By consulting relevant materials and combining the author’s work experience, the factors affecting the service life of graphite tubes are as follows:
(1) The type of graphite tube is different and the service life will be different:
The graphite tube is divided into pyrolysis graphite tube and common high-density graphite tube. Under the same conditions, the pyrolysis graphite tube has a longer service life than the ordinary high-density graphite tube. Because the graphite tube has porous characteristics, in the ordinary high-density graphite tube, the liquid sample easily penetrates into the graphite tube wall, resulting in a large contact area between the element to be tested and carbon, and the graphite tube is susceptible to high temperature oxidation damage; pyrolytic graphite tube is a pyrolysis of a high-density graphite tube, divided into pyrolytically coated graphite tubes and fully pyrolytic graphite tubes, and has a metallic luster surface. The sample penetrates less in the tube wall. Due to the small contact area, the surface of the graphite tube is not easily oxidized at high temperature, and the formation of carbide is inhibited and the formation of sensitivity is improved, the service life have also been extended.
Pyrolytic coated graphite tubes are pyrolyzed on the surface of high-density graphite tubes; graphite tubes with fully pyrolytic graphite have a long service life even without inert gas protection.
(2) The life of the graphite tube will also be affected by the difference in the sample:
For different analytical samples, the heating process and temperature settings are also different, and the service life of the graphite tubes will be greatly different. When measuring low-temperature elements, such as Pb, Zn, K, Mg and other elements, the temperature of the graphite tube is lower, and the loss of the graphite tube is smaller; when measuring high-temperature elements, such as Mn, Cu, Ag, etc.,The temperature of the graphite
tube is very high, and the loss of the graphite tube is large.
(3) Effect of sample concentration and injection volume on the service life of graphite tubes:
If the concentration of the test sample is high, the injection volume is large, and under high temperature conditions, depositing impurity carbides will be generated in the graphite tube and the graphite cone, which affects the resistivity of the graphite tube, resulting in an increase in the heating current of the graphite furnace and accelerating the graphite tube. Aging, greatly shortening the service life of the graphite tube.
Therefore, for unknown samples or high-concentration samples, it should be diluted and then measured. Try to use the flame method to test the concentration. Do not use the graphite furnace method for measurement at first. Otherwise, the graphite tube and atomic absorption spectrometer may be damaged.
(4) The effect of the acid content and type of the sample on the life of the graphite tube:
First, the increase in the content of the same kind of acid will cause the life of the graphite tube to be shortened. The appropriate acidity in the sample should be maintained. For example, for pyrolysis tubes, 1.5% acid medium is generally the best acidity. If 3 to 5% acid medium is used, it will damage the inner wall coating of pyrolytic graphite tube.
In addition, the effects of different types of acids on the life of graphite tubes are also different. At the same concentration of different kinds of acids, the oxidizing acid has great damage to the graphite tube. For example, the oxidizing property of perchloric acid is strong. If the sample containing perchloric acid is analyzed, the service life of the graphite tube is obviously shortened.
The acid analyzed in the samples analyzed by Daya Bay and Ling’ao Nuclear Power Plant is only nitric acid, and the final sample has a nitric acid concentration of about 0.2%, which will not have a significant impact on the life of the graphite tube.
(5) The effect of abnormal injection on the life of the graphite tube caused by the offset of the syringe position:
The accuracy of the syringe position directly affects the smooth and accurate performance of the analysis process, as well as the life of the graphite tube. If the needle position is offset, the syringe can not be accurately inserted into the sample hole of the graphite tube, and the sample is injected outside the graphite tube, which will cause damage to the outer surface of the graphite tube, especially there are graphite tubes with pyrolytic coating only inside the wall.
(6) The effect of the flow rate of the exhaust extraction system on the life of the graphite tube:
Flow control of the exhaust extraction system is an essential factor in the service life of the graphite tube. Too low a flow rate causes residual steam in the ashing stage to damage the graphite tube; too high aspiration flow will draw air into the graphite furnace, causing oxidation damage to the graphite tube.
(7) Temperature monitoring abnormality:
During the use of the graphite furnace, impurities may deposit on the temperature detecting hole or cause the filter of the temperature control system, which reduces the detection efficiency, so that the actual temperature of the graphite tube is higher than the temperature set by the heating program, resulting in the graphite tube too fast aging damage.
The position of the detection probe is misaligned with the detection hole, which also causes the same problem.
(8) Rationality of the heating procedure:
Unreasonable heating procedures will directly affect the number of times the graphite tube is used, and sometimes it may cause sudden breakage of the graphite tube, especially when the graphite tube is air burning. The temperature rise procedure of the test process should be set strictly in accordance with the test conditions of each element.
(9) Effect of electrode guide rail on the service life of graphite tube:
Generally, the graphite furnace is divided into two electrodes, one graphite cone is embedded in each electrode, and the graphite tube is clamped by two graphite cones; and the two electrodes are mostly fixed one by one, and the other is movable left and right.
After heating, the graphite tube can be extended by one millimeter than normal temperature. If the electrode guide rail can not slide, the thermal stress of the graphite tube will act on the graphite tube, causing the graphite tube to crack and break. The solution is simple and movable. Add some lubricant to the electrode slide rails.
(10) The impact of the installation of graphite tubes on their service life:
If the graphite tube is not on the same axis as the graphite cone after installation, or the contact with the graphite cone is not good, the electrical resistivity between the graphite tube and the graphite cone will increase, resulting in an increase in the heating current, thereby accelerating the aging and shortening of the graphite tube. The service life of the graphite tube.
After replacing the graphite cone, if the installation is not correct, the graphite tube is too tight, and the graphite tube will be cracked and broken after being heated and expanded.
(11) cooling system
The graphite furnace will heat up during the measurement process and need to be cooled to equilibrium temperature (near room temperature) for the next measurement. It can also protect the graphite tube and graphite cone from oxidative damage at high temperatures. Too cold or too fast a flow rate will cause condensation of graphite cones and graphite tubes, damage to graphite tubes and graphite cones, and reduce their service life; when cooling is insufficient, graphite tubes and graphite cones will also contact the air at a high temperature, excessive oxidation.
(12) Effect of protective gas on the service life of graphite tubes:
Since the graphite tube is easily oxidized at a high temperature, it is required to be protected by an inert gas, and argon gas is generally selected as a shielding gas for the graphite furnace. Impure argon gas and small flow rate will cause aging of the graphite tube and reduce the service life.
Argon is not pure: the purity of argon should be greater than or equal to 99.9%. When the cylinder pressure is lower than 10 bar, there are more argon impurity gases remaining and need to be replaced; after replacing the cylinder, the air in the pipeline should be purged.
The flow rate of argon gas is small: improper setting or solenoid valve jam may cause small flow and insufficient protection, resulting in a decrease in the service life of the graphite tube.
When the graphite tube is replaced, the graphite cone is not cleaned, resulting in poor contact between the graphite tube and the graphite cone, which may cause air leakage at the end of the graphite tube, causing sample vapor to diffuse through the gap to the outer surface of the graphite tube end, corroding the graphite tube.
