2001 opticalinst...

August ‘171 Optical Instruments

2001 SpectrometersInstrument Machinery

Chp20:

Movies from this presentation can be access athttp://www.shsu.edu/~chm_tgc/sounds/sound.html


Instrument ComponentsComponents of various types of instruments for optical spectroscopy. Spectroscopic instruments consist of five components, (1) a stable source of radiation energy; (2) a wavelength selector that isolates a limited region of the spectrum for measurement; (3) one or more sample container; (4) a radiation detector, which converts radiant energy to a measurable electrical signal; and (5) a signal processing and readout unit i.e., computer.

(a)Arrangement for absorption measurement. The radiation of the selected wavelength is send through the sample and the transmitted radiation is measured by the detector/signal processor

(b) Configuration for fluorescence measurement. Two wavelength selectors are needed to select the excitation and the emission wavelengths. The selected sources radiation is incident on the sample and the radiation emitted is measured, usually at right angles to avoid scattering.

(c) Configuration for emission spectroscopy. A thermal sources of energy, such as flame or plasma, produces an analyte vapor that emits radiation isolated by the wavelength selector and converted to an electrical signal by the detector.


Varian Cary Spectrophotometer

The Varian Cary 50 scanning UV-Vis instrument scans wavelength with the speed of comparable to diode array technology. The scan speed for the UV-Vis instrument can go up to 24,000 nm/min. with a resolution 1.65 nm bandwith. The lamp is a Xenon flash lamp technology with a range of 190nm to 1100 nm. Diffraction grading with blaze angle of 8.6° at 240nm.


Spectroscopic SourcesSource must generate a beam of radiation that is sufficiently powerful to allow detection and measurements. A continuum source (spectral continuum) provides a broad distribution of wavelengths within a particular spectral range. A line sources emits a limited wavelength range.

Spec20: Tungsten/halogen. Cary 50 & Fluorolog Spex3: Xenon Flash lamp

190nm – 1100nm

Other source of lamp include Low-pressure Mercury arc lamp for LC and Lasers.


Optical MaterialsThe cell windows, lenses, mirrors and wavelength-selecting elements in an optical spectroscopic instrument must transmit radiation in the wavelength region being measured. Shown are wavelength range for several optical materials.

Simple glass can be used in the visible region. Fused silica or quartz is needed for the UV region. Halide salts are often used in the IR region but have complication by being water soluble and also expensive.

Spectrophotometer cuvettes, polystyrene with stopper have optical windows of 340 – 800 nm


Sample ContainersSample containers, which are usually called cells or cuvettes must be made from material that is transparent in the spectral region of interest.


Monochromator (Tuneable)A monochromator is an optical device that can filter or transmit a mechanically selectable narrow band of wavelengths of light or other radiation chosen from a wider range of wavelengths available at the input. The monochromator can be tune by turning the crystal and allows selected wavelengths to be tuned or changed with great precision.


MonochromatorA monochromator can use either the phenomenon of optical dispersion in a prism, or that of diffractionusing a diffraction grating, to spatially separate the colors of light. It usually has a mechanism for directing the selected color to an exit slit. Usually the grating or the prism is used in a reflective mode. A reflective prism is made by making a right triangle prism (typically, half of an equilateral prism) with one side mirrored. The light enters through the hypotenuse face and is reflected back through it, being refracted twice at the same surface. The total refraction, and the total dispersion, is the same as would occur if an equilateral prism were used.


Wavelength SelectorA device to restrict the radiation being measured to a narrow band is called a monochromator. The monochromator (a) grating monochromator; (b) prism monochromator.

Filters may also be used to restrict the incident wavelength. These may either be interference or absorption filters. The scheme above shows the schematic cross section of an interference filter.How a grating works: http://www.shsu.edu/%7Echm_tgc/sounds/flashfiles/grating.swf


Resolution, Dispersion and Efficiency

Dispersion measures the ability to separate wavelength difference by Dl through difference in angle, Df (radiants).

