Detector spiking

Peak width definition (threshold) Lets the computer know how to distinguish between true peaks and detector spikes. 

With conventional nonspectroscopic detectors, other methods must be used to identify the solutes. One approach is to spike the sample by adding an aliquot of a suspected analyte and looking for an increase in peak height. Retention times also can be compared with values measured for standards, provided that the operating conditions are identical. Because of the difficulty of exactly matching such conditions, tables of retention times are of limited utility. 

Caffeine in coffee, tea, and soda is determined by a solid-phase microextraction using an uncoated silica fiber, followed by a GC analysis using a capillary SPB-5 column with an MS detector. Standard solutions are spiked with G3 caffeine as an internal standard. 

Figure 9 Three-dimensional cationic CITP of (A) blank (B) lysozyme (LYSO), creatinine (CREAT), conalbumin (CAL), y-amino-n-butyric acid (GABA), and ovalbumin (OVA) (C) OVA spiked with CREAT and GABA. Capillary 90 cm (length to the detector, 70 cm) x 75 p i.d. leader 10 mM potassium acetate and acetic acid with 0.3% HPMC, pH 4.75 terminator 10 mM acetic acid sample 10 to 30 mg/ml proteins dissolved in leader without HPMC voltage 20 kV. (From Gebauer, P. and Thormann, W.,. Chromatogr., 558, 423, 1991. With permission.)...

Consider you have forgotten to switch on multi-read 28 with your CCD detector and the raw data are full of cosmic-ray spikes. How do you remove them without spoiling the image ... 

The linearity of the detector can be obtained by diluting the analyte stock solution and measuring the associated responses, while the linearity of the analytical method can be determined by making a series of concentrations of the analyte from independent sample preparations (weighing and spiking) [15]. It is also essential that the basic calibration linear curve be obtained by using independent samples, and not by using samples that have been prepared by dilution and injected into HPLC/GC, or spotted on one TLC plate. 

Van Hall, S afr anko, and Stenger [ 51 ] have also pointed out that strong brines interfere with the method by producing fogs which maybe counted as carbon dioxide, while in cases where the flame ionisation detector is being used, large spikes appear in the recorded curve [105]. 

Lee [42] determined pentachlorophenol and 19 other chlorinated phenols in sediments. Acidified sediment samples were Soxhlet extracted (acetone-hexane), back extracted into potassium bicarbonate, acetylated with acetic anhydride and re-extracted into petroleum ether for gas chromatographic analysis using an electron capture or a mass spectrometric detector. Procedures were validated with spiked sediment samples at 100,10 and lng chlorophenols per g. Recoveries of monochlorophenols and polychlorophenols (including dichlorophenols) were 65-85% and 80-95%, respectively. However, chloromethyl phenols were less than 50% recovered and results for phenol itself were very variable. The estimated lower detection limit was about 0.2ng per g. 

If the perturbations are in the form of spikes of an irregular nature, the problem is likely to be detector contamination. Such spikes are especially observed when dust particles have settled into the FID flame orifice. Of course, the problem may also be due to interference from electrical pulses from some other source nearby. Regular spikes can be due to condensation in the flow lines causing the carrier, or hydrogen (FID), to pulse, or they can be due to a bubble flow meter attached to the outlet of the TCD, as well as the electrical pulses referred to above. Baseline perturbations can also be caused by pulses in the carrier flow due to a faulty flow valve or pressure regulator. 

LASs are UV absorbing compounds and detection is usually performed using an UV detector operating at a wavelength of 214 nm. Limits of detection (LOD) of about 1 p,g L-1 [8] in spiked ground water after 200-fold enrichment and 10 p,gL 1 [4] for wastewater (100-fold enrichment) were achieved. 

Quantitative data can be obtained by means of the use of an internal or external standard, whose exact concentration can be checked by means of spectrophotometric detector, which measures spe-cihc absorbance or molar absorbance. Spiked authentic samples can be used to measure the limit of detection (LOD) and the limit of quantihcation (LOQ). 

Detector sensitivity is one of the most important properties of the detector. The problem is to distinguish between the actual component and artifact caused by the pressure fluctuation, bubble, compositional fluctuation, etc. If the peaks are fairly large, one has no problem in distinguishing them however, the smaller the peaks, the more important that the baseline be smooth, free of noise and drift. Baseline noise is the short time variation of the baseline from a straight line. Noise is normally measured "peak-to-peak" i.e., the distance from the top of one such small peak to the bottom of the next. Noise is the factor which limits detector sensitivity. In trace analysis, the operator must be able to distinguish between noise spikes and component peaks. For qualitative purposes, signal/noise ratio is limited by 3. For quantitative purposes, signal/noise ratio should be at least 10. This ensures correct quantification of the trace amounts with less than 2% variance. The baseline should deviate as little as possible from a horizontal line. It is usually measured for a specified time, e.g., 1/2 hour or one hour and called drift. Drift usually associated to the detector heat-up in the first hour after power-on. 

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