Array detection

The array detector (collector) consists of a number of ion-collection elements arranged in a line each element of the array is an electron multiplier. Another type of array detector, the time-to-digital converter, is discussed in Chapter 31. 

Streak cameras and multianode microchannel plate photomultipliers (MCP-PMs) interfaced to a polychromator also permit multiwavelength fluorescence decay measurements, the spectral response of both being determined by the photocathode composition. 

Farrens and Song 40) have replaced the original spark source with a picosecond diode laser in a multiplexed dual wavelength T-formatfluorometer.(41)With an overall instrumental response width of ca. 300 psec full-width half-maximum (FWHM), near-IR fluorescence lifetimes as low as 75 psec in the case of l,l -diethyl-4,4 carbo-cyanine iodide (DCI) (excitation 660 nm) and decay components as low as 48 psec in the case of 124 kDa oat phytochrome (excitation 752 nm) were reported. 

The near-IR offers many attractions when working with sensors owing to low transmission losses in optical fibers, compatibility with the wealth of optoelectronics 

Sensors based on the fluorescence quenching ofrhodamine 6G in resins by iodide ions(43) and in Nafion polymer by metal ions in solution 44,45) have been demonstrated. However, complex fluorescence decay mechanisms often hinder interpretation in lifetime-based sensing and much progress is still to be made in this area before the true potential of lifetime-based sensing becomes a reality. For example, rhodamine 6G in 

Nafion is thought to form dimers giving rise to a biexponential decay. 44,46) Using diode laser excitation at 670 nm, the fluorescence of oxazine in Nafion and its quenching by copper ions has been shown to give rise to a complex fluorescence decay.(47) Despite such complications there is still room for optimism. For example, Zen and Patonay(48) have demonstrated a pH sensor based on cyanine dye fluorescence intensity in Nafion excited with 30 mW diode laser excitation at 780 nm. 

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Kidder L H, Levin I W, Lewis E N, Kleiman V D and Heilweil E J 1997 Mercury cadmium telluride focal-plane array detection for mid-infrared Fourier-transform spectroscopic imaging Opt. Lett. 22 742-4... 

Where space is not a problem, a linear electron multiplier having separate dynodes to collect and amplify the electron current created each time an ion enters its open end can be used. (See Chapter 28 for details on electron multipliers.) For array detection, the individual electron multipliers must be very small, so they can be packed side by side into as small a space as possible. For this reason, the design of an element of an array is significantly different from that of a standard electron multiplier used for point ion collection, even though its method of working is similar. Figure 29.2a shows an electron multiplier (also known as a Channeltron ) that works without using separate dynodes. It can be used to replace a dynode-type multiplier for point ion collection but, because... 

The array system is discussed in Chapter 29. With array detection, resolution of m/z values depends both on the analyzer and the collector. Historically, the method for recording ions dispersed in space was to use a photographic plate, which was placed in the focal plane such that all ions struck the photographic plate simultaneously but at different positions along the plate, depending on m/z value. This method of detection is now rarely used because of the inconvenience of having to develop a photographic plate. 

A second use of arrays arises in the detection of trace components of material introduced into a mass spectrometer. For such very small quantities, it may well be that, by the time a scan has been carried out by a mass spectrometer with a point ion collector, the tiny amount of substance may have disappeared before the scan has been completed. An array collector overcomes this problem. Often, the problem of detecting trace amounts of a substance using a point ion collector is overcome by measuring not the whole mass spectrum but only one characteristic m/z value (single ion monitoring or single ion detection). However, unlike array detection, this single-ion detection method does not provide the whole spectrum, and an identification based on only one m/z value may well be open to misinterpretation and error. 

A powerful tool now employed is that of diode array detection (DAD). This function allows peaks detected by UV to be scanned, and provides a spectral profile for each suspected microcystin. Microcystins have characteristic absorption profiles in the wavelength range 200-300 nm, and these can be used as an indication of identity without the concomitant use of purified microcystin standards for all variants. A HPLC-DAD analytical method has also been devised for measurement of intracellular and extracellular microcystins in water samples containing cyanobacteria. This method involves filtration of the cyanobacteria from the water sample. The cyanobacterial cells present on the filter are extracted with methanol and analysed by HPLC. The filtered water is subjected to solid-phase clean-up using C g cartridges, before elution with methanol and then HPLC analysis. 

