GC-MS (Gas chromatography-mass spectrometry) is an instrument that combines the use of gas chromatography and mass spectrometry. Mass spectrometry can perform effective qualitative analysis, but is powerless for the analysis of complex organic compounds; while chromatography is an effective separation and analysis method for organic compounds and is particularly suited to conduct the quantitative analyses of organic chemical compounds, but qualitative analyses are much more difficult. Therefore, the effective combination of these two will certainly provide chemists and biochemists with an efficient qualitative and quantitative analysis tool for complex organic compounds. The combination of two or more methods like this is called a coupling technique, and the instrument that combines a gas chromatograph and a mass spectrometer is called a gas-mass spectrometer.
During the 1950s, Roland Gohlke and Fred mclafferty first developed gas chromatography-mass spectrometry couplers. However, the mass spectrometers used during that time were bulky and fragile, could only be used as stationary laboratory devices, and were not suitable for commercial dissemination.
The first commercially available four-stage rod mass spectrometer for gas analysis was built in 1964 by Bob Finnigan and P.M. Uthe.
In 1966 syntex attempted to acquire the California subsidiary of American Electronics Associates (eai), of which Robert Finnigan was a director. The acquisition failed, but it prompted Finnigan to suggest to mike story that they should form their own company to develop a new quadrupole gas chromatography-mass spectrometry (GC-MS) technique.
In 1967, the Finnigan instrument corporation (fic) was established. With Robert Finnigan as president, mike story was in charge of mass spectrometer design. Shortly afterward, t. z. chu, the general manager of Varian's gas chromatography division, joined the group.
In 1968 fic sent the prototypes of the first GC/MS to Stanford and Purdue Universities. fic eventually rebranded itself as Finnigan corporation and continues to be the world leader in the development and production of GC-MS systems. Finnigan firmly believed that the combined GC-ms systems for chromatographic applications would provide a significant market for new enterprise instrumentation.
In 1978 Yost and Enke developed the triple quadrupole technique.
In 1980 Finnigan introduced the first commercial hyperbolic quadrupole mass spectrometer, the MAT44.
In early 1981 Finnigan produced the first commercial triple quadrupole mass spectrometer sold to Shell (Shell Development Company Emeryville Research Center) for $300k, pioneering the commercial triple quadrupole. 1990 gc8000-md800 GC-MS was officially released. In the same year, Finnigan joined the Thermo Fisher family.
Gas chromatography-mass spectrometer equipment is extensively used in the isolation and characterization of complex constituents.Due to the high resolution of GC and the high sensitivities of mass spectrometry, they are effective instruments for the qualification and quantification of pharmaceuticals and a metabolite in biological specimens. The basic parts of a mass spectrometer are: an ion source, a mass filters and a detection, which are placed in a vacuum master pipe.
GC-MS device is mainly composed of the following parts: chromatographic part, gas-mass interface, mass spectrometer part (ion source, mass analyzer, detector), and data processing system.
The chromatographic section is basically the same as the general chromatograph, including the column chamber, gasification chamber and carrier gas system. Except for special needs, most of them are no longer equipped with detectors, but MS is used as the detector. In addition, the chromatographic section also has a split/non-split injection system, programmed temperature rise system, automatic pressure and flow control system, etc. The main role of the chromatographic section is separation, where the mixture sample is separated into individual components under suitable chromatographic conditions and then entered into the mass spectrometer for identification. Chromatographs operate at atmospheric pressure, while mass spectrometers require high vacuum, so if the chromatograph uses a packed column, it must pass through an interface device - a molecular separator - that removes the chromatographic carrier gas and allows the sample gas to enter the mass spectrometer. If the chromatograph uses a capillary column, the capillary can be inserted directly into the mass spectrometer ion source because the carrier gas flow rate in the capillary is much smaller than that of a packed column and will not destroy the mass spectrometer vacuum.
The gas-mass interface is the connecting component from GC to MS. The most common connection method is the direct connection method, where the capillary column is directly introduced into the mass spectrometer and sealed with a graphite gasket (85% Vespel + 15% graphite). The interface must be heated to prevent condensation of the separated components, and the interface temperature is generally set to the maximum value of the GC program ramp-up.
Mass spectrometer is both a general-purpose and selective detector. It ionizes the sample molecules in the ion source section to form ions and fragment ions, which are then separated by the mass analyzer according to the mass-to-charge ratio, and finally the signal is generated in the detector section and amplified and recorded to obtain the mass spectra.
1. Ion source
The function of the ion source is to receive the sample to produce the ion, and the common ways of ionization are.
Electron impact ionization, EI
EI is one of the most commonly used ion sources. Organic molecules are bombarded by a stream of electrons (generally 70 eV in energy) and lose an outer electron to form positively charged molecular ions (M+), which further fragment into various fragment ions, neutral ions or free radicals. Under the action of an electric field, the positive ions are accelerated, focused, and analyzed in a mass analyzer.
(1) Simple structure, easy to operate.
(2) The spectrum is characteristic and the molecular fragmentation of the compound is large, which can provide more information and is very beneficial to the identification and structure resolution of the compound.
