In terms of a standard food label then there isn’t much on there as small as sodium. Proteins, starches and fibres are Mummy and Daddy Molecules, fats are pretty grand, and even sugars have something to throw around. However, the element sodium (usually the only element to be found on a label) is a diminutive little thing – particularly as it will almost certainly be in the form of the sodium ion, which, for those with a ruler, has an ionic radius of about 0.1nm (or 0.1 x10-9m).
In some foods you will find sodium up to percentage levels, and although it is essential for life, in humans it is also strongly associated with hypertension and heart disease when consumed above recommended levels. It might be small but it has a big impact! Therefore, to allow consumers to make appropriate choices it has to be declared on a food nutritional label, and therefore we need to analyse food for the sodium content.
The most common way that sodium is introduced into food is by the use of salt. Salt has been used as a food preservative for many thousands of years, and as an enhancer of flavour for almost as long. Indeed, the chemical symbol for sodium, Na, is ultimately derived from the Natron Valley in northern Egypt, which was a major source of salt for the Ancient Egyptians. Throughout recorded history salt has been significant, both culturally and as a traded commodity. It appears in religious texts, in Chinese documents dating back over 4500 years ago, and is the root of the English word “salary”. Sodium, in the form of salt, has affected the course of human civilisation!
However, although salt, or sodium chloride, is usually the major source of sodium in the diet, it is analysis of sodium itself that is required for food labelling. There are a number of reasons for this. Firstly, in a complex food product then it is not possible to analyse sodium chloride per se – one can test for either the sodium bit or the chloride bit, but not the two together. Secondly, there are non-salt sources of dietary sodium. Sodium might be naturally occurring within a food. It might also be within ingredients such as sodium bicarbonate, sodium citrate, sodium tartrate or monosodium glutamate (MSG). So, sodium doesn’t necessarily come from salt: and we must remember that it is the sodium bit that gives rise to high blood pressure, not the chloride. Therefore, we must measure the total sodium content separately from everything else.
Despite our need to analyse solely for sodium, I do have to point out that when it comes to the value presented on a nutritional label, we do convert all the sodium that we find into an equivalent sodium chloride (salt) value. This is an attempt to demystify the food label. If people think of sodium then it might be as a tiny slice of metal burning up on a bowl of water as part of a chemistry lesson – a bit exotic. If people think of salt then it is something that they sprinkle on their chips – reassuringly familiar. Therefore, we declare all the sodium as if it were friendly salt rather than the hyper-reactive metal.
There are very many ways of measuring the sodium content of a food sample. Ion chromatography is an option, although not one commonly found within the UK testing market. Ion selective potentiometry could be used, but there are inevitable matrix issues that preclude its use as a generic method. Almost always, food testing laboratories rely on spectrometric techniques; flame photometry, atomic absorption spectroscopy, or optical emission spectroscopy.
The technique requiring the least technically advanced equipment is undoubtedly flame photometry (FP). A flame photometer is not really much more than a very clever and high-tech camping stove. If a very fine spray of a solution containing sodium is introduced to a finely controlled gas flame (methane, propane or butane are all suitable fuels) then the excited sodium atoms will give off a bright orange light of a specific wavelength. If the flow of the liquid is constant, then it is possible to construct a calibration curve of intensity of light given off (emitted) against sodium concentration.
No-one can deny that flame photometry is an effective approach. However, its simplicity (and occasional apparent malevolence) tends to require a genuine level of technical competence by the analysts using the equipment. The calibration range is usually relatively small, and when in use the instruments tend to require regular calibration checks to account for signal drift. Blockages in inlet tubing and aspirators are not uncommon, and the technique can be less reliable for some sample matrices. Nonetheless, when used correctly flame photometry will provide effective, rugged and fit-for-purpose data for the sodium content of foods for labelling purposes.
Atomic absorption spectroscopy (AAS) is definitely a step up the technological ladder. In some ways it retains some features of flame photometry inasmuch as a solution containing sodium is aspirated into a flame. However, this is an air-acetylene flame, which would not be appreciated on many camp sites. In addition, a sodium lamp is used to direct a beam of light, of a very specific wavelength, across the flame. It is the amount of that light which is absorbed by the sodium atoms in the flame, rather than that which is emitted, that is indicative of the sodium concentration of the aspirated solution.
Once set up, the determination of sodium in food by AAS is an excellent approach to the question at hand. However, anything involving acetylene requires careful thought and consideration, and again, a genuine level of technical competence can be most beneficial. Ionisation of sodium can be an issue and the addition of a suitable suppressant, such as caesium chloride, can be recommended. Having said that, with a very large calibration range, stability of signal and the high-energy flame helping restrict interferences, AAS can be exceedingly rugged, effective and sensitive process for this purpose.
The third and most expensive technique that is routinely found in food testing laboratories is inductively coupled plasma optical emission spectrometry (ICP-OES). This is another step up in technology, although the principles will still appear familiar to the flame photometrist. Sample solutions are aspirated into a plasma (rather than a flame) and the emission of light of a wavelength specific to sodium can be measured. ICP-OES has the largest laboratory footprint of the three techniques, also requiring high vacuum and a supply of a gas, usually argon or nitrogen. Ionisation issues can be dealt with similarly to AAS, and the very high energy of the plasma helps minimise interferences and give quite sensitive responses with a very wide dynamic range for calibration purposes.
There are undoubted advantages and disadvantages associated with each of these analytical approaches. A laboratory’s method of choice may well be determined by capacity, facilities and capital expenditure requirements rather than any technical demand. There have been suggestions that for some food matrices there may be differences in the results obtained by differing methods, but generally speaking sodium analysis should give a reliable result regardless of the equipment used.