GPA 2261-13天然气组分分析-GC法

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FOREWARD

GPA 2261 provides the gas processing industry a method for determining the chemical composition of natural gas and similar gaseous mixtures using a Gas Chromatograph (GC).

The precision statements contained in this standard are based on the statistical analysis of round-robin laboratory data obtained by Section B.

This standard was developed by the cooperative efforts of many individuals from industry under the sponsorship of GPA Section B, Analysis and Test Methods.

Throughout this publication, the latest appropriate GPA Standards are referenced

“Copyright? 2013 by Gas Processors Association. All rights reserved.

No part of this Report may be reproduced without the written consent

of the Gas Processors Association.”

1. SCOPE

1.1 This standard covers the determination of the chemical composition of natural gas and similar gaseous mixtures within the ranges listed in Table 1, using a Gas Chromatograph (GC). The three columns represent the original Table 1, but separate the values to three distinct groups. The first group is concentrations lower than the data obtained from the round-robin project (RR-188). The second group is concentrations used in the round-robin project (RR-188). The equations listed in the precision statement in this standard cover the range listed in the middle column, after outliers were removed. The third group is concentrations higher than the data obtained from the round-robin project (RR-188).

The precision statement in this standard utilizes equations derived from a regression of the data in RR-188 and is detailed in GPA TP-31. The precision statement criterion applies only to values listed in Section 10, Table 6.

1.2 Components sometimes associated with natural gases, i.e., helium, hydrogen sulfide, water, carbon monoxide, hydrogen and other compounds are excluded from the main body of the method. These components may be determined and made a part of the complete compositional data. Refer to Appendix A.

Table I Ranges of Natural Gas Components Covered

Component Lower

Region

Round

Robin

Higher

Region

Nitrogen 0.01 - 0.1 0.1 - 30 > 30 Carbon Dioxide 0.01 - 0.1 0.1 - 30 > 30 Methane 0.01 - 40 40 - 100 N / A Ethane 0.01 - 0.1 0.1 - 10 > 10 Propane 0.01 - 0.1 0.1 - 10 > 10 Isobutane 0.01 - 0.25 0.25 - 4 > 4

n-Butane 0.01 - 0.25 0.25 - 4 > 4 Isopentane 0.01 - 0.12 0.12 - 1.5 > 1.5

n-Pentane 0.01 - 0.12 0.12 - 1.5 > 1.5

* Hexanes Plus 0.01 - 0.1 0.1 - 1.5 > 1.5

* Heptanes Plus 0.01 - 0.1 0.1 - 1.5 > 1.5 *Data from round –robin was only obtained for Hexanes

Plus

Table Note: Uncertainty in the Lower region can easily be ten times greater and in the higher region two to three times greater than the center column.

NOTE 1 – Components not listed in Table 1 may be determined by procedures outlined in Appendix A or other applicable analytical procedures. Refer to Appendix A. 2. SUMMARY OF METHOD

2.1 Components to be determined in a gaseous sample are physically separated by gas chromatography and compared to calibration data obtained under identical operating conditions. A fixed volume of sample in the gaseous phase is isolated in a suitable inlet sample system and entered onto the column.

2.2 The full range analysis of a gaseous sample may require multiple runs to properly determine all components of interest. The primary run is on a partition column to determine air, methane, carbon dioxide, ethane and heavier hydrocarbons. When oxygen/argon content is critical in the unknown sample, or is suspected as a contaminant, a secondary run should be made to determine oxygen/argon and nitrogen in the air peak on the partition column. When carbon dioxide content in the unknown sample does not fall within the calibrated range on the partition column, a secondary run should be made to determine carbon dioxide content. When helium and/or hydrogen content are critical in the unknown sample, a secondary run should be made to determine helium and/or hydrogen.

2.2.1 These analyses are independent and may be made in any order, or may be made separately to obtain less than the full range analysis. The configuration can consist of a single or multiple GC’s to accomplish this. Refer to Appendix A.

2.3 Response factors or response curves derived from calibration data are essential to accurately determine the composition of an unknown sample. The reference standard blend and the unknown samples must be run using identical GC operating conditions.