(3) The obtained molecular ion peaks are not strong and sometimes cannot be identified.
(4) The method is not suitable for high molecular weight and thermally unstable compounds.
Chemical ionization, CI
The CI in GC-MS mixes the reaction gas (methane, isobutane, ammonia, etc.) with the sample in a certain ratio and then performs electron bombardment. The methane molecules are first ionized to form primary and secondary ions, and these ions then react with the sample molecules to form (M+1) ions that are one mass number larger than the sample molecules or called quasi-molecular ions. The excimer ion may also lose an H2 to form an (M-1) ion.
(1) Strong energy exchange does not occur as in EI, with less chemical bond breakage, and a simple spectral shape.
(2) The molecular ion peak is weak, but the (M+1) peak is strong, which provides molecular weight information.
(3)Field ionization (FI) is suitable for the ionization of volatile molecules, such as carbohydrates, amino acids, peptides, antibiotics, amphetamines, etc. It can produce strong molecular ionization peaks and quasi-molecular ionization peaks.
Field desorption ionization, FD
FD is used for compounds that are highly polar, difficult to gasify, and unstable to heat.
Negative ion chemical ionization, NICI
NICI is an ionization method developed on the basis of positive ion MS, which gives characteristic negative ion peaks with a high sensitivity (10-15 g).
2. Mass analysis
In a gas chromatograph, mass analysis is used to separate the ions generated in the ionization chamber by the mass-to-charge ratio (m/z) for mass spectrometric detection. Common mass analyzers are :
Quadrupole analyzer consists of four parallel cylindrical electrodes, which are divided into two groups, respectively with DC voltage and AC voltage of a certain frequency. After the sample ions enter the electric field along the axial direction between the electrodes, they oscillate between the electrodes of opposite polarity, and only the ions with mass-to-charge ratios in a certain range can pass through the quadrupole and reach the detector, while the rest of the ions collide with the electrodes due to excessive amplitude and are neutralized by the discharge and then are drawn away. Therefore, by changing the voltage or frequency, ions with different mass-to-charge ratios can reach the detector in turn and be separated and detected.
Sector mass analyzer
Magnetic-sector mass analyzer is accelerated by the electric field into the magnetic field, the motion of the ion orbit bent, ion orbit deflection can be expressed by the formula: when H, V is certain, only a certain mass-to-charge ratio of ions can pass through the slit to reach the detector.
The magnetic-sector mass analyzer is characterized by low resolution and difficulty in resolving ions of the same mass and different energies.
Double-focusing mass assay
Double-focusing mass assay consists of an electrostatic analyzer and a magnetic analyzer. The electrostatic analyzer allows ions with a certain energy to pass through and focus according to different energies, which successively enter the magnetic analyzer and are focused twice, greatly improving the resolution.
Ion trap detector
The principle of an ion trap detector is similar to a quadrupole analyzer but lets the ions stored in the well, change the electrode voltage so that the ion moves both up and down to enter the detector by means of a small hole at the base.
The function of the detectors are to transform the ion rays into electrical signatures and to enlarge the signals, the detector commonly used is an electron multiplier. When the ions hit the detector caused by the multiplier electrode surface ejected some electrons, were ejected due to the potential difference was accelerated to the second multiplier electrode, ejected more electrons, thus continuous action, each electron collision next electrode can eject 2 ~ 3 electrons, usually electron multiplier has 14 levels multiplier electrode, can greatly improve the detection sensitivity.
Since the mass spectrometer must work under vacuum conditions, the vacuum level directly affects the performance of the gas mass spectrometer. The general vacuum system consists of two vacuum stages, the pre-stage vacuum pump and the high vacuum pump. The main function of the pre-stage vacuum pump is to provide a running environment for the high vacuum pump, which is usually a mechanical rotary vane pump. The high vacuum pump mainly includes oil diffusion pump and turbo molecular pump, and the main application is turbo molecular pump at present.
The overall performance indicators of the gas chromatograph are the following: mass range, resolution, sensitivity, mass accuracy, scanning speed, mass axis stability, and dynamic range.
Mass range refers to the lowest and highest masses that can be detected and determines the application range of the instrument, depending on the type of mass analyzer. The quadrupole mass analyzer has a lower mass range of 1 to 10 and an upper mass range of 500 to 1200.
Resolution is the ability of a mass spectrometer to resolve two adjacent ion masses, and the type of mass analyzer determines the resolving power of the mass spectrometer. The resolution of a quadrupole mass analyzer is generally the resolution per mass.
Sensitivity: Gas mass analyzers generally use octafluoronaphthalene as the compound for sensitivity testing, and ions with mass number 272 are selected to be expressed as the root mean square (RMS) signal-to-noise ratio of 1 pg octafluoronaphthalene. The level of sensitivity is not only related to the performance of the gas chromatograph, but the test conditions also have an impact on the results.
Mass accuracy is the accuracy of ion mass determination and depends on the type of mass analyzer as much as resolution. Quadrupole mass analyzers are low-resolution mass spectrometers with a mass accuracy of 0.1u.
