Instrumental methods use machines to detect and measure substances

1. A chemical test, such as a flame test, depends on a person seeing and judging a colour, a precipitate or a sound.
2. An instrumental method uses a machine to measure a property of the sample, and a computer usually records the result.
3. Flame emission spectroscopy heats a sample in a flame and measures the light given out, which identifies the metal ions present.
4. Gas chromatography separates the substances in a mixture and records the time each one takes to pass through the instrument.
5. The result is a chart or table of measurements, which is then compared with results for known substances.

Instrumental methods are sensitive, accurate and rapid

1. They are sensitive, so they detect substances present in very small amounts, even in a tiny sample.
   1. A flame test needs enough of the metal ion to colour the flame, whereas flame emission spectroscopy detects traces far too small to see.
2. They are accurate, giving measurements close to the true value, so they identify substances reliably and can show how much is present.
   1. The result does not depend on a person's judgement of a colour, so a yellow-orange flame cannot be mistaken for an orange-red one.
3. They are rapid, so results come quickly and a machine can analyse many samples one after another, day and night.
4. The main drawback is that the instruments are expensive to buy and need trained people to run them.

Identifying substances by matching a reference chart

1. In flame emission spectroscopy, the light from the flame is split into a line spectrum, a set of bright lines at particular wavelengths.
2. Each metal ion gives its own pattern of lines, like a fingerprint.
3. To identify an unknown sample, compare its lines with reference spectra for known metal ions, drawn on the same scale.
4. A metal ion is present only if every line in its reference spectrum appears in the spectrum of the sample.
   1. Lines left over after one match belong to another ion, so one spectrum can identify each ion in a mixture.

Identifying substances by matching a reference table

1. In gas chromatography, each substance leaves the instrument after its own retention time, measured under fixed conditions.
2. The instrument records a peak for each substance, so the number of peaks shows how many substances the sample contains.
3. The retention times from the sample are compared with a reference table for known substances, measured under the same conditions.
4. A substance is identified when its retention time matches a reference value.
5. A value that matches no reference belongs to a substance that cannot be identified from the data given.