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Inka Orko, Jukka Lehtomaki, E& Sa idell & Mona Arnold Analysis of odorous gases with simultaneous GC-MS and sensory determination Injection unit L r Cold trap GC oven OSTI Columns Restrictor capillary Humified air MS A TECHNICAL RESEARCH CENTRE OF FINLAND ESP00 1995

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Inka Orko, Jukka Lehtomaki, E& Sa idell & Mona Arnold

Analysis of odorous gases with simultaneous GC-MS and

sensory determination

Injection unit

L

r Cold trap GC oven

O S T I

Columns

Restrictor capillary

Humified air

MS

A

TECHNICAL RESEARCH CENTRE OF FINLAND ESP00 1995

VTT TIEDOTTEITA - MEDDELANDEN - RESEARCH NOTES 1710

Analysis of odorous gases with simultaneous GC-MS and sensory

determination

Inka Orko, JuMta Lehtomiiki, Erik Sandell & Mona Arnold VTT Chemical Technology

TECHNICAL RESEARCH CENTRE OF m A N J 3 ESP00 1995

ISSN 12354605 Copyright 0 Valtion teknillinen tutkimuskeskus (V'IT) 1995

JULKAISIJA - UTGIVARE - PUBLISHER

Valtion teknillinen tutkimuskeskus (V"'), Vuorimiehentie 5, PL 2000, 02044 VTT puh. vaihde (90) 4561, telekopio (90) 456 4374

Statens tekniska forskningscentral (VTT), Bergsmansvagen 5, PB 2000, 02044 v?T tel. vke l (90) 4561, telefax (90) 456 4374

Technical Research Centre of Finland (VTT), Vuorimiehentie 5, P.O.Box 2000, FIN42044 VTT, Finland phone internat. + 358 0 4561, telefax + 358 0 456 4374

VTT Kemiantekniikka, Ympikistotekniikka, Betonimiehenkuja 5, PL 1403, 02044 v?T puh. vaihde (90) 4561, telekopio (90) 456 7022

VTT Kemiteknik, Miljoteknik, Betongblandargriinden 5, PB 1403, 02044 VTT tel. vke l (90) 4561, telefax (90) 456 7022

V l T Chemical Technology, Environmental Technology, Betonimiehenkuja 5, P.O.Box 1403, FIN42044 V?T, Finland phone internat. + 358 0 4561, telefax + 358 0 456 7022

Technical editing Kerttu Tirronen

VTT OFFSETPAINO, ESP00 1995

DISCLAIMER

Portions of this document may be illegible in electronic image products. Images are produced from the best available original document.

Orko, Inka, Lehtomiiki, Jukka, Sandell, Erik & Arnold, Mona. Analysis of odorous gases with simultaneous GC-MS and sensory determination. Espoo 1995, Technical Research Centre of Finland, V'IT Tiedotteita - Meddelanden - Research Notes 1710. 23 p. + app. 2 p. UDC 628.5 12:66.099.7:53.083 Keywords analyzing, odors, gases, industrial plants, emission, off-gases, odor control, odor

detections, odor emissions, methods, detectors, measuring instruments

ABSTRACT Industrial odorous off-gases can consist of hundreds of different compounds giving cause to odour annoyance in the vicinity of the odour-emitting plant. For the identification of the odorous components in the gas, traditional analytical methods are not always sufficient since the odour threshold values cannot often be found in literature.

This report decribes the development of a GC-MS sniffing port method for identifying odorous compounds in off-gases. In the method the sample is injected into a gas chromatograph and divided into two flows. The compounds in these sample flows are separated in two identical columns and detected simultaneously in a mass spectrometer and by sensory means. The olfactory detections are marked in the iongram and the odorous compounds are identified.

Tenax TA adsorbent is generally used for collecting the odorous sample for analysis. The compounds are released fkom the adsorbent for analysis by thermal desorption.

The report also decribes a case study where the GC-MS sniffing port method was applied to a gaseous emission from a food factory. Over ten odorous compounds could be identified.

