Instrumental Analysis Exam 2 - Star

Most commercially available atomic absorption spectrophotometers utilize a double-beam spectrometer in which the source radiation is passed intermittently through and then around the flame. The purpose of this is to be sure that the background signal that is recorded includes any light radiation that is emitted by the flame itself and most importantly to determine the radiation emitted power (Pr) of the lamp itself. In the typical double-beam spectrophotometer used in AAS a ____ is used to alter

a. A chopper has alternating open spaces to allow radiation light to pass through it and the flame, and then mirrored surfaces which direct light to a mirror and around the flame. It's rotational frequency is timed to be synchronous with the detector amplifier so the detection electronics are alternately recording the absorbed light intensity (P) and the radiant source intensity (Pr)

Unlike other techniques which utilize absorption spectroscopy (e.g. UV-vis, IR) and have radiation sources that produce a continuum of radiation, in atomic absorption spectroscopy, the most common radiation sources are Hollow Cathode Lamps (HCLs) and Electrodeless Discharge Lamps (EDLs). These radiation sources produce ____ radiation which is characteristic of the element you wish to determine (and which is contained within the lamp you are using).

a. Both EDLs and HCLs are constructed using pure samples of the element of interest, although sometimes multiple elements can be combined in a single lamp that may be useful for up to 4-5 different elements. As the atoms of these elements in the lamp are excited, the radiation they produce contains discrete wavelengths of light that are characteristic of the electronic transitions in the element of interest

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

a. Cold-vapor atomization is almost exclusively for mercury analysis

When analyzing samples with limited volumes, one would generally use an atomic absorption spectrometer equipped with a/an ____ atomizer.

a. Electrothermal (sometimes called graphite furnace) atomizers require very small volumes of samples. While a flame atomizer utilizing nebulizer with uptake capillary, flow spoilers, etc. might require 1 mL of sample or more per minute, meaning that only one aliquot of the sample is placed in the atomizer prior to analysis and there is no continuous consumption of sample

A water analysis laboratory provides the determination of magnesium and calcium in well water. Your overhead is approximately $1,200/week. Using your autosampler tray, you can process 400 samples/day in flame mode and 150 sample/day using furnace mode. Operating at full capacity, (a) what is your "breakeven" price in dollars per measurement in flame mode at full capacity? (b) What is your "breakeven" price per measurement in furnace mode at full capacity?

a. If we assume a 5 day work week the lab can process flame mode = (5 days/work work)(400 samples/day) = 2000 samples/work week. To break even in flame mode the lab would have to charge "breakeven" = $1,200/(2000 samples)/week = $0.60/sample b. If we assume a 5 day work week the lab can process furnace mode = (5 days/work work)(150 samples/day) = 750 samples/work week. To break even in flame mode the lab would have to charge "breakeven" = $1,200/(750 samples)/week = $1.60/sample

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 temperatures in flame atomic absorption spectroscopy (FAAS) are required. The highest temperature flames that are available in FAAS are those in which acetylene is used with what oxidant?

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

Unlike inductively coupled plasma techniques, atomic absorption spectroscopy is subject to a number of chemical interference. One of these interferences occurs when chemical compounds in the sample form compounds in the atomized sample that do not readily absorb light that is characteristic of the element of interest. One type of agent that can be used to reduce chemical interferences is called a ___ agent.

a. a releasing agent will form a chemical compound with the interfering species and prevent it from reacting with the analyte

The precision of atomic absorption spectroscopy is limited mostly by what step in the process in the photo

a. atomization - In atomic absorption spectroscopy, 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 atomic absorption spectroscopy more than any other process in the analysis

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 concentration of elements (dissolved plus particulate) is desired, or filtering prior to analysis is not practical. In this case, a laboratory that relies on a pneumatic nebulizer for sample introduction would be best served by using what type of

a. 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.

