Instrumental Analysis Exam 1

Differentiate between dispersive and non-dispersive spectrometers.

Dispersive spectrometers use gratings/prisms to spatially separate light; non-dispersive use filters or FT to isolate wavelengths without spatial separation.

List and describe three types of atomic spectroscopy.

Atomic Absorption (AA), Atomic Emission (AE), Atomic Fluorescence (AF); each uses atoms' interaction with light differently.Atomic Absorption measures light absorbed by atoms, Atomic Emission measures light emitted from excited atoms, Atomic Fluorescence analyzes light emitted from atoms after excitation.

What are the four major types of noise in spectroscopy?

Thermal noise, shot noise, 1/f (flicker) noise, and environmental (interference) noise.

Define thermal noise and its origin.

Thermal noise results from random motion of electrons in resistive components; it?s frequency-independent.

Explain how shot noise is generated.

Shot noise arises from the random arrival or emission of charge carriers like electrons or photons.

Define refraction and explain how it affects spectroscopy.

Refraction is bending of light as it passes between media with different refractive indices, affecting how light interacts with optical components.

Describe the working principle of a photomultiplier tube (PMT).

PMTs emit electrons from a photosensitive surface; these are amplified through dynodes to generate a strong current.

What is 1/f noise and why is it hard to eliminate?

1/f noise arises from poorly understood mechanisms like drift or flickering; its power decreases with frequency.

What is a multiplex spectrometer and what advantages does it offer?

A multiplex spectrometer collects all wavelengths at once and decodes them via Fourier Transform, offering speed and better S/N.It allows simultaneous analysis of multiple wavelengths, enhancing speed and signal-to-noise ratio in spectroscopy.

Compare simultaneous and sequential spectrometers.

Simultaneous spectrometers measure many wavelengths at once (e.g., multiple detectors); sequential scans one wavelength at a time. Simultaneous spectrometers provide faster measurements by capturing all wavelengths concurrently, while sequential spectrometers focus on one wavelength at a time to improve resolution.

Describe the function of a Rowland circle in a polychromator.

A Rowland circle is a geometry where a concave grating and multiple detectors align for simultaneous wavelength focusing.

How can interference noise be minimized?

Grounding, shielding, and signal averaging can reduce interference noise.
Additionally, using differential signaling and proper circuit design techniques helps mitigate noise.

State Beer’s Law and define its variables.

Beer’s Law states that the absorbance of light passing through a solution is directly proportional to the concentration of the absorbing species and the path length of the light. The equation is A = εlc, where A is absorbance, ε is molar absorptivity, l is path length, and c is concentration.

What causes deviations from Beer’s Law?

Deviations arise from high concentrations, stray light, polychromatic radiation, and chemical changes in analyte. At high concentrations, the solution may scatter light, stray light can affect the measurement accuracy, polychromatic light can lead to incorrect absorbance readings, and chemical changes can alter the absorbance characteristics.

What is the main purpose of sample preparation in analytical chemistry?

To isolate, concentrate, and clean up the analyte to make it measurable by instrumental methods.Sample preparation in analytical chemistry aims to optimize the analyte for analysis, ensuring accurate and reliable results by removing interferences and enhancing detection sensitivity.

Why is absorbance preferred over transmittance?

Absorbance is logarithmic and more linearly related to concentration compared to transmittance.Absorbance provides a more direct and accurate measure of concentration, allowing for easier interpretation of results in quantitative analysis.

Describe how Soxhlet extraction works.

Soxhlet extraction recycles solvent through a sample continuously, concentrating analytes without repeated manual extraction.The Soxhlet apparatus uses a siphon mechanism to repeatedly wash the solid sample with solvent, allowing for efficient extraction of compounds into the solvent where they can be collected for analysis.

Define instrumental detection limit (IDL).

IDL is the smallest signal distinguishable from noise by the instrument alone, typically based on S/N = 3.The instrumental detection limit (IDL) is the lowest quantity of an analyte that can be reliably detected by an instrument, defined by a signal-to-noise ratio of 3:1. It indicates the sensitivity of an analytical method in identifying small concentrations of a substance.

How does a Michelson interferometer work in an FT-IR system?

A Michelson interferometer splits and recombines light using moving and fixed mirrors to create interference patterns. It measures the intensity of light as a function of the path length difference, allowing for the collection of spectral information from a sample. The Michelson interferometer operates by dividing a beam of light into two paths, one of which is reflected off a moving mirror while the other reflects off a fixed mirror. The beams are recombined to form interference patterns, enabling the determination of the sample's spectral characteristics based on the interference of the light waves.

What is the difference between IDL and method detection limit (MDL)?

MDL includes all sample processing and is typically higher than IDL; it's the smallest amount reliably detected in a matrix. MDL (Method Detection Limit) takes into account the variability of the entire analytical procedure, including sample preparation and processing, leading to a higher limit than IDL. It reflects the lowest concentration of an analyte that gives a statistically valid result in a specific sample matrix.

Define signal-to-noise ratio (S/N).

S/N = signal amplitude divided by noise amplitude; critical for determining detection limits.

Compare continuum and line sources with examples.