Differences between flame and plasma excitation sources in trace metal analysis systems

An excitation source is an essential requirement for trace metal determinations. The two well established techniques are Flame Atomic Absorption Spectroscopy (AAS) and Inductively Coupled Plasma Spectroscopy (ICP) both of which require an excitation source.AAS commonly makes use of a flame whereas ICP employs a plasma excitation source.

A question now arises that are both flame and plasma same. The answer is that though both serve the same purpose but in several respects they are vastly different from one another. The present article highlights some of the differences between them.

Flame

The two common techniques for sample excitation in AAS analysis technique are flame and graphite furnace atomization. Flame version is mostly used in laboratories. The processes taking place inside the flame on sample introduction are discussed in the article under reference.

A flame results from an exothermic reaction between the fuel gas(acetylene) and the oxidant gas(air or nitrous oxide).The accompanying flame involves cleavage of C-H bonds of acetylene with a release of the bond energy. This energy is partially used in raising the kinetic energy of atoms and the rest emits as heat and light.

A flame is hottest at its upper tip and varies in colour from bluish at base to yellowish-red at top. A flame is seen as yellowish-red due to the glow of heated un-burnt carbon soot particles present. As hot gases are lighter they are pushed upwards by the cooler surrounding gases. This gives the flame its shape. Flame temperatures generally range from about \(2100^0- 2300^0 C\) for air- acetylene flames. However in presence of excess oxygen a colourless or bluish flame is seen.

A flame or fire requires essentially a combination of three conditions-a source of heat for ignition, fuel for burning and supply of oxidant to sustain combustion. Absence of any of the three will extinguish the flame.

Plasma

All are familiar with the three states of matter- solid, liquid and gas. Plasma has been termed as the fourth state of matter. It is essentially a very hot gas, say, at temperatures ranging between \(6000^0C\)– \(8000^0C\).Due to the excessive energy some of the electrons are knocked away from the atoms. This results in free movement of electrons and ionized atoms which make plasma a conductive medium and an emitter of UV radiation due to re-combination of electrons with gas ions.

Plasma can be generated from a single gas without introduction of a fuel or oxidant gas and it does not require a high temperature heat source for its generation. The required energy for initiation and sustenance in most ICP plasmas makes use of an alternating field generated with the help of an induction coil typically operating at 27 or 40 MHz frequencies. Argon is the most commonly used inert gas. The plasma sustains itself as long as gas is passing through the torch and RF power is applied to the coil. If any of the conditions is not fulfilled the plasma ceases to exist. The plasma is shaped like a toroid above the torch. The shape and size are governed by factors such as torch geometry, argon flow rate and the RF power excitation frequency. Sample enters the plasma as an aerosol and the elements present in the sample begin to radiate their characteristic excitation frequencies. ICP systems provide advantage of simultaneous detection of the elements present in the sample without matrix interferences and the linear dynamic range is also several orders of magnitude larger than AAS methods. This feature helps determination of elements at much lower concentrations and in a fraction of time compared to the AAS technique.

Both the excitation sources are popular in trace metal laboratories and their adoption is governed by factors such as cost, sample loads and required levels of detection.