TABLE OF CONTENTS

ABSTRACT ............................................................................................................ 3

PREFACE ................................................................................................................ 4

1 INTRODUCTION ............................................................................................... 6

2 SAMPLING OF ODOROUS GASES ................................................................. 7 2.1 SAMPLING IN A SAMPLE CONTAINER ............................................... 7 2.2 CONCENTRATIVE SAMPLING ............................................................... 8

3 ANALYSIS ....................................................................................................... 10 3.1 SAMPLING, DESORPTION AND INJECTION ..................................... 1 1 3.2 SEPARATION IN COLUMN AND DETECTION .................................. 11 3.3 SENSORY DETECTION AND THE SNIFFING PORT ......................... 12 3.4 GC-MS SNIFFING PORT APPARATUS ................................................ 14

4 DEVELOPMENT OF GC-MS SNIFFING PORT SYSTEM AT VTT ............ 15

5 CALIBRATION ................................................................................................ 17

6 CASE STUDY: ODOROUS GASES EMITTED FROM A FOOD FACTORY18

7 CONCLUSIONS ................................................................................................ 21

REFERENCES ..................................................................................................... -22

APPENDICES

AN IONGRAM OF THE CASE STUDY ........................................................ 1

TABLE OF THE COMPOUNDS IDENTIFIED ............................................. 2

5

I INTRODUCTION

As techniques for reducing emissions have developed, the requirements for decreasing malodorous emissions have become more stringent. Emissions of malodorous gases do not usually pose a health risk, but they can cause significant nuisance problems in the surroundings of the emission source.

The intensity of the odour of a gaseous emission is affected by both the concentration of the odorous compounds in the flue gas and the volumetric quantity of the emission. The intensity of the odour does not increase linearly, since the response of the human sense of smell is logarithmical. Individuals have varying sensitivity to different odours. Also, long-term exposure to odour may cause sensory saturation and adaptation. Thus the quality of the odour and the individual characteristics as well as previous experiences of a person influence how the odour is perceived. Odour concentration is expressed in odour units (OU/m3 = odour unit/m3), and it is determined with an olfactometer by an odour panel [l].

Compounds which typically generate offensive smell, are e.g. aldehydes, amines, ketones, and reduced sulphur compounds. The main sources of odour emissions in Finland are the sulphate (kraft) pulp industry, food industry, feed production industry, chemical industry, foundries and solvent industry. The emissions from various industrial plants, even within the same branch of industry applying the same process, usually differ from each other in their chemical consistency. Therefore, the odour control systems have to be individually designed and implemented for each odour-emitting plant. It is vital to have knowledge of what odorants are present in the emitted gases.

Odorous compounds are typically very volatile and their molecular weight is relatively low. Therefore, gas chromatographic (GC) separation followed by detection with e.g. a flame ionization detector (FID) and/or mass spectrometer (MS) has typically been used for their chemical analysis. The effect of each compound on the total odour can be evaluated by comparing their concentrations to the odour threshold concentration values found in literature. However, this method is not sufficiently reliable, because the odour threshold limits have not been determined for all compounds. In addition, there are odorous compounds which are easily masked or lost in chromatographic analysis. Sensory detection, i.e. ”sniffing” of the sample gas flow separated in a chromatograph provides valuable additional information about the sample.

6

The great number of compounds in odorous off-gases and their different polarity, volatility and molecular weight make sampling, chemical analysis and evaluation of the impact of a certain individual compound on the total odour difficult. These problems can often be solved, or at least facilitated, with simultaneous chemical detection and sensory determination which take place subsequent to the gas chromatographic separation.

This report focuses on a system in which a sniffing port has been coupled to a gas chromatograph-mass spectrometer (GC-MS), and the separated compounds are determined by sensory means at the sniffing port. Furthermore, the development of a simultaneous GC-MS sniffing port apparatus is described and different sampling methods are overviewed. A case study is reported including a description of the method and the apparatus as well as the results. The performance of the method for industrial off-gases is evaluated.

2 SAMPLING OF ODOROUS GASES

Sampling of odorous gases may be carried out with a few different methods. A very simple and useful method is to collect the sample with a pump into a sample container of inert material. This way the sample can be stored as close to its original consistency as possible. The gas sampling can also be carried out with enrichment techniques based on adsorption or absorption. In these methods, a large volume of sample gas is transferred through a sorbent material in which the target compounds are retained.

2.1 SAMPLING IN A SAMPLE CONTAINER

Gas samples can be collected into sample containers which are then transported to the laboratory for analysis. The container material should be odour-fiee and both chemically and physically stable and retentive in order to prevent the compounds from adsorbing onto the walls of the container or diffusing through the wall. If the sample is led directly through a gas pump, those parts of the pump, which get into contact with the sample, should be made of an inert material.

