Chromatogram combination
from differing instruments, differing modes of analysis,
even including non chromatographic data measured by any physical instrument

The concept is shown in one example: it covers high precision natural gas analysis complete from methane to hydrocarbons C₁₂, inert gases, one polar organic trace, one highly polar inorganic trace, including all important physical values like gas density and all calorific values.
The repeatability standard deviation “s” reaches +- 0.002 mole-% absolute for methane using 4 repetitions. The accuracy for the calorific values is better than 0.02 % absolute.

Concept:

The gas sample is taken under main line pressure of 30 to 60 bar directly without any pressure reducer or any other large volume instrumentation into a high safety gas cylinder three times. The sample cylinder finally contains 500 ml sample under main line pressure which shows no sample taking error larger than detectable by a analysis quality of s = + - 0.002 % absolute for methane. The sample temperature is adjusted to + 20 degr. C and delivers a constant micro gas flow through the inlet system of a micro process GC instrument. Here a series of at least five consecutively repeated chromatograms are taken and the quantitative data for the inert gases and the hydrocarbons up to C6 integrated and stored in a process GC EXPORT data file. This file contains besides GC data also calculated physical gas quality values. The Micro GC uses two differing capillaries under isothermal conditions and uses two micro detectors. The substance values are automatically calibrated and electronically stored as mole-%. The physical data are calculated under user defined units based on standard pressure and standard temperature. 15 data lines are automatically written for each single run into a Process GC EXPORT file
(= PGC file). Its data structure differs necessarily from the chromatography standard data structure in normal chromatography EXPORT files because of the strict limitation for values from only 11 chemical substances (including one substance group called C₆plus) and 4 physical data. This PGC file structure is shown in Table 1 below.

 Natural gas contains hundreds of further substances not taken by the micro process GC instrument. These are hydrocarbons above C₆ and not ending at C₄₀, but above C₁₂...C₁₄ in very low concentrations only. Their measurement is necessary only for very special reasons because of slow long range effects in wide ranging gas line systems. Besides hydrocarbons natural gas may contain alcohol traces (methanol, diols, triols) and water at trace concentration. The alcohol and the water concentrations are of technical importance thus their value is needed. The higher hydrocarbons in the range C₆ to C₁₂ identifies the gas source. It is considered as the individual fingerprint of a gas type.
There are further traces in natural gas including solid micro particles and higher poly aromatic hydrocarbons as complex as in raw mineral oils, but at so low concentrations, that their quantitation is outside the needs of a daily routine task. They can be measured by PLC. PLC is applicable to gas analysis

The alcohols and the higher hydrocarbons are quantitized by temperature programmed capillary gas chromatography using a flame ionization detector. In case the gas contains sulfur compounds, a further extra gas chromatographic analysis is needed using sulfur specific detection.

The water concentration is measured only in constantly flowing gas under line pressure because of otherwise serious data falsification. Water traces can be correctly quantitized in non polar gases using physical modes of analysis. The special instruments deliver water values in many differing units but can give results also in grams per m³. Knowing the gas analysis results the water data are transformed into mole-%.

The temperature programmed capillary GC data are automatically written into a standard EXPORT file, but in order to get qualified trace quantities for the higher hydrocarbons and correct alcohol values, the GC analysis needs high sample volumes. This falsifies the values for methane, even for ethane and propane and thus of course also all higher hydrocarbon trace values. This is a typical error caused by oversampling. The values for methane are necessarily far outside the quantitative working range.
 

No Problem at all !

In the PGC EXPORT file data set of the micro process GC are calibrated values for the mole-% of methane, ethane, propane.
In the Standard GC EXPORT file data are values of the higher hydrocarbons, which are RELATIVELY correct, that is: the relation of the trace compounds to each other are accurate, as these signals are NOT outside the quantitative working range. The detector, signal amplifier and integration program reported correct relative data. It is only necessary, to insert the correct process GC values C₁, C₂, C₃ into the FID based EXPORT data file.  This is a most simple task for an evaluation software, which combines two EXPORT data sources. It is only necessary to assure, that the sample taken by the process micro GC is absolutely equal to the sample taken by the temperature programmed FID capillary instrument. The same way other analytical values are integrated: the water concentration, possible higher alcohols, possible sulfur compounds. The water value must be taken in the minute the last sample flush was given to the external gas sample cylinder. With this procedure we can transmit the very high accuracy and precision from the calibrated micro process GC analysis into the complete gas analysis data, although the flame ionization detector quantitation attached to a thick film 15 m capillary GC analysis would never offer a repeatability standard deviation of +- 0-002 % for methane and thus for the natural gas energy data. However by automatic replacement of wrong FID values by the correct and calibrated PGC values for the main compounds the very high level of repeatability and the excellent level of accuracy because of calibration at a +- 0.002 % absolute standard deviation pushes the complete and combined analytical values to the quality level of the non complete process GC by micro capillary chromatography. We needed some time until we understood the basis of this fact.

Very high accuracy and precision for natural gas analysis pays back, as energy prizes are high and growing.

In flowing gas a very high accuracy and precision of quantitative analytical data is normally not possible, as the gas composition is not constant. The opposite statement results from less high accuracy and precision. We got immediately a factor ten less good precision when changing the sample source from static (gas cylinder) to dynamic (gas line). Equal findings exist in the water trace analysis: river water in (sorption stabilized) bottles can be analyzed quite well reproducible. On-line river water trace analysis by direct connection from the river to the water analyzer is less well qualified.
This is in full agreement with the basic conditions we found during the development of the
“error detector sf4” and shows, how sensitive the sf4-standard deviation values react on smallest changes in the sample composition during a consecutively repeated series run.

Table 1: PGC EXPORT data file, fundamentally differing from a standard GC EXPORT file

Table 2: FID capillary GC  EXPORT data file, a standard GC EXPORT file. Only the first 7 data lines from a total of 62 lines are shown:

Table 3: combined EXPORT data files, water inserted, values are used for a series of reports upon demand, including figure 4 below. NOTE, that “area-%” is replaced by “mole-%”

Graphics reproduction of parts of the combined data set as a LOG / LIN line chromatogram.
Y-axis = mole-% containing the calibrated quantitative data of the natural gas analysis.
X-axis = linear retention time positions of the peaks from the capillary gas chromatogram of the over sampled but by EXPORT file combination data .
Practically the time scale is a temperature programmed one and in fact LOG with respect to the substance type is the correct scaling.
For technical reasons the reproduced line chromatogram graphics does not show the real peak density of 93 traces, which represent the gas fingerprint for source identification. For a real critical gas type comparison the line chromatogram graphics is the best basis. Figure 4 below is only part of the complete one.