ELS detector

HPLC coupled with an evaporative light scattering (ELS) detector has been used as an universal detection technique also valid for quantitative analysis. Alcohol ether sulfates were analyzed by this method with good results and also at very low concentrations [295]. 

The product purity was >95% as determined by H NMR. The checkers analyzed each sample by LC/MS using an ELS detector. For steps 2-6, the mass of the major peak was consistent with the expected mass of the desired product. Product purity as determined by ELS integration of the LC were as follows product 2, 89.9% product 3, 91.6% product 4, 89.8% product 5, 90.4%. 

For the cationic surfactants, the available HPLC detection methods involve direct UV (for cationics with chromophores, such as benzylalkyl-dimethyl ammonium salts) or for compounds that lack UV absorbance, indirect photometry in conjunction with a post-column addition of bromophenol blue or other anionic dye [49], refractive index [50,51], conductivity detection [47,52] and fluorescence combined with postcolumn addition of the ion-pair [53] were used. These modes of detection, limited to isocratic elution, are not totally satisfactory for the separation of quaternary compounds with a wide range of molecular weights. Thus, to overcome the limitation of other detection systems, the ELS detector has been introduced as a universal detector compatible with gradient elution [45]. 

A simple and efficient alternative to the traditional UV detection of amino acids and related compounds is nowadays represented by the evaporative light scattering (ELS) detector, which allows the direct chromatographic separation, with no need for preliminary derivatization. In the field of glycopeptides-based CSPs, it was applied for the first time in the chromatographic resolution of carnitine and 0-acylcarnitine enantiomers on a TE CSP [61]. The considered compounds are nonvolatile solids and gave optimal ELS response under a variety of experimental conditions (buffered and unbuffered mobile phases, flow-rates from 0.5 to 1.5 mL/min, different kind and... 

Another possibility is to use the evaporative light scattering (ELS) detector, which can be used independently of the composition or variations of the elution solvent mixture (presence of salt solutions is the only limit), however, it is not yet so diffused, even if its earlier applications date back to early 1980s. 

A classic HPLC analysis of sugars is made by means of an isocratic elution with R1 detection hence the only degree of freedom is the choice of the column on which the level of separation depends. More recent issues that do not allow the use of isocratic elution are the use of an ELS detector or an electrochemical detector (ED). 

The ELS detector was previously also referred to as a mass detector, pointing to the fact that the response is (mainly) determined by the mass of the sample rather than by its chemical structure. Van der Meeren et al., though, demonstrated that the ELSD calibration curves of phospholipid classes were also dependent on the fatty acid composition (52). The dependence on the fatty acid composition is, however, completely different in nature and much less pronounced than for UV detection. The reason for this behavior is to be found in the partial resolution of molecular species, even during normal-phase chromatography. Thus, the peak shape depends not only on the chromatographic system but also on the fatty acid composition and molecular species distribution of the PL sample (47). Because it was shown before, based on both theoretical considerations and practical experiments, that the ELS detector response is generally inversely proportional to peak width (62,104), it follows that the molecular species distribution of the PL standards used should be similar to the sample components to be quantified. It was shown that up to 20% error may be induced if an inappropriate standard is used (52). 

Some detectors are not compatible with gradient elution, such as the electrochemical detector or the refractometric detector. The latter one is a universal detector, which gives a response for almost all sample compounds, but also for the mobile phase components. The only universal detector that can be used for gradient elution is the evaporative light-scattering (ELS) detector, but it is approximately two orders of magnitude less... 

High-performance liquid chromatography is performed using a Hewlett-Packard 1090 chromatograph equipped with a ternary-solvent delivery system, an autoinjector with a 0 -20- u.L injection loop, an oven compartment, and a diode-array UV detector. An ELS detector (Alltech Associates, Deerfield, IL) is connected in series to the UV detector. Hexane, 2-propanol, and water were used for the analysis of nonionic surfactants. Water and tetrahydrofuran (THF) are used for the analysis of anionic surfactants. No preliminary sample preparation is used other than dilution. The nonionic surfactants are diluted 1 40 (v/v) with hexane. The anionic surfactants (alkyl ether sulfates and synthetic and petroleum sulfonates) are diluted 1 20 (v/v) with water-THF (50 50). The calcium sulfonate surfactants were diluted 1 20 (v/v) with a THF-38% hydrochloric acid solution of pH 1. Hydrochloric add is required to prevent salt precipitation by converting any excess water-insoluble caldum carbonate into water-soluble calcium chloride. All diluted samples are... 

The diode-array UV and ELS detectors are connected in series. The UV signals are monitored at 230 and 254 nm. The operating conditions of the ELS detector are optimized for maximum detector response and stable baseline. Surfactants with UV absorbance are detected by both detectors, whereas the UV-trans-parent surfactants could only be detected by the ELS detector. 

Figure 6. Comparison of size distribution curves ofLudox silicas obtained from Flow I with a conventional UV detector and an ELS detector.

The accuracy of the breakthrough method depends on the ability to detect the surfactant in the mobile phase. A refractive index (RI) detector is most often required since MLC usable surfactants should not absorb UV light. An evaporative light scattering (ELS) detector can also be used successfiilly since most surfactants are solids at room temperature. 

Thus far the only difference between the UV and El detectors is that the observed signal for the former is a measure of transmitted intensity I, while that for the latter is a measure of the absorbed intensity — but it turns out that is of little consequence for the present purpose. Thus, for example, a fluorescence detector records a signal that is a measure of the absorbed intensity of the exciting radiation like the El case, but it is also a concentration dependent detector like the simple UV absorption detector. The difference between the two types arises rather in the relationship between the analyte concentration delivered by the mobile phase (c ) and that within the absorption cell or El source in the former case c = c (see equation [4.2]), but the situation is very different in the El case where the mass spectrometer vacuum pumps continuously remove the analyte from the El source. In fact c ei represents an instantaneous steady state value, a compromise between the flow rate of A into the source and the pumping rate out of the source here instantaneous means simply that the establishment of the steady state value c gi occurs on a timescale appreciably shorter than that of the chromatographic peak. Then at this steady state ... 

Analytical conditions The polar hydroperoxides were first separated from non-polar material by solid-phase extraction on anunonium columns, followed by reversed phase HPLC on C-18 columns using either methanol 2-propanaol (90 10, v/v) or methanol-2-propanol dichloromethane (80 10 10, v/v). The post-column CL reagent contained isoluminol and microperoxidase in borate buffer (pH 10)-methanol (30 70, v/v). Methyl linoleate hydroperoxides were used as external standards for quantification by CL and UV detectors, and 1,3-diolein by ELS detector. 

Figure 8.65 Simultaneous separation of a basic drug and its counterion utilizing UV and ELS detectors in series. Separator column Acclaim Trinity PI, 3 pm column dimensions 50 mm X3 mm i.d. column temperature 30°C eluent 20 80 (v/v) MeCN/30 mmol/L...

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