Optimizing the Sensitivity of an Evaporative Light Scattering Detector

The evaporative light scattering detector (ELSD) is a universal, mass-based detector for HPLC, ultraperformance liquid chromatography (UPLC), gel permeation chromatography (GPC), and supercritical fluid chromatography (SFC) that has received a considerable degree of acceptance over the last two decades. It can provide quantitative information about essentially all compounds in the sample and does not require that the compound(s) of interest contain a chromophoric, fluorophoric, electroactive, or other type of functional group to provide a response. The sensitivity of the evaporative light scattering detector (ELSD) is several orders of magnitude greater than other “universal” detectors such as the refractive index detector, and allows for the use of gradient elution.1

The ease of operation of the detector and the sensitivity that the ELSD can provide has made it the detector of choice in a broad range of fields such as drug discovery and high-throughput screening. The ELSD is commonly used for the detection of such diverse compounds as phospholipids, carbohydrates, small peptides, nutraceuticals, and synthetic polymers.

The sensitivity of the ELSD, like that of all HPLC detectors, is a critical issue for many chromatographers. While the sensitivity of the detector is dependent on many of the same factors as other detectors (e.g., the compound should be eluted as a sharp peak), there are a number of specific issues that relate to the sensitivity of the ELSD. As is discussed in detail here, perhaps the most critical condition for which to optimize the sensitivity is to employ a low temperature for the evaporation of the mobile phase, especially for compounds that have a significant vapor pressure and/or are thermally labile.2,3 In addition, a number of design features are described that can optimize the sensitivity of the detector, such as the selection of the appropriate nebulizer and the use of a focusing gas in the optical cell.

Experimental

All chromatograms were collected on a variety of SEDEX LT-ELSD (low-temperature-evaporative light scattering detectors) (SEDERE, SA, Alfortville, Cedex, France). The column, solvents (HPLC grade), and conditions for each chromatogram are indicated in Figures 1–5.

Steps in the detection of an analyte via ELSD

There are three discrete stages in ELSD detection (Figure 1):

Figure 1 - The three steps involved in low-temperature evaporative light scattering detection.

  1. Nebulization. The eluent from the HPLC system is forced through a narrow orifice with a stream of gas flowing through a Venturi tube that sheaths the mobile phase to form small droplets that can be easily evaporated. A flow of N2 at a pressure of 2–4 bar is typically used.
  2. Vaporization. The mobile phase is removed from the nebulized droplets that contain the compound(s) of interest. The evaporation is performed by passing the droplets through a heated tube.
  3. Detection. The stream of solute particles that exits the vaporization chamber enters the optical cell where the amount of light scattering, which is related to the mass of the compounds of interest, is measured. The sensitivity of the detector is ultimately determined by the number and shape of particles that are detected by the light scattering chamber in a given period.

While each of these steps is described as an individual process, the reader should recognize that the detector is an integrated system, and if efforts are taken to optimize one of the steps, these factors may reduce the effectiveness of another step. As is typical with multistep detection processes, optimization of the overall process will therefore require that a series of compromises be made.

Role of temperature in optimizing sensitivity via ELSD

Role of temperature in optimizing sensitivity via ELSD sample contains compounds that are volatile and/or thermo labile, a low temperature is desirable for vaporization of the mobile phase since it will minimize the loss of analyte and hence optimize the sensitivity.4

The appropriate temperature for the detection of a given compound can be empirically determined by collecting the chromatogram at various temperatures. Figure 2 presents the chromatograms of caffeine, which is thermosensitive (it sublimes), using 30 °C and 50 °C to evaporate the mobile phase. At 30 °C, the peak height is approx. 10 times greater than when 50 °C is employed. Similarly, Figure 3 demonstrates that the sensitivity of the detector is improved by greater than a factor of 10 for urea (another thermosensitive compound) when the temperature is lowered from 39 °C to 25 °C.

Figure 2 - Detection of caffeine at 30 and 50 °C evaporation temperatures. ODS column: 30 × 4.6 mm, particle size: 5 μm, mobile phase: 70:30 water/acetonitrile, flow rate: 1 mL/min.

Low-temperature detection with gradient separation via ELSD

Figure 3 - Detection of urea at various evaporation temperatures. Column: Asahipak (Showa Denko, K.K., Tokyo, Japan) 5 μm, NH2; mobile phase: CH3CN/H2O (85:15); flow rate: 1 mL/min.

A major benefit of ELSD vis-à-vis other “universal” detection methods (e.g., the use of a refractive index detector) is that gradient mobi le phases can be used. An example of evaporative light scattering detection with a gradient is shown in Figure 4, where glucose and sucrose can be readily determined at the 5-ppm level.* (Note: The background noise of the detector, which is due to issues such as unevaporated solvent particles and residue after evaporation of the mobile phase, sorbent that has been removed from the column packing, and particulate matter from the mobile phase, is a major determinant in the sensitivity of the evaporative light scattering detectors. An example of the level of noise with the gradient used for the separation depicted in Figure 4 is shown in Figure 5.)

