Question

a) what factors limit the frequency range covered by a fourier transform spectrometer

b) Why are molecules particulary bad sources of spectral interfrence in atomic spectroscopy

Answer 1

(a) The important factors that affect any spectrum collected through FT-NMR spectrometer

1. Spectrometer Frequency 2. Acquisition time 3. Pulse Width 4. Recycle Delay 5. Number of Points 6.Sweep Width  

(1) Spectrometer Frequency: The Radio frquency pulse has a characteristic frequency, called the spectrometer frequency , which is dependent upon the nucleus you wish to observe and the magnetic field strength of the spectrometer. NMR spectrometers named accorind to the the frequency at which hydrogen atoms resonate.

(2)Acquisition time:The time it takes to acquire the  Free Induction Decay is called the acquisition time.’. For most nuclei of relaxation times are within the range of 10-1 to 10 seconds.

(3)Pulse Width:Application of a short radio frequency pulse at the appropriate frequency will rotate the bulk magnetization by a specific angle. Pulses are generally described by this angle of rotation.The angle of rotation is dependent on the intensity of the pulse and the width of the pulse.

(4) Recycle Delay:It is an important factor and plays a role in obtaining accurate integration.

(5) Number of Points:The tiny analog signal emitted from the sample (in microvolts) is amplified, mixed, filtered, and attenuated prior to digitization, which is required for further computer processing.The digital converter converts the analog FID into a series of discrete points along the FID signal envelope. This is the number of points . In general, the more points used to define the FID, the higher resolution.

(6) Sweep Width :The frequency domain spectrum has two important factors associated with it; the spectrometer frequency and the spectral width ( sweep width). The spectral width in Hz will change depending upon the spectrometer used. Based on definitions and the rule of digitalization, that if the spectral width in the spectrum, is increased, the acquisition time in the FID, decreases for a fixed number of points.

(b)Spectral interferences are caused by the presence of another atomic absorption line or a molecular absorbance band close to the spectral line of element of consideration . Common spectral interferences are due to molecular emissions .That is mainly from the oxides of other element present in the sample.

Molecular species may scatter light over a wide wavelengths.When this type of non-specific adsorption overlaps the atomic absorption of the analyte, background absorption occurs.

Using background correction method, this problem can be reduced.A deuterium source lamp is used for background correction which comes already fitted into the Atomic absorption instrument by Manufacturer.
Measuring and subtracting the background absorption from the total measured absorption to determine the true atomic absorption.