3. APPARATUS

3.1 Chromatograph - Any Gas Chromatograph may be used as long as the specifications for repeatability and reproducibility stated in Section 10 within the round-robin test component ranges listed in Table 1are met or exceeded. The equipment described in this section has been proven to meet the above requirements; however other configurations including portable and online may be acceptable.

3.1.1 Detector- The Thermal Conductivity Detector (TCD) has proven to be a reliable and universal detector for this method.

3.1.2 Sample Inlet System- A gas sampling valve capable of introducing sample volumes of up to 0.500 ml may be used to introduce a fixed volume into the carrier gas stream at the head of the analyzing column. The

Analysis for Natural Gas and Similar Gaseous Mixtures by Gas Chromatography

sample volume should be repeatable such that successive runs meet the precision requirements of Section 10.

NOTE 2 – The sample size limitation of 0.500 ml or smaller is selected relative to linearity of detector response and efficiency of column separation. Larger samples may be used to determine low-quantity components in order to increase measurement accuracy.

3.1.3 Chromatographic Columns

3.1.3.1 Partition Column - This column must separate nitrogen (air), carbon dioxide, and the hydrocarbons methane through n-Pentane. (or n-Hexane when a C7 plus analysis is performed). Silicone DC 200/500, 30% by weight on 80/100 mesh Chromosorb P, acid washed,

packed into 30’ x 1/8” SS tubing has proven to be satisfactory for this purpose.

3.1.3.2 Precut Column – A backflush column similar to the partition column described in 3.1.3.1. This column must be of the same diameter and long enough to clearly separate the hexanes plus or heptanes plus fraction from the lighter components. Figure 1A shows an example chromatogram of a natural gas mixture using the precut column for grouping the hexanes and heavier (heptanes and heavier in Figure 1B).

3.1.3.3 Pressure Buffer Column- A lightly loaded column placed between the detector inlet and the column switching/sampling valve (Figure 2A, Column 3) may help to position the hexanes and heavier peak to provide better resolution. This column is usually 1 wt% Silicone 200/500 between 12” and 40” long. (Figures 2A and 2B show a typical column switching/sampling valve arrangement).

NOTE 3 – The arrangements of columns, detectors and valves depicted in Figure 2A and 2B have been determined to meet or exceed the performance criteria of this standard. (See Section 10, “Precision”.)

3.1.4 Temperature Control -The chromatographic columns and the detector should be maintained at temperatures consistent enough to provide repeatable peak retention times and compositional precision within the limits described in Section 10 during the reference standard and corresponding sample runs.

3.2 Carrier Gas - The contaminants in the carrier gas must be limited to levels that are known not to interfere with the analysis or cause maintenance problems with the GC. Refer to manufacturer for recommendations regarding carrier gas quality

3.2.1 Pressure and Flow Control Devices- These devices should maintain flow rate consistent enough to provide repeatable peak retention times and compositional precision within the limits described in Section 10 during the reference standard and corresponding sample runs. Two Stage regulators with stainless steel diaphragms have been shown to be satisfactory for this purpose.

Figure 1A Chromatogram of early backflush of

hexanes and heavier (C6+).

Figure 1B Chromatogram of early backflush of

heptanes and heavier (C7+).

3.3 Sample Conditioning Systems - GPA 2166 gives guidance for proper design and use of sample conditioning systems. The sample conditioning system should not cause the GC precision to fall outside the requirements in Section 10.

NOTE 4 – Valves and sample introduction system must be maintained at a temperature above the hydrocarbon dew point of the calibration blend and unknown samples. Supplemental heating may be required to accomplish this. Refer to GPA 2166 for guidance.

3.4 Integration System- The integration system should be configured to properly integrate all peaks of interest. Integration systems can not correct for inadequate component separation. The integration system should not cause the GC precision to fall outside the requirements in Section 10.

Figure 2A Two Six port valves used for sample

injection and precut backflush.

Figure 2B One Ten port valve used for sample

injection and precut backflush.

4. NATURAL GAS QUALITY ASSURANCE

4.1 Determination of Linear Range - GPA 2198 describes procedures to establish the linear range of a GC system. This process is necessary to determine the proper calibration and analytical procedures for each instrument.

4.2 Fidelity Plot - GPA 2198 describes the procedure to create a Fidelity Plot. The Fidelity Plot is a tool that can be used to monitor the validity of calibration standards and performance of GC systems.