What is fluorescence a. emission of excited electrons that keep the original spin b. emission of excited electrons where the light lasts a long time c. absorption of excited electrons that changes the spin d. emission of excited electrons that change the spin

a. emission of excited electrons that keep the original spin

The main advantage of fluorescence over UV-vis absorption spectroscopy is.. a. its sensitivity b. its compatibility with separation techniques c. its compatibility with most analytes d. none of the above

a. its sensitivity

To collect a fluorescence emission spectrum... a. the excitation wavelength is held constant and the emission intensity is measured as a function of the emission wavelength b. the emission is measured at one wavelength while the excitation wavelength are scanned c. both the excitation and the emission wavelengths are simultaneously scanned with a small wavelength difference between them

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

The analysis of the x-ray beam by diffraction is similar to spectrum analysis carried out with a diffraction grating a. true b. false

a. true

For the quantum number "I" equal to 2, is an "ml" value of a -1 permitted? a. yes b. no

a. yes

Is the following quantum numbers (n, l, ml, ms) possible? 4, 1, -1, +1⁄2) a. yes b. no

a. yes

For a p sublevel, "I" equals __. a. 0 b. 1 c. 2 d. 3

b. 1

What is the correct representation for an orbital which has an "n" value of 3 and an "I" value of 1? a. 3s b. 3p c. 3d d. 3f

b. 3p

The higher pressures and temperatures in the Hollow Cathode Lamp (HCL) relative to the temperatures in the flame allow for absorptions by the analyte to occur within the narrower bandwidth of an HCL emission. a. True b. False

b. False

What is phosphorescence? a. emission of excited electrons that keep the original spin b. a photoluminescence where the light lasts a long time c. any excited electrons that are emitted from a molecule d. absorption of excited electrons that keep the original spin

b. a photoluminescence where the light lasts a long time

Which of the following can be done to avoid loss of intensities of x-rays due to absorption of long wavelength x-rays? a. apparatus must be contained in a chamber b. air in the chamber must be replaced by helium c. inert gas atmosphere must be provided d. proper slits must be used

b. air in the chamber must be replaced by helium

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? b. atoms moving towards the detector will have shorter wavelengths, atoms moving away will have longer wavelengths c. atoms moving toward the detector will have longer wavelengths, atoms moving away will have shorter wavelengths d. atoms moving toward or away from the detector wavelengths will increase e. atoms moving toward or away from the detector wa

b. atoms moving towards the detector will have shorter wavelengths, atoms moving away will have longer wavelengths

When the 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? a. no change to line b. becomes more narrow c. becomes more broad d. width approaches 0.3 nm

b. becomes more narrow

Which of the following is the disadvantage of silicon semiconductor detector? a. low stability b. can be operated only at low temperatures c. have low count-rate d. low resolution

b. can be operated only at low temperatures

In energy dispersive system, the energy level of pulses is related to which of the following? a. amount of sample b. element involved c. concentration of the element d. number of atoms

b. element involved - in energy dispersive system, the energy level is related to the element involved. The number of pulses is related to the concentration of the element involved

The intermolecular process in which a molecule crosses to a lower electronic state without emitting radiation is called? a. vibrational relaxation b. internal conversion c. external conversion d. intersystem crossing e. predissociation f. dissociation

b. internal conversion

In analyzing solid samples which are of great historical significance, or which needs to be preserved for other examination, sample introduction techniques that are non-destructive or minimally disturb the sample are used. What type of sample introduction method for atomic spectroscopy would be least destructive to the sample? a. ultrasonic nebulization b. laser ablation c. electrothermal vaporization d. pneumatic nebulization

b. laser ablation - Laser ablation uses a focused beam to convert solid samples intp plumes of vapor which are then swept into the spectrometer, usually an inductively coupled plasma mass spectrometer (ICP-MS). The damage to the sample is minimal, so bulk solid samples can remain largely intact

An analyst needs to determine what trace elements are present in rubber for tires. The laboratory uses a spectrometer with an arc source (electrical discharge) in the sample introduction system. What problems is the analyst likely to encounter in analyzing these samples? a. rubber samples are difficult to dissolve b. rubber is an insulator, so to conduct electricity it would have to be mixed with another conducting material c. rubber samples are soft and hard to grind

b. rubber is an insulator, so to conduct electricity it would have to be mixed with another conducting material