Scan speed, defined as the maximum number of masses scanned per second, is a fundamental parameter for data acquisition and has a significant impact on obtaining reasonable spectra and good peak shapes.
Mass axis stability refers to the extent to which the mass scale is shifted within a certain time under certain conditions, and is generally mostly expressed as the change of a certain mass measurement value within 24h.
The dynamic range determines the concentration range of the gas chromatograph.
TIC is similar to GC spectra and is used for quantification. l Repetitive scanning method (RSM) - repeated scanning at certain intervals, automatic measurement, and calculation to produce mass spectra of individual components, which can be characterized. l Masschromatography (MC) - recording of ion intensity with a certain mass-to-charge ratio over time. The ion intensity of the ions is plotted against time. A mass chromatogram similar to the total ion flow chromatogram is available for any mass number in a selected mass range.
Selectedion monitoring performs single or multiple ion detection of a selected characteristic mass peak or peaks and obtains a curve of the intensity of these ion flows over time. The detection sensitivity is 2 to 3 orders of magnitude higher than that of total ion flow detection.
Mass spectrogram is a bar graph of the relationship between the mass-to-charge ratio of positively charged ion fragments and their relative intensities. The strongest peak in the mass spectrogram is called the base peak, and its intensity is specified as 100%, and the other peaks are used to determine their relative intensities.
The working principle of GC-MS is as follows:
For the proper operation of the mass spectrometer instrument, it is necessary to compose a vacuum chamber for a high vacuum system. The instrument uses a high performance turbomolecular pump with a pre-stage vacuum pump to form a two-stage vacuum unit to ensure the required vacuum. The analyzed sample is separated by a capillary column and enters the ion source. An electronically powered standard configuration (EI) is used to generate positive ions, which are fed into the quadrupole system by the action of the push-repulsion, focus, and lead electrodes. The quadrupole forms a high-frequency electric field under the combined action of high-frequency voltage and positive and negative voltages. Under the action of the scanning voltage, only ions conforming to the equation of motion of the quadrupole field can reach the ion detector through the center of quadrupole symmetry, which is then amplified by an ion flow amplifier to produce a mass spectrometry signal. The mass spectra are obtained, and the composition of the unknown sample is identified by interpreting the spectra or performing a spectral library search.
A typical GC-MS system is shown in the figure. The sample to be analyzed is initially separated by a GC column with a carrier gas (hydrogen or helium), and the components from the column are transferred through the GC-MS interface module to the ion source unit of the MS module, where they are ionized to form ions that are analyzed by the mass analysis in the MS module. The data obtained from the analysis is processed and displayed by the data processing module of the GC-MS platform, and database search and comparison are performed. The data obtained from the analysis are processed and displayed by the data processing module of the GC-MS platform, and the database search and comparison are performed. The entire process sequence involved in the analysis is controlled and coordinated by the instrument control module of the GC-MS platform.
Classification of gas chromatographs can be divided according to the operating principle of the mass analyzer:
Carrier gas system
1. Use of carrier gas
The gas purity must reach 99-999% and be filled with special cylinders. Insufficient purity of carrier gas or insufficient amount of remaining carrier gas will cause an excessive abundance of m/z28 spectrum, and according to the quality of the carrier gas used, the carrier gas should be replaced when the pressure of the cylinder is reduced to several MPa to prevent the pollution of the gas circuit by the residue at the bottom of the bottle.
2. Carrier gas purification
General carrier gas into the chromatography before the need to be purified to remove the residual hydrocarbon compounds, oxygen, water, and other impurities in the carrier gas, to improve the purity of the carrier gas, extend the life of the column, reduce the loss of stationary phase column, and largely reduce the background noise, so that the baseline is more stable. It is recommended to install a high-capacity deoxygenation tube and carrier gas purifier or use a composite carrier gas purification tube. The purification unit should be replaced promptly. When the deoxygenation tube is used for too long, the adsorbed oxygen will enter the instrument with the carrier gas, resulting in an excessive abundance of the m/z32 spectrum. Commercially available deoxygenation tubes are usually saturated with nitrogen and must be installed with helium gas to purge the nitrogen inside the deoxygenation tube and in the line before connecting to the instrument.
Confirmation of mass spectrometry vacuum leak and leak detection
1. Leak Confirmation
Whether there is air leakage in the mass spectrometry vacuum of GC-MS can be judged from the pressure and the background pattern of air/water. If the instrument reaches a stable state, in general when the column flow rate is 1ml/min, the forepressure should be less than 50mTorr, ion gauge pressure is less than 7e-5Torr, and if the pressure is too large, there may be leakage; m/z18, 28, 32 and 44 is the characteristic peak of air/water, the background spectrum of air/water under normal circumstances; if m / z28 peak is much higher than m / z18 peak, and the ratio of the peak with m / z32 in line with the ratio of nitrogen and oxygen in the air, it can be judged that there is a small leak; if m / z28, 32 two peaks are abnormally high, the total ion flow intensity of more than 10 of the eighth power, the leak is serious, at this time to immediately turn off the filament, otherwise it will cause the filament to break.
