1
Read Chapter pp. 1-22 Problems 1,7,8,9 and 10
Analytical chemistry deals with methods for determining the chemical composition of samples of matter: a measurement science. Two types: classical (or so-called "wet" chemical methods) and instrumental methods. Qualitative analysis = information about the identity of atomic or molecular species. What is present in the sample? Quantitative analysis = numerical information as to the relative amount of atomic or molecular species. How much is present in the sample?
Classical methods include solubility tests, odors, optical activity, melting points, etc. Distillation, extraction and precipitation often used in classical methods to separate the analyte from the complex sample. Instrumental methods involve studying the physical properties of analytes. Conductivity, electrode potential, light absorption or emission, masstocharge ratio are properties often probed. Highly efficient chromatographic (HPLC, GC) and electrophoretic methods used for analyte separation in modern day measurements prior to analyte detection.
Analyte + Matrix = Complex Sample
Complex Sample
How to get from here to here? Very few instrumental analysis methods can provide both qualitative and quantitative information about individual analytes in a complex sample!
Analyte 1 + Analyte 2 + Analyte 3 + Matrix
Analyte 1
Analyte 2
Analyte 3
Separation of sample components followed by detection is usually necessary!
Overall Process of an Instrumental Measurement
Absorption and emission of radiation (h). [UV/Vis and fluorescence spectroscopy of molecules, and atomic absorption and emission]
Stimulus
Response
Vibrations of molecules. [FTIR] Electrical potential and current. [electrochemistry] Mass-to-charge ratio. [mass spectrometry] Separation science. [GC, HPLC and CE]
Energy source
System under
study
Basic Design of an Instrument for Chemical Analysis
Detector or output transducer Broad energy source e.g., broad band light source Perturbation signal selector e.g., wavelength selector (filter) Sample holder Electrical readout device e.g., computer
Data Domains
An instrument is a communication device between the "chemical system" and the user (usually some electrical signal).
Chemical system = intensity of light, density, pressure, size, chemical composition, etc. Analog signals = electrical signals discrete or continuous in amplitude (current, voltage or charge). Time Domain Signals = frequency, pulse width, phase information stored in time domain. (Susceptible to electrical noise) Digital signals = two-level scheme to represent electrical signals (Hi-Lo).
Defining the What Problem
accuracy is required? How much sample is available? What is the concentration range of the analyte? What components of the sample will cause interference (matrix effect)? What are the physical and chemical properties of the sample? How many samples are to be analyzed? What information is desired qualitative or quantitative?
Performance Characteristics Analytical Figures of Merit
Precision absolute standard deviation, relative standard deviation or coefficient of variance (measure of the reproducibility of a measurement). Bias absolute systematic error or relative systematic error (measure of the accuracy of a measurement). Sensitivity calibration or analytical sensitivity (response magnitude change with concentration change). Detection limit minimum amount detectable with a certain level of confidence. Linear dynamic range concentration range over which a linearly changing instrumental response is observed. Selectivity measure of how selective the instrumental response for one analyte is over another.
1.
Precision (reproducibility)
s= (Xi - X )
i=1 N 2
X i = value for each measurement X = mean or average of all measurements N = number of measurements
(absolute standard deviation)
N -1
s RSD = X
(relative standard deviation)
s CV = 100% X
(coefficient of variance)
X=
X
i=1
N
i
N
(mean or average)
2.
Bias (accuracy)
bias = xt
= mean concentration of the sample xt = true concentration of a sample (a reference)
3.
Sensitivity, Linear Dynamic Range and Limit of Detection
theoretical actual
Signal LOD S = mC + Sbl
...