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Gas Mass Spectrometry (GC-MS) Detection Features and Applications

Gas-mass spectrometry (GC/MS) is widely used in the separation and identification of complex components, which has the high resolution of GC and the high sensitivity of mass spectrometry. GCMS can quickly obtain molecular weight information for synthetic compounds without sample preparation.

Mass spectrometry (MS), that is, the mass spectrum, the molecules of the substance form charged particles through physical action or chemical reaction under high vacuum, and some charged particles can be further broken. The mass-to-charge ratio of each ion is called the mass-to-charge ratio. After the ions with different mass-to-charge ratios are separated one by one by the mass separator, the mass-to-charge ratio and relative intensity of each ion are measured by the detector, and the resulting spectrum is called mass spectrum. Mass spectrometry is an analytical method that ionizes analytes and then separates them according to the mass-to-charge ratio of the ions to achieve the purpose of analysis by measuring the peak intensity of the ions.

High-performance gas chromatograph pyrolysis injection analysis is that under certain conditions, macromolecular organic matter follows a certain cracking law, that is, a specific sample can produce specific cracking products and product distribution, using high-efficiency gas chromatography to analyze and identify cracking products, according to This characterizes the original sample.

First, the basic principle of gas mass spectrometry (GC-MS):
The polymer sample is placed in a cracker, and under strictly controlled operating conditions, it is rapidly pyrolyzed at high temperature to generate volatile small molecular products, and then the cracked products are sent to a gas chromatograph for separation and analysis. Because the composition and relative content of cracked fragments are closely related to the structure of the polymer to be tested, the cracking chromatogram of each polymer has its own characteristics, so the cracking chromatogram is also called the thermal cracking fingerprint chromatogram.

Second, gas mass spectrometry (GC-MS) requirements for the cracker:
1. Due to the different pyrolysis temperatures and different pyrolysis products, the pyrolysis temperature control should be accurate and repeatable.
2. Different substances require different pyrolysis temperatures, and the pyrolysis temperature should be adjustable.
3. The cracker has a large heat capacity and a fast heating rate.
4. The volume of the cracker and the interface is small to reduce the dead volume and prevent the chromatographic peak from broadening.
5. No catalytic reaction to cracking reaction, preventing disproportionation reaction and secondary reaction.

Three, gas mass spectrometry (GC-MS) cracker type:
1. Tube furnace cracker:
The tube furnace cracker is usually made of a quartz tube heated by an outer wall, heated by an electric heating wire, the cracking temperature is 300-1000 ℃, and the constant temperature precision is high. When the furnace temperature reaches the set temperature, the sample is placed in the platinum boat, and the platinum boat is sent to the cracking furnace with a push rod, and the sample is not in contact with the tube wall.
The tubular furnace cracker has a simple structure, can quantitatively inject samples, is easy to operate, and can continuously adjust the cracking temperature. However, the heating rate is not adjustable, and the dead volume is large, which is prone to secondary reactions.

2. Hot filament cracker:
The hot wire cracker is usually made of platinum wire or nickel-chromium wire with a diameter of 0.2 to 0.5 mm and a length of about 50 mm wound into a spiral shape. The sample is coated on the metal hot wire, and the hot wire is heated to the required temperature with a stable voltage, so that the Sample lysis.
The hot filament cracker has simple structure, short heating time and less secondary reaction. However, it is not easy to quantitatively inject samples, and is generally only used for qualitative analysis.

3. Curie point cracker:
Curie point cracker is a high-frequency induction heating cracker, which uses ferromagnetic material as heating element. When it is placed in a high-frequency electric field, it will absorb radio frequency energy and heat up rapidly. When it reaches the Curie point temperature, the ferromagnetic substance becomes paramagnetic substance, no longer absorbs radio frequency energy, and the temperature stabilizes at the Curie point temperature. When the temperature drops after the high frequency power supply is cut off, the ferromagnetism resumes. By attaching the sample to the heating element, the sample can be lysed at the Curie point temperature.
Different ferromagnetic materials have different Curie point temperatures, and a heating element with the required temperature can be obtained by adjusting the composition of the ferromagnetic alloy.