2. Leak detection
Leak detection of cylinder and gas pipeline is checked by Snoop leak detection solution for each pipeline contact point, with special attention to the cylinder head part. Every time you change the gas cylinder, you must check the cylinder and the gas inlet joints with the Snoop leak test solution to ensure that there is no leakage from the cylinder and the gas inlet line. Open the cylinder, adjust it to a certain pressure, close the pressure of the gas chromatograph inlet, close the main valve of the cylinder, open the pressure divider, and if there is any gas leakage, the pressure of the pressure divider will drop significantly after some time. Regularly check the pressure gauge (every hour) and the pressure decrease, which can be used to find out whether the gas main line is leaking.
GC part of the inspection, GC part of the air leakage will usually occur in the internal gas carrier tube joints, spacer positioning nuts, column nuts, and other locations. An appropriate amount of acetone can be applied to the above locations, one location at a time, in order of priority by the principle of being close to and far from the MS section. After an appropriate period, observe the peaks in the background pattern, and if there is a steep and significant climb at m/z58 and m/z43, this indicates that there is an air leak at the location where the acetone has just been applied.
The MS section can be checked to determine whether the leak is in the GC or the MSD using the following method: first, observe the background spectrum of air/water, then cool all the heating zones of the GC, remove the end of the column connected to the GC inlet, block the column port with a waste spacer, wait 15-20 min, and observe the background spectrum of air/water once again, if both are essentially the same, the leak exists in the MSD or at the column nut at the end of the GC/MSD transmission line; if the results are significantly different, the leak exists in the GC section. The method for finding air leaks in the MSD was similar to that in the GC section, by applying acetone to the locations where leaks could occur, one location at a time, always starting with the seal that had recently been opened, which was the likely location of the air leak. The air leakage in the MSD section is more likely to occur at the column nut at the end of the transmission line, where repeated changes in column temperature may cause loosening; on the other hand, when loading the capillary column, the nut should not be screwed too tightly, otherwise, it will easily crush the graphite ring and cause air leakage, which is usually tightened by hand and then tightened by a quarter turn with a wrench. wrench to screw a quarter turn.
GC-MS injection system
1. Injection spacer
Should use high quality low loss and high temperature resistant injection spacer, when replacing the injection spacer, first reduce the column temperature to below 50℃, turn off the temperature and flow rate of the injection port (FinnigantraceGCUltra) gas chromatograph with gas leak protection function, if the flow rate is not turned off, when the injection port nut is unscrewed, a large amount of carrier gas will be lost, and all the heating part of the gas chromatography will be automatically turned off and need to be turned on again). (The instrument can be turned on only when it is turned on again). When replacing the spacer, note that the inlet nut should not be screwed too tightly, otherwise the spacer will be compressed and the rubber will lose its elasticity, and the needle will cause a perforation effect and shorten the service life of the inlet pad. After doing the gas leak detection "leakcheck", if the nut is screwed to the right, the instrument will show: "leakcheckpassed".
Inlet spacer should be replaced in time depending on the situation micro-leakage is not easy to detect, generally speaking micro-leakage can cause the target analyte peak early, because the oxygen in the air when the furnace temperature is higher will oxidize the column stationary solution, so that the loss of stationary solution increased, reducing the column efficiency, but only through a long comparison to find; relative to the large amount of air leakage will cause the retention time is extended. General automatic injection about 100 needles that should be replaced after the injection pad, manual injection and less, depending on the injection technique.
2. Lining tube and quartz cotton
The liner should be selected depending on the type of injection port, the sample volume, injection mode (splitorsplitless), solvent type and other factors. In particular, the liner should not be mixed, and the top and bottom should not be reversed during installation.
The cleanliness of the liner tube directly affects the detection limit of the instrument, so attention should be paid to the inspection of the liner tube. If the replaced liner tube is not too dirty, it can be cleaned with anhydrous methanol or acetone ultrasonication, taken out and dried, and then used again; if it is too dirty, it should be cleaned with detergent and then solvent, and then the liner tube should be silanized and used again.
Silanized quartz wool should be used, which is generally finer and more brittle than untreated quartz wool. Untreated quartz wool has serious adsorption on analytes, especially polar compounds, and requires high concentration of the target material to reach adsorption saturation before the sample can be analyzed. Used quartz wool should be discarded and not reused.
Chromatographic columns in GC-MS
1. Selection and aging of chromatographic columns
The choice of chromatographic column is generally considered from the type of stationary fluid, length, caliber and film thickness of four aspects, capillary column stationary fluid is divided into non-polar, weak polarity, medium polarity and strong polarity, the higher the polarity of the column stationary fluid, the lower the upper limit of the use of temperature, with the increase in column temperature, the greater the degree of loss of stationary fluid, the principle of choosing stationary fluid is generally low polarity is not used as far as possible to choose high polarity; column length can be solved with a short column, not a long column; small caliber column larger caliber has better separation, peak shape and sensitivity are also good. The column length can be solved with a short column without a long column; small caliber column larger caliber has a better separation, peak shape and sensitivity are also good, but the caliber is small column capacity is also small, depending on the analytical requirements; fixed solution film thicker can withstand the larger volume of the sample, separation of isomers, better separation than the film, but the column loss is also more serious, the actual operation of the temperature is also lower than the film.
