Mass Dependent Detectors

The detector of choice for preparative work, the RI detector can be used only in isocratic systems. It is very sensitive to turbulence, temperature, or 

The fasting growing application of a mass detector is in the burgeoning field of LC/MS in which the HPLC is connected through an evaporative interface into a mass spectrometer. The MS detector is by far the most sensitive, versatile, and expensive detector used in an HPLC system. The information it provides can yield a definitive identification of the separated compounds, but requires extensive data acquisition and interpretation, computer treatment, and expertise in operation. It will be covered in more detail in Chapter 15. 

---

There can be no compromises concerning column bleed in GC-MS. Column bleed generally contributes to chemical noise where MS is used as the mass-dependent detector, and curtails the detection limits. The optimization of a particular S/N ratio can also be effected in GC-MS by selecting particularly thermally stable stationary phases with a low tendency to bleed. For use in trace analysis, stationary phases for high-temperature applications have proved particularly useful (Figures 2.79 and 2.80). Besides the phase itself, the film thickness also plays an important role. Thinner films and shorter columns exhibit lower column bleed. 

Noun + adjective time-dependent changes, mass-selective detector... 

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). 

When such different techniques as in Table 1 are compared, there is always the problem of different sensitivities for different aspects of the distribution. If, for example, information about the high molar mass tail is of importance, PCS may be the method of choice. It may also be incorrect to regard the SEC distribution as the true molar mass distribution as it may suffer from calibration problems, solute-column interactions, peak broadening, and a molar mass dependence of the contrast factor d n/dc, and hence the detector sensitivity. 

Sample molecules that are too large to enter the pores of the support material, which is commercially available in various pore dimensions, are not retained and leave the column first. The required elution volume Ve is correspondingly small. Small molecules are retained most strongly because they can enter all the pores of the support material. Sample molecules of medium size can partly penetrate into the stationary phase and elute according to their depth of penetration into the pores (Fig. 7.3). No specific interactions should take place between the molecules of the dendrimer sample and the stationary phase in GPC since this can impair the efficiency of separation by the exclusion principal. After separation the eluate flows through a concentration-dependent detector (e.g. a UV/VIS detector) interfaced with a computer. One obtains a chromatogram which, to a first approximation, reflects the relative contents of molecules of molar mass M. If macromolecules of suitable molar mass and narrow molar mass distribution are available for calibration of the column, the relative GPC molar mass of the investigated dendrimer can be determined via the calibration function log(M) =f( Vc). 

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. 

To overcome the problems associated with classical SEC of complex polymers, molar mass-sensitive detectors are coupled to the SEC instrument. Since the response of such detectors depends on both concentration and molar mass, they have to be combined with a concentration-sensitive detector. The following types of molar-mass-sensitive detectors are used frequently [25-28] ... 

The column flow into the ESMS system is best kept to a few jiL/min., and it is common to split the flow from all but capillary columns so that the larger portion is carried through a UV detector to a fraction collector. For example, with a 1 mm diameter column with a flow of 40 pL/min., effluent will typically be split so that about 5 pL flows to the mass spectrometer and 35 pL goes to a fraction collector. There is no loss in sensitivity of detection in such stream splitting because the ESMS acts as a concentration-dependent detector. 

Detectors can also be grouped into concentration-dependent detectors and mass-flow-dependent detectors. Detectors whose responses are related to the concentration of solute in the detector cell, and do not destroy the sample, are called concentration-dependent detectors, whereas detectors whose response is related to the rate at which solute molecules enter the detector are called mass-flow-dependent detectors. Typical concentration-dependent detectors are TCD and GC-FTIR. Important mass-flow-dependent detectors are the FID, thermoionic detector for N and P (N-, P-FID), flame photometric detector for S and P (FPD), ECD, and selected ion monitoring MS detector. 

Neither the MS or the UV-vis detector is very good at predicting the amount of material present. A third detector that is mass dependent is sometimes included to quantify the peak of interest. One common detector used in both SFC and FIPLC is the evaporative light-scattering detector or (ELSD) (5-8). There are a number of different ELSDs on the market. It is important to use one that is compatible with SFC and is capable of operating during steep composition gradients. 

Figure 2.15 FID detector (a) and NPD detector (b). The electrometers used with detectors allow the measurement of very small intensities. The response of these mass flow dependant detectors are unaffected by make-up gas.

For heavy ions, a phenomenon called the pulse height defect (PHD) seems to have an important effect on energy calibration. As a result of the PHD, the relationship between pulse height and ion energy is mass dependent. In semiconductor detectors, experiments have shown that the PHD depends on the... 

ECD Characteristics. Compound-specific destructive concentration or mass-flow detector depending on operation compound sensitivities differ over a wide range useable primarily with isocratic RP-HPLC including some buffer salts, pre- or postcolumn derivatization can be used to increase number of measurable compounds. 

---