Analytical Techniques and Instrumentation

Mass Spectrometry or MS Technique:

Before discussing GC-MS & ICP-MS, let us first take a look at mass spectrometry. Mass spectrometry is the analytical technique that provides the most structural information for the least amount of analyte material. It provides qualitative and quantitative information about the atomic and molecular composition of inorganic and organic materials. Mass spectrometer is a highly reliable and most powerful detector which possesses unique and distinct advantages which include:

  • Increased sensitivity
  • Reduced background interference
  • Excellent specificity
  • Identification of unknowns
  • Confirmation of the presence of suspected compounds
  • Information about molecular weight

All mass spectrometers have essentially the following seven components:

  • Inlet sample system
  • Ion source
  • Ion acceleration system
  • Mass (ion) analyzer
  • Ion collection system
  • Data handling system
  • Vacuum system connected to components 1 - 5

We will not go into the detail working of these systems as it will only make it difficult for most non-technical readers.

Just remember that the mass spectrum is a fingerprint of each compound because no two molecules are fragmented and ionized in the same manner when undergoing electron impact ionization. When mass spectra are reported, the data are normalized by assigning the most intense peak a value of 100. Other peaks are reported as percentage of the base peak.

Mass Spectrometry or MS Technique:

The combination of GC and MS techniques make available the benefits and advantages of both analytical fields. GC does the separation and MS does the identification and quantitation. Benefits include low detection limits, and high information content provided from mass spectra of organic compounds.

There are three requirements of a GC-MS interface: (1) the volume of gas from GC must be reduced to that compatible with the inlet of the mass spectrometer (2) the spectra of analytes should be obtained often on the order of milliseconds (3) a final requirement is a data system capable of handling the volume of data generated by a fast-scanning mass spectrometer.

GC-MS technique is used for organic compounds, their identification and quantitation while ICP-MS handles mostly inorganic analytes like metals and elements.

ICP-MS technique:

Inductively coupled plasma mass spectrometry or ICP-MS is an analytical technique used for elemental determination. This technique has many advantages because of its superior detection capabilities, higher throughput, minimum matrix interference and ability to obtain isotopic information.

The sample is typically introduced into ICP plasma as an aerosol, either by aspirating a liquid or dissolved solid sample into a nebulizer. Once the sample aerosol is introduced into the ICP torch, it is completely desolvated and the elements in the aerosol are converted first into gaseous atoms and then ionized towards the end of the plasma. Once the elements in the sample are converted into ions, they are then brought into the mass spectrometer via the interface cones. The interface region in the ICP-MS transmits the ions traveling in the argon sample stream at atmospheric pressure into low pressure region of mass spectrometer. The ions from ICP source are then focused by the electrostatic lenses in the system. The metal ions are positively charged, so the electrostatic lenses also positively charged serve to collimate the ion beam into the slit of the mass spectrometer. There the ions are separated by their mass to charge ratio. The ability to filter ions on their mass to charge ratios allows ICP-MS to supply isotopic information, since different isotopes of the same element have different masses.

Since ICP-MS first became commercially available over 25 years ago, ICP-MS has become a widely used tool, for both routine analysis and for research in a variety of areas. ICP-MS is a fast, multi elemental technique and has extremely low detection limits. We are routinely using it to detect and quantify arsenic, lead, mercury and others, in crude oils and blending streams down to 5-10 ppb levels.