Newly purchased capillary commercial column (such as DB series capillary column) has generally been aging well before the factory, so the use of generally do not need a long time aging, the new column aging is generally not connected to the mass spectrometry or other detectors, set a program to warm up the program to go two to three times to meet the needs of the analysis, which program to warm up the starting temperature to low, generally set to 50 ℃, the temperature can be selected below the upper limit of the column temperature of 30 ℃ or in When the old column is aging, it can be connected to mass spectrometry or other detectors, and the temperature of the program can be higher than the usual temperature, but not more than the upper limit of the temperature allowed for the use of the column.
2. Use and storage of chromatographic column
Chromatographic column should pay attention to the temperature indicated in the manual, can not exceed the upper limit of the temperature of the column, otherwise, it will cause the loss of fixed solution, but also can cause pollution of the detector. To set the allowable use temperature, in case of an artificial or unexplained sudden temperature rise, GC will automatically stop the temperature rise to protect the column. Oxygen, inorganic acids and bases, and mineral acids can cause damage to the column fixative, and these types of substances should be prevented from entering the column.
Chromatographic column is usually removed after the ends of the column are inserted in the unused sample pad, if only temporarily removed for a few days can be put in the dryer.
3. Installation of the column
The installation of the chromatographic column should be operated in accordance with the instructions, cutting the application of special ceramic slices, the cutting surface should be flat. Different sizes of capillary columns choose different sizes of graphite gaskets, note that the graphite gaskets used to connect the inlet end and the mass spectrometry end are different, do not mix. Don't put on the nut of the column joint too tightly, too tightly crush the graphite ring but easy to cause gas leakage, generally tighten by hand and then use a wrench to tighten a quarter turn can be. Before connecting the mass spectrometer, let the end of the column be inserted into a small beaker with organic solvent to see if there are bubbles overflowing and the flow rate is comparable to the set value. It is strictly forbidden to bake the column at high temperature when no carrier gas is passed, so as not to damage the column by oxidation loss of the stationary solution.
GC-MS intermediate state check and precautions during injection operation
1. GC-MS instrument intermediate state inspection
Instrument status directly affects the detection limit of analytes, qualitative and quantitative, in addition to the prescribed cycle of metrological verification, should also be regularly verified during the period, according to the use of the instrument can be verified quarterly or monthly. Gas chromatograph period verification can include: instrument detection limit (sensitivity), analyte retention time repeatability (stability), data precision, linear range and other aspects. The mass spectrometry part can also confirm whether the instrument needs calibration by observing whether the characteristic ions of calibration gas FC43 are normal. The sample analysis can be performed only when the instrument is in good condition through verification.
2. Precautions for GC-MS injection operation
Gas phase volume reagents are generally small in molecular weight and volatile, so throughout the injection process, in order to fully ensure the accuracy and reliability of the analytical results, solvent volatilization should be avoided as much as possible to ensure that the concentration of the analyzed sample solution remains unchanged. First, to ensure that the room temperature is as constant as possible, so that the standard solution series and the sample solution series are analyzed and determined under the same conditions; second, when the automatic injection, the vial cap pad used is used once, and the used cap pad is easy to cause the volatilization of the fixing reagent and increase the concentration of the analyte to be measured, and the higher the concentration of the analyte to be measured, the greater the error of the sample.
In recent years, gas chromatography and mass spectrometry technology has been developed rapidly and widely used in various fields, becoming one of the effective means to analyze complex mixtures. In the process of using the instrument, a variety of faults often occur, affecting the normal analysis and testing work, therefore, how to quickly and accurately determine the cause of the fault, and timely to be eliminated, is the instrument operators often face and urgently need to solve the problem. Therefore, how to quickly and accurately determine the causes of failure and timely troubleshooting is a problem that instrument operators often face and need to solve. Combining years of experience and experience in the use of operation, maintenance and repair, I summarize and summarize the common failure phenomena of gas chromatography and mass spectrometry, the possible causes of failure and troubleshooting methods for the reference of instrument operators.
Trouble phenomena related to mass spectrometer tuning, possible causes of the trouble and troubleshooting methods.
1. when the tuning parameters change, the change of the tuning peak intensity lags behind the possible causes of the failure and troubleshooting methods.
a. ion source is contaminated, the exclusion method is to ion source in turn with methanol, acetone ultrasonic cleaning for 15min.
b. Pre-quadruplex rod is contaminated, the exclusion method is the pre-quadruplex rod in turn with methanol, acetone ultrasonic cleaning for 15min.
c. ion source components are not installed in place, the circuit is not connected, the exclusion method is to remove the ion source, reinstallation.
2. Tuning mass spectrometer, the need for excessive ion energy and push repulsion voltage generated by the possible causes of failure and troubleshooting method
a. high ion energy is too high due to the ion source is contaminated, the repulsion voltage is too high in the pre-four rod, a four-stage rod is contaminated, the exclusion method is the ion source, pre-four rod, four-stage rod in turn with methanol, acetone ultrasonic cleaning each 15min and maintenance.
b. The tuning of the mass spectrometer has not reached a good state, the method of elimination is to retune the mass spectrometer.
