Difference between ICP and ICP-torch

Difference between ICP and ICP-torch  

                        BY

Farooq Khalid (Philosopher in Chemistry)

 

The objectives of this article are

TO understand how ICP is utilised as an ion source in ICP-ms.

 

 After reading this article you will be able to understand the working, mechanism, assembly and use of the ICP torch as an ion source.

  ICP ( inductively coupled plasma ) is a technique by the use of which we can analyse all the metallic and non-metallic elements. For the very first time, this technique came in 1960 followed by some amendments in 1964. The first commercial instrument came in 1970 and the more efficient one came in 1973. As per the name, it is quite obvious that ICP involves the production of plasma and for this purpose, we need a source and that source is ICP

It consists of three concentric tubes, the function of each tube is different and the purpose of each tube is specific. In all of these tubes, Ar gas flows at different rates. The flow rate through the innermost tube, inner tube, and the outermost tube is 1L/min, 1_5L/min, and 15L/min respectively. The outermost tube is greater in diameter and length and is made up of quartz which provides insulation from plasma.

The Ar gas which flows through the innermost tube is known as carrier gas because it carries the sample into the base of the plasma. similarly, the Ar gas which flows through the inner tube is known as plasma gas or support gas or auxiliary gas because this involves the production of plasma. Ar gas which flows through the outermost tube is called coolant gas because it provides insulation for tubes from plasma. The Ar gas which flows through the outermost tube comes up from the upper tube in tangential form as a result of which plasma gets centralised and stabilised, this is known as the Reeds Vortex Stabilization Technique.

Three copper-made coils are turned but not straight to produce an oscillating magnetic field. oscillation Produces plasma. RF generator uses a high-frequency current. i,e 40MHz,30.27MHz or 27.12MHz but most commonly 27.12MHz is utilised which produces a power of 1 to 5 kW. A current of 100A passes through the three copper coils which produce an oscillating magnetic field perpendicular to the direction of the flow of current. Oscillation is the main reason for the production of plasma.

Ar gas is noble gas or inert gas then how does it produce plasma?

Small tesla discharge occurring between the two cadmium electrodes provides a seedling electron which produces the Ar ions. these Ar ions and electrons tend to move into the circular path in a direction perpendicular to the magnetic field and hence cause a current which is known as Eddy Current. Interaction of Ar atoms, ions, and electrons results in a temperature of  6000 to 8000k.

Combining ICP with Mass Spectrometry

Inductively coupled plasma-mass spectrometry is unique among the flame and plasma spectroscopy techniques in the ability to discriminate between the mass of the various isotopes of an element where more than one stable isotope occurs. Enrichment of one of the isotopes permits the calculation of the concentration of the element. Isotope dilution is the most reliable method of accurate determination of elemental concentration. The conventional method of sample introduction for inductively coupled plasma-mass spectroscopy is by aspiration, via a nebulizer, into a spray chamber. Approximately 1 mL of sample is required per analytical run, about 99% of which is wasted.

Recently, low-cost, low uptake rate and high-efficiency nebulizers have been employed to combat this problem. The high-efficiency nebulizer operates more efficiently at 10-200 L/min.

Schematic of an ICP-MS system

The schematic diagram depicted above detail a two- or three-stage differentially pumped interface used to extract ions from the atmospheric pressure plasma into the low-pressure mass spectrometer. The gas expands behind the first orifice, and a portion passes through a second orifice in the skimmer cone. A series of ion lenses, maintained at appropriate voltages, are used to direct the ions into the quadrupole mass analyzer. The ions are transmitted through the quadrupole based on their mass to charge ratios and then detected by an electron multiplier.

The inductively coupled plasma-mass spectrometry system is considered a sequential multi-element analyzer that has scan times less than 20 ms for one sweep. The signal intensity is a function of the number of analyte ions in the plasma and the mass-dependent transport through the mass spectrometer.

The most important advantages of ICP-MS include multi-element capability, high sensitivity, and the possibility to obtain isotopic information on the elements determined. The isotopes of argon, oxygen, nitrogen, and hydrogen can combine with themselves or with other elements to produce isobaric interferences.

                                                                       Plasma torch

The plasma used in an ICP-MS is made by partially ionizing argon gas (Ar → Ar⁺ + e⁻). The energy required for this reaction is obtained by pulsing an alternating electrical current in wires that surround the argon gas.

After the sample is injected, the plasma's extreme temperature causes the sample to separate into individual atoms (atomization).

Advantage of argon

Making the plasma from argon, instead of other gases, has several advantages. Argon also has a higher first ionization potential than all other elements except He, F, and Ne. Because of this high ionization energy, the reaction (Ar⁺ + e⁻ → Ar) is less energetically favourable than the reaction (M⁺ + e⁻ → M). This ensures that the sample remains ionized (as M⁺) so that the mass spectrometer can detect it.

However, it is important to note that whichever form of argon is purchased, it should have a guaranteed purity of 99.9% Argon at a minimum.

Summary

1: commonly nebulizer is used to aspirate the sample into the base of the torch. Most commonly pneumatic nebulizer, cross flow nebuliser, barbiginton, and ultrasonic are used.

2: nebuliser converts the sample into aerosols which are concentrated by Peltier cooled double pass or cyclonic spray chamber .it converts the sample into limit size because larger size can cause noisy effects.

3: sample is passed through the innermost tube, to achieve the greater temperature of the plasma it is important to have the aerosols of uniform size.

4: these ions are produced in a normal analytical zone which is 10 to 20mm above the conduction coils. Now to introduce these ions from the atmospheric pressure of the torch to the reduced pressure of the mass spectrometer, these ions are passed through sample cold cones usually 1mm in diameter having small orifices. The gas expands beyond these orfices and passes through the skimmer cone which is 4mm in diameter.

5: To focus these ions on the quadrupole mass analyser acting as a detector  series of the lenses maintained at a suitable temperature are used which separate these ions based on mass by charge ratio

6: quadrupole mass analyser is used for resolution of better than unit mass for a compound having a molecular mass of 300.

7: Ar is a cheaper and more abundant noble gas as compared to other which was formed by the radioactive decay of potassium.

8: Ar has greater first ionization potential so the reaction involving the formation of Ar atom from Ar ion is less favourable as compared to the formation of a molecule from a molecular ion. So molecule remains in an ionised form which is easier to detect by the spectrometer.

9: ICP_ms has a span time of 20ms for one sweep so it is very rapid and also known as a sequential multi-element analyser.