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 spectrophotometer

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المزاج : الحمد لله رايقه
الدفعة : احلـــى ناس 25
التخصص : طبيه
نقاط : 0
تاريخ التسجيل : 23/07/2008

مُساهمةموضوع: spectrophotometer   23/7/2008, 6:57 pm


A spectrophotometer optically determines the absorbance
or transmission of characteristic wavelengths
of radiant energy (light) by a chemical species
in solution. Each molecule absorbs light at certain
wavelengths in a unique spectral pattern because of
the number and arrangement of its characteristic functional
groups, such as double bonds between carbon
atoms. According to the Beer-Lambert law, the
amount of light absorbed at these wavelengths is directly
proportional to the concentration of the chemical
species. Spectrophotometers are generally used to determine
solution concentrations for quantitative
analyses or, with more sophisticated instruments, for
enzyme or reaction kinetics assays.

Principles of operation
The basic components of a spectrophotometer are a
light source, a wavelength monochromator, focusing
devices, a cuvette, a photodetector, and a display device
(see Fig. 1 on page 2). Light passes through amonochromator
that separates the light into its component wavelengths.
Slits then isolate a bandpass (the segment of
the spectrum isolated by the monochromator) of the
wavelength needed for measurement, improving its
purity. Next, the light passes through the sample, and
a portion of the radiant energy is absorbed; the amount
depends on the nature and concentration of the sample.
The portion of the bandpass that is not absorbed is
transmitted to a photodetector, which converts light
energy to electrical energy that can be registered on a
meter or digital readout. When a photodiode array
detection system is used in place of a photodetector,
light passes through the sample first, before reaching the monochromato

Light sources
The light source typically yields a high output of
polychromatic light over a wide range of the spectrum.
Tungsten-filament bulbs, with a wavelength range of
about 330 to 900 nm, are used for the visible region.
They are generally useful for measuring moderately
dilute solutions in which the change in color intensity
varies significantly with changes in concentration

[Monochromators
The monochromator isolates radiant energy of a desired
wavelength either by placing colored filters in the
beamor by dispersing the beaminto its components and
placing in the light path a slit that lets pass only anarrow
wavelength band (0.05 to 20 nm). This process is called
spectral isolation. On some units, the wavelength is
selected by adjusting the slit position manually. Other
units can be programmed to automatically scan through
a range of wavelengths. If placed in the light path to
intercept light that has passed through the sample, the
monochromator can be used to present the spectrum of
transmitted light to a diode-array detector

[Focusing devices
Combinations of lenses, slits, and mirrors can be
inserted into the light path to render the light rays
parallel or to isolate narrow portions of the light beam
or its spectrum. Variable slits also permit adjustments
in the total radiant energy reaching the detector. The
Ebert and Czerny-Turner monochromators and their
variations are combinations of prisms or gratings and
focusing devices
.
Cuvettes
The cuvettes, or absorption cells, are round, square,
or rectangular and are constructed fromglass, quartz, or
plastic.All sample cells cause some loss of light intensity
owing to reflectionat the surface, inhomogeneities of the
cuvette wall, and scattering losses in the solution. As
a result, cuvettes must be optically matched if more
than one is used in the course of an analysis. Cuvettes
are chosen for transparency in the spectral wavelengths
of interest. For measurements in the visible
region, cuvettes of optical glass are sufficient; however,
optical glass absorbs light below 350 nm, and
more expensive quartz or fused silica must be used for
these wavelengths. The sample cuvettes are placed in
a darkened analysis chamber; some chambers have
rotating carousels that can hold several cuvettes.
Some spectrophotometers have automatic samplers,
and some have continuous-flow assemblies
[size=9]linked to chromatographs) to measure the absorbance
of a stream of liquid
.
Detectors
Any photosensitive device can be used as a detector
of radiant energy. The photocell and phototube are the
simplest photodetectors, producing current proportional
to the intensity of the light striking them; phototransistors
and photodiodes can also be used.
Measuring low-intensity light requires photomultiplier
tubes (PMTs), which are used in spectrophotometers
with a narrow bandpass and in instruments that
must record fast changes in light emission (scanning
spectrophotometers). Light striking the PMT generates
electrons through a series of dynodes that amplify
the signal severalfold. PMTs respond rapidly, have
excellent sensitivity, and have a longer working life
than other detectors

Display devices
The data from a detector is displayed by a readout
device, such as an analog meter, a light beam reflected
on a scale, or a digital display (i.e., light-emitting diode
[LED] or video monitor). The output can also be transmitted
to a computer or printer.
Microcomputers have been added to spectrophotometers
to act as system controllers and to perform
data analysis and storage. Multiwavelength data
processing methods help distinguish overlapping spectra
in samples with multiple components


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العمر : 35
الموقع : السودان-مدني
العمل/الترفيه : خريج جامعة الجزيرة/الانترنت والبلياردو
المزاج : تمام والحمد لله
الدفعة : 26
التخصص : هندسة طبية
الوسام :
نقاط : 26
تاريخ التسجيل : 30/05/2008

مُساهمةموضوع: رد: spectrophotometer   31/7/2008, 2:39 am

شكرا لكي اختي الكريمة على هذا الموضوع الرائع وان شاء الله نرى منك المزيد والجديد عن الاجهزه الطبيه وانا اهتم بذلك كثيرا لانه تخصصي .. تقبلي مروري ومع التحية..

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spectrophotometer
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