Instrumental- Exam 2 Socrative

spin quantum number (ms)

+1/2, -1/2

Characteristics of electrothermal atomizer

- electrically heated graphite furnace - sample is introduced into the furnace by means of micropipette - furnace is heated in three or more steps to properly atomize the sample - the average time of analyte path is several seconds - the operator must determine reasonable time and temp for each stage of the analysis

For a p orbital, "l" equals...

1

Assume you own a water analysis lab and a service you provide is the determination of magnesium and calcium in well water. Your overhead is approximately $1200/week. Using your autosample tray, you can process 150 samples/day in furnace mode. Operating at full capacity, what is your breakeven price in dollars per measurement if you ran all of your samples in furnace mode at fully capacity?

150 samples/ day (5 days/ work week) = 750 samples/ week --> $1200/750 = $1.60 per sample

What is the correct representation for an orbital which has an "n" value of 3 and an "l" value of 1?

3p

How many quantum numbers are needed to describe the energy state of an electron in an atom?

4

Assume you own a water analysis lab and a service you provide is the determination of magnesium and calcium in well water. Your overhead is approximately $1200/week. Using your autosample tray, you can process 400 samples/day in flame mode. Operating at full capacity, what is your breakeven price in dollars per measurement if you ran all of your samples in flame mode at fully capacity?

400 samples/ day (5 days/ work week) = 2,000 samples/ week --> $1200/2000 = $0.60 per sample

What is the correct representation for an orbital which has an "n" value of 4 and an "l" value of 2?

4d

How many orbitals are in the d subshell?

5; can hold 10 electrons

What is a chromophore?

A group of atoms in a compound responsible for the absorption of EM radiation

Atoms are in constant motion. What is the effect of this motion on the wavelength(s) observed by a detector when these molecules emit radiation?

Atoms moving towards the detector will have a shorter wavelength, atoms moving away will have longer wavelengths: atom is moving in all possible directions --> Doppler broading

The determination of the dissolved elements in natural water samples is normally completed by first filtering an aliquot of the sample to remove trace elements which are associated with particulate matter or large colloids. Sometimes, however, the total (dissolved plus particulate) concentration of dissolved elements is desired or filtering prior to analysis is not practical. In this case, a lab that relies on a pneumatic nebulizer for sample intro would be best served by using what type of nebulizer for analysis of total concentration of dissolved elements in natural water samples with some particulate content?

Babington: Since the sample in the Babington nebulizer does not flow through a very narrow capillary prior to atomization, this nebulizer is resistant to clogging (which thus reduces instrument downtime). While less ideal than a Babington nebulizer, the cross-flow nebulizer might also be adequate in this situation. Concentric tube nebulizers rely on very narrow glass or quartz capillary tubes that the sample solution must pass through and often clog. If these are used, the sample often must be filtered prior to analysis.

In atomic absorption spectroscopy what types of sample introduction are used?

Both continuous and discrete: In flame atomic absorption spectroscopy (FAAS), the most common type of sample intro system uses a nebulizer to continuously produce an aerosol from the sample solution. When graphite furnace atomic absorption spec (GFAAS) is used, aliquots of the sample are pipetted, either robotically or manually using a micropipette, into the electrothermal vaporizer

T/F: The main difference between spectrophotometers and photometers is the light source.

False; Spectrophotometers have monochromator or spectrographs for wavelength selection. The spectrophotometer can be used for wavelength scanning or for multiple wavelength selection. The photometer is restricted to one or a few wavelengths.

When hydride generation is utilized for sample intro, NaBH4 is often used to generate the volatile species of interest that is then swept into the absorption or emission spectrometer. What is the purpose of the borohydride?

It provides a source of hydride ions that react with the analyte to produce a volatile hydride (eg arsine gas, AsH3): ex. In the determination of arsenic by hydride generation, samples are pretreated with potassium iodide to convert all of the arsenic to the +3 oxidation state. Reaction with hydride ions from the NaBH4 produce arsine gas

Why is an electrothermal atomizer more sensitive than a flame atomizer?