Energy dispersive system uses which of the following detectors? a. optical detector b. semiconductor detector c. thermistor d. bolometer

b. semiconductor detector - energy dispersive system uses semiconductor detectors. It consists of an excited source and a sample

Given is the diagram of energy dispersive system. Identify the unmarked component a. lens b. specimen c. sample holder d. energy analyzer

b. specimen

To collect a fluorescence excitation spectrum... a. the excitation wavelength is held constant and the emission intensity is measured as a function of the emission wavelength b. the emission is measured at one wavelength while the excitation wavelength are scanned c. both the excitation and the emission wavelengths are simultaneously scanned with a small wavelength difference between them d. fluorescence emission is measured at a right angle to the excitation beam

b. the emission is measured at one wavelength while the excitation wavelength are scanned

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 remarkably free of background radiation d. the concentration of analyte can be reduced by using ultrasonic nebulizer

b. the plasma contains a high concentration of electrons

What is the correct representation for an orbital which has an "n" value of 4 and an "I" value of 2? a. 4s b. 4p c. 4d d. 4f

c. 4d

How many orbitals in the d subshell? a. 1 b. 3 c. 5 d. 7

c. 5

In atomic absorption spectroscopy what types of sample introduction systems are used? a. discrete b. continuous c. both

c. both - In flame atomic absorption spectroscopy (FAAS), the most common type of sample introduction system uses a nebulizer to continuously produce an aerosol from the sample solution. When graphite furnace atomic absorption spectroscopy (GFAAS) is used, aliquots of the sample are pipetted into the electrothermal vaporizer

In inductively coupled plasma atomic emission spectroscopy (ICP-AES or just ICP), highly reproducible measurements are made during the analysis of liquid samples because as the same is introduced into the ICP-AES, the sample signal is scanned repeatedly and the collected signals are averaged. What type of sample introduction system would be most suitable for ICP-AES? a. discrete b. continuous c. neither d. both are equally suitable

c. continuous - Continuous sample introduction 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 introduction system for ICP-AES.

A radiationless process in which a molecule loses electronic energy while transferring that energy to the solvent or another solute is called... a. vibrational relaxation b. internal conversion c. external conversion d. intersystem crossing e. predissociation f. dissociation

c. external conversion

When hydride generation is utilized for sample introduction, sodium borohydride (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? a. it acts as an oxidizing agent b. it acts as a buffer to control the pH of the sample solution c. it provides a source of hydride ions that react with the analyte to produce a volatile hydride (e.g. arsine gas, AsH3)

c. it provides a source of hydride ions that react with the analyte to produce a volatile hydride (e.g. arsine gas, AsH3)

The crystal used as x-ray grating has ___ dimensional lattice arrays? a. one b. two c. three d. four

c. three

How many quantum numbers are needed to describe the energy state of an electron in an atom? a. 1 b. 2 c. 3 d. 4

d. 4

Which of the following components makes use of a thin metal foil to isolate a nearly mono-energetic excitation beam? a. the x-ray tube b. secondary fluorescence sources c. slit d. filters

d. filters - filters make use of a thin metal foil to isolate a nearly mono-energetic excitation beam. Nickel is one of the metals that is used in filters

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... a. vibrational relaxation b. internal conversion c. external conversion d. intersystem crossing e. predissociation f. dissociation

d. intersystem crossing

Why is an electrothermal atomizer more sensitive than a flame atomizer? a. electrothermal atomizer is an electrically heated graphite furnace b. the sample is introduced into the furnace by means of a micropipette c. the furnace is heated in three or more steps to properly atomize the sample d. the average residence time of analyte in the optical path is several seconds e. the operator must determine reasonable time and temperature for each stage of the analysis

d. the average residence time of analyte in the optical path is several seconds