€

λΔλ

= nN,

λ = wavelength, Δλ = difference between two wavelength (resolution)n = diffraction order (integer), N = number of grooves

€

Δφ

Δλ=

n dcosφ

φ = reflection angle, d = distance between adjacent grooves

Efficiency of grating determines what wavelength are allowed to pass.

Resolution measures the ability to separate two closely spaced peaks.

€

η = E λo (grating) E λ (mirror)

E λo = irradiance at a particular wavelength diffraction

E λ = irradiance of same wavelength reflected by mirror


Monochromator bandwidthIncreasing monochromator bandwidth broadens the band and decreases the apparent absorbance. Note the narrower the bandwidth, the better the resolution.


Detector and Photoelectric EffectSpectroscopic information is detected in some manner and converted to a measurable quantity. The theory behind these detector is the photoelectric effect.


Detector (Transducer)Spectroscopic information is detected in some manner and converted to a measurable

quantity. A detector is a device that indicates the existence of some physical

phenomenon. A transducer is a type of detector that converts various types of

chemical and physical quantities into electrical signals such as electrical charges,

current, or voltage.


PhotoDiode Array DetectorPhotodiode arrays are example of multichannel instruments for UV/Vis absorption.

These instruments are usually single-beam design with the the photodiode array placed in the focal plane of the spectrograph. The detector allow the measurement of an entire spectrum (900nm - 300nm) in 1 s.


DetectorCharged Couple Devices: Stores photo-generated charge in a 2-dimensional array

Infrared DetectorThermocouple: Junction between two different electrical conductors

Ferroelectric material: material polarizes as function of temperature pyroelectriceffect

Photoconductive detector: semiconductor change conductivity with IR radiation

Optical SensorOptical Fiber: medium that carries light by total internal reflection

Opotodes: optical electrodes, optic fibers with sensors tips

Attenuated Total Reflectance:Sampling technique in which radiation enters the device through a set of mirrors. The sample is placed above a crystal with a high refraction index and absorbs part of the infrared light.


Photometry Configuration

Instrument designs for UV/Vis photometers or spectrophotometers.

(a)single-beam instrument: Light from filter or monochromator passes the sample or reference cell before the photodetector.

(b) double-beam in space instrument: light from the filter or monochromator is split into two beams that simultaneously pass through the reference and sample cell before the matching photodetector

(c) the double beam-in-line instrument: light is alternately sent through reference and sample cells before striking a single photodetector.


Double-Beam SpectrophotometerInstrument designs for UV/Vis photometers or spectrophotometers. Double-beam in space instrument: light from the filter or monochromator is split into two beams that simultaneously pass through the reference and sample cell before the matching photodetector


Spectrometers


Spec20, spectrophotometerhttp://www.dartmouth.edu/~chemlab/techniques/spectrometer.html

Usage: The instrument was introduced Bausch & Lomb in 1954. It is mostly used to determine the composition of solutions via the measurement of the absorption at specific wavelengths. It is commonly used to determine the concentration of substances in solution by measurement of absorbance at a particular wavelength and comparison to a standard of known concentration. A grating based spectrophotometer that uses a single-beam 20 nm bandwidth in the visible light region.

The instrument has has a mirrored scale graduated from 0-100% by 1% and from 0-infinite Optical Density (logarithmic scale in red). The wavelength scale is graduated from 350-1000 nm by 5 nm with numbered major divisions every 25 nm. The light sources is a Tungsten lamp with a grating monochromator and a photomultiplier detector.

UV photometers and spectrometers


Spec20, spectrophotometerhttp://www.dartmouth.edu/~chemlab/techniques/spectrometer.html

UV photometers and spectrometers


http://www.thermo.com/eThermo/CMA/PDFs/Product/productPDF_53056.pdf

Genesys 10S UV-Vis Spectrophotometer


Multichannel InstrumentsPhotodiode arrays are example of multichannel instruments for UV/Vis absorption.

These instruments are usually single-beam design with the the photodiode array placed in the focal plane of the spectrograph. The detector allow the measurement of an entire spectrum (900nm - 300nm) in 1 s.


