Spectrum diode array

In one instrument, ions produced from an atmospheric-pressure ion source can be measured. If these are molecular ions, their relative molecular mass is obtained and often their elemental compositions. Fragment ions can be produced by suitable operation of an APCI inlet to obtain a full mass spectrum for each eluting substrate. The system can be used with the effluent from an LC column or with a solution from a static solution supply. When used with an LC column, any detectors generally used with the LC instrument itself can still be included, as with a UV/visible diode array detector sited in front of the mass spectrometer inlet. 

A more definitive identification may be obtained by combining retention characteristics with more specific information from an appropriate detector. Arguably, the most information-rich HPLC detectors for the general identification problem are the diode-array UV detector, which allows a complete UV spectrum of an analyte to be obtained as it elutes from a column, and the mass spectrometer. The UV spectrum often allows the class of componnd to be determined but the... 

Diode-array UV detector A UV detector which monitors all wavelengths simultaneously and therefore allows a complete UV spectrum to be obtained instantaneously. The alternative, a dispersive UV detector, monitors one wavelength at a time and thus requires a considerable amount of time to record a complete spectrum. 

Photodiodes are the modem analogues to photocells. They increase their electrical resistance under light impact which, as part of an electric circuit, can be measured easily. Many current instruments display diode arrays instead of a single diode. Tens of photodiodes are arranged in a tight area. They are exposed to the sample bound spectrum where they respond to the color that corresponds to their positions in the diode array. A rapid, periodically performed electrical interrogation of all diodes (sequence periodicity in the order of milliseconds) reveals a quasi-stationary stable spectrogram. More sophisticated than photodiodes are phototransistors. They amplify internally the photoelectric effect, but the sensitivity of a photomultiplier cannot be achieved. 

UV detection, diode-array detector (DAD) and fluorescence have been the detection techniques used, coupled to HPLC for the analysis of OTC. UV detection is set at 355 nm [49-51], 350 nm [40], or at 353 nm [52], Using the diode array detector [49] offers advantages that the target peak can be identified by its retention time and absorption spectrum. Compared to UV detection, fluorescence detection is generally more specific and is less interfered by other compounds in the sample matrix [51]. A HPLC method with electrochemical detection has also been suggested recently. Zhao et al. [53] described HPLC with a coulometric electrode array system for the analysis of OTC, TC, CTC, DC, and methacycline (MC) in ovine milk. An amper-ometric detection coupled with HPLC was developed by Kazemifard and Moore [54] for the determination of tetracyclines in pharmaceutical formulations. 

In order to observe a short-lived species it may be necessary to employ a rapid-scanning spectrometer, such as a diode-array instrument (Sms for a 240nm-800nm spectrum). In addition, the absorbances of electrogenerated species can be very small and signal-averaging or phase-sensitive detection may be necessary to achieve the required signal-to-noise ratio (cf. EMIRS and FTIR). 

FIGURE 13.9 The HPLC diode array UV absorbance detector. When a mixture component elutes from the column, not only the chromatography peak but the entire UV absorption spectrum for that component can be recorded. 

A diode array spectrophotometer is one that utilizes a series of photodiodes to detect the fight intensity of all wavelengths after the fight has passed through the sample. See Figure 8.9. The advantage is that an absorption spectrum can be measured in a matter of seconds. 

For the characterisation of the biodegradation intermediates of C12-LAS, metabolised in pure culture by an a-proteobacterium, Cook and co-workers [23] used matrix-assisted laser desorption/ionisation (MALDI)-time of flight (TOF)-MS as a complementary tool to HPLC with diode array detection and 1H-nuclear magnetic resonance. The dominating signal in the spectrum at m/z 271 and 293 were assigned to the ions [M - H] and [M - 2H + Na]- of C6-SPC. Of minor intensity were the ions with m/z 285 and 299, interpreted to be the deprotonated molecular ions of C7- and C8-SPC, respectively. 

Figure 3.7 Diode array detector. Light from the lamp passes through the flow cell and to a holographic reflectance grating and the resulting spectrum is focused on the diode array. Detectors frequently have a spectral range of 190-800 nm and can offer bandwidths as low as 1.0 nm.

A general approach to the analysis of multicomponent analytes bearing chromophores was demonstrated with a mixture of nitrophenylhydrazines (250). In a FLA system the mixture was preconcentrated by SPE on Q x bonded silica, followed by desorption with a buffer and detection by UW on a diode array. The spectrum, resolved for three components, had RSD 1.43% for 11 samples containing 2 x 10 5 M of 250c. The method allowed up to 40 samplings per hour527. 

Figure 8.5. Palm of the hand. Fluorescence spectrum (Fb-mode, full line) and diffuse reflectance absorption spectrum (dashed line, normalized to X = 700 nm, reference filter paper) measured with a sensitized diode array spectrometer.

The first measurement we make when starting a fluorescence study is not usually a fluorescence measurement at all but the determination of the sample s absorption spectrum. Dual-beam differential spectrophotometers which can record up to 3 absorbance units with a spectral range of 200-1100 nm are now readily available at low cost in comparison to fluorimeters. The wide spectral response of silicon photodiode detectors has made them preeminent over photomultipliers in this area with scan speeds of a few tens of seconds over the whole spectral range being achieved, even without the use of diode array detection. 

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