Shimadzu Evaporative Light Scattering LC Detector LTII (ELSD-LTII)

Shimadzu Evaporative Light Scattering LC Detector LTII (ELSD-LTII)

In the history of high-performance liquid chromatographs, which dates to the early 1960s, refractive index detectors (RI detectors) have often been used as general-purpose detectors. RI detectors enable the detection of components that do not possess UV absorbance and give a proportional relationship between the heights of detected peaks and the quantities of detected components. So, in comparison with absorbance detectors (UV detectors), they offer advantages such as the ability to ascertain unknown component quantities and obtain molecular weight distributions for macromolecules. On the other hand, they also have various disadvantages. For example, they cannot be used for gradient analysis, the baselines they produce are susceptible to the influence of fluctuations in the ambient temperature, their sensitivity is low compared to that of UV detectors, and they are prone to giving negative peaks, which make quantitative analysis difficult. Furthermore, with both UV and RI detectors, in cases where the solvent peaks of the analyzed samples appear at the start of the chromatogram, it is sometimes not possible to detect target substances with short elution times. Therefore, RI detectors cannot truly be described as general-purpose detectors. The principle of evaporative light scattering detectors (ELSD), which solve these problems, is extremely simple. The target components are converted to a fine spray by a nebulizer and heated so that only the mobile phase is evaporated. Light is directed at the remaining target substances and the scattered light is detected. ELSDs can detect almost all components that are less volatile than the mobile phase. These detectors first appeared in 1966, but were subsequently overshadowed by high-performance liquid chromatographs, which advanced significantly at that time. They were first commercialized in the early 1980s, and the basic technology of modern-day ELSDs was  established in the mid-1980s. In addition to describing the operating principles and practical benefits of ELSDs, this article uses the ELSD-LT 2, a product that achieves greater sensitivity, speed, and convenience than conventional products by incorporating the latest technology, to present application examples that utilize ELSD characteristics.

What Is an Evaporative Light Scattering Detector (ELSD)?

The target substances separated in a column are, together with the mobile phase, converted to a fine spray by a nebulizer, and this spray is carried to a drift tube. In the drift  tube, heat is applied so that only the mobile phase is evaporated. The remaining target substances that were in the mobile phase are converted to minute solid particles and are carried to the detection unit. In the detection unit, the target substance particles cause the light emitted from a light source to be scattered. This scattered light is measured by a photo multiplier and the target substances are thereby detected. The intensity of the signal detected in the ELSD can  be represented by the following equation:

(Signal intensity) = a x (Quantity of target substance)b

Here, "a" and "b" are constants that are determined by a variety of factors, such as the size of the particles, the concentration and type of the target substances, the gas flow rate, the mobile phase flow rate, and the temperature of the drift tube. In principle, ELSDs are capable of analyzing all substances that have an evaporation temperature lower than that of the mobile phase, and can attain roughly the same level of detection sensitivity for any compound. For this reason, they are well-suited to the detection of components such as sugars, fats, surfactants, synthetic macromolecules, and steroids, as these components have low light absorbance, making them difficult to detect with UV detectors. If the target substances are nonvolatile, detection is possible down to the nanogram level in nearly all cases.

ELSDs and RI Detectors:

Like RI detectors, ELSDs are classified as general-purpose detectors but they differ from RI detectors in the following ways:

  1.  They are 5 to 10 times more sensitive than RI detectors.
  2.  They support the use of the gradient elution method.
  3. They are not easily affected by changes in the ambient temperature.
  4.  They are not affected by interference due to solvent peaks.
  5.  Time is not required to allow for the instrument and the baseline to stabilize.

Although the gradient elution method is effective for the batch analysis of, for example, multiple components in natural products, RI detectors cannot be used for this because of fluctuations in the baseline caused by changes in the refractive index of the mobile phase.

With ELSDs, baseline fluctuations do not occur in gradient elution, meaning this method can be used to perform the efficient, high-sensitivity analysis of multiple components.

This feature of ELSDs is useful for the following types of analysis:

  1. The analysis of compounds that cannot be detected with UV detectors: Carbohydrates (sugars), sugar alcohols, alcohols, terpenoids, surfactants, natural macromolecules, and synthetic macromolecules
  2.  The batch analysis of compounds for which the gradient elution method is difficult to use ecause absorbance occurs only in the short wavelength region:

          Fats, phospholipids, glycerides, fatty acids, natural macromolecules, and synthetic macromolecules Also, because ELSDs can be applied to all                 aspects of the methods used for LC/MS, including the mobile phases, they can be used as substitute detectors in place of expensive LC/MS            instruments in the screening of compounds.

Features of ELSD-LT2:

  • High-Sensitivity Detection of Semi-Volatile Substances Achieved with Low-Temperature Evaporation of Mobile Phase:

With an ELSD, the larger the nebulized droplets, the higher the evaporation temperature must be set in order to evaporate them. If analysis is performed at a low temperature, the larger droplets that are not evaporated create scattered light that gives rise to a high level of noise. The ELSD-LT2 solves this problem by incorporating a glass cell with a unique structure. Minute droplets that leave the ELSD-LT2 nebulizer are carried by the nebulizer gas stream through the glass cell and into the drift tube. Larger droplets, however, are not carried by the nebulizer gas stream and adhere to the inside surface of the glass tube, where they change to liquid form. This liquid accumulates in a siphon tube and is subsequently discharged. There is always waste liquid in the siphon section, so all of the nebulizer gas flows into the drift tube (siphon split method). In this way, the larger droplets that cause noise are separated selectively, and the smaller droplets are efficiently carried into the drift tube. This technology makes it possible for the ELSDLT2 to suppress noise, even at low evaporation temperatures. Because mobile phases can be evaporated at low drift tube set temperatures in the range of 35°C (organic solvent mobile phases) to 40°C (aqueous mobile phases), efficient, high sensitivity analysis is possible for nearly all compounds. Also, with the ELSD-LT2, assisting gas is  projected in a cylindrical shape centering on the drift tube outlet. This increases the concentration of the target substances that reach the detection unit from the drift tube and, consequently, increases sensitivity. In this way, with both a reduction in noise achieved through low-temperature evaporation technology and an increase in peak intensity achieved through superior detection technology, the ELSD-LT2 realizes high-sensitivity detection.

  • More User Friendly :

Setting the drift tube temperature and gain is the only preparation required to perform analysis with the ELDS-LT2. It is also possible to turn off the lamp and stop the nebulizer after the completion of analysis using the auto power-down function. The use of a long-life LED lamp and the auto power-down function enable reductions in the frequency of lamp replacement and the consumption of nebulizer gas, and thereby making it possible to reduce running costs. Furthermore, an automatic cleaning function for the drift tube helps make maintenance easier.