Quality Assurance Flashcards
What are different internal standards that can be used? (3)
- Same substance, but mass-labeled (or somehow different)
- Different substance (chemically similar)
- Same substance (spiking)
standard solutions have a known ____ and ____.
purity; concentration
What is a stock standard?
standard of HIGH CONCENTRATION, long storage life
- used to prepare working standards
A calibration curve is ____ _____ while spiking is ____ _____.
external calibration; internal calibration
When would a standard be determined to be expired?
The measurements of that standard differ from a newer standard by more than 10% (SANCO)
The storage condition of standards will affect:
stability/shelf life
True/False: stock standards are used for spiking, calibration, and comparison with samples during analysis
False; stock standards used to prepare working standards for that purpose
the linear relationship established by running standards should be:
a non-weighted regression curve (all points carry equal weight, do not force through intercept, etc)
What is the blank used for, in preparing an external calibration curve?
Subtract signal of blank from signal for each calibration point (correction)
How should the spacing and range of the standards be done, according to IUPAC?
spacing: depends on precision level required; should be evenly spaced
range: should be 0 to 150% of expected test result
How should calibration samples be run?
Random order
According to IUPAC, what is the minimum number of standards needed for a calibration curve?
5 (excluding 0)
What are residuals an indicator of?
Whether the calibration curve is acceptable
The residuals should be in what sort of distribution? What if this is not the case?
Randomly distributed around zero (regression curve of residuals is straight line y=0)
if TREND exists: problem with method, need to reexamine
List the steps of preparing an external calibration curve
- Prepare blank + standard samples (need triplicates)
- run in random order
- Calculate mean and SD for each point, correct using blank
- Graph and establish regression curve of best fit
What are residuals?
Difference between MEASURED value and CALCULATED value (using calibration curve)
What is the “linear range” of a signal on a calibration curve?
the concentration range at which SIGNAL INTENSITY is directly proportional to concentration (linear relationship)
(the blank signal + 3SD)/slope is the:
LOD
What is IDL? What is MDL?
IDL: instrument detection limit
MDL: Method detection limit
True/False: A good instrument should have a high LOD
False: a higher LOD means that it cannot detect lower concentrations confidently
Differentiate IDL and MDL:
IDL: instrument limit, expressed as amount injected in machine for analysis; instrument blanks are run (only through machine)
MDL: for entire procedure, expressed as concentration in food matrix intially; procedural blanks are run (taken through all steps of procedure)
What is LOQ? Define it.
limit of quantification
smallest level that can be quantified with acceptable precision/accuracy
10SD from concentration in procedural blank
What is LOD? Define it.
what is the criteria?
Limit of Detection
smallest level that produces a SIGNIFICANTLY DIFFERENT signal from the background noise/blank
(can be distinguished/detected)
3SD from response obtained on blanks
If the result is below the LOD, what do you do?
report as below LOD
Procedural blanks are also known as:
matrix blanks
Why might calibration curves prepared using standard samples be inaccurate for comparison with a food sample?
Matrix effect; calibration curve does not correspond to signal obtained from food sample
Spiking is known as the:
standard addition method
What do you report if the result is above LOD but below LOQ?
signal is sufficient to indicate presence (detectable), but not enough to be quantified.
Report as BELOW LOQ
Can a result be above the LOQ but not the LOD?
No. LOD < LOQ
How can the matrix affect the analysis results?
Increase or decrease signal, give higher or lower concentrations than actual amounts
You can only report the level of the analyte if the result was above both the ____ and the ____.
LOD, LOQ
The spiking solution added should be: (2)
- small amount (assume volume stays constant)
- highly concentrated (so can add small amount)
What is the standard addition method?
Add KNOWN AMOUNTS of standard solution (same as analyte) to sample -> measure response before/after each addition (x, x+1, x+2, x+3…) -> establish curve, determine original amount (x)
Or, can separate sample into many SUBSAMPLES, add increasing amounts to each
What is the internal standard method?
Adding fixed amount of ANOTHER CHEMICAL to sample
The ____ is the same over a range of concentrations, which is used in internal standard calculations.
RF (response factor)
The substance added in the internal standard method should: (2)
have similar chemical properties
Not interfere with analyte analysis
The internal standard method aims to deal with: (3)
Matrix effect
Injection variability
Analyte loss
What is used as an IS in ICP-MS?
rare elements (Yttrium, Rhodium, Indium, Scandium)
What could we use as an IS in GC-MS or LC-MS?
Mass labelled surrogate
what is the response factor?
ratio of (signal A/IS) / (concentration A/IS)
“Trueness” is also known as ____. How do you assess it? (4)
Recovery
- Comparison of results with known standard (certified ref material)
- Compare results with those from a method that works, has little error
- compare with results from interlaboratory tests
- spiking samples (based on blanks or positive blanks)
Where can you source certified materials in Canada? In the US?
Canada: NRC Certified Reference Materials
US: NIST
What is mass labelling?
Using a modified version of the compound, with C13 or deuterium instead of normal C or H (molecule will have different molecular mass, so can be differentiated from original)
