Sensitivity, concentration detector

Experimental SEC conditions require highly sensitive concentration detectors giving a detector response, which is linearly related to polymer concentration. The most common detector for monitoring polymer concentration in the eluent is the differential refractometer (DRI). The response of the detector to polymer concentration does not depend on polymer molecular weight except for very low polymers. 

Since the TCD Is a flow sensitive (concentration) detector, the low flow rates generally associated with capillary systems should enhance the observed response of a low volume (30) TCD. The observed response, l.e. peak height, or peak area, is Inversely proportional to the gas flow rate at the detector (assuming the same carrier gas flow rate through the column, with any additional gas being Introduced as a make-up flow just prior to the detector). Optimum detector response should be obtained at those flow rates Just sufficient to efficiently sweep-out the dead volumes associated with the detector and connecting tubing. 

For measuring the inert species, some of which are present in the majority of gases, the thermal-conductivity detector (TCD) is often the detector of choice for gas analyses. Since the TCD is a concentration detector and its sensitivity is lower than that of mass-flow detectors such as the flame-ionization detector (FID), relatively high concentrations of compounds in the carrier gas are needed. This means that packed columns, with their high loadability, are still quite popular for such analyses. 

The spectrometer is behaving as a concentration-sensitive detector as the signal intensity remains constant as the flow rate increases. If it were mass-sensitive, the detector response would increase. 

The trend in modern SEC is to couple one or several other detectors (in particular molecular weight sensitive) to the concentration detector providing complementary... 

SFC-SFC is more suitable than LC-LC for quantitation purposes, in view of the lack of a suitable mass-sensitive, universal detector in LC. Group quantitation can be achieved by FID. The ideal SFC-SFC system would consist of a short (10-30 cm) packed-capillary primary column, interfaced to a long (5-10m) open-tubular column, but such a combination is difficult to realise, due to the different flow-rates required for each column type. Coupled SFC-SFC is often configured with a solute concentration device prior to valve switching on to the SFC. The main approaches to this concentration stage are the use of absorbent material or cryofocusing. Davies el at. [924] first introduced two-dimensional cSFC (cSFC-cSFC), and its use has been reported [925,926]. 

The progress made in interfacingHPLC instruments with mass spectrometry has been a significant development for laboratory analyses in the pharmaceutical industry. The low concentrations of test drugs in extracts of blood, plasmas, serums, and urine are no problem for this highly sensitive HPLC detector. In addition, the analysis is extremely fast. Lots of samples with very low concentrations of the test drugs can thus be analyzed in a very short time. At the MDS Pharma Services facility in Lincoln, Nebraska, for example, a very busy pharmaceutical laboratory houses over 20 LC-MS units, and they are all in heavy use daily. 

A more sensitive viscometer than the drop-time glass capillary method is also needed in size exclusion chromatography (SEC) such as the gel permeation chromatographic (GPC) analysis of polymer molecular weight distribution (MWD). In an SEC system, a concentration detector is commonly used for providing the weight concentration profile of the polymer elution curve. 

Sensitivity can be associated with the slope of the calibration curve. It is also dependent on the standard deviation of the measurements. The higher the slope of your calibration curve the higher the sensitivity of your detector for that particular component, but high fluctuations of your measurements will decrease the sensitivity. The more selective the detection, the lower is signal/noise and the higher the sensitivity. The detector response is linear if the difference in response for two concentrations of a given compoimd is proportional to the difference in concentration of the two samples. 

Because of their high sensitivity, fluorescence detectors are particularly useful in trace analysis when either tire sample size is small or the analyte concentration is extremely low. Although fluorescence detectors can become markedly nonlinear at concentrations where absorption detectors are still linear in response, their linear dynamic range is more than adequate for most trace analysis applications. Unfortunately, fluorometric detectors are often susceptible to background fluorescence and quenching effects that can plague all fluorescence measurements. 

The wide acceptance and success of this technique have been due to such features as simplicity, rapidity of analysis, high sensitivity of detector systems, efficiency of separations, varied applications and the use of very small samples (microgram or smaller). Presently GC is finding use in the concentration of impurities in the parts per million (ppm) and parts per billion (ppb) range and in addition to the actual measurement of impurities at these levels. Without the use of GC many analytical problems could not be solved or would involve more intricate and time-consuming techniques. 

While single-column methods work quite well, these methods have two main drawbacks. First, when coupling turbulent-flow directly to a mass spectrometer, the mobile-phase effluent (4 or 5 mL/min) has to be split to make the effluent flow compatible for mass spectrometers (< 1 mL/min). Although narrow-bore TFLC columns (0.5 mm i.d.) can be operated at 1-1.5 mL/min, only some mass spectrometers are capable of operating at these flow rates. The splitting will result in lower detection limits. The lower sensitivity is not due to the detection limit of the mass spectrometer, which acts as a concentration detector, but to the fact that the analyte is more diluted when contained in the higher TFLC flows. There is also more mobile-phase waste to dispose. Of course, this may not be a major drawback, if one were to collect fractions... 

To be effective, the detector must be capable of responding to concentration changes in all of the compounds of interest, with sensitivity sufficient to measure the component present in the smallest concentration. There are a variety of HPLC detectors. Not all detectors will see every component separated by the column. The most commonly used detector is the variable ultraviolet (UV) absorption detector, which seems to have the best combination of compound detectability and sensitivity. Generally, the more sensitive the detector, the more specific it is and the more compounds it will miss. Detectors can be used in series to gain more information while maintaining sensitivity for detection of minor components. 

A sensitivity test solution is used to check the sensitivity of the detectors. The used test chemicals should not adsorb on the column material at low concentration levels. It is recommended that test solutions supplied from the instrument manufacturer are used. If these are not available, the test chemicals presented in Table 3 are suitable for testing also the sensitivity of detectors, which are used most often for the detection of CWC-related chemicals (24). 

A column performance test is used to ensure the proper condition of columns and the stability of retention parameters. Because CWC-related chemicals greatly differ both chemically and physically from each other, the test chemicals have been selected so that their physical, chemical, and retention properties are different, and so that they elute evenly over the whole chromatogram. The use of the following chemicals in a column performance test is recommended trimethylphosphate, 2,6-dimethylphenol, 5-chloro-2-methylaniline, tri-n-butylphosphate, dibenzothiophene, malathion, and methyl stearate. The concentration of test chemicals depends on the sensitivity of detectors. The... 

In summary, peak area is the preferred measurement especially if there are any changes in chromatographic conditions, such as partition ratio, temperature, or sample introduction method, that can cause changes in peak height or width (but not area). However, peak height measurements are less affected by overlapping peaks, noise, and sloping baselines. It must also be remembered that the commonly used concentration detectors are flow sensitive and prone to errors if areas are used for quantitative analysis. 

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