In the USA, a standardized method applies the sampling of gaseous samples in special containers. These containers are constructed of stainless steel and they are cleaned in a certain way [2, 3, 41. Steel is considered to be an adequately inert material, at least in respect of alkanes and sulphides. However, carbonyl and amine compounds may be adsorbed onto the walls of the steel container. Steel containers are quite heavy and unresilient and thus may prove to be impractical when sampling takes place in the field.

Resilient sample bags are fabricated from numerous different membrane materials. The most common membrane materials in use are Teflon, Tedlar, PVC and Mylar. Teflon and Tedlar are inert in respect of alkanes, sulphides, and carbonyls, but amines may adsorb onto the wall of the sample bag. Sample bags made of PVC or Mylar are known to be inert to all the above-mentioned compounds. These materials are also regarded odour-free and they can be used in numerous applications requiring gas sampling. Sample bags are in general light and easy to handle. The problem with sample bags, as well as unpressurized containers, is that they can hold only a limited volume of gas. The volumetric size of the containers cannot be further enlarged because of practical aspects. It is also known that a small sample quantity reduces the sample match. Therefore the suitability of the sampling in the sample bags must be considered in each case [5 ] .

Even in an inert container the sample may undergo alteration. This can be due to the reactions between different compounds in the container. Therefore, for highly reactive compounds, another sampling method should be considered. Chemical transformation may also occur photochemically due to sunlight. This should be taken into account when storing the sample.

2.2 CONCENTRATIVE SAMPLING

The previously described sampling method is applicable when the compounds are present in detectable concentrations, i.e. at least a few micrograms per cubic meter of air [4, 61. When lower concentrations are to be measured, concentration or enrichment of the sample is necessary. Selective concentration of the sample has some significant advantages: it improves the analytical detectability, decreases the interference of the gas matrix and makes it possible to increase the total volumetric quantity of the sample, which improves the match of the sample [5 ] . Storing concentrated samples is also more convenient as they require much less space. Also, the stability of the samples is improved. Concentrative sampling techniques are especially applicable when low concentrations of odorants with very low odour threshold limits are to be detected.

8

Various enrichment methods are used for concentrating the sample: absorption in liquid, cold trap method and adsorption on the surface of a solid material. If also a sensory determination of the sample is desired, it must be possible to revaporize the sample from the concentrated form to its original consistency. Of the above listed methods, this requirement is best fulfilled by adsorption onto a solid material. Combined with thermal desorption, this sampling method is a good choice for overall VOC analysis.

Concentration of gaseous samples onto a solid adsorbent is a common and conventional sampling method at VTT. In the VTT method, the so-called trap, where the compounds of the sample are adsorbed, is an adsorption tube which generally is made of glass or steel. The tube is packed with solid sorbent or sometimes with activated carbon. The method is quite simple and economical. Mixtures of different sorbents can be used in order to concentrate multiple types of compounds. Also, very selective sorbent traps can be prepared [5]. The most commonly used adsorbent material at VTT is Tenax TA.

The adsorbed sample is desorbed for analysis either by thermal desorption or extraction. Thermal desorption is the preferable method for odorous gas samples, since no extra compounds have to be added. Figure 1 shows an arrangement where a gas chromatograph is connected to a thermal desorption unit.

transfer

-- tube

Pressure gauge

Pressure T regulator

t Carrier inlet

Detector Injector

Capillary column

Figure I . Gas chromatograph coupled with a thermal desorption unit [6].

9

In thermal desorption the sorbent tube, which contains the sample, is placed in the desorption trap. The trap is heated as quickly as possible (in a few seconds), the flow of the carrier gas is minimized, and the sample is vaporized for analysis. Usually the sample is analyzed by first separating the components of the sample in a gas chromatograph and then detecting the separated compounds with a flame ionization detector or a mass spectrometer. The sample is transferred fiom the thermal desorption unit into the gas chromatograph through a cold trap. The stability of the sorbent and the quality of the sample usually determine the highest desorption temperature. A very high yield can be achieved with thermal desorption [ 5 ] .

Adsorption onto a solid adsorbent is not always applicable to concentrating compounds with very low molecular weight, since these compounds can be purged along with the gas flow and driven out of the sorbent. Also adsorption of compounds with very high molecular weight may not be comprehensive. The adsorbed compounds may also react on the sorbent surface andor dissolve. The sampling flow has to be adequately small in order to avoid pressure impacts on the sampling efficiency. Sample flow humidity can be retained in some sorbents so effectively that it has to be removed before entering the sorbent trap. [5]. With Tenax TA such problems seldom occur.

