Analytical Instrumentation II CTEC 2431

DOUBLE BEAM SPECTROPHOTOMETERS

Double beam spectrophotometers use a beam chopper to separate the reference beam from the sample beam.

Beam Chopper

The chopper is divided into three equal segments.  One segment is clear which will pass light through the wheel, one segment is mirrored which will reflect light along a different path, and the third segment is opaque so that no light is transmitted nor reflected.

Optical Path of Double Beam Spectrophotometers

Electronics of Double Beam Spectrophotometers

 

Advantages of double beam spectrophotometers

• speed of operation
• automatic compensation for variation in lamp output
• solvent absorption at various 8
• changes in detector sensitivity
• spectra scan

Most instruments have various options and selections which can be made during an analysis.

The mode switch allows the operator to choose what data the readout gives . The three major choices are absorbance (a log amplifier converts transmittance to absorbance), % T, or directly into concentration units.

The slit adjustment allows the operator to adjust the slit width of the instrument.

For instruments which have gratings which can be automatically turned the scan speed (wavelength/min) can be adjusted.  Higher scan speeds are used for surveys while slower speeds are used to look at areas in more detail.

The chart recorder is used to record the data of scan instruments. The paper speed and range (mv) can be adjusted to match the output of the spectrophotometer to give a spectra scan.

Spectra scans have two coordinated: the X axis is normally wavelength, and the Y axis is either absorbance or transmittance.   The resulting graph is called a spectra scan.  A spectra scan is similar to a fingerprint of a compound.