Analytical Chem 1-5, Miami University CHM 375 Exam 1

How does stray light effect Beer's Law?

Beer's law should follow a linear relationship of absorbance with concentration. Stray light will decrease the measured absorbance. This flattens the curve causing a non-linear relationship.

Why is a high power laser as the excitation source advantageous for fluorescence but not absorbance?

Fluorescence (F=k[c]) is proportional to the power P* of the excitation source. (this is part of k) Absorbance is dependent on the ration of P*/P, as P* increases P will also increase.

Mt. Dew and quinine interaction

Quinine fluoresces at 450 nm. Mt. Dew is a yellow solution which will absorb light at that wavelength. This will decrease the amount of light that reaches the detector.

Why is the fluorescence measured at 90* to the source light?

We measure at 90 degrees to the source light because we will receive only the fluorescent radiation, and not the transmitted source radiation.

Raman scattering

due to the scattering of light energy below or above the wavelength of the laser excitation.

Polarizability

the ease with which the electron distribution in the atom or molecule can be distorted (needed for Raman scattering)

For the valence electron shell energy diagram for Na, which wavelength transition is most intense and why (Fig 10.57)? Consider the atomic number for Na. Write the orbital electron configuration.

1s2 2s2 2p6 3s1. The most intense wavelength transition is 3s to 3p at 589.6nm because it is the most common transition for the 3s electron to make with the lowest energy required, so it is the most efficient. The lower energy means more atoms are populating the 3p excited state.

What is Beers law?

A = ε bC (ε = Molar absorptivity; b=length; C=concentration; A=Absorbance)

How does a hollow cathode lamp work? What is the importance of M*-->M+hv

A cathode and anode are used to create an electrical current to ionize the gas (Helium or Argon at low pressure). Positive gas ions hit the negative cathode, which is made out of the analyte element. The positive gas ions get shot out and give off radiation in their excited state. The radiation emitted (In the UV/Vis spectra) characteristic of the metal of the cathode, which can be detected through the transparent window. Impurities in the metal will show up on the spectra. The most sensitive wavelength is usually used. M* represents the excited particle, and the energy given off (hv) is the energy released by the particle as it returns to ground state.

Why can flame emission be used for alkali metals but not transition metals?

Alkali metals are easily excited meaning excitation energy is low. Flame emission has a temperature high enough to make N*/N ratio high. Transition metals have a higher excitation energy so a hotter sources is needed.

What is the difference between atomization and excitation and do we want to minimize or maximize N*/No of the Boltzmann equation?

Atomization is the emission the given off by electrons when they move from an excited energy level state to a lower energy level. The intensity of emission is directly proportional to the N* of electrons in the excited state. We want to maximize N*/No during atomization (more excited electrons).

What is electrothermal atomization? Give an advantage and disadvantage.

Electrothermal atomization uses and electrically heated graphite ference instead of a flame. It eliminates the sample matrix and has a better LOD than flame AA. It is more expensive.

What is external conversion and how can the quenching effect be minimized?

External conversion is the interaction between solvent molecules and the compound of interest, causing the compound in the excited state to return to the ground state without fluorescing. Cooling the solution decreases this interaction minimizing the quenching effect.

What is electrothermal atomization and indicate one advantage and one disadvantage of this approach?

For electrothermal atomization, a graphite tube heated by resistive heating is used instead of a flame. First, the aqueous sample becomes a dried residue through thermal energy. The temperature is increased, which converts organic matter to CO2 and H2O, and vaporizes inorganic materials. The organic matter is removed by inert gas flow. Another temperature increase atomizes the remaining inorganic sample analyte. The advantage of this is increased sensitivity. The disadvantage is it decreases precision. It is better to use this method with a smaller sample.

What is flame atomization and why is the burner slot quite long (10cm)? Consider Beer's law: A = ε bC

For flame atomization, the sample is made into an aerosol and sprayed over a flame to create a dry aerosol of small, solid particles. The flame's thermal energy then causes the solid particles to form a vapor of free atoms. The burner slot is so long in order to increase sensitivity (absorbance increases linearly with length, as shown in Beer's law).

Describe two advantages of Raman over IR spectroscopy.

IR spectroscopy cannot look at aqueous solutions, but Raman scattering radiation can. Raman scattering can see lower levels of analytes, so IR is not as sensitive as Raman.