4.3 Control Charts -GPA 2198 describes the use of Control Charts. Control Charts can be used to monitor each component in the calibration blend and the GC performance over time.

4.4 Precision Test- Section 10 of this document establishes the precision requirements of this standard.

5. SAMPLE INTRODUCTION

5.1 Sample Introduction-The sample introduction must be performed in the same manner for calibration and subsequent unknown samples. It is acceptable to either perform a purged or evacuated introduction. Successive runs must be repeatable and not contain contamination from the previous injection. Refer to Appendix A for discussions on linearity, calibration and other related topics.

5.1.1 Purged Introduction - Determine the rate and duration of the purge. Perform alternate injections using a suitable reference blend and instrument carrier gas. Perform alternate injections of each material at various purge rates and purge durations. Note the rate and duration of each purge test and the component concentrations from each run. Repeatability of each component must meet the criteria listed in Section 10, “Repeatability” on the sample runs for the purge rate to be acceptable. Results from the carrier gas blank run must not contain carryover (individual peaks) greater than 0.01 un-normalized mol % from the previous injection of sample for the duration to be sufficient. Once this has been established, this rate and duration should be used for all calibration and analytical runs.

5.1.2 Evacuated Introduction - Evacuate the sample entry system and observe the vacuum gage or manometer for pressure changes indicating a leak. Leaks must be repaired before proceeding. Determine the pressure to be used for injections. Perform alternate injections of a suitable reference blend and carrier gas. Make replicate runs at the selected pressure. Repeatability of each component must meet the criteria listed in Section 10, “Repeatability”. Use this pressure for calibration and analytical runs. Results from the carrier gas blank run must not contain carryover (individual peaks) greater than 0.01 un-normalized mol % from the previous injection of sample.

5.1.3 Equilibration- All sample injections must be performed in the same manner for known and unknown sample compositions. The sample introduction system must be allowed to equilibrate prior to operation of the gas sample valve.

5.2 Preparation and Introduction of Sample – Samples must be properly conditioned prior to analysis. GPA 2166 gives guidance on proper heating of sample containers and sampling systems. Refer to GPA 216

6.

NOTE 5 – To ensure representative samples are obtained in the field, refer to GPA Publication 2166.

5.2.1 Sample connections and tubing used in the sample entry system of the GC must be composed of material that does not cause sample distortion. Stainless Steel and Nylon 11 have proven to perform in this manner. Rubber and other plastic tubing must not be used since these materials readily absorb hydrocarbons.

6. CALIBRATION PROCEDURE

6.1 Calibration

6.1.1 Response factors for the components of interest are determined in accordance with the calculations discussed in Section 8. This can be accomplished by

various means. Either single level calibration(s) using one or more certified reference blend(s) or a multi-level calibration using at least three certified reference blends is acceptable.

6.1.2 Procedures discussed in Section 4 and the calibration type will determine the calibrated range. All components in the unknown samples should lie within the calibrated range for a specific GC.(See Section 10, “Precision”.)

6.1.3 Calibration should be verified on a set frequency. Verifications can utilize a single blend or multiple blends. At least two runs should be made to verify repeatability. If the calculated concentrations deviate by more than the precision requirements for repeatability listed in Section 10, or the un-normalized total deviates by more than 1% from 100 %, instrument maintenance or recalibration may be necessary. First verify the calibration blend is valid, then verify the instrument is operating properly (repair as required), and then recalibrate if necessary.

6.1.4 Fidelity plots and Control Charts, as described in GPA 2198, are excellent tools to monitor instruments and calibration blends.

6.2 Calibration types

6.2.1 Single Level Calibration(s)

6.2.1.1 One or more certified gas reference standard blends of known composition are used to determine response factors for anticipated component ranges in the unknown samples. The results from the “Linearity Check” and the response factors determined for each component can be used to identify the calibrated range for concentrations anticipated in the unknown samples.

6.2.1.2 One gas reference standard blend of known composition may be used to determine response factors for each component. Unknown samples are analyzed and the results determined from the response factors derived from the reference standard blend.

