Quantitative Analysis There are three important stages in a GC analysis, 1. The preparation of the sample. 2. The development of the separation and the production of the chromatogram 3. The processing of the data and the presentation of the results. Each stage is equally important and if not carried out correctly the results will be neither precise nor accurate. Sample preparation can be very simple involving no more that diluting a known weight of

Relative to Actual Separation Ratio for Two Closely Eluting Peaks    In contrast, the separation ratio need only be in excess of about 1.035 on the high efficiency columns (10,000 theoretical plates), before accurate retention measurements can be made on the composite curve. It follows that,: Considerable care must be taken when accessing closely eluting peaks. If the resolution is inadequate, measurements must be taken on the individual solutes, chromatographed separately. Quantitative Analysis from Retention Measurements A consequence of the above discussion on composite peak envelopes is that if the retention times of a pair of solutes are accurately known, then the measured retention time of the composite peak will be related to the relative quantities of each solute present. It follows that an assay of the two components could be obtained from accurate retention measurements only. This method of analysis was shown to be feasible and practical by Scott and

bacterial action, aging, rancidity or decomposition. In addition, tests that identify the area or country in which the food was processed or grown may also be needed. The source of many plants (herbs and spices) can often be identified from the peak pattern of the chromatograms obtained directly from head space analysis. Similarly, unique qualitative and quantitative patterns from a GC analysis will often help identify the source of many alcoholic beverages. Unfortunately, food analysis involves the separation and identification of very complex mixtures and the difficulties are compounded by the fact that the components are present at very low concentrations. Thus, gas chromatography is the ideal (if not only) technique that can be used successfully in food and beverage assays and tests. The potential carcinogenity of the aromatic hydrocarbons make their separation and analysis extremely important in environmental testing. However, the aromatics can pose

for the separation of hydrocarbons (e.g. OV101, which is also a polyalkyl-siloxane, is widely used in packed columns). The flow velocity of 20 cm/sec., appears to have been taken from the ratio of the column length to the dead time. Thus, due to the pressure correction the actual effective linear velocity would be much less than that (see Dispersion in Chromatography Columns ). Helium was used as the carrier gas which was necessary to realize the high efficiencies with reasonable analysis times. The FID detector provided the necessary to wide quantitative dynamic range. The column temperature was held at 35˚C for 15 min. to effect the separation of the low boiling, low molecular weight hydrocarbons, the temperature was then increased to 200˚C at 2˚C/min. and finally held at 200˚C for 5 min. to ensure the complete elution of the higher boiling components. An excellent separation is obtained giving clearly separated peaks for the marker

to automation either appropriately designed hard-wired equipment or by the use of a laboratory robot. The hard wired device is generally inflexible, the laboratory robot, on the other hand, can be programmed to carry out many different types of analysis. The separation itself has some interesting properties. Free acids are very readily adsorbed onto active sites on the support which can result in very asymmetric peaks and, as a result of the strong adsorption, significant quantitative losses can occur. In the above example, the effect of the adsorptive sites on the support is reduced by blocking them with phosphoric acid. Phosphoric acid is very involatile and thus can tolerate the high temperature and although it is active enough to block the adsorption sites it is not active enough to cause sample decomposition. It is seen that the peaks exhibit excellent symmetry for free acids. Teraphthalic acid has also been used for this purpose to deactivate the

nbsp; The quantitative evaluation of seven duplicate analyses is shown in Table 5.   Table  5.  Reproducibility of the Quantitative Analysis by Peak Area Normalization of a Five Component Synthetic Mixture (Isocratic High Speed Separation) Compound          Normalized Peak Area        Mean Mean s s % p-Xylene 9.8 9.8 9.6 9.8 9.8 9.6 9.7 0.10 1.1   Anisole