Discussion on the maintenance of gas chromatography mass spectrometer - Database & Sql Blog Articles

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The development of technology, gas chromatography mass spectrometry is widely used, most users are able to operate the instrument proficiently, but should be special for use.

Some problems, routine maintenance of the instrument and how to proceed

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Introduction. and

The accuracy and reliability of the data, the state of the instrument is a very important influencing factor, the correct use and maintenance can not only ensure the good state of the instrument, but also extend the service life of the instrument and consumable parts. The author of this article is based on years of instrumentation

To Finnigan trace

Ultra, trace DSQ quadrupole gas chromatography mass spectrometer as an example, from the carrier gas system, mass spectrometry vacuum system, injection system, column use, mass spectrometry system maintenance, instrument period verification, etc. . Other models of gas chromatography mass spectrometers are also available as a reference.

1 carrier gas system

1·1 use of carrier gas

Gas purity must reach 99.999%, and it is filled with special steel cylinder. If the carrier gas is not pure enough, or the remaining carrier gas is not enough, the m / z28 line abundance will be too large. According to the quality of the carrier gas used, when the cylinder When the pressure is reduced to several MPa, the carrier gas should be replaced to prevent contamination of the gas path by the residue of the bottom of the bottle.

1·2 carrier gas purification

Generally, the carrier gas needs to be purified before entering the chromatogram to remove residual hydrocarbon compounds, oxygen, water and other impurities in the carrier gas to improve the purity of the carrier gas, prolong the service life of the column, and reduce the loss of the column stationary phase. Reduce background noise to a greater degree and make the baseline more stable. It is recommended to install a high capacity deaerator and carrier gas purifier or a composite carrier gas purifier. The purification device should be replaced in time, the deoxidation tube should be used for a long time, and the adsorbed oxygen will enter the instrument with the carrier gas, resulting in the spectral line abundance of m / z32 is too large. Commercially available deoxidizers are typically saturated with nitrogen. When installed, helium must be purged with helium in the deoxygenation tube and in the line before being connected to the instrument.

2 Identification and leak detection of mass spectrometer vacuum leakage

2·1 Confirmation of leakage

Whether there is an air leak in the mass spectrometer vacuum can be judged from the background map of pressure and air/water. If the instrument reaches a steady state, when the column flow rate is 1 ml/min, the fore pressure should be less than 50 mTorr, and the ion gauge pressure should be less than 7e-5 Torr. If the pressure is too high, it may be There are leaks; m / z18, 28, 32 and 44 are characteristic peaks of air / water, the background map of air / water is normal as shown in Figure 1; if shown in Figure 2, the peak of m / z28 is much higher than the peak of m / z18 And the ratio of m / z32 peak to the ratio of nitrogen and oxygen in the air, it can be judged that there is a small leak; if Figure 3, m / z28, 32 two peaks are abnormally high, the total ion current intensity exceeds 10 8th power , the gas leakage is serious, at this time, the filament should be turned off immediately, otherwise the filament will be broken.

2·2·1 Leak detection of cylinders and gas lines Check each line contact with Snoop leak test, paying special attention to the head portion of the cylinder. For each gas cylinder change, the cylinder and inlet contacts must be thoroughly inspected with Snoop leak detection fluid to ensure that there are no air leaks in the cylinder and intake line. Open the cylinder, adjust to a certain pressure, close the pressure of the gas chromatograph inlet, close the valve of the cylinder, open the pressure divider valve, if there is a leak, the pressure of the partial pressure gauge will drop significantly after a period of time. Regularly check the pressure gauge (hourly) and the pressure drop to find out if the gas main line is leaking.

2·2·2 Inspection of the GC section The air leakage in the GC section usually occurs at the internal carrier pipe joint, the spacer locating nut, and the column nut. The above position can be applied with an appropriate amount of acetone, one position at a time, in accordance with the principle of being close to and far from the MS portion. After the appropriate time, observe the peak map in the background map. If there is a steep, significant climb at m/z58 and m/z43, there is an air leak at the location where the acetone has just been applied.

2·2·3 MS section inspection The following method can be used to determine whether the leak is in the GC or in the MSD: first observe the air/water background spectrum, then cool all the heating zones of the GC/MS, then remove the column and At the end of the GC inlet connection, block the column port with a waste septum, wait 15~20 min, and observe the air/water background spectrum again. If the two are basically the same, the leak exists in the MSD or GC/ The column nut at the end of the MSD transmission line; if the results are significantly different, the leak is present in the GC section. The method of finding air leaks in the MSD is similar to the GC section, where acetone is applied at the location where the leak may occur, and each time a position is always started from the recently opened seal, which is the most likely place for air leaks. After smearing a position, observe the change of the peak map in the background map to judge. Some of the MSD air leakage is more likely to occur at the column nut at the end of the transmission line, where loosening may occur due to repeated changes in the oven temperature. On the other hand, the nut may not be tightened too tightly when the capillary column is installed. Otherwise, it is easy to crush the graphite ring, causing air leakage. Under normal circumstances, the hand is tightened, and then a quarter turn can be screwed with a wrench.

