JNTU B.Pharmacy IV INSTRUMENTAL METHODS OF ANALYSIS Previous Question Paper (September-2021 - R17)

For measuring low-intensity transmitted radiation in spectrophotometry, the Photomultiplier Tube (PMT) is the most sensitive detector. It consists of a photo-emissive cathode, a series of dynodes, and an anode, all enclosed in an evacuated glass envelope.

The working principle is based on the photoelectric effect and secondary electron emission:

• Incident photons strike the photo-emissive cathode, causing the emission of electrons.
• These primary electrons are accelerated toward a series of dynodes, each maintained at a higher positive potential than the previous one.
• When an electron strikes a dynode, it releases multiple secondary electrons, creating an electron cascade.
• This process repeats across several dynodes, resulting in an amplification factor of up to 10^6 to 10^7.
• The final pulse of electrons is collected at the anode, producing a measurable current proportional to the intensity of the incident radiation.

Due to this high internal gain, PMTs are exceptionally effective at detecting very low light levels with high signal-to-noise ratios.

The Simultaneous Equation Method (Vierordt’s method) is used for the analysis of two drugs in a mixture when their absorption spectra overlap. It relies on the principle of additivity of absorbances, where the total absorbance of a mixture at a specific wavelength is the sum of the individual absorbances of the two components.

The method requires determining the absorptivity values for both drugs at two selected wavelengths, λ1 and λ2. The concentrations are calculated using the following equations:

Where Cx and Cy are concentrations of drugs X and Y, A1 and A2 are absorbances of the mixture at λ1 and λ2, and ax1, ax2, ay1, ay2 are absorptivities of X and Y at the respective wavelengths.

The necessary conditions for this method include:

• The two drugs must not react chemically with each other.
• Each drug must obey the Beer-Lambert Law at the selected wavelengths.
• The wavelengths chosen (λ1 and λ2) should ideally be the λmax of each drug respectively to maximize sensitivity.
• The ratio of absorptivities of the two drugs at the selected wavelengths must be significantly different (i.e., ax1/ax2 ≠ ay1/ay2) to ensure the equations are solvable and provide accurate results.