Refractive index detectors detection

Degassed and preswelled Bio-Gel P-6 and Sephacryl S-200 were packed in self-made glass columns (70 X 1.5 cm 140 X 1.5 cm) and equilibrated for 20 hr with H20(dest.) -t- 0.002% NaN3 to prevent microbial growth. The mass of eluted fractions was detected with a differential refractive index detector (Waters 403 RI, sensitivity 8). 

FIGURE 16.23 Inulln isolated from small (—). medium ( ) and large (A) tubers separated on P-6 (140 X 1.5 cm) flow rate 0.33 ml/min eluenf. H20(dest) + 0.002% NaNa mass detection Waters 403 R differential refractive index detector, sensitivity 8X applied sample solution volume I ml of a 20-mg/ml aqueous inulin solution. 

Sodium dodecyl sulfate present in hydrophilic ointments has been determined by TLC on silica gel with flame ionization detection, which was considered better than the colorimetric method. TLC is preferred to HPLC in this case due to the low sensitivity of the refractive index detector that makes difficult the analysis of small amounts of sodium dodecyl sulfate [284]. 

The flow sensitivity of a detector will also be one of the factors that determines the long term noise and thus will influence the sensitivity or minimum detectable concentration of the detector.lt is usually measured as the change in detector output for unit change in flow rate through the sensor cell. Again, the refractive index detector is the most sensitive to flow rate changes. 

Detection of the these types of compounds are sometimes difficult as many components of cosmetic products are aliphatic, do not possess a UV chromophore and are not easily reacted to give fluorescent derivatives. Providing the concentration of the component of interest is sufficiently high, then a refractive index detector is often used. If... 

Adsorption chromatography using small particle silica or alumina has also been employed in the separation of biologically meaningful substances. Phospholipids, for example, have been separated on silica (38). One of the big problems for such substances is detection, since many of the compounds are not U.V. active. Generally, the refractive index detector is employed for isocratic operation, and the moving wire detector for gradient operation. Formation of U.V.-active derivatives is also possible (39). 

Detection requirements in preparative-scale chromatography also differ from analytical erations where detectors are selected for their sensitivity. Sensitivity is not of overriding importance in preparative-scale chromatography the ability to accommodate large column flow rates and a wide linear response range are more useful. The sensitivity of the refractive index detector is usually quite adequate for prqtaratlve work but the ... 

Sample preparation, injection, calibration, and data collection, must be automated for process analysis. Methods used for flow injection analysis (FLA) are also useful for reliable sampling for process LC systems.1 Dynamic dilution is a technique that is used extensively in FIA.13 In this technique, sample from a loop or slot of a valve is diluted as it is transferred to a HPLC injection valve for analysis. As the diluted sample plug passes through the HPLC valve it is switched and the sample is injected onto the HPLC column for separation. The sample transfer time typically is determined with a refractive index detector and valve switching, which can be controlled by an integrator or computer. The transfer time is very reproducible. Calibration is typically done by external standardization using normalization by response factor. Internal standardization has also been used. To detect upsets or for process optimization, absolute numbers are not always needed. An alternative to... 

Detection is also frequently a key issue in polymer analysis, so much so that a section below is devoted to detectors. Only two detectors, the ultra-violet-visible spectrophotometer (UV-VIS) and the differential refractive index (DRI), are commonly in use as concentration-sensitive detectors in GPC. Many of the common polymer solvents absorb in the UV, so UV detection is the exception rather than the rule. Refractive index detectors have improved markedly in the last decade, but the limit of detection remains a common problem. Also, it is quite common that one component may have a positive RI response, while a second has a zero or negative response. This can be particularly problematic in co-polymer analysis. Although such problems can often be solved by changing or blending solvents, a third detector, the evaporative light-scattering detector, has found some favor. 

Detection in 2DLC is the same as encountered in one-dimensional HPLC. A variety of detectors are presented in Table 5.2. The choice of detector is dependent on the molecule being detected, the problem being solved, and the separation mode used for the second dimension. If MS detection is utilized, then volatile buffers are typically used in the second-dimension separation. Ultraviolet detection is used for peptides, proteins, and any molecules that contain an appropriate chromophore. Evaporative light scattering detection has become popular for the analysis of polymers and surfactants that do not contain UV chromophores. Refractive index (RI) detection is generally used with size exclusion chromatography for the analysis of polymers. 

Detectors have been discussed at some length in Chapter 3. Here we will highlight some of the improvements that are being made in HPLC detectors to help improve detectability. Examples of such detectors are a UV detector that allows simultaneous recording of absorbance at many wavelengths and a refractive index detector that can perform IQ, OQ,... 

Laser-based refractive index detector, Cuprammonium reagent,4-Aminobenzoic acid reagent, Indirect detection methods for cyclodex-trins, and sugar phosphates Reversible derivatization using 2-amino-pyridine ... 

In the recent review on column LC by Majors et al. (21), a survey on the use of detector types was carried out in the same manner as that for the use of the various separation modes already mentioned. The results, shown in Table VI, were tabulated for the periods 1982-83 and 1980-81. The increased use of electrochemical and refractive index detectors is significant in these data. The authors speculated that the increased use of refractive index detectors resulted from the increased number of publications on the separation of carbohydrates. The increased use of electrochemical detection is probably a function of many different factors cell designs that are easier to use, expanding sales... 

Refraction is described in Section 20-4 A refractive index detector responds to almost every solute, but its detection limit is about... 

Several points had to be taken into consideration when choosing the mobile phase. First, the mobile phase had to dissolve the silver nitrate properly while at the same time being sufficiently nonpolar for the elution of saturated TGs. Second, the mobile phase had to be inert to silver ions so that no reaction would take place. Third, the refractive index of the mobile phase had to be different from that of the TGs, since TGs were to be detected using a refractive index detector. For these reasons, mobile phases such as propionitrile, acetonitrile, methanol, and 2-propanol were tested. The mobile phase that gave the best separation results was methanol-2-propanol (3 1 v/v) with dissolved silver nitrate. 

In the beginning, the refractive index detector was the most used detection system, although it has two important drawbacks (1) solvent gradients cannot be used, and (2) it has low sensitivity and different responses to saturated and highly unsaturated TGs (112). Moreover, use of the ultraviolet (UV) detector is difficult, because the most adequate solvents also absorb in the same range and therefore cause an important baseline drift with gradient elution systems (106). 

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