Chromatograph detection systems

Fodor-Csorba K, Dutka F. 1986. Selectivity and sensitivity of some thin-layer chromatographic detection systems. J Chromatogr 365 309-314. 

Photoionization a gas chromatographic detection system that uti-hzes an ultraviolet lamp as an ionization source for analyte detection. It is usually used as a selective detector by changing the photon energy of the ionization source. 

Ultraviolet spectrophotometers have been used as gas chromatographic detection systems mainly after condensation of the chromatographic effluent. Systems are capable of detecting naphthalene at 10 8g by scanning every 20 sec from 165 to 220 nm. Use of a monochrometer permits selectivity. Reactions producing chemiluminescence are known. 

The substrate and/or products must show an appreciable response to the chromatographic detection system. 

A dynamic headspace gas-partitioning method has been used to assess the effect of dissolved organic carbon on the value of H for mirex (Yin and Hassett 1986), and with a gas chromatographic detection system for analysis, this is applicable to native water samples containing mirex. 

Table 4.9 Chromatographic detection systems and their detection principles.

Crown ethers have undergone limited investigation as components in ion chromatographic detection systems. One example is the work of Jane and Shih, who coated a piezoelectric quartz crystal with dibenzo-16-crown-5-oxydodecanoic acid. The detector was used for cation and anion detection after separation on a diaza-18-crown-6-based separator column with nonionic eluents. The frequency response of this detector for both cations and anions, due to cation complexation and anion association with the resulting complex, was as reproducible and sensitive as standard conductimetric detection, but peak broadening resulted from a relatively large cell volume. 

Aqueous distillates are extracted, usually with dichloro-methane (DCM), concentrated to small volumes, generally in a Kuderna-Danish evaporator, and examined by gas chromatography (GC) using a specific detection system. Additional chromatographic cleanup may be required, depending on the complexity of the sample and specificity of the chromatographic detector. 

Chromatographic systems were finally coupled with relatively inexpensive, yet powerful, detection systems with the advent of the quadrupole mass selective detector (MSD). The operational complexity of this type of instrumentation has significantly declined over the last 15 years, thus allowing routine laboratory use. These instruments... 

It is often difficult to define where sample extraction ends and cleanup procedures begin. Sample extracts may be injected directly into a gas or liquid chromatograph in certain cases, but this will be dependent on the analyte, sample matrix, injection, separation and detection system, and the limit of determination (LOD) which is required. It is also more likely that matrix-matched calibration standards will be needed in order to obtain robust quantitative data if no cleanup steps are employed. 

The principal limitation in the use of electrophoretic techniques is the lack of availability of suitable detection systems for quantitative analysis and unequivocal identification of pesticide analytes. Traditionally, either ultraviolet/visible (UVA IS) or fluorescence detection techniques have been used. However, as with chromatographic techniques, MS should be the detection system of choice. A brief comparison of the numbers of recent papers on the application of GC/MS and LC/MS with capillary elec-trophoresis/mass spectrometery (CE/MS) demonstrates that interfaces between CE... 

Finally, the integration of biochemical or biosensor methods with conventional chromatographic analyses should not be overlooked. For example, the use of im-munoaffinity columns prior to chemiluminescence or the use of biosensor detection systems following the chromatographic step may provide useful solutions to speciflc analytical needs. 

For detection, MS is rapidly becoming the method of choice for multiclass, multiresidue analysis owing to its many advantages, recent improvements in technology, and availability of cost-effective commercial instrumentation. Detection systems in general are continually being improved, and in combination with the improvements in chromatographic instruments and techniques, an exceptionally low limit of detection (LOD) is possible for pesticide residues. 

The internal standard and analyte should be resolved chromatograph-ically to baseline (except for isotoplcally labelled samples when mass discrimination or radioactive counting are used for detection), elute close together, respond to the detection system in a similar way, and be present in nearly equal concentrations. 

Yamauchi, S., Nakai, C., Nimura, N., Kinoshita, T., and Hanai, T., Development of a highly sensitive fluorescence reaction detection system for liquid chromatographic analysis of reducing carbohydrates, Analyst, 118, 773,1993. 

HPLC-QFAAS is also problematical. Most development of atomic plasma emission in HPLC detection has been with the ICP and to some extent the DCP, in contrast with the dominance of the microwave-induced plasmas as element-selective GC detectors. An integrated GC-MIP system has been introduced commercially. Significant polymer/additive analysis applications are not abundant for GC and SFC hyphenations. Wider adoption of plasma spectral chromatographic detection for trace analysis and elemental speciation will depend on the introduction of standardised commercial instrumentation to permit interlaboratory comparison of data and the development of standard methods of analysis which can be widely used. 

---