Element selective detector, HPLC

Evaporative LC-FTIR is rapidly gaining industrial acceptance as a useful tool in low-MW additive analysis. HPLC has also been coupled with various element-selective detectors. There is significant demand for speciation information for many elements, and the separation ability of chromatography coupled to ICP-MS offers the analyst a versatile tool for such studies. It is apparent that ICP-MS is increasingly being employed for chromatographic detection. Several modes of GC, SFC, LC and CE have been hyphenated with ICP-MS for improved detection limits compared to other traditional methods of detection such as UV-VIS spectroscopy. Inorganic speciation deserves more attention. 

Organochlorine pesticides and OPPs have been determined mainly using GC, because of the stability and volatility that most of them show under chromatographic conditions and, particularly, the availability of element-selective detectors that display high selectivity for OCPs (electron-capture detector, ECD), and OPPs (flame photometric detector, FPD, and nitrogen phosphorus detector, NPD). Mass spectrometry-based detection is also more popular in GC than in HPLC (1,2,12,16). 

A number of very useful and practical element selective detectors are covered, as these have already been interfaced with both HPLC and/or FIA for trace metal analysis and spe-ciation. Some approaches to metal speciation discussed here include HPLC-inductively coupled plasma emission, HPLC-direct current plasma emission, and HPLC-microwave induced plasma emission spectroscopy. Most of the remaining detection devices and approaches covered utilize light as part of the overall detection process. Usually, a distinct derivative of the starting analyte is generated, and that new derivative is then detected in a variety of ways. These include HPLC-photoionization detection, HPLC-photoelectro-chemical detection, HPLC-photoconductivity detection, and HPLC-photolysis-electrochemical detection. Mechanisms, instrumentation, details of interfacing with HPLC, detector operations, as well as specific applications for each HPLC-detector case are presented and discussed. Finally, some suggestions are provided for possible future developments and advances in detection methods and instrumentation for both HPLC and FIA. 

Their Spectraspan VI that appeared on the market in 1984 appears to be an ideal element selective detector for HPLC, though very few papers have thus far utilized that specific model in HPLC-DCP studies. It is arguable that MDLs via HPLC-DCP must, by the very nature of the DCP, always be Inferior to the HPLC-ICP system. We have not found this always to be the case, and Indeed in a very recent HPLC-DCP study of various chromium containing species, the HPLC-DCP MDLs appeared somewhat better (lower) than those reported by HPLC-ICP methods (22). We have already suggested some possible explanations for these observations, though nobody has yet made a full, direct comparison of HPLC-DCP HPLC-ICP MDLs for even a few metal species. 

Verification of the methodology was achieved by using the echelle spectrometer system as an element selective detector for both anion exchange and reverse phase HPLC. The anion exchange work involved the separation and detection of the ethylenediaminetetraacetic acid (EDTA) and nitrilotriacetic acid (NTA) complexes of Ca, Cu, and Mg, duplicating the work of Fraley et al. [51]. The reverse phase HPLC work consisted of the separation of acetylacetonate (AcAc) complexes of a number of transition metals employing a C-18 column and a 50 50 methanol water mobile phase. 

Drushel [58] and others [31,59] have described the needs of the chromatographer in the area of detectors. Specific texts concern detection in quantitative GC [54], diode-array detection in HPLC [48], selective detectors [39] and element-specific chromatographic detection by AES [60], electrochemical detectors [61] and laser detectors [62]. 

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. 

Supercritical fluid chromatography has some of the same characteristics of both HPLC and gas chromatography (GC). Packed column SFC uses the same column technology as HPLC, and when used with binary or tertiary solvents, has a broad range of applicability [1]. This range is much broader than GC, because compounds need not be volatile or thermally stable. As in GC, SFC can be coupled to most modern chromatographic detectors, such as element-specific detectors. These detectors are often very selective for... 

The use of HPLC coupled to an element-specific detector for speciation analysis has been reviewed, as have the HPLC methods for some specific elemental species. " A representative selection of speciation methods based on HPLC separation is given in Table 6. The main separation modes used are ion exchange (anion and cation) and... 

In general, QE-AAS, AES, AES, microwave induced plasma (MIP) and ICP-MS are used as detectors rather than the less specific FID, FPD, and ECD. The overriding reason for this is the greater sensitivity and selectivity afforded by the element-specific detectors, without which it would not be possible to determine the chemical species of interest at the low concentrations generally present in biological and environmental samples. The other main detection method that has been used is mass spectrometry in its various configurations, but particularly electrospray ionization (ESI) and atmospheric chemical ionization (APCI), which are used with HPLC and CE separations, and... 

In closing this section, it should be emphasized that element selective detection in HPLC is an area now undergoing very rapid exploration and development, and significant Improvements in HPLC-ICP/ DCP detection limits with resultant practical applications should be realized in the near future. If a then relatively inexpensive element selective spectroscopic detector could be developed for conventional or microbore HPLC, with a successful Interface providing low MDLs, this could result in a more widespread acceptance and application of both the overall Instrumentation and final techniques. 

For several years, the Winchester Engineering and Analytical Center (WEAC) of the U.S. Food and Drug Administration (FDA) has evaluated and applied a variety of GC/HPLC-DCP approaches for true element specific and selective detection. Element selective, by and large, suggests that the response is relatively unique for a certain/specific element, but that other interferents may also produce similar responses. Element specific, by and large, suggests that the response on that particular detector is unique for that particular element, whatever its species or form. 

Plasmas compare favourably with both the chemical combustion flame and the electrothermal atomiser with respect to the efficiency of the excitation of elements. The higher temperatures obtained in the plasma result in increased sensitivity, and a large number of elements can be efficiently determined. Common plasma sources are essentially He MIP, Ar MIP and Ar ICP. Helium has a much higher ionisation potential than argon (24.5 eV vs. 15.8 eV), and thus is a more efficient ionisation source for many nonmetals, thereby resulting in improved sensitivity. Both ICPs and He MIPs are utilised as emission detectors for GC. Plasma-source mass spectrometry offers selective detection with excellent sensitivity. When coupled to chromatographic techniques such as GC, SFC or HPLC, it provides a method for elemental speciation. Plasma-source detection in GC is dominated by GC-MIP-AES... 

While validating a production process, several steps were listed as they pertained to each of the components of manufacturing equipment, process conditions, personnel, and so forth. These key elements multiply rapidly when it comes to analytical methods validation. Take, for example, HPLC — the most commonly used method of analysis. A typical analytical method would involve use of columns, pumps, heaters, detectors, controllers, samplers, sensors, recorders, computers, reagents, standards, and operators — put together as a system. Each of these components and systems needs independent validation, followed by a validation of the system. Note that when this equipment is used to manufacture a product such a therapeutic proteins wherein HPLC techniques are used for the purification purpose, then all additional requirements of a manufacturing system also apply, including, but not limited to, the requirement that the equipment be of a sanitary kind. This limits the choice for manufacturers, and these considerations should be taken into account in the first selection of equipment. 

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