3 injection system

3·1 injection septum

3·1·1 High-quality low-bleed and high-temperature injection septa should be used. When replacing the injection septum, first lower the column temperature to below 50 °C, and turn off the inlet temperature and flow rate (Finnigan trace GC Ultra GC) The instrument has a gas leakage protection function. If the flow rate is not closed, when the inlet nut is unscrewed, a large amount of carrier gas is lost, and all the heating parts of the gas chromatograph are automatically turned off, and need to be restarted to be turned on). When replacing the septum, be careful not to tighten the inlet nut too much, otherwise the septum will be pressed tightly, the rubber will lose its elasticity, and the needle sticking will cause the punching effect and shorten the service life of the injection pad. Finally, the leak detection “leak check” is performed. If the nut is screwed properly, the instrument will display “leak check passed”.

3·1·2 Injecting septum should be replaced in time according to the situation. It is difficult to detect the micro-leakage. Generally speaking, the micro-leakage can cause the peak of the target analyte to advance, because the oxygen in the air will oxidize the column fixation when the furnace temperature is high. Liquid, so that the loss of fixed solution is increased, reducing the efficiency of the column, but it can be found through long-term comparison; a relatively large amount of air leakage will cause the retention time to be extended. Generally, the injection pad should be replaced after about 100 injections, and the manual injection is even less, depending on the injection technique.

3·2 liner and quartz wool

3·2·1 Liner should be selected according to the type of inlet, sample injection volume, split mode (splitor splitless), solvent type and other factors. In particular, the splitter does not split the liner, so be careful not to mix it.

3·2·2 The cleanliness of the liner directly affects the detection limit of the instrument. The liner should be inspected. If the liner is replaced, it can be ultrasonically cleaned with anhydrous methanol or acetone. Use; if it is too dirty, it should be cleaned with detergent and then solvent, then the liner is silanized and reused.

3·2·3 Silicified quartz wool should be used. This quartz wool is generally thinner and more brittle than untreated. Untreated quartz wool is highly adsorbed on analytes, especially polar compounds, and requires high concentration. The target can be analyzed after the adsorption is saturated. Used quartz wool should be discarded and cannot be reused.

4 column

4·1 Column selection and aging

4·1·1 The choice of column generally depends on the type, length, caliber and film thickness of the fixative. The fixed liquid of the capillary column is non-polar, weakly polar, medium-polar and strong. The higher the polarity of the column fixative, the lower the upper limit of the temperature. The higher the loss of the fixant with the increase of the column temperature, the principle of selecting the fixative is generally to use the polarity as low as possible. High; column length can be solved with short column without long column; small diameter column has better resolution with larger diameter, peak shape and sensitivity are also good, but the diameter is small, the column capacity is also small, should be considered According to the analysis requirements; the fixed liquid film can bear a large injection volume, separate the isomers, and has better separation than the film, but the column loss is also serious, and the maximum temperature of the actual operation. Also lower than the film.

4·1·2 Newly purchased capillary commercial columns (such as DB series capillary columns) are generally aged before leaving the factory, so generally do not need long-term aging before use, the new column aging is generally not connected to mass spectrometry or other detectors, set one The temperature programming program can meet the analysis needs two to three times. The starting temperature of the temperature programming program is low, generally set to 50 ° C, and the maximum temperature can be selected to be lower than the upper limit of the column temperature of 30 ° C or the highest temperature used in normal times. The heating rate is slow, generally set to 5 ° C / min. Old columns can be connected to mass spectrometry or other detectors when aging. The maximum temperature for programming can be higher than the highest temperature normally used, but it should not exceed the upper temperature limit allowed by the column.

4·2 Column use and preservation

4·2·1 When using the column, pay attention to the minimum and maximum temperatures indicated in the manual. Do not exceed the upper temperature limit of the column. Otherwise, the fixed solution will be lost and the detector will be polluted. To set the maximum allowable temperature, the GC will automatically stop warming to protect the column if it is suddenly heated by human or for unknown reasons. Oxygen, inorganic acids and bases, and mineral acids can cause damage to the column fixative. These substances should be prevented from entering the column.

4·2·2 After the column is removed, the two ends of the column are usually inserted into the unused sample pad. If it is only temporarily removed for several days, it can be placed in the desiccator.
4·3 Column Installation

The installation of the column should be carried out according to the instructions. When cutting, special ceramic chips should be applied and the cutting surface should be flat. Different sizes of capillary columns are used for different sizes of graphite gaskets. Note that the graphite gasket used at one end of the inlet and the end of the mass spectrometer is different. Do not mix. The length of the capillary entering the end of the inlet depends on the liner used. The instrument company provides a special comparison tool. Similarly, the length of the capillary entering the end of the mass spectrometer also needs to be compared with the special tools provided by the instrument company. The column joint nut should not be too tight, too tight to crush the graphite ring, but it is easy to cause air leakage. Generally, the hand is tightened and then tightened with a wrench for a quarter turn. Turn on the mass before inserting the mass into the small beaker containing the organic solvent to see if there is any bubble overflow and the flow rate is equal to the set value. It is strictly forbidden to bake the column at a high temperature when no carrier gas passes, so as to avoid damage to the column caused by oxidation loss of the fixing solution.