When a general screening of compounds is needed it is desirable that as many types of compounds as possible can be concentrated. This type of sampling requires an overall enrichment method. Adsorption onto a solid adsorbent is often the simplest and most economical method. In many cases it also has sufficient capacity for concentrating a sample of many different substances.

3 ANALYSIS

As previously mentioned, the enric,,ment of gas samples onto an adsorbent followed by thermal desorption and separation of the sample in a gas chromatograph has become an established combination of methods. It is also often the only practical way of analyzing samples which contain numerous components. The use of synthetic capillary polymers enables effective enrichment of a gaseous sample; even concentrations of less than 1 pg/m3 can be analyzed [7]. As capillary columns have become more common, also the accuracy of the analysis has further increased. The detection takes place in a mass spectrometer. Sometimes the analysis is complemented by additional spectroscopic data with IR, NMR or UV analyses of the substances {ti].

A sniffing port coupled to a gas chromatograph has been largely used for analyzing the flavours (aromas) of foodstuff and also for analyzing their packing materials and residual flavours. The methods used in flavour analysis can

10

probably also be applied in industrial odour emission analytics, as the detected compounds are volatile and the concentrations of the odourants can be very low.

High concentrations of interfering non-odorous compounds may complicate sampling and analysis. If the concentrations of odorants are very low, they might be masked by co-eluting major components in a normal gas chromatographic analysis. In addition to that, it is often difficult to reach sufficiently high separation efficiency and selectivity with routine equipment. It may be impossible to separate all components in only one gas chromatographic run because of the low sample capacity of the capillary column [9]. Also, resolving numerous very different types of compounds from the same sample can be problematic. In these cases, it may be necessary to collect the sample in a few different matrices and then run the analyses separately with different configurations and settings of the gas chromatograph. Different detectors may also have to be used.

3.1 SAMPLING, DESORPTION AND INJECTION

The application of porous polymers and support-bonded chromatographic phases as trapping material and the subsequent thermal desorption have proved to be a suitable method for carrying out a combined GS-MS analysis for volatile organic compounds. A general problem is transferring thermally desorbed compounds into the capillary column. In order to maintain high resolution, the sample should be transferred to the beginning of the column as a narrow band. For this reason, the sample is usually released from the sorbent by heating and then collected to a cold trap with the help of inert carrier gas. The cold trap is heated very rapidly and the sample is transferred as a narrow band into the column [lo].

Direct injection can only be used for detecting major components with high vapour pressure. The injection of concentrated liquid samples is problematic since capillary columns with high efficiency may begin to overload even in the range of approximately 100 ng per single component. These limits are reached quickly, and especially if the sample contains larger amounts of non-volatile materials, the performance of the column might deteriorate [lo].

3.2 SEPARATION IN COLUMN AND DETECTION

The column, the temperature programme and the detector are chosen according to the quality of the sample. A polar column is used for polar substances and an apolar column for apolar compounds. High separation capacity and selectivity can be achieved by using a capillary column. Because of the high separation capacity

11

of capillary columns, the selectivity of stationary phases is not of the same importance as if packed columns were used [l 11.

A very important factor when carrying out difficult separations and quantitative analyses is proper adjustment and control of the temperature.The quality of the sample and its thermal stability have an influence on the required temperature programme.

A flame ionization detector (FID) is often used for general detection. Also selective detectors can be used. There are specific detectors for at least sulphur, nitrogen and phosphor-containing compounds. The separated compounds can also be detected and identified with a mass spectrometer (MS) or a mass selective detector (MSD).

3.3 SENSORY DETECTION AND THE SNIFFING PORT

Sensory determination of the odorous components of a sample takes place in a sniffing port. The sniffing port can be coupled to a GC-MS system, for example, so that the compound separated in a column is transferred simultaneously to the detector and to the sniffing port. In practice, the flow is divided into two separate flows by a splitter prior to the two identical columns. One flow is then directed to the instrumental detector and the other one to the sniffing port. The responses of the human and instrumental detectors are then correlated with each other, and the peaks of odorous components can be pointed out in the chromatogram.

