Types of Machines Flame: absorption and emission Graphite Furnace: absorption and emission Plasma: emission ICP: inductively coupled plasma ICP-MS: inductively coupled plasma mass spectrometry
The Flame Depending on the element, different temperatures are required. So different fuel and oxidants are employed. For typical elements, acetylene is the fuel and air is the oxidant. But nitrous oxide, oxygen may be used as the ox. Hydrogen and cyanogen may be the fuel.
Graphite Furnace Graphite furnace AA machines are more expensive and more tempermental, but they have advantages. The amount of sample required is very small, and autosamplers are used to ensure a very small, yet consistent amount is injected into the furnace. Sample size is 1-100 L The sensitivity is greater than flame.
Graphite Furnace In a graphite furnace, the sample is injected into a platform, where it can reside for several seconds before being atomized. This long residence time results in the higher sensitivity and low sample requirement. Again, it is wise to use an autosampler to take full advantage of the capabilities and sensitivities.
Graphite Furnace In the chamber, a sample is first dried at a low temperature to remove the solvent. If necessary to remove organic matter, it will then undergo pyrolysis (charring) at a higher temperature (1000 C) Finally, the sample will be atomized. Absorption and emission are both used in graphite furnaces.
ICP In ICP, much higher temperatures are employed to create a plasma. Of course, these are very expensive machines! They utilize emission.
ICP: Creating the Plasma Argon gas is ionized by a spark from a Tesla coil. The ionized argon gas is heated to 6,000-10,000 C by the collision of free electrons with the argon atoms.
ICP: The Sample Meanwhile, the sample is injected into the system which aerosolizes it. The aerosol is then dried by being carried through a heated tube by argon gas (not the plasma). The analyte then reaches the plasma. The analyte is then atomized, excited, and its emission is measured.
ICP-MS In ICP-MS, a mass spec is coupled to the ICP so that extremely low detection limits are obtained.
Temperature Effects You know that as the temperature increases, the energy of atoms increases. So as the temp goes up, the number of atoms in an excited state also increases. This is very important for emission techniques. The rule of thumb is that for every 10K increase in temp, there will be a 4% increase in excited state atoms, and hence a 4% increase in emission intensity.
Temperature Effects Why is this? If the baseline temp is increased, a higher population of atoms will be in the excited state. For emission, the energy released when these excited state atoms relax is measured. So the more atoms in the excited state, the more energy is released and measured.
Temperature Effects Temperature has a much lower effect on absorption measurements. For absorption, there are still mostly ground state atoms even at higher temperatures. Since absorption measures the ground state atoms becoming excited state, there is very little difference as the temp is raised slightly.
Interferences There are several types of interferences that affect a measurement of abs or emission. Chemical Spectral Ionization Matrix
Chemical Interferences Chemical interferences occur when another substance is present in the sample that suppresses the atomization of the analyte. Anions like sulfate and phosphate can bind to metal ions like calcium to lower the amount of calcium atoms that actually atomize. So the calcium reading would be low (sound familiar?).
Chemical Interferences To prevent this, another metal such as La3+ is added to preferentially remove the anion. EDTA and other agents may also be employed. These are called “releasing agents” as the remove the interfence, releasing the analyte so that it may be fully atomized. Having a higher temp and higher fuel ratio may also help.
Spectral Interferences This occurs when another substance has a signal that overlaps with the analyte signal. This would result in high measurements. If you know this, or deduce this from control samples, the best thing is to choose another wavelength to monitor.
Ionization Interferences This occurs usually with alkali metals, as they have low ionization energies. As the atoms become excited, they don’t just get excited, a fraction will also ionize! Of course, this would lead to low measurements as the ionized atoms do not give off the same signal. Na is typically 5% ionized, while K is up to 33% ionized!
Ionization Interferences If you want correct measurements of alkali metals, then an “ionization suppressor” is a must. Strangely enough, an even “easier” to ionize additive like CsCl is added! The ionization of the Cs atoms yields an electron rich environment in the flame. This then suppresses the ionization of Na or K as they will pick up an electron as easily as they will lose it (Le Chatelier’s Principle).
Matrix Interferences If the matrix for the sample is very different than the solvent system for the calibration standards, then matrix effects may result. This may be due to chemical, spectral, or ionization interferences present in the complicated sample matrix. The method of standard additions is commonly employed to compensate for simple matrix effects.