6.2.1.3 More than one gas reference standard blend of known composition may be used to determine response factors for each component. The composition of these standards should cover the anticipated range of compositions in the unknown samples. Unknown samples are analyzed and the results determined from the certified reference blend more closely matching the unknown.

6.2.1.4 The calibrated range, when within the ranges listed in Section 10, must meet the precision requirements listed in the column “Reproducibility”.

6.2.2 Multi-level Calibration

6.2.2.1 Multi-level calibrations may be used for single components, select components, or the full range of components in each standard. A multi-level calibration with three or more gas reference standard may be used to determine response factors for component(s) of interest. The results from the “Linearity Check” and response factors determined for each component can be used to identify the calibrated range for concentrations anticipated in the unknown samples.

6.2.2.2 The calibrated range, when within the ranges listed in Section 10, must meet the precision requirements listed in the column “Reproducibility”.

NOTE 6 – See Appendix A for more information on linearity, calibrations, and other related topics.

7. ANALYTICAL PROCEDURE

7.1 Precut Backflush Method for Nitrogen, Carbon Dioxide, Methane, and Heavier Hydrocarbons - Using the same instrument conditions and sample introduction technique that were used in the calibration run(s) for the unknown sample, obtain a chromatogram through n-pentane with hexanes and heavier eluting as the first peak in the chromatogram.

7.1.1 This is accomplished by the GC system configured as shown in Figures 2A and 2B.

7.1.2 The sample is loaded into the sample loop as determined in Section 5 and allowed to equilibrate. The sample is injected by valve actuation. The lighter components, including n-pentane, move through the pre-column and into the analytical column. Column switching must occur before hexanes and heavier material exit the pre-column. The exact valve timing must be determined for each GC system.

7.1.3 The pre-column is initially in series upstream of the analytical column to isolate the hexanes plus. After the valve switch the pre-column is in series downstream of the analytical column, with flow reversed to back-flush the hexanes plus into a single peak. See Figure 1A.

7.1.4 This recommended approach to the hexanes and heavier separation has two distinct advantages: (1) better precision of measuring the peak area, and (2) a reduction in analysis time over the non-precut (single) column approach.

7.1.5 To perform this procedure as a heptanes plus analysis the valve timing must be adjusted so that the valve switch occurs after the elution of normal hexane from the pre-column onto the analytical column. See Figure 1B.

7.1.6 In order to reduce the pressure disturbance from the valve actuation on the plus fraction peak, a delay or buffer column may be utilized. A column between 12” and 40”, with 1% DC 200/500 on Chromosorb P has been found effective.

8. CALCULATIONS

8.1 Determine the peak areas of each component for the reference standard blend and unknown sample.

8.2 Response factors are calculated for each component using peak areas from the reference standard blend in accordance with the following relationship:

K = Ms / Ps where: K - Response factor

Ms – Mol % of component in reference standard Ps -Peak area in arbitrary units for reference standard

8.3 Concentrations are calculated for each component in accordance with the following relationship:

Mu = Pu x K where:

Mu - Mol% of component in unknown

Pu- Peak area of each component in unknown sample

K - Response factor as determined in 8.2

9. REPORTING AND NORMALIZATION

9.1 Normalization is the process of forcing the sum of the concentrations of components to the desired total. This is accomplished by multiplying each component by the normalization factor. This factor is determined as follows:

F norm = Σunnormalized / Σdesired

where:

F norm = normalization factor

Σunnorm = unnormalized total Σdesired = desired total

9.2 Normally the desired total is 100%, except in cases such as secondary analyses such as those described in Appendix A, Section A-1.2. Refer to Appendix A and Table 5 below.

9.3 Reporting is commonly to two decimal places due to limitations on equipment. TCD detectors typically have a linear dynamic range of 10,000:1. Numbers are calculated to three decimal places and then rounded up when the third digit is 5 or higher.

10. PRECISION

10.1 The repeatability and reproducibility statements for this standard are from the statistical data obtained in a GPA RR-188. The testing program included ten samples comprised of ten components analyzed by six laboratories. The standard as revised has been statistically evaluated under ISO and ASTM protocols. The documentation of the statistical evaluation may be found in GPA TP-31.

10.2 To determine the precision for any component at a specific concentration, use the formulae shown in Table 6 and substitute the mole percent of the component for x.