The human sense of smell is very sensitive to certain substances. Thus it is sometimes possible to detect compounds, the response of which in the instrumental detector is so low that no peak can be observed. In these cases the sniffing port gives truly new information about the sample.

The gas eluated from the chromatograph has a high linear velocity, but a very low volume flow and mass transfer rate, especially if narrow-bore capillary columns are used. This would make the sniffing event in the cone rather doubtful, since the compounds might be lost in inhaling the air from outside the cone as well. Therefore, the eluent gas has to be diluted with a larger air flow. The sensory determination can further be facilitated, if the dilution air is humidified. The sample should be well-ventilated from the surroundings of the sniffing port. An illustration of the sniffing port and the gas flows is presented in Figure 2.

12

GC COLUMN

.(I+ ( NOSE

GOOD VENTILATION

Figure 2. The sample eluatedfiom the column is mixed with odourless, humidiJied air. The exhaustflow is ventilatedfiom the vicinity of the nose [12].

When the odorous compound eluating from the sniffing port initiates a sensory response in the human detector, the time is recorded from the beginning to the end of the response. If possible, also the quality and intensity of the sensory response is described with adjectives. Alternatively, the results can be collected on a moving paper recorder with known velocity, an audio recorder, or a real-time computer.

Sometimes sensory saturation and adaptation cause problems. As a result the human detector can no longer detect separate peaks. This situation should be recognized and solved, for example by analyzing several parallel samples.

3.4 GC-MS SNIFFING PORT APPARATUS

In a certain GC-MS sniffing port-apparatus the sample is thermally desorbed from the sorbent tube and transferred through an injector into the columns. The apparatus contains two columns, which are made by cutting a 50 meter-long quartz capillary column into two equally long pieces. The column ends are connected to each other with a soft silicone hose. The columns are mounted through a cold trap placed inside the oven. Their free ends can be coupled to separate detectors, e.g. to the FID and to the sniffing port. Since the columns are of equal length, the odours can be detected at the sniffing port in principle at the same time as they eluate into the FID and get recorded in a chromatogram as peaks. Also, an electron capture detector (EC), a mass spectrometer (MS), or a flame photometric detector can be used for chemical determination. When an MS is used, there is a delay of about 5-1 5 seconds between the detection at the sniffig port and in the instrument. Figure 3 shows a GC apparatus with two columns [13].

N2

DEWAR WESSEL

L- SNIFF DETECTOR (FIG. 2)

Figure 3. Schematic diagram of a .GC apparatus with two columns. 1. & 2. Temperature control, 3. Swagelog-Jitting, 4. Gas valve, 5. Sample tube, 6. Heating oven. [I31

14

4 DEVELOPMENT OF GC-MS SNIFFING PORT SYSTEM AT VTT

The sniffing port at VTT was coupled to a GC-MS apparatus.

The GC-MS apparatus consists of a Tekmar LSC 2000 sample injection unit, a Hewlett-Packard 2890 series I1 gas chromatograph, and a double-focusing Jeol SX 102 mass spectrometer. These main parts of the apparatus are connected to each other in the above-mentioned order. A maximum of 100 ml of gaseous sample can be injected. The gaseous sample is injected into a Tenax TA-adsorbent tube in the adsorption unit, fiom which it is flushed into a capillary cold trap (120 "C) by means of heating and a helium gas flow. The sample is enriched at the same time, since only a certain volatility range of organic compounds is retained on Tenax TA. When the desorption of the sample into the cold trap is completed, the trap is heated very rapidly and the sample is transferred as a coherent front to the analytical column. The temperature programmme of the chromatograph is started and the components of the sample are separated fiom each other. The separated compounds are then transferred to the mass spectrometer, in which they are ionized and detected. A real-time iongram is printed on a computer terminal. The iongrams and the mass spectra of the compounds can be reproduced by means of the software of the mass spectrometer. The compounds are identified by searching in an internal substance library which contains the data of more than 50000 compounds. The components can be quantified based on the area of the peak.

Coupling the sniffing port to the MS-GC apparatus took place by drilling a hole through the oven wall of the GC and mounting an approximately 15 cm long copper tube through the hole. The flow fiom the sample injector to the gas chromatograph was divided into two similar apolar DB-1-columns (length 30 m, inner diameter 0.32 mm). The end of one column was projected out of the oven through the copper tube and was directed to the sniEng port. The other column was connected to a 10 meter-long, non-phaseous, deactivated silica capillary (inner diameter 0.25 111111) acting as restrictor. This arrangement was necessary for balancing the pressure deviation between the MS and the sniffing port. The other end of the capillary was then coupled to the mass spectrometer. Glass couplings were used for all the column connections.

