View
238
Download
2
Tags:
Embed Size (px)
Citation preview
Phosphorus MeasurementsPhosphorus Measurements
The Technique Detection Limits Wallastonite
The Technique Detection Limits Wallastonite
Ascorbic Acid TechniqueAscorbic Acid Technique
Ammonium molybdate and antimony potassium tartrate react in an acid medium with orthophosphate-phosphorus to form an antimony-phospho-molybdate complex.
This complex is reduced to an intensely blue-colored complex by ascorbic acid.
The reaction is slow and the complex is not stable and thus analysis must be performed after 10 minutes and before 30 minutes.
Ammonium molybdate and antimony potassium tartrate react in an acid medium with orthophosphate-phosphorus to form an antimony-phospho-molybdate complex.
This complex is reduced to an intensely blue-colored complex by ascorbic acid.
The reaction is slow and the complex is not stable and thus analysis must be performed after 10 minutes and before 30 minutes.
InterferenceInterference
Barium, lead, and silver interfere by forming a precipitate.
The interference from silica, which forms a pale-blue complex is small and can usually be considered negligible.
Arsenate is determined similarly to phosphorus and should be considered when present in concentrations higher than phosphorus.
Barium, lead, and silver interfere by forming a precipitate.
The interference from silica, which forms a pale-blue complex is small and can usually be considered negligible.
Arsenate is determined similarly to phosphorus and should be considered when present in concentrations higher than phosphorus.
Sample PreparationSample Preparation
No pretreatment Measures orthophosphates
Sulfuric acid treatment Measures hydrolyzable and orthophosphates
Persulfate digestion (strong oxidant) All phosphorus converted to orthophosphates Measures total phosphorus
No pretreatment Measures orthophosphates
Sulfuric acid treatment Measures hydrolyzable and orthophosphates
Persulfate digestion (strong oxidant) All phosphorus converted to orthophosphates Measures total phosphorus
Detection LimitsDetection Limits
What controls our ability to measure small concentrations of phosphorus?
How could we determine if the answer we get is meaningful?
Expected analytical range is 10 g/L to 1 mg/L as phosphorus
What controls our ability to measure small concentrations of phosphorus?
How could we determine if the answer we get is meaningful?
Expected analytical range is 10 g/L to 1 mg/L as phosphorus
Types of Detection LimitsTypes of Detection Limits
Instrument detection limit (IDL) instrument noise
Method detection limit (MDL) instrument noise sample preparation
Practical quantitation limit (PQL) routinely achievable detection limit with reasonable
assurance that any reported value greater than the PQL is reliable
5 times MDL
Instrument detection limit (IDL) instrument noise
Method detection limit (MDL) instrument noise sample preparation
Practical quantitation limit (PQL) routinely achievable detection limit with reasonable
assurance that any reported value greater than the PQL is reliable
5 times MDL
Which dominates?
Instrument Noise for a Spectrophotometer
Instrument Noise for a Spectrophotometer
What measurements are involved in obtaining a concentration reading from a spectrophotometer? _____________
_____________ _________________________________ _________________________________
_____________ _____________
What measurements are involved in obtaining a concentration reading from a spectrophotometer? _____________
_____________ _________________________________ _________________________________
_____________ _____________
Reference (P0)Reference (P0)
Lamp intensityLamp intensity
Absorbance of cuvetteAbsorbance of cuvette
StandardsStandards
SampleSample
Absorbance of reference solution
sample preparation
What are the limitations at low concentrations?
What are the limitations at low concentrations?
ReferenceReference
P0P0
Photons strike diode and produce a voltage responsePhotons strike diode and produce a voltage responseVoltage is digitizedVoltage is digitizedDigital Calculations → absorbance Digital Calculations → absorbance
bc =log P
PA o
Po - _________ light intensity P light intensity after passing through
sample As C 0 P __ Describe the journey after light leaves
sample to computer ______________________________________ ___________________ ________________________________
Po - _________ light intensity P light intensity after passing through
sample As C 0 P __ Describe the journey after light leaves
sample to computer ______________________________________ ___________________ ________________________________
Minimum Detectable AbsorbanceMinimum Detectable Absorbance
Suppose a 12 bit Analog to Digital Converter is used. What is the smallest absorbance that can be measured?
Suppose a 12 bit Analog to Digital Converter is used. What is the smallest absorbance that can be measured?
bc =log P
PA o bc =log
P
PA o 00011.0
12
2log
12
12
A 00011.0
12
2log
12
12
A
What if P0 is digitized into 200 intervals?
0022.0199200
log
A 0022.0199200
log
A
12 bit ( ) means _____ intervals4096122122
Additional Instrument Limitations
Additional Instrument Limitations
Differences in ___________ Fluctuations in ______ intensity
Power supply Warm up time
Repeatability of Cuvette ___________ Sample carryover if using sipper cell
Differences in ___________ Fluctuations in ______ intensity
Power supply Warm up time
Repeatability of Cuvette ___________ Sample carryover if using sipper cell
CuvettesCuvettes
LampLamp
alignmentalignment
Method Detection LimitMethod Detection Limit
"Method detection limit" is the smallest concentration that can be detected above the noise in a procedure and within a stated confidence level.
