Lesson 2. Errors in Chemical Analysis

Accuracy

Closeness of an experimental value to true/ accepted value

determined exactly

Accuracy can never be _____________________.

Absolute/ Experimental error (E)

Relative error (Er) or the ratio of absolute error to the true value

Percentage error = Er x 100

Measures of Accuracy

Precision

• Reproducibility of a set of measurements

• Closeness of data to other data that have been obtained in exactly the same way

Standard deviation (SD or s)

Variance (s2)

Relative standard deviation (RSD)

Coefficient of variation (CV)

Spread or range (w)

Measures of Precision

reproduced

Definition of Reliability:

• The extent to which the results can be ____________ when the research is repeated under the same conditions.

supposed to measure

Definition of Validity:

• The extent to which the results really measure what they are ___________________.

consistency of results

Assessment of Reliability:

• By checking the ___________________ across time, across different observers, & across parts of the test itself.

established theories

Assessment of Validity:

• By checking how well the results correspond to __________________ & other measures of the same concept.

not always valid

Relationship between Reliability and Validity:

• A reliable measurement is _____________________: the results might be reproducible, but they’re not necessarily correct.

generally reliable

Relationship between Reliability and Validity:

• A valid measurement is _____________: if a test produces accurate results, they should be reproducible.

precision

Reliability is similar to _________________.

accuracy

Validity is similar to ___________________.

Accidental or Random Errors

Types of Errors in Experimental Data

• other term for indeterminate errors because it is naturally occurring, intangible, and innate

precision

Types of Errors in Experimental Data

Indeterminate Errors are errors that:

a. affect the _____________ of measurement

b. are due to sudden change in a _________________________

a = ?

experimental conditions (ex: temperature, humidity, and voltage fluctuations)

Types of Errors in Experimental Data

Indeterminate Errors are errors that:

a. affect the _____________ of measurement

b. are due to sudden change in a _________________________

b = ?

Indeterminate Errors

Types of Errors in Experimental Data

• Due to the limited ability of the analyst to control or make corrections for external conditions

averaging

Types of Errors in Experimental Data

• Indeterminate Errors can be minimized by ______________ several readings

Systematic Error

Types of Errors in Experimental Data

• other name for Determinate Error

Determinate Error

Types of Errors in Experimental Data

• Errors affect the accuracy of results may be constant or variable

• Presumably can be either avoided or corrected

Instrument Errors

Types of Errors in Experimental Data

Determinate Error

• Imperfections in measuring devices

• Non-calibrated or poorly calibrated measuring tools

• Instabilities in their power supplies

• Zero-effect” – requires a “zero correction”

• Eliminated by calibration

Method Errors

Types of Errors in Experimental Data

Determinate Error

• Non-ideal chemical or physical behavior of analytical systems

• Incomplete reaction in chemical methods

• Flaws in experimental design

• Often difficult to detect

Personal/Human Errors

Types of Errors in Experimental Data

Determinate Error

• Result from the carelessness, inattention or personal limitations of the analyst

• Can be reduced by experience and care of the analyst in the physical manipulations involved

• Errors in reading scale: Parallax reading, Incorrect Significant figures

Gross Error

Types of Errors in Experimental Data

• Results of human mistakes in using instruments, calculating measurement and recording data

• Occur occasionally

• Often large and may cause a result to be either high or low (outliers)

• Can introduce error in analysis that may be critical

Analysis of Standard Samples

Detection of determinate errors

• Analysis of standard reference materials (SRM)

Independent Analysis

Detection of determinate errors

• A second independent and reliable analytical method can be used in parallel with the method being evaluated

Blank Determinations

Detection of determinate errors

• All steps of the analysis are performed in the absence of a sample; reveal errors due to interfering contaminants from the reagents and vessels employed in the analysis

Variation in Sample Size

Detection of determinate errors

• Varied sample sizes tested may give different result

Practical or Technical Grade

Grade Classification of Reagents:

• chemicals of relatively low purity (70-95%)

• used in the preparation of cleaning solutions

Reagent Grade

Grade Classification of Reagents:

• analytical grade

• chemicals of high purity (≥ 95%)

• most common grade used in quantitative analysis

Primary Standard Grade

Grade Classification of Reagents:

• highest grade

• chemicals of exceptionally high purity (≥ 99.9%)

USP Grade

Grade Classification of Reagents:

• chemicals that conform to the tolerances for contaminants that pose human health hazard set by USP

• may contain considerable amounts of non-hazardous impurities

Special—Use Chemicals

Grade Classification of Reagents:

• with very stringent specifications with regard to specific impurity or physical property used for specific applications (eg. HPLC grade reagents)