The sniffing port was constructed simply by placing the end of the column into the humidified dilution air flow. The separate fiactions are mixed thoroughly with the dilution air before entering a glass b e l which serves as a sniffing mask. The column travels a short path from the gas chromatograph to the dilution air hose inside the copper tube in order to prevent the sample flow fiom cooling down very rapidly and condensing at the same time in the column before it is eluated. The humidification of the dilution air is carried out by bubbling the gas through water.

injection unit c I I cold trap GC oven

I I 1 restrictor capillary

L I humified air

MS

Figure 4. GC-MS snzfingport apparatus at m.

The most suitable dilution air flow proved to be 50 mumin. The odour perception became stronger as the dilution air flow was increased up to 50mUmin. An additional increase of the dilution air flow did not have any further effect on the intensity of the sensory stimulus. Inert and odourless Teflon was selected for the material of the dilution air flow line. The sample outflow from the column to the sniffing port was less than 1 ml/min, measured at 30 "C.

16

5 CALIBRATION

Test runs revealed that sensory and mass spectrometric determinations cannot be totally synchronized when temperature programming is used. The main reason for this is that the columns "swell" as the temperature rises, thus decreasing the flow through the columns. The temperature difference between the column ends at the mass spectrometer and at the sniffing port is not constant. It is important to know the intervals between the retention times throughout the measurement range in order to be able to attach the sensory detections to the iongrams. Therefore a retention time calibration is needed.

A gas standard was prepared containing eight different compounds, the retention times of which covered the measurement range comprehensively, e.g. diethylether, butanal, isobutanol, 2-pentanone, methyl-isobutylketone, isobutylacetate, xylene, and a-pinene. The standard was prepared by injecting pure liquid compounds into a sampling bag filled with a known volume of pure nitrogen gas 99.999 % ). The concentration of each compound was approximately 90-100 mg/m . (3

A PID (Photo Ionization Detector) was connected to the end of the column normally directed to the sniffing port. The detector is very sensitive to organic compounds. All of the compounds in the standard could be detected with the PID.

2.0 ml of the gas standard was injected into the sample injection unit. The compounds were detected with both the mass spectrometer and the PID. The retention time at the sniffing port was plotted as a function of the retention time of the mass spectrometer. The plotted curve shows the correspondence between the retention times of the mass spectrometer and the sensory detections. The calibration curve is shown in Figure 5.

CALIBRATION CURVE OF A SNIFFING PORT

retention time in mass spectrometer / min. 20

15 -

10 -

5 -

A I 1 I

15 v

0 5 10

retention time at sniffing port /min.

20

Figure 5. Calibration curve for the retention times at snifingport and in MS.

6 CASE STUDY: ODOROUS GASES EMITTED FROM A FOOD FACTORY

An off-gas sample fiom the exhaust gases of a food factory was taken with a vacuum pump into a Tedlar sample bag. The sample was analyzed with an olfactometer according to the VDI 3881 guideline and also with the GC-MS sniffing port apparatus.

The programme settings of the sample injection unit, the temperature programme and the mass spectrometer of the GC-MS-sniffing port apparatus are presented in Table 1.

18

Table I . Settings of the GC-MS-snifing port apparatus.

Settings

- purge 6 rnin

- dry purge 2 min

- capillary cool down to -120 "C

- desorption 4 min at 250 "C

- injection 2 rnin at 250 "C

- bake (cleaning of the sorbent trap) 8 min at 250 "C

-30 "C 5 rnin

-20 "C/min up to 110 "C

-10 "C/min up to 250 "C

-250 "C 5 rnin

- ionization current 300 pA

- voltage 70 V

- acceleration voltage 10 kV

- rate of amplification 1.3 kV

- configuration BE

100 ml of the sample was injected through a Tekmar injection unit into the GC- MS sniffing port apparatus. The compounds were separated in the gas chromatograph and sniffed at the sniffing port. The observations were written down and the corresponding retention times of the mass spectrometer were obtained from the calibration curve. The iongram is presented in Appendix 1. The odour detections are marked with arrows in the iongram. The identified odorants are shown in Table 2. Also, the odour threshold limit of each compound is pres- ented in the table, whenever it was found in literature. The identification of the odour-generating compounds in the sample was not comprehensive, because not all odours could be correlated to the iongram.

