Comparative advantages of modern analytical techniques over classical techniques

What are Analytical tools and techniques?

Analytical chemistry tools and methods refer to techniques used for the detection, identification, characterization and quantification of chemical compounds. These methods are commonly used in biology for research, development and quality control of pharmaceutical products. In practice, separation, identification or quantification may constitute the entire analysis or be combined with another method. Separation isolates analytes. Qualitative analysis identifies analytes, while quantitative analysis determines the numerical amount or concentration.

The methods used allow quantitative or qualitative, more or less invasive and destructive analysis of samples, and typically require sophisticated instrumentation.

The properties analyzed are mass, chemical composition, molecular structure, radioactivity, interactions between molecules, etc. Applications range from the identification of molecules present in a sample to the validation of production methods.

Analytical chemistry consists of classical, wet chemical methods and modern, instrumental methods. Classical qualitative methods use separations such as precipitation, extraction, and distillation. Identification may be based on differences in color, odor, melting point, boiling point, solubility, radioactivity or reactivity. Classical quantitative analysis uses mass or volume changes to quantify amount. Instrumental methods may be used to separate samples using chromatography, electrophoresis or field flow fractionation. Then qualitative and quantitative analysis can be performed, often with the same instrument and may use light interaction, heat interaction, electric fields or magnetic fields. Often the same instrument can separate, identify and quantify an analyte.

Analytical chemistry is also focused on improvements in experimental design, chemometrics, and the creation of new measurement tools. Analytical chemistry has broad applications to medicine, science and engineering.

In this blog we are going to discuss about advantages of modern analytical tools and techniques over classical. We will be going to study both one by one.

Classical Techniques

Classical analysis, also termed wet chemical analysis, consists of those analytical techniques that use no mechanical or electronic instruments other than a balance. The method usually relies on chemical reactions between the material being analyzed (the analyte) and a reagent that is added to the analyte. Wet techniques often depend on the formation of a product of the chemical reaction that is easily detected and measured. For example, the product could be coloured or could be a solid that precipitates from a solution.

Classical Qualitative analysis

Classical qualitative analysis is performed by adding one or a series of chemical reagents to the analyte. By observing the chemical reactions and their products, one can deduce the identity of the analyte. The added reagents are chosen so that they selectively react with one or a single class of chemical compounds to form a distinctive reaction product. Normally the reaction product is a precipitate or a gas, or it is coloured. Take for example copper(II), which reacts with ammonia to form a copper-ammonia complex that is characteristically deep blue. Similarly, dissolved lead(II) reacts with solutions containing chromate to form a yellow lead chromate precipitate. Negative ions (anions) as well as positive ions (cations) can be qualitatively analyzed using the same approach. The reaction between carbonates and strong acids to form bubbles of carbon dioxide gas is a typical example.

Prior to the qualitative analysis of any given compound, the analyte generally has been identified as either organic or inorganic. Consequently, qualitative analysis is divided into organic and inorganic categories. Organic compounds consist of carbon compounds, whereas inorganic compounds primarily contain elements other than carbon. Sugar (C₁₂H₂₂O₁₁) is an example of an organic compound, while table salt (NaCl) is inorganic.

Classical Quantitative analysis

Classical quantitative analysis can be divided into gravimetric analysis and volumetric analysis. Both methods utilize exhaustive chemical reactions between the analyte and added reagents. As discussed above, during gravimetric analysis an excess of added reagent reacts with the analyte to form a precipitate. The precipitate is filtered, dried, and weighed. Its mass is used to calculate the concentration or amount of the assayed substance in the analyte.

Volumetric analysis: it is also known as titrimetric analysis. The reagent (the titrant) is added gradually or stepwise to the analyte from a buret. The key to performing a successful titrimetric analysis is to recognize the equivalence point of the titration (the point at which the quantities of the two reacting species are equivalent), typically observed as a colour change. If no spontaneous colour change occurs during the titration, a small amount of a chemical indicator is added to the analyte prior to the titration. Chemical indicators are available that change colour at or near the equivalence point of acid-base, oxidation-reduction, complexation, and precipitation titrations. The volume of added titrant corresponding to the indicator colour change is the end point of the titration. The end point is used as an approximation of the equivalence point and is employed, with the known concentration of the titrant, to calculate the amount or concentration of the analyte.

