Chapters 7 & 11 Instrumental Analysis

Front 1

Define: Monochromator

Back 1

A device for varying the wavelength of light over a considerable range. "scanning" a spectrum.

Front 2

Define: Silicon diode transducer

Back 2

Consist of a reverse biased pn junction formed on a silicon chip. p doped silicon (holes) n doped silicon (electrons)

Front 3

What is the function of the monochromator?

Back 3

Rotating the grating allows different wavelengths of light to be focused on the exit slit.

Front 4

What is the difference between a transmission grating and a reflection grating?

Back 4

Transmission grating-light travels through the grating and is dispersed. Reflection grating-light is reflected from the grating surface and is dispersed.

Front 5

What principle do echellette gratings operate on?

Back 5

Light of different wavelengths striking the Echellete grating are reflected at a different angle due to constructive and destructive interference.

Front 6

What are four characteristics of an ideal transducer?

Back 6

1. High sensitivity. 2. High signal to noise ratio. 3. Fast response time. 4. Zero output signal in the absence of illumination.

Front 7

Explain in detail how a silicon diode transducer operates.

Back 7

The reverse bias creates a depletion layer that reduces the conductance of the junction to nearly 0. If radiation is allowed to impinge on the chip, however, holes and electrons are formed in the depletion layer and swept through the device to produce a current that is proportional to radiant power.

Front 8

What three phenomena are optical spectroscopic methods based upon?

Back 8

Absorption, Fluorescence, Emission

Front 9

What are the five components used to construct optical instruments?

Back 9

1. stable source of radiant energy. 2. transparent container for holding the sample. 3. A device that isolates a restricted region of the spectrum for measurement. 4. A radiation detector. 5. Signal processor and read out.

Front 10

Define continuum sources. Give an example of a specific light source for the UV, visible, and infrared spectral regions.

Back 10

Continuum sources: sources which emit radiation that changes in intensity only slowly as a function of wavelength. -deuterium lamp for the ultraviolet region -tungsten filament lamp is used for the visible region -inert solid heated 1500 and 2000 Kelvin for the infrared region.

Front 11

Define line sources. What type of spectroscopy uses a line source. Is a Mercury lamp a line source?

Back 11

Line sources: emit a limited number of lines or bands or radiation each of which spans a limited range of wavelengths. Atomic absorption spectroscopy. Yes: Mercury lamps provide a few sharp lines in the UV and visible regions.

Front 12

In general, how are the two types of radiation transducers differentiated from each other?

Back 12

One responds to photons, one responds to heat.

Front 13

Basis of operation for photon transducer: Photovoltaic cells

Back 13

The radiant energy generates a current at the interface of a semiconductor layer and a metal.

Front 14

Basis of operation for photon transducer: Photo tubes

Back 14

Radiation causes emission of electrons from a photosensitive solid surface.

Front 15

Basis of operation for photon transducer: Photomultiplier tubes

Back 15

Contain a photoemissive surface as well as several additional surfaces that emit a cascade of electrons when struck by electrons from the photosensitive area.

Front 16

Basis of operation for photon transducer: Silicon photodiodes

Back 16

In which photons increase the conductance across a revers-biased pn junction.

Front 17

Basis of operation for photon transducer: Charge transfer transducers

Back 17

The charges developed in a silicon crystal as a result of absorption of photons are collected and measured.

Front 18

Which has a higher sensitivity: a phototube or a photomultiplier tube?

Back 18

PMT (pohotomultiplier tube) is better than the phototube for the measurement of low radiant power.

Front 19

What is a photodiode array?

Back 19

One-dimensional transducer in which the photosensitive elements are arranged in a line. Contains electronic circuitry that makes it possible to determine the electrical output signal from each of the photosensitive elements either sequentially or simultaneously.

Front 20

What does CCD stand for?

Back 20

Charge-Coupled Device

Front 21

List three advantages of mass spectrometry.

Back 21

1. Detection limits a thousand times better than optical methods. 2. Simple spectra. 3. Measurement of atomic isotopic ratios.

Front 22

Describe the process of mass spectrometry.

Back 22

Atomization- bonds within molecules are broken with external energy. Ionization- conversion of a substantial fraction of the atoms formed to a stream of ions (usually singly charged positive ions). Separation-separating the ions formed on the basis of their mass-to-charge ratio (m/z) where m is the mass of the ion in atomic mass units and z is its charge. Counting-counting the number of ions of each type.

Front 23

What does m/z stand for? What is the most common value for z? Is m/z usually equal to the mass of the ion?

Back 23

Mass-to-charge ratio. 1. Yes.

Front 24

How are components of the sample ionized?

Back 24

Components of the sample are converted into gaseous ion by bombardment with electrons, photons, ions, or molecules.

Front 25

Describe one advantage and one disadvantage of the Faraday cup.

Back 25

Advantage: Simple and inexpensive. Disadvantage: Slower

Front 26

Describe how an electron multiplier works.

Back 26

-similar to the photomultiplier tube for UV radiation. -each dynode is held at a successively higher voltage. -the cathode and the dynodes have Cu/Be surfaces from which bursts of electrons are emitted when struck by energetic ions or electrons.

Front 27

Which ions hit the detector first in a time of flight mass analyzer, smaller or larger mass ions?

Back 27

Lighter particles arrive at teh detector earlier than heavier ones.

Front 28

Describe the construction and operation of the quadrupole mass analyzer and contrast it with a time of flight mass analyzer.

Back 28

Compact, less expensive. High scan rate-the entire mass spec can be obtained in less than 100 ms. Four parallel cylindrical rods serve as electrodes. Opposite rods are connected electrically. Ions are accelerated into the space between the rods. Only ions having within a narrow range of m/z values can survive the trip. Due to low mass fragmentation of a soft source, compounds with the same molecular weight appear on the same protonated mass and can hardly be differentiated using a quadrupole mass filter. High-resolution time of flight, compounds with different atomic composition appear on their exact protonated mass allowing isobaric species to be distinguished.

Front 29

How much time does it take for a quadupole mass analyzer to acquire a mass spectrum?

Back 29

Less than 100 ms.

Front 30

What function does the ICP torch have in ICPMS?

Back 30

Atomizes (breaks apart molecules) and ionizes the sample.

Front 31

Why has ICPMS become an important and widely used analytical method?

Back 31

Low detection limits, high degree of selectivity, reasonably good precision and accuracy.

Front 32

How are lasers used as a means of sampling solids for ICPMS?

Back 32

High intensity radiation vaporizes the sample. A flow of argon carries the vapors into the ICP-MS

Front 33

What type of interferences are encountered in atomic mass spectrometry?

Back 33

Isobaric Interferences: Two elements that have isotopes having substantially the same mass. Polyatomic Ion Interferences: Polyatomic species formed from interactions between species in the plasma and species in the matrix of the atmosphere. Oxide and Hydroxide Species Interference: Oxides and hydroxides formed from the analyte itself, the matrix components, the solvent, and the plasma gases.

Front 34

What is the purpose of the radiofrequency induction coil? Does the instrument really "touch" the sample in order to heat it?

Back 34

Supplies power to the plasma. No.

Front 35

Give an example of an isobaric interference that might be present during the analysis of cobalt in a geologic sample.

Back 35

The isotopes of Cobalt (59)Co and (60)Co overlaps with (50)Ni (60)Ni Isobaric interference.