Detection methods/detectors universal

Universal and selective detectors, linked to GC or LC systems, have remained the predominant choice of analysts for the past two decades for the determination of pesticide residues in food. Although the introduction of bench-top mass spectrometers has enabled analysts to produce more unequivocal residue data for most pesticides, in many laboratories the use of selective detection methods, such as flame photometric detection (FPD), electron capture detection (BCD) and alkali flame ionization detection (AFID) or nitrogen-phosphorus detection (NPD), continues. Many of the new technologies associated with the on-going development of instrumental methods are discussed. However, the main objective of this section is to describe modern techniques that have been demonstrated to be of use to the pesticide residue analyst. 

The most widely regarded approach to accomplish the determination of as many pesticides as possible in as few steps as possible is to use MS detection. MS is considered a universally selective detection method because MS detects all compounds independently of elemental composition and further separates the signal into mass spectral scans to provide a high degree of selectivity. Unlike GC with selective detectors, or even atomic emission detection (AED), GC/MS may provide acceptable confirmation of the identity of analytes without the need for further information. This reduces the need to re-inject a sample into a separate GC system (usually GC/MS) for pesticide confirmation. Through the use of selected ion monitoring (SIM), efficient ion-trap or quadrupole devices, and/or tandem mass spectrometry (MS/MS), modern GC/MS instruments provide LODs similar to or lower than those of selective detectors, depending on the analytes, methods, and detectors. 

Perhaps the most revolutionary development has been the application of on-line mass spectroscopic detection for compositional analysis. Polymer composition can be inferred from column retention time or from viscometric and other indirect detection methods, but mass spectroscopy has reduced much of the ambiguity associated with that process. Quantitation of end groups and of co-polymer composition can now be accomplished directly through mass spectroscopy. Mass spectroscopy is particularly well suited as an on-line GPC technique, since common GPC solvents interfere with other on-line detectors, including UV-VIS absorbance, nuclear magnetic resonance and infrared spectroscopic detectors. By contrast, common GPC solvents are readily adaptable to mass spectroscopic interfaces. No detection technique offers a combination of universality of analyte detection, specificity of information, and ease of use comparable to that of mass spectroscopy. 

The concept of peak capacity is rather universal in instrumental analytical chemistry. For example, one can resolve components in time as in column chromatography or space, similar to the planar separation systems however, the concept transcends chromatography. Mass spectrometry, for example, a powerful detection method, which is often the detector of choice for complex samples after separation by chromatography, is a separation system itself. Mass spectrometry can separate samples in time when the mass filter is scanned, for example, when the mass-to-charge ratio is scanned in a quadrupole detector. The sample can also be separated in time with a time-of-flight (TOF) mass detector so that the arrival time is related to the mass-to-charge ratio. 

An alternative to derivatizing carbohydrates is the use of indirect photometric detection. In this method, a detectable co-ion in the electrolyte is added to the buffer system generating a steady state absorbance signal in the detector. As the analyte ions migrate in front of the detector window, they displace the detectable co-ion and cause a decrease or negative response in the detector signal. This method provides universal detection of all anions or cations. Since most carbohydrates are not ionized... 

For the cationic surfactants, the available HPLC detection methods involve direct UV (for cationics with chromophores, such as benzylalkyl-dimethyl ammonium salts) or for compounds that lack UV absorbance, indirect photometry in conjunction with a post-column addition of bromophenol blue or other anionic dye [49], refractive index [50,51], conductivity detection [47,52] and fluorescence combined with postcolumn addition of the ion-pair [53] were used. These modes of detection, limited to isocratic elution, are not totally satisfactory for the separation of quaternary compounds with a wide range of molecular weights. Thus, to overcome the limitation of other detection systems, the ELS detector has been introduced as a universal detector compatible with gradient elution [45]. 

