Homework 4: Chapters 10-11

Assigned 15 February 2006; Due 20 February 2006; Quiz on 22 February 2006

Atomic emission spectroscopy (Chapter 10)

Atomic mass spectroscopy (Chapter 11)

  1. If you had a solution that contained In, Cd, and Sn, would you prefer to use by ICP-AES or ICP-MS if solution concentration was not an issue?

The stable isotopes for In are 113 and 115. Cd has a 113 isotope, and Sn has a 115 isotope. The use of ICP-AES eliminates mass overlaps Cd and Sn with In so would be preferable.

  1. What are the components used in an ICP torch?
  • Ar gas
  • 5-20 L/min
  • 3 quartz tubes
  • RF source
  • 0.5-2 kW
  • 27 or 41 MHz

The ionization is initiated by Tesla coil. Ions and e- interact with magnetic field with resulting flow and heating. The quartz is cooled by Ar.

  1. What different sample introduction methods are used in atomic emission spectroscopy?

•Nebulizers used for liquid samples provide a cross flow of Ar producing fine droplets carried by gas into plasma. Ultrasonic method can also be combined with a nebulizer

•Electrothermal vaporization uses a water cooled furnace to vaporize the sample

•Ablation can be performed with a laser to eject sample from a source

  1. What are the types of interference that can be observed in ICP-MS?

•Spectroscopic interference

  • Same m/z for ions in plasma

•Isobaric interference

  • Elements with isotopes of similar mass

In (113 and 115) with 113Cd and 115Sn

40Ar and 40K

58Ni and 58Fe

Ti (62-66) with 62Ni, 63Cu, 64Zn, 65Cu, and 66Zn

•Polyatomic interference

  • Formed in plasma due to matrix or atmosphere
  • Mainly for m/z>82
  • Can include dimers (O2, N2, Ar2) as ions

Examples are

InterferenceAnalyte

38Ar1H39K

40Ar40Ca

40Ar16O56Fe

35Cl16O51V

40Ar12C52Cr

23Na40Ar63Cu

40Ar35Cl75As

•Oxide and hydroxide

  • Formation of MO+ and MOH+ species
  • Interferes with elements 16 and 17 mass units above M
  1. What is the difference between sequential and simultaneous detection in AES?

In sequential detection the detector moves from one element line to the next and often has different components for the detection of UV and Visible light. In simultaneous detection a collection of different wavelengths are made at the same time, requiring more CCD or PMT detectors

  1. What is detected by mass spectroscopy?

The ion mass to charge ratio is detected. With a constant charge, namely 1, the mass can be determined.

  1. How is an ion’s mass to charge ratio determined in a quadrupole?

A quadrupole has two paired electrodes. An alternating current is applied that selects a single mass to charge ratio, acting as a mass filter. The potential can be varied to change the selected mass to charge ratio.

  1. What are some transducers used in mass spectroscopy?

An electron multiplier uses secondary electrons emission after interaction of ion with a surface to detect the primary ion. This is similar in concept to PMT. A Faraday cup operates when an xxiting ion strikes a collecter electrode. The electrode is surrounded by cage that collects scattered ions and secondary electrons. The results are independent of ion energy.