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

UNIT3 Sampling, Calibration & Error Flashcards

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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
Q

What are some ways to minimize calibration error?

A

Certified reference materials
Seperation techniques
Internal standard
External Standard

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26
Q

What is an internal standard?

A

A known amount of reference species is added to sample standard and blanks
The ratio of analyte signal to reference species is recorded

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27
Q

What is the calibration curve of an internal standard plotting?

A

Ratio of the reading analyte to reading of fixed
X: analyte of concentration
Y: recorded ratio

28
Q

Why do we use an internal standard?

A

Compensate for variations of measurement

  • > regression model
  • > compare R^2
  • > calculate unknown
29
Q

What are some figuresof merit?

A

sensitivity
detection limit
linear dynamic range

30
Q

Calibration sensitivity

A

Slope of curve

X indicates conc differences

31
Q

Analytical sensitivity

A

Ratio of calibration curve to std dev. of analytical at given analyte conc.

32
Q

Detection Limit

A

Lowest concentration that can be reported within a certain level

33
Q

How do you calculate detection limit?

A

y=3Sb1Sb=std. dev. of blank

–> calculate corresponding concentration

34
Q

Linear Dynamic Range

A

Max, where calibration is no longer applicable

–> upper limit is concentration of analytical signal deviates 5% from linearity

35
Q

What is standard deviation?

A

Measure eof precision

36
Q

What are the standard deviations at 1,2,3?

A

μ+1σ=68.3%
μ+2σ=95.4%
μ+3σ=99.7%

37
Q

Lower std dev

A

Increase precision

Lower standard error

38
Q

Standard error

A

S/√ N

39
Q

Relative Standard Deviation

A

RSD=S/x

40
Q

Spread or Range?

A

w=largest value-smalled value

41
Q

Sources of error

A

Instrumental–> correctable, periodic
Method–> most serious
Personal –> overcome by instrumentation

42
Q

Absolute Error

A

Difference between easured value and true value

E=X(measured)-X(true)

43
Q

Relative Error

A

Aboslute error devided by true value
Er= (Xi-Xt)/Xt
percent, ppm

44
Q

Error can be…

A

constnat, independent of sample size

45
Q

As sample size decreases

A

Error becomes more serious

46
Q

What is a blank determination

A

solution contains all solvents and reganets of analyte, reveals error

47
Q

What is error?

A

The measurement of uncertainty

48
Q

What is normal distribution?

A

distribution of replicates of analysis

49
Q

Can random error be eliminated?

A

No, it is uncertain

Cannot be detected

50
Q

Why is it important for analytical results

A

errors have serious effect and must be detected for accuracy

51
Q

What is error cause by?

A

Experimenter
Faulty calibration standardization
Random variation

52
Q

What is measured data?

A

Est. of true value

53
Q

What is analytical data w/o qualtity of unknown

A

WORTHLESS

54
Q

Replicates

A

Sample of same size, analyzed the same way

55
Q

Central Value

A

Mean: the average

Median

56
Q

Median

A

The middle value in a set of data arranged in numerical order

57
Q

What central value method is useful when there is a n outlier

A

Median

58
Q

Outlier

A

A significantly different value

59
Q

Accuracy

A

Closeness of a measured value to the true or accepted value

60
Q

How is accuracy expressed?

A

absolute or relative error

61
Q

What is percent recovery?

A

Measured value/reference value

62
Q

Precision

A

Closeness of replicates of results in the same way

63
Q

How is precision expressed?

A

Std. dev., variance, coefficient of variation

64
Q

What is systematic error

A

Affects measurement accuracy, can cause the mean to differ from true/accepted value

65
Q

Random Error

A

Indeterminate error

Affects measurement precision

66
Q

What is Gross error

A

Outliers
Caused by humans
Determined statistically

5) Analytical Chemistry (11 decks)

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