InstrumentationInstrumentationUV VIS SpectrometersUV VIS Spectrometers

Introduction Introduction

A source that generates electromagnetic radiation A dispersion device that selects a particular

wavelength from the broad band radiation of the source

A sample area A detector to measure the intensity of

radiation

In principle, all spectrophotometers consist of four major In principle, all spectrophotometers consist of four major sub-units:sub-units:

In addition, there are other optical components such as lenses or mirrors that relay the light through the instrument.

Light sourceLight source

The ideal source:

* The noise from the lamp is often the limiting factor in overall instrument noise performance.

Constant intensity over all wavelengths in the UV-Visible region

Low noise * Long term stability.

The deuterium arc lampThe deuterium arc lamp Good intensity continuum in the UV

region with useful intensity in the Visible region.

Modern lamps have low noise characteristic.

Typically a lamp will have a "half-life" (the time for the intensity to fall to half the initial value)

The tungsten-halogen lampThe tungsten-halogen lamp

Good intensity over the part of the UV and the whole of the Visible range.

It has very low noise and low drift.

The detectorThe detector

A device which converts a light signal into an electrical signal. Ideally it should give a linear response, with low noise and high sensitivity.

A spectrophotometer detector must have low noise characteristic at low intensity levels so that it is possible to determine accurately small differences between blank and sample measurements.

PhotodiodesPhotodiodesAdvantage greater dynamic range solid state devices - robust. Principle of operation

– Light falling on the semiconductor material allows electrons to flow through it, thereby depleting the charge in a capacitor connected across it. The amount of charge needed to recharge the capacitor at regular intervals is proportional to the intensity of the light.

A spectrum is obtained by electronically scanning the array.

The diode-array The diode-array spectrophotometerspectrophotometer An array of detectors and a "reverse" optics configuration:

the dispersion device comes after the sample. An advantage of the reversed optics configuration is that

only light traveling along the axis from source to inlet slit before the dispersion device can reach the detector; light from other angles cannot.

Light of all wavelengths falls on the diode-array and is measured simultaneously, that is, data acquisition is done in parallel.

Despersive devicesDespersive devices

Certain devices cause different wavelengths of light to be dispersed at different angles.

Two types of dispersion device are commonly used:

PrismPrism

Advantage: simple and inexpensive to make

Disadvantages: the dispersion is angularly non-linear they are temperature-sensitive.

Holographic gratingsHolographic gratings

Are made from glass blanks onto which are ruled very narrow grooves.

The dimensions of the grooves are of the same order as the wavelength of light which is to be dispersed.

The prepared blank is then coated with a very thin layer of aluminum to create a mirror.

Light falling on the grating is reflected at different angles depending upon the wavelength.

Holographic gratingsHolographic gratings

Advantage: Gives a linear angular dispersion with wavelength, Temperature insensitive.

Disadvantage Light is reflected in different orders which overlap A concave holographic grating combines the two functions

of dispersing and focusing light at the same time.

OpticsOptics

Optical components used to relay and focus light through the instrument are either lenses or concave mirrors.

Simple lenses are inexpensive but suffer from

chromatic aberration ( light of different wavelength is not focused at exactly the same point in space).

At each optical surface 5 - 10% of the light is lost by absorbance or reflection.

OpticsOptics

Achromatic lenses combine multiple lenses of different glasses with different refractive indices into a compound lens which is largely free of chromatic aberration. Such lenses are used in cameras. They offer good performance but at relatively high cost.

Concave mirrors are less expensive to make than achromatic lenses and are free from chromatic aberration. Their one disadvantage is that the aluminum surface may be easily corroded, causing a loss in efficiency.

The Conventional Scanning The Conventional Scanning SpectrophotometerSpectrophotometer

A single detector and a "forward" optics configuration: the dispersion device comes before the sample.

The sample area must be completely covered to prevent ambient light reaching the detector.

To measure at different wavelengths or to measure a spectrum it is necessary to rotate the dispersion device. Data acquisition therefore is sequential.

