Instrument calibration and sample analysis. Silicon was determined in the extracts by AAS,
using a nitrous oxide-acetylene flame. The atomic absorption spectrometer initialization
procedure followed the manufacturer's recommended values (i.e., for gas flows and
lamp current). The 251.6-nm silicon wavelength was used. After the instrument
warm-up period, the gas flow, nebulizer uptake, and burner head alignment adjust-
ments were made while aspirating a Dow Corning © 200 fluid-MIBK solution con-
taining 60 to 70 •xg Si/ml. The final optimization adjustments for best sensitivity and
signal-to-noise ratio were made while aspirating the water-saturated blank solvent and
calibrating solutions.
Calibration in both wet and dry solvent solutions was necessary because silicon is very
sensitive to small changes in flame temperature. The water which saturates the MIBK
during the solvent extraction step alters the temperature and fuel-oxidant stoichiometry
of the N20-C2H2 flame. The atomic absorption response for silicon in wet MIBK is
perhaps reduced to 70% as compared to the same quantity of silicon in dry MIBK.
Dry MIBK silicon calibration solutions were prepared from Dow Corning © 200 fluid,
which has a polymer repeating unit molecular weight of 74. ! g/unit of Me2SiO. It
contains 37.9 percent silicon. Viscosity grades between 100 centistokes and 1000 cen-
tistokes are permissible for use as standards. For a typical standard, 50 mg of DC 200
fluid was weighed into a 250-ml flask partially filled with MIBK. DC 200 fluid tends
to adhere to dry glass surfaces and may require a long contact time with the solvent to
ensure complete solubility of the entire sample. An exact weight of 50 mg is not
essential, but that weight in 250 ml will result in a silicon concentration of 75 }zg of
silicon per milliliter of solvent, or 75 mg/kg Si on a wt/volume basis.
The calibration plot of Si concentration vs. absorbance was linear from the zero inter-
cept through a silicon concentration of 75 mg/kg provided that the atomic absorption
spectrometer was set up under optimum conditions. The exact silicon concentration was
calculated using the following equation:
Sample weight (milligrams + 0.1 mg) x 28.1 x 1000
74.1 x 250ml = micrograms of Si per ml
This weight/volume silicon standard may be serially diluted with MIBK to produce
calibrating solutions lower in silicon content as required.
To make the wet solvent standard calibration solutions, an aliquot of the standard was
shaken with an excess of water, which was centrifuged out, prior to use. For example,
to a 25-ml aliquot of calibrating solution, five ml of water and 0.25 ml of concentrated
HCI reagent were added. Dry standards can be stored and will remain constant, but
wet standards deteriorate on standing and so were made anew for each day's testing. In
addition to the standards, the background solvent which is aspirated into the instru-
ment between samples must also be water-saturated. A supply of MIBK stored over
water was maintained to use for background solvent.
Samples were aspirated into the AA spectrometer, and the absorbance values at 251.6
nm were recorded along with the absorbance values for the wet calibrating solutions. A
calibration plot of absorbance vs. silicon concentration was constructed. The concentra-
tion of silicon in the sample extracts was recorded from the calibration plot, and then
Instrument calibration and sample analysis. Silicon was determined in the extracts by AAS, using a nitrous oxide-acetylene flame. The atomic absorption spectrometer initialization procedure followed the manufacturer's recommended values (i.e., for gas flows and lamp current). The 251.6-nm silicon wavelength was used. After the instrument warm-up period, the gas flow, nebulizer uptake, and burner head alignment adjust- ments were made while aspirating a Dow Corning © 200 fluid-MIBK solution con- taining 60 to 70 •xg Si/ml. The final optimization adjustments for best sensitivity and signal-to-noise ratio were made while aspirating the water-saturated blank solvent and calibrating solutions. Calibration in both wet and dry solvent solutions was necessary because silicon is very sensitive to small changes in flame temperature. The water which saturates the MIBK during the solvent extraction step alters the temperature and fuel-oxidant stoichiometry of the N20-C2H2 flame. The atomic absorption response for silicon in wet MIBK is perhaps reduced to 70% as compared to the same quantity of silicon in dry MIBK. Dry MIBK silicon calibration solutions were prepared from Dow Corning © 200 fluid, which has a polymer repeating unit molecular weight of 74. ! g/unit of Me2SiO. It contains 37.9 percent silicon. Viscosity grades between 100 centistokes and 1000 cen- tistokes are permissible for use as standards. For a typical standard, 50 mg of DC 200 fluid was weighed into a 250-ml flask partially filled with MIBK. DC 200 fluid tends
to adhere to dry glass surfaces and may require a long contact time with the solvent to
ensure complete solubility of the entire sample. An exact weight of 50 mg is not
essential, but that weight in 250 ml will result in a silicon concentration of 75 }zg of
silicon per milliliter of solvent, or 75 mg/kg Si on a wt/volume basis.