10.3 Repeatability is the expected precision within a laboratory using the same equipment and same analyst. Repeatability is the difference in analyzed values between two sequential runs. Reproducibility is the expected precision when the same method is used by different laboratories using different equipment and different analysts. Reproducibility is the difference between two analyzed values. Neither value represents the difference between an analyzed value and the certified value listed on a blend. (Refer to 10.6 and 10.7).

Table VI Component Ranges for Precision Limits

10.4 The following example calculations show the repeatability and reproducibility for two different blends. The Ranges from the previous precision statement are used in the two examples. Example 1 lists the lower concentration from the original precision statement range of each component and Example 2 lists the higher concentration from the original precision statement range for each component along with the repeatability and reproducibility calculated for those values.

10.5 The values shown in these calculations are in mol percent. These values are the mol % of the component plus or minus the value determined from the appropriate equation. That is to say, if the value is 1.00 and the precision value is 0.02, results that are between 0.98 and 1.02 are acceptable and values that are above or below that range are not acceptable and fail to meet the precision criteria of this standard. When the result is less than 0.01, use 0.01 as the lowest precision value.

Example 1

Example 2

10.6 Performance evaluations commonly use the repeatability and reproducibility of laboratory results compared to a certified blend. This precision statement is based on the data contained in GPA RR-188 and the statistical evaluation described in GPA TP-31. This treatment of data compared laboratory results independent of the certified blend values. Therefore, performance evaluations must either compare the laboratory results in the same manner by using the reproducibility values described in Table 6 and subsequent example calculations, or use the Performance Evaluation Acceptance Criteria listed below.

10.7 The ability of an instrument to match the certified values from a gravimetric blend referred to as is the Performance Evaluation Acceptance Criteria. The blend uncertainty must be known to use this approach. The reproducibility and the uncertainty of the calibration blend are used to determine the Performance Evaluation Acceptance Criteria.

Where:

CV B is the certified value of component in blend

PE is the Acceptance Criteria for component

R is the method reproducibility for component

U B is the blend uncertainty of component

In Example 3, we use the blend from Example 2, with a 1% Certified Reference Blend used in an audit. For more information, refer to Section 11, “Definitions”.

Example 3

Mol % U B Reproducibility PE Nitrogen 7.70 0.077 0.44 0.45 Methane 86.40 0.86 0.14 0.88 CO2 7.90 0.079 0.24 0.25 Ethane 9.70 0.097 0.067 0.12 Propane 4.30 0.043 0.054 0.07

Iso-butane 1.00 0.010 0.018 0.02

N-butane 1.90 0.019 0.045 0.05

Iso-pentane 0.45 0.0045 0.020 0.02

N-pentane 0.42 0.0042 0.020 0.02 Hexanes

Plus 0.35 0.0035 0.030 0.03 From the example above, if the laboratory result for methane is between 85.52 and 87.28 mol % it would be deemed acceptable. For hexanes plus, a result between 0.32 and 0.38 mol % would be acceptable.

10.8 If the Blend Uncertainty is not known, this approach is not acceptable. Instead, compare the individual laboratory results to the robust mean of those results plus or minus the reproducibility of the method. Using the values from Example 2, if the mean result for methane is 86.66 mol %, then acceptable results will be between 86.52 and 86.8 mol %. In example 2, if the hexanes plus mean result is 0.37, acceptable results will be between 0.34 and 0.40 mol %. Refer to TP-31.

11. DEFINITIONS

Analytical Column– The column in the early backflush configuration that separates all compounds of interest except the “Plus” fraction. This is the longer of the two DC200/500columns.

Calibrated Linear Range – An experimentally determined range of concentrations for a component on a particular instrument. (Refer to GPA 2198”)

Carrier Gas– The gas used to deliver the sample to the detector.

Carryover– Components that are left in the GC system from a previous run. Column – The part(s) of the GC system used to separate components from each other.

Detector– The device used to detect the presence and determine the amount of each component within a mixture.

Effluent– A component that has exited the analytical column.

Elute – The act of a component leaving the column.

GC System – The equipment used in gas chromatography, including the sample inlet system, sample conditioning system, outlet tubing, analytical columns, carrier gas tubing, and detectors.