The average residence time of analyte in the optical path is several seconds: In the flame, there is a high flow rate with which the fuel and the oxidants are being fled in order to maintain the flame, so the atoms are zipping through very quickly (microseconds). In the furnace, they evaporate and stay in the optical path for a longer time. So, the furnace is a more sensitive technique

T/F: When comparing hydrogen and deuterium lamps as sources of UV radiation, D2 lamps generally produce higher intensity radiation.

True

For the quantum number "l"=2 is an ml value of -1 permitted?

Yes

Is the following quantum numbers (n, l, ml, ms) possible: 4, 1,-1, +1/2 ?

Yes

What is phosphorescence?

a photoluminescence where the light lasts a long time, emission of excited electrons that change the spin

Which of the following statements is consistent with an electric absorption being broad? a. an electronic absorption includes vibrational and rotational structure b. hydrogen bonding causes an electronic absorption to be broad c. the absorption of a photon is slower than the timescale of molecular vibrations d. electronic transitions are always localized on a single atomic center

a. an electronic absorption includes vibrational and rotational structure

The precision of atomic absorption spec is limited mostly by what step in the process?

atomization (labelled dissociation in picture) - where the atoms are being formed: In AAS, several key processes occur in the analysis of the sample. These include atomization, in which the sample is converted from solid or liquid form into atoms, ions, or molecular species in the flame or electrothermal atomizer. The process of atomization is complex, and impacts the precision of AAS more than any other process in the analysis.

Why are ionization interferences less severe in ICP than in flame emission spectroscopy? a) atomization occurs in chemically inert Ar environment b) the plasma contains a high concentration of electrons c) the plasma is twice as hot as conventional flame d) the plasma is remarkable free of background radiation e) the conc of analyte can be reduced by using ultrasonic nebulizer

b) the plasma contains a high concentration of electrons - shifts equilibrium towards atoms

Select everything that can be used as a beamsplitter for UV-Vis absorption measurements: a. matching light sources b. double slit c. rotating chopper d. mirrors e. matching detectors f. fiber optics

c. rotating chopper d. mirrors f. fiber optics

Most methods of analysis for elemental mercury rely on the absorbance of light at 254 nm by elemental mercury. In atomic absorption spec, when mercury is determined, what type of atomization technique is normally used?

cold vapor: Cold vapor atomization is used almost exclusively for Hg analysis. Samples are digested so that all Hg is oxidized to Hg+2, and then a reductant (normally SnCl2) is used to convert the Hg to its elemental form. The gaseous elemental Hg is then passed through a cell which sits in place of the flame on an atomic absorption spec and irradiate with light at 254 nm. Coincidentally, the same apparatus used for cold-vapor atomization is also often used for hydride generation atomization, which can be used to determine low concentrations of arsenic, selenium, and other hydride forming species.

In ICP AES, highly reproducible measurements are made during the analysis of liquid samples because as the sample is introduced into the ICP AES, the sample signal is scanned repeatedly and the collected signals are averaged. Which type of sample intro system would be most suitable for ICP?

continuous: Continuous sample intro systems constantly feed the sample into the instrument without disruption. The signal, if the instrument is operating properly, is homogenous over nearly the entire time the sample aliquot is introduced, allowing it to be measured at almost any point in time. Discrete systems, such as electrothermal vaporization, place small aliquots of the sample into the instrument, and the signal produced is a transient one and present only for a short period of time. Only during that short period of time is the signal produced and replicate analyses on the same aliquot of the sample are not possible. Pneumatic nebulization is the most common sample intro system for ICP-AES.

When analyzing samples with limited volumes, one would generally use an atomic absorption spectrometer equipped with a(n) ____________ atomizer.

electrothermal: (sometimes called graphite furnace) atomizers require very small volumes of sample. While a flame atomizer utilizing a nebulizer w uptake capillary, flow spoilers, etc might require 1 mL of sample/min, an electrothermal atomizer requires only 1-2 uL of sample. The sampling process is discrete, meaning that only one aliquot of the sample is placed in the atomizer prior to analysis and there is no continuous consumption of sample.