Figure 5.3 Analysis of 100 ml of (a) surface water and (b) drinking water sample spiked with 0.1 pig/ml of microcystins, using column-switching HPLC 1, microcystin-RR 2, microcystin-YR 3, microcystin-LR. Reprinted from Journal of Chromatography A, 848, H. S. Lee et al, On-line trace enrichment for the simultaneous determination of microcystins in aqueous samples using high performance liquid chromatography with diode-array detection , pp 179-184, copyright 1999, with permission from Elsevier Science.

C. Aguilar, I. Feirer, R Bonnll, R. M. Marce and D. Barcelo, Monitoring of pesticides in river water based on samples previously stored in polymeric cartridges followed by on-line solid-phase extraction-liquid cliromatography-diode array detection and confirmation by atmospheric pressure chemical ionization mass spectrometry . Anal. Chim. Acta 386 237-248 (1999). 

R. M. Marce, H. Prosen, C. Crespo, M. Calull, R Boirull and U. A. Th Brinkman, Online ti ace enrichment of polar pesticides in environmental waters by reversed-phase liquid cliromatography-diode array detection-particle beam mass spectrometry , J. Chromatogr. 696 63-74 (1995). 

S. Lacorte, J. J. Vreuls, J. S. Salau, R Ventura and D. Barcelo, Monitoring of pesticides in river water using fully automated on-line solid-phase extraction and liquid cliro-matography with diode array detection with a novel filrtation device , J. Chromatogr. 795 71-82(1998). 

C. Aguilar, R Bomtll and R. M. Marce, Determination of pesticides by on-line trace enrichment-reversed-phase liquid cliromatogr aphy-diode-array detection and confirmation by particle-beam mass spectrometry , Chromatographia 43 592-598 (1996). 

I. Eeirer, V. Pichon, M. C. Hennion and D. Barcelo, Automated sample preparation with exti action columns by means of anti-isoproturon immunosorbents for the determination of phenylurea herbicides in water followed by liquid chi omatography diode array detection and liquid clrromatogi aphy-atmospheric pressure chemical ionization mass spectrometry , 7. Chromatogr. Ill 91-98 (1997). 

Figure 15.3 Separation of tricyclic antidepressants by using multidimensional LC-LC. Peak identification is as follows DOX, doxepin DES, desipramine NOR, noitryptylene IMI, imipramine AMI, amiti yptyline. Adapted from Journal of Chromatography, 507, J. V. Posluszny et al., Optimization of multidimensional high-performance liquid cliromatography for the deterTnination of drugs in plasma by direct injection, micellar cleanup and photodiode array detection , pp. 267 - 276, copyright 1990, with permission from Elsevier Science.

Determined by HPLC using diode array detection of a sample taken directly from the reaction mixture. b (< = 1, CHClj). 

Watmough, N.J.. Turnbull, D.M., Sherratt. H.S.A. Bartlett. K. (1989). Measurement of the acyl-CoA intermediates of p-oxidation by hplc with on-line radiochemical and photodiode-array detection. Application to the study of [U- C]hexadecanoate by intact rat liver mitochondria. Biochem. J. 262,261-269. 

Keller, H. R. and Massart, D. L., Artefacts in Evolving Factor Analysis-Based Methods for Purity Control in Liquid Chromatography with Diode-Array Detection, Ana/yt/ca Chimica Acta 263, 1992, 21-28. 

NURMI T, ADLERCREUTZ H (1999) Sensitive HPEC method for profiling phytoestrogens using coulometric array detection application to plasma analysis, Analytical Biochemistry, 274, 110-17. 

GUO c, CAO G, SOFIC E and PRIOR R L (1997) High-performance liquid Chromatography coupled with coulometric array detection of electroactive components in fruits and vegetables Relationship to oxygen radical absorbance capacity, J Agric Food Chem, 45, 1787-96. 

