound, a second transition metal compound, an activator wherein the activator comprises an aluminoxane compound, an organoboron, an organoborate compound, an ionizing ionic compound, a solid oxide treated with an electron-withdrawing anion or any combination thereof, and an optional co-catalyst, with an olefin monomer and an optional olefin comonomer in a reactor within the polymerization reactor system under polymerization reaction conditions to produce an olefin polymer; determining a concentration of the first transition metal compound in a solution comprising the first transition metal compound and the second transition metal compound, the concentration determined via the steps of: (i) submitting a sample of the solution to a sample chamber; (ii) irradiating the sample in the chamber with a light beam at a wavelength or a range of wavelengths in the UV-visible spectrum; and (iii) generating a sample absorbance profile of the sample, subtracting a reference absorbance profile of the second transition metal compound in a reference solution from the sample absorbance profile to result in a first transition metal compound absorbance profile, and correlating the first transition metal compound absorbance profile to a standard to determine the concentration of the first transition metal compound in the solution; and adjusting a flow rate of the first transition metal compound into the reactor when the concentration of the first transition metal compound in the solution has reached a predetermined level, wherein the solution comprising the first transition metal compound and the second transition metal compound is: a feed stream to a catalyst preparation vessel; a liquid or homogeneous catalyst system; a solution prepared from a heterogeneous or supported catalyst system; or a solution prepared from a sample mixture from the reactor. contacting a catalyst system comprising a first transition metal compound, a second transition metal compound, an activator wherein the activator comprises an aluminoxane compound, an organoboron, an organoborate compound, an ionizing ionic compound, a solid oxide treated with an electron-withdrawing anion or any combination thereof, and an optional co-catalyst, with an olefin monomer and an optional olefin comonomer in a reactor within the polymerization reactor system under polymerization reaction conditions to produce an olefin polymer; determining a concentration of the first transition metal compound in a solution comprising the first transition metal compound and the second transition metal compound, the concentration determined via the steps of: (i) submitting a sample of the solution to a sample chamber; (ii) irradiating the sample in the chamber with a light beam at a wavelength or a range of wavelengths in the UV-visible spectrum; and (iii) generating a sample absorbance profile of the sample, subtracting a reference absorbance profile of the second transition metal compound in a reference solution from the sample absorbance profile to result in a first transition metal compound absorbance profile, and correlating the first transition metal compound absorbance profile to a standard to determine the concentration of the first transition metal compound in the solution; and (i) submitting a sample of the solution to a sample chamber; (ii) irradiating the sample in the chamber with a light beam at a wavelength or a range of wavelengths in the UV-visible spectrum; and (iii) generating a sample absorbance profile of the sample, subtracting a reference absorbance profile of the second transition metal compound in a reference solution from the sample absorbance profile to result in a first transition metal compound absorbance profile, and correlating the first transition metal compound absorbance profile to a standard to determine the concentration of the first transition metal compound in the solution; and adjusting a flow rate of the first transition metal compound into the reactor when the concentration of the first transition metal compound in the solution has reached a predetermined level, wherein the solution comprising the first transition metal compound and the second transition metal compound is: a feed stream to a catalyst preparation vessel; a liquid or homogeneous catalyst system; a solution prepared from a heterogeneous or supported catalyst system; or a solution prepared from a sample mixture from the reactor. of the first transition metal compound into the reactor when the concentration of the first transition metal compound in the solution has reached a predetermined level, wherein the solution comprising the first transition metal compound and the second transition metal compound is: a feed stream to a catalyst preparation vessel; a liquid or homogeneous catalyst system; a solution prepared from a heterogeneous or supported catalyst system; or a solution prepared from a sample mixture from the reactor.