Gravimetric analysis: a method of quantitative chemical analysis in which the constituent sought is converted into a substance (of known composition) that can be separated from the sample and weighed. The steps commonly followed in gravimetric analysis are (1) preparation of a solution containing a known weight of the sample, (2) separation of the desired constituent, (3) weighing the isolated constituent, and (4) computation of the amount of the particular constituent in the sample from the observed weight of the isolated substance.

Errors made in gravimetric analyses usually relate to the purity of the isolated constituent. In general, the compounds that are precipitated are very insoluble, and negligible error results from the incompleteness of precipitation. Obtaining a precipitate that is 100 percent pure and is exactly of the composition represented by a chemical formula is, however, considerably more difficult. All gravimetric methods are subject to some degree of error because of this difficulty.

Use of Classical Techniques 

Various analytical techniques may be employed to obtain the chemical composition of a mineral. Quantitative chemical analyses mainly use so-called wet analytical methods (e.g., dissolution in acid, flame tests, and other classic techniques of bench chemistry that rely on observation), in which the mineral sample is first dissolved. Various compounds are then precipitated from the solution, which are weighed to obtain a gravimetric analysis. A number of analytical procedures have been introduced that provide faster but somewhat less accurate results.

To ensure an accurate chemical analysis, the selected sample, which might include several minerals, is often made into a thin section (a section of rock less than 1 mm thick cemented for study between clear glass plates). To reduce the effect of the impurities, an instrumental technique, such as electron microprobe analysis, is commonly employed. In this method, quantitative analysis in situ may be performed on mineral grains only 1

 micrometre (10⁻⁴ centimetre) in diameter.

Advantages of Classical Techniques

It's important to know the quantity of all or part of a sample for several reasons. If you're performing a chemical reaction, quantitative analysis helps you predict how much product to expect and to determine your actual yield. Some reactions take place when the concentration of one component reaches a critical level. For example, an analysis of radioactive material might indicate there is enough of a key component for the specimen to undergo spontaneous fission!

Quantitative analysis is crucial to the formulation and testing of food and drugs, as it is used to measure nutrient levels and provide an accurate accounting of dosage. It is also critical in determining the level of contaminants or the impurity of a sample. While qualitative analysis might be able to determine the presence of lead in the paint on a toy, for example, quantitative analysis detects how much concentration exists. Medical tests rely on quantitative analysis for information about a patient's health. For example, quantitative analysis techniques can determine blood cholesterol levels or the ratio of lipoproteins in plasma or the amount of protein excreted in urine.

Aside these volumetric and gravimetric analysis also plays an important role:

The major advantage of volumetric titration is that it is a simple and cost-effective method. Volumetric titration is not sophisticated, so skill is not required to handle it. Possible sources of error are readily checked in gravimetric analysis since filtrates can be tested for completeness of precipitation and precipitates may be examined for the presence of impurities. It is an absolute method; it involves direct measurement without any form of calibration being required. Gravimetric analysis was used to determine the atomic masses of many elements to six figure accuracy.

These are some highlighted benefits of classical analysis which still allow us to use these techniques in many fields.  

But with time and developing technology we need to find some new, more faster, and more accurate tools due to which these classical methods are more or less ignore due to some of their drawbacks.

Limitations of Classical Techniques

As an analyst, we are required to achieve results as close as possible to true values, by applying the correct procedures. Whether it's during lab work or while doing research. But in fact, errors in an analysis are still common. An understanding of the theory of error in chemical analysis becomes very important.

Limitations of Quantitative Analysis Methods

Before we start further, we need to know the factors that influence the limitations of the analytical methods, namely:

• Accuracy
• Accuracy (precision)
• Source of error
• Chemistry involved in the analysis process

Disadvantage of Gravimetric method:

•       The chief disadvantage is that it requires meticulous time consuming.
•       The chemist often prefers modern instrumental methods when they can be used.
•       Gravimetric analysis usually only provides for the analysis of a single element, or a limited group of elements, at a time.