Hydrocarbon Dew Point – The temperature (pressure) at a given pressure (temperature) at which a particular gaseous hydrocarbon mixture begins to condense into the liquid phase.

Integration System– The hardware and software used to calculate peak areas.

Linearity– The ability to obtain test results within the precision limits of the standard for components of interest, using a single response factor for each component.

Linear Range– The range of concentrations where the peak area is proportional to the component mol % for a particular component.

Linearity Check–A process that verifies the degree of nonlinearity for an analytical instrument (Refer to GPA 2198)

Molecular Sieve – A device used to separate a particular component from the rest of a mixture.

Normalized Mol % – T he sum of mol % determined for a mixture, adjusted to 100 %.

Partition Column– A column that separates by liquid partitioning, gas-liquid chromatography, such as the DC200/500.

Peak Windows – The expected time period for a particular component to elute from the column.

Performance Evaluation Acceptance Criteria – A range that acceptable instrument test result must fall within defined by the root sum square of the method reproducibility and uncertainties of the performance evaluation blend. Refer to GPA 2198.

Plus Fraction– A group of components that are lumped together after the last speciated component. In a “C6

Plus” analysis, this is all components that elute after normal pentane on frontal flow.

Porous Polymer Column– A column that separates utilizing polymer beads, gas solid chromatography, such as Porapak Q or Hayesep Q.

Pre-Column– The column in the early backflush configuration that lumps the “Plus” fraction components into a single peak. This is the shorter of the two DC200/500 columns.

Retention Time– The amount of time between sample introduction and elution for a particular component.

Repeatability– The expected precision for a test result when the same method is used utilizing the same equipment and analyst. Values for “Repeatability” can be found in Section 10, “Precision”.

Reproducibility – The expected precision for a test result when the same method is used utilizing different equipment and/or analysts. Values for “Reproducibility” can be found in Section 10, “Precision”.

Response Factor– The response factor is calculated by dividing the peak area for a particular component by the corresponding mol % of the reference standard blend. This factor is then used to determine the mol % of the component in an unknown gas sample.

Robust Mean – The statistical mean of a set of values after outliers have been removed. Refer to TP-31 for guidance on outlier rejection. Sample/Calibration Run – The act of analyzing a gaseous mixture, from sample introduction to elution.

Sample Conditioning System – The portion of the sample system that removes contaminants from the sample.

Sample Inlet/Entry System– The portion of the sample system where the sample is received from a sample container.

Sample System– The equipment used to prepare and introduce a sample onto the pre-column, including the sample inlet/entry system and the sample conditioning system

Thermal Conductivity Detector (TCD)– A detector that may use a wheat-stone bridge to determine the amount of each component. The carrier gas passes over an element with a current running through it, and the sample stream passes over a similar element with the same current running through it. The resistance of each element is measured and the difference between the two coupled with expected retention times is used to determine the amount of each component present.

Un-Normalized Mol % - Un-normalized mol % is the sum total mol % of the components determined for a mixture. (See Normalized Mol %.)

A-1Linearity

Section 4, Appendix C and GPA 2198 discuss Linearity and list procedures to determine the linear range and calibration requirements of GC systems. When it is anticipated that the range of concentrations of components in the unknown samples will not fall in the linear calibrated range of the instrument, it is necessary to make corrections for this. Two means of accomplishing this are through multi-level calibration (calibration curve) or secondary analysis.

A-1.1 Calibration Curves (Multi-level Calibration)

A-1.1.1Calibration Curves Using Multiple Calibration Blends

Once linearity has been determined for a GC, as described in Section 4, and the linear range is found to be inadequate for the range of unknown sample concentrations anticipated, calibration curves for any component may be determined by using multiple calibration blends.

Duplicate injections of at least three concentration levels for the desired component should be made. If the values on duplicate runs agree within the tolerances in Section 10, “Repeatability”, the response factor should be calculated as follows for each concentration level:

K= C n

A n

where

K = Response factor

C n = Concentration of component n

A n = Peak area in arbitrary units of component n

Calibration curves may now be developed by plotting response factors versus concentration. Any program capable of generating a polynomial curve fit may be used.

A-1.1.2Calibration Curves Using Partial Pressures of Pure Components

Once linearity has been established for the instrument as described in Section 4, calibration curves for any component to be measured in the unknown sample may be determined by using pure components.