Why is the symmetric stretch for CO2 not IR active, but is Raman active

IR requires a permanent dipole moments which does not occur in a symmetrical compound. The symmetrical stretch does not effect the change in electron distribution (polarizability) which cause Raman scattering.

If Po is 100, and transmitted power is 50, what is P is Po is 50?A=log(Po/P)

If Po is 100, P is 50. If Po is 50, P is 25. Both ratios are equal to 2.

How do organic solvents influence atomic absorbtion?

Increasing the % of organic solvents reduces surface tension and increases sample transport rate. This modestly increases absorbance

What is surface enhanced Raman, and what the advantage?

SERS involves absorbing analyte on rough metal surfaces or nanoparticles to enhance the Raman signal. This allow ppb solutions to be detected.

Bases on the color wheel how do you pick the LED light to measure te absorbance of a solution?

The LED should be complementary in color to the solution should be use. Complementary colors are opposite each other on the color wheel.

M*/M+* => M/M+ + hv

The equation represents excited metal species dropping back to the ground state. M*/M+* : excited metal atom/ion M/M+ : are ground state metal atom/ion hv=E

Fluorescence sensitivity can be increased by monitoring the light emission from a larger volume of the cuvette. How can this be done?

You can increase sensitivity by making the slits horizontal instead of vertical; it increases the volume so that there is a longer path for the light to go through (b). There is no need to change the dimensions.

LOQ equation

intercept + 10(precision)/ slope(ie sensitivity)

ICP ppb concentration calculation

m1v1 = m2v2 m= moles/L v= mass/density

Advantages of Raman over IR

water can be used solvent, so aqueous solution can be analyzed glass and quartz cuvettes can be used for liquid or solids

What is the difference between atomic emission spectrum and molecular visible spectrum?

AES shows discrete line, only certain electronic transitions are possible MVS is a continuous signal, rotational, vibrational, and electronic transitions are noted

Why can't TNT be determined by fluorescence?

4. TNT cannot be determined by fluorescence because the 3 nitrate groups withdraw elections, pulling them aw/ay from the pi ring, making them more difficult to excite. The electrons are already at a higher energy level due to the interaction electron withdrawing groups, making them harder to excite. The withdrawing groups bind the electrons away from the ring.

Why is a hollow cathode lamp used as a source for AAS instead of a continuous source such as a D2 lamp and monochromator to generate monochromatic light?

A hollow cathode lamp is used instead of a continuous light source because the bandwidth of a continuous source is 1000x larger than the atomic absorption line. The hollow cathode lamp gives a transmission percentage of less than 100 and an absorbance greater than 0, where a continuous source would give you a reading of 100% transmittance and 0 absorption.

What is the limitation of an LED light source without a monochromator and how does a photodiode array instrument fix this problem?

A monochromator breaks light into a narrow range of wavelengths, LED lights have a broad range of wavelengths. Beer's Law linearity is only valid using narrow bandwidth source. A PDA has multiple detectors to measure light over narrow wavelengths.

What is a monochromator and why is it important in AAS, considering the AAS instrument design?

A monochromator is a method which allows a specific wavelength of radiation to be radiated by splitting up polychromatic light. A monochromator is important in AAS because you need to be able to separate the wavelengths in order to tell what wavelengths you are measuring absorbances at. A flame has a ton of different wavelengths, more than an LED light would give. The result will therefore be more specific. It is important to have a monochromator when using a flame.

A significant lmitation to Beers law can be noted if a light emitting diode (LED) source is used with no monochromator which is a common colorimeter design. For example, the wavelegnth specification for a green LED is 515nm maximum intensity with a 30nm peak width. What is this limitation? What is a monochromator? Why is this rarely a problem with a photodiode array (PDA) instrument?

A monochromator is a method which allows a specific wavelength of radiation to be radiated by splitting up polychromatic light. Without a monochromator, an LED will have lower resolution, meaning a slight deviation from wavelength will result in a dramatically different measurement. The photo diode array uses multiple diodes to can detect absorbance at different wavelengths, so there is no need for a monochromator to narrow down the emitted light to a smaller range of wavelength.

What is a plasma and its approximate temperature? What is the temperature of an acetylene torch? Which is generally more desirable to use for atomic emission?

