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