Difference Between Positive and Negative Ionization in Mass Spectrometry

The key difference between positive and negative ionization in mass spectrometry is that the positive ionization forms positively charged ions, whereas the negative ionization forms negatively charged ions.

Mass spectrometry or MS is a technique in analytical chemistry that measures the mass-to-charge ratio of ions. The end result of this technique comes as a mass spectrum that appears as a plot of intensity. Further, this plot is drawn as a function of the mass-to-charge ratio. For mass spectrometry, the instrument we use is a mass spectrometer. When we introduce our sample into this instrument, sample molecules undergo ionization. Here, choosing the proper ionization technique is very important because it has a great impact on the end result. If we use a reagent gas, e.g. ammonia, it will cause ionization of sample molecules to form either only positive ions or only negative ions, depending on the setup of the instrument.

1. Overview and Key Difference
2. What is Positive Ionization in Mass Spectrometry
3. What is Negative Ionization in Mass Spectrometry
4. Side by Side Comparison – Positive vs Negative Ionization in Mass Spectrometry in Tabular Form
5. Summary

What is Positive Ionization in Mass Spectrometry?

Positive ionization in mass spectrometry involves the formation of positive ions for the determination of the mass-to-charge ratio of the sample molecules. We call this positive ion mode in mass spectrometry. We can denote this positive ion as M-H⁺. In this technique, we can detect ions in a high yield.

Difference Between Positive and Negative Ionization in Mass Spectrometry

Figure 01: A Mass Spectrum

The ionization process is as follows:

GH⁺ + M ⟶ MH⁺ + G

A good example of positive ionization is methane ionization. We can write down the chemical equation for this ionization as follows:

CH₄ + e ⟶ CH₄⁺ + 2e ⟶ CH₃⁺ + H

What is Negative Ionization in Mass Spectrometry?

Negative ionization in mass spectrometry involves the formation of negative ions for the determination of the mass-to-charge ratio of the sample molecules. We call this negative ion mode in mass spectrometry. Further, we can denote this negative ion as M-H^–. In this technique, we can detect these ions in a high yield. The ionization process is as follows:

GH^– + M ⟶ MH^– + G

What is the Difference Between Positive and Negative Ionization in Mass Spectrometry?

Mass spectrometry or MS is a technique in analytical chemistry that measures the mass-to-charge ratio of ions. There are two ways of ionizing sample molecules to determine this ratio: positive ionization and negative ionization. The key difference between positive and negative ionization in mass spectrometry is that the positive ionization is the process that forms positively charged ions, whereas the negative ionization is the process that forms negatively charged ions. Furthermore, the general formula for positive ionization in mass spectrometry is GH⁺ + M ⟶ MH⁺ + G, while the general formula for negative mass spectrometry is GH^– + M ⟶ MH^– + G.

The following infographic summarizes the difference between positive and negative ionization in mass spectrometry.

Summary – Positive vs Negative Ionization in Mass Spectrometry

Mass spectrometry or MS is a technique in analytical chemistry that measures the mass-to-charge ratio of ions. Moreover, positive ionization and negative ionization are the two ways of ionizing sample molecules to determine this ratio. In summary, the key difference between positive and negative ionization in mass spectrometry is that the positive ionization is the process that forms positively charged ions, whereas the negative ionization is the process that forms negatively charged ions.

Reference:

1.“Mass Spectrometry.” Wikipedia, Wikimedia Foundation, 30 Sept. 2019, Available here.
2. Large, Robert, and Hans Knof. “A Comparison of Negative and Positive Ion Mass Spectrometry.” Wiley Online Library, John Wiley & Sons, Ltd, 7 Apr. 2005, Available here.