A plasma is an ionized gas with positive ions and free electrons. Plasma provides better atomization and increases population in excited states as a more intense source of excitation. The temperature of the plasma is between 6,000-8,000 Kelvin at the height of the flame, where the emission is measured. It is about 10,000 Kelvin at the base. The acetylene torch temperature is between 2,100-24,000 Celsius.

With the lego colorimeter what equation is use to measure absorbance?

A=-log T where transmittance T=P*/P where P*/P is the ratio of power of the sample/power of the source.

One limitation of Beer's law is it is generally considered linear at analyte concentrations of 0.01M or less .

At higher concentrations the particles of an analyte are no longer independent of each other. These interactions can change the absorption of the analyte. Using low concentrations helps keep the refraction index constant.

what is the advantage to signal averaging

Averaging the signal-to-noise ratio helps to imporove the final spectrum when multiple spectra are collected for a single sample. As the sum of signals increases (nS) the noise (N) at any point also increases by the squar root of the number of spectra.

What is self-absorption and why does it often control the limit of linearity for a calibration curve in ICP atomic emission?

Because the hottest part of the flame is towards the center of the flame, there are excited analyte atoms concentrated at the flame's center. If an excited atom in the center emits a photon returning to its ground state, then a ground state atom in the outer flame region may absorb the photon, so self-absorption occurs. High concentrations of analyte increase the probability that self-absorption will occur, and causes an inversion in the emission band. Ionization limits linearity for small concentrations, and self-absorption limits the linearity for larger concentrations, because self-absorption will limit the ability to accurately detect emission.

Wterm-3hich compounds favor fluorescence?

Fluorescence is favored by aromatic compound, and electron donating groups. (quinine and tryptophan)

Why is the use of a high power laser as the excitation source advantageous for fluorescence but not absorbance? What are the terms making up k in equation 10.28?

Fluorescence is measured as return from the excited state to ground state, a higher power laser may be used on higher energy molecules that would not fluoresce with a lower-powered laser. Increasing the power for absorbance isn't advantageous because it will not change the absorbance; it is a ratio of the Power of the origin and the Power that is transmitted. k' is the product of 2.303, k, quantum yield, molar absorptivity, path length, and concentration.

Explain the atomization process indicating how an aqueous analyte is converted to a free atom from the actual species that absorbs light.

In most cases, the analyte is in a solution. Atomization requires the stripping away of the solvent, evaporation of the analyte, and dissociation of the analyte into free atoms using thermal energy, typically flame atomization and electrothermal atomization. For flame atomization, the sample is made into an aerosol and sprayed over a flame to create a dry aerosol of small, solid particles. The flames thermal energy then causes the solid particles to form a vapor of free atoms. For electrothermal atomization, a heated graphite tube is used instead of a flame. The aqueous sample becomes a dried residue. The temperature becomes hotter, which converts organic matter to CO2 and H2O, and vaporizes inorganic materials. A second temperature increase atomizes the analyte.

List 3 examples of error, and ways to avoid them?

Indeterminate error comes from. 3 sources noise in thermal detectors, noise in photon detectors, and positioning of the sample cell. Relative uncertainty from photon detectors is large for very low and very high absorbances. Uncertainty from noise in photon detectors is large for very low absorbanes. At ver low absorbances the positioning of the sample cell can also cause a large amount of uncertainty. Low concentrations lead to an increase in the fluctuation sin the intensity from the source.

What is the boltzmann equation

N*= N(gi/go)e^(Ei/kT) where gi and g0 are statistical factors that account for the number of equivalent energy levels for the excited state and the ground state, Ei is the energy of the excited state relative to a ground state energy, E0, of 0, k is Boltzmann's constant (1.3807E-23 J/K), and T is the temperature in kelvin. From equation 10.31 we expect that excited states with lower energies have larger populations and more intense emission lines. We also expect emission intensity to increase with temperature.

The raman signal is not due to the absorbance of light or emission of light. What is the Raman signal due to ? Describe the light source and its wavelength that generates the Raman signal?

The Raman signal is due to shifts in vibrational frequency. The photons in the emitted light have a different energy than the absorbed light, so the photons are Raman scattered. This difference in energy relates to the vibrational energy and can therefore be used to measure the vibrational energy of the molecule. The light source is always a high-power laser, preferably one which is monochromatic and stable. The five most commonly used lasers include wavelengths ranging from 1064nm-488nm. We are using a laser with a 532nm wavelength.

