ASTM D7622 Standard Test Method for Total Mercury in Crude Oil Using Combustion and Direct Cold Vapor Atomic Absorption Method with Zeeman Background Correction
9. Preparation of Standards
9.1 Working Standard Set - Prepare a set of standards that are appropriate to the range settings on the instrument in use. An example of such a set follows:
9.1.1 Place 30 mL of a dilution solution in a volumetric flask (100 mL volume). Then place 2 mL of a standard sample of mercury ions NIST or other standard reference material issuing bodies' traceable standard solution (concentration C1 = 1.0 g/L). Bring the contents of the volumetric flask up to the mark with the dilution solution and stir thoroughly. This results in a mercury concentration of 20 mg/L (C2). The solution may be stored in a refrigerator during 6 months.
9.1.2 Place 30 mL of a dilution solution in a volumetric flask (100 mL volume). Then place 10 mL ofa standard sample of mercury solution C2 = 20 mg/L. Bring the contents of the retort up to the mark with the dilution solution and stir thoroughly. This makes a 2 mg/L = 2000 µg/L (C3). The solution may be stored in a refrigerator during 3 months.
9.1.3 Place 30 mL of a dilution solution in a volumetric flask (100 mL volume). Then place 10 mL ofa standard sample of mercury solution C3 = 2.0 mg/L. Bring the contents of the retort up to the mark with the dilution solution and stir thoroughly. This makes a 200 µg/L (C4). The solution may be stored in a refrigerator during 3 months.
9.1.4 Place 30 mL of a dilution solution in a volumetric flask (100 mL volume). Then place 10 mL ofa standard sample of mercury solution C4 = 200 µg/L.) Bring the contents of the retort up to the mark with the dilution solution and stir thoroughly. This makes a 20 µg/L (C5). The solution may be stored in a refrigerator during 1 month.
NOTE 3 - The standard preparation procedure given in Test Method D7623 is also acceptable.
10. Instrument Preparation
10.1 Assemble the instrument and check for leaks in the system in accordance with the manufacturer's instructions. Follow the instrument manufacturer's recommended procedure to optimize the performance of the instrument.
10.2 Choose the proper sample heating mode based upon the expected mercury concentration and the sample matrix.
10.3 Samples boats and charcoal should also be pretreated in the muffle furnace before initial use.
10.4 A satisfactory blank should have stable response and a signal that corresponds to the equivalent of < 3 ng/g mercury.
11. Calibration Procedure
11.1 Spread approximately 0.2 g of charcoal in the bottom ofa cooled sample boat. Using a micropipetter and tip, transfer 100 µL of the dilution solution on to a charcoal media. Place the sample boat into the furnace area and start the instrument. Repeat this step four times to obtain blank value.
11.2 Spread approximately 0.2 g of charcoal over bottom of a cooled sample boat. Using a micropipetter and tip, transfer 100 µL of standard solution 200 µg/L on to a charcoal media. Place the sample boat into the furnace area and start the instrument. Repeat this step for 20 µg/L and 2000 µg/L to span the effective range of response. Create a calibration curve by assigning the appropriate mass of mercury introduced into the instrument with the corresponding response. Follow manufacturer's recommendations to use available software tools that automate the calculations.
11.3 Periodic Calibration Verification and Recalibration - In accordance with Practice D6792, analyze a control sample on a periodic basis. Results obtained for the control sample must be within established limits. If a control check sample result is out of control, all results obtained since the last successful control check must be rejected and the calibration procedure repeated.
