Quiz 1 Flashcards
(131 cards)
1
Q
What concentration quantifies as a major substituent in a sample?
A
If it’s greater than 1%
2
Q
What concentration quantifies as a minor substituent in a sample?
A
0.1-1%
3
Q
What concentration quantifies as a trace substituent in a sample?
A
less than 0.1%
4
Q
Can tiny concentrations be bad?
A
Yes, for example arsenic in water
5
Q
What is a SRM?
A
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.
6
Q
What is a SRM used for?
A
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.
7
Q
How would you use a SRM?
A
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.
8
Q
Name two analytical methods the NIST uses to analyze SRM
A
Instrumental neutron activation analysis and graphite furnace atomic absorption spectometry
9
Q
Does a high precision imply accuracy?
A
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.
10
Q
How are distribution curves useful?
A
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.
11
Q
What is the single most important characteristic of any result from an analytical method?
A
It is the statement of it’s uncertainty.
12
Q
Explain how me might analyze chocolate for caffeine and theobromine
A
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.
13
Q
How do we determine the amount of theobromine and caffeine in our chromatography graph?
A
By creating a calibration curve using known quantities of each analyte.
14
Q
How do we construct a calibration curve?
A
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.
15
Q
How do wo determine the equation of the calibration curve (ie how do we create the best fit line)?
A
Using the method of least squares
16
Q
What are the two assumption in the method of least squares?
A
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).
17
Q
What does the method of least squares do?
A
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.
18
Q
How does the method of least squares work?
A
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.
19
Q
Using the data points on table 4-7 pg105 calculate the best fit line that minimizes di.
A
20
Q
What is a standard solution?
A
Solutions with known amount of protein (analyte) and reagent.
21
Q
What is a blank solution?
A
Solutions with no protein (analyte) but with all other reagents.
22
Q
What instrument is used to determine protein conc in plasma
A
Instrument: Use UV-vis-spectrophotometer to measure light absorbance in the plasma, as the light absorbance is directly proportional to the amount of protein.
23
Q
What is the first step in creating a calibration curve for protein conc in plasma?
A
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.
24
Q
What is the second step in creating a calibration curve for protein conc in plasma?
A
We would create blank solutions- solutions with plasma but no protein, and then record the UV absorbances in these samples.
25
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.
26
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.
27
Your analytical method is as good as?
Your calibration curve
28
When creating a calibration curve what's the minimum amount of data needed?
At least 6 concentrations and two replicates of each.
29
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.
30
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.
31
What is quality assurance?
Quality assurance is what you can do to verify the analytical result in terms of accuracy and precision.
32
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.
33
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.
34
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.
35
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.
36
What are the three basics of QA?
Raw data, treated data, and results
37
Define raw data
individual measurements for example: area under peaks in chromatography.
38
Define treated data
Concentrations (or measurements) derived from a calibration curve made from raw data.
39
Define results
Quantities reported after statistical analysis of data, ex mean, standard deviation, confidence interval
40
What are the three steps in the Quality Assurance process?
Use objectives, specifications, and Assessment
41
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.
42
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.
43
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.
44
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.
45
Define specifications
Is how good numbers must be and what precautions are required in the analytical procedure.
46
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?
47
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.
48
What is false results?
This is when even well executed procedures sometimes produce false conclusions.
49
What is a false positive?
Is when results say concentration exceed a limit when it is actually below.
50
What is a false negative?
When results says concentration is below a limit when it is acc above.
51
What is two things to consider when choosing an analytical method?
Selectivity and sensitivity
52
What is selectivity?
the ability of an analytical method to distinguish the analyte from other species in a sample.
53
What is sensitivity?
Basically the detection limit of the analytical method must be lower than the expected conc of your unknown otherwise results are bad.
54
What are acceptable blank values?
Blank values that can be used to subtract for the analytical measurements you have
55
What are method blanks?
Is a sample containing all components except the analyte and is prepared the exact same way as the analytical sample.
56
What are field blanks?
Similar to method blank, but it was at the field site where the sample was taken, this ensures any contaminant in the sample will enter the blank as well.
57
Whats the purpose of acceptable blanks?
Tell us if analyte was picked up/contaminated so we can get correct measurement for analyte in our sample.
58
What is spike recovery
This is a specification needed as sometimes the response of an analyte can be decreased by something else in the sample ie the matrix.
59
What is a spike?
A spike is a known quantity of analyte added to a sample and then has the response measured to ensure it's the same as the calibration curves expected response. For example, if drinking water is found to contain 10.0 mg/L of nitrate, a spike of
5.0 mg/L could be added. Ideally, the concentration in the spiked portion found by analysis
will be 15.0 mg/L. If a number other than 15.0 mg/L is found, then the matrix could be interfering with the analysis.
60
What are calibration checks?
If you have a large number of samples you check the instruments is continuing to to work properly every ten samples instead of just doing em all w out checking.
61
What is a quality control sample?
Samples given to an analyst as "unknowns" to check their performance, the analysts results are compared to the known value of that sample to make sure she is doing her job right.
