Detectors used in array spectrophotometer

DETECTOR USED IN ARRAY
SPECTROPHOTOMETER
PRESENTEDBY
SURIYAPRIYA.K
ISTM.PHARM
DEPARTMENTOFPHARMACEUTICALANALYSIS
KMCHCOLLEGEOFPHARMACY
COIMBATORE

1. Introduction
• Array detectors `
• Photodiode
• Diode
2. Photodiode Array Spectrophotometer
• Working
• Advantages and disadvantages
3. Charged coupled device
• Working
• Advantages and disadvantages
• Process

INTRODUCTION :
Array detectors are capable of determining the spatial
distribution of particles ,whether photons ,neutrons ,atoms , or
others have long existed.
More recently, this technology has been applied to mass
spectrometry for the simultaneous detection of multiple ions of
differing mass to charge (M/Z) values .

• Diode is an electronic device with two transmitting
terminals that allows electric current to flow in one
direction while blocking current in the opposite direction.
• A diode is a light-emitting diode, ex : LED
• The positive end of a diode is called anode , and the
negative end is called cathode.

DIODE ARRAY
DETECTOR
The diode array detector is a
multichannel detector capable
of simultaneous measurement of
all wavelengths of dispersed
radiation. It comprises of
an array of silicon photodiodes on
a single silicon chip.

• A photodiode is a semiconductor p-n junction device that
converts light into an electrical current .
• The current is generated when photons are absorbed in the
photodiode.
• Photodiodes usually have a slower response time as their
surface area increases.
• It consists of a transparent window that allows light to fall on
it.
• Example : Solar cell

• A diode is a two-terminal electronic component with asymmetric
conductance;
• It has low resistance to current in one direction, and high
resistance in the other.
i.e. A diode is an electrical component acting as a one-
way valve for current .
SILICON DIODE

• A silicon diode is a semiconductor that as positive and negative
polarity , and can allow electrical current to flow in one direction while
restricting it in another.
• The element silicon , in it is pure form, acts as an electrical insulator.
TO BE CONTINUED….

• To enable it to conduct electricity, minute amounts of other
elements-in a process known as doping are added to it.
• A diode is a p-n junction under forward bias, current flows from n- si
to p-si, under reverse bias, no current flows boundary is called a
depletion layer or region.
• N-type silicon has no conduction electron-p or as doped
• P-type silicon has a ‘hole’ or electron vacancy or b doped.
TO BE CONTINUED....

• A photodiode is a semiconductor device that converts light into current.
• The current is generated when photons are absorbed in the photodiode.
• Both single beam and double beam instruments uses only single detector
to monitor only one wavelength at a time.
• A photodiode is designed to operate in reverse bias.
• The photodiode array is a solid- state device and is more secure and
reliable than PMT (Photomultiplier tube).

• A photodiode array(PDA) is a linear range of discrete photodiodes on a
photodiodes on an integrated circuit(IC).
• For spectroscopy it is placed at the image level of a spectrometer to allow
simultaneous detection of a wavelength.
• Array detectors are particularly useful for recording total UV absorption
spectra of samples flow cell.
• In a diode array spectrometer the source radiation passes through the
sample and is dispersed by diffraction grating and the amount of the
dispersed light is estimated for each wavelength.

• The photodiode ranges from 128,256,512 and 1024. Each
photodiode acts as a capacitor initially holding a fixed amount
of charge.
• Light striking a photodiode discharges the diode.The
magnitude discharge depends on the amount of light striking
the photodiode
TO BE CONTINUED…

ADVANTAGES OF PHOTODIODE
1. Low noise
2. Spectral response from 190nm to 1100nm(silicon),longer wavelength with other
semiconductor materials.
3. Low cost
4. No high voltage required
5. Long life time
6. Excellent linearity of output current as a function of incident light.

DISADVANTAGES
1. Response time for many designs is slower.
2. Much lower overall sensitivity.
3. Small area.
4. Photon counting only possible with specially designed, usually
cooled photodiodes , with special electronic circuits.

• A charge-coupled device (CCD) is an integrated circuit containing an array
of linked, or coupled, capacitors
• It is under the control of an external circuit, each capacitor can transfer
its electric charge to a neighboring capacitor. CCD sensors are a major
technology used in digital imaging.
• It is highly susceptible detectors which are mainly used for detection of
extremely low intensity light signals.
• These are similar to diode array detectors but instead of diodes that consist
of an array of photo capacitors arranged in one or two dimensional array.

• The photo capacitors comprise of thousands or even
millions of detectors elements also called pixels.
• Simultaneously detection of emitted light from lowest to
highest wavelength is possible.
• An image is projected through a lens onto the capacitor
array (the photoactive region), causing each capacitor to
accumulate an electric charge proportional to
the light intensity at that location.

• Once the array has been exposed to the image, a control circuit
causes each capacitor to transfer its contents to its neighbor
(operating as a shift register).
• The last capacitor in the array dumps its charge into a charge
amplifier, which converts the charge into a voltage.
• By repeating this process, the controlling circuit converts the entire
contents of the array in the semiconductor to a sequence of
voltages.

• In digital device, these voltages are then sampled, digitized, and
usually stored in memory; in an analog device.
• In a CCD for capturing images, there is a photoactive region
(an epitaxial layer of silicon), and a transmission region made out of
a shift register.
TO BE CONTINUED….

• Low noise
• Accuracy (both linearity and stability)
• Dimensionally stable
• Regular grid of pixels.
• No chemical processing is needed.

• Demagnification is a major issue
• Vary with application
• Very expensive
• Lower sensitivity because of less
photosensitive area in each pixel.

Exposure : Converts light into an electronic charge at discrete sites called
pixels, photo sites, photodiodes or capacitors.
Charge transfer: One of the CCD architectures is used.
Charge to voltage: Signal conversion from analog to digital.

• https://www.hitachi-
hightech.com/global/products/science/tech/ana/lc/basic/course7.html
• Module-6 unit-4uv/visible spectroscopy book
• https://teaching.shu.ac.uk/hwb/chemistry/tutorials/molspec/uvvisab3.htm
• https://www.google.com/search?q=principle+of+pda+detector&source=lmns
&bih=655&biw=393&client=ms-android-xiaomi&prmd=isnv&hl=en-
US&sa=X&ved=2ahUKEwiamNjw5MvvAhU7hUsFHYdcAnEQ_AUoAHoECAA
QAw

Detectors used in array spectrophotometer

Detectors used in array spectrophotometer

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