CHM 3210 Instrumental Analysis Lecture Notes

Flashcards based on lecture notes for CHM 3210, covering key concepts in instrumental analysis.

According to the lecture, what two steps consume the most time for Analytical Chemists during Sample Analysis?

Sample Preparation (61%) and Data Management (27%).

According to the lecture, what two factors contribute the most to errors generated during sample analysis?

Sample Processing (30%) and Operator errors (19%).

What is an instrument in the context of instrumental analysis?

Assemblage of components for making a measurement of concentration/amount, or identification/characterization of matter.

Name some quantitative figures of merit for analytical methods.

Precision, Bias, Sensitivity, Detection Limit, Concentration Range, and Selectivity.

Name some qualitative figures of merit for analytical methods.

Speed, Convenience / ease of use, Required skill, and Cost.

Define Precision.

Reproducibility without regard for closeness to true value.

Define Accuracy.

Correctness / trueness of the measurement.

Define Bias.

A way to quantify accuracy, measuring how much a value deviates from the true mean.

How is accuracy established ?

Comparing the experimental results to a “known” or “true” value.

What are the three types of errors discussed in the lecture?

Random, systematic, and gross errors.

Describe Random errors.

Equally likely to be positive or negative; precision is a measure of random error.

Describe Systematic errors.

One-directional (always high or always low); bias is a measure of systematic error.

Describe Gross errors.

Screw-ups such as switching sample IDs, spilling, bleeding into your vial, etc.

What is the formula for uncertainty propagation in addition and subtraction?

The absolute uncertainties are added: Δf = [(Δx)2 + (Δy)2 + (Δz)2 + … ]1/2

What is the formula for uncertainty propagation in multiplication and division?

The relative uncertainties are added: Δf / f = [(Δx / x)2 + (Δy / y)2 + (Δz / z)2]1/2

What is a detection limit?

The lowest concentration of analyte that the analytical process can reliably detect.

What is the formula for calculating the Limit of Detection (LOD)?

LOD = Blk avg + 3*SDblank

What are the two types of Detection Limits described in the lecture?

Instrumental Detection Limits and Method Detection Limits.

Define Instrumental Detection Limits (IDLs).

What the instrument can detect in a simple matrix when decoupled from the sample preparation process.

Define Method Detection Limits (MDLs).

What can be detected in a real sample, factoring in all steps in the sample introduction process.

What are some goals of Sample Preparation?

Analyte loss, Preserve representativeness, Minimize contamination, Remove matrix components, and Get concentration into an appropriate range.

What is the formula for calculating Percent Spike Recovery?

100 * (SSR - NATIVE)/SA

Name some dissolution and extraction processes for organic constituents from soil.

Soxhlet extraction, Supercritical fluid extraction and Sonication with a solvent.

Name some preparations for elemental constituents.

Leaching with acids, complete dissolution with HF, and Destroy/oxidize organic matter.

Name three sources of contamination in the sample preparation process.

Reagents, Containers, and Atmosphere.

What are the Calibration Methods described in the lecture?

External Calibration, Standard Addition Methods, and Internal Standard Methods.

What is an "internal standard"?

A substance is added to samples and standards used to correct for drift and matrix effects.

What are the requirements for an internal standard (IS)?

Technique must be multicomponent, No interferences, Internal standard must emulate drift and matrix effect behavior, and Internal standard is not native in the sample.

What is the line slope of the calibration used for?

Sensitivity is the slope of the calibration curve.

Define dynamic range.

From LOQ (Limit of Quantitation) to LOL (Limit of Linearity).

Define Noise.

Noise is the unwanted component of the measured signal that affects and limits measurements of the chemically relevant signal component.

What are some types of noise?

Thermal, Shot, 1/f (flicker) noise, and Interference (environmental).

What is the S/N formula?

S/N = mean signal / standard deviation of noise.

What criteria should be in place for the resolution for a spectrometer?

The Rayleigh criterion

What is a detection limit?

The concentration of analyte that the analytical process can reliably detect

What are the main three purpose of spectroscopy?

Qualitative identification, Characterization of energy level structures, and Quantitative analysis

Describe light?

Particle and wave characteristics

Name some properties of electromagnetic radiation.

Reflection, Transmission, Absorption, Dispersion, Refraction, Scattering, Polarization, Emission, and Diffraction.

What is Refraction?

Change of direction when light passes from one medium to another.

What is Dispersion?

Apparent “spreading out” in distance or angle when light is diffracted or refracted.

What is Diffraction?

Constructive and destructive interference pattern that is formed when light passes through an opening of size d which is ~ same order as wavelength.

What is Transmission?

When light passes through a medium without a net change.

What is Absorbtion?

Is when the energy of the EMR is transferred to atoms or molecules of the absorbing medium, which subsequently wind up in higher energy states.

What is Emission?

Is the opposite of absorption: an atom or molecule in an excited state gives off a photon and returns to a lower- lying energy state

Source, wave length selector, sample holder, photoelectric transducer, and signal processor

Components of instruments for optical spectroscopy

Dispersive or Non-dispersive

Spectrometers

What is a monochromator?

(a) Thermal- based on the ability to conduct the electrons or photons within its spectral band.

What instrument is a polarimeter?

(Laser) – used a monochromator and light source to determine concentration .

What is spectral bandwidth?

Light exiting a wavelength selector will have a Gaussian distribution

What is a multiplex spectrometer?

All frequencies follow the same optical path, a “time domain” interferogram is converted into a frequency domain spectrum by FT

What is a laser?

Intense, coherent, monochromatic, polarized, collimated sources, stimulated emission of radiation from a highly populated excited state.

What is and deal transducer?

High sensitivity, high signal to noise ratio, constant response over large range of wavelengths, fast response time and zero signal in absence of illumination

Photoelectric effect-based detectors: what is Phototubes?

Individual photons evoke a “quantum response” of charge carriers

Used for low light intensity and photon counting applications

Photoelectric effect based detectors: photomultiplier tubes (PMT’s)

What does Silicon Photodiode do?

Used to fabricated into arrays (PDA’s) for simultaneous measurements. Design parameter: bigger PDAs are more sensitive, but have slower response time!

What does Charge Coupled Devjce CCD do?

Based on semiconductors and photoelectric effect. Array of pixels

What is UV/Vis Absorption?

Based on measurements of Transmittance (T) or the Absorbance (A) of solutions.

What is the Chemical Deviations from Beer's Law?

Deviations from Beer’s Law when a system can undergo dissociation/association and are dependent the Ka (constant) for the reaction.

What are are the Main types of Instrumentation for molecular obsorbtion ?

Four types of instrumentation for Molecular Absorption: 1) single beam 2) double beam in space 3) double beam in time 4) multichannel

Two types of re-emission of hν after excitation are:

Molecular Fluorescence Spectroscopy -Theory Jablonski Diagram different lifetimes

Structure Fluorescence: Which of the Following reacts better?

Fluoresces better- fluoresces C better than he will

Atomic Spectroscopy: How can an analyst determine one element for another ?

Atomic Spectroscopy is based on spectral response of atomic species which absorb, emit, or fluoresce in a relatively limited number of discrete lines at λ’s characteristic of the element