chapter 10 - atomic emission spectroscopy

emission spectroscopy

requires the use of very high temperatures of atomization sources to excite atoms

**there is no need for a source lamp

advantages of AES

• show lower susceptibility to chemical interferences

• good emission spectra result for must elements under a single set of excitation conditions

• spectra for many elements can be recorded simultaneously

• atomization is high

• chemical interference is low

• oxidation is low

advantages of plasma sources

• they permit determination of low concentrations of elements that tend to form refractory compounds

• permit the determination of nonmetals

• plasma emission methods have concentration ranges of several orders of magnitude

• few chemical interference

• atomization occurs in a chemically inert environment, enhancing the lifetime of the analyte by preventing oxide formation

• the temperature cross section of the plasma is relatively uniform (self-absorption and self-reversal are not observed as often)

• plasma produces significant ionization, making it an excellent source for ICPMS

sources for emission spectra

• plasmas

• arcs

• sparks

**are complex and comprise hundreds/thousands of lines

plasma

→ a homogeneous mixture of gaseous atoms, ions, and electrons at a very high temperature

***popular source for plasma is argon

types of plasma

• inductively couples plasma (ICP)

• microwave induced plasma (MIP)

• direct coupling plasma (DCP)

what is the ICP source?

torch; consists of 3 concentric quartz tubes through which streams of argon gas flow, as spark is created in the argon, creating Ar+ and e-. the induction coil has a high AC current through wires at 27 or 41 MHz radio frequency. the current ins the wires sets up a magnetic field around the wire

what do ions do in the alternating B field of ICP?

ions oscillate around the annular region: positive ions are going one way while negative electrons are going the other way. as the AC changes positive to negative, they spin the other way. this heats up the region up to 10,000 K

what is the optimal observation region in ICP? what is the desolvation/atomization region?

• 8000-6000 K

• more than 10,000 K

what are disadvantages of ICP?

• it is an “argon hog” with argon having a flow rate of 10-15 L/min

• there is an increased probability of spectral interference so they require higher resolution

• expensive optical equipment is required

• the procedure require less operator skill to yield satisfactory results

• the excited state populations are small relative to ground states

• ICP depends strongly on temperature

what is a refractory compound?

a compound that is highly resistant to thermal decomposition

how are wavelengths selected on the post-excitation end of the AE instrument?

a monochromator is used to see the emission as atoms return back to their ground states as they rise in the plasma and cool. there are very few interferences, and this allows for simultaneous analysis of multiple atoms

is MIP or ICP more inexpensive?

MIP is cheaper because the plasma can’t be sustained with nitrogen or air where ICP needs argon

when should one use internal standards in ICP?

when they need highly accurate/precise results, they can also be used when drift needs to be accounted for

why are ionization interferences less severe in ICP than they are in flame emission?

the large electron concentration from ionization of the argon maintains a fairly constant electron concentration in the plasma

how do you find the concentration of an unknown using an emission calibration curve?

use the equation conc_x = (b*conc_spike)/(m*vol_spike)

be able to draw a block diagram of the components within a successful ICP instrument capable of doing elemental analysis