BÜCHI Labortechnik AG
www.buchi.com
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1.2 PRINCIPLE OF OPERATION
The unique detection principle of evaporative light scattering
detection involves a three-step process: nebulization of the
column effluent to form an aerosol, solvent evaporation within
a heated drift tube, and detection of the non-volatile solute
particles in the light scattering cell.
NEBULIZATION
EVAPORATION
DETECTION
NEBULIZATION
The column effluent from an HPLC separation enters the
nebulizer, where it is mixed with a steady stream of
nebulizing gas (usually nitrogen) to form an aerosol. The
aerosol consists of a uniform distribution of droplets whose
size is dependent on the gas flow rate used for the analysis.
The lower the gas flow rate used, the larger the resulting
droplets will be. Larger droplets scatter more light and
increase the sensitivity of the analysis, but they are also more
difficult to evaporate in the drift tube. There will be an
optimum gas flow rate for each method which will produce the
highest signal-to-noise ratio.
Lower mobile phase flow rates will require lower gas flow
rates for optimal nebulization. Substitution of a 2.1mm i.d.
column for your standard 4.6mm i.d. column will allow you to
greatly reduce the mobile phase flow rate while also
increasing the sensitivity of the analysis.
EVAPORATION
Evaporation of the volatile components in the aerosol occurs
in a heated, stainless steel drift tube. The optimum drift tube
temperature setting for an application will depend on mobile
phase composition, mobile phase flow rate, and on sample
volatility. Highly organic mobile phases require lower drift
tube temperatures for evaporation than highly aqueous
mobile phases. Lower mobile phase flow rates require lower
drift tube temperatures than higher mobile phase flow rates.
Semi-volatile analytes require the use of much lower drift
tube temperatures to obtain optimum sensitivity. The
optimum temperature should be determined by observing
the signal-to-noise ratio with respect to temperature.
Within the Model 3300 ELSD drift tube, a PTFE-coated,
stainless steel impactor plate is located three inches from
the nebulizer. The plate is perpendicular to the aerosol path
so as the aerosol contacts the plate, larger droplets exit
through the drain tube on the side panel. The remaining
droplets pass around the impactor and travel through the
drift tube to the optical cell for detection. The removal of the
larger droplets allows operation of the Model 3300 ELSD at
low temperatures, making it ideal for the analysis of semi-
volatile compounds.
Non-volatile impurities in the mobile phase or nebulizing gas
will produce noise. Using the highest quality gas, solvents,
and volatile buffers (preferably filtered) will greatly reduce
baseline noise. Noise will also increase if the mobile phase
has not been completely evaporated. Detector settings must
be carefully selected to ensure adequate mobile phase
evaporation.
DETECTION
The non-volatile sample particles emerge from the drift tube
in the mobile phase vapor and enter the light scattering cell.
In the optical cell, sample particles scatter light emitted by a
laser light source. The scattered light is detected by a silicon
photodiode, generating a signal proportional to sample
mass. This signal is sent to the analog output for data
collection. The advanced design of the Model 3300 ELSD
optical components
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provides excellent sensitivity for HPLC
a
nalysis.
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US Patent #6229605; other patents pending
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