Lecture 11: Molecular Absorption & Atomic Spectrometry

what are the qualitative applications of ultraviolet/visible spectometry?

-usually measured using dilute solution of the analyte

why is single point standard additions methods more risky than multiple point methods?

There is no check on linearity with single-point methods, and results depend strongly on the reliability of one measurement

atomization

a sample is volatilized and decomposed to produce gas-phase atoms and ions, is required as the first step in all atomic spectroscopic procedures.

the standard additions method

can be helpful in counteracting matrix effects.

• The multiple-additions method

chosen for spectrophotometric analyses.Several increments of a standard solution are added to sample aliquots of the same size. Each solution is then diluted to a fixed volume before measuring its absorbance.

• Infrared spectroscopy

identifying pure organic and inorganic compounds because, with the exception of a few homonuclear molecules such as O2, N2, and Cl2, all molecular species absorb infrared radiation.

blank, or additive,

interference produces an effect that is independent of the analyte concentration. These effects could be reduced or eliminated if a perfect blank could be prepared and analyzed under the same conditions.

• Analyte, or multiplicative,

interferences change the magnitude of the analyte itself. Chemical, physical, and ionization interferences are examples.

ionization suppressant

is an easily ionized species that produces a high concentration of electrons in a flame and represses ionization of the analyte

why is Infrared spectroscopy less satisfactory tool for quantitative analytes than ultraviolet and visible spectroscopy

lower sensitivity and frequent deviations from Beer's law.

What is the effect of stray radiation?

may cause false peaks when a spectrophotometer is being operated at its wavelength extremes

• To nebulize

means to convert a liquid into a fine spray or mist.

• Fourier transform infrared (FTIR) spectrometers

offer the advantages of high sensitivity, resolution, and speed of data acquisition.

What are the two major types of atomic spectrometry?

optical atomic spectroscopy and atomic mass spectroscopy

noise

refers to random variations in the instrument due to electrical fluctuations and also variables such as the temperature of the solution, the position of the cell in the light beam, and the output of the source

• Bernoulli's principle

states that as a fluid flows through a tube with a restricted area such as the nozzle in Figure 26-9, the fluid velocity increases and its pressure decrease. The lower pressure at the tip causes liquid to be drawn to the tip.

§ Continuous atomizers

such as plasmas and flames that introduce samples as a continuous stream.

§ Discrete atomizers

such as the electrothermal atomizer that are used to inject individual samples using a syringe or autosampler.

Variables that influence the absorption spectrum of a substance?

the nature of the solvent, the pH of the solution, the temperature, high electrolyte concentrations, and the presence of interfering substances

What is molecular absorption spectroscopy?

used for qualitative and quantitative analyses

What is use of Atomic spectroscopic methods?

used for qualitative and quantitative determination of more than 70 elements

• Syringe pumps and electroosmosis

used to induce flow in flow injection systems. used to accomplish separations, titrations, and kinetic methods

what is UV-visible absorption?

used to monitor titrations and to study the composition of complex ions.

most common atomization methods are

§ inductively coupled plasmas (ICPs) § flames § electrothermal atomizers

Important characteristics of UV and visible molecular absorption spectroscopy

§ wide applicability § relatively low detection limits § high sensitivity § moderate to high selectivity § good accuracy § ease and convenience

What is the relationship of the total absorbance of a solution and the sum of the individual components of a solution?

• The total absorbance of a solution at any given wavelength is equal to the sum of the individual components in the solution.