Diode array detector sensitivity

Photodiodes produce an electric field as a result of pn transitions. On illumination a photocurrent flows that is strictly proportional to the radiation intensity. Photodiodes are sensitive and free from inertia. They are, thus, suitable for rapid measurement [1, 59] they have, therefore, been employed for the construction of diode array detectors. 

Diode array detectors consist of silicon integrated circuit (IC) chips incorporating up to one or two hundred pairs of photodiodes and capacitors. Each photodiode measures about 0.05 x 0.5 mm and is sensitive to... 

Peak purity tests are used to demonstrate that an observed chromatographic peak is attributable to a single component. Mass spectrometry is the most sensitive and accurate technique to use for peak purity evaluation because of the specific information derived from the analysis. However, a good number of HPLC methods use mobile phase conditions that are incompatible with mass spectrometry detection. In this case, PDA spectrophotometers using peak purity algorithms may be used to support the specificity of the method. Almost all commercially available diode array detectors are equipped with proprietary software that will perform these calculations. Although this technique is more universal in application to HPLC methods, the data provided is neither particularly... 

Some spectrophotometric techniques work to enhance sensitivity or utility in other ways. The advent of semiconductor diode array detectors permits entire spectra to be acquired simultaneously instead of one wavelength band at a time. Also, automated spectrophotometric analyzers originally developed for clinical use have been adapted for use at sea when many samples... 

Ostermeyer (87) described the rapid and sensitive determination of acesulfame, saccharin, aspartame, SA, and BA in fishery products. Aspartame is determined directly in the aqueous extract of samples. For the other additives, cleanup of the extract is performed by anion-exchange chromatography. Ostermeyer used a Nucleosil 100-5 C lg (Macherey-Nagel) (250 X 4 mm, 5 /xm ) column and 0.02 mol/L KH2P04 acetonitrile as mobile phase and 219 nm detection using a diode array detector. Recoveries of spiked samples at concentrations of 50-400 mg sweetener/kg and 50-4000 mg preservative/kg were in the range 84-102% for all analyzed substances (87). The HPLC conditions for the determination of OAs are summarized in Table 5. 

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. 

The need for a reverse-field polarity power supply is at least two-fold 1) It permit a complete spectral analysis of the substance under study. By reversing field polarity, the substance zones can be run forward and backward in front of the detector as many times as needed. Incremental changes as small as 1- or 2-nm in wavelength can be used to maximize instrumental sensitivity, thus allowing coverage of the entire spectral range. In fact, this feature provides the same functions as a diode array detector, albeit somewhat slower. Proteins and peptides have almost identical spectral characteristics, however, when other functional groups are attached to it, is possible to observe more than one maximal absorbance peak. For example,... 

Spectroscopic detectors measure partial or complete energy absorption, energy emission, or mass spectra in real-time as analytes are separated on a chromatography column. Spectroscopic data provide the strongest evidence to support the identifications of analytes. However, depending on the spectroscopic technique, other method attributes such as sensitivity and peak area measurement accuracy may be reduced compared to some nonselective and selective detectors. The mass spectrometer and Fourier transform infrared spectrometer are examples of spectroscopic detectors used online with GC and HPLC. The diode array detector, which can measure the UV-VIS spectra of eluting analytes is a... 

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