Detectors by Esayas AAU

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Types of detectors in AAS/AES

• By Group III

April 29, 2013
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Outline

• Introduction
• Detectors
• Phototube
• Photomultiplier tube
• Charge coupled device
• Linear diode array
• Charge-transfer device
• Instrumental Noise in Detectors

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Introduction

• Atomic absorption is the process that occurs when
  a ground state atom absorbs energy in the form of
  light of a specific wavelength and is elevated to
  an excited state.
• The basic instrumentation for atomic absorption
  requires a primary light source, an atom source,
  a monochromator to isolate the specific
  wavelength of light to be used, a detector to
  measure the light accurately, and electronics to
  treat the signal, and a data display or logging
  device to show the results.

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Introduction

• Atomic emission spectroscopy is a process in
  which the light emitted by excited atoms or ions
  is measured.
• The basic instrument used for atomic emission is
  very similar to that used for atomic absorption
  with the difference that no primary light source
  is used for atomic emission.
• One of the more critical components for atomic
  emission instruments is the atomization source,
  because it must also provide sufficient energy to
  excite the atoms as well as atomize them.

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Introduction
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Introduction

• Atomic Absorption ? it measures the radiation
  absorbed by the unexcited atoms that are
  determined.
• Flame Emission ? it measures the radiation
  emitted by the excited atoms that is related to
  concentration.

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Introduction

• AAS and AES measurements require that the energy
  contained in a photon to be converted into a
  measurable electrical signal.
• Any photosensitive device may be used as a
  detector provided that it is
• Responsive to the characteristic wavelength that
  is being used
• Sensitive enough to measure the change in radiant
  energy caused by any absorption or emission of
  the sample.

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Types of detectors in AAS/AES

• Phototube
• Photomultiplier tube
• Photodiode arrays
• Charge-transfer devices.

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Phototube

• A phototube consists of an evacuated glass or
  quartz chamber containing an anode and a cathode.
• Cathode surfaces are composed of materials that
  readily give up electrons Group I metals such as
  Cs work well of this purpose.
• A relatively large potential is placed across the
  anode and cathode, usually 90V, and the gap is
  referred to as a dynode.
• Electrons contained in the cathode are released
  as photons with a sufficient energy strike the
  surface. This causes electrons to move through
  the low-pressure gap to the anode, which produces
  a current.

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Photomultiplier tube

• In PMTs numerous dynodes are aligned in a
  circular or in a linear manner.
• Most of the electrodes act as both an anode and a
  cathode with each dynode having a potential
  difference of 90 V thus, the potential
  increases by 90 V as an electron goes from one
  electrode to the next.
• This causes considerable amplification of a weak
  signal compared to a photon tube that does not
  amplify the signal.

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Photomultiplier tube
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Photodiode Arrays

• PDAs are silicon based multichannel array
  detector.
• A Si diode consists of two pieces of Si, one
  positively doped (p-doped) and one negatively
  doped (n-doped), with the functional area (the
  depletion region) being at the interface between
  the two media.
• The positively doped end was created by addition
  of a Group III element, such as Al.
• The negatively doped end of a diode is a similar
  piece of silicon where a Group V element, such as
  phosphorus is added to the silicon.

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Photodiode Arrays

• Combining the p-doped and n-doped silica into one
  device (called a p-n junction) creates a diode
  that can be operated in two modes, forward and
  reverse bias.

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Photodiode Arrays
Forward bias mode, a positive potential is placed
on the p-doped semiconductor and a negative
potential is placed on the n-doped semiconductor.
This results in the holes and electrons moving
toward the center of the device where they can
combine and reduce the resistance in the diode
here the device acts as a conductor.
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Photodiode Arrays

• In Reverse bias mode, Charges on the electrodes
  are reversed and the holes in the p-doped
  semiconductor are drawn toward the negative
  terminal while the electrons in the n-doped side
  are drawn toward the positive terminal.

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Photodiode Arrays
when a photon of sufficient energy strikes the
depletion region, holes and electrons are created
that then migrate to their respective terminals
generating a current.

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Photodiode Arrays

• Linear Diode Array is collection of Silicon
  diodes arranged in linear manner.
• LDA allows the simultaneous detection of a wide
  number of wavelengths.

