Molecular Spectroscopy Q&A

Molecular Spectroscopy

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Frequently Asked Questions for FTIR and UV-Vis

FTIR Frequently Asked Questions

Q: Is FTIR affected by the environment?
Q: How deep does the infrared light penetrate at the position of contact between the prism and sample during ATR measurements?
Q: What sort of sample concentration should I use for measurements by the KBr pellet method or diffuse reflection method?
Q: I can see noise near 4000 to 3400 cm-1 and 2000 to 1300 cm-1 in the spectrum. How can I get rid of it?
Q: Can you tell me why the spectrum baseline is curved?
Q: MCT detectors used in infrared microscopes have different wavenumber ranges. On what basis should I select my detector?
Q: MCT detectors are said to be high-sensitivity detectors. In what circumstances are they needed?
Q: What is the maximum size that can be measured with an infrared microscope?
Q: I get extremely high noise with microscope transmission measurements. What is it caused by?

UV-Vis Frequently Asked Questions - Introduction

What is spectroscopy?
What is an instrument?

UV-Vis Frequently Asked Questions - Light and Theory

What is light?
Why is the wavelength of light important?
How are the different light wavelengths classified?
What light wavelength regions are used?
How does “absorption” spectroscopy work?
What type of information does a spectrophotometer produce?
What are the different data collection modes of a typical spectrophotometer?
How does a spectrophotometer measure a sample?
What is “molar absorptivity”?
How do you use the Beer-Lambert Law to perform quantitative analysis?
What is the difference between “amount” and “concentration”?
Why is quantitative analysis always done in Absorbance rather than %Transmission?
What types of chemistry can a UV/Vis spectrophotometer measure?
What is the relationship between absorption spectroscopy and polyaromatic ring structure?
How does pi orbital conjugation influence peak wavelength?
What is the relationship between absorption spectroscopy and sigma orbital bonding?
What is the influence of functional groups?
Does molecular size matter if it is not pi electrons?
Electronic theory of Near Infra-Red (NIR) chemistry?
What types of chemistry can a Near Infra-Red (NIR) spectrophotometer measure?

UV-Vis Frequently Asked Questions - Instrument Design

How does a spectrophotometer function?
What are the light source requirements for a spectrophotometer?
What types of light sources are used in a typical spectrophotometer?
What is the output of a halogen lamp?
What is the energy distribution of a deuterium lamp?
What does a low-pressure mercury lamp do?
What is “lamp switching” in a spectrophotometer?
What component detects the light in a spectrophotometer?
How does a photomultiplier tube work?
What is a detectors spectral sensitivity curve?
How does a silicone photodiode detector work?
What does an InGaAs detector sensitivity curve look like?
How does a PbS photoconductive detector work?
What is the function of the monochromator?
Which is better: prism or diffraction grating?
How do diffraction gratings work?
What is a “blazed angled” diffraction grating?
What is a diode array spectrophotometer?
What goes on under a spectrophotometer’s cover?
How are single beam and double beam instruments different?
How does optical design affect instrument time stability?
How do spectrophotometers use the blank?
How is a double beam spectrophotometer different from a single beam instrument?
How does a beam chopper work?
What is a high performance (HP) spectrophotometer?

UV-Vis Frequently Asked Questions - Resolution and Stray Light

What is resolution?
How does resolution (slit width) influence spectral peak height and shape?
What is stray light?
How does stray light influence high absorbance measurements?
How does stray light result in varied absorbance ranges on different instruments?
How is 8 Plus Absorbance Even Possible?
How does noise change between fixed slit and servo slit mode?
How does noise influence high absorbance measurements?

UV-Vis Frequently Asked Questions - Scan Speed

How long does it take to scan a spectrum?
How fast can I scan and get noise free data?
How are instrument parameters influenced by scan speed?

UV-Vis Frequently Asked Questions - Cuvettes

What is a typical sample?
How should cuvettes be cleaned?
What is meant by cell or solvent cut-off?
What are “long path” cuvettes and why are they used? (sample too dilute)
What are “short path” cuvettes and why are they used? (sample too concentrated)

UV-Vis Frequently Asked Questions - Reflection Spectroscopy

What are the different types of reflection?
What are the different modes of measuring reflection?

UV-Vis Frequently Asked Questions - Solid Sample Transmission

My sample’s not a liquid, now what do I do?
What are the relationships between different types of transmitted light?
How do clear and diffuse sample spectra compare on a linear transmission accessory?
What is a linear transmission measurement?
How does sample thickness influence linear transmission measurements?
How do integrating spheres work?
What is a total transmission measurement?
How does an integrating sphere make a total transmission measurement?
How do clear and diffuse sample spectra compare on a total transmission accessory?
How does an integrating sphere make a diffuse transmission measurement?
How do clear and diffuse sample spectra compare on a diffuse transmission accessory?
Can I measure linear transmission measurements on integrating spheres?
How do I baseline correct when measuring a coating on a solid sample?

UV-Vis Frequently Asked Questions - Specular Reflection

How do you measure relative specular reflectance?
Can you measure absolute specular reflection without a calibrated reference mirror?

UV-Vis Frequently Asked Questions - Relative Diffuse Reflectance

What is relative diffuse reflectance?
How do you measure relative diffuse reflectance?

UV-Vis Frequently Asked Questions - Relative Total Reflectance

What is relative total reflectance?
How do you measure relative diffuse reflectance?
An example of total reflectance measurement