Attach the pure component to the sample entry system and evacuate the entry system to less than 1 mm of mercury. Using the partial pressure range suggested in Table A-l, inject at least three partial pressures in duplicate and capture data including Barometric Pressure at the time of the injection. When concentrations on duplicate runs meet the criteria listed in Section 10, “Repeatability”, calculate the response factor as follows:

K= (Pi) (100)

(Po) (A)

Where

K = Response factor

Pi = Partial pressure of pure component in mm of

mercury to nearest 0.5 mm

Po = Barometric pressure in mm of mercury to

nearest 0.5 mm

A = Peak area of pure component in arbitrary

units

Calibration curves can now be developed by plotting response factors versus concentration. Most integration software packages have this feature built-in, but if this feature is not available, other programs capable of generating a polynomial curve fit may be used.

Table A-1

Component Partial

Pressure mm

of Hg (Pi)

Barometric

Pressure mm of

Hg (Po)

Pi/Po * 100

Oxygen 100 750 13.33

Nitrogen 100 750 13.33

Methane 500 750 66.67

Carbon

Monoxide

100 750 13.33

Carbon Dioxide 100 (650)* 750 13.33

Ethane 200 (450)* 750 26.67

Propane 100 (200)* 750 13.33 Isobutane 100 (100)* 750 13.33

n-Butane 100 (100)* 750 13.33 Isopentane 50 (50)* 750 6.67

n-Pentane 50 (50)* 750 6.67

*Partial Pressures in parentheses are the maximum pressures to be used to determine response factors. Exceeding these pressures could result in low response factors caused by compressibility of the pure component.

A-1.2Secondary Analyses

Secondary analyses may be used instead of calibration curves (as in the case of Carbon Dioxide on a Porous Polymer column.) or for determination of compounds not determined by the partition column run. The secondary analysis or run may occur separately or simultaneously to the primary analysis or run. When more than one component is determined, add all component concentrations and normalize to 100%. When a single component is determined, it is acceptable to keep that component concentration whole as described below:

F norm= 100- C n

100

Where

F norm = Normalization Factor

C n = Concentration of component n

APENDIX A - Calibrations

All components determined in the primary analyses or run are then multiplied by N, and the single component held whole.

A-1.3Other Documentation– Instrument logbooks, Maintenance logbooks, User Manuals, Calibration Records, QA/QC records, Analytical Methods and SOP’s are documents that form the analytical audit trail. These documents may either be maintained electronically or in written form.

An ideal GC detector will provide a linear response across all sample component concentrations. In this case, a

calibration standard with any concentration of the component of interest could be run and a calibration response factor could be determined:

Example: A calibration standard has 80 Mole % Methane. When the sample is run on the GC, it generated a peak area of 80,000. The response factor for Methane at 80% concentration is:

K methane = Mole % Methane ÷ Peak

Area = 80 ÷ 80,000 = 0.001

When the detector was perfectly linear, and an unknown sample was run and generated a peak area for Methane of 40,000, the Methane concentration in Mole % would be:

Peak Area x K methane = 40,000 x 0.001 =

40 Mole % Methane

A graph of Mole % concentration to peak area would be linear (a straight line): However, many chromatograph detectors are not linear in their response. A graph of Mole % concentration to peak area would not be linear:

In this example,

Calibrating to 80 Mole % would yield the following response factor:

KF Methane = Mole % Methane ÷ Peak

Area Methane

= 80 ÷ 96,000 = 0.000833

Using the above calibration factor to analyze a sample with 40% Methane would yield the following result:

Mole% Methane = Peak Area

Methane * KF Methane

= 64,000 * 0.000833 = 53.333 Mole %

Methane

Since the difference between the actual value (40%) and the reported value from calibration to 80% Methane (53.333%) exceeds the reproducibility limits established in Section 10 it would be necessary to have separate calibration standards for samples containing 80% Methane and samples containing 40% Methane.

The linearity check is used to determine the number of calibration standards that are needed to analyze all the expected sample compositions.

When more than one calibration is required, this can be achieved by having a separate calibration method for each expected sample composition, a multi-level calibration for all components or a multi-level calibration for the components that are not linear.

