Instrumentation detectors and integrators

The ultimate in selectivity in HPLC detection is seen with the use of mass-spectrometric detection, and for many applications this could be seen as the ideal detection method. However, more mundane considerations such as size of the instrumentation and limited budgets combine to reduce HPLC-MS to a relatively small number of applications which most effectively exploit its unique properties. When such practical constraints are taken into account, the real detector coimected to the HPLC system usually turns out to be a device that is a compromise, and its performance characteristics need to be taken into account during the development of many analyses just as much as the performance of the column or any other component of the HPLC system. For example, lack of detection selectivity may require extra method development to completely resolve an interfering peak, or lack of sensitivity could force the inclusion of an extraction-concentration step in an analytical method to achieve detectable levels of analyte. 

A listing of the most common types of HPLC detector is given in Table 6.1, along with some of their properties. By far the most widely used HPLC detector is the LTV absorbance detector. This is due to a combination of factors firstly, although sensitivity is not good compared to other detector types (e.g. fluorescence, electrochemical), sensitivity is adequate for the majority of HPLC analyses secondly, a great many compounds are detectable by UV absorbance measurements thirdly, simplicity of construction and economies of scale mean that UV absorbance detectors 

Detector type Sensitivity Selectivity Range of application Characterise Gradient solute elution Relative cost 

---

INSTRUMENTATION DETECTORS AND INTEGRATORS 6.5 The refractive index detector... 

Lloyd, D. K., Instrumentation detectors and integrators, in High Performance Liquid Chromatography (W. J. Lough and I. W. Wainer, eds.), Blackie Academic Professional/Chapman Hall, London, 1996, pp. 120-125. 

Lloyd, D.K. Instrumentation Detectors and integrators. In High Performance Liquid Chromatography, Lough, WJ., Wainer, I.W., Eds. Blackie Academic Professional/ Chapman HaU London, 19% 120-125. 

Studies of the effect of 14 MeV neutron irradiation on semiconductor electronic components have an essential role, because many electronic instruments are used in intense fast neutron fields. Radiation effects can be observed at a fast neutron fluence in the order of 10 /cm at 20°C. Changes in the main characteristics of electronic circuits depend strongly on the type of semiconductor components. For example, definite changes were observed in the operating characteristics of Si(Li) detectors, diodes, transistors, and integrated circuits after the irradiation with 14 MeV neutrons up to 10 /cm fluence. Further investigations are needed to study the effect of self-recovery of the irradiated Si(Li) detectors and integrated circuits. 

Electrical Concerns Electrical requirements for installing two- to four-gas chromatographs are similar to those for a single-gas chromatograph. Each instrument should be on its own 15-20-A circuit. Try to keep related electrical devices (integrators, computers, etc.), except electrically acmated devices, on the same circuit. Detector and integrator cables need to be shielded and located 6 in. or more away from the electrical lines. The gas lines, particularly copper lines, should be 6-12 in. (15-30 cm) away from the power lines—they can pick up electrical current if they are too close to the power hues. 

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. 

One area of analytical chemistry which is currently developing rapidly is the automation of methods. Some degree of automation has been used for a number of years in instruments such as automatic burettes coupled to absorptiometric or electrometric end-point detectors, and in data output devices which provide continuous pen recording or signal integration facilities. The major features of recent developments include the scope for instrumental improvements provided by solid-state electronic circuits and the increasing application of digital computers (Chapter 13). 

A simple system is comprised of an isocratic pump, a manual injector, a UV detector, and a strip-chart recorder. A schematic diagram of an HPLC instrument is shown in Fig. 15.4. This simple configuration is rarely used in most modern laboratories. A typical HPLC system is likely to consist of a multi-solvent pump, an autosampler, an on-line degasser, a column oven, and a UV/Vis or photodiode array detector all connected to and controlled by a data-handling workstation. Examples of modular and integrated systems are shown in Fig. 15.5. Some of the important instrumental requirements are summarized in Table 15.2. 

The core components of a CE instrument are a power supply, a detector, and devices that allow for temperature control of the capillary and sample compartment. A wide variety of commercial CE instruments are available, from simple modular systems to fully integrated automated systems under computer control. 

Instrument e.g., cooling system, capillary types and lengths possible, type of detectors, instrument sensitivity, software and integration functions, maintenance status of instrument and qualification status, definition of instrument cleaning procedures... 

Production Gas Detection Instruments. A family of portable instruments has been developed for the detection and monitoring of CO levels in air (7 > The instrument family consists of a direct reading detector with LCD display of actual CO concentration and a personal CO dosimeter. Both the detector and the dosimeter measure the accumulated CO exposure of personnel in industrial environments and provide both visible and audible alarms if instantaneously unsafe levels of CO are encountered. An accompanying support console is used for integrated cumulative CO dosage readout and battery charging. 

Most HPLC instruments are on line with an integrator and a computer for data handling. For quantitative analysis of HPLC data, operating parameters such as rate of solvent flow must be controlled. In modern instruments, the whole system (including the pump, injector, detector, and data system) is under the control of a computer. 

---