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Daniele Naviglio » 1.Analytical Chemistry


Analytical Chemistry

“Analytical Chemistry is the art of separating and recognising different substances and determining the constituents of a sample”

(Whilelm Ostwald, 1894)

Since then, Analytical Chemistry has evolved from an art to a science. It is not an end in itself but is applied to basic chemistry research, and has practical application in industry, medicine and other branches of science based on the general study of materials.

The central role of Analytical Chemistry


Fields of Application

  • the efficiency of smog control systems fitted on motor vehicles is assessed by measuring the hydrocarbons, nitrogen oxide and carbon monoxide in the exhaust fumes in parts per million (Environmental Analytical Chemistry);
  • measuring the amount of various analytes (electrolytes or glucose in blood) helps in the diagnosis of possible diseases or checks on the evolution of specific diseases (Clinical Analytical Chemistry);
  • determining the amount of nitrogen in foods helps to evaluate protein content (Food Analytical Chemistry);
  • archaeologists identify sources of volcanic glass (obsidian) from concentrations of minor elements taken from different sites (Geological Analytical Chemistry);
  • civil engineers determine the efficiency of oxidative processes in refluents by monitoring the breakdown of pharmaceuticals like amoxycillin, diclofenac and DMZ (Environmental Analytical Chemistry);
  • in the field of biological research, DNA and proteins need to be sequenced (Biological Analytical CHemistry);
  • materials engineering involves study at the molecular level of mixes and substances which are used for different aspects of engineering (Materials Analytical Chemistry).

These few examples help us realise what a central role Analytical Chemistry plays.

Analytical Chemistry

“Analytical chemistry is the branch of chemistry concerned with the identification, chemical-physical composition and the qualitative and quantitative analysis of the components in a given sample”.

Some of the most frequently-encountered terms in analytical chemistry are:

sample: the object of the analytical procedure (for example: a blood sample);

analyte: the substance that is of interest in the analysis (for example: amount of hemoglobin in blood);

matrix: the constituents, apart from the analyte, of the given sample (for example: all the constituents of blood except hemoglobin);

method: the procedure used for the analysis, which may be standard or non-official;

qualitative analysis: reveals the presence and chemical identity of the analyte in a sample;

quantitative analysis: establishes, in numerical terms, the quantity of one or more analytes in a sample;

revealability limit: the minimum quantity of analyte that can be determined through analysis;

sensibility: the appreciable variation in quantity of analyte depending on the technique used for analysis.

“The process of analysis often requires preliminary operations (treatment of sample) to transform or eliminate other constituents in the sample that are not of interest (the matrix) so there is no interference.”

from Wikipedia

Analytical chemistry (cont.)

“Both quantitative and qualitative chemical analysis can be based on chemical reactions between reagents to form a product and /or on determining the chemical -physical barriers in an analyte. Most chemical analysis these days is of the quantitative type. Qualitative analysis is a procedure that is only used for macroscopic or mesoscopic investigation (or in the classroom). However, it is completely inadequate for revealing how much analyte is present if the amounts are microscopic. If this is the case, instrumental methods of analysis have to be used.”

from Wikipedia

Analytical chemistry as a “forma mentis” of the science researcher

Based on the information given above, we can define the current objectives of Analytical Chemistry:

  • to take problems arising in other disciplines and provide an answer;
  • to provide qualitative and quantitative answers to problems posed by other areas of scientific research;
  • to assess the validity of the answers given;
  • to assess the temporal and spatial value and validity of analytical responses by comparing results from different laboratories and by repeating the procedures over time.

Analytical Chemistry: a practical, laboratory-based course

The aim of this course is to help students develop the mindset of the chemiscal analyst and to provide them with the basic knowledge for solving probblems relating to Analytical Chemisty, mainly in the food industry. Students should also be aware that the knowledge and techniques they learn can be successfully applied to other fields.

Since this is a course about practical application of knowledge, students are encouraged to attend the laboratory sessions which include quite a lot of practical activities.

Analytical chemistry and food technology

One of the main aims of food legislation is to make sure human life and health are protected. This can be achieved through proper application of the HACCP system (hazard analysis critical control point). The main points of this system are:

  • identifying every hazard and making sure they are prevented, eliminated or reduced to acceptable levels;
  • identifying the chemical control points during the stages where control is essential for preventing or eliminating a risk or for reducing it to acceptable levels;
  • setting critical control points, the critical limits for differentiating between acceptability and inacceptability for the prevention, elimination or reduction of identified risks;
  • application of efficient surveillance procedures at critical control points;
  • choosing and implementing corrective course of action when surveillance reveals that a critical point is out of control.

Critical control limits

In many cases, it is current legislation that dictates critical control limits but they can also be set arbitrarily:

  • acidity of oil for human consumption is measured using acid/base titration;
  • the number of peroxides in oil for human consumption is measured using iodometric titration;
  • moisture levels in pasta, bread and other food matrices are measured using gravimetric testing;
  • fat quantities in cheese are measured by extraction with solvents and then weighing.