Table 2. Odorous compounds detected in odorous of-gases @om a food factory.

Odorous compound, Odour threshold Odour quality, detected identified

mg/m3, E141

Butanal 0.20 sweet, aldehyde-like

Acetic acid, methyl ester 0.7-63 sweet

2,6-dimethyl- 1 -heptene fishfood-like

Pentylcyclopropane musty, animal food-like

Hexanal 0.043 grass-like

II I strong, offensive I 4-hexene-2-one

I O.Ol1 I rotten H Dimethyldisulfide

2-methyl- 1,4-pentadiene solvent-like

Pentanal 0.034 sweet

Benzene 1.5 nauseous, noxious, strong

2-butenal 1.7 solvent -1i ke

Acetic acid 0.09 sour, tart, bitter

The total odour concentration determined 60 000 oulm3.

with an olfactometer was

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7 CONCLUSIONS

Chemical analysis can, when successfully accomplished, give quantitative as well as qualitative information about the odorous gas sample. Still, the compound or the compounds which create the odour in the sample cannot always be recognized. However, the odorous compound can often be identified by combining sensory evaluation with chemical analysis. The compounds can also be quantified at the same time.

The method applied in this work is evidently suitable for industrial odorous off- gases, provided they are not too concentrated or toxic. However, not too accurate results can be expected from the sensory determination, since several uncertanties are involved. First of all, the sample is concentrated in the sampling phase and then separated in the column, and before entering the sniffing cone it is diluted again. Since the flows in the cone are not controlled, this causes the true concentrations to be somewhat varying. Because the flow in the sniffing port is quite low, the odorous compound might be diluted with the outside air or even get lost, too. Therefore sensory detection at the sniffing port can give only qualitative results. Secondly, it may happen that a sensory detection is left unidentified, i.e. it cannot be pointed out in the iongram. In such a case the sample would have to be reanalyzed using different analytical settings or even a different detector, which is often laborous and time consuming. On the other hand, new information can be gained this way, and even the unexpected and otherwise undetected compounds may be recognized in the sample. Still, even in the simplest case the analysis tends to be costly.

The sampling and the panellist performance seem to be the critical points in the method. Care should be taken that all compounds of interest are retained on the sampling medium. Also, the panellist should orientate for his task carefully by training with a standard mixture. Using several panellists would further improve the quality of the results. It was also noted that "sniffing" is quite tiring , and the panellist should rest well between the runs in order to avoid loss of alertness.

On the whole, the method at its best gives totally new information about the sample. When the main odorants in the emission are identified the most suitable method for reducing odorous emissions can be selected taking into account the characteristics of the odorous compound, such as its pH-value, solubility and concentration. Combined with olfactometric determination, a good picture of the odorous off-gas is attained and a plant-specific solution for the odour nuisance problem can be reached.

REFERENCES

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6. Dublin, T. & Thone, H.J. Thermal adsorption - capillary gas chromatography for the quantitative analysis of dimethyl sulphate, diethyl sulphate and ethylene oxide in the workplace. J. of Chromatography 456 (1988), p. 233-239.

7. Smith, R. & Drummond, I. Trace determination of carbonyl compounds in air by gas chromatography of their 2,4-dinitrophenylhydrazones. Analyst 1 04 (1 979) 9, p. 875-877.

8. Nitz, S., Kollmansberger, H. & Rrawert, F. Determination of sensorial active trace compounds by multi-dimensional gas chromatography combined with different enrichment techniques. J. of Chromatography 471 (1989), p. 173-185.

9. Schomburg, G., Husmann, H., Podmaniczky, L., Weeke, F. & Rapp, A. Coupled gas chromatographic methods for separation, identification, and quantitative analysis of complex mixtures: MDGC, GC-MS, GC-IR, LC-GC. In: Schreier, P. Analysis of Volatiles, Methods, Applications. Berlin 1984, Walter de Gruyter, p. 121-150.

10. Nitz, S. & Julich, E. Concentration and GC-MS analysis of trace volatiles by sorption-desorption techniques. In: Schreier, P. Analysis of Volatiles, Methods, Applications. Berlin 1984, Walter de Gruyter, p. 15 1-1 70.