• PMTs have higher sensitivity, a larger dynamic
  range, and lower signal to noise ratios.

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Charge-Transfer Devices

• There are two types of charge transfer devices,
  charge injection devices and charge coupled
  devices.
• Both of these overcome the disadvantages of LDAs
  by amplification (through timed storage or
  accumulation) of the original signal.

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Charge-Transfer Devices

• Charge injection devices use n-doped silicon.
• When a photon of sufficient energy strikes a bond
  in the silicon, a hole and an electron are
  created.
• The electron migrates to the positively charged
  substrate and is removed from the system.
• The hole migrates to a potential well that is
  created under the most negatively charged
  electrode.

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Charge-Transfer Devices

• The system acts as a capacitor because of the -5V
  and -10V applied potential.
• After a sufficient amount of time (when a large
  number of holes have been created and collected)
  the applied electrical potential on the -5V
  electrode is removed and a voltmeter
  (potentiometer) is used to measure the capacitor
  potential under the -10V electrode.

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Charge-Transfer Devices

• Amplification of the signal, or more specifically
  accumulation of holes, can be completed in two
  ways,
• By initially measuring a significant amount of
  time and
• By measuring and deciding to continue the
  collection process.

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Charge-Transfer Devices

• Charge coupled devices (CCDs) are designed in an
  opposite manner as compared to charge injection
  device.
• In CCDs, p-doped silicon is used and the
  electrodes atop the semi-conductive material are
  positively charged.
• Photons strike the doped silicon, holes are
  neutralized and removed by the metal substrate
  below the pixel and electrons migrate to the
  potential well where they are stored.

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Charge-Transfer Devices
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Charge-Transfer Devices

• Each pixel acts as a capacitor where its
  potential can be measured by removing the applied
  potential and measuring the capacitor potential
  with a voltmeter.
• Arrays are read from left to right by shifting
  the potential on the electrons and measuring the
  ?V transferred between the electrodes.

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Charge-Transfer Devices

• The measurement is destructive, so only one
  measurement is possible.
• This is not a disadvantage in FAAS or FAES
  measurements since a manual measurement only
  takes seconds to obtain, and doubling or tripling
  the reading time is relatively insignificant.
• Due to high thermal noise charge injection
  devices are usually cooled to liquid nitrogen
  temperatures.

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Instrumental Noise in Detectors

• Most analysts attempt to determine if an analyte
  is present or not, and in doing so must measure
  the smallest amount of an analyte in a sample
  near the detection limit of the instrument.
• The detection limit is basically the minimum
  concentration that one can measure above the
  noise in an instrument.

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Instrumental Noise in Detectors

• Environmental noise consists of factors in the
  immediate lab environment that will affect an
  instrument or sample.
• If an instrument is sensitive to vibrations, such
  as an NMR, it should not placed in an area of
  high vibration, such as next to an elevator.
• Its not possible to measure in ppm-, ppb-, and
  ppt-level concentration measurements in an
  environment where the analyte of interest is
  present in high concentrations (such as in the
  air near a metal smelter).

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Instrumental Noise in Detectors

• Instrumental noise is common and in many cases
  can be avoidable or managed.
• Thermal noise (also referred to as Johnson noise)
  results from the thermal agitation of electrons
  in resistors, capacitors, and detectors.
• It can be overcome by cooling specific components
  of the instrument such as is done in advanced
  detectors (i.e. CIDs).

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Instrumental Noise in Detectors

• Shot noise results from a current being generated
  by the premature movement of electrons across a
  junction. Photons arrive discretely,
  independently and randomly. Dark current noise
  (Nd)thermally inducedfiringof the detector
• Flicker noise(optical) results from random
  fluctuations in current and is inversely related
  to frequency. It is overcome by electronically
  modulating the detector output signal to a higher
  frequency where less noise is present (i.e. from
  102 Hz to 104 Hz).

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Summary

• Any photosensitive device may be used as a
  detector provided
• Responsive to the characteristic wavelength and
• Sensitive enough to measure the change in radiant
  energy caused by any absorption of the sample.
• Types of detectors in AAS/AES includes
  Phototube, photomultiplier tube, photodiode array
  and charge-transfer devices.

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