Whether a GC detector is linear for a component or not linear for a component is determined by whether it can be analyzed within the reproducibility limits outlined in Section 10.

Linearity curves can be established by running multiple calibration standards of various compositions. In this case, the actual Mole% is plotted against the peak area.

Linearity curves can also be established by running the same calibration standard under various partial pressures. In this case, the Mole% value is determined by the following formula (see Appendix A, A-1.1.2):

APENDIX B – Linearity Discussion

Partial Pressure Mole % Equivalent = Mole % x Inj P / Max P

Where:

Max P = the normal sample loop pressure that samples are injected in

absolute pressure

Inj P = the sample loop pressure that the sample was injected in absolute

pressure

Notes:

The above calculation does not take compressibility into account. To be accurate, the compressibility factor should be included in the calculation.

Max P and Inj P must be expressed in the same absolute pressure units.

Refer to GPA 2198 for more detailed instruction in calibrating with non-linearity in mind

C-1Run Analysis for Nitrogen, Methane, Carbon Dioxide, and Ethane

The porous polymer column must completely separate methane, carbon dioxide and ethane to baseline as shown in the example chromatogram. The linearity of this system must be determined to be linear to be an acceptable alternative to the calibration curve technique described in Appendix B. This system can be used as part of a multi-column GC, as in the case of some portable GC’s.

C-2.Determination of Carbon Monoxide

This component is encountered in association with oxygen, nitrogen, carbon dioxide and the conventional hydrocarbons in the effluent streams from combustion processes such as insitu combustion, manufactured gas and many varied types of stack gases. No extra equipment is necessary to determine carbon monoxide since it elutes shortly after methane on the molecular sieve run. If a calibration gas is available containing carbon monoxide, obtain a response factor as for methane on the molecular sieve column. However, should a gas blend not be available, a calibration curve should be developed using pure carbon monoxide to determine the extent of the nonlinearity, if present.

C-3 Determination of Hydrogen and Helium

When hydrogen is to be separated from helium, a 20’ molecular sieve 5A column using nitrogen or argon as a carrier gas may be used. Low temperature, 40°C (104°F) or less is necessary to effect this separation.

When hydrogen is present, it will elute on the standard molecular sieve run, using helium as a carrier gas, just before oxygen. The hydrogen response is downscale (negative) rather than upscale (positive). Signal polarity must be reversed for the hydrogen peak to be recorded upscale. The sensitivity and precision of measurement will be poor under these conditions due to similar thermal conductivity values for hydrogen and helium. If a calibration gas blend is available containing hydrogen and helium, it should be used to obtain response factors however, if this is not the case, the pure components, hydrogen and helium, may be used to develop response factors in the manner set forth in Appendix A, A-1.

C-4.Determination of Hydrogen Sulfide

As indicated earlier in this text, to be absolutely sure of the hydrogen sulfide content of a gas, determinations should be made at the sample source. However, in the case where a field measurement has not been made and although corrosion of the sample bottle may have resulted in some loss of hydrogen sulfide, a measurement of the in-place component may be made by gas chromatography. It is necessary to charge a sample of pure hydrogen sulfide to the column prior to charging the unknown gas. As soon as the pure hydrogen sulfide has cleared the column, the unknown gas should be charged. (All calibrations should be done the same way, that is, each partial pressure charge of pure hydrogen sulfide must be preceded by a full sample loop of pure hydrogen sulfide.) A column that has proved satisfactory for this type of analysis is the Silicone 200/500 column. It is most convenient since this is the recommended column for determining the hydrocarbons in a natural gas analysis. Hydrogen sulfide elutes between ethane and propane with good resolution.

CAUTION - Extreme care must be taken when working with hydrogen sulfide due to the very toxic nature of the gas. The best ventilation possible must be maintained in the laboratory.The Maximum Allowable Concentration that a person may be exposed to without approved respiratory protection equipment is 10 ppm for an eight hour working period.When the exposure lasts through the working day, concentrations as low as 15 ppm may cause severe irritation to the eyes and respiratory tract. Exposure of 800 to 1,000 ppm may be fatal in a few minutes. The nose must not be depended upon to detect the presence of hydrogen sulfide, as 2-15 minutes of exposure will cause the loss of smell.

APENDIX C – Supplementary Procedures