62
What are Standard Operation Procedures (SOP)
stating what steps will be taken and how they will be
carried out. For example, if a reagent has “gone bad” for
some reason, control experiments built into your normal procedures should detect that something is wrong and your results should not be reported. It is implicit that everyone follows the
standard operating procedures. Adhering to these procedures guards against taking short cuts which then produce false results.
63
What are the two components of the assessment?
1) Collect data which show that the analytical procedure is in accordance with the specifications, and see if final results meet the defines use objective.
64
What is an API?
It's an active pharmaceutical ingredient
65
Define method validation
is the Process of proving the analytical method is acceptable for the purpose (use objective).
66
What tests does method validation require in order to be compliant with health companies?
Method specificity
Linearity
Accuracy
Precision
Range
Limit of detection
Limit of quantification
Robustness
67
Define method specificity?
Ability of an analytical method to distinguish analyte from everything else that might be in the sample.
68
Define electrophoresis
Is an analytical method in which substances are separated by differing rates of migration in an electrical field.
69
Define baseline seperation
Is a requirement for method specificity, means that the detector signal returns to its baseline before the next compound reaches it.
70
Say a peak has not returned to baseline and merges with the next peak in your electrophoresis reading, what does this indicate?
Indicates that the impurity has not been separated from the other components of the compound, meaning the sample is still impure.
71
Define linearity in a graph
Is a measure of how proportional the response is to the quantity of analyte, and how well the calibration curve follows a straight line.
72
How do you test the linearity of a calibration curve?
You do this by testing 5 standard solutions that (0.5x to 1.5x) the expected conc of your unknown and see if the calibration curves estimated conc of your target sample lines up with your samples conc. Prepare each standard and analyze each three times and prepare three blanks to correct for additional reagent in order to make this curve.
73
What are the two measures of linearity?
The square of correlation coefficient R^2, and the y intercept of the calibration curve (after blank subtracted to get corrected standard) being near 0.
74
What are three way to demonstrate accuracy?
Analyze an apprporiate SRM, compare results from two diff anal methods, spike a blank sample and analyze it to see of you get the same conc as your addition, add standard additions of analyte to unknown and see if method can quantify it.
75
What is the most common way to test accuracy of ur anal method?
spiking as sometimes you don't have an srm or two methods.
76
Define precision
How well replicates agree with one another (small s)
77
What are autosamplers?
is a devoce that is hooked up to anal method and provides reproducible samples again and again, helps ensure precision of method.
78
Define instrument precision
is repeatability of instrument when it quantifies the same sample again and again.
79
Define interlabratory precision
Measure of reproducibility by different people in different labs, this is worse than measurements made in one lab and get worser when conc of analyte decreases.
80
What is the coefficient of variation?
Is a variable where the sd is divided by the mean and multiplied by 100. Lower it is the more precise your measurements
81
What is the horwitz trumpet?
A graph that shows that when conc is 1 ppm, the coefficent of variation is 16%, but when it's 1 ppb, the coefficent of variation is 45%.
82
Define range
The concentration interval over which linearity, accuracy, and precision are all acceptable.
83
Define dynamic range
conc range over which there is a measurable response.
84
define linear range
Conc range over which the calibration curve is linear
85
What is a limit of detection?
This is the smallest analyte concentration that is statistically significantly different from the blank.
86
What is the chance that a blanks analytical measurement falls above the limit of detection?
1% chance for a false positive
87
What is the chance that an analytical sample whos conc is at the detection limit will be analyzed as a false negative?
50%
88
What is the lower detection limit quantified?
3 standard deviations/calibration line
89
Define the lower limit of quantification?
The smallest analyte concentration that
can be measured with reasonable accuracy
90
What is the lower limit of quantification quantified?
10 s/m
91
If there is an analyte with the minimum detectable conc in it (equal to the LOD), can we measure this with accuracy?
No, we can only say whether it's there or not.
92
Define a reporting limit
this is the conc below which regulations say that a given analyte is reported as not detected
93
If a package says something is not detected, is it actually not detected?
No, it is still there, it's just below the reporting limit so regulations allow them to say there is none, this can be 5-10x higher than the detection limit.
94
What is an example of a detection limit?
The detection limit for trans fat is 0.5 g, if the conc of trans fat per serving size is less than this companies can say there is none on the nutrition facts.
95
Define matrix
Anything else in the sample that's not the analyte.
96
Define robustness
Is the ability of your analytical method to not be affected by small changes, ex: turning lab temp down a few degrees.
97
Define matrix effect
A change in the analytical signal caused by anything in the sample that is NOT the analyte
98
How can you analyze a sample with an unknown concentration if the matrix affect is distorting your true results?
You use the standard addition method in which you add small standard amounts of the target analyte to that same unknown and measure the detector response it receives and using the increase with those values we can estimate what the concentration of our unknown is.
99
Describe internal standards
If an instrument response varies slightly every analysis we cannot make a calibration curve so instead we add a known amount of an internal standard (a diff substance than the analyte) and analyse the detector response of each which are directly porportional to the concs- we then deduce the detector response and then solve for the analyte conc through math.
