Sensitivity LC detectors

Detector development has gone hand in hand with column development in all forms of chromatography. The rapid development of GC arose solely from the swift availability of sensitive detecting systems and it was not until sensitive LC detectors were introduced that could the rapid advances in LC could take place. Highly sensitive detectors have provided accurate concentration profiles of eluted... 

Practical aspects. The detector is frequently used in LC and TLC to detect compounds which show fluorescence or can be made fluorescent by derivatization. It is one of the most sensitive LC detectors. 

The fluorescence detector is probably the most sensitive LC detector and is often used for trace analysis. Molecules are excited by electromagnetic radiation to produce luminescence and this effect is called photoluminescence. 

Both the sensing device of the LC detector and the associated electronics can be temperature sensitive and cause the detector output to drift as the ambient temperature changes. Consequently, the detecting system should be designed to reduce this drift to a minimum. In practice the drift should be less than 1% of FSD at the maximum sensitivity for 1°C change in ambient temperature. 

The UV detector is the most popular and useful LC detector that is available to the analyst at this time. This is particularly true if multiwavelength technology is included in the genus of UV detectors. Although the UV detector has definite limitations, particularly with respect to the detection of non-polar solutes that do not possess a UV chromaphore, it has the best combination of sensitivity, versatility and reliability of all the detectors so far developed for general LC analyses. 

Detector selectivity is much more important in LC than in GC since, in general, separations must be performed with a much smaller number of theoretical plates, and for complex mixtures both column separation and detector discrimination may be equally significant in obtaining an acceptable result. Sensitivity is important for trace analysis and for compatibility with the small sizes and miniaturised detector volumes associated with microcolumns in LC. The introduction of small bore packed columns in HPLC with reduced peak volume places an even greater strain on LC detector design. It is generally desirable to have a nondestructive detector this allows coupling several detectors in series (dual... 

SFC-SFC is more suitable than LC-LC for quantitation purposes, in view of the lack of a suitable mass-sensitive, universal detector in LC. Group quantitation can be achieved by FID. The ideal SFC-SFC system would consist of a short (10-30 cm) packed-capillary primary column, interfaced to a long (5-10m) open-tubular column, but such a combination is difficult to realise, due to the different flow-rates required for each column type. Coupled SFC-SFC is often configured with a solute concentration device prior to valve switching on to the SFC. The main approaches to this concentration stage are the use of absorbent material or cryofocusing. Davies el at. [924] first introduced two-dimensional cSFC (cSFC-cSFC), and its use has been reported [925,926]. 

Coupled LC-LC can separate high-boiling petroleum residues into groups of saturates, olefins, aromatics and polar compounds. However, the lack of a suitable mass-sensitive, universal detector in LC makes quantitation difficult SFC-SFC is more suitable for this purpose. Applications of multidimensional HPLC in food analysis are dominated by off-line techniques. MDHPLC has been exploited in trace component analysis (e.g. vitamin assays), in which an adequate separation for quantitation cannot be achieved on a single column [972]. LC-LC-GC-FID was used for the selective isolation of some key components among the irradiation-induced olefinic degradation products in food, e.g. dienes and trienes [946],... 

The progress made in interfacingHPLC instruments with mass spectrometry has been a significant development for laboratory analyses in the pharmaceutical industry. The low concentrations of test drugs in extracts of blood, plasmas, serums, and urine are no problem for this highly sensitive HPLC detector. In addition, the analysis is extremely fast. Lots of samples with very low concentrations of the test drugs can thus be analyzed in a very short time. At the MDS Pharma Services facility in Lincoln, Nebraska, for example, a very busy pharmaceutical laboratory houses over 20 LC-MS units, and they are all in heavy use daily. 

The R1 detector is another basic detector in general use in HPLC. Although the sensitivity is generally lower than that of most other LC detectors, R1 detectors are especially useful for non-chromogenic compounds such as sugars [8], high molecular weight polymers [9], and some pharmaceuticals present in animal feeds [10], R1 has also been applied to microbore columns for the analysis of carbohydrates [11] and small molecules [12],... 

The method of complete electrolysis is also important in elucidating the mechanism of an electrode reaction. Usually, the substance under study is completely electrolyzed at a controlled potential and the products are identified and determined by appropriate methods, such as gas chromatography (GC), high-performance liquid chromatography (HPLC), and capillary electrophoresis. In the GC method, the products are often identified and determined by the standard addition method. If the standard addition method is not applicable, however, other identification/determination techniques such as GC-MS should be used. The HPLC method is convenient when the product is thermally unstable or difficult to vaporize. HPLC instruments equipped with a high-sensitivity UV detector are the most popular, but a more sophisticated system like LC-MS may also be employed. In some cases, the products are separated from the solvent-supporting electrolyte system by such processes as vaporization, extraction and precipitation. If the products need to be collected separately, a preparative chromatographic method is use-... 

