RA-4500 High Blanks
The following are the possible causes of high blank values:
- Contaminated reagent or deionized water
- Contaminated reagent or DI bottle
- Contamination inside the instrument
- Rise in the concentration of mercury at the atmosphere (air)
Carry out the following steps in order to narrow down the cause of the contamination.
1. Measure DI water using normal method settings, except change all of the reagents, other than stannous chloride, to "0.0" mL in the software. Leave the Stannous at the default amount.
If the blank value is OK, then one of the reagents, excluding the stannous, and/or the environment are contaminated, go
to step 4.
If the blank value is still high the problem is with the DI water, stannous chloride, or the cap, proceed to step 2.
2. If the result in Step 1 is not good then measure the stannous chloride by filling the test tube with 1 mL of stannous chloride and run the test tube using the method settings in step 1.
If the blank value is good, the problem is the DI water.
If the blank value is still high, the problem is with the stannous, the cap, the reaction tubing going to the detector, the
condenser, or the detector, proceed to Step 3.
3. If the result is not good after step 2, reduce the stannous reagent amount in the software method to "0.0", and rerun the same test tube and solution using the modified method.
If the blank is good, then it is confirmed that the stannous chloride reagent and the pump tube for the stannous are
contaminated.
If the blank value is still high, the problem is with the cap, tubing going to the detector, condenser, or the detector. Follow each of these procedures to clean the system:
4. If the blank value is good after step 1, start back with the default method in the software and set the quantities of the reagents other than stannous chloride solution to "0.1" mL, and the "Waiting time" and "Heating Time and "Cooling Time to "000". Fill the tube with 5 mL of DI water and make a measurement.
If the blank value decreases, it is confirmed there is contamination with one of the other reagents (not stannous) or the reagent pump tubes, proceed to step 5.
If the blank value did not change there is contamination with the room air, the tubing going to the detector, the condenser, or the detector flowcell. (A mercury concentration of several tens of ng/m3 or more is expected.) Follow each of these
procedures to clean the system:
5. If the blank value decreases with the test in step 4, the cause is contamination of one of the digestion or quenching reagents and/or tubing used for the digestion. To discern whether it is the reagents or the tubing do a blank value test following the method settings in Step 1, but this time pipette all the other reagents into the test tube with the DI water and run the tube.
If the blank value goes down, the cause is a contaminated reagent tubing. In that case, pour nitric acid (1+10) into a reagent bottle, put all the reagent lines in that solution and carry out the "Reagent Filling" step in the software and leave the instrument sitting with the nitric acid in the tubing overnight.
If the blank value does not change, or changes very little, the contamination is with one of the digestion reagents, proceed to Step 6.
6. If the blank value is still high after the test in Step 5, the contamination is in one of the digestion reagents or the quenching reagent. To discern which reagent(s) has the contamination, put 1 mL of each reagent in its own clean test tube and run the Step 1 settings on each tube. The test tubes that produce signal are the reagent(s) with the mercury contamination.
If one of your reagents is contaminated check the raw reagents, the reagent solution storage bottle, the utensils used to measure the chemicals during reagent preparation, and the bottles/beakers used to prepare the reagent solution. With low level analysis it is recommended to have dedicated mercury preparation glassware and utensils.Related Articles
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