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5) Analytical Chemistry > UNIT3 Sampling, Calibration & Error > Flashcards

UNIT3 Sampling, Calibration & Error Flashcards

1
Q

What is sampling?

A

Process of obtaining a representative fraction

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2
Q

What are the steps to determine the sample?

A

Identify population –> gross sample (homogenization)–> Laboratory sample

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3
Q

What is a uniased mean estimate

A

All members have equal probability of being included

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4
Q

Unbiased variance estimate

A

every sample is equally likely to be drawn

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5
Q

What is Calibration?

A

The relationship between the analytical response and the analyte concentration

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6
Q

What do calibrations require?

A

Purified agents

Standardized methods

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7
Q

What do calibrations compare?

A

Compare a property of analyte with that of a standard

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8
Q

What is a series of concentrations of standard solutions?

A

Serial dilution

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9
Q

How do you prepare a serial dilution?

A

Seperatelt from the sample via purified chemicals

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10
Q

What can you obtain from a calibration function?

A

Graph or mathematical formula

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11
Q

What is the Least squares method?

A

Used to find the mathematical eq for calibration

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12
Q

What is the X and Y of a least squares method?

A

X- abscissa

Y- Ordinate

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13
Q

What is the equation for the regression model?

A

y=mx+b

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14
Q

What is the coefficient of determination and what does it mean?

A

R^2=0-1

Needs to be larger than 0.9

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15
Q

What is a blank?

A

Contain solvent and reagent but no analyte

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16
Q

What is the ideal blank? Does it exist?

A

The analytical response =0

–> DOES NOT EXIST

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17
Q

What is the corrected response?

A

Sample response-blank response

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18
Q

What is the corrected reading?

A

The corrected response, used for calibration

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19
Q

Why do we need to be careful with calibrations? What is the matrix effect?

A

Need to be careful to avoid contamination

The matrix effect can interfere with a sample migth enhance/reduce analyte response

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20
Q

What is the difference between measured data and predicted data?

A

A Residual

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21
Q

What is the equation for a residual?

A

=Yi-(AXi+B)

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22
Q

A lower amount of residuals results in?

A

Higher accuracy

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23
Q

What should the trendline be for residuals?

A

0

Cannot be over 20

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24
Q

What do you plot residuals against?

A

Concentration

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25
What are some ways to minimize calibration error?
Certified reference materials Seperation techniques Internal standard External Standard
26
What is an internal standard?
A known amount of reference species is added to sample standard and blanks The ratio of analyte signal to reference species is recorded
27
What is the calibration curve of an internal standard plotting?
Ratio of the reading analyte to reading of fixed X: analyte of concentration Y: recorded ratio
28
Why do we use an internal standard?
Compensate for variations of measurement - > regression model - > compare R^2 - > calculate unknown
29
What are some figuresof merit?
sensitivity detection limit linear dynamic range
30
Calibration sensitivity
Slope of curve | X indicates conc differences
31
Analytical sensitivity
Ratio of calibration curve to std dev. of analytical at given analyte conc.
32
Detection Limit
Lowest concentration that can be reported within a certain level
33
How do you calculate detection limit?
y=3*Sb1*Sb=std. dev. of blank | --> calculate corresponding concentration
34
Linear Dynamic Range
Max, where calibration is no longer applicable | --> upper limit is concentration of analytical signal deviates 5% from linearity
35
What is standard deviation?
Measure eof precision
36
What are the standard deviations at 1,2,3?
μ+1σ=68.3% μ+2σ=95.4% μ+3σ=99.7%
37
Lower std dev
Increase precision | Lower standard error
38
Standard error
S/√ N
39
Relative Standard Deviation
RSD=S/x
40
Spread or Range?
w=largest value-smalled value
41
Sources of error
Instrumental--> correctable, periodic Method--> most serious Personal --> overcome by instrumentation
42
Absolute Error
Difference between easured value and true value | E=X(measured)-X(true)
43
Relative Error
Aboslute error devided by true value Er= (Xi-Xt)/Xt percent, ppm
44
Error can be...
constnat, independent of sample size
45
As sample size decreases
Error becomes more serious
46
What is a blank determination
solution contains all solvents and reganets of analyte, reveals error
47
What is error?
The measurement of uncertainty
48
What is normal distribution?
distribution of replicates of analysis
49
Can random error be eliminated?
No, it is uncertain | Cannot be detected
50
Why is it important for analytical results
errors have serious effect and must be detected for accuracy
51
What is error cause by?
Experimenter Faulty calibration standardization Random variation
52
What is measured data?
Est. of true value
53
What is analytical data w/o qualtity of unknown
WORTHLESS
54
Replicates
Sample of same size, analyzed the same way
55
Central Value
Mean: the average | Median
56
Median
The middle value in a set of data arranged in numerical order
57
What central value method is useful when there is a n outlier
Median
58
Outlier
A significantly different value
59
Accuracy
Closeness of a measured value to the true or accepted value
60
How is accuracy expressed?
absolute or relative error
61
What is percent recovery?
Measured value/reference value
62
Precision
Closeness of replicates of results in the same way
63
How is precision expressed?
Std. dev., variance, coefficient of variation
64
What is systematic error
Affects measurement accuracy, can cause the mean to differ from true/accepted value
65
Random Error
Indeterminate error | Affects measurement precision
66
What is Gross error
Outliers Caused by humans Determined statistically

5) Analytical Chemistry (11 decks)

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