Simultaneous Determination of Manganese and Nickel in Steel by Inductively Coupled Plasma Atomic Emission Spectrometry

READING (FROM HARRIS TEXT):

• • 550-552 (An Overview)

  • 557-558 (Inductively Coupled Plasma)

  • 561 (Multi-element Detection with Atomic Emission)

  • 563-564 (Detection Limits)

  • 565-566 (Virtues of the Inductively Coupled Plasma)

  • 818-819 (Dissolving Inorganic Materials with Acids)

  •  822-823 (Liquid Extraction Techniques, note Figure 28-12 is the same instrument you'll be using) 

OBJECTIVES:

• • To simultaneously determine the mass percentages of nickel and manganese in a sample of steel powder.

  • To become familiar with the operation of an inductively coupled plasma atomic emission spectrophotometer.

  • To become familiar with the principles of microwave digestion.

This experiment will be an introduction to the inductively coupled plasma atomic emission spectrometer (ICP-AES), also known as ICP-OES for optical emission spectrometer. The first commercial ICP-AES was made available in 1975, and it is now commonly used as a very powerful instrument for the determination of one or more elements in a sample. The ability to simultaneously determine the concentrations of multiple elements in a sample is what sets the ICP-AES apart from the less expensive atomic absorption (AA) spectrometer.

The principle of the ICP-AES is quite simple. The sample is exposed to the extremely high temperature of an argon plasma (up to 10 000 K) that breaks the sample into atoms, ionizes these atoms, and electronically excites the resulting ions. When the excited electrons in these ions fall back to lower energy levels, they emit light. The wavelengths of light emitted by a particular element serve as a “fingerprint” for that element. Therefore by measuring the wavelengths of light emitted by our sample, we can identify the elements in the sample; and by measuring the amount of light emitted by a particular element in our sample, we can determine the concentration of that element.

The diagram above shows the basic design of the ICP-AES instrument. The sample solution is pumped by a peristaltic pump into the nebulizer where it is broken into an aerosol of fine droplets by a fast stream of argon gas. From the nebulizer it passes through the spray chamber (which eliminates the larger droplets) and on to the quartz plasma torch. The plasma ionizes and excites the atoms of the sample. Emitted light from the ions in the plasma then passes through the entrance window to the monochromator where it is separated into its various wavelengths (colors).  The monochromator is a high-resolution “Echelle” design that makes use of both a diffraction grating and a prism to generate a two-dimensional pattern of individual wavelengths of light. This light hits the charge-coupled device (CCD) detector, similar to what you find in a digital camera, where thousands of individual picture elements (pixels) capture the light and turn it into a digital signal that we can measure.

The figure above shows both a diagram of the plasma torch and a picture of the torch compartment on our instrument. A plasma is simply a conducting gas consisting of a combination of positively charged ions and their respective electrons. In our case the plasma is made up of argon ions and electrons. The plasma is initiated by a spark from a tesla coil, and is maintained by a high-frequency electrical current in the induction coil powered by an RF (radio frequency) power supply operating with a power of 0.5 to 2.0 kW at 40.68 MHz. The RF current in the coil generates a magnetic field that causes the ions and electrons to flow in a circular path. This induced current results in collisions between particles and extreme ohmic heating to temperatures of 6000 to 10000 K. A tangential flow of argon gas protects the quartz torch from overheating in this extreme environment.

The picture below shows our Varian ICP-715-ES spectrometer. The cooling unit to the right of the instrument maintains a constant flow of chilled water through the induction coil.

Overall the ICP-AES is a very easy instrument to use, even for beginners, yielding very accurate results. As previously mentioned, ICP-AES is most useful if multiple elemental determinations must be completed on a single sample. Detection limits are typically in the low parts-per-billion range (1 ppb = 1 ng/mL), and sometimes as low as a few parts per trillion (pg/mL). The addition of an autosampler can increase both productivity and precision. The downside of this instrument is the expense.  Aside from the initial purchase price ($60k to $150k), the instrument is very expensive to run due to the high rate of argon consumption (~18 L/min).

MICROWAVE DIGESTION

In order to introduce the samples into the instrument, they will have to be dissolved in aqueous solution. Many types of samples can be digested (decomposed and dissolved) in nitric acid with heating. Traditionally this has been done on a hotplate in a fume hood. However, digestion can instead be carried out in a much safer and more efficient fashion using a laboratory microwave oven. These instruments have become popular for digestion, extraction, and organic synthesis, and working with multiple samples is much easier than when using hotplates. While some applications have used conventional kitchen microwave ovens, laboratory-grade instruments offer sophisticated temperature sensing and programming, and are much safer due to proper venting, vapor sensing, and pressure relief.

Microwave systems heat by exciting the rotation of dipoles within a liquid. Compounds such as water and other polar liquids absorb microwave radiation rapidly, subjecting the sample to rapid heating and elevated pressures. Heating is very efficient since the energy is transferred directly to the liquid instead of through a hot plate and beaker as is in the traditional method. Samples digest or dissolve in a short period of time.

We will be performing acid digestions of the steel samples in a simple-to-use microwave instrument called the MARS 6, a very popular instrument manufactured by CEM. A picture of the unit is shown below, and more information can be found on their website. Sample temperature is measured by an infrared sensor which allows us to program the instrument to heat and cool the sample to specific temperatures over defined time periods.

REFERENCES

1. Skoog, D.A.; Holler, F.J.; Crouch, S.R.; Principles of Instrumental Analysis (6th ed.) Belmont, CA: Brooks/Cole, 1998, pp 255-258, 266-269.

2. Cresswell, S.L,; Haswell, S.J.; "Microwave Ovens - Out of the Kitchen," Journal of Chemical Education, 2001, 78(7), 900-904.

3. MARS 6 Microwave Reaction System Operation Manual,  CEM Corporation, 2011.