Photo-Diode-Array

JUSTESEN u, KNUTHSEN p and LETH T (1998) Quantitative analysis of flavonols, flavones, and flavanones in fruits, vegetables and beverages by high-performance liquid chromatography with photo-diode array and mass specfrometric detection, /C/u matogr A, 799, 101-10. 

Photo diode array Inaccurate wavelength Sensitivity and linearity problems. 

Fig. 1.14 LC-MS analysis of a mixture of daidzein and acetophenone with a ESC source (A) ESI, (B) APCI, (C) photo diode array detection.

Modern drug discovery approaches involve HTS, where, applying full automation and robotics, hundreds of molecules can be screened using several assays within a short time, and with very little amounts of compounds. In order to incorporate natural products in the modern HTS programmes, a natural product library (a collection of dereplicated natural products) needs to be built. Dereplication is the process by which one can eliminate recurrence or re-isolation of same or similar compounds from various extracts. A number of hyphenated techniques are used for dereplication, e.g. LC-PDA (liquid chromatography-photo-diode-array detector). 

The variety of detection modes available for HPLC analysis that provide additional information about the eluent as it exits the column greatly facilitates unknown characterization. The majority of analytical methods for phenolic compounds includes HPLC with spectrophotometric-based detection techniques (UV absorption, fluorescence, photo diode array—PDA) as well as HPLC with electrochemical detection. 

Amakura Y, Okada M, Tsuji S, Tonagai Y. 2000. High performance liquid chromatographic determination with photo diode array detection of ellagic acid in fresh and processed fruits. J Chromatogr A 896 87-93. 

The use of LC also has increased in use in recent years, driven by greater sensitivities of the detectors. Traditional ultraviolet (UV) and photo diode-array detectors were frequently employed in triazine analysis, but advances in source designs have provided efficient coupling of MS with LC. The advantage of LC is the ability to analyze polar metabolites not amenable to analysis using GC. Recent progress in LC/MS/MS instrumentation has enabled the direct aqueous injection (DAI) of a water sample without prior cleanup. 

Figure 10. Spectral image as seen by the photo diode array camera...

Figure 12. Fiber optic image rotator and photo diode array assembly...

Csillag K, Vereczkey L, Gachalyi B (1989) Simple high-performance liquid chromatographic method for the determination of tolbutamide and its metabolites in human plasma and urine using photo-diode array detection. J Chromatogr 490 355-363... 

Zgorka, G. Dawka, S. 2001. Application of conventional LW, photo-diode array (PDA) and fluorescence (FL) detection to analysis of phenolic acids in plant material and pharmaceutical preparations. J. Pham. Biomed. Anal. 24 1065-1072. 

Welkera (2002) developed an HPLC method to analyze enviromnental samples for their content of cylindrospermopsin (CYL) based on HPLC with photo diode array detection as an alternative to costly LC-MS approaches. A gradient from 0% to 50% aqueous methanol(+0.05% trifluoroacetic acid) in 20 minntes proved to be highly reproducible with respect to peak height, peak area, and retention time of pnrified CYL. Good linearity of peak area response was found for 1-300 ng CYL on colnmn. For a good performance, the duration of equilibration prior to individual runs was crucial. Extraction from cell material (culture and bloom) was efficiently done with pure water in one extraction step, and CYL contents determined matched well with results previously obtained by LC-MS. 

The easiest way to perform specificity for any HPLC method is to perform this test in conjunction with a forced decomposition study. The utilization of mass spectrometry (MS) detector (in series) after a Photo Diode Array (PDA) detector to obtain more information is encouraged (in terms of mass-to-charge ratio of parent ions, initial fragmentation pattern, and peak purity). 

Charge Coupled Devices (CCD), and Photo Diode Arrays (PDA).. . 89... 

Another kind of linear solid state position sensitive detectors are the Photo-Diode-Arrays (PDA s), which are different from CCD s. A PDA consists of an array of separate photodiodes, each with an associated capacitance and a multiplexing read-out system (see Fig. 21). The charges collected in each cell are simply switched to the output, one by one. Unlike in the case of CCD s, the photosensitive elements are separated completely from the transfer circuity. 

The selection of an appropriate mobile phase is another aspect of concern. Most often the commonly used LC mobile phase is not compatible with thermospray LC/MS, for instance owing to the use of non-volatile buffers can be left out or replaced by volatile ones. In other cases the buffers are present for retention time reproducibility, which mostly is not very important for identification. In other applications however a correspondence between UV or fluorescence peaks and MS identification is obligatory, which makes mobile phase changes unattractive. In this respect it is often overlooked that LC-UV and LC/MS give different responses as a result of different detection principles. For not too complex samples a UV photo-diode array detector can be used to link up the chromatographic peaks obtained under different mobile phase conditions. To cut short, despite many successes also many potential problems are met in LC/MS to which tailor-made creative solutions are needed. 

Laboratory instruments based on photo-diode arrays (PDAs) are not commonplace. The trend seems to be application in process control to avail the analyst of their rapid response. 

Several very small photo-diode arrays are commercially available. The first commercial units were built by Ocean Optics (Dunedin, FL), but they are now available from several sources. These devices are based on the light (postsample, either diffusely reflected or transmitted) impinging upon a very small fixed grating then onto a series of diodes with minimum spacers between them. While the number of diodes and resolution of the array is limited, the small size and high speed of this type of instrument may compensate for the lack of resolution in a variety of applications and may provide certain advantages in the appropriate setting. 

Agilent 1100 liquid chromatographic system equipped with thermostated autosampler, binary pump, photo diode array detector and Chemstation software (Agilent, USA). 

Quantitation of 1,4BD by high-performance liquid chromatography (HPLC) with photo-diode-array detection was described by Duer et al. (2001). The 1,4BD was extracted from the biological matrix with acetonitrile and derivatized with 3.5-dinitrobenzoyl chloride. This method has a linear range from 3 to 250 rng L with a limit of quantitation of 10 rng L . Also, as was mentioned previously, the United Chemical Technologies GHB SPE column method has been modified to include 1,4BD analysis (Kraner et al., 2000). 

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