Modern Analytical Techniques

Modern Analytical Chemistry is used in the analysis of light energy emitted by electrons, atoms, ions, or molecules at their ground state. Analytical chemistry methods refer to techniques used for the detection, identification, characterization and quantification of chemical compounds. These methods are commonly used in biology for research, development and quality control of pharmaceutical products. The methods used allow quantitative or qualitative, more or less invasive and destructive analysis of samples, and typically require sophisticated instrumentation. The properties analyzed are mass, chemical composition, molecular structure, radioactivity, interactions between molecules, etc. Applications range from the identification of molecules present in a sample to the validation of production methods.

Methods of Modern Analytical techniques

1)  1. Nuclear magnetic resonance spectroscopy (NMR) is one of the most powerful analytical tools for determining the molecular structure of an organic compound. Nuclear Magnetic Resonance (NMR) spectroscopy is an analytical chemistry technique used in quality control and research for determining the content and purity of a sample as well as its molecular structure.

 

• Excite the nuclei sample into nuclear magnetic resonance with the help of radio waves to produce NMR signals. These NMR signals are detected with sensitive radio receivers. The resonance frequency of an atom in a molecule is changed by the intramolecular magnetic field surrounding it.

• When molecules are placed in a strong magnetic field, the nuclei of some atoms will begin to behave like small magnets. ... The resonant frequencies of the nuclei are then measured and converted into an NMR spectrum that displays all of the right frequencies as peaks on a graph.

• It’s very expensive for routine analysis but is invaluable in designing and analysing new molecules or finding the structure of natural molecules that the drug industry might find useful in developing new pharmaceutical products.

2. Infra-red spectroscopy can help to determine molecular structure and identify an organic compound.

Each molecule has a 'fingerprint' pattern of absorption of different infrared frequencies.

Alcohols and ether structure and naming (c) doc b. The technique can be used to determine alcohol (ethanol) concentrations in breath - spectroscopic breathalyser! A sample of a molecules is scanned with a wide range of infrared wavelengths/frequencies.

Depending on the structure of the molecule, each wavelength/frequency is either transmitted without interaction or absorbed giving the 'dips' in the infrared spectrum.

Each infrared spectrum is different, so again we have a 'fingerprint' pattern that can be used for identification of quantitative analysis.

The fingerprint' infrared spectrum of ethanol ('alcohol') is shown below.

The 'dip' at 3000 cm-1 is due to the infrared absorption by the hydroxy group (the OH in CH3CH2OH).

The y-axis is transmittance - how much infrared radiation of that wavelength is allowed through unabsorbed.

The x-axis is the reciprocal of the wavelength in cm-1 (a convenient 'historic' scale).

3. Electroanalytical Chemistry

A group of quantitative analytical methods that are based upon the electrical properties (electrical response) of a solution of the analyte (chemical system) when it is made part of an electrochemical cell.

Chemical System: Electrolyte; measuring electrical circuit; Electrodes

Uses of Electroanalytical Chemistry

Electroanalytical techniques are capable of producing very low detection limits. Electroanalytical techniques can provide a lot of characterization information about electrochemically addressable systems.

– Stoichiometry and rate of charge transfer.

– Rate of mass transfer.                                                               

– Extent of adsorption or chemisorption.

– Rates and equilibrium constants for chemical reactions.

Advantages compared to other methods

Inexpensive.

Used for ionic species not total concentration.

Responds to ionic activity rather than concentration.

Ion selective electrodes and developing of the measuring devices in voltammetry made wider spread of the methods.

4. X-Ray Diffraction (XRD Analysis) 

X-ray diffraction analysis (XRD) is a technique used in materials science to determine the crystallographic structure of a material. XRD works by irradiating a material with incident X-rays and then measuring the intensities and scattering angles of the X-rays that leave the material. A primary use of XRD analysis is the identification of materials based on their diffraction pattern. As well as phase identification, XRD also yields information on how the actual structure deviates from the ideal one, owing to internal stresses and defects.

 How does it Work?