MATTILA p and KUMPULAINEN J (2002) Determination of free and total phenolic acids in plant-derived foods by HPLC with diode-array detection, JAgric Food Chem, 50, 3660-67. 

Hong, V. and Wrolstad, R.E., Use of HPLC separation/photodiode array detection for characterization of anthocyanins, J. Agric. Food Chem., 38, 708, 1990. Osmianski, J. and Lee, C.Y., Isolation and HPLC determination of phenolic compounds in red grapes. Am. J. Enol. Vitic., 41, 204, 1990. 

Muller, H., Determination of the carotenoid content in selected vegetables and fruit by HPLC and photodiode array detection, Z. Lebensm. Enters. Forsch. A, 204, 88, 1997. 

Ferruzzi, M.G. et ah. Analysis of lycopene geometrical isomers in biological microsamples by liquid chromatography with coulometric array detection, J. Chromatogr. B Biomed. ScL Appl. 760, 289, 2001. 

Cortes et al.. Identification and quantification of carotenoids including geometrical isomers in fruit and vegetable juices by liquid chromatography with ultraviolet-diode array detection, J. Agric. Food Chem., 52, 2203, 2004. 

Hong, V. and Wrolstad, R.E., Use of HPLC separation/photodiode array detection for characterization of anthocyanins, J. Agric. Food Chem., 38, 708, 1990. 

Chirinos, R. et al., High-performance liquid chromatography with photodiode array detection (HPLC-DAD)/HPLC-mass spectrometry (MS) profiling of anthocyanins from Andean mashua tubers (Tropaeolum tuberosum Ruiz and Pavon) and their contribution to the overall antioxidant activity, J. Agric. Food Chem., 54, 7089, 2006. 

Hvattum, E., Determination of phenolic compounds in rose hip (Rosa canina) using liquid chromatography coupled to electrospray ionisation tandem mass spectrometry and diode-array detection, Rapid Commun. Mass Spectrom., 16, 655, 2002. 

Saito, K., A new enzymatic method for extraction of precarthamin from dyer s saffron (Carthamus tinctorius) florets, Z. Lebensmitt. Untersuch. Forsch., 197, 34, 1993. Cserhati, T. et ah. Separation and quantitation of colour pigments of chili powder (Capsicum frutescens) by high-performance liquid chromatography-diode array detection, J. Chromatogr. A, 896, 69, 2000. 

Spectrophotometric resolution for the discrimination of individual colorant molecules found in mixtures is lower than that of chromatographic techniques such as TLC or HPTLC and even low-cost paper chromatography. More expensive but more accurate determinations may be made by RP-HPLC, IP-HPLC with UV-Vis, and diode array detection. ... 

Recently a new method was developed for the complete liquid chromatographic separation and diode array detection of standard mixtures of the 14 most frequently used synthetic colorants. Protocols for RP-HPLC - " and IP-HPLC techniques have been extensively described and the techniques were compared with micellar electrokinetic capillary chromatography, - which has been shown to be suitable for the analysis of synthetic colorants. 

Five synthetic and five natural colorants were identified and quantified in lyo-philized dairy products and fatty foods using an automatic method based on solid phase extraction using a stationary phase followed by RP-HPLC C,g columns for the sequential retention of colorants and diode array detection. Lyophilization of the samples coupled with the separation procedure provided clean extracts despite the complexity of the food matrices and preserved the sample for at least 2 months without changes in colorant concentrations. The detection limits achieved for the colorants were found in a wide range from 0.03 to 75 pg/g of the lyophilized sample, according to the limits established by the European Union. ... 

Diode Array Detection in HPLC, edited by Ludwig Huber and Stephan A. George... 

J.K. Strasters, H.A.H. Billiet, L. de Galan.B.G.M. VandeginsteandG. Kateman, Evaluation of peak-recognition techniques in liquid chromatography with photodiode array detection. J. Chromatog., 385 (1987) 181-200. 

B.G.M. Vandeginste, R. Essers, T, Bosman, J. Reijnen and G. Kateman, Three-component curve resolution in HPLC with multi wavelength diode array detection. Anal. Chem., 57 (1985) 971-985. 

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