Mass flow detectors

FID Universal detector Mass-flow-dependent detector ... 

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. 

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. 

Catalytic activity for the selective oxidation of H2S was tested by a continuous flow reaction in a fixed-bed quartz tube reactor with 0.5 inch inside diameter. Gaseous H2S, O2, H2, CO, CO2 and N2 were used without further purification. Water vapor (H2O) was introduced by passing N2 through a saturator. Reaction test was conducted at a pressure of 101 kPa and in the temperature range of 150 to 300 °C on a 0.6 gram catalyst sample. Gas flow rates were controlled by a mass flow controller (Brooks, 5850 TR) and the gas compositions were analyzed by an on-line gas chromotograph equipped with a chromosil 310 coliunn and a thermal conductivity detector. 

The catalytic reforming of CH4 by CO2 was carried out in a conventional fixed bed reactor system. Flow rates of reactants were controlled by mass flow controllers [Bronkhorst HI-TEC Co.]. The reactor, with an inner diameter of 0.007 m, was heated in an electric furnace. The reaction temperatoe was controlled by a PID temperature controller and was monitored by a separated thermocouple placed in the catalyst bed. The effluent gases were analyzed by an online GC [Hewlett Packard Co., HP-6890 Series II] equipped with a thermal conductivity detector (TCD) and carbosphere column (0.0032 m O.D. and 2.5 m length, 80/100 meshes), and identified by a GC/MS [Hewlett Packard Co., 5890/5971] equipped with an HP-1 capillary column (0.0002 m O.D. and 50 m length). 

Most detectors are of the differential type, that is their response is proportional to the concentration or mass flow rate of the eluted component. They depend on changes in some physical property of the gas stream, e.g. thermal conductivity, density, flame ionization, electrolytic conductivity, P-ray ionization, in the presence of a sample component. The signal from the detector is fed to a chart recorder, computing integrator or... 

Catalysts were tested for oxidations of carbon monoxide and toluene. The tests were carried out in a differential reactor shown in Fig. 12.7-1 and analyzed by an online gas chromatograph (HP 6890) equipped with thermal conductivity and flame ionization detectors. Gases including dry air and carbon monoxide were feed to the reactor by mass flow controllers, while the liquid reactant, toluene was delivered by a syringe pump. Thermocouple was used to monitor the catalyst temperature. Catalyst screening and optimization identified the best catalyst formulation with a conversion rate for carbon monoxide and toluene at room temperature of 1 and 0.25 mmolc g min1. Carbon monoxide and water were the only products of the reactions. 

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,... 

G = Gas Filters MFC = Mass Flow Controllers R = Reactor Setup CG = Carrier Gas FID = Flame Ionization Detector... 

The FID detector, for which the signal depends on the instantaneous mass flow, is essentially free from variations due to flow rate that can cause errors in detectors whose response is a function of the instantaneous molar concentration (Fig. 2.11). 

Figure 2.11—(a) FID detector (b) NPD detector and (c) effect of flow rate on detector signal and difference between the mass flow detector and concentration dependent detector. 1, normal situation (constant flow) 2, mass flow detection (i.e. FID) with an interruption in the flow rate (the area remains constant) 3, TCD detection with an interruption in the flow rate (the area does not represent the mass of the compound flowing through the detector). 

The detector, irrespective of its nature, is required to have a number of fundamental properties. It should give a response that is proportional to the instantaneous mass flow, be sensitive, have a small inertia, be stable with time and yield very low background noise. 

Altitude Response. Pressure response is an issue that needs to be addressed for every instrument deployed on an aircraft. First, it must be decided how chemical abundances are to be reported. If standard practice is followed and they are reported as mixing ratios, then it must be determined whether the instrument is fundamentally a mass- or a concentration-depen-dent sensor, because this definition determines the first-order means by which instrument response is converted to mixing ratios as a function of pressure. In this context, a mass-sensitive detector is a device with an output signal that is a function of the mass flow of analyte molecules a concentration-sensitive detector is one in which the response is proportional to the absolute concentration, that is, molecules per cubic centimeter. 

For sensors that are truly mass sensitive and for which the mass flow of sample through the sensing element is held constant as a function of pressure (for example, by use of electronic mass-flow controllers), instrument response is proportional to the mixing ratio independent of the pressure. For concentration-sensitive detectors, such as simple spectrophotometric instruments measuring absorbance or fluorescence, instrument response is a function of the absolute concentration, and the response will decrease for a constant mixing ratio as the pressure decreases. For example, the response of a pulsed fluorescence SO instrument sampling air containing a fixed... 

The need for more complicated considerations in conversion of instrument response to mixing ratios usually arises when instruments that are based on mass-sensitive detectors are used. Common reasons are either that the mass flow is not held constant or that the process whereby the flow of analyte molecules is converted to an electrical signal changes as the pressure changes. These effects are illustrated by a discussion of the pressure response of two instruments commonly used to measure atmospheric trace gases, both based on detection schemes that are inherently mass sensitive. 

However, even the use of mass-flow controllers may not be sufficient to stabilize instrument response as the pressure changes. For many detectors, chamber pressure varies with altitude even if the sample mass flow is maintained at a constant value. This variance may cause a change in the... 

Some altitude effects on the operation of chromatographic instruments are anticipated. To achieve reproducible retention times for identifying compounds, mobile-phase flows need to be controlled so that they are independent of ambient pressure. Detectors may also respond to changes in pressure. For example, the electron capture detector is a concentration-sensitive sensor and exhibits diminished signal as the pressure decreases. Other detectors, such as the flame ionization detector, respond to the mass of the sample and are insensitive to altitude as long as the mass flow is controlled. 

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