100
What is the step after assessment in quality assurance?
Method validation, which proving that the analytical method is acceptable for the purpose it was used.
101
Chemical analysis is meaningless unless ------
we begin with a meaningful sample
102
Define representative sample
a sample that is representative of the lot.
103
Define heterogenous material
is material that has different compositions place to place
104
Define homogenous material
Material has same analyte conc everywhere
105
Define Sampling
Process of selecting a representative bulk sample from a lot
106
Define a lot
is the total material from which the sample is taken
107
How do we go from a representative bulk sample to a homogenous lab sample?
By grinding the bulk sample to a fine powder and thorughly mixing it, then using aliquots of it for analysis.
108
What is the proper procedure for taking field samples?
Proper labelling on the bottle/glass jar (including date, time, and GPS) and using clean equipment (acid washed polyethylene/teflon bottles)
109
Why is sample transport and storage important?
If the analyte conc changes between the collection of the sample and the analysis, ur results r useless- you must maintain the integrity of the sample.
110
What should you avoid to maintain the integrity of the sample?
Contamination (mineral dusts)- do this by putting a lid on the sample
Analyte loss- for ex adsorption (avoid glass), volatilization (put lid on container immediately and avoid headspace), keep sample in ice chest and maintain temp less than 10 degrees. Prevent analyte degradation.
111
What should you do to prevent analyte degradation? (ex: herbicide)
To avoid herbicide for ex in a soil sample add 5% HNO3 (acid) to stop microbial degradation. BUT, make sure the nitrate doesn't affect other components of the sample.
111
What should you do to prevent analyte degradation?
To avoid herbicide for ex in a soil sample add 5% HNO3 (acid) to stop microbial degradation. BUT, make sure the nitrate doesn't affect other components of the sample.
112
Explain why cleaning of equipment is essential even when new through a real world example
Scientists measured Mn in blood samples and put aliquots into unwashed tubes and some into washed tubes, the concentration of Mn in unwashed tubes was higher showing that even new it still had impurties (Mn)
113
In regards to a sample, what should the lab book include?
How sample was collected, how it was transported and stored, and how it was prepared for analysis.
114
Define sample preparation
Series of steps that convert the lab sample to a form suitable for analysis
115
How would you prepare a sample such as biomass for analysis?
You dry the leaves at 110 to remove h20, grind them (through steel,ball mill, agate, or boron carbide mill), then dissolve the sample through acid digestion or fusion.
116
If analyte is iron is it acceptable to use a steel grinder?
No
117
What acids are used to dissolve inorganic materials
HCL, HBr, HF, dilute H2SO4, HNO3, ad HCLO4- but they must be pure!!!! they also dissolve metals by redox rxn.
118
What elements will react with acids and cause loss of analyte?
If sulfur, fluorine, or carbonate they might react with acid to form volatile species
119
Name some volatile metal halides
HgCl2, SnCl4
120
What do you do to dissolve substances that don't dissolve in acid
You dissolve them with a hot, molten, inorganic flux
121
How do you dissolve inorganic substances through flux?
Take the finely powdered unknown, mix it with 20 times its mass of solid flux (sodium oxide, sodium carbonate, K2S2O7) and melt in a pt crucible at 300-1500 degrees. Then place in beaker with highly pure HNO3 to dissolve analyte.
122
How do you decompose organic substances?
Through dry or wet ashing
123
Define dry ashing
example of this is halogens in coal- you put coal and ammonium nitrate through a big microwave and then the halides released dissolved in the acid Nh4Co3, and then were measured through ion chromatography.
124
Define wet ashing
You place oyster tissue inot a teflon vessel, add HNO3 and H2so4, microwave, cool and analyze after putting in a vol flask. After this is done all organic material oxidizes giving a clear solution.
125
Name other sample preparation techniques
dissolving the sample, extracting the analyte from the marix, concentrating the analyte, converting the analyte into a measurable form, and removing/masking interfering species
126
What is liquid extraction?
Analyte is dissolved in solvent but the solvent doesn't interfere w the matrix.
127
Describe a liquid extraction technique
taking pesticide and putting in sepratory funnel with cyclohexane (non polar compound) helps remove matrix, extracts analyte, and concentrates it.
128
Whats an issue with the seperatory funnel (liquid liquid) extraction technique?
Can form an emulsion if you shake too vigorously.
129
Describe microwave assisted extraction
you place soil and acetone and hexane in a teflon lined bomb and heat to 150 degrees C (higher than boiling points of both solvents) The pesticides in the soil dissolve and are then analyzed by chromatography
130
Describe supercritical fluid extraction
Uses a supercritical fluid as the extraction solvent, most commonly CO2 and disposal is easy (just vaporizes). Pressurized
fluid is pumped through a heated extraction vessel. Fluid can be left in contact with the
sample for some time or it can be pumped through continuously. At the outlet of the extraction vessel, the fluid flows through a capillary tube to release pressure. Exiting CO2 evaporates, leaving extracted analyte in the collection vessel. Alternatively, the CO2 can be bubbled
through a solvent in the collection vessel to leave a solution of analyte.