Flame Atomic Absorption SpectroscopyAtomic absorption spectroscopy (AAS)

determines the presence of metals in liquid

samples. It also measures the concentrations of

metals in the samples, with concentrations range

in the low mg/L range (ppm).

In their elemental form, metals will absorb

ultraviolet light when they are excited by heat.

Each metal has a characteristic wavelength that

will be absorbed. The AAS instrument looks for

a particular metal by focusing a beam of UV

light at a specific wavelength through a flame

and into a detector. The sample of interest is

aspirated into the flame. If that metal is

present in the sample, it will absorb some of the

light, thus reducing its intensity. The instrument

measures the change in intensity. A computer

data system converts the change in intensity

into an absorbance.

More details in next chapter


Infrared SpectroscopyInfrared (IR) spectroscopy is a chemical analytical technique, which measures the infrared intensity versus wavelength (wavenumber) of light. Based upon the wavenumber, infrared light can be categorized as far infrared (4 ~ 400cm-1), mid infrared (400 ~ 4,000cm-1) and near infrared (4,000 ~ 14,000cm-1). Older IR instrument were dispersive double-beam designs. Fourier transformed infrared (FTIR) detect all the wavelength and are the standard type of equipment in today’s lab. Most FTIR are single beam mode.

The advantage of FTIR vs. dispersive spectrometers include better speed and sensitivity, better light-gathering power, more accurate wavelength calibration, simple mechanical design, and the virtual elimination of the problems of stray light and IR emission.


Fourier Transformed InfraredAn interferometer utilizes a beam splitter to split the incoming infrared beam into two optical beams. One beam reflects off of a flat mirror which is fixed in place. Another beam reflects off of a flat mirror which travels a very short distance (typically a few millimeters) away from the beam splitter. The two beams reflect off of their respective mirrors and are recombined when they meet together at the beam splitter. The recombined signal results from the “interfering” with each other. Consequently, the resulting signal is called interferogram, which has every infrared frequency “encoded” into it. When the interferogram signal is transmitted through or reflected off of the sample surface, the specific frequencies of energy are adsorbed by the sample due to the excited vibration of function groups in molecules.

The infrared signal after interaction with the

sample is uniquely characteristic of the sample. The

beam finally arrives at the detector and is measure

by the detector. The detected interferogram can

not be directly interpreted. It has to be “decoded”

with a well-known mathematical technique in term of

Fourier Transformation. The computer can perform

the Fourier transformation calculation and present

an infrared spectrum, which plots absorbance (or

transmittance) versus wavenumber.


Fourier TransformFourier analysis is a procedure in which a curve is decomposed into a sum of sine and cosine waves, called a Fourier series. A curve can be mathematically be decomposed by the series:

y = ao sin(0wx) + bo cos(0wx) + a1 sin(1wx) + b1 cos(1wx) + ) + a2 sin(2wx) + b2 cos(2wx) + ...

Y = S [an sin(nwx) + bn cos(nwx) ]


FT and S/NSignal-to-noise ratio (often abbreviated SNR or S/N) is an electrical engineering measurement, defined as the ratio of a signal power to the noise power corrupting the signal. A ratio higher than 1:1 indicates more signal than noise. Signal averaging is a signal processing technique applied in the time domain, intended to increase the strength of a small-amplitude signal that is buried in noise. By averaging trials, the signal-to-noise ratio is usually increased.


Signal AveragingSignal averaging is a signal processing technique applied in the time domain, intended to increase the strength of a signal relative to noise that is obscuring it. By averaging a set of replicate measurements, the signal-to-noise ratio, S/N, will be increased, ideally in proportion to the square root of the number of measurements.

Ideally it is assumed that-• Signal and noise are uncorrelated.• Signal strength is constant in the replicate measurements.• Noise is random, with a mean of zero.• Under these assumptions let the signal strength is

calculated by the following equation.

SN

= nSnσ 2

= n Sσ

Where, S= signal strengthσ = standard deviation of a single measurment (or noise, N)n = number of signals added together


SummarySpectrometers, Electromagnetic Radiation and chemical information

http://faculty.sdmiramar.edu/fgarces/LabMatters/Instrument.htm

2001 opticalinst...

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