Aromatic compounds such as benzene are easily detected by UV spectrophotometry at 260nm using a mercury lamp which has strong emission at 354nm. What is the molecular functional group involved, and the type of electronic transition at 260nm?

The aromatic ring is easily detected by UV spectrophotometry in part due to the functional groups called chromophores. The C=C double bond absorbs radiation at a higher energy than a single carbon bond. The pi-->pi* transition occurs at 260nm.

Why must the excitation wavelength in Raman spectroscopy be carefully chosen?

The excitation wavelength must be carefully chosen in order to limit the photo-decomposition fluorescence and the fact that colored samples and some solvents can absorb the Raman scattered radiation.

how to avoid the matrix effect when conducting ICP and AAS

The heat from ICP eliminates the matrix effect so an external calibration curve will give better results. AAS is only 2000 C so the matrix effect can be avoided by using standard addition.

How does the ICP torch work?

The inductively coupled plasma (IPC) source provides a means for converting an analyte into a free gaseous atom and exciting the atom. The plasma in atomic emission creates hear from electrons and argon ions moving through the gas. Plasmas have much higher temperatures than flames. There are three concentric quartz tubes in the IPC torch, which are surrounded at the top by a radio-frequency induction coil. The analyte sample and argon are mixed by a nebulizer and carried through a central capillary tube to the plasma. A spark from the Tesla coil initiates plasma formation. The radio-frequency coils provide an alternating radio-frequency current to make a fluctuating magnetic field to induce the argon ions and electrons to move in a circular path. Collisions with unionized gas cause resistive heating, and thus the high temperatures created by the plasma. Because of the high temperatures, the quartz must be insulated from the plasma by a flow of argon.

Why are the monochromator and photodiode array detector important components of an ICP instrument?

The monochromator is important because it allows the light to be separated into various wavelengths, and each wavelength is directed to an individual sensor, so that several elements can be analyzed. The photodiode array detector is important for detecting all the wavelengths of light that were separated by the monochromator.

The signal intensity can be considered to be proportional to the probability of such an energy transition. Explain why the peaks for the anti-stokes peaks are smaller than the Stokes peaks.

The peaks for stokes are higher than anti-stokes because stokes emission is favored over anti-stokes, because the ground vibrational level is more highly populated.

What is the quantum yield of a molecule and how is it affected by temperature and why?

The quantum yield of a molecule is a number which quantifies the fluorescence efficiency, represented by the fraction of excited state molecules returning to the ground state by fluorescence. The fluorescent quantum yield is anywhere between 1 and 0, 1 being when every molecule in an excited state undergoes fluorescence, and 0 being none undergo fluorescence. Temperature affects the quantum yield indirectly; as temperature increases, quantum yield decreases due to increased external conversion from more frequent collisions between the molecules and the solvent.

Would the x-axis of Figure 18-2 change if the excitation source was a diode laser (785nm) instead of the argon laser (488nm)? Why or why not?

The x-axis would not change no matter the excitation source, because Raman shifts are independent of wavelengths.

Why are the IR and Raman spectra in 18-4 similar but show differences? Explain what must happen to the molecule for an IR transition to be observed and what must happen to the molecule to see a Raman transition peak.

They are similar because the overall mechanism for measuring the vibrational modes is the same, but the mechanisms by which they go about measuring those modes are different. For IR spectroscopy it requires a change in charge distribution, but for Raman scattering, the distribution of electrons around a specific bond and re-emission of scattered radiation as the bond returns to a normal state are measured.

Why do UV-VIS spectra show broad bands while the bands in IR spectra are sharper?

UV/Vis radiation can change the energy level of valence electrons, while also changing the vibrational energy of atoms. The absorption bands from the electron absorption/energy level change and the vibrational energy can become blended together so that it appears as though there is a broad band. IR radiation can only change the vibrational energy of atoms and molecules without the change of electron energy levels, which makes sharper bands.

One limitation of Beers law is it is generally considered valid (linear) at analyte concentrations of 0.01M or less. What is this explanation?

When analyte concentrations increase, analyte particles are no longer independent of each other, which is going to affect the way particles interact with light. When there are higher concentrations of analyte, less light is being transmitted through the sample, until PT is close to the value of stray radiation (Pstray). Pstray then contributes more to the value of the transmitted light, making the absorbance less than expected (or transmitted light more than expected).

LOD equation

intercept + 3( precision)/ slope( ie sensitivity)