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Accred Qual Assur (2001) 6:815Q Springer-Verlag 2001 REVIEW PAPER

Kevin David Cleaver The analysis of process gases: a review

Received: 1 March 2000Accepted: 31 March 2000

A presentation summarising the keyissues highlighted in this paper was firstgiven at the NederlandsNormalisatie-instituut Gas AnalysisSymposium & Exhibition, 79 November1999, Evoluon Eindhoven,The Netherlands.

K.D. CleaverBOC Gases, The Priestley Centre,The Surrey Research Park, Guildford,Surrey, GU2 5XY, UKe-mail: kevin.cleaver6uk.gases.boc.comTel.: c44-1483 244308

Fax: c44-1483 450741

Abstract A general review of keyissues involved in the analysis of

process gases is presented. Thereasons for such measurements which include safety, quality, envi-ronmental and economic factorsare considered. The technical is-sues arising from these measure-ments are dependent upon a varie-ty of factors, including the overallsampling system, the type of analy-tical instrumentation, methods ofdata collection and the specifiedcalibration protocols. The use ofgas calibration cylinders as transfer

standards is detailed and issues of

stability and traceability to refer-ence material discussed.

Keywords Process 7 Gas 7Analysis 7 Sampling 7 Traceability

Introduction

A process gas may be regarded as any gas produced bya chemical or physical process, or a gas that is used asan integral part of a process. Examples are the steamreformation of a natural gas feedstock to produce hy-drogen and carbon monoxide or the distillation of air atcryogenic temperatures to produce nitrogen, oxygenand argon. The use of gases in medicine, such as nitrous

oxide in anaesthesia, or the use of gaseous calibrationcylinder standards as transfer standards used in envi-ronmental or emissions monitoring is also widespread.

The need for the analysis of process gases arises pri-marily from an industrial requirement for: Reliable and accurate data to enable process control

and optimisation Materials and product evaluation Quality control.The data obtained allows: Compliance with legislation

Contract specifications International specifications and standards.

Examples of analysis performed to ensure regulato-ry compliance include ambient air quality and stackemission monitoring for pollutants such as higher ox-ides of nitrogen, sulphur dioxide, carbon monoxide andozone, automotive exhaust emission monitoring andthe calibration and use of analysers for evidential pur-poses such as ethanol/air breath analysers.

Within the natural gas industry fiscal monitoring,

i.e. the analysis of oil and gas steams to determine calo-rific value, is a key part of the quality system requiringtraceable gas calibration standards. These analyseshave a significant and direct impact upon oil and gascompanies profitability and the consumer. Further-more, there is a widespread need for analysis to moni-tor occupational exposure levels to gases in the workplace. Toxic gases, such as silane, arsine and phosphine,are widely used in the manufacture of semiconductordevices in gas cabinets and containment chambers. Thewhole operation is, however, conducted under clean

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room conditions, with re-circulated air, so gas monitor-ing is essential. The monitoring of flammable or explo-sive atmospheres and medical gases, such as anaesthe-tics or inhaled nitric oxide, is also very important toensure process safety.

Finally, all industrial organisations survive on their

ability to compete on the quality, price and the efficientdistribution and delivery of their products into the glo-bal market-place. The analysis of gases, in the contextof quality control, includes the optimisation of outputand minimisation of rejects and the reduction of oper-ating costs. All of these factors ensure that customersatisfaction is achieved and that products are fit forpurpose.

Industry types

A wide range of industries has a need to use and ana-lyse process gases. The breadth of use may be illus-trated by listing the key market sectors that either pro-duce, use, or analyse process gases, namely: Chemical and petrochemical Environmental, including both ambient air and stack

emission monitoring Scientific and engineering research organisations, in-

cluding universities and national laboratories Medical institutions, including hospitals The food processing and drinks industries, where

gases such as nitrogen are used to enhance the shelflife of products by reducing oxidation and carbondioxide is widely used in soft drinks and alcoholicbeverages

The microelectronics industry, which includes semi-conductor manufacture and telecommunications

Fabrication industries, including the motor, ship andaircraft industries

Power generation, particularly the nuclear industry,for example advanced gas reactors (AGRs)

Instrument manufacturers (OEMs).

