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Interpreting
Electrospray Mass Spectra |
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Key to the "Average" Mass Spectrum |
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-The Key
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| A | The Y axis is labeled relative intensity. This is the intensity relative to the tallest peak in the spectrum with the tallest peak set to 100%. | |||
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| B | The X axis is mass divided by charge, m/z. For example if the mass of a molecule is 2000 u and the molecule posses two proton adducts its m/z value is equal to(2000+2)/ 2, the m/z value read on the spectrum is 1001. | |||
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| C | This is the tallest peak in the spectrum also known as the "base peak" | |||
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| D | A spectrum will have a certain number of counts associated with the tallest peak in the spectrum. This number can be used to gauge the relative intensity or concentration of the analyte. One should be forewarned that the count number is relative and can be adjusted with the multiplier gain and strictly speaking cannot be related to concentration without an internal standard. Counts will also be affected by spray needle and over all source maintenance. | |||
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| E | All of the peaks in a spectrum are just that, peaks and should not be referred to as ions, strictly speaking they are peaks in a spectrum. Why are "mass spec. types" so strict? | |||
| Note: | ||||
| 1. | It is common to overhear people talking about the "molecular ion" while pointing at an electrospray peak. Molecular ions are not generally observed in the electospray ionization process. A molecular ion is formed by the loss of an electron. In the electrospray process, ionization is accomplished by the loss or gain of a proton (or other adduct), some refer to this ion as the "pseudo molecular ion." | |||
| 2. | The electrospray process usually produces a population of multiply charged molecules and this population is accurately reflected in the intensity of the peaks in the spectrum. | |||
| 3. | In positive ion mode the number of charged species normally observed in an electospray spectrum is reflected in the number of basic sites on a molecule that can be protonated at low pH. | |||
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