If a solute of interest does not contain a chromophore, it may be detected by indirect UV detection. Indirect detection is a technically simple and sensitive method for the detection of compounds with little inherent detector response. Indirect UV detection is a nondestructive technique, in which the analyte is characterized in native form. Indirect detection is a universal detection mode, with few requirements as to the exact nature of the analyte. The properties of indirect detection have been reviewed by Yeung.22 Indirect detection is particularly attractive for the analysis of biological compounds. Optical systems are the same for direct or indirect detection the only difference is that, in indirect detection, the mobile phase, rather than the analyte, contains a UV chromophore. 

In contrast to component-specific detectors, such as ultraviolet (UV) absorbance and fluorescence, conductivity detection is a universal detection method. This means that a bulk property (conductivity) of the buffer solution is continuously measured. A migrating ionic component locally changes the conductivity and this change is monitored. As such, conductivity detection is universally sensitive because, in principle, all migrating ionic compounds show detector response, although not to the same extent. 

Irrespective of the detection method used, the analysis of VOCs often involves the presence of interferences in the chromatogram. This is normally due to VOCs present in the atmosphere, such as normal laboratory solvents (acetone, dichloromethane, chloroform, acetonitrile, or methanol) which can be detected, especially when using a universal detector such as a mass spectrometer which operates under vacuum. This problem is eliminated when the laboratory of analysis is free of solvents and is isolated, especially from urban areas. 

In principle, mineral oil (see also Chapter 6) is also a group parameter. All compounds that are extracted, are not removed during clean-up, pass the chromatographic column and are detected by the universal detector are said to be mineral oil. The influence on changing parameters of the method will not be as significant as with the EOX-determination. All mineral oil compounds are very non-polar. Hence care has to be taken. 

According to another classification, the detectors can be divided into specific, universal and mixed. For the choice of the detection method, the properties of sample and mobile phase as well as experimental requirements are decisive however, sometimes several types of instruments based on the same detection principle are marketed. When judging the detectors one considers primarily (i) linearity, dynamic range and sensitivity, (ii) erroneous responses like noise and drift, caused in the former by the instability of operational variables such as temperature variations, pulsating eluent flow, etc. (iii) response distortion due to hydraulic broadening and skewing of the sample zone as well as response delay. 

Recently, electrochemical detection methods, namely, conductimetry, amperometry, and potentio-metry, have also become accessible. All three variants of electrochemical detection are intrinsically simpler than the optical methods, and their success depends highly on the electrode materials and designs used. Conductivity detection relies on measurement of the differences between the conductivities of the analyte and the separation electrolyte this provides a direct relationship between migration times and response factor, and makes this detector universal. On the contrary, amperometric detection is restricted to electroactive species and potentiometric detection is not possible for certain small ions with multiple charges. Conductimetric detection works better for inorganic compounds since the higher mobility of... 

Generally, UV detection is used for the determination of most environmental pollutants. However, the use of UV detectors in CE for metal ion and anion analysis is not suitable due to the poor absorbance of UV radiation by metal ions and anions. The most common method to solve this problem is indirect UV detection. The main advantage of the indirect UV detection method is its universal applicability. The complexation of metal ions with Ugands also increases the sensitivity of their detection. The complex-ing agent is either added to the electrolyte (in situ, online complexation) or to the sample before the introduction... 

Despite the many progresses made recently, any single or multidimensional LC separation is only the first step in molecular characterization of complex polymers. Possible local polydispersity of the fractions obtained in the course of separation must be understood properly and the detector data have to be processed accordingly. Therefore it is important to understand the limitations of each separa-tion/detection method is well as the advantages. Furthermore, at the moment no universal approach does exist and customized procedure must be developed to solve each particular complex polymer system of interest. 

As mentioned before, a unique advantage of SFC is the fact that a wide variety of detection methods can be applied (see Table 11). Besides the traditional LC detection (e.g., UV) the use of GC detectors, especially, enables relatively sensitive, universal (FID), as well as selective [e.g.. electron capture detectors (BCD), thermoionic detectors (TID)] methods of detection. 

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