Instrumental Analysis

Introduction

New technology affects all our lives, and the lives of analytical chemists are no different. As electronics and computers develop, new machines can be built that are capable of detecting unknown substances, much faster and in far smaller amounts than traditional 'test-tube' chemistry could ever do. In this unit you can find out about one machine we use to detect and identify unknown elements in a chemical sample, and another that is used to identify unknown compounds.

Atomic spectrometers

Spectrometers are expensive machines that analyse the energy (in the form of electromagnetic radiation, such as light) absorbed or given out by a sample. Atomic spectrometers are used to detect which elements are in a sample.

The sample is heated in a flame. Any molecules of compounds that exist in the sample are broken down at this stage. The energy from the flame causes electrons in the atoms of the sample to jump into higher energy levels (shells).

When the 'excited' electrons fall back to a lower energy level (shell), they give out a pulse of energy that we detect as light of a characteristic wavelength. Because there are a number of different electrons in the atoms of each

element

An element is a substance made from only one type of atom. An element cannot be broken down into any simpler substances.element, pulses of light at a number of different wavelengths are given out and can be detected by the spectrometer. Look at Fig.1 below to see what happens inside the

atom

An atom is the smallest particle of an element that can still be defined as that element.atom.

Figure 1.	An excited electron!

The energy given out at the different characteristic wavelengths is called the emission spectrum of the element. Each element has its own characteristic emission spectrum. We can see these as lines in a spectrum or as peaks when analysed by a detector and fed into a computer.

Figure 2.	The emission spectrum of mercury.

We can use this method to tell us which elements are present by matching the emission spectrum to a database of the emission spectra of elements. This can be done by computer. This analysis can also show how much of each element is present. For example, we can now detect mercury in a sample of water even if just 0.000 000 001 g is present!

Which of the following are advantages of using atomic spectrometers to analyse a sample of water that you suspect to be polluted?

It is fast.
It analyses compounds as well as elements.
It is cheap to buy the spectrometer.
It is accurate.
It can detect very small quantities.

Complete the following statement, explaining how electrons are involved in creating an emission spectrum.

The electrons are by the flame. In other words, they energy and jump into energy levels (or shells). When they return to energy levels (shells) they energy which corresponds to the difference in energy between the two energy levels (shells).

Atomic spectrometers are used in other industries, besides the water industry, to monitor samples. For example, in the

steel

Steel is a metal composed mainly of iron, with a little carbon and sometimes other metals, mixed in.steel industry they are used to analyse precisely the amounts of trace elements present in steel to control its quality.

Visible–ultraviolet spectrophotometers

We use visible–ultraviolet spectrophotometers to analyse which compounds are present in a sample. The sample is not broken up by any harsh treatment in the machine, such as heating it in a flame. Instead light is shone on the sample, then the wavelengths that are absorbed are analysed. You can see this process in Fig.3 below.

Figure 3.	A simplified model of a visible–ultraviolet spectrophotometer.

The pattern of wavelengths that is absorbed by the sample is called its absorption spectrum. This absorption spectrum can be matched or 'fingerprinted' against known samples, and thus be identified. Even complicated mixtures of compounds can be successfully analysed in this way.

If a

compound

A compound is a substance made up of more than one type of atom.compound does not absorb light in the visible–ultraviolet part of the spectrum, this technique will not work. However, we can often make it react with other compounds to give products that do absorb light in the right range. Then we can detect these compounds.

Visible–ultraviolet spectrometers are used in the water industry to analyse levels of nitrates and phosphates in water supplies.

Which of the following would visible–ultraviolet spectrophotometers be good at detecting in the water industry?

Mercury
Fertilizers
Lead
Detergents

Summary

Nowadays we can use modern instruments to detect and measure very small amounts of elements and compounds in samples.

An atomic spectrometer heats samples in a flame and analyses the light given out. It is used to identify unknown elements in a sample.

A visible–ultraviolet spectrophotometer analyses light that has passed through a sample. It is used to identify unknown molecules present in a sample.

These machines are used to monitor and control water quality, but also have many other uses, for example in forensic science and in hospitals.

Exercises