Product Review
In the spray
The words "light scattering" in the phrase "evaporative light scattering detector" are something of a misnomer. Yes, the signal measured in an evaporative light scattering detector (ELSD) is based on light scattered from analyte particles, but don't think for a minute that these detectors are in any way similar to the classical light scattering detectors that follow the theories of Rayleigh, Fraunhofer, or Mie for particle size or absolute molar mass determinations. These two detectors have only the names in common ELSDs, which genNew Text proportional to the analyte concentration were originally called evaporative mass detectors Table 1 lists representative light scattering detectors which are used in various chromatographic separations In ELSDs, effluent from the column is nebulized to form an aerosol, which then enters a heated tube where the mobile phase evaporates and leaves particles of nonvolatile analytes. The process limits S0003-2700(97)09025-2 CCC: $14.00
© 1997 American Chemical Society
with ELSDs
Evaporative light scatteringfillsin the gaps left by other chromatographic detectors. the applicability of evaporative light scattering to analytes that are less volatile than the solvent. The analyte particles then enter the detector's optical cell, where they cause a light beam to be scattered; the scatter is detected by a photodetector, providing the output signal. Essentially, two designs of ELSDs are used. In the original design, the aerosol is introduced directly into the heated tube. All of the sample is sent to the optical cell
with the original design, which provides maximum sensitivity and reproducibility for nonvolatile analytes. This first category includes detectors from Alltech and Polymer Laboratories. The newer ELSD design uses a spray chamber (glass or metal, similar to those used in ICP-AES instruments) between the nebulizer and the evaporation tube. The purpose of the spray chamber is to remove the larger droplets from the aerosol, thus producing a narrower droplet size distribution. The smaller droplets are easily evaporated, thus reducing noise and in turn increasing the detector's sensitivity. Operating temperatures are also lower because of the removal of the larger aerosol droplets, which account for a few percent to more than 90% of the column effluent, prior to evaporation. This second design category is ideal for semivolatile analytes and includes detectors from EuroSep SEDEX and Alltech (with an optional adapter)'
Analytical Chemistry News & Features, September 1, 1997 5 6 1 A
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Table 1 . Summary of representative evaporative light scattering d e t e c t o r s .
Product Company
I 500 ELSD Alltech Associates 2051 Waukegan Rd. Deertield, IL 60015 847-948-8600
I DDL 31 Eurosep Instruments Distributed by: Bodman Industries P.O. Box 2421 Aston, PA 19014
I PL-EMD 960 I Sedex 55/65 ELSD Polymer Laboratories Richard Scientific Amherst Fields Research Pk. 366-A Bel Marin Keys Blvd. 160 Old Farm Rd. Novato, CA 94949 Amherst, MA 01002 800-840-3030 413-253-9554
URL Operating temperature range
www.alltechweb.com To140°C
www.bodman.com To180°C
www.polymerlabs.com To 200 °C
Nebulization gas Source
Nitrogen or air Laser diode, 670 nm
Nitrogen or helium Halogen lamp
Nitrogen or air Multiwavelength
Detectors Flow rate
Photomultiplier tube Up to 4 mL/min aqueous or nonaqueous
Photomultiplier tube 0.1-2.0 mL/min nonaqueous; 0.1-1.0 mL/min aqueous
Control
Silicon photodiode 0.1 2.0 mL/min nonaqueous; 0.1-1.0 mL/min aqueous (up to 5 mL/min with adapter) Microprocessor and keypad
Microprocessor and keypad
Microprocessor
Options Reader service no.
SFC, low-temperature adapter 401
Preparative 402
Heated transfer line 403
610-459-5600
'The primary requirement for detection [with ELSDs] is a volatility difference between the mobile phase and the analyte," says John Koropchak, a professor at Southern Illinois University. The mobile phase is often a solvent such as acetonitrile or methanol/water. As long as the temperature is not too high, the analyte remains as particles. The size of the particles depends on the initial size and concentration in the droplets, and the light scattering increases roughly in proportion to the size of the particles. The ELSD is largely a universal detector—that is, it can detect anything that's not too close to the volatility of the mobile phase. "There aren't too many truly universal detectors for liquid chromatography. Refractive index detection and mass spectrometry may be the only other ones—and many labs can't afford mass spectrometers. Evaporative light scattering just has a nonvolatility requirement," says Koropchak. Typical detection limits are on the order of 1 ug/mL or a ppm, which makes them more sensitive than refractive index detectors. In some instances the ELSDs are as much as 10 to 1000 times more sensitive thcin ci refrtictive index detector Because the ELSD response depends only weakly on the optical properties of the analytes (refractive index and density of the analyte particle), these detectors can be used in situations where the other conventional HPLC detectors won't work. This 562 A
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case is especially true for gradient separations for which a refractive index detector would be completely unsuitable. ELSDs can also be used when the analyte does not have a UV chromophore or—the opposite problem—when the mobile phase does have a UV chromophore. Yet another advantage of the ELSD is its insensitivity to ambient temperature fluctuations. Richard Devereaux of Richard Scientific says, "Evaporative light scattering detection is suddenly becoming popular, particularly with the advent of combinatorial chemistry. Because it is a mass detector, its response is always proportional to the mass present and is independent of the presence or absence of chromophores in molecules. An ELSD thus offers the opportunity for quickly assessing the purity of synthetic organics without having to construct calibration curves
