Instrumental Analysis - Exam 3 questions with
verified answers
A graphite furnace can reach very high temps. Why is this bad?
Ans✓✓✓ It can become so hot that it glows white, which leads to
increased noise especially in the visible and UV range (350-800) which
affects elements in this range (such as calcium).
AAS Spectral interference can be caused by overlapping absorption
lines from different elements (ie. vanadium and aluminum). How can
you handle this. Ans✓✓✓ 1. Choose a different wavelength
2. Remove the interfering element.
Define the base and molecular ion peak. Ans✓✓✓ The base peak is the
most abundant peak in the mass spectrum and is scaled to 100. The
molecular ion peak is typically the farthest peak to the right (ignoring
isotopic ratios and +1 peaks) and represents the MW of your analyte.
Describe a magnetic sector MS. Ans✓✓✓ A high voltage accelerates
the ions, which are exposed to a magnetic field that changes the radius
of curvatures based on m/z. All of the different m/z reach the detector
at the same time, but at different spots, so you need to select your ions
before doing this.
Describe a quadrupole mass analyzer. Ans✓✓✓ An electric field is
generated by a constant DC voltage and an oscillating RF voltage
resulting in an AC potential superimposed on the DC potential. The
,stability and path of the ion is voltage dependent (we control it)
allowing us to select for the m/z we want, while others wont reach the
detector.
Describe chemical ionization (CI) Ans✓✓✓ An excess reagent gas
(methane, isobutane) is added to the chamber, electron bombardment
(same as EI) occurs, and the sample becomes ionized indirectly. This is a
soft ionization, so there are simpler mass spectra which is easier to
determine the MW of your sample.
Describe direct analysis in real time (DART). Advantages and
disadvantages. Ans✓✓✓ This produces electronically excited gases
that ionize atmospheric molecules which interact with sample
molecules to produce analyte ions. These analyte ions are pulled to the
mass analyzer by a charged lens.
Pros:
- simple spectra, simple operation, cool
Cons:
- matrices concerns, specificity of detection can be lower
Describe electron ionization (EI) for MS. Ans✓✓✓ Electrons are
emitted from a heated metal filament (W) and accelerated towards an
anode. Molecules become ionized and fragmented due to these high
energy electrons. This is a hard ionization technique and can be hard to
distinguish sometimes.
, Describe electrospray ionization. Ans✓✓✓ The analyte solution is
passed through a small metal capillary and an electric field conveys
charges as the droplets form a mist. The potential difference pulls
droplets towards mass analyzer.
Describe Fast atom bombardment (FAB) Ans✓✓✓ A beam of fast-
moving neutral atoms is directed onto a metal plate coated with a
sample. The KE is dissipated in ways that lead to ionization. The beam is
usually an inert gas such as xenon or argon.
Describe glow discharge atomization. Ans✓✓✓ Sample with the
analyte is placed on a cathode, ionized argon gas bombards the sample,
and ejected atoms are pulled into the path of radiation by a vacuum
system.
Describe how a continuum background correction works with AAS.
Ans✓✓✓ You use a continuum radiation source in addition to the HCL
source. Two "scans/trials?" are ran, one with each radiation source. The
continuum shows only the background absorption. When the HCL
source is used, it collects both background and atomic absorption. So
by subtracting the two, you can correct for the background absorption.
Describe how a two-line background correction works in AAS.
Ans✓✓✓ You use a close nonresonance line from the HCL to
determine background absorption and then compare to the resonance
line.
verified answers
A graphite furnace can reach very high temps. Why is this bad?
Ans✓✓✓ It can become so hot that it glows white, which leads to
increased noise especially in the visible and UV range (350-800) which
affects elements in this range (such as calcium).
AAS Spectral interference can be caused by overlapping absorption
lines from different elements (ie. vanadium and aluminum). How can
you handle this. Ans✓✓✓ 1. Choose a different wavelength
2. Remove the interfering element.
Define the base and molecular ion peak. Ans✓✓✓ The base peak is the
most abundant peak in the mass spectrum and is scaled to 100. The
molecular ion peak is typically the farthest peak to the right (ignoring
isotopic ratios and +1 peaks) and represents the MW of your analyte.
Describe a magnetic sector MS. Ans✓✓✓ A high voltage accelerates
the ions, which are exposed to a magnetic field that changes the radius
of curvatures based on m/z. All of the different m/z reach the detector
at the same time, but at different spots, so you need to select your ions
before doing this.
Describe a quadrupole mass analyzer. Ans✓✓✓ An electric field is
generated by a constant DC voltage and an oscillating RF voltage
resulting in an AC potential superimposed on the DC potential. The
,stability and path of the ion is voltage dependent (we control it)
allowing us to select for the m/z we want, while others wont reach the
detector.
Describe chemical ionization (CI) Ans✓✓✓ An excess reagent gas
(methane, isobutane) is added to the chamber, electron bombardment
(same as EI) occurs, and the sample becomes ionized indirectly. This is a
soft ionization, so there are simpler mass spectra which is easier to
determine the MW of your sample.
Describe direct analysis in real time (DART). Advantages and
disadvantages. Ans✓✓✓ This produces electronically excited gases
that ionize atmospheric molecules which interact with sample
molecules to produce analyte ions. These analyte ions are pulled to the
mass analyzer by a charged lens.
Pros:
- simple spectra, simple operation, cool
Cons:
- matrices concerns, specificity of detection can be lower
Describe electron ionization (EI) for MS. Ans✓✓✓ Electrons are
emitted from a heated metal filament (W) and accelerated towards an
anode. Molecules become ionized and fragmented due to these high
energy electrons. This is a hard ionization technique and can be hard to
distinguish sometimes.
, Describe electrospray ionization. Ans✓✓✓ The analyte solution is
passed through a small metal capillary and an electric field conveys
charges as the droplets form a mist. The potential difference pulls
droplets towards mass analyzer.
Describe Fast atom bombardment (FAB) Ans✓✓✓ A beam of fast-
moving neutral atoms is directed onto a metal plate coated with a
sample. The KE is dissipated in ways that lead to ionization. The beam is
usually an inert gas such as xenon or argon.
Describe glow discharge atomization. Ans✓✓✓ Sample with the
analyte is placed on a cathode, ionized argon gas bombards the sample,
and ejected atoms are pulled into the path of radiation by a vacuum
system.
Describe how a continuum background correction works with AAS.
Ans✓✓✓ You use a continuum radiation source in addition to the HCL
source. Two "scans/trials?" are ran, one with each radiation source. The
continuum shows only the background absorption. When the HCL
source is used, it collects both background and atomic absorption. So
by subtracting the two, you can correct for the background absorption.
Describe how a two-line background correction works in AAS.
Ans✓✓✓ You use a close nonresonance line from the HCL to
determine background absorption and then compare to the resonance
line.
