Chromatograms, instrument quality

The usefulness of broad spectrum analysis is based upon being able to observe the changes in water quality data represented by differences between chromatograms. Thus, the data analysis involves the interpretation of large quantities of chromatographic data at one time. Therefore, sample and instrument quality assurance becomes extremely critical for reliable comparison of interchromatographic data (J). 

True profile analysis requires scanning over the whole mass range for the acquisition of all data on excreted compounds. Quantitation has been more challenging on a quadrupole instrument because total ion current peaks are seldom a single component and extracted-ion chromatograms (EICs) when recovered from scanned data are of poor quality due to the lower sensitivity of scanning GC-MS. Thus, we developed profile analysis based on SIM of selected analytes but tried to ensure the components of every steroid class of interest were included. For ion traps the fundamental form of data collection (in non-MS/MS mode must be full -scans). Thus, the quantitative data produced are EICs obtained from scanned data. The EICs are of the same ions used for SIM in quadrupole instruments and the calibration external standards are the same. 

Simpler instrumentation may be used and the effect of the quality of instrumentation on the resulting chromatogram is reduced. 

Although classical calibration is widely used, it is not always the most appropriate approach in chemistry, for two main reasons. First, the ultimate aim is usually to predict the concentration (or independent variable) from the spectrum or chromatogram (response) rather than vice versa. The second relates to error distributions. The errors in the response are often due to instrumental performance. Over the years, instruments have become more reproducible. The independent variable (often concentration) is usually determined by weighings, dilutions and so on, and is often by far the largest source of errors. The quality of volumetric flasks, syringes and so on has not improved dramatically over the years, whereas the sensitivity and reproducibility of instruments has increased manyfold. Classical calibration fits a model so diat all errors are in the response [Figure 5.4(a)], whereas a more appropriate assumption is that errors are primarily in the measurement of concentration [Figure 5.4(b)]. 

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