Figure 4 - Detection of glucose and sucrose. Column: carbohydrate, 5 μm, 150 × 4.6 mm; mobile phase: H2O/CH3CN gradient, T = 0 min, 80% B, T = 10 min, 50% B, T = 11 min, 80% B, T = 14 min, 80% B; flow rate: 1.0 mL/min.

Importance of instrument design on sensitivity

Although the analyst can rapidly determine the optimum temperature to maximize the sensitivity, a number of other instrument-related features can be employed to enhance the sensitivity of the detector. These include:

1. Use of a nebulizer to match the appropriate flow rate. The role of the nebulizer is to form droplets from the mobile phase. Since the HPLC flow rate can range from the low μL/min range to several mL/min, the design of the nebulizer is a critical factor. As an example, a nebulizer that is designed to optimize the nebulization of mobile phase at a flow rate of 0.1 mL/min will not do a very good job if the flow rate is 2 mL/min. If the nebulizer is designed to handle low flow rates, relatively large particles will be formed when a considerably higher flow rate is used. This will, of course, require a higher temperature to vaporize the mobile phase. Similarly, if a nebulizer that is designed for a 2-mL/min flow rate is used with a 0.2-mL/min flow rate, the particles that are formed will be very small.

It is suggested that the instrument manufacturer provide a set of nebulizers to cover the flow rate range typically used for HPLC. In this regard, nebulizers for capillary chromatography (~2 μL/min to 75 μL/min), low HPLC flow (~20 μL to 1 mL/min), HPLC (~200 μL/min to 2.5 mL/min), and high HPLC flow (~1 mL/min to 5 mL/min) should be available so that the analyst can then select the most appropriate nebulizer for the flow rate.

2. Use of a drift tube to remove large particles after nebulization. The nebulizer typically creates a stream with a broad range of particle sizes. Since it is desirable that only the smaller particles enter the evaporation region (so that a lower evaporation temperature can be employed and higher S/N can be reached, i.e., sensitivity), it is desirable that the large particles be eliminated. A simple way to eliminate the large particles is to allow the nebulized particles to traverse a drift tube (Figure 5) in which the larger size particles will fall to the bottom of the drift tube and can then be drained from the system.

3. Design of the oven to minimize the pathlength and ensure rapid heating. The oven should be designed to maximize the heat transfer to the coil through which the nebulized particles traverse during the evaporation process. All materials should be specifically selected and configured to the minimum temperature required to evaporate the mobile phase.

4. Focusing of the particles. The particle flux that exits the evaporation tube in the ELSD is directed to the optical cell in which the light scattering of the solid particles is monitored. The signal from the detector can be increased by using a secondary flow of inert gas to sheathe the particles and focus the particles in the center of the cell to optimize detection. An additional benefit of this design is to minimize the deposition of the particles on the walls of the detector cell, reducing the need for periodic cleaning of the cell.

Conclusion

Figure 5 - Special drift tube designed to eliminate large eluent particles.

The sensitivity provided by an ELSD is extremely dependent on the design of each of the components of the detector. The nebulizer should be selected to match the flow rate of the mobile phase, the drift tube should be designed so that large particles do not enter the oven, and the particles from the oven should be focused into the measurement head.

These features allow the analyst to employ a lower temperature in the oven used to evaporate the mobile phase, so that even thermolabile, volatile, and semivolatile compounds can be readily detected at exceedingly low levels. Typically, low ppm levels of volatile compounds can be readily detected, even when low temperature evaporation is employed.

References

  1. Dreux M, Lafosse M, Morin-Allory L. LC·GC Int March 1996.
  2. Herbreteau B, Morin-Allory L, Lafosse M, Dreux M. J High Res Chromatogr 1990; 13:343.
  3. Lafosse M, Elfakir C, Morin-Allory L, Dreux M. J High Res Chromatogr 1992; 15:312.
  4. Pennanec R. LC·GC Sept 2004.

*The sensitivity of an evaporative light scattering detector is typically defined as the minimum detectable quantity of an analyte that can be detected with a given S/N. Many manufacturers of ELSD systems present the minimum sensitivity by collecting a chromatogram with a considerably larger S/N (e.g., S/N = 30 or higher) using a high concentration and then extrapolating to determine the concentration that would provide a S/N of 3. This approach is not ideal, since this assumes that the signal at the maximum is due entirely to the compound of interest. If sensitivity specifications are employed, they are obtained from a chromatogram with the selected S/N criteria, rather than via extrapolation.

Dr. Pennanec is Applications Engineer, SEDERE, SA, Alfortville, Cedex, France, and Dr. Froehlich is President, Peak Media, 10 Danforth Way, Franklin, MA 02038, U.S.A.; tel.: 508-528-6145; e-mail: pfpeakmedia@msn.com.

Comments