Standard addition (Lecture 18) Flashcards
What is quality assurance?
Quality assurance is what we do to get the right answer for our purpose with sufficient accuracy and precision to support subsequent decisions.
What is method validation?
Method validation is the process of proving that an analytical method is acceptable for the intended purpose. This includes determining: specificity, accuracy, precision, detection limit, quantitation limit, etc.
How can you demonstrate accuracy?
- Analyse a certified reference material under identical conditions to your experimental sample. You will find the value of the reference material within the precision of your method.
- Compare results from two or more different analytical methods. They should agree within their expected precision.
- Analyse a blank sample spiked with a know addition of analyte. The matrix must be the same as your unknown. If you cannot prepare a blank with the same matrix as the unknown, then use standard additions of analyte to the unknown.
In standard addition what is added to the unknown?
Known quantities (spikes) of analyte are added to the unknown.
What does the increase of signal from standard addition allow us to calculate?
The increase in signal allows us to calculate how much analyte was in the original unknown.
What does the method of standard addition require?
This method requires a linear response to analyte concentration.
What is standard addition used for?
Used for samples with unknown or complex compositions that affect the analytical signal. We hence cannot create standards and blanks whose composition matches that of the unknown sample to prepare a “traditional” calibration curve.
What is the matrix effect?
A change in signal caused by anything in the sample other than the analyte.
What equations are associated with standard addition?
(conc of analyte in initial solution) / (conc of analyte plus standard in final solution) = (signal from initial solution) / (signal from final solution)
[X]i / ([X]f + [S]f) = I(x) / I(x+s)
What are the equations associated with volume dependence?
[X]f = [X]i x ( Vi/(Vi + Vs) ) [S]f = [S]i x ( Vs/(Vi + Vs) ) The quotient (initial volume/final volume), which relates final concentration to initial concentration, is called the dilution factor. It comes directly from the solution dilution formula.
What is the method for standard addition with variable total volume?
- We begin with an unknown solution and measure the analytical signal.
- Then we add a small volume of concentrated standard and measure the signal again.
- We add several more small volumes of standard and measure the signal after each addition.
Standard should be concentrated so that only small volumes are added and the sample matrix is not appreciably altered.
When is standard addition with variable total volume used?
If the analysis does not consume solution.
What is the equation associated with standard addition with variable total volume?
I(x+s) x ( (Vi + Vs)/Vi ) = I(x) + I(x)/[X]i x ( [S]i x V(s)/V(i) )
Where I(x+s) x ( (Vi + Vs)/Vi ) is on the y axis, [S]i x V(s)/V(i) is on the x axis and [X]i is the x intercept.
What is the method for standard addition with constant total volume?
- Equal volumes of unknown are pipetted into several volumetric flasks.
- Increasing volumes of standard are added to each flask.
- Each sample is then diluted to the same final volume.
- For each flask, a measurement of analytical signal, IS+X, is then made.
Every flask contains the same concentration of unknown and differing concentrations of standard.
When is standard addition with constant total volume used?
If the analysis consumes some of the solution.
