An assay method to measure arsenic using a MHS-10 and AA200 and Mercury Hydride System.
Sample Preparation -Sample Weights to be used:
Heads…………….. 0.3 g
Cu Con……………..0.1 g
Zn Con……………..0.2 g
Knelson…………….0.3 g
Core…………………0.1 g
STANDARD MP-1a ……………0.1 g
Cu Con Check Sample………..0.1 g
- Weigh the samples into 250 ml low form beakers.
- Add 10 mL Acid Mix (1:9 v/v CONC H2SO4:HNO3) and digest on a hot plate. Start the digestion on one switch and immediately turn to two. Remove each sample when the fumes are prolific and no liquid remains in the beaker. Do not over dry.
- Allow the samples to cool. Add about 40 ml of distilled H20, ~1mL (a squirt) of 3:2 H2SO4 and cover with a watch glass. Bring the samples to a boil on a bare plate. Remove, cool, and transfer to a 200 mL volumetric flask.
- Mix samples by inverting 15 times.
Cell Alignment Procedure:
- Install the arsenic burner head equipped with the quartz cell bracket. The bracket must be tilted out of the light beam path.
- Remove a lamp and install the arsenic lamp.
- Turn on the AA200.
- Select the MHS method, and then touch OK.
- Select TOOLS, then SELECT METHOD, then ARSENIC, then OK twice, and finally SET UP INSTRUMENT. Wait until the instrument has finished the SET UP routine.
- Install the quartz cell and tilt the bracket into position. Slip a piece of cardboard into the rear slot of the burner compartment and turn out the room lights.
- Carry out a visual burner adjustment until the maximum amount of light beam passes through the center of the quartz cell by using the vertical, horizontal and angle adjustment knobs.
- Select the FLAME page.
- Touch the AUTO ZERO GRAPH button.
- Fine tune the cell alignment by making incremental adjustments to the burner’s vertical, horizontal and angle adjustment knobs to obtain minimum absorbance.
- Tilt the quartz cell away from the burner, ignite the flame by touching the ON/OFF button, and then tilt the cell back into the flame.
- The fuel setting will be richer than the normal AA operation and the cell should glow orange. If the cell is not hot enough, a low absorbance will be obtained.
- Allow 45 min. – 1 hour for warm-up and stabilization before starting the analysis.
Readjustment of the A.A. Settings Prior to Reading the Samples
- Repeat steps #8 – #10 from the cell alignment procedure to realign the cell.
- Repeat the wavelength by touching the LAMP page, then SET UP INSTRUMENT button. The arsenic wavelength tends to drift as the lamp warms up. This is critical.
Determination of the Samples
- Run a minimum of 3 – 4 blanks until a stable base line is achieved.
- Remove the reaction flask from the flange and rinse the reaction tip with distilled water and dry it. This must be done every time a reaction flask is changed. Attach a second blank and repeat the 40 sec. purge cycle (step #1). Once the system is purged, read the second blank as in step #2.
- Touch ANALYZE BLANK, and then touch OK. When the AGC disappears, wait an additional 1-2 seconds* then actuate the plunger and hold it down for 8 seconds. Release the plunger and wait until the measuring cycle is complete (20 seconds). Record the absorbance reading. *This is critical otherwise low and unstable results will occur.
- Touch the Analyze page. Attach the reaction flask with the blank sample (10 mL 25 % HCL sol. only) to the flange. Make sure Argon is bubbling in the reductant reservoir. Push ANALYZE SAMPLE, then OK button and wait until the measuring cycle is complete (20 seconds). Push the ANALYZE, then OK button again and wait another 20 seconds. The reaction flask is now purged of all the air.
Standard Calibration
- Attach the reaction flask containing standard #1 to the flange. Purge the system for 40 seconds using the ANALYZE SAMPLE button (two 20 second cycles). *Touch the Standard window and then touch 1: 0.42 % then touch OK. Touch the ANALYZE STANDARD button again. When the AGC disappears wait an additional 1-2 seconds then actuate the plunger for 8 seconds then release. *The appropriate standard must be selected prior to running. Run two or more standards to insure the 0.42 % setting is reproducible. Record the peak area values for std #1.
- *Run the 0.84 % std. as above. Record the peak area. Run another 0.84% std if the graph and absorbance is suspect.
Reading the Unknown
- Read the unknown samples by staggering the duplicates i.e. #1, #3, #5, #7.
- Run a 0.42 % std. every 4-5 samples. If the reading is between 0.40 % – 0.44 % proceed to the next set of unknowns. If the curve has drifted, establish a new std. curve.
- Read unknown samples #2, 4, 6, 8 then run a 0.42 % std.
- If the Borohydride flask needs refilling, bubble Argon through it for 5 minutes and run blanks to establish a baseline and standards to reset the curve. Continue reading the unknown samples.
End of the Run
- Once the run is complete, disconnect the Borohydride reservoir and reconnect the distilled water reservoir. Depress the plunger for a few seconds to rinse any residual Borohydride from the transfer tubes in the MHS-10 instrument. Rinse the reaction tip inside and out with distilled water. Allow Argon to run for an additional minute then shut it off.
- Touch the FLAME page then the ON/OFF button to turn the flame off. Touch the LAMP page, then INSTALL LAMPS, then the ON/OFF check mark, then touch OK to turn the lamp off. Turn the power switch off and allow the system to cool down before disassembling.
- Attach an empty reaction flask but leave it loose so the seal is not damaged.
- Rinse the reaction flasks four times under warm tap water, invert and allow to drain prior to putting them away.
Note: It is recommended to periodically remove flash back arrestor and rinse it with 25 % HCl and distilled water to eliminate any NaBH4 residue.
Test for Peak Time
If the Argon flow has been changed a Peak Time Check must be performed.
- Simply look at the graph after the 0.84 % standard has been run. The Peak reading should occur at or before the 8 second mark and the entire sample should be passed through the system in 20 seconds (i.e. the graph should be returned to the baseline level after the 20 second mark)
- If these times are exceeded, the Argon flow must be increased slightly. If the Peak is abrupt or the Hydrogen coming out of the ends of the cell catches fire, the Argon flow must be reduced slightly.
Calculations
HEAD SAMPLE…………………0.3 g @ 100 uL…………READING divided by 6
CU CON SAMPLE …………….0.1 g @ 50 uL…………DIRECT READING
ZN CON SAMPLE……………..0.2 g @ 100 uL…………READING divide by 4
KNELSON SAMPLE………….0.3 g @ 100 uL…………READING divide by 6
CORE SAMPLE…………………0.1 g @ 100 uL…………READING divide by 2
Formula: A 50 uL aliquot from a 0.1 g sample = A Direct Reading
Standard MP-1a 0.84 %
Reagents
1. 25 % HCL v/v
2. 2 % NaBH4 sol. in 1 % KOH (5 g NaBH4 + 2.5 g KOH into 250 mL)
3. Acid Mix (1:9 v/v H2SO4:HNO3)
4. 3:2 H2SO4