Mass spectrometry is a powerful analytical technique used to quantify known materials, identify unknown compounds within a sample and to elucidate the structure and chemical properties of different molecules. The complete process involves the conversion of the sample into gaseous ions, with or without fragmentation, which are characterized by their mass / charge ratio (m / z) and their relative abundance.
This technique basically studies the effect of ionizing energy on molecules. It depends on the chemical reactions in the gas phase in which the sample molecules are consumed during the formation of ionic and neutral species.
Principle of Measurement
The mass spectrometer consists of six basic components: the inlet system, the ion source, the separator, the detector, the vacuum system, and the electronics/controller . It is one of the most powerful and versatile analyzers on the market. Mass spectrometers can analyze most components that can be vaporized and are compatible with the analyzer’s operating temperature; it is a universal analyzer.
The mass spectrometer’s inlet system introduces a small sample into the ion source by using a flow-by capillary or a membrane. The spectrometer’s design must ensure that the introduced sample is a true representation of the fluid; therefore, sample fractionation must be avoided. The spectrometer’s inlets are commonly heated to vaporize the liquid samples and to maintain gas samples above their dew points. The ion source bombards the sample with electrons, forming a gaseous mixture of ionic fragments that drift into the analyzer. Positive ions are measured by injecting them into the separator. The separator separates the ions according to their mass-to-charge ratio. There are two types of separators: the quadrupole and the magnetic sector. In the quadrupole separator, the fragments are separated by a combination of DC electric fields and radio frequency. The magnetic sector separator uses a magnetic field to separate the ion beam, which tends to form a segment of a circular orbit. Each type has its advantages and disadvantages. But typically, the quadrupole is smaller, faster, more reliable, and less expensive then the magnetic sector type.
On the other hand, the magnetic sector type is more accurate and more stable and requires less frequent calibration. The mass spectrometer detector produces a voltage that is proportional to the ion beam that strikes it, which in turn is proportional to the amount of a particular component in the sample. The vacuum system creates a very low pressure in the system so as to avoid the collisions between the ions and gas molecules. This also increases the probability that the ionizing electrons do ionize the sample.
The system’s electronics, including a personal computer (PC), identify the results from the detector and compare their fingerprint to a database, thereby identifying the components. The PC controls the analyzer, tunes the system, and calculates the component concentrations.
The mass spectrometer has a fast and linear response. What may take ten minutes to analyze with another method, takes ten seconds with a mass spectrometer. It provides a highly reproducible measurement for a given set of conditions, offers an equal sensitivity to all components, and can measure up to 16 different components from a single stream.
The mass spectrometer provides continuous measurement and is used for gases, vapors, and liquids. The unit measures from ppb (but typically ppm) to 100 percent and has a sensitivity of about 0.01 percent with a measuring error of +/- 1 to 2 percent. However, it requires a sample that can be vaporized (when measuring chlorine or acid gases, the sample should be dry).
The analyzer of a mass spectrometer must be tuned every one to two months, and it is essential that maintenance personnel receive extensive training in its maintenance. Maintenance is complex, and the mass spectrometer’s initial cost is high.