3. When the tuning parameters change, the instrument response is not obvious possible causes of the failure and troubleshooting methods.
Ion source short-circuit or circuit is not connected, the method of elimination is to remove the ion source, using a multimeter to measure the circuit connection between the components is normal.
4. The shape of the tuning peak is not good, there is a shoulder peak generated by the possible causes of failure and elimination methods:
a. The mass spectrometer tuning has not reached *good state, the exclusion method is to retune the mass spectrometer.
b. The ion source is contaminated, the method of elimination is to clean the ion source with methanol and acetone ultrasonication for 15 min each in turn.
c. The analyzer is faulty or broken. Exclusion method is to inspect the appearance of the analyzer for any defects or damage.
5. When tuning, there is no reference peak, the possible causes of the failure and troubleshooting methods:
a. There is no reference sample in the reference sample bottle of perfluorobutylamine, the solution is to add the reference sample of perfluorobutylamine in the built-in reference sample bottle of the mass spectrometer.
b. The pipeline of the reference sample is blocked, the solution is to remove the pipeline and clean it with acetone ultrasonication.
If the height of the air peak m/z 28 is higher than 10% of the helium peak m/z 4, it indicates an air leak.
6. The appearance of irregular, rough tuning peaks generated by the possible causes of failure and elimination methods.
a. ion source is contaminated, the method of elimination is to ion source in turn with methanol, acetone ultrasonic cleaning each 15min.
b. Aging of the filament, eliminate the method is to replace the filament.
c. The tuning of the mass spectrometer has not reached the *optimal state, the method is to retune the mass spectrometer.
7. m/z 18, 28, 32 peaks greater than 10% helium peak m/z 4 Possible causes of failure and methods of elimination.
a. Air leakage, exclusion method is leak detection, check the connection of the column.
b. impurity enrichment in the cylinder due to imminent helium depletion, to be eliminated by replacing the carrier gas cylinder and installing a degassing device.
c. the newly cleaned ion source is not dried, the exclusion method is to set the ion source temperature of 250 ℃ to bake the ion source.
d. The column is contaminated, the exclusion method is to age the column.
8. When the filament is in good condition, no ion generation Possible causes of failure and troubleshooting methods.
a. the ion source needs to be recalibrated, the method of elimination is to use the calibration tool to recalibrate the ion source.
b. The air leakage is serious, eliminate the method is to detect the leakage and tighten the connections.
9. When tuning the mass spectrometer, the high mass peak m/z 502, 614 does not show the possible causes of the failure and troubleshooting methods.
The short circuit of the pre-quadrupole is removed and blown dry with helium or nitrogen.
Failure phenomena related to calibration and sensitivity, possible causes of failure and troubleshooting methods
1. The mass scale of the mass spectrometer can not be calibrated to produce the possible causes of the failure and troubleshooting methods.
a. The mass spectrometer is not optimally tuned, the method of elimination is to retune the mass spectrometer.
b. The ion source temperature is too high or too low, troubleshooting method is to set the ion source temperature at 180~220℃.
C.If the height of the air peak m/z 28 is greater than 10% of the height of the helium peak m/z 4, it indicates an air leak.
d. The energy of the emitted electrons is not appropriate, and the exclusion method is to set the energy of the emitted electrons to 70eV.
2. Possible reasons for the failure of low sensitivity and methods of elimination
a. The tuning of the mass spectrometer does not reach *good condition, the exclusion method is to re-tune the mass spectrometer.
b. The mass scale of the mass spectrometer is not calibrated, the method of elimination is to recalibrate the mass scale of the mass spectrometer.
c. The ion source is contaminated, the exclusion method is to clean the ion source with methanol and acetone ultrasonication for 15 min each in turn.
d. The ion source temperature is too high or too low, resulting in sample decomposition or adsorption in the ion source, the exclusion method is to adjust the ion source temperature.
e. The depth of the column into the ion source is not appropriate, the exclusion method is to adjust the depth of the column into the ion source.
f. faulty shunt feeder and valve, the method of elimination is to check the feeder and valve.
g. the column efficiency is reduced, the exclusion method is to replace the column.
h. Injector is contaminated, exclude the method is the liner tube in turn with methanol, acetone ultrasonic cleaning for 15 min each or replace the liner tube
i. Detector voltage is too low, exclude the method is the detector voltage should be 350 ~ 450V
j. Air leakage, the exclusion method is to check the height of the air peak m/z 28, if greater than 10% of the height of the helium peak m/z 4, indicating that there is air leakage, using a syringe to drop acetone at each interface, by observing the intensity of the acetone molecular ion peak m/z 58 changes, to further identify the exact location of the leak.
3. No noise in the mass color latent diagram (in a flat line) Possible causes of failure and methods of elimination
If the detector voltage is too low, the exclusion method is to increase the detector voltage.
