Causes and elimination of common faults in graphite furnace

Foreword: This paper takes Z-2000 type as the example, and introduces the common faults and troubleshooting methods in the analysis of graphite furnace as follows:
( 1 ) Four factors that affect the reproducibility of graphite furnace analysis. The pretreatment of the sample is completely incomplete.
2 Is the temperature setting procedure reasonable and unreasonable?
3 Is it appropriate to adjust the injector?
4 Is the performance of the graphite tube intact?
( 2 ) Influencing factors of graphite tube 1 In summary, the performance and state of the graphite tube directly affect the reproducibility of the sample test; corresponding to the instrument of a certain model, the graphite tube used is not only the outer dimensions It must be consistent, and its resistance R should be consistent (the resistance of the graphite tube used in the Z-2000 original suction is 30 milliohms). This is because the graphite furnace power supply is generally a constant current source power supply mode. That is to say, once a certain temperature program is fixed, the power supply current I of the graphite furnace is correspondingly changed, so the graphite tube thermal power P=I2× R; It can be seen from the formula that the change of the resistance value R of the graphite tube directly affects the thermal power P. When the graphite tube is used for a certain number of times, the wall of the tube becomes thinner and the resistance increases, and the inner and outer walls of the tube produce a "honeycomb"-like physical change (see Figure-1); at this time, the temperature generated by the graphite tube is much higher than the original Set value; when this happens, the biggest unfavorable factor is: the actual temperature of the graphite tube in the ashing stage is higher than the original set temperature, and some low temperature elements (such as Pb, Cd, etc.) tend to cause at this stage. The “ashing loss” causes the absorbance to drop, which is the most common and most overlooked fault in the analysis of graphite furnaces. In addition, since the inner tube wall is honeycomb-shaped, the sample is easily immersed in the deep part of the tube after injection, and the coexisting material and the element to be tested are not easily burned or dissociated completely during the ashing, atomization, and scavenging stages, thereby generating a memory effect. .

Figure - 1
2 In another case, the appearance and resistance of the graphite tube do not change much, but because the acidity in the sample is too high or the concentration of the matrix modifier used is too large, the pyrolysis graphite tube is coated near the inlet. The layer is destroyed, so the syringe is slightly irregular during the injection, that is, the needle does not form a concentric circle with the inlet, and the sample is easily absorbed by a part; it also affects the reproducibility of the test. The appearance of this graphite tube is shown in Figure-2.

Figure - 2
3 Another hidden fault is that the coating on the inner wall of the pyrolysis tube is “skinned” due to the acidity or the effect of the modifier. The skin is just in the light path and oscillates under the disturbance of the carrier gas, and the position occurs from time to time. Changes, which directly affect the reproducibility of the test; and such hidden faults are often difficult to detect, only to remove the tube to observe the bright spot; (see Figure-3)

Figure - 3
( 3 ) Influencing factors of graphite ring <br>When the blank value of the graphite tube is too high, and the air burning is invalid several times, and the change of the new tube is not obvious, the graphite ring is often contaminated by the sample; The setting is unreasonable, the sample is sputtered onto the inner wall of the graphite ring during drying or ashing; since the temperature of the graphite ring is not high when the temperature is raised (the resistance value is too small), the sputtering is in the ring. The sample cannot be burned out. As time goes by, the residual material gradually accumulates on the inner wall of the ring. Therefore, the material remaining when it is burned or atomized is baked due to the far-infrared action of the graphite tube, and the calculation of the measured value is involved. , causing measurement errors. In addition, when the deposit is too thick, the graphite tube and the graphite ring are in poor contact, and the graphite tube is first brightened from both ends at the beginning of the atomization (therefore, the contact resistance between the two ends is large). Figure 4 is a real photo of the contaminated graphite ring.

Figure - 4
Since the price of the graphite ring is relatively expensive, in addition to the measures to replace the new graphite ring, you can try to clean the graphite ring again.
the way is:
1 Remove the contaminated graphite ring together with the graphite electrode. Note that the graphite ring cannot be removed separately; since the graphite ring can not be removed once it is removed from the electrode, the graphite ring is in contact with the electrode after the secondary loading. Poor contact occurs and the contact surface is ignited (the heating current is too large).
2 Repeat the rubbing of the joint between the graphite ring and the graphite tube with an ethanol swab.
3 Find a complete old graphite tube at one end (of course, the new tube is best), hold one end of the graphite tube with your thumb and forefinger, place the entire other end vertically into the graphite ring, and make the fingers move back and forth (similar to the drill Fire-like) until the graphite ring interface is bright (see Figure-5)
4 Finally, use an ethanol swab to repeatedly clean the polished part of the graphite ring.
The above cleaning method is equally applicable to other structurally similar atomic absorption instruments.

Figure - 5
( 4 ) Excessive “ false background ” absorption problem when measuring graphite furnace <br>In the analysis of graphite furnace, it is sometimes found that the background value is high during atomization and higher in the removal stage, it seems that there is a very high background. Absorption, but there is no such high background in the actual sample; and as the temperature of the graphite furnace gradually decreases, this background value takes a long time to gradually return to the baseline (ie zero) position, as shown in Figure -6:

Figure - 6
There are two reasons for this failure:
1 Graphite electrode leaks from the inlet and outlet into the carrier gas pipeline due to poor sealing or improper installation. These traces of moisture are carried by the carrier gas to the inner wall of the graphite electrode. At the instant of atomization, the water is atomized. Rapidly low into water vapor and adsorbed on the quartz window (due to the low temperature of the quartz window) caused a false background absorption, and over time, the moisture attached to the quartz window was gradually evaporated, so the background The baseline returns to zero again. Judgment method: After the end of the cleaning phase, remove the quartz window and observe that there is no steam on the quartz plate. Solution: Replace the rubber seal at the bottom of the electrode or add raw tape. The appearance of the graphite furnace electrode and the quartz window is shown in Figure-7.

Figure - 7
2 The graphite furnace body deviates from the optical axis; the background (blue line) does not have much at the beginning of atomization, but the background suddenly becomes very high after the atomization ends, and the background gradually returns to zero until the end of the temperature rise. The reason for this failure is that the beam emitted by the graphite tube and the cathode lamp is not in a concentric circle; the graphite tube will expand after being thermally expanded, and this extension can be up to 1 mm. When the graphite tube does not deviate from the optical axis, the tube and The optical axis maintains a concentric state. This extension does not affect the absorbance value. However, when the graphite tube deviates from the optical axis, the tube will block the light due to the extension of the heat, which will produce a "false background". The greater the degree, the larger the false background value. Sometimes the quality of the graphite tube itself (such as substitutes) is not good, it will produce similar faults (see the left side of Figure-8, red is the atomization absorption signal of the sample, blue is the false background signal)

Figure - 8
3 Judgment method: Remove the graphite tube and observe whether the spot of the cathode lamp on the graphite ring on the left side is concentric with the graphite ring.
Figure 9 is an example photograph of the graphite furnace from the optical axis (the spot is illuminated in the lower right corner of the graphite ring).
Solution: Re-adjust the four horizontal screws of the graphite furnace platform. (Because the adjustment steps are more complicated, they will not be described in detail here)
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