Instrumental Analysis - Test 2

What is an LED

"Light Emitting Diode" Enhanced light formation, limited use as IR or red/ Low intensity

What type of transition is UV/Vis spectroscopy? What type of transition is IR? What type of transition is Nuclear Resonance?

1. Bonding Electrons 2. Rotation/ Vibration of molecules 3. Spin of nuclei in magnetic field.

4. Your AAS instrument gives you an absorption value of 0.321. What is the corresponding transmittance?

A = -logT T = 10-A T = 10-0.321 = 0.478

What is a plasma?

A gaseous mixture containing a significant amount of cations and electrons.

1. Explain in your words what is a hollow cathode lamp and how it works?

A hollow cathode lamp is a line source used in AAS. A high voltage across the anode and cathode causes gas to ionize and accelerate into the cathode. This plasma strips atoms from the cathode (made of the element of interest). These atoms are excited by further collisions, return to a lower state, and emit photons in the process.

What would be an advantage of using inductively coupled plasma atomic emission spectroscopy (ICP-AES) rather than atomic absorption spectroscopy?

Advantage - can detect many elements simultaneously and at generally lower detection limits. No need of a radiation source since the plasma is the source in ICP-AES. Higher temperatures mean better atomization and good excitation for a wide range of elements. The plasma is inert which reduces the formation of oxides. The temperature is more uniform in the torch.

14. How does an X-ray tube work?

An evacuated tube contains a cathode (electron source) and a metal anode (target). Electrons from the cathode bombard the metal target; some of the resulting energy is released as X-rays, which pass through a beryllium window in the tube.

Could you think of a way to circumvent the interference problem? Or to decrease it?

Answers will vary. Valid answers are: use blanks, measure a different isotope, modify the solvent of the sample (remove O), etc.

Bonus question: Where would ArO+ come from? Why is that a problem?

Ar is the inert gas in the plasma and O comes from the sample and/or solvent

Bonus question (where does the Argon come from?

Argon plasma

2. Explain what is hydride generation and detail 1 benefit of this technique for sample introduction.

As, Sb, and Se react with NaBH4 to form volatile hydrides. Using this technique, the target species can be transferred into the gas phase for easier atomization. The advantages are easier atomization and isolation from the sample matrix, hence reduced interference.

Propose one method of atomization suitable for atomic absorption spectroscopy

Flame atomization (oxyacetylene, natural gas, hydrogen gas), electrothermal atomization (graphite tube)

Propose one method of atomization suitable for atomic absorption spectroscopy

Flame atomization (oxyacetylene, natural gas, hydrogen gas), electrothermal atomization (graphite tube)

What is the principle behind a time of flight mass filter?

Following ionization, all ions are accelerated to essentially the same energy before they enter the drift tube. However, the velocity will depend the masses of the ions (E = ½mv2). The differing velocities of the ions cause separations in time of when the ions reach the end of the drift tube.

3. What is the frequency corresponding to a photon of this wavelength (207.5 nm)?

Frequency ν = c/λ ν = 3*108 ms-1 / 207.5*10-9 m ν = 1.4*1015 Hz

What are the advantages of transducers?

High sensitivity, high signal to noise ratio, fast response time Reality: Dark current

5. Why are plasmas used in atomic emission spectroscopy and flames in atomic absorption spectroscopy?

Higher degree of atomization and ionization Higher degree of excitation (optical spectroscopy)

Considering all 3 options: AAS, ICP-AES and ICP-MS, knowing that you need to measure K, Mg, Ca, Sr, Ba, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, S, Cd, Pb, and P, what method would you use? JUSTIFY! By providing 2 arguments for your choice

ICP-AES is faster than AAS (many metals quantified at once) and it offers lower detection limits. However, S and P are non-metals and would only show up with ICP-MS. The ICP-MS spectrum is generally easier to interpret and is prone to less interference (with the exception of oxide/hydroxide interference, etc).

Why is slit width important?

If the slit and gap width are the same size, then the image will just fill the exit slit as long as the monochromater is set at the wavelength of radiation.

What is the difference between a level 3 and level 4 laser?

In a 3 level laser the energy transition responsible for the laser radiation is between an excited state and the ground state whereas in a 4 level laser the energy transition is between an excited state and an excited ground state. This is much easier to achieve between excited state and excited ground state than excited state and ground state. Hence much less pumping energy is necessary to create the population inversion

How does a laser work?

