Mass Spectrometry

Question: What is it?

Answer: Mass spectrometry is a powerful analytical technique used to quantify known materials, to identify unknown compounds within a sample, and to elucidate the structure and chemical properties of different molecules. Detection of compounds can be accomplished with very minute quantities (as little as 10-12g, 10-15 moles for a compound of mass 1000 Daltons). This technique basically studies the effect of ionizing energy on molecules.

Question: What is a Mass Spectrometer?

Answer: A mass spectrometer is an instrument that measures the masses of individual molecules that have been  converted into ions, i.e., molecules that have been electrically charged.

As molecules are very small. it is not practical or convenient to measure their masses in kilograms, or grams.

For example, the mass of a single hydrogen atom is approximately 0.0000000000000000000000000166 grams (1.66 X 10-24grams).

Basic Principle

A mass spectrometer generates multiple ions from the sample under investigation, it then separates them according to their specific mass-to-charge ratio (m/z), and then records the relative abundance of each ion type.

The first step in the mass spectrometric analysis of compounds is the production of gas phase ions of the compound. This molecular ion undergoes fragmentation.

The ions are separated in the mass spectrometer according to their mass-to-charge ratio, and are detected in proportion to their abundance

A mass spectrum of the molecule is thus produced. It displays the result in the form of a plot of ion abundance versus mass-to-charge ratio.

Ions provide information concerning the nature and the structure of their precursor molecule.

In the spectrum of a pure compound, the molecular ion, if present, appears at the highest value of m/z (followed by ions containing heavier isotopes) and gives the molecular mass of the compound.

Components

Ion Source: For producing gaseous ions from the substance being studied.

Analyzer: For resolving the ions into their characteristics mass components according to their mass-to-charge ratio.

Detector System: For detecting the ions and recording the relative abundance of each of the resolved ionic species.

In Addition: A sample introduction system is necessary to admit the samples to be studied to the ion source. Computer is required to control instrument, acquire and manipulate data.

Processes performed by mass spectrometry

  1. Produce ions from the sample in the ionization source.
  2. Separate these ions according to their mass-to-charge ratio in the mass analyzer.
  3. Eventually, fragment the selected ions and analyze the fragments in a second analyzer.
  4. Detect the ions emerging from the last analyzer and measure their abundance with the detector that converts the ions into electrical signals.
  5. Process the signals from the detector that are transmitted to the computer and control the instrument using feedback.

Question: What is it used for?

Answer: To identify, quantify, and elucidate structures of chemical compounds.

For example:

• Analyze for environmental pollutants.

• Detect and identify the use of steroids in athletes.

• Perform forensic analyses such as conformation of drugs.

• Perform ultrasensitive multi element inorganic analyses.

• Identify structures of biomolecules, such as carbohydrates.

• Sequence biopolymers such as proteins and oligosaccharides.

• Establish the elemental composition of semiconductor materials.

• Determine the composition of molecular species found in space.

Question: How does it work?

Answer: A sample, which may be a liquid or vapor, enters the vacuum chamber through an inlet. Depending on the type of inlet and ionization techniques used, the sample may already exist as ions in solution, or it may be ionized in conjunction with its volatilization or by other methods in the ion source.

Atoms or molecules are passed into a beam of high-speed electrons.

The high-speed electrons knock electrons off the atoms or molecules being analyzed and change them to positive ions.

An applied electric field then accelerates these ions through a magnetic field, which deflects the paths of the ions.

The amount of path deflection for each ion depends on its mass with the most massive ions are deflected the smallest amount, which causes the ions to separate.

The gas phase ions are sorted in the mass analyzer according to their mass-to-charge (m/z) ratios and then collected by a detector.

In the detector the ion flux is converted to a proportional electrical current.

The data system records the magnitude of these electrical signals as a function of m/z and converts this information into a mass spectrum.

Question: How to read the spectrum?

Answer: A Mass spectrum appears as a series of peaks/signals distributed along the x-axis of the spectrum.

Each of these signals corresponds to the relative abundance of an atom within the molecule being observed. The position of each signal in the spectrum gives the m/z ratio.

Question: What is the use of Mass Spectrometry In Forensics?

Answer: Mass Spectrometry is used to confirm the identify of unknown, such as illegal drugs. Mass Spectrometry “fingerprints” the components so that they can be matched to existing known spectra.