Inductively Coupled Plasma Atomic Emission Spectrometer (ICP-AES)

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

Reading:

pp. 106-108, 479-482, 486-487, and 492-494 in Harris text

Objective:

Introduction:

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.  

Figure 1.

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.

Figure 2.

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 left of the instrument maintains a constant flow of chilled water through the induction coil.  Note also the “quick plasma off” button on the front of the instrument that allows the user to extinguish the plasma torch at a moment’s notice.

Figure 3.

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).

Method of Standard Additions:

In this experiment we will be making use of the method of standard additions for calibration of the instrument.  Normally, calibration is performed by running a series of pure standard solutions of known concentration through the instrument (Mn and Ni standards in our case) and generating a calibration curve of signal versus concentration.  Then we run our sample through the instrument and determine the concentration of our analyte by matching our sample signal to the calibration curve generated with the standards.  Ideally, our standard solutions should be similar to our sample solution since other components of the sample solution (the matrix) can interfere with our analyte signal (a matrix effect).  Many times, however, it is difficult to prepare standard solutions with a matrix similar to that of our sample.  Instead, we can add known amounts of standard directly to the sample solution.  In this way the standard is put in the same matrix environment as the analyte in the sample.  We then are interested in the increase in signal due to the addition of the standard.  Typically we add several different amounts of standard solution to our sample and then generate a curve of signal versus the added analyte concentration.  Our original analyte concentration in the sample can then be determined by calculating the x-intercept of the resulting curve.  Study the standard additions plot below until you understand why we can say the concentration of analyte in our original sample is 18 ppm.  

Figure 4