4. The possible causes of excessive noise and troubleshooting methods
a. ion source is contaminated, the method of elimination is the ion source in turn with methanol, acetone ultrasonic cleaning each 15min.
b. The power supply system generates miscellaneous peaks, eliminate the method to install the power supply purification device.
Fault phenomena related to chromatograms and mass spectra, causes of faults and troubleshooting methods
1. Possible reasons for the occurrence of flat loss of peak generation failure and methods of elimination
a. Sample overload in the column, exclusion is to split the sample or dilute the sample.
b. Detector overload, the method of elimination is to reduce the detector voltage.
2. Retention time instability generated by the failure of the possible causes and methods of elimination
a. degradation of the stationary phase of the capillary column, the exclusion method is to cut off the end of the capillary column 0.5m or replace the column.
b. Injector leakage, the elimination method is to improve the sealing condition of the injector.
c. Leakage of the carrier gas pipeline, exclude by leak detection and tightening.
3. Possible causes and troubleshooting methods for low sensitivity and poor peak shape of high boiling point compounds
a. The ion source temperature is too low, causing the sample to be adsorbed, the elimination method is to increase the ion source temperature.
b. The temperature of the gas chromatography interface is too low, and the solution is to increase the temperature of the gas chromatography interface so that it is in line with the final temperature of the ramp-up procedure.
c. The final temperature of the GC ramp-up procedure is too low, and the solution is to increase the final temperature of the GC ramp-up procedure.
4. Possible causes of peak dragging and troubleshooting methods
a. The temperature of the injector is too low, and the elimination method is to increase the temperature of the injector.
b. The carrier gas flow rate is too small, the exclusion method is to increase the carrier gas flow rate.
c. The liner tube and column are contaminated. The method of elimination is to clean the liner tube with methanol and acetone ultrasonically for 15 min each in turn and age the column.
5. Possible reasons for the appearance of skewed peaks or variant peaks and methods of elimination
a. The scanning speed is too low, resulting in an insufficient number of scans for each peak, the method of elimination is to increase the scanning speed and make the number of scans for each peak greater than 6 times as far as possible.
b. The chromatographic peak is too narrow, and the method of elimination is to change the chromatographic conditions.
c. The tuning of the mass spectrometer has not reached the optimal state, the method of elimination is to retune the mass spectrometer.
6. Possible causes of failure arising from incorrect isotope ratios and methods of elimination.
a. the mass site scale of the mass spectrometer is not calibrated, the exclusion method is to recalibrate the mass scale of the mass spectrometer.
b. incorrect ratio of each mass peak after tuning the mass spectrometer, the exclusion method is to retune the mass spectrometer.
c. Air leakage, the exclusion method is to check the height of the air peak m/z 28, if greater than 10% of the height of the helium peak m/z 4, indicating that there is an air leak, using a syringe to drop acetone at each interface, by observing the intensity change of the acetone molecular ion peak m/z 58.
7. The possible causes of the failure of the separation of the wide peak is too weak and the method of elimination
a. ionization source temperature, current is too high (more than the cracking temperature and ionization current), the exclusion method is to adjust the ion source temperature, current.
b. Chemical ionization air pressure is too high or too low (for chemical ionization source), the exclusion method is to adjust the chemical ionization air pressure.
8. Possible reasons for the loss of isotope peaks in the mass spectrometry map to produce the fault and the method of elimination
a.the mass scale of the mass spectrometer is not calibrated, the exclusion method is to recalibrate the mass scale of the mass spectrometer.
b. The mass spectrometer is not tuned to a optimal state, the method of elimination is to retune the mass spectrometer.
c. The ion source is contaminated, the method of elimination is to clean the ion source with methanol and acetone ultrasonication for 15 min each in turn.
d. The voltage of the detector is too low, exclude the method is to increase the voltage of the detector.
e. Detector failure, exclude the method is to check the sensitivity of the detector.
9. Possible causes and troubleshooting methods for poor reproducibility of mass spectrometry
a. The ion source is contaminated, the solution is to clean the ion source with methanol and acetone ultrasonically for 15 min.
b. The ion source heater is unstable, eliminate the method is to replace the ion source heater.
c. The filament is damaged, the method is to replace the filament.
d. The tuning of the mass spectrometer is not optimal.
e. The mass scale of the mass spectrometer is not calibrated, troubleshooting method is to recalibrate the mass scale of the mass spectrometer.
f. Air leakage, the exclusion method is to check the height of the air peak m/z 28, if greater than 10% of the height of the helium peak m/z 4, indicating that there is an air leak, using a syringe to drop acetone at each interface, by observing the intensity of the molecular ion peak m/z 58 of acetone changes, to further identify the exact location of the leak.
10. Possible causes and troubleshooting methods for large interference peaks in total ion flow chromatograms
a. air leakage, elimination method is to check the height of the air peak m/z 28, if greater than 10% of the height of the helium peak m/z 4, indicating that there is air leakage, using a syringe to drop acetone at each interface, by observing the intensity change of the molecular ion peak m/z 58 of acetone, to further identify the exact location of the leakage.
b. faulty quality of the carrier gas, exclusion of which is to replace the carrier gas.
c. the sample is contaminated, the exclusion method is to improve the sample pre-treatment method.