11. Gunther, W., Klocner, K., Sclegelmilch, F. & Roukeria, S. Comparison of GLC capillaries. In: Schreier, P. Analysis of Volatiles, Methods, Applications. Berlin 1984, Walter de Gruyter, p. 93-108.

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12. Hangartner, M., Hartung, J., Paduch, M. Pain, B.F. & Voorburg, J.H. Improved recommendations on olfactometric measurements. Environmental Tech. Letters 10 (1989), p. 231-236.

13. Veijanen, A., Lahtipera, M., Paukku, R, KZiriainen, H. & Paasivirta, J. Recent development in analytical methods for identification of off-flavour compounds. Water Sci. Tech. 15 (1983), p. 161-168.

14. van Gemert, L.J. Compilation of odour threshold values in air, supplement V, TNO 1984,49 p.

23

Appendix I . Iongram of the case study (see Chapter 6); an exemplary run of simultaneous GC-MS sniflng port detection.

Ln N

C

E (z, m m N

0 + 1s) 6l m

.-

.. Q) rn C m w I- CY

+ 3 Q c, 3 0

n c m a, t

A I L O E m -Ln

L O m e 3(9 0- a,

C N \ - - E

-

-

-2

6 . l - - -- a,

LD

tn

m

N

Appendix 2. Table of the compounds identiJied in the simultaneous GC-MS sniflng port detection, including their refenrence numbers, retention times and detected concentrations. The moment of sensory detection of each odorous compound has been marked below the time axis of the previous iongram with reference numbers.

.

~~ ~

Ref. I Compound I Retention time I Concentration No. I 1 min

1 Butanal 9.47 2.7

2 Acetic acid, methyl ester 8.02 0.73

3 2,6-dimethyl- 1 -heptene 15.51 < 0.1

4 Pentylcyclopropane 14.42 3.2

I I 14.22 I 3.2 6 1 4-hexene-2-one I 13.69 I <0.1

7 I Dimethyldisulfide 1 13.32 I <0.1

8 2-methyl- 1,4-pentadiene 12.17 1.6

9 Pentanal 12.05 1 .o 10 Benzene 11.51 18

l1 I 2-butenal I I le7 12 I Aceticacid I 10.35 I 1.6

Vuorimiehentie 5, P.0.Box 2000, FIN42044 VTT, Finland Phone internat. + 358 0 4561 Telefax + 358 0 456 4374 Date Project number

V'IT Tiedotteita 1710 VIT-TIED-1710

December 1995 KET93 1 Author(s)

Orko, Inka Lehtomiiki, Jukka Sandell, Erik Arnold, Mona

Name of project

Commissioned by

Technical Research Centre of Finland (V'IT) 1

Title Analysis of odorous gases with simultaneous GC-MS and sensory determination

4bstract

Industrial odorous off-gases can consist of hundreds of different compounds giving cause to odour annoyance in the vicinity of the odour-emitting plant. For the identification of the odorous components in the gas, traditional analytical methods are not always sufficient since the odour threshold values cannot often be found in literature.

This report decribes the development of a GC-MS sniffing port method for identifying odorous compounds in off-gases. In the method the sample is injected into a gas chromato- graph and divided into two flows. The compounds in these sample flows are separated in two identical columns and detected simultaneously in a mass spectrometer and by sensory means. The olfactory detections are marked in the iongram and the odorous compounds are identified.

Tenax TA adsorbent is generally used for collecting the odorous sample for analysis. The compounds are released from the adsorbent for analysis by thermal desorption.

The report also decribes a case study where the GC-MS sniffing port method was applied to a gaseous emission from a food factory. Over ten odorous compounds could be identi- fied.

ktivity unit V l T Chemical Technology, Environmental Technology, Betonimiehenkuja 5, P.O.Box 1403, FIN42044 VTT, Finland

SSN and series title 12354605 VTT TIEDO'TTEITA - MEDDELANDEN - RESEARCH NOTES

SBN 951-384864-9

:lass (UDC) 628.512:66.099.7:53.083

iold by VTT Information Service P.0.Box 2000, FIN-02044 VTT, Finland Phone internat. + 358 0 456 4404 Fax + 358 0 456 4374

Language English

Keywords analyzing, odors, gases, industrial plants, emission, off-gases, odor control, odor detections, odor emissions, methods, detectors, measuring instruments

Pages 23 p. + app. 2 p.

Price group A