Sensitivity and Detection Limits

The discrepancy in sensitivity of the GC-UV method, given by detection limits in Table 2, can primarily be derived from the differences in the geometry of the gas flow cell and from the separation columns (packed or capillary) used.

In comparison with the infrared wavelength region, the absorptivities are up to 1000 times higher in the UV wavelength region. Also, the fact that the noise level could be kept equal to or lower than for FTIR detectors ought to imply a considerably more favourable signal-to-noise ratio. The specification concerning the noise level, along the time scale, given

Figure 6 A contour plot showing the result obtained at the analysis of a petroleum product: the second derivatives are plotted and, as indicated, the single, double and polyaromatic compounds appear clearly.

for the GC-UV Inscan 175 spectrophotometer, is about 0.5 x 10~4 peak to peak.

For a detection limit defined as the amount giving rise to an absorption of 1.5 x 10~4 AU, and expressed as the minimum amount of the compound per second that could be detected, the following values were calculated: perchloroethylene 55 pg s_1, naphthalene 3.3 pgs"1, mesitylene 8.9 pgs"1, carbon disulfide 4.8 pg s_ 1, acetone 49 pg s_ 1 and benzene 5.4 pg s_ 1.

The noise level can be lowered by smoothing and by taking the average absorption values within a wavelength range. However, it has been shown that this noise reduction is most effectively obtained using average derivatives values. The derivatives are in these cases differentials with a rather large gap value

(2-6 nm) along the wavelength scale. The noise level obtained, using this mode, was & 1 x 10~5 AU peak to peak, which gives a calculated detection limit for naphthalene of ~0.5pgs~1.

The limit of quantification (MIQ: minimum identification quantity) is usually defined as the lowest quantity when the hit rate of the unknown is still found among the first five proposals. This depends on the size of the spectrum library and, at this stage, the available reference spectrum library is probably not large enough to make any closer estimations. However, clear identifications can be made in the mid pg range, provided that the compound possesses a functional group with relatively high absorptivity values (e.g. aromatics). For the GC-UV method the introduction of an additional sensitivity level, namely a minimum of classification quantity (MCQ), appears to be desirable. The suggestion is to define MCQ as the minimum quantity necessary for giving the right functional group at the first library search proposal.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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