AAS and FES (Ch 10, 7th e, WMDS) AES and ICP Both are emission methods. In the older AES the excitation is by an electrical discharge through the sample. In ICP, the excitation occurs in an argon plasma torch.
AAS and FES (Ch 10, 7th e, WMDS) AES Let’s discuss AES first. Both AC and DC currents may be used for the excitation. The sample may be positioned in any of the electrode configurations shown in Figure 10.1. Electrodes are generally made of high purity graphite which can be machined to the desired shapes and exhibits very few emission lines.
AAS and FES (Ch 10, 7th e, WMDS) AES The DC arc produces both emission lines and molecular spectra. The electrodes are heated by the DC current to high temperatures so the spectra include significant background radiation. DC arc is best suited to qualitative or semiquantitative analyses. The use of an internal standard is valuable in minimizing the effects of arc instability and the sample matrix.
AAS and FES (Ch 10, 7th e, WMDS)
AAS and FES (Ch 10, 7th e, WMDS)
AAS and FES (Ch 10, 7th e, WMDS) AES The AC arc offers the advantages of lower operating temperatures and increased precision, but generally are less sensitive.
AAS and FES (Ch 10, 7th e, WMDS) Microprobe The use of a laser to focus on a very small portion of the sample and examine the emission lines produced. Because the cross sectional area of the sample is so small due to the properties of the laser, this method is essentially “nondestructive”, because the cross section has a diameter of ~ 50m.
AAS and FES (Ch 10, 7th e, WMDS) ICP - induced coupled plasma. A more recent development in emission spectroscopy is that of induced coupled plasma (ICP). A plasma consists of a very hot mixture of partially ionized gas with a relatively high concentration of positive and negative ions. In ICP this plasma is generally produced with a flowing stream of argon gas, so the ions are Ar+n species and electrons. The flowing mixture of ions and electrons are 'ignited' by a spark from a Tesla coil and it is the resulting collisions between ions and un-ionized gas to produce the high temperatures (as high as 10,000K).
AAS and FES (Ch 10, 7th e, WMDS) Flame Profile of an ICP torch
AAS and FES (Ch 10, 7th e, WMDS) ICP A distinct advantage of this method over FES is that the greater energy allows the excitation to higher levels and increases the population of excited atoms. The latter increases the intensity of the emitted lines, allowing for increased sensitivity.
AAS and FES (Ch 10, 7th e, WMDS) ICP. The purpose of the spray chamber is to make sure that only droplets in a narrow size range make it through into the plasma. Most of the sample drains away from the chamber, the rest is carried into the plasma and instantly excited by the high temperatures (5000-10,000 K)
AAS and FES (Ch 10, 7th e, WMDS) ICP Within the plasma most atoms become ionized with ~99% efficiency. Either ICP-optical emission spectrometry (ICP-OES) or ICP mass spectrometry (ICP-MS) may be used to analyze samples. ICP-OES uses UV and visible spectrometry to image the plasma at the exact wavelength of ionic excitation of the element of interest. This is a well-established technique.
AAS and FES (Ch 10, 7th e, WMDS)
AAS and FES (Ch 10, 7th e, WMDS) ICP ICP-MS is a more recent addition to ICP technology where a mass spectrometer is used to separate and analyze the atoms produced in the plasma. A portion of the ionized gas from the tail of the ICP torch is introduced into the vacuum system of a MS.
AAS and FES (Ch 10, 7th e, WMDS) Introduction of the ICP ionized gas into the vacuum of the MS
AAS and FES (Ch 10, 7th e, WMDS)