This is how analytical chemistry fits in to the food industry.

Analytical chemistry answers two basic questions:

1. What analytes are present in the sample?

2. What is the concentration of analyte in the sample?

There are two different types of analysis:

Qualitative analysis → The process of identifying what types are present in the sample.

Quantitative analysis → The process that serves to establish in numerical terms the quantity of one or more of the components in a sample.

Inorganic qualitative analysis


Quantitative analysis

The course will mainly cover traditional analytical techniques


Classical and Instrumental Analytical Chemistry

Classical Analytical Chemistry (uses traditional laboratory materials like glassware, scales etc.)

Traditional analytical methods → Gravimetric and titrimetric methods are particularly useful for determining the main components, that is, those up to 10-3 10-4 M.

Instrumental analytical techniques → Useful for determining substances present in trace amounts (less than 10-5 10-6 M) and they are carried out by measuring a physical signal either directly from the analyte or from one of its derivatives produced by chemical transformation.

Instrumental Analytical Chemistry (using electric-powered tools).

Theoretical knowledge underpinning Analytical Chemistry

Analytical chemistry uses knowledge from General and Inorganic chemistry and applies it to its own ends.

Basic knowledge:

Theory of solutions: solute, solvent, solution.

Homogeneous and heterogeneous solutions.

Colligative properties.

Acid-base reactions.

Precipitation reactions.

Complexation reactions.

Rédox reactions.

Chemical balance.

Titration curves.

End point of titration indicators.

PH of water solutions.

Tampon solutions.

Concept of mass and weight.

Scales.

Principle of titrimetric method

An analyte reacts with a reactive in solution, called the titrant, using any chemical reaction possible that gives us the required answer, namely, what is the volume of reactive necessary to make the analyte react completely?

Principle of titrimetric method (cont.)

Different types of reaction can be used:

  • neutralization (acid/base)

H+ + OH- → H2O

  • redox

Ce4+ + Fe2+ → Ca(EDTA)2+

  • complexation

Ca2++EDTA → Ca(EDTA)2+

  • precipitation

Ag+ + Cl- →  AgCl

The end point has to be demonstrated clearly, either with visual indicators or with instrument methods.

Principles of gravimetric analysis

Gravimetric analysis involves measuring how much analyte is present in a sample before and /or after drying. The instrument needed therefore, is an analytical balance. A classic gravimetric analysis measures the moisture present in a sample, the amount of water being calculated as the difference in weight between the original sample and once it has been dried.


 Principles of inorganic qualitative analysis

Inorganic qualitative analysis refers to a systematic procedure used to confirm the presence of ions or other elements by using different chemical reactions such as:

selective: applied to only a few analytes;

specific: reactions which are only applicable when looking for a single analyte.

Principles of instrumental analysis

Based on the measurement of physical (e.g. absorption of light) and chemical (e.g. oxidability) properties of an analyte and its derivatives, obtained through chemical-physical transformation.

The instrument measures these properties and sends a signal that varies according to analyte concentration. Electronics and computing have favoured the development of instrumental methods to the extent that:

  • baseline limits for investigation have been lowered;
  • smaller samples can be used;
  • lower concentration levels can be dealt with.

Quantification is through the dose-response curve, or standardisation curve.

Variation of technical progress in Analytical science from World War II to present day


Traditional methods and instrumental methods compared


Choosing the right method

This is the most important stage in qualitative and quantitative analysis, and requires experience and, above all, intuition. Of vital importance are:

  1. defining the problem;
  2. a survey of the scientific literature to see if the problem has been dealt with before;
  3. ability to slavishly apply a method;
  4. ability to make modifications to the method and demonstrate their validity;
  5. assessing the reliability of the results to make sure they are accurate and precise.

Choosing the right method


Features of scientific method

Analytical chemistry falls into the category of scientific methods and so is considered a proper science. This is why is has to satisfy the requirements of reproducibility and universality, making it possible for anyone who wants to check the results to do so. It is a science within everyone’s reach.

  • detection or revelation limit: minimum amount of analyte that can be distinguished from a zero value with any confidence;
  • sensitivity: variation of signal from instrument in response to the unit of concentration (slope of dose-response curve);
  • linearity interval: concentration interval of analyte whereby the instrument response is directly proportional to the concentration;
  • specificity: absence of interference in the determination.

Features of scientific method (cont.)

We need to consider the precision and accuracy of the method.

Precision: the result is repeatable if the experiment is carried out repeatedly. It depends on random errors and is represented by standard deviation.

Accuracy: concordance between the result obtained and the real value. It depends on random error as well as system errors.

A high level of accuracy and precision requires large amounts of time and money. This is why the chosen method is often a compromise between accuracy and cost.

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