11. Possible reasons for the gradual increase in the total ion flow chromatogram generating the malfunction and methods of elimination
a. Loss of stationary phase from the column (characteristic peaks of m/z 207, 281), elimination by aging or replacing the column.
b. Air leakage, the exclusion method is to check the height of the air peak m/z 28, if it is greater than the height of the 10% helium peak m/z 4, it indicates an air leakage, use a syringe to drop acetone at each interface, and further identify the exact location of the leakage by observing the intensity change of the molecular ion peak m/z 58 of acetone.
12. Slow decline of total ion flow chromatogram Possible causes of the malfunction and methods of elimination
a. The purge valve is closed, and the rule of thumb is to open the purge valve.
b. The purge flow rate is too low, troubleshooting method is to increase the purge flow rate.
13. Possible reasons for the failure of the chromatographic peak width and the method of elimination
a. The temperature of the injector is too low, and the method of elimination is to increase the temperature of the injector.
b. The sample in the column is overloaded, and the method of elimination is to split the sample.
c. The temperature of the gas chromatography is too slow, and the method of elimination is to change the temperature rise procedure of the gas chromatography.
After the installation of gas chromatography-mass spectrometry instrument, engineers are to test the instrument for commissioning, will issue a complete commissioning test report. The following areas need to be noted.
Issued a tuning report passed. Pay attention to the air background intensity, the size of the peak intensity of the standard, the air background intensity reflects the good or bad vacuum, the lower the intensity should be better, tuning with the peak intensity of the standard reflects the sensitivity of the instrument, should be sufficient intensity. Pay attention to the change of air background intensity and the standard peak intensity in the later use process should be relatively stable.
S/ N tests are generally performed using standards after installation and a test report is issued. The test results should reach the value of their claimed parameters. Sensitivity is a mandatory test item that must be examined by various instruments, reflecting the overall performance of the instrument, divided into absolute sensitivity and relative sensitivity, which are generally given by the instrument manufacturer on the instrument performance parameters. The so-called absolute sensitivity refers to the amount of sample required to obtain a controlled mass spectrometry signal on the recorder (g); the relative sensitivity refers to the minimum concentration of trace substances that can be detected (ppm). Sensitivity is related to ionization efficiency, mass transfer efficiency, scanning method, scanning speed, detector gain, injection method, and other factors, so in the acceptance of the instrument, it is necessary to keep a good check (many manufacturers in their promotional materials give a high value of sensitivity, most of them are in the acceptance of the instrument to play other indicators of the rubric, through the adjustment of other indicators to show the visual high sensitivity).
In the acceptance of GC-MS instruments, the absolute sensitivity is generally detected using octa fluoro decalin. 1pg octa fluoro decalin m/ z272S/ N>180 (RMS), which is expressed here as the root mean square signal-to-noise ratio, and in the instrument acceptance process, the manufacturer's settings are generally the signal value with a very short period (the baseline of the instrument can even appear straight in a very short period), which visually The sensitivity of the instrument is greatly improved. If we use the peak-to-peak signal-to-noise ratio, which is extremely intuitive, the value obtained will be much lower than the sensitivity given by the instrument (of course, it is undeniable that mathematically the root-mean-square representation is indeed higher than the average height ratio). As for the scanning method, the selected ion scan (SI M) is much more sensitive than the full ion scan (SCAN).
The problem of air leakage is a frequently encountered problem. A good vacuum is a prerequisite for MS analysis, so the vacuum condition of the instrument should be given enough attention. Generally, a vacuum leak test can be performed during tuning, and if it passes, it indicates that the vacuum is not leaking.
Generally, under certain conditions, the magnitude of the drift of the mass scale within a certain time, generally more to 8h or 12h of the measured value of a certain mass, generally controlled at ± 0. 1u/ 12h, which can reduce the frequency of instrument calibration and increase the stability of the instrument. The instrument's mass range: 1-11200au.
Resolution reflects the ability to distinguish between two adjacent masses and is the ability of the instrument to separate ions of different masses and focus the same ions. It is worth mentioning here that since sensitivity is almost inversely related to resolution, the resolution of the instrument must be determined before determining the sensitivity of the instrument, otherwise it will also produce a visually high sensitivity.
The scanning speed is determined by the type and construction parameters of the mass analyzer and, of course, by the computer interface. Scanning rate of the instrument: 60s/ scan.
Graphite gaskets of various sizes ( for 0. 25 columns, are the graphite gaskets at both ends of the inlet and detector the same? ), shunt-unshunt liner, capillary cutter, gas leak detector, mechanical pump oil, ion source cleaning kit, capillary seal ferrules, copper and plastic tube cutters. 5 ml and 25 ml sample purge tubes (2 sets), 2 each of the corresponding inlet seal syringes, and trap tubes.
The following are the common brands of GC-MS (in no particular order).
7. Skyray Instrusment
9. Sunyu Hengping Instrument
12. HEXIN MASS SPECTROMETRY