In a first step the system is pumped into an excited state (e.g. by electrical discharge). An inversion is created with more species in the excited state than in the ground state. Then in a second step, emission of photons is stimulated by injected on a photon. The stimulated emission results in emission of photons in phase with the original photon and hence amplification of the light

8. What does ICP-MS stand for?

Inductively coupled plasma mass spectrometry

Selenium is a challenging element in ICP-MS because of interferences in particular 78Se+ has an interference with 38Ar40Ar+. 10. What is this type of interference called?

Isobaric interference - 2 elements have isotopes of the same mass

What is a wavelength selector?

It narrows the bandwidth, which increases the sensitivity and provides selectivity to absorption and emission methods.

What are disadvantages of XRF?

Less sensitive than optical methods. Difficulties with lower atomic number elements.

AAS requires a radiation source. There exist line and continuum sources, please explain the difference? Please name 1 line and 1 continuum source

Line sources produce discrete atomic lines. Continuum sources produce a broad spectrum of relatively constant intensity. Continuum sources are blackbody radiators that have innumerable atomic and molecular oscillations. Line sources: hollow cathode lamp, lasers. Others: electrodeless discharge lamp (page 239), Hg vapor lamp, Na vapor lamp Continuum sources - 2H (deuterium), Xe arch, C arch, W(tungsten) filament.

11. Arguing with your colleague, please provide 2 advantages of atomic mass spectrometry over atomic emission spectrometry.

Mass spectrometry is less prone to interference than optical spectroscopy. The spectra are much less complex and "noisy" in MS, resulting in lower detection limits.

9. What does mass spectrometry measure?

Mass spectrometry measures mass to charge ratio.

What kind of energy transitions correspond to gamma ray emissions?

Nuclear state transitions

6. Give a disadvantage of atomic absorption spectroscopy over atomic emission spectroscopy.

One element per source, so AAS is much slower. Fewer elements can be quantified overall. Detection limits are higher in AAS.

What kind of energy transitions are involved in atomic emission spectroscopy?

Outer electron transitions

7. What is the energy difference between a photon of 204.0 nm and 207.3 nm (in joules)?

Photon 1 (204.0 nm): ν = c/λ = 3*108 ms-1 / 204.0*10-9 m = 1.47*1015 Hz E = hν = (6.63*10-34 Js)*(1.47*1015 s-1) = 9.74*10-19 J Photon 2 (207.3 nm): ν = c/λ = 3*108 ms-1 / 207.3*10-9 m = 1.45*1015 Hz E = hν = (6.63*10-34 Js)*(1.45*1015 s-1) = 9.59*10-19 J Difference: E1 - E2 = 9.74*10-19 J - 9.59*10-19 J = 1.55*10-20 J

Why are flames used preferentially in atomic absorption spectroscopy while plasmas are more popular for atomic emission spectroscopy?

Plasmas atomize and excite to higher levels causing photon emission. Flames atomize and excite only enough to permit absorption of photons.

One of the elements considered is iron. However, 56Fe is prone to an interference. In fact 40ArO+ has the same mass to charge ratio as 56Fe+. What is this type of interference called?

Polyatomic species interference. Oxides interference. It ISN'T isobaric because that involves different elements with the same mass (14Cand 14N)

Name two kinds of Wavelength selectors

Prisms and Gratings

Name one other type of mass filter (besides time of flight)

Quadrupole, double focusing

12. What is the principle behind a time of flight mass filter?

The ion created in the source (e.g. plasma) is accelerated and passed through a drift tube. The time it takes for the ion to cover a certain distance (fly through) is measured. The higher the mass to charge ratio, the less acceleration is imparted to the fragment, and the longer it takes to cover the distance.

What are examples of peak broadening?

Uncertainty, doppler, pressure, electric or magnetic field effects.

Finally your lab is being asked to help out in another forensic investigation, aiming at identifying a thief by matching red glass found on him to red glass broken during the heist. Red glass often contains selenium, so you could use the same technique than for the supplement investigation above. However, this time you are not allowed to destroy the sample. 13. What would be your technique of choice: ICP-MS or X-ray fluorescence? Please justify using the physical principles involved.

XRF would be a non-destructive technique. It involves transitions of inner electrons hence no atomization (and resulting destruction) would be necessary.

What advantages do gratings have over prisms?

cheaper, better separation, more linear dispersion

What is disadvantages of MS?

very expensive, not free from interference.

How does a quadrupole work?

voltages are ran across ion source, in which there are unstabe and stable electrons, which then enter the transducer.