There are many variations in designs based on this concept but, in general, there are four important groups:

Single BeamSingle Beam

The simplest design. To make a measurement, the blank is first placed in

the instrument and measured to Then the sample is placed in the instrument and

measured

Double BeamDouble Beam

Designed to eliminate drift by measuring blank and sample virtually simultaneously.

A "chopper" alternately transmits and reflects the light beam so that it travels down the blank and the sample optical paths to a single detector.

The chopper causes the light beam to switch paths at about 50 Hz causing the detector to see a "saw tooth" signal of Io and I which are processed in the electronics to give either transmittance or absorbance as output.

Double BeamDouble Beam

Double BeamDouble Beam

The advantage of the double beam design is high stability because reference and sample are measured virtually at the same moment in time

The disadvantages are higher cost, lower sensitivity because throughput of light is poorer because of the more complex optics and lower reliability because of the greater complexity.

Split BeamSplit Beam

The split beam spectrophotometer is similar to the double beam spectrophotometer

Uses a beam splitter instead of a chopper to send light along the blank and sample paths simultaneously to two separate but identical detectors.

Thus blank and sample measurements can be made at the same moment in time.

Split BeamSplit Beam

Split BeamSplit Beam

The advantage of this design is good stability (though not as good as a double beam

instrument because two detectors can drift independently)

Low noise ( though not as low as a single beam instrument because the light is split so that less than 100% passes through the sample).

The Diode-Array The Diode-Array

SpectrophotometerSpectrophotometer Uses an array of detectors and a "reverse" optics

configuration Light of all wavelengths falls on the diode-array

and is measured simultaneously, that is, data acquisition is done in parallel.

A spectrum is obtained by electronically scanning the array.

Diode-array spectrophotometers can be designed as single-, double- or dual-beam but, in practice, the advantages of the single beam design combine well with diode-array detection.

The Diode-Array SpectrophotometerThe Diode-Array Spectrophotometer

Diode-Array Advantages:Diode-Array Advantages:

The diode-array spectrophotometer has the following advantages over conventional mechanical scanning spectrophotometers:

Fast spectral acquisition Simultaneous multi-wavelength measurement Wavelength reproducibility High sensitivity Measurement Statistics Very high reliability and ruggedness Open sample area

Fast Spectral AcquisitionFast Spectral Acquisition

Speed is the best known advantage of diode-array spectroscopy.

Data is acquired in parallel, the detectors are read-out by "electronic scanning", and microprocessors and computers are used to process data;

a spectrum can be obtained in as little as 0.1 sec.

Simultaneous Multi-Simultaneous Multi-

wavelength Measurementwavelength Measurement as measurements are made at different wavelengths

at the same time. Conventional spectrophotometers can make multi-wavelength measurements but there is a time differential between each measurement .

If an impurity is present, the absorbance will be different.

High SensitivityHigh Sensitivity

High light throughput result in low noise Wavelength averaging : The signal-to-noise ratio improves as

more data points are averaged. Time averaging: ( conventional spectrophotometers acquire

data serially and diode-array spectrophotometers acquire data in parallel, equivalent scan times are 30.0 and 0.1 sec respectively). noise can be theoretically reduced by the square root of number of measurements With a diode-array spectrophotometer, longer scan times are practical (with a conventional spectrophotometer, they rapidly become impractical because they are so long that productivity is very low and drift errors may become significant.)

Measurement StatisticsMeasurement Statistics

Measurement times of 0.5 or 1.0 sec are used and 5 or 10 spectra are measured and averaged together.

In addition, the standard deviation for each data point is calculated; this is a measure of the reliability of the data point.

The statistical information on the reliability of data points is important for "chemometric" techniques such as least squares with maximum likelihood.

High Reliability and RuggednessHigh Reliability and Ruggedness

Our diode-array spectrophotometers are designed with the minimum of moving parts -- just the shutter and an air fan for cooling --

with a short and rigid optical bench The spectrograph that contains key optical

components (grating and diode-array) is sealed with a nitrogen atmosphere.

Diode Array spectrophotometers Diode Array spectrophotometers areare:

reliable require virtually no maintenance low down-time cost of ownership is very low very rugged

Scanning instruments have many moving parts and are therefore inherently less reliable.