What is fluorescence?

emission of excited electrons that keep the original spin

A radiationless process in which a molecule losses electronic energy while transferring that energy to the solvent or another solute is called:

external conversion

The intermolecular process in which a molecule crosses to a lower electronic state without emitting radiation is called:

internal conversion

The process in which a molecule in one spin state changes to another spin state with nearly the same total energy (e.g. singlet--> triplet) is called:

intersystem crossing

When wavelength of emitted radiation is set to 300 nm, what happens to the width of the line when the lifetime of the excited state increases?

it becomes more narrow --> Heisenberg uncertainty principle: - infinite time = line is extremely sharp - short time =broad peak

The main advantage of fluorescence over UV-Vis absorption spectroscopy is:

its sensitivity

orbital angular momentum (l)

l= 0 (s), 1(p), 2(d) ... n-1 (subshell) - shape of electron cloud next to nucleus

In analyzing solid samples which are of great historical significance or which need to be preserved for other examinations, sample intro techniques that are nondestructive or minimally disturb the sample are used. If a chemist was determining the trace element concentrations in the growth rings of trees to evaluate historical pollution patterns, what type of sample intro method for atomic spec would be least destructive to the samples?

laser ablation: Laser ablation uses a focused laser beam to convert solid samples into plumes of vapor which are then swept into the spectrometer, usually inductively coupled plasma mass spec. The damage to the sample is so minimal, so bulk solid samples can remain largely intact .

magnetic quantum number (ml)

ml= -l, -l+1, ... 0, ... l-1, l - get 2l-1 numbers - rotation of electron cloud shape

When comparing filters and monochromators as wavelength selectors, which one is better suited for qualitative analysis or structural studies?

monochromators

principle quantum number (n)

n= 1, 2, 3, 4 ... (shell) - how far e-s are from nucleus

Pauli exclusion principle in terms of quantum numbers

no two electrons will have all 4 of the same quantum numbers

An aqueous solution of a dye has a strong absorption with λmax= 464 nm. The color of the solution is:

orange

When the analyte is likely to form refractory oxides or other resilient species that will not be converted into atomic forms during atomization, higher flame temps in flame atomic absorption spec (FAAS) are required. The highest temperature flames that are available are those in which acetylene is used with what oxidant (there is a table associated with this question)? What is the most practical oxidant used with acetylene?

oxygen: In FAAS, the hottest flames are available when using pure oxygen and acetylene as the oxidant and fuel respectively. From a practical purpose, most analyses that require higher flame temps use acetylene and nitrous oxide. The flame is easier to control, in part because the burning velocity is less than for acetylene and oxygen.

What part of double-beam-in-time spectrophotometers is used for 100% transmittance adjustment?

reference cell

A novice analyst needs to determine what trace elements are present in rubber samples that are used to make automotive tires. The lab they work in uses a spectrometer with an arc source (electrical discharge) in the sample intro system for their spectrometry. What problems is the analyst likely to encounter in analyzing these samples?

rubber is an insulator, so in order to conduct electricity (and thus produce an arc discharge), it would have to be mixed with another conducting material: Materials that are nonconducting are difficult to analyze if one relies on sample intro sources that rely on electrical discharge. A better approach, if available, would be to digest an aliquot of the sample and use a technique that allows for the analysis of liquids.

What part of double-beam-in-time spectrophotometers is used for 0% transmittance adjustment?

selector mirror

What part of double-beam-in-space spectrophotometers is used for 0% transmittance adjustment?

shutter

To collect a fluorescence excitation spectrum:

the emission is measured at one wavelength while the excitation wavelengths are scanned

To collect a fluorescence emission spectrum:

the excitation wavelength is held constant and the emission intensity is measured as a function of the emission wavelength

What is red shift?

the shifting of an absorption to lower energy

In which region of the EM spectrum does an absorption at 177 nm come?

vacuum-UV

How do values of λ max for the pi* <-- pi transitions vary among a series of conjugated polyenes?

values shift to longer wavelength as the number of C=C double bonds increases