Instrumentation and techniques

It is perhaps unsurprising, given the breadth of applica-tions, that a very wide range of analytical instrumenta-tion is used for process gas analysis. This instrumenta-tion utilises broad based techniques such as massspectrometry and infrared spectroscopy, etc., chroma-tography, and also specific sensors used in analyserslike paramagnetic and zirconia analysers, fuel and elec-trolytic cells. The following review, while not exhaus-tive, indicates the diversity of approach and the con-tinuing developments to be found in this challengingfield.

Infrared and Raman spectroscopy

Vibrational spectroscopy, particularly infrared spec-troscopy, has been used for monitoring the environ-ment. A number of devices have been developed forthe analysis of gases. These include a portable FT-IR

real time gas analyser [1], an IR imaging volatile or-ganic carbon field sensor [2], an intercavity diode lasernear-IR spectrophotometer [3], and a dual-cell extrac-tive FT-IR ambient air monitor [4]. Trace gas analysishas been performed using a FT-IR equipped with amultipass cell and Grams/386 software [5]. FT-IR hasbeen used for the time-resolved air monitoring of me-thyl bromide concentrations following the fumigationof buildings [6] and the detection of ammonia and hy-drogen chloride in flue gases [7]. The determination ofconsumer exposure to volatile solvents during paintstripping operations [8] has also been determined byFT-IR. FT-IR has been used as a continuous emission

monitor for stack gas analysis at an oil refinery [9] andincinerator [10]. Sampling techniques and FT-IR meth-ods for the analysis of non-methane organic gases inautomotive exhaust have been detailed [11] and thegases from aircraft engine exhaust analysed by meansof an on-board FT-IR spectrophotometer [12]. Fourseparate analysers are normally used to test vehicle em-issions for carbon monoxide, carbon dioxide, oxides ofnitrogen (NOX) and total hydrocarbons, the results ob-tained from an FT-IR analyser have been comparedwith those obtained from the separate analysers [13].FT-IR has also been used in a study of the catalyticconversion of NOX [14]. The use of a ground-based FT-

IR spectrophotometer as a remote-monitoring devicefor pollutants such as CFCs has been reported [15]. Re-views covering the remote sensing of the Earths atmos-phere from space using IR [16], remote monitoring us-ing far-IR [17] and the use of mid-IR tuneable diodelasers for monitoring trace gases in the atmosphere andthe remote sensing of exhaust gases [18] have also beenreported.

In situ FT-IR spectroscopy has been used to monitorthe production of air toxics during the pyrolysis andcombustion of benzene and o-dichlorobenzene. The ef-fects of temperature and chlorine concentration on theformation of polynuclear aromatic hydrocarbons were

discussed [19]. NIR spectroscopy with fibre optics hasbeen used in on-line determinations of hydrocarbongases at a petrochemical plant in mixtures of ethane,ethene, propane and propene [20].

Raman spectroscopy has been used in some indus-trial applications; Lipp and Grosse have reported mon-itoring dichlorosilane distillation by Raman spectrosco-py [21] with data acquisition times of only 1030 s re-quired. Chlorosilane monitoring has now moved to atleast the pilot plant stage. Gervasio and Pelletier havecharacterised the use of Raman spectroscopy to moni-

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tor the synthesis of phosphorus trichloride from phos-phorus and chlorine [22].

Gas chromatography (GC)

General reviews covering aromas [23], environmentalsamples [24], multiple detector alternatives to multiplecolumns [25], anaesthetic gases [26], and a book whichdiscusses the theory and practice of headspace sam-pling with GC [27], have recently appeared in the liter-ature. A review with eleven references has appeared onthe use of metal capillary GC columns as an alternativeto fused silica columns [28]. Other column develop-ments have included a charcoal-based column capableof resolving light hydrