Transcript How could we measure ANC?

Phosphorus Measurements
 The
Technique
 Detection Limits
 Wallastonite
Ascorbic 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.
Interference
 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 Preparation
 No
pretreatment
 Measures
 Sulfuric
acid treatment
 Measures
 Persulfate
 All
orthophosphates
hydrolyzable and orthophosphates
digestion (strong oxidant)
phosphorus converted to orthophosphates
 Measures total phosphorus
Detection Limits
 What
controls our ability to measure small
concentrations of phosphorus?
 How
could we determine if the answer we
get is meaningful?
analytical range is 10 mg/L to 1
mg/L as phosphorus
 Expected
Types of Detection Limits
 Instrument
detection limit (IDL)
 instrument
 Method
noise
detection limit (MDL)
 instrument
noise
 sample preparation
 Practical
Which dominates?
quantitation limit (PQL)
 routinely
achievable detection limit with
reasonable assurance that any reported value
greater than the PQL is reliable
 5 times MDL
Instrument Noise for a
Spectrophotometer
 What
measurements are involved in
obtaining a concentration reading from a
spectrophotometer?
Reference (P0)
 _____________
Lamp intensity
 _________________________________
Absorbance of reference solution
 _________________________________
Absorbance of cuvette
 _____________
Standards
sample preparation
 _____________
Sample
 _____________
What are the limitations at low
concentrations?
 Po
- _________
Reference light intensity
 P light intensity after passing through
sample
Po
A  log = bc
 As C  0 P __
P0
P
 Describe the journey after light leaves
sample to computer
 ______________________________________
Photons strike diode and produce a voltage response
 ___________________
Voltage is digitized
Digital Calculations → absorbance
 ________________________________
Minimum Detectable Absorbance
 Suppose
a 12 bit Analog to Digital
Converter is used. What is the smallest
absorbance that can be measured?
12 bit ( 212) means _____
4096 intervals
A  log
Po
P
= bc
 212 
  0.00011
A  log

12
 2  1
What if P0 is digitized into 200 intervals?
 200 
A  log
  0.0022
 199 
Additional Instrument
Limitations
 Differences
in ___________
Cuvettes
 Fluctuations in ______
Lamp intensity
 Power
supply
 Warm up time
 Repeatability
of Cuvette ___________
alignment
 Sample carryover if using sipper cell
Method 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?
Measuring 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
MDL  st n1,
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?
 Are
sample preparation errors significant?
 Variability in reagent blank (reference
sample)
 Results
in a calibration curve with nonzero
intercept
 Sample
 Ultra
contamination
pure water
 Acid washed plastic or glass ware
 Airborne contamination
Decreasing the IDL
 May
or may not decrease the MDL
 How can you improve an estimate of a
parameter?
 Use more ________!
diodes
 How could you use a section of the
spectrum?
extinction
standards to determine _________
________ array
______
coefficient
 Take an average of all the predicted
concentrations?
 Use
Arrays!
ε
c
A
bc
A
bε
Maximum Detection Limit
 Chemistry
 reagent
limitations (stoichiometry)
 reaction by-products
 Instrument
limitations
 Maximum detection limits are easily
surmounted by __________
dilution
Wallastonite
 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 Results
 Why
Fraction of P Removal
are long
retention times
needed?
 What is the
mechanism?
5 mg phosphorus/L influent
1.00
0.90
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.00
Arst, Gifford, Smith
Goehring, et al
0
20
40
60
Retention time (hours)
80
Wallastonite Research (Proposal)
 Quantify
phosphorus removal as a function
of time in batch tests
 Phosphorus concentration (100 mg/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
Expectations
fraction remaining
1
0.8
0.6
0 (mg/5 mL)
10 (mg/5 mL)
30 (mg/5 mL)
100 (mg/5 mL)
300 (mg/5 mL)
1000 (mg/5 mL)
0.4
0.2
0
0
20
40
60
time (minutes)
80
100
Prelab
 You
will be creating 1 mL standards by
diluting a stock of 100 mg P/L (1, 3, 10, 30,
100 mg P/L)
 Reagent dilution problem
Spectral Analysis
The initial extinction coefficient arrays are obtained
from the slope of the linear regression line for A(l) =
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
