INTRODUCTION:
Gasification of bio-mass and wastes can result in a wide range of gaseous compounds. Some of these can be noxious and even toxic. The levels can be relatively high reaching hundreds of ppm for some of the more volatile components. Among some of the compounds of that have been noted are:
- Benzene
- Toluene
- Ethylbenzene
- m,p-Xylenes
- o-Xylene
- Naphthalene
- 1-Methylnaphthalene
- Acenaphthene
- Acenaphthylene
- Anthracene
- Fluorene
- Pyrene
- Phenol
- 3-Methylphenol
- Furfural
- Catechol
- Di-n-Butylphthalate
- Bis-(2-Ethylhexyl)phthalate
Many other compounds are possible depending on the type of feedstock.
In order to address the widest variety of compounds a GC/MS method has been developed. One of the more challenging features of the methodology is creating a wide variety of quantitative standards. This paper describes the analytical approach in some detail.
GENERAL APPROACH:
Some compounds are of very specific interest. In particular benzene, toluene, ethylbenzene and xylenes (BTEX) are always of great interest -- especially when lignins or aromatic based plastics are present. These compounds can be purchased in a certified gas standard, but only up to about 20 ppm at any reasonable pressure. At higher pressure the heavier compounds tend to precipitate. For toluene, ethylbenzene and xylene, the 20 ppm standard is usually sufficient for use with effective gasification. These compounds are generally at or below 20 ppm in the syngas stream. It appears that single or double methylated benzenes are not particularly stable to thermal degradation. Benzene, however, often exceeds 20 ppm by a substantial margin. Hence, additional standards are needed for benzene. Beyond these BTEX compounds there are other gas standards commercially available, but these are for use in ambient air testing. The concentrations for these compounds are generally 1 ppm or less. Several other compounds (especially naphthalene) are in concentrations well in excess of 1 ppm. These kinds of standards both not very useful and prohibitively expensive.
Gas standards can be created dynamically using permeation tubes or diffusion tubes. These devices provide a constant source of pure compound. The rate of permeation or diffusion depends on the construction of the device and the temperature. The constant flow of pure compound is carefully diluted with a known amount of gas (usually nitrogen or air). If the rate of permeation or diffusion is known and the gas flow has been carefully calibrated, the concentration of the gas can be calculated. For many liquid compounds, the device can be calibrated gravimetrically by running the device at a constant temperature for several hours.
One of the most important compounds to monitor is benzene. This compound is often found at concentrations up to several thousand ppm. This compounds happens to be one of the most convenient for creating suitable standards using a diffusion device. It is also one of the more convenient compounds for analysis by GC/MS. It appears in a wide variety of environmental standard mixes and has a well behaved mass spectrum. Benzene has a unique mass spectrum with a strong molecular ion at 78 amu. This ion can be used to calibrate for benzene with little danger of interference from other gases. By using an appropriate diffusion device and flow rates, dynamic standards between a few hundred and a few thousand ppm can be generated. Linearity and detection limits can be readily tested by analyzing these dynamic standards.
Generating other standards are not nearly as convenient – especially for compounds with low volatility. Nevertheless, by injecting dilute mixes with known ratios between benzene and other compounds of interest, relative response factors (RRF’s) can be generated. To give the widest flexibility of this approach, these RRF’s are generated based on the Total Ion Count (TIC) data. These tend to be fairly closely related to the total mass being injected – at least for compounds of similar chemical structure. These RRF’s can also account for differences in volatility. These RRF’s can be used to give good estimates for the concentration of many other compounds. An especially convenient approach is to use the benzene in the sample similar to an Internal Standard. The benzene content of the sample is calculated from the 78 ion response and a dynamic RF for the sample is calculated from the TIC area and the calculated benzene concentration. The RRF’s are then used to estimate the concentration of other compounds using the formula:
XC ~ (XBenzene/TICAreaBenzene) * (TICAreaC/RRFC)
Where:
XC = Concentration of Compound C in ppm
XBenzene = Concentration of Benzene in ppm
TICAreaBenzene = TIC Area of Benzene
TICAreaC = TIC Area of Compound C
RRFC = Relative Response Factor of Compound C to Benzene
As it turns out, benzene is one of the most ubiquitous and persistent of all gasification by-product gases. With few exceptions, syngas from feedstocks containing any woody bio-mass (contains lignins) or waste (usually contains plastic), will contain measurable amounts of benzene. Hence, direct calibration for benzene combined with indirect calibration for other compounds using benzene as an “internal standard” will work well for almost any bio-mass or waste derived syngas. Where benzene does not exist the estimation can still be made. The factor XBenzene/TICAreaBenzene from the most recent calibration check is substituted into the equation.
SELECTING A DIFFUSION DEVICE:
Diffusion rates have to balanced with calibrator flow rates and desired concentrations. A VICI 450 calibrator was used for the making the dynamic standards. This is pictured below:

