Mass flow sensitive detector

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)... 

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. 

There are other major problems with peak assignment on the basis of the areas. These problems relate to the reproducibility of peak area measurements under widely varying conditions. Ideally, the area of a peak remains constant even if its capacity factor varies. However, varying the conditions may affect the peak areas. If the column temperature is changed in GC, then the flow rate may be affected. Peak areas will change (by a constant factor) if concentration-sensitive detectors such as the hot wire detector (H WD katharo-meter) are used, but not with mass flow sensitive detectors (such as the flame ionization detector, FID). 

The two detectors should have similar types of sensitivities (e.g. two concentration sensitive detectors or two mass flow sensitive detectors). 

Most of today s popular detectors are so-called mass-flow sensitive detectors , i.e. the recorded signal (h0) is proportional to the mass of solute passing through the detector per unit time, in other words proportional to the product of the solute concentration (cmax) and the volumetric flow rate (F). Therefore, we find for mass-flow sensitive detectors with eqn.(7.25) ... 

Consequently, if mass-flow sensitive detectors are used, a reduction of the diameter of capillary columns will give rise to a decrease in sensitivity. This effect is enhanced by the tendency to use thicker films of stationary phase (larger d in columns with larger diameters. The limited sensitivity of the detection is a disadvantage of the use of narrow-bore capillary columns in GC. 

Reducing the diameter of capillary columns leads to an enhanced sensitivity if concentration-sensitive detectors are used, but to a reduced sensitivity in combination with mass-flow sensitive detectors. 

Detectors can be mass flow-sensitive (refractive index detectors, ELSDs) or concentration-sensitive (UV-Vis and fluorescence detectors). The former respond to the amount (mass) of analyte passing through the detector per unit of time (calculated by multiplying the eluent flow rate by the analyte concentration in the eluent), while the latter respond to analyte concentration. In mass flow-sensitive detectors, the response (signal amplitude) is proportional to the amount of sample component reaching the detector per unit of time. 

For concentration-sensitive detectors S = AFAV for mass flow-sensitive detectors S =A/W where A = peak area, W = mass of analyte, F = eluent flow rate... 

Detectors are either concentration sensitive or mass flow sensitive. The signal from a concentration-sensitive detector is related to the concentration of the solute in the detector and is decreased by dilution with a makeup gas. The sample is usually not destroyed. Thermal conductivity, argon-ionization, and electron capture detectors are concentration sensitive. In mass-flow-sensitive detectors, the signal is related to the rate at which solute molecules enter the detector and is not affected by the makeup gas. These detectors usually destroy the sample, such as flame ionization and flame thermionic detectors. Sometimes two-column GC is used to increase resolution, by taking cuts of eluents from an initial column and directing them to a second column for secondary separation. The first detector must be nondestructive or else the eludnt split prior to detection, with a portion going to the second column. 

The mass spectrometer is a mass flow sensitive detector. The peak area is independent of the mobile phase flow rate although the ionization efficiency of a LC-MS interface may be affected by the flow rate. Mass spectrometers are more and more miniaturized. They are routinely hyphenated to separation instmments, (GC-MS, LC-MS, SFC-MS, TLC-MS, CE-MS). 

Proper flow adjustments for any detector-related gases are essential to quantitative GC, as both fluctuations or long-term drifts will effect performance of both concentration and mass-flow sensitive detectors. For example, it was shown [25] that even the fluctuations in the atmospheric pressure could cause some deviations in peak areas with the flame ionization detector. Yet another systematic study of detection variables [26] reinforces the importance of instrumental control with the flame detectors. Wherever highly quantitative results are expected, frequent calibrations with appropriate standards are urgent. 

The flame ionization detectors (as well as the other flame detectors) can be used equally well with packed and capillary columns. Different considerations may apply to other detector types. The well-known classification of chromatographic detectors into the concentration-sensitive and the mass-flow-sensitive types is highly relevant in this respect. A response enhancement [108] to the mass-flow-sensitive detector types is given as... 

The lower detection limit of a detector is defined as the minimum amount of compound detectable at a given signal-to-noise ratio. Ideally the detection limit should be determined independently of the chromatographic separation system using standard dilution devices. Detection limits are often expressed in g/s (mass flow-sensitive detectors) or g/mL (concentration-sensitive detectors) to obtain a value which is independent of the measuring conditions (flow rate, etc.). 

Practical aspects. All compounds which contain oxidizable carbon can be detected. A linear range over seven decades and a detection limit of a few pg/s of carbon make this mass flow-sensitive detector very suitable for GC. It has also found increased application in SFC when CO2 is used as the mobile phase. Decompression from the supercritical state to atmospheric pressure is performed by a restriction capillary at the outlet of the separation column. 

The mass flow sensitive detector (e.g. FID) first translates its input "quantity of substance" into the variable "electric charge". M (g) of substance generates the charge ... 

The slope of the detector calibration curve (Fig. 58) is known as sensitivity. In the case of concentration dependent detectors, sensitivity has the dimension [A ml g", and for mass flow sensitivity detectors... 

Concentration-Sensitive and Mass-Flow-Sensitive Detectors.. 270... 

Electrospray is a low-flow-rate technique. When the flow rate is increased for a given sample concentration, the analyte ion signal does not increase. So in terms of sample concentration, sensitivity remains constant, but in terms of mass flow the sensitivity drops when the flow rate of the sample solution is increased. This is an unusual situation in mass spectrometry because a mass spectrometer operating in the electron ionization or chemical ionization mode is a mass-flow-sensitive detector. 

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