The calibration plot of Si concentration vs. absorbance was linear from the zero inter-
cept through a silicon concentration of 75 mg/kg provided that the atomic absorption
spectrometer was set up under optimum conditions. The exact silicon concentration was
calculated using the following equation:
Sample weight (milligrams + 0.1 mg) x 28.1 x 1000
74.1 x 250ml = micrograms of Si per ml
This weight/volume silicon standard may be serially diluted with MIBK to produce
calibrating solutions lower in silicon content as required.
To make the wet solvent standard calibration solutions, an aliquot of the standard was
shaken with an excess of water, which was centrifuged out, prior to use. For example,
to a 25-ml aliquot of calibrating solution, five ml of water and 0.25 ml of concentrated
HCI reagent were added. Dry standards can be stored and will remain constant, but
wet standards deteriorate on standing and so were made anew for each day's testing. In
addition to the standards, the background solvent which is aspirated into the instru-
ment between samples must also be water-saturated. A supply of MIBK stored over
water was maintained to use for background solvent.
Samples were aspirated into the AA spectrometer, and the absorbance values at 251.6
nm were recorded along with the absorbance values for the wet calibrating solutions. A
calibration plot of absorbance vs. silicon concentration was constructed. The concentra-
tion of silicon in the sample extracts was recorded from the calibration plot, and then
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Instrument calibration and sample analysis. Silicon was determined in the extracts by AAS,
using a nitrous oxide-acetylene flame. The atomic absorption spectrometer initialization
procedure followed the manufacturer's recommended values (i.e., for gas flows and
lamp current). The 251.6-nm silicon wavelength was used. After the instrument
warm-up period, the gas flow, nebulizer uptake, and burner head alignment adjust-
ments were made while aspirating a Dow Corning © 200 fluid-MIBK solution con-
taining 60 to 70 •xg Si/ml. The final optimization adjustments for best sensitivity and
signal-to-noise ratio were made while aspirating the water-saturated blank solvent and
calibrating solutions.
Calibration in both wet and dry solvent solutions was necessary because silicon is very
sensitive to small changes in flame temperature. The water which saturates the MIBK
during the solvent extraction step alters the temperature and fuel-oxidant stoichiometry
of the N20-C2H2 flame. The atomic absorption response for silicon in wet MIBK is
perhaps reduced to 70% as compared to the same quantity of silicon in dry MIBK.
Dry MIBK silicon calibration solutions were prepared from Dow Corning © 200 fluid,
which has a polymer repeating unit molecular weight of 74. ! g/unit of Me2SiO. It
contains 37.9 percent silicon. Viscosity grades between 100 centistokes and 1000 cen-
tistokes are permissible for use as standards. For a typical standard, 50 mg of DC 200
fluid was weighed into a 250-ml flask partially filled with MIBK. DC 200 fluid tends
to adhere to dry glass surfaces and may require a long contact time with the solvent to
ensure complete solubility of the entire sample. An exact weight of 50 mg is not
essential, but that weight in 250 ml will result in a silicon concentration of 75 }zg of
silicon per milliliter of solvent, or 75 mg/kg Si on a wt/volume basis.
The calibration plot of Si concentration vs. absorbance was linear from the zero inter-
cept through a silicon concentration of 75 mg/kg provided that the atomic absorption
spectrometer was set up under optimum conditions. The exact silicon concentration was
calculated using the following equation:
Sample weight (milligrams + 0.1 mg) x 28.1 x 1000
74.1 x 250ml = micrograms of Si per ml
This weight/volume silicon standard may be serially diluted with MIBK to produce
calibrating solutions lower in silicon content as required.
To make the wet solvent standard calibration solutions, an aliquot of the standard was
shaken with an excess of water, which was centrifuged out, prior to use. For example,
to a 25-ml aliquot of calibrating solution, five ml of water and 0.25 ml of concentrated
HCI reagent were added. Dry standards can be stored and will remain constant, but
wet standards deteriorate on standing and so were made anew for each day's testing. In
addition to the standards, the background solvent which is aspirated into the instru-
ment between samples must also be water-saturated. A supply of MIBK stored over
water was maintained to use for background solvent.
Samples were aspirated into the AA spectrometer, and the absorbance values at 251.6
nm were recorded along with the absorbance values for the wet calibrating solutions. A
calibration plot of absorbance vs. silicon concentration was constructed. The concentra-
tion of silicon in the sample extracts was recorded from the calibration plot, and then
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