The flow rate for the VICI 450 could be varied from a low of about .6 L/minute to a high of about 11 L/minute. In many gasification applications the concentration of benzene ranges from a low of around 50 ppm to a high of 5000 ppm. Hence, the most desirable “mid-range” needed is around 500 to 1000 ppm. In order to achieve approximately 500 ppm at a mid-range flow of 2.0 L/min at diffusion rate of:

In order to achieve such a high diffusion rate, a large diameter, relatively short diffusion tube was needed. Diffusion rate is approximated by:
r = 1.90 x 104 T Do M (A/L) log10(P/(P-p))
Where:
r = rate of diffusion (ng/min)
T = Temperature (oK)
Do = Diffusion coefficient (~0.0849 for benzene)
M = Molecular weight (g/mole)
A = Area of diffusion capillary (cm2)
L = Length of diffusion path (cm)
P = atmospheric pressure (mm Hg ~ 625 @ Denver)
P = vapor pressure of chemical at T (mm Hg ~483 mm @ 66oC)
A Type D diffusion vial with a 25.4 mm tube length was selected. It is pictured below:

The diffusion device was altered by adding a lead weight to the glass legs to keep it from flipping over in the VICI 450.
CALIBRATION OF DIFFUSION DEVICE:
Multiple runs of approximately 2 hours were made with the Type D diffusion vial. The VICI 450 was operated at 66oC to a high vapor pressure for benzene and to melt other important compounds (like phenol) if they were to be also calibrated by diffusion.
The results for six runs are shown below:

The diffusion rate was 3.5292E+6 ng/min. This was very close to the desired diffusion rate. The VICI 450 was operated at three different settings.
CALIBRATION OF THE VICI 450 FLOW:
The VICI 450 flow was measured using a wet test gas meter. The various flows were recorded and used to calculate the dynamic benzene concentrations. In all cases the diffusion device was held at 66oC. The actual diluted gas temperature was found to be virtually invariant and very close to 25oC. The calculated concentrations are shown below:

DYNAMIC RANGE CHECK:
Five dynamic standards (S2-1, S2-3, S2-5, S2-9 and S1-9) and the one 20 ppm certified standards were analyzed by GC/MS in duplicate. The areas (ion 78 and TIC) were plotted against concentration to test for linearity. These are shown below:


The curve showed excellent linearity (R2 = 0.9946 and 0.9933 for TIC and ion 98 respectively) out to 1200 ppm. This is generally adequate for most samples. The range can be extended by:
- Using a fair linear approximation out to 2500 ppm with correlations of 0.991 and 0.986 for TIC and ion 78 respectively, or
- If greater accuracy is needed, using a quadratic curve where the correlations are 0.9988 and 0.9985 for TIC and ion 78 respectively. This would require routinely updating the curve with at least two standards (counting a force through zero).
CREATING RRF’s:
A methanol/acetone solution of benzene and several other compounds commonly found in the syngas was gravimetrically prepared. It was diluted to put all of the TIC area responses at the upper end of the linear range of the GC/MS (40 to 60 million counts). The standard was injected and the relative response factors were calculated. The data are shown below:

It is interesting to note that the response by GC/MS is highly dependent on the mass and the complexity of the compound. This makes sense in that more complex molecules tend to give more ions. Hence, if very accurate data is needed for other compounds of interest additional RRF’s should be developed. This could actually be done after the testing is concluded if needed.
SUMMARY:
GC/MS can be used quite effectively for monitoring a wide range of compounds in syngas. Calibration can be a challenge, but by using permeation tubes and diffusion tubes a wide range of compounds can be calibrated directly. Other compounds can be estimated by using RRF's to compounds of known calibration such as benzene.