Quiz 1 Flashcards
What concentration quantifies as a major substituent in a sample?
If it’s greater than 1%
What concentration quantifies as a minor substituent in a sample?
0.1-1%
What concentration quantifies as a trace substituent in a sample?
less than 0.1%
Can tiny concentrations be bad?
Yes, for example arsenic in water
What is a SRM?
Are materials produced by the NIST (National institute of standards and technology) with an attached composition including the amount of all components within the material (with standard deviations) as well as the methods used to analyze the material.
What is a SRM used for?
An SRM is used to help develop accurate methods of analysis, to calibrate measurement systems and test instrumentation and methods, and to ensure the long-term adequacy and integrity of measurement quality assurance programs.
How would you use a SRM?
If you use an analytical method to analyze an SRM and the concentration you obtain do not match the ones the SRM states then you know your analytical method is incorrect, allows you to test and fix analytical method.
Name two analytical methods the NIST uses to analyze SRM
Instrumental neutron activation analysis and graphite furnace atomic absorption spectometry
Does a high precision imply accuracy?
No, measurements can have a high precision and therefore be very reproducible but they could all be measurements far off the actual accurate value. A smaller s (deviation) does not imply greater accuracy.
How are distribution curves useful?
For example if selling light bulbs can create curve with data on how long amounts of light bulbs burnt, allows us to say on average how long light bulbs will burn as we cannot say exactly how much a light bulb will burn.
What is the single most important characteristic of any result from an analytical method?
It is the statement of it’s uncertainty.
Explain how me might analyze chocolate for caffeine and theobromine
Through chromatography have a chocolate extract sample go through a chromatography column which then separates both analytes as theobromine elutes faster than caffeine due to caffeine interacting with hydrocarbons on SIO2 molecules that are packed in the column. Then have these substances go through UV light and then a detector to create graphs with spikes for both analytes. The peaks of the compounds will tell us the quantities of each.
How do we determine the amount of theobromine and caffeine in our chromatography graph?
By creating a calibration curve using known quantities of each analyte.
How do we construct a calibration curve?
Can do this by injecting standard solution with known concentrations of each analyte into the column and then measuring the detector response to construct a curve from these results. Then using equation derived from curve you can calculate exactly how much analyte corresponds to the observed detector response.
How do wo determine the equation of the calibration curve (ie how do we create the best fit line)?
Using the method of least squares
What are the two assumption in the method of least squares?
That the standard deviation is the same for all data, and that the error in our y values (detector response) is greater than that of our X values (known concentration).
What does the method of least squares do?
We use the method of least squares to draw the “best” straight line through experimental data points that have some scatter and do not lie perfectly on a straight line.
How does the method of least squares work?
You square the vertical deviations (di) (the space from the Y point you obtained and the Y point that the line on the data corresponds too)- this is seen through eqn yi-y. We then square all of these deviations in order to obtain only positive numbers, we then pick a best fit line that minimizes this value using arithmatic.
Using the data points on table 4-7 pg105 calculate the best fit line that minimizes di.
What is a standard solution?
Solutions with known amount of protein (analyte) and reagent.
What is a blank solution?
Solutions with no protein (analyte) but with all other reagents.
What instrument is used to determine protein conc in plasma
Instrument: Use UV-vis-spectrophotometer to measure light absorbance in the plasma, as the light absorbance is directly proportional to the amount of protein.
What is the first step in creating a calibration curve for protein conc in plasma?
We would first prepare standard solutions that span the range of concentrations for the unknown- we would do this by putting a fixed amount of protein in the plasma therefore creating fixed concentrations and then recording the UV absorbance of these samples.
What is the second step in creating a calibration curve for protein conc in plasma?
We would create blank solutions- solutions with plasma but no protein, and then record the UV absorbances in these samples.
What is the third step in creating a calibration curve for protein conc in plasma?
Subtracting the average absorbance of the blank from each measure absorbance of the standard to get the corrected absorbance for just the plasma.
What is the fourth step in creating a calibration curve for protein conc in plasma?
