Flow sensitivity

Bulk property detectors such as the refractive index detector or the dielectric constant detector are often particularly pressure sensitive and for that reason wide diameter exit tubes are strongly recommended, however, such detectors are not useful for multidimensional analyses. It follows that the pressure sensitivity of the detector should be specified by the manufacturer. The pressure response Dp should be given as the output in mV for unit pressure change in ihe detector cell. The pressure response should be given in both mV/psi and mV/kg/m2, It is also recommended that the pressure noise is given in terms of that pressure change which would provide a signal equivalent to the noise, i.e. 

With such information, the effect of pressure fluctuations on detector noise could be directly assessed. In order to allow the chromatographer to determine whether a detector could be used in multidimensional analyses, the absolute pressure tolerance of the detector should also be given, which can be deHned as the maximum working pressure of the detector where there is no loss of mobile phase due to cell leakage. The maximum working pressure of a detector Pj) should be given in both psi and kg/m. For a normad operation the detector Pj) should lie between 30 and 100 psi. For a detector to be employed for multidimensional analyses, however, pressures at least 2000 to 3000 psi are often necessary. 

The flow sensitivity of a detector is. as stated previously, closely associated with its pressure sensitivity, but can be due solely to flow changes and independent of the pressure in the detector ceil. Experimentally, however, it can be extremely difficult to determine the flow sensitivity of a detector under conditions where there is no accompanying pressure change in the cell. Nevertheless, the flow sensitivity of a detector needs to be known particularly where flow programming development is likely to be employed. The flow sensitivity of the detector also determines the flow control that is necessary with respect to the pump that is to be employed with the chromatograph. The conditions for 

Detectors also have a limited temperature range over which they can operate satisfactorily. The maximum/minimum operating temperatures which the detector can satisfactorily function should also be given. 

The Response Index - The response index of a detector is a measure of its linearity and has been given the symbol r. It is a dimensionless constant and for a truly linear detector should take the value of unity. In practice the value of r should be between 0.98-1.02 and a knowledge of its value is very important where accurate quantitative analyses are required. If r is known. 

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Table 7-4 shows flow patterns and applications of some commercially available impellers. Generally, the axial flow pattern is most suitable for flow sensitive operation such as blending, heat transfer, and solids suspension, while the radial flow pattern is ideal for dispersion operations that require higher shear levels than are provided by axial flow impellers. Myers et al. [5] have described a selection of impellers with applications. Further details on selection are provided by Uhl and Gray [6], Gates et al. [7], Hicks et al. [8] and Dickey [9]. 

The benefit of WAC over SAC is the extremely high exchange capacity and lower (almost theoretical) regeneration efficiency. There is some additional cost in capital equipment and higher resin prices, but this is more than compensated for by lower operating costs. Weak acid cation resin capacity is flow-sensitive, so flows must match design criteria. The overall dealk/degasser/BX is the most popular IX process of its kind in the world today, followed by BX/SBA(C1). 

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. 

In general terms, electrospray ionization is considered to be concentration-sensitive at Tow flow rates and mass-flow-sensitive at high flow rates, while APCI is considered to be mass-flow-sensitive. Low and high are both subjective terms and require investigation as part of method validation. 

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)... 

Figures 4 and 5 give a broad indication of the relevant biomechanical properties of a number of flow sensitive biomaterials. In the case of the data shown in Fig. 5, the surface mechanical properties are lumped into a single measure of the surface integrity. Admittedly, in view of what has been said in the introduction about the viscoelastic nature of the wall material, the information given in Figs. 4 and 5 are oversimplistic. The data in Fig. 5 are based on reported critical minimum stresses (often expressed in terms of the mean bulk fluid stresses) at which physical damage is first observed. Figure 6 gives an indication of the...

The variation in wall thickness and the development of cell wall rigidity (stiffness) with time have significant consequences when considering the flow sensitivity of biomaterials in suspension. For an elastic material, stiffness can be characterised by an elastic constant, for example, by Young s modulus of elasticity (E) or shear modulus of elasticity (G). For a material that obeys Hooke s law,for example, a simple linear relationship exists between stress, , and strain, a, and the ratio of the two uniquely determines the value of the Young s modulus of the material. Furthermore, the (strain) energy associated with elastic de-... 

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. 

Rate Meter Measures the instantaneous volume flow rate through the sampling systems. An example would be a rotameter or venturi meter. Used to set precise flow rate for flow sensitive sampling devices. 

The mass spectrometer is a mass-flow sensitive device, which means that the signal is proportional to the mass flow dm/dl of the analyte, i.e. the concentration times the flow-rate. It is only now possible to realise the high (theoretically unlimited) mass range and the high-sensitivity multichannel recording capabilities that were anticipated many years ago. Of considerable interest to the problem of polymer/additive deformulation are some of the latest developments in mass spectrometry, namely atmospheric pressure ionisation (API), and the revival of time-of-flight spectrometers (allowing GC-ToFMS, MALDI-ToFMS, etc.). 

Thermal conductivity (TCD) io-9 104 450 non-destructive, temperature and flow sensitive... 

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,... 

J. Henry, R. Ion spray mass spectrometric detection for liquid chromatography a concentration- or a mass-flow-sensitive device ... 

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. 

Flow sensitivity is the rate of change of signal displacement with flow rate resulting from step changes in flow rate. Flow sensitivity is solvent dependent. 

The flow sensitivity of the electrode has the same origin, as has been pointed out previously. A stagnant (Prandtl) boundary layer of thickness 5 forms around the spherical electrode (radius ro) placed in the liquid of kinematic viscosity v which is moving with linear velocity U. 

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