What is C such that I can be 99% confident that C > 0?
"Method detection limit" is the smallest concentration that can be detected above the noise in a procedure and within a stated confidence level.
What is C such that I can be 99% confident that C > 0?
Measuring the MDLMeasuring the MDL
Make a standard that is near the MDL Divide it into at least 7 portions. Process each portions through all sample
preparation and analysis steps Calculate the MDL using the equation
Make a standard that is near the MDL Divide it into at least 7 portions. Process each portions through all sample
preparation and analysis steps Calculate the MDL using the equation
,1 nstMDL ,1 nstMDL
n is the sample size, s is the standard deviation,=0.01 is generally the required confidence, t is the student t distribution
n is the sample size, s is the standard deviation,=0.01 is generally the required confidence, t is the student t distribution
Is the MDL > IDL?Is the MDL > IDL?
Are sample preparation errors significant? Variability in reagent blank (reference
sample) Results in a calibration curve with nonzero
intercept Sample contamination
Ultra pure water Acid washed plastic or glass ware Airborne contamination
Are sample preparation errors significant? Variability in reagent blank (reference
sample) Results in a calibration curve with nonzero
intercept Sample contamination
Ultra pure water Acid washed plastic or glass ware Airborne contamination
Decreasing the IDLDecreasing the IDL
May or may not decrease the MDL How can you improve an estimate of a
parameter? Use more ________! How could you use a section of the
spectrum? Use standards to determine _________
________ ______ Take an average of all the predicted
concentrations?
May or may not decrease the MDL How can you improve an estimate of a
parameter? Use more ________! How could you use a section of the
spectrum? Use standards to determine _________
________ ______ Take an average of all the predicted
concentrations?
diodesdiodes
extinction coefficient array
extinction coefficient array bc
Aε
bc
Aε
ε
A
bc
ε
A
bc
Arrays!
Maximum Detection LimitMaximum Detection Limit
Chemistry reagent limitations (stoichiometry) reaction by-products
Instrument limitations Maximum detection limits are easily
surmounted by __________
Chemistry reagent limitations (stoichiometry) reaction by-products
Instrument limitations Maximum detection limits are easily
surmounted by __________ dilutiondilution
WallastoniteWallastonite
Wallastonite (calcium metasilicate mixed with ferrous and aluminum metasilicate) tailings can be used to effectively remove phosphorus from solution.
These tailings are waste products generated during wallastonite ore mining in Northern New York.
Wallastonite (calcium metasilicate mixed with ferrous and aluminum metasilicate) tailings can be used to effectively remove phosphorus from solution.
These tailings are waste products generated during wallastonite ore mining in Northern New York.
Wallastonite Column ResultsWallastonite Column Results
Why are long retention times needed?
What is the mechanism?
Why are long retention times needed?
What is the mechanism?
0.000.100.200.300.400.500.600.700.800.901.00
0 20 40 60 80Retention time (hours)
Fra
ctio
n of
P R
emov
al
Arst, Gifford, Smith
Goehring, et al
5 mg phosphorus/L influent
Wallastonite Research (Proposal)Wallastonite Research (Proposal)
Quantify phosphorus removal as a function of time in batch tests
Phosphorus concentration (100 g/L) Wallastonite concentrations (0, 10, 30, 100,
300, 1000) mg per 7 mL phosphorus solution
Batch contact times (1, 5, 15, 30, 60, 90) minutes
Quantify phosphorus removal as a function of time in batch tests
Phosphorus concentration (100 g/L) Wallastonite concentrations (0, 10, 30, 100,
300, 1000) mg per 7 mL phosphorus solution
Batch contact times (1, 5, 15, 30, 60, 90) minutes
ExpectationsExpectations
0
0.2
0.4
0.6
0.8
1
0 20 40 60 80 100
time (minutes)
frac
tion
rem
ain
ing
0 (mg/5 mL)10 (mg/5 mL)30 (mg/5 mL)100 (mg/5 mL)300 (mg/5 mL)1000 (mg/5 mL)
PrelabPrelab
You will be creating 1 mL standards by diluting a stock of 100 g P/L (1, 3, 10, 30, 100 g P/L)
Reagent dilution problem
You will be creating 1 mL standards by diluting a stock of 100 g P/L (1, 3, 10, 30, 100 g P/L)
Reagent dilution problem
Spectral AnalysisSpectral Analysis
The initial extinction coefficient arrays are obtained from the slope of the linear regression line for A() = f(c)
Uses general least squares regression to add multiples of extinction coefficient arrays for each component to obtain the best curve fit for the sample
A better estimate of the extinction coefficient is obtained by interpolating between adjacent standards
Repeat least squares regression analysis
The initial extinction coefficient arrays are obtained from the slope of the linear regression line for A() = f(c)
Uses general least squares regression to add multiples of extinction coefficient arrays for each component to obtain the best curve fit for the sample
A better estimate of the extinction coefficient is obtained by interpolating between adjacent standards
Repeat least squares regression analysis