Classical LC detectors (refractive index, fixed wavelength UV absorbance at 254 or 280 nm) have lacked the sensitivity to allow direct analysis of cannabi-noids in biological fluids. However, recent development of variable wavelength absorbance detectors extending into the 195-220nm UV region and of fluorescence detectors for HPLC led the authors to initiate... 

Likewise, the luminescence properties of many analytes can be altered in the presenoe of surfactant aggregates (4,7.,8.). Consequently, addition of micelle-forming surfactants (present either in the LC mobile phase or added post-column) can improve the sensitivity of fluorimetric LC detectors (49,482). Micellar spray reagents have been utilized to enhance the fluorescence densitometric detection of dansylamino acids or polycyclic aromatic hydrocarbons (483). The effect was observed for TLC performed on cellulose or polyamide stationary phases with the micellar spray reagent being either CTAC, SB-12, or NaC (483). More recently, use of nonionic Triton X-100 has been found to improve the HPLC detection of morphine by fluorescence determination after post-column derivatization (486) as well as improve the N-chlorination procedure for the detection of amines, amides, and related compounds on thin-layer chromatograms (488). 

The introduction of small bore packed columns in LC [7] reduced the peak volume still further and placed an even greater strain on LC detector design. Due to the relatively lower sensitivity of LC detectors compared with that of GC detectors, the LC detector sensor volume was forced down to a level which, for present day technology, may well be the practical limit for many types of LC detectors. This interaction between detector design and column design continues to this day and probably will do so for many years to come. 

This classification is satisfactory for GC detectors but as there is only one LC detector that is mass sensitive [8] (the transport detector), which at the time of writing this book is also not commercially available, this manner of classification is of little use for LC detectors. 

The pressure sensitivity of a detector is extremely important as it is one of the detector parameters that determines both the long term noise and the drift. As it influences long term noise, it will also have a direct impact on detector sensitivity or minimum detectable concentration together with those other characteristics that depend on detector sensitivity. Certain detectors are more sensitive to changes in pressure than others. The katherometer detector, which is used frequently for the detection of permanent gases in GC, can be very pressure sensitive as can the LC refractive index detector. Careful design can minimize the effect of pressure but all bulk property detectors will tend to be pressure sensitive. 

The wire transport system has many attractive characteristics as an LC detector, but for general use, its sensitivity needs to be increased by at least an order of magnitude. Furthermore, the overall system needs to be simplified to render it more reliable, less expensive and easier to operate. 

The association of a spectrometer with a liquid chromatograph is usually to aid in structure elucidation or the confirmation of substance identity. The association of an atomic absorption spectrometer with the liquid chromatograph, however, is usually to detect specific metal and semi-metallic compounds at high sensitivity. The AAS is highly element-specific, more so than the electrochemical detector however, a flame atomic absorption spectrometer is not as sensitive. If an atomic emission spectrometer or an atomic fluorescence spectrometer is employed, then multi-element detection is possible as already discussed. Such devices, used as a LC detector, are normally very expensive. It follows that most LC/AAS combinations involve the use of a flame atomic absorption spectrometer or an atomic spectrometer fitted with a graphite furnace. In addition in most applications, the spectrometer is set to monitor one element only, throughout the total chromatographic separation. 

The four most commonly used LC detectors are the UV detector, the fluorescence detector, the electrical conductivity detector and the refractive index detector. Despite there being a wide range of other detectors to choose from, these detectors appear to cover the needs of 95% of all LC applications. This is because the major use of LC as an analytical technique occurs in research service laboratories and industrial control laboratories where analytical methods have been deliberately developed to utilize the more straight forward and well established detectors that are easy and economic to operate. LC detectors are more compact than their GC counterparts and need much less ancillary support. Most operate solely on the mobile phase and need no other fluid supplies for their effective use. All LC detectors are 3-5 orders of magnitude less sensitive than their GC counterparts and thus sensor contamination is not so severe, and generally less maintenance is required. 

Whilst the object of this chapter has been to show the extent and type of HPLC technique that is used today in today s environmental laboratories, there are a number of less routine techniques that may or may not have an impact on routine environmental monitoring. One of the most potentially important of these is the use of LC-MS. The problems associated with using LC-MS for trace analysis are twofold one is the usual LC-MS problem of interfacing the second is that of sensitivity of detector. The interfacing problem may well continue to have partial (compared with GC-MS interfacing) solutions such as FAB, and thermospray, etc. However, even given the advances arising from electrospray interfaces the answer may well be to move away from LC-MS to supercritical fluids and SFC-MS. 

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