The development history of mass spectrometry (MS) dates back to the beginning of the 20th century. The original mass spectrometer was mainly used to determine the atomic mass of elements or isotopes. With the development of ion optics theory, mass spectrometers have been continuously improved, and their applications are constantly changing. It has been widely used in the determination of inorganic compounds and organic compounds in the late 1950s. After the 1980s, the development of new disciplines and technologies such as materials science, precision mechanics, electric vacuum and computers has promoted the advancement of high-performance mass spectrometer manufacturing. Today, mass spectrometry technology has been extensively applied to chemical analysis tests.
Basic theory of mass spectrometry
(1). Principles of mass spectrometry
Mass spectrometry is an analytical method for ionizing a substance to be measured by mass spectrometry, separating the mass-to-charge ratio of ions, and measuring the intensity of various ion peaks for analysis purposes. Quality is one of the intrinsic characteristics of matter. Different substances have different mass spectra, ie mass spectrometry. With this property, qualitative analysis (including molecular mass and related structural information) can be performed; and peak intensity and compound content represented by it can be quantified.
(2). Composition of the mass spectrometer
The mass spectrometer is generally composed of a vacuum system, a sample introduction system, an ion source, a mass analyzer, a detector, and a data processing system.
- Performance indicators of mass spectrometer
There are many indicators for measuring the performance of a mass spectrometer. These indicators include resolution, mass range, sensitivity, quality stability, mass accuracy, and more. There are many types of mass spectrometers, and the representation methods of their performance indicators are not identical. The main indicators are as follows.
The resolution of the mass spectrometer represents the ability of the mass spectrometer to separate adjacent two masses. Commonly represented by R. It is defined as if a mass spectrometer can just separate the two masses of ions M and M+∆M at mass M. The resolution of the mass spectrometer is R=M/ ∆ M.
2.2. Quality range
The mass range is the range of ionic mass-to-charge ratios that can be measured by the mass spectrometer. For most ion sources, the ions ionized are singly charged ions. Thus, the mass range is actually the range of molecular weights that can be determined; for electrospray sources, the ions formed are multi-charged, and although the mass range is only a few thousands, the molecular weight that can be determined can be more than 100,000. The size of the mass range depends on the mass analyzer. The upper limit of the mass range of the quadrupole analyzer is generally around 1,000, and some can reach 3,000, while the time-of-flight mass analyzer can reach hundreds of thousands. Due to the different principles of mass separation, different analyzers have different mass ranges. It doesn’t make sense to compare each other. The same type of analyzer reflects the performance of the mass spectrometer to some extent.
Sensitivity represents the amount of sample required to produce a certain signal-to-noise ratio of molecular ion peaks at a given resolution for a given sample.
2.4. Quality stability
The quality stability mainly refers to the condition that the quality of the instrument is stable during operation, and is usually expressed by the mass unit of mass drift within a certain period of time. For example, the quality stability of an instrument is: 0.1 amu / 12 hr, which means that the instrument drifts within 12 hours without a mass drift of more than 0.1 amu.
2.5. Quality accuracy
Mass accuracy refers to the accuracy of mass determination. Commonly represented by relative percentage. Mass accuracy is an important indicator of high-resolution mass spectrometers and does not make much sense for low-resolution mass spectrometers.
- Tandem mass spectrometry and applications
3.1. Tandem mass spectrometry
Two or more mass spectra connected together, are called tandem mass spectrometry. The simplest tandem mass spectrometry (MS/MS) consists of two mass spectra connected in series, with the first mass analyzer (MS1) pre-separating or energizing the ions and analyzing the results by a second-stage mass analyzer (MS2). The most basic functions of MS/MS include the description of the relationship between the parent ion in MS1 and the daughter ion in MS2.
MS/MS has many advantages in mixture analysis. When mass spectrometry is combined with gas chromatography (GC) or liquid chromatography (LC), it can be identified even if the chromatogram fails to completely separate the material. The MS/MS can select the parent ion from the sample for analysis without interference from other substances.
3.2. Combined technology
Chromatography can be used as a sample introduction device for mass spectrometry, and the sample is initially separated and purified. Therefore, chromatography/mass spectrometry can separate and analyze complex systems. Since chromatography can give retention times of compounds, mass spectrometry can give molecular weight and structural information of compounds, so it is very effective for the identification and determination of compounds in complex systems or mixtures.
To be continued in Applications of mass spectrometry in chemical analysis.