Detector flow sensitivity

The Detector Flow Sensitivity - The detector flow sensitivity is the output in mV that corresponds to unit change in flow rate it is reported in mV/ml/min and has been designated the symbol Dq. The flow sensitivity of the detector ultimately determines the flow rate control required by the chromatographic pump. 

Detector Sensitivity, or Minimum Detectable Concentration Pressure Sensitivity Flow Sensitivity Temperature Sensitivity... 

The pressure sensitivity of a detector will be one of the factors that determines the long term noise and thus can be very important. It is usually measured as the change in detector output for unit change in sensor-cell pressure. Pressure sensitivity and flow sensitivity are to some extent interdependent, subject to the manner in which the detector functions. The UV detector, the fluorescence detector and the electrical... 

In this study, the effect of mobile-phase flow rate, or more accurately, the rate of flow of liquid into the LC-MS interface, was not considered but as has been pointed out earlier in Sections 4.7 and 4.8, this is of great importance. In particular, it determines whether electrospray ionization functions as a concentration-or mass-flow-sensitive detector and may have a significant effect on the overall sensitivity obtained. Both of these are of great importance when considering the development of a quantitative analytical method. 

A solution oontaining 0.5 mg mM of an analyte gives a detector response (based on peak height) of 48 3 arbitrary units when analysed by LC-MS at a flow rate of 0.75 ml min". At a flow rate of 1.00 ml min", the detector response was 49 3 arbitrary units. Is the mass speotrometer behaving as a conoentration- or mass-flow-sensitive detector ... 

Mass-flow-sensitive detector A detector for which the intensity of response is proportional to the amount of analyte reaching it. 

Mass-sensitive detector see Mass-flow-sensitive detector)... 

The second most widely used detector in HPLC is the differential refractometer (RI). Being a bulk property detector, the RI responds to all substances. As noted in Table 3 the detection limits are several orders of magnitude higher than obtained with the UV detector. Thus, one turns to the RI detector in those cases in which substances are non-UV active, e.g. lipids, prostaglandins. In addition, the RI detector finds use in preparative scale operation. Finally, relative to the UV detector, the RI is significantly more temperature and flow sensitive and cannot be used in gradient elution. 

Principles and Characteristics The main reasons for hyphenating MS to CE are the almost universal nature of the detector, its sensitivity and the structural information obtainable, including assessment of peak purity and identity. As CE is a liquid-phase separation technique, coupling to the mass spectrometer can be achieved by means of (modified) LC-MS interfaces. Because of the low flow-rates applied in CE, i.e. typically below lOOnLmin-1, a special coupling device is required to couple CE and the LC-MS interface. Three such devices have been developed, namely a... 

Important performance characteristics of UV/Vis detectors are sensitivity, linearity, and band dispersion. These are controlled by design of the optics and the flow cell—more specifically by spectral bandpass, stray light characteristics, and the volume and path length of the flow cell. 

Degassing of the eluent is important for trouble-free operation and highly sensitive detection,2 otherwise the eluent may become supersaturated with air that is released as bubbles in the pump check valves or the detector flow... 

Manufacturers publish their product s performance characteristics as specifications, which are often used by the customer for comparison during the selection process. Table 1 shows the specifications of an Agilent 1100 Series Quaternary Pump, which is quite representative of other high-end analytical pumps. Note pulsation is particularly detrimental to the performance of flow-sensitive detectors (e.g., mass spectrometer, refractive index detector). Differences in dwell volumes and composition accuracy between HPLC systems might cause problems during method transfers. 

It is well known that UV detectors used in liquid chromatographs are concentration-sensitive devices. Injection of the same mass of a particular compound onto two columns with identical plate number and length but different inner diameters, will result in a higher response from the column with the smaller inner diameter. The gain in the signal is inversely proportional to the square of the ratio of the inner diameters of the two columns. The situation is different for a mass spectrometer, which is a mass-flow sensitive detector. Under constant flow conditions,... 

