ASTM D7623 Standard Test Method for Total Mercury in Crude Oil Using Combustion-Gold Amalgamation and Cold Vapor Atomic Absorption Method
9. Preparation of Mercury Standards
9.1 L-cysteine Solution, 0.001 m % - Weigh approximately 0.010 g of L-cysteine to the nearest milligram onto a glassine weighing paper. Quantitatively transfer the contents of the paper to a 1 L volumetric flask. Add approximately 800 mL of water. Add 2 mL of nitric acid into the flask with a pipette. Swirl to dissolve the solids. Fill to the mark with water. The L-cysteine solution stabilizes the ionic mercury and is used to prepare working standards.
9.2 Mercuric Chloride Stock Solution - Weigh 0.0677 g of mercuric chloride onto weighing paper. Quantitatively transfer the material to a 500 mL volumetric flask using the L-cysteine solution prepared in accordance with 9.1. Fill to the mark with L-cysteine solution and mix. This results in a mercury concentration of 100 mg/kg.
9.3 Working Standards Set - Prepare a set of mercury standards that are appropriate to the range settings on the instrument in use. An example of such a set follows.
9.3.1 With a 5 mL pipette, transfer an aliquot of the mercuric chloride stock solution to a 100 mL volumetric flask. Fill the flask to the mark with L-cysteine solution and mix. This makes a 5 mg/kg standard.
9.3.2 With a 1 mL pipette, transfer an aliquot of the mercuric chloride stock solution to a 100 mL volumetric flask and fill up to the mark with L-cysteine solution. Mix. This makes a 1 mg/kg standard.
9.3.3 Perform appropriate dilutions of the 1 mg/kg standard with L-cysteine solution to prepare 0.1 mg/kg and 0.01 mg/kg mercury standards.
10. Instrument Preparation
10.1 Assemble the instrumental 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 sensitivity range and sample heating mode based upon the expected mercury concentration and the sample matrix.
10.3 Reagents used for the analysis of hydrocarbon samples should be placed in the 750 °C muffle furnace overnight prior to use. Sample boats should also be pretreated in the muffle furnace before initial use.
10.4 The instrument should be run through several heating cycles after being powered up to attain a new blank signal. A satisfactory blank should have stable response and a signal that corresponds to the equivalent of <0.1 ng/g mercury in the sample.
11. Calibration Procedure
11.1 Choose the working standard to match the sensitivity range to which the instrument is set. For example, a sensitivity range with a maximum of 2 ng of mercury would require using the nominal 0.01 mg/kg standard solution. A sensitivity range of 20 ng would use the 0.1 mg/kg standard and so on.
11.2 After the instrument has cycled several times and a stable response has been attained, initiate the analysis with no sample boat in the furnace and record the blank value.
11.3 Using a micropipetter and tip, transfer 50 µL of standard solution to a cooled sample boat. Place the sample boat into the furnace area and start the instrument. Repeat this step for 100 µL, 150 µL, and 200 µL aliquots 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 the manufacturer's instructions to use available software tools that automate the calculations.
11.4 A reagent blank is subtracted from the response of the samples to compensate for trace mercury signal associated with the sample boat and combustion reagents.
11.4.1 Remove a sample boat and treated reagent(s) from the muffle furnace and allow to cool in a container that is purged of ambient mercury.
11.4.2 Prepare the reagent blank by adding the cooled reagent(s) in the recommended portions to a sample boat.
11.4.3 Open the inlet cover. Place the sample boat into the instrument's combustion furnace and run on the same heating cycle as that used for samples. Record the reagent blank for subtraction from the mercury response of the samples. The data handling software of the instrument may perform this calculation for each sample in the batch.
NOTE 3 - An indicated problem with linearity of the instrument during calibration can result from contamination of the CRM or calibrating agent as the container becomes depleted. It is therefore recommended that the CRM or calibrating agent be discarded when less than five grams remain in the container.
11.5 Analyze, as samples, portions of a CRM, reference crude oil, or calibrating agent chosen to represent the level of mercury in the samples to be tested. Use the mercury values for calibration. These values must have been calculated previously from the certified mercury values. Continue analyzing until the results from five consecutive determinations fall within the repeatability interval of the test method. Calibrate the instrument according to the manufacturer's instructions using these values. Analyze, as samples, two CRM reference crude oils or calibrating agents that bracket the range of values to be tested. The results obtained for these samples must be within the stated precision limits of the CRM, reference crude oil, or calibrating agent or the calibration procedure shall be repeated. Record all calibrations.
11.6 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, or all results obtained since the last successful control check must be rejected and the calibration procedure repeated.
