25 Mass Spectrometry and Hyphenated Methods*

Mass spectrometry (MS) measures the mass-to-charge ratio (m/z) of gas-phase ions. These ions come from a sample of interest. The quantitative amount of each ion is measured. The ions may be molecular ions, representing the full (but ionized) structure of an analyte, or may be fragment ions from the breakdown of the molecule during ionization. A mass spectrum is a plot of signal intensity versus m/z. When the m/z of a molecular ion is not sufficient to unambiguously identify an analyte, the fragmentation pattern (i.e. molecular weights of different ionized pieces of the analyte) observed in a mass spectrum may enable unique identification.

Mass spectrometers measure the exact mass of an ion. As such, mass spectrometers are sensitive to isotopes (e.g. methane, CH₄, is 16.034 amu with ¹²C and 17.035 amu with ¹³C) and mass spectra will have isotopic peaks. Molecular weights that differ by less than one atomic mass unit can be resolved.

Instrument Design

The main components of a mass spectrometer are its ionization source, which converts a condensed-phase sample to gaseous ions; its mass analyzer (e.g. quadrupole, time-of-flight), which separates ions based on their m/z; and its detector, which quantifies the ions for each m/z ratio.

Ionization sources and analyzers are discussed more in Chapters 26 and 27, respectively.

Detectors. The ion detectors used for MS are typically electron multipliers. For example, a positive ion from the analyzer strikes a conversion cathode and ejects electrons from the impact. An applied potential gradient then accelerates these electrons between discrete dynodes, along a continuous horn-shaped dynode, or within a plate-like array of microchannels  to produce more secondary electrons with each impact. The amplification is up to 10⁶-10⁸-fold and produces a measurable electrical current at an anode for an ion strike. Alternatively, a microchannel plate may be coupled to a phosphorescent screen, where light is produced when the secondary electrons hit the screen. The light is imaged on a CCD camera.

Hyphenated Techniques

In addition to its utility as a standalone technique, mass spectrometry (MS) is frequently combined with other analytical techniques. These methods are called hyphenated techniques and include GC-MS and LC-MS, among others. The MS serves as a detector for the chromatography. It is the largest and most costly chromatography detector, but provides by far the most information about analyte identities, as well as the best sensitivity for a wide scope of analytes.

The technical challenge with hyphenated methods is to go from the relatively high pressure of the LC or GC to the vacuum required to separate ions based on m/z. The interface between the ionization source and analyzer must solve this challenge. This high pressure-to-vacuum transition is usually accomplished through a series of skimmers between chambers of progressively higher vacuum. Ions are electrostatically directed through small apertures while neutral molecules are removed by vacuum pumps.

MS can also be combined with itself in the form of tandem mass spectrometry (abbreviated MS/MS or MS2). The first MS analyzer selects for an ion of particular m/z, which is then fragmented, with the fragment m/z to be detected by the second MS analyzer.

Molecular and Elemental Analysis

Although the above discussion has focused on molecular analysis, MS can also be used for elemental analysis in the form of ICP-MS. The ICP produces atoms that the MS then analyzes based on m/z. Detection limits for elements are lower than those for ICP-OES or ET-AAS in most cases.

Connections

• The electron multiplier ion detectors operate analogously to a PMT (Ch. 5), with the main difference being that an incident ion (rather than incident photon) produces the first electrons in the amplification cascade.
• MS is useful for molecular analyte detection at the end of an LC (Ch. 20) or GC column (Ch. 21), or CE capillary (Ch. 23).
• The sample introduction and atomization in ICP-OES (Ch. 10) and ICP-MS instruments are analogous.

Post-Reading Questions

1. Calculate the mass-to-charge (m/z) ratio for an acetate ion made up of only ¹²C, ¹⁶O, and ¹H. You will need to look up a table of exact atomic masses online.
2. What are two reasons that a 100% pure sample of a single molecule is unlikely to give a single peak in a mass spectrum?
3. What are the main components of a mass spectrometer?
4. Which of the following is a typical internal pressure for a MS: 10^–6 kPa or 101 kPa?
5. What is a substantial technical challenge for hyphenated techniques?

Topic Learning Objectives

The chapter is a primer for the following learning objectives, which will be covered in lecture and/or with additional assigned reading:

• Be able to sketch a mass spectrum.
• Appreciate how MS provides structural information.
• Understand how to distinguish between singly and doubly charged organic ions, and between isobars.
• Draw simple block diagrams for mass spectrometers and LC-MS and GC-MS instruments.
• Understand why high vacuum is necessary and how it is practically achieved in instruments.
• Know different strategies for tandem MS measurements.