When signal is the peak height, the sample size is the mass flowrate through the detector at the peak maximum, for the mass flow sensitive detector. For the concentration-sensitive detector, the sample size is the concentration in the detector at the peak maximum. Table 5.3 gives the equations for response factor in terms of weight, M, of compound injected. 

The flow sensitivity indicates that while a detector may be accurately balanced in terms of resistance and voltage, the reference flow only reduces the flow sensitivity of the analytical column flow by a factor of three to four. Much of the drift due to flow changes comes from the flow controllers. One type commonly used today has been found to have a mass flowrate proportional to absolute temperature. Flow induced noise, however, can come from column temperature fluctuations. Even if the front of the column is fed from a perfect flow source, a temperature change in the column will lead to a viscosity change in the carrier gas. Since the gas is compressible, a transient flow change occurs in the detector. Needless to say, a fluctuation in column temperature also leads to a fluctuation in the bleed level, which affects the recorder baseline by a much more direct process. 

FLOW REQUIREMENTS. The carrier gases used are nitrogen or argon containing methane at 5 - 10% of the total volume. The methane reduces the concentration of metastable argon and promotes thermal equilibrium of the electrons. The ECD is/is not a flow-sensitive detector. Many believe that column bleed and traces of oxygen in the carrier gas are responsible for flow and temperature dependence. It is prudent to see if the system is dependent. 

Several gas chromatographic detectors are sensitive to changes in the flowrate of the carrier gas. Any changes in flow rate cause the baseline to be displaced. These displacements make quantification quite difficult especially since the response of certain detectors such as thermal conductivity also changes with changes in flowrate. When an accuracy of 1% in quantitative analysis is required, the flowrate should not fluctuate more than 0.2 percent (see Chapter 4). 

Pulse polarographic studies have been described using a microcell of 0.5 mL capacity, which analyzed two 1,4-benzodiazepines, with the lowest detection limit reported to date being 10-20 ng/mL of blood [199]. Detailed construction of the cell and electrode assembly was also described (shown in Fig. 26.16). Further miniaturization of this type of three-electrode cell is not practical hence further increases in sensitivity will have to rely on electrochemical detector flow cells of microliter capacity such as those used in conjunction with liquid chromatography (see Chap. 27). 

Due to the high sensitivity it is favorably to couple a nanoHPLC to an ESI-source. As mass spectrometers are concentration dependent detectors, the sensitivity of an instrumental setup is mostly determined by the peptide concentration of the eluate but not by the peptide amount. Thus a nanocolumn with a flow rate of 300 nL/min provides an about thousand times higher sensitivity than a microbore column with a flow rate of 300 (xL/min. As an alternative to buying a nanoHPLC system it is also possible to use a relatively inexpensive flow splitter after the pump and before the injection valve and the column. Thereby the flow rate can be reduced to use a capillary column (flow rate 4 (xL/min) on an analytical HPLC system or a nanocolumn on a capillary HPLC system. Instead of a flow-splitter it is preferred to couple a nanoHPLC to an ESI-source. Thereby, the flow rate is split according to the column backpressure, i.e., mostly the column volumes if the same packing materials are used. However, these low-cost setups are less reliable than a nanoHPLC and the reproducibility is worse. 

Commercially available HPLC instrumentation was originally designed for use with standard-bore columns (4.6 mm I.D.). Detector flow cells were optimized for maximum sensitivity with these analytical columns, injectors were designed to introduce microliter quantities of sample, and pumps were designed to be accurate and reproducible in the milliliter flow-rate ranges commonly employed with standard-bore columns. However, these instruments are not well suited for use with small-bore columns, as the dispersion introduced by the large volumes is detrimental to the separation. In addition, the reproducibility and accuracy of the pumping system at the low flow rates required are questionable. 

Figure 16. Electropherogram illustrating the flow-programmed separation of the same ply d(A) 40-60 mer sample presented in Figure IS. The sample was separated at IS kV and the potential was reduced to l.S kV as radiolabeled sample reached the detector. The sensitivity improvement...

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