4. Laser cracker. This is a new type of cracker, which will gradually be widely used with technological breakthroughs.

Fourth, gas mass spectrometry (GC-MS) features:
1. High separation efficiency: Most pyrolysis gas chromatographs use capillary chromatographic columns, which can effectively separate complex pyrolysis products, especially the small differences between macromolecular organics and trace components in polymer materials. Sensitively reflect on the cracking chromatogram to find the corresponding features.
2. High sensitivity: The pyrolysis gas chromatograph generally adopts the hydrogen flame ionization detector, which has high sensitivity.
3. Small amount of sample: The amount of sample is generally in the order of μg to mg, which is very beneficial for the detection of only trace samples.
4. Fast analysis speed: The typical analysis period is 30min. When the cleavage product is very complex, one analysis can be completed in 1 to 2 hours.
5. Large amount of information: It can carry out qualitative and quantitative analysis, and can also conduct research on the relationship between cracking conditions and cracking products, the relationship between sample structure and cracking products, cracking mechanism and reaction kinetics.
6. Wide range of applications: It is suitable for all kinds of samples without pretreatment, whether it is viscous liquid, powder, fiber and elastomer, etc., or cured resin, coating and vulcanized rubber, etc., can be directly injected and analyzed.
7. Easy to popularize: The pyrolysis injector has a simple structure and can be used for separation and analysis when combined with a gas chromatograph.
8. Can be connected online with various spectroscopic instruments: Any spectroscopic instrument that can be connected online with a gas chromatograph can be connected online with a pyrolysis gas chromatograph.

Five, gas mass spectrometry (GC-MS) application:

It is suitable for the separation and analysis of substances with large molecular weight, complex structure, difficult volatile and insoluble substances.

In pharmaceutical analysis, the flash evaporation technique can be used to analyze the volatile components in Chinese herbal medicines. The so-called flash evaporation means that before the sample is cracked, the sample is rapidly heated at a lower temperature (lower than the cracking temperature of the sample) to evaporate the volatile components to obtain a chromatogram. The sample is then cracked at high temperature to obtain a cracked chromatogram. In this way, important information on volatile components in the sample can be obtained, which is very useful in the qualitative identification of the sample.

The identification of polymers by pyrolysis-gas chromatography is carried out by comparing the pyrograms of unknown samples and standard samples, a so-called “fingerprint” identification. Fingerprints of standard samples can be stored in a computer database or obtained by parallel experiments with unknown samples during identification. Regardless of the method, the spectra being compared are required to be obtained under the same experimental conditions.

Although the fingerprint identification method is intuitive and convenient, it is not strict, and sometimes it is often difficult to accurately judge some polymers with similar structures.

Gas mass spectrometry (GC-MS) using multidimensional pyrolysis gas chromatography is also a well-established method for polymer identification. Using methylsilicone and PEG-20M double capillary column system, the cleavage products with the set retention time window on the methylsilicone column are switched to the PEG-20M column for analysis, and the characteristics of olefin polymers and nylons can be obtained. Spectrum.

There is also an internal standard identification method, that is, using polystyrene as a reference polymer to crack with an unknown sample, and then calculate the retention time of the sample product relative to styrene, and then compare it with the corresponding result of the standard sample.

In conclusion, the identification of polymers by pyrolysis-gas chromatography is a very effective method. The fingerprint identification method is intuitive and convenient. Characteristic peak identification method is a better method, but structural identification of characteristic peaks is required. The engineers of Jinjian Laboratory believe that using the internal standard method and identifying the characteristic peaks may not need to identify the structure of the characteristic product, but can ensure a certain reliability. The better way is to use the spectral library for comparison, which is convenient and fast.