5 Maintenance of mass spectrometry

5·1 Power on and off

Turn on the GC when starting up, then open the mass spectrometer, set the appropriate ion source temperature and transmission line temperature, do not forget to open the vacuum compensation, otherwise the vacuum is difficult to reach below 50mTorr. Turn off the transmission line temperature before shutting down. The ion source temperature should be reduced to below 175 °C. After the molecular turbo pump speed drops, the instrument will prompt “OKto turn off power” to turn off the power.

5·2 need to determine before the experiment

(1) Whether the ion source reaches the specified temperature is generally recommended to use 250 °C; (2) When the column flow rate is 1 ml/min, the front pole pressure is normally between 30 and 50 mTorr; (3) Turn on the filament to see the air/water Background, confirm that the vacuum is not leaking.

5·3 setting of parameters related to mass spectrometry

Under the premise that the sensitivity can meet the requirements, the emission current is generally 100 uA, and the detector gain is 1. If it is trace analysis, it is necessary to increase the sensitivity. The emission current can be set to 200 to 300 uA, and the gain is set to 2 or 3. The higher the value, the higher the sensitivity, but the life of the associated components will decrease. The scanning range depends on the nature of the target compound. It can cover the range of debris. It is not necessary to set it too wide. Otherwise, the sensitivity will be sacrificed. The sampling rate is usually 3 to 5 spectra per second. The start time setting takes into account the retention time and solvent delay of the first target peak, neither losing the compound nor hiding the solvent.

5·4 Diagnosis and tuning

Mass spectrometry generally has a diagnostic function. If the sensitivity of the instrument drops sharply or the spectrum is abnormally abnormal, a diagnosis should be made to help find the cause. Note that the front pole pressure must be below 50 mTorr for the full diagnosis. If the instrument is normal, each test should pass (PASSED). If any test fails, click on the details and save the page to facilitate contact with the instrument's service engineer for repair. Before tuning the mass spectrometer, first adjust the range of MASS to 50~650, turn on the filament and calibration gas, observe the characteristic peak of m /z100, and see if it is clean around. If there is interference ion, increase the column temperature and advance. At the sample temperature, the impurities are driven out first to ensure that the m/z100 is clean around before tuning. During the tuning process, if the correction of the gain portion fails, the characteristic peak of m / z 502 cannot be found, or the voltage of the electron multiplier tube increases too much, close to 100 volts, it is likely that the ion box, lens or pre-rod is too dirty. Need to be cleaned.

5·5 cleaning of ion source and pre-rod

Prepare the relevant tools and reagents before cleaning, then open the case, carefully unplug the cable connected to the ion source, loosen the screw, and remove the ion source. Remove the main quadrupole before taking the pre-rod, place it on the dust-free paper, and then remove the pre-rod to be washed. Be careful when handling the entire process. Second, avoid dust entering the cavity. The components of the ion source are separated, and the filaments, circuit boards, and black ceramic rings are not cleanable in all components of the ion source. The ion box and its support, three lenses, stainless steel heating block and pre-rod need to be scrubbed with alumina, and the 600-mesh alumina powder is adjusted into a paste with glycerin or deionized water, scrubbed with a cotton swab, and the components are scrubbed with emphasis. The inner surface, the passage of ions. After the alumina is scrubbed, rinse it with water, then soak it with deionized water, methanol and acetone, then ultrasonically clean it. After the ion source is combined, install the pre-rod and quadrupole, and then carefully install the ion source. Cover the chassis and clean it.

6 Precautions for instrument intermediate status check and injection operation

6·1 Instrument intermediate status check

The state of the instrument directly affects the detection limit, qualitative and quantitative of the analyte. In addition to the metrological verification according to the specified cycle, it should be periodically checked during the period, and it can be checked quarterly or monthly according to the instrument usage. The contents of the verification during the GC/MS can include: instrument detection limit (sensitivity), repeatability (stability) of analyte retention time, precision of data, linear range and so on. It can be verified by repeated injections of a series of certified standard substance solutions. The mass spectrometry part can also confirm whether the instrument needs correction by observing whether the characteristic ions of the calibration gas FC43 are normal. Sample analysis can only be performed by verifying that the instrument is in good condition.

6·2 Precautions for injection operation

The gas phase constant volume reagent generally has a small molecular weight and is volatile. Therefore, in order to ensure the accuracy and reliability of the analysis result throughout the injection process, the solvent evaporation should be avoided as much as possible to ensure the concentration of the sample solution to be analyzed. First, we must 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 is used, the vial pad used is preferably used once, and the used bottle cap is used. It is easy to cause the volatilization of the constant volume reagent to increase the concentration of the analyte, and the sample with higher concentration of the analyte is more error.

In summary, the operator of the quadrupole GC/MS should be as familiar as possible with the performance of each part of the instrument and gradually gain experience in daily use. Proper use and good maintenance not only ensure the instrument's good condition, but also ensure the accuracy and reliability of the analytical data, while extending the life of the instrument and reducing the cost of depreciation and maintenance.