Crystals are regular arrays of atoms, whilst X-rays can be considered as waves of electromagnetic radiation. Crystal atoms scatter incident X-rays, primarily through interaction with the atoms’ electrons. This phenomenon is known as elastic scattering; the electron is known as the scatterer. A regular array of scatterers produces a regular array of spherical waves. In the majority of directions, these waves cancel each other out through destructive interference, however, they add constructively in a few specific directions, as determined by Bragg’s law:

2dsinθ = nλ

Where d is the spacing between diffracting planes, θ{\displaystyle \theta } is the incident angle, n is an integer, and λ is the beam wavelength. The specific directions appear as spots on the diffraction pattern called reflections. Consequently, X-ray diffraction patterns result from electromagnetic waves impinging on a regular array of scatterers.

X-rays are used to produce the diffraction pattern because their wavelength, λ, is often the same order of magnitude as the spacing, d, between the crystal planes (1-100 angstroms).

XRD Benefits and Applications

XRD is a non-destructive technique used to [2]:

• Identify crystalline phases and orientation
• Determine structural properties:
  
  - Lattice parameters
  - Strain
  - Grain size
  - Epitaxy
  - Phase composition
  - Preferred orientation
• Measure thickness of thin films and multi-layers
• Determine atomic arrangement

5. Thermal Analysis

The study of the relationship between a sample property and its temperature as the sample is heated or cooled in a controlled manner.

Thermal analysis is a general term defining a technique used to analyze the time and temperature at which physical changes occur when a substance is heated or cooled. Each technique is defined according to the types of physical changes being analyzed.

When evaluating material characteristics, it is necessary to use different techniques or a combination of multiple techniques depending on the purpose.

Several methods are commonly used – these are distinguished from one another by the property which is measured:

.Dielectric thermal analysis): dielectric permittivity and loss factor

.Differential thermal analysis: temperature difference versus temperature or time

.Differential scanning calorimetry: heat flow changes versus temperature or time

.Dilatometry: volume changes with temperature change

.Dynamic mechanical analysis: measures storage modulus (stiffness) and loss modulus (damping) versus temperature, time and frequency

.Evolved gas analysis: analysis of gases evolved during heating of a material, usually decomposition products

.Laser flash analysis: thermal diffusivity and thermal conductivity

.Thermogravimetric analysis: mass change versus temperature or time

.Thermomechanical analysis: dimensional changes versus temperature or time

It is usual to control the temperature in a predetermined way - either by a continuous

increase or decrease in temperature at a constant rate (linear heating/cooling) or by carrying out a series of determinations at different temperatures (stepwise isothermal measurements). 

Benefits of Modern Techniques

 Modern methods of instrumental analysis have evolved from the earlier classical methods which were dependent on measurement of mass or volumes. Such classical methods were time-consuming and were limited by errors both determinate and indeterminate.

Modern instrumental methods came to the aid of the analytical chemist by providing the following distinct benefits:

• Time saving – Analysis time has been reduced from several hours or days to even minutes

• Improvement in accuracy of results by elimination of errors introduced due to personal bias

• Improvement in sensitivity leading to trace analysis at ppt or even femto- mole levels. Such sensitivities could not be imagined using the conventional classical approach.

• Fast decisions in case of online analysis during manufacturing operations or deciding on viability before taking commercial decisions.

• A small amount of a sample is needed for analysis.

• Determination by instrumental method is considerably fast.

• Complex mixture can be analyzed either with or without their separation.

• Sufficient reliability and accuracy of results are obtained by instrumental method.

• When non-instrumental method is not possible, instrumental method is the only answer to the problem.

REFERENCES

• Braun, Robert Denton. "Chemical analysis". Encyclopedia Britannica, 1 Apr. 2016, https://www.britannica.com/science/chemical-analysis. Accessed 15 June 2021.

• Conventional methods of quantitative analysis, Himanshu Saxena, Apr. 06, 2015, available at : https://www.slideshare.net/HimanshuSaxena15/conventional-methods-of-quantitative-analysis

• What are the benefits offered by Modern Analytical Instrumental methods over Non-Instrumental methods?, Dr. Deepak, March 10, 2015.

• https://www.pharmastuff4u.com/2012/12/advantages-and-limitation-of-instrumental-methods.html?m=1

PUBLISHED BY   

ROHAN BEDKE, VAISHNAVI BIRADAR, MD MUZZAMMIL HUSSAIN, IMRAN AHMAD, MANOJ JADHAV.

STUDENTS, DEPARTMENT OF CHEMICAL ENGINEERING AND TECHNOLOGY, VIT, PUNE