Analytical Chemistry Exam 1

Interference

A species that causes an error in an analysis by increasing or decreasing the quantity being measured

Matrix

Everything in the unknown other than the analyte

Masking

Transformation of an interference into a fam that is not detected

Null Hypothesis

States that two data sets are drawn from the dame populations and any observed differences arise from random errors.

Standard Operating Procedures

Specific set of instructions that describes how a particular task should be performed

Quantiative Analysis

Relative amount of each substance in a sample

Qualitative Analysis

Identity of the elements/compounds in a sample

Accuracy

Closeness of the measurement to the true or accepted valuePre

Precision

reproducibility of the measurements

Population Mean

The mean for the population which represents the total number of measurements in the population

Sample Mean

Authentic average of a subset of the population (limited sample drawn from population)

Systematic error

Arises from a flaw in equipment or the design of an experiment. Always above or below the true value. It can be identified and eliminate with a better experiment

Random Error

Arises from uncontrolled (and may uncontrollable) variables in the measurement. Equally likely to be positive or negative. The error can be reduced by a better experiment but cannot be completely eliminated.

Gross Error

An accidental error caused by a significant departure from the procedure. The error may be so significant that data must be rejected or the experiment redone.

Three ways (there are four) of detecting systematic errors

1. analysis of standard reference materials. Use t-statistical to see whether the true value falls within the CI of your data

2. Independent analysis: A second independent analytical method that is reliable

3. Blank determination

4. Round Robin Experiment

How can systematic error be eliminated

Systematic errors can be minimized by regular maintenance of equipment

An electronic balance that is placed on a countertop near an area with frequent air movements and vibrations, such as a high traffic area in the laboratory

random error

the use of a 10.00 mL volumetric flask with a piece of dust lodged in its interior

Systematic error

The use of reference weight density of 8.0 g/cm³ to make a buoyancy correction when a balance has been calibrated with a reference weight that has a density of 7.8 g/cm³

Systematic error

Outline steps in a chemical analysis

1. Formulating the question

2. Selecting the analytical procedures/select method

3. Sampling-obtain a representative materials analyze

4. sample preparation

5. Analysis

6. Reporting an Interpretation of results

7. Drawing conclusions

Analytical Chemistry

Branch of chemistry that deals with separations, identification, and determination of components in a sample. Field of chemistry that deals with the use and development of tools and processes for examining and studying chemical substances. Seeks ever improved methods to measure chemical composition of natural and artificial materials. Improves the reliability of existing techniques to meet demands for better chemical measurements which arise constantly in our society.

Examples of Analytical Chemistry Applications

Concentrations of O2 and CO2 determined in millions of blood samples every day and used to diagnose and treat illness.

Sample

Portion of material taken for analysis

Analyte

The component of a sample; major, minor, or trace component we’re interested in measuring

Assay

Process of determining how much of a given sample is the material indicated by its name

Sampling

Process of collecting a small mass of material whose composition accurately represents the bulk of the material being sampled

• homogeneous

• heterogeneous

Replicates

Portions of a material of approximately the same size that are carried through an analytical process

Masking

Transformation of an interference into a form that’s not detected

Quantitative analysis

How much analyte is present in the sample?

Qualitative Analysis

Is a particular analyte present in the sample?

Chemical Identification

What is the identity of an unknown chemical in a sample?

Structural Analysis

What is the atomic/molecular mass, composition, or structure of the analyte?

Property Characterization

What are some chemical or physical properties of the analyte?

Spatial analysis

How is the analyte distributed throughout a sample?

Time-dependent analysis

How does the amount of an analyte or a property of the analyte change over time?

Gravimetric methods

determines the mass of the analyte or some compound chemically related to it CO

Volumetric Methods

volume of a solution containing sufficient reagent to react completely with the analyte is measured

Electroanalytical methods

Measurement of electrical properties such as potential, current, resistance, and charge

Spectroscopic Methods

Measurement of the interaction between electromagnetic radiation and analyte atoms or molecules or production of such radiation by analytes

Chromatography

Separates analytes and measures the quantity of each compoundC

Calibration Curve

Measure detector response for known concentrations of each analyte

Good Laboratory practices (GLP)

A set of guidelines that promotes proper work and conduct within the laboratory. The purpose is to give confidence that the final results are a valid representation of a sample. GLPs accomplished by using standard operating procedures (SOPs). SOP is a standard operating procedure is a specific set of instructions that describes how a particular task should be performed.

Examples of SOP

• a procedure for synthesizing a chemical

• a safety protocol

• a method of calibrating an instrument

• a written description of an experiment

• handling of chemicals

• the use of a laboratory notebook

Experimental error

Every measurement (physical and chemical) has some uncertainty

Sources of Systematic Errors

• Instrumental errors- caused by nonideal instrument behavior

• Method errors- arise from nonideal chemical or physical behavior of analytical systems

• Personal errors- result from carelessness, inattention or personal limitation of the experiment

Ways of eliminating Systematic errors and personal errors

regular maintenance of equipment. Use standard addition or internal standard to correct for matrix effects. Good laboratory practices and care and self-discipline.

Absolute uncertainty (absolute error)

the margin of uncertainty associated with a measurement (e=x-u)

Relative Uncertainty

is a more useful quantity than the absolute uncertainty

Error Propagation

a method that can be used to help identify the major contributions to random errors in an analysis

Gaussian Distribution

If an experiment is repeated a great many times and if the errors are purely random. The results tend to cluster symmetrically about the average value. The more time the experiment is repeated the more closely the results approach a Gaussian distribution.

Mean

the sum of a set of results divided by the number of values in the set.

standard deviation

measures how closely data are clustered about the mean

F-test

compares standard deviation, s. If Fcalc > Ftable, then reject null hypothesis

t-test

compares mean, tcalc > ttable then reject null hypothesis

Confidence Interval (CI)

the range of values within which the population mean, u, is expected to lie within a certain probability

Confidence Level (CL)

Degree of certainty you wish

Grubbs Test: Check for outliers

a statistical test to decide whether to discard a datum that appears discrepant, Gcalc> Ctable then reject the null hypothesis

Type I error (alpha error)

when you conclude the model and experimental value are not the same when they really are equivalent

Type II error (beta error)

when you conclude that a results is the same as your model, but the result is actually part of an entirely different data distribution

Method of least squares

use to draw the “best” straight line through experimental data points that contain some scatter D

Deviations

can be positive or negative. used to minimize magnitude irrespective of sign, square the deviation

Determinates

final solution for m and b

Residual plots

further confirms whether your data really fits within the line

Calibration curves

shows the response of an analytical method to known quantities of analyte. St

Standard solutions

contain known concentrations of analyte Bl

Blank solutions

contain all reagents and solvents use in the analysis, but contain no deliberately added analyte

Linear range

analyte concentration range over which response is proportional to concentration D

Dynamic range

concentration range over which there is a measurable response to analyte, even if the response in not linear