Chemistry lecture 35

INSTRUMENTAL CHEMICAL ANALYSIS

1. INTRODUCTION: BASIC PRINCIPLES AND TECHNIQUES

The need of the sophisticated analytical instruments and determinations using them is almost a routine process for the modern chemical laboratories. It has been a vast expanding area of knowledge as the instrument and computer manufacturers are producing analytical machines, which are in ever-increase of power and scope.

Basically, chemical analysis can be divided into three broad categories as given below, which are  almost  invariably  applied  to  major  areas  such  as  Fundamental  Research,  Product Development, Product Quality Control, Monitoring & Control of Pollutants, Medical & Clinical Studies, etc:

QUALITATIVE ANALYSIS:

Chemical analysis which just identifies one or more species present in a sample

QUANTITATIVE ANALYSIS:

Chemical analysis which finds out the total amount of the particular species present in a sample

STRUCTURAL ANALYSIS:

Chemical analysis which helps in finding the spatial arrangement of atoms in a molecule and the presence or position of certain organic functional groups in a given compound

Chemical analysis has some basic steps like, choice of method, sampling, preliminary sample treatment, separations, final measurement and assessment of results. It is with the first step viz. choice of method, care should be exercised to select the proper instrument to carry out fruitful analysis. A wrong selection at this point will lead to a meaningless analysis. Selection of the instrument is such important criteria!

2.  CLASSIFICATION OF THE ANALYTICAL TECHNIQUES

In a broad sense the techniques for the chemical analysis can be classified as follows:

Analysis through spectroscopy

Analysis through chromatography

Analysis through thermal energy

Analysis through x-ray techniques

Analysis through microscopy

Analysis through electrochemical techniques

Analysis through miscellaneous techniques

This classification is based on the interactions of molecules with various forms of energy like
electro-magnetic radiation, heat (thermal energy) and with matters like electrons. Each technique has specific principle, mode of operation, advantages and disadvantages.

3. Nuclear Magnetic Resonance Spectroscopy (NMR):

Principle: In NMR substances absorb energy in the radio frequency region of the electromagnetic spectrum under influence of a strong magnetic field.

Applications: The application lies mostly in the identification and structural analysis of organic compounds and thus, it is mostly a tool for qualitative analysis. It gives valuable information regarding the position of the functional groups in a molecule and provides distinguished spectra for the isomer. Much precise information on the structure of the compounds can be obtained using the same technique with other magnetic nuclei like C¹³, O¹⁷, the instrumentation being the same except that the sweep of the magnetic field is varied.

Disadvantages: Very expensive and the instrumentation is complex and needs exceptional skills to operate.  Its  sensitivity  ranges  from  moderate  to  poor,  however,  can  get  clear information using C¹³ or O¹⁷ NMR. The usage of the solvents is limited and in most of the situations deuterated solvents are required.

4. ANALYSIS THROUGH CHROMATOGRAPHY

The technique through which the chemical components present in complex mixtures are separated, identified and determined is termed as chromatography. This technique is widely used like spectroscopy and is a very powerful tool not only for analytical methods but also for preparative methods. Compounds of high grade purity can be obtained by this method. Chromatography can be simply defined as follows:

Based on the mobile phase this technique can be simply classified into two categories as:
Liquid Chromatography and Gas Chromatography. The column which holds the stationary phase (which in the form of small particles of the diameter of the order in microns), plays unique role in these processes. Usually silica is the base material for producing this phase.

4.1 LIQUID CHROMATOGRAPHY (LC/HPLC)

Principle: Early liquid chromatography was carried out in long glass columns with wide diameter. The diameters of the stacked particles inside the column were of the order of 150-200 microns range. Even then, the flow rates (eluent time) of the mobile phase with the analyte were very slow and separation times were long - often several hours!

The HPLC technique can be divided into four main categories depending on the nature of the processes that occur at the columns as follow:

4.1.1 High-Performance Adsorption Chromatography: Here the analyte species (components to be analysed) are adsorbed onto the surface of a polar packing. The stationary phase consists of finely divided solid particles packed inside a steel tube. If the component mixture is eluted through this tube with the mobile phase, different components present in the mixture adsorb to different degrees of strength and they become separated as the mobile phase moves steadily through the column.

4.1.2  High-Performance  Partition  Chromatography: It is the most  widely  used  liquid chromatographic  procedures  to  separate  most  kinds  of  organic  molecules. Here the components present in the analyte mixture distribute (or partition) themselves between the mobile phase and stationary phase as the mobile phase moves through the column. The stationary phase actually consists of a thin liquid film either adsorbed or chemically bonded to the surface of finely divided solid particles.

It finds wide applications in various fields, viz., pharmaceuticals, bio-chemicals, food products, industrial chemicals, pollutants, forensic chemistry, clinical medicine, etc.

GAS CHROMATOGRAPHY (GC)

Principle: Here an inert carrier gas (Helium or Nitrogen) acts as the mobile phase. This will carry the components of analyte mixture and elutes through the column. The column usually contains an immobilized stationary phase. The technique can be categorised depending on the type of stationary phase as follow:

Gas Solid Chromatography (GSC) - here the stationary phase is a solid which has a large surface area at which adsorption of components of the analyte takes place. The separation is possible based on the differences in the adsorption power and diffusion of gaseous analyte molecules. The application of this method is limited and is mostly used in the separation of the low-molecular-weight gaseous species like carbon monoxide, oxygen, nitrogen and lower hydrocarbons.

Gas Liquid Chromatography (GLC) - this is the most important and widely used method for separating and determining the chemical components of volatile organic mixtures. Here the stationary phase is a liquid that is immobilized on the surface of a solid support by adsorption or by chemical bonding. The separation of the mixture into individual components is by distribution ratio (partition) of these anayte components between the gaseous phase and the immobilized liquid phase. Because of its wide applications most of the GCs are configured for the GLC technique.