Mass-sensitive detector

Under many experimental conditions, the mass spectrometer functions as a mass-sensitive detector, while in others, with LC-MS using electrospray ionization being a good example, it can behave as a concentration-sensitive detector. The reasons for this behaviour are beyond the scope of this present book (interested readers should consult the text by Cole [8]) but reinforce the need to ensure that adequate calibration and standardization procedures are incorporated into any quantitative methodology to ensure the validity of any results obtained. 

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

Column and detector properties determine the minimum amount of a component that can be reliably distinguished from the background noise. If we arbitrarily select a signal to noise ratio of 4 as the minimum value for the confident determination of a peak in a chromatogram then for a mass sensitive detector the minimum detectable amount is given by... 

For a detector to be of use in quantitative analysis, the signal output should be linear with concentration for a concentration-sensitive detector and with mass for a mass-sensitive detector. Some detectors have an additional time constant purposely introduced to remove the high-frequency noise. This should always taken into consideration, since a slow detector response can significantly broaden and attenuate chromatographic peaks relative to those actually sensed. Moreover, a versatile detector should have a wide linear dynamic range so that major and trace components can be determined in a single analysis, over a wide concenua-tion range. 

GC detectors can be grouped into concentration-sensitive detectors and mass-sensitive detectors. The signal from a concentration-sensitive detector is related to the concentration of solute in the detector, which does not usually destroy the sample. Mass-sensitive detectors usually destroy the sample, and the signal is related to the rate at which solute molecules enter the detector. The response of a mass-sensitive detector is unaffected by make-up gas, while that of a concentration-sensitive detector will lower with make-up gas. A summary of some important characteristics of the GC detectors specifically used in drug residue analysis is presented in Table 23.1. 

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. 

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. 

Quartz crystal microbalance — The quartz crystal microbalance (QCM) or nanobalance (QCN) is a thickness-shear-mode acoustic wave mass-sensitive detector based on the effect of an attached foreign mass on the resonant frequency of an oscillating quartz crystal. The QCM responds to any interfacial mass change. The response of a QCM is also extremely sensitive to the mass (density) and viscoelastic changes at the solid-solution interface [i-vi]. 

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] ... 

Coupling with Molar-Mass-Sensitive Detectors... 

As has been pointed out, for SEC of complex polymers no simple correspondence exists between elution volume and molar mass. It is, therefore, useful to determine the molar mass not via a calibration curve but directly from the SEC effluent. This can be done by using molar-mass-sensitive detectors based on Rayleigh light scattering or intrinsic viscosity measurements [45]. 

As has been discussed, the combination of SEC and molar-mass-sensitive detectors is a powerful tool for the analysis of complex polymers. However, it is important to distinguish between claimed versus actual capabilities and between potential expectations and demonstrated performances. Tables 1 and 2, taken from a critical review of different techniques, summarize the informational content and additional details of SEC-light scattering and SEC-viscometry coupling... 

Table 1. SEC analysis using molar-mass-sensitive detectors ...

Gores F, Kilz P (1993) Copolymer characterization using conventional SEC and molar mass-sensitive detectors. In Provder T (ed) Chromatography of polymers, chap 10. ACS Symp Ser 521, American Chemical Society, Washington, DC... 

The introduction of molar mass-sensitive detectors overcame problems in SEC analysis of various polymer topologies, if the chromatographic technique is able to separate them properly (S). 

CHROM software. It is also possible to use molar mass sensitive detectors in this setup to get independent of calibrations or assumptions for the calculation of molar masses for copolymers with unknown structures. Multidetection analysis for independent copolymer composition calculations (S) and 2D segment distributions are already fully implemented in the PSS 2D-CHROM software package. 

An alternative is to determine the polymers molecular weight/mass in the SEC eluent in situ, by use of on-line molecular mass sensitive detectors. Two such detectors are commercially available, the light scattering detector and the viscosity detector. These detectors are usually used in series with a mass concentration detector and require specialised data handling/software to compute the outputs from the twin detectors and to produce molecular weight/masses and distributions. 

Often it is required to detect compounds with no or only very weak chromophores such as sugars and amino acids. Refractive index detectors and mass sensitive detectors can be used but they are relatively insensitive in the context of biological sample concentrations. Indirect detection using a UV or fluorescent eluent can also be employed. However, the most common approach is the use of derivatisation. Derivatisation of some chemically reactive moiety on the analyte can be performed in two modes. In post-column derivatisation the sample is separated first and then reacted with a flowing stream of derivatising reagent being pumped into... 

Gel permeation chromatography (GPC) is the established method for the determination of molar mass averages and the molar mass distributions of polymers. GPC retention is based on the separation of macromolecules in solution by molecular sizes and, therefore, requires a molar mass calibration to transform elution time or elution volume into molar mass information. This kind of calibration is typically performed with narrow molecular mass distribution polymer standards, universal, or broad calibration methods or molar-mass-sensitive detectors like light-scattering or viscosity detectors. 

FIA Uses existing equipment Saves eluent No separation Limited time gain Not applicable for copolymers/blends Requires molar mass sensitive detectors Only primary information (cone., Mw, TV) Needs method change Needs special software Samples difficult to separate Utilize existing instruments... 

---