Make a graph of protein analyzed versus corrected absorbance, and then through the method least squares create a calibration curve of it.
Your analytical method is as good as?
Your calibration curve
When creating a calibration curve what’s the minimum amount of data needed?
At least 6 concentrations and two replicates of each.
What is the linear range vs the dynamic range?
The linear range is the area in the calibration curve where response is proportional to analyte concentration, the dynamic range is not linear and harder to extrapolate from. We do not take values from the dynamic range.
Imagine your unknown concentration comes out to 28 ug, but your last measured sample in your calibration curve in 26ug, can you just extrapolate further than the 26ug and get a value from the calibration curve eqn?
no, It is not reliable to extrapolate any calibration curve, linear or nonlinear, beyond the measured range of standards. Measure standards in the entire concentration range of interest.
What is quality assurance?
Quality assurance is what you can do to verify the analytical result in terms of accuracy and precision.
What happened in the International Measurement Evaluation Program (Belgium)
Some A chemists calculated lead conc in water, they then asked 181 diff labs to do the same with any analytical method of their choice.
What was the certified range of lead in the water? What as the average concentration from the results of the other 181 labs?
The certified range of lead in the water was (62.3 +/- 1.3) the average of the results was less than this.
How can the average of lead be lower? As in, how can a sample lose analyte?
lead can be absorbed through glass surfaces as glass surfaces contain silica, this is why you use teflon surfaces when containing an analyte.
How can the average of lead be higher? As in, how can a sample gain analyte?
You could put sample in a jar and leave it unopened and lead from the surroundings could enter the sample, or you cold have dirty glassware. This is why we clean glassware with nitrogen.
What are the three basics of QA?
Raw data, treated data, and results
Define raw data
individual measurements for example: area under peaks in chromatography.
Define treated data
Concentrations (or measurements) derived from a calibration curve made from raw data.
Define results
Quantities reported after statistical analysis of data, ex mean, standard deviation, confidence interval
What are the three steps in the Quality Assurance process?
Use objectives, specifications, and Assessment
Define analytical use objectives
This states the purpose for which results will be used or customers needs, after stating the purpose this determines what kind of data you’re collecting and what way you’re collecting it. For example, a use objective might be minimizing the arsenic in water to 0.0001%, now we know we need a very precise analytical method to measure arsenic in water.
What are questions you must ask after obtaining a use objective? Give an example.
For example in landfarming we must ask what information we need (in this case we need measurements of hydrocarbon in soil), ask how accurate and precise results have to be, and address practical constraints such as speed and cost.
What are the compromises you must make when addressing a use objective?
Need to make the best compromise between speed, cost, and acceptable accuracy and precision.
Say you’re analyzing a soil sample, what device is better a GC-MS or a handheld device?
As the GC-MS is very expensive to take one sample, and we are taking samples out in the field which introduces a lot of variables, it’s better to use the handheld device as it’s more cost effective and allows us to take way more samples.
Define specifications
Is how good numbers must be and what precautions are required in the analytical procedure.
What questions do specifications need to be made for?
How we should take the representative sample
How many samples are needed
Do we need to take special precautions when collecting the sample- for ex: preventing degradation or volatilization of the sample.
Specifications for this might be making sure the sample is not in heat, taking a deep sample so it’s uncontaminated, compacting the sample carrier so there is no air space to allow hydrocarbons to escape. We store the sample in the dark to prevent degradation.
what level of accuracy and precision
will satisfy the use objectives? What rate of false positives or false negatives is acceptable?
What does accuracy and precision address in Quality assurance?
It addresses what level of accuracy and presicion is required, whether to use an SRM to test accuracy and precision, what reagent purity is needed, and which model of apparatus to use as well as what tolerances are acceptable.
What is false results?
This is when even well executed procedures sometimes produce false conclusions.
What is a false positive?
Is when results say concentration exceed a limit when it is actually below.
What is a false negative?
When results says concentration is below a limit when it is acc above.
What is two things to consider when choosing an analytical method?
Selectivity and sensitivity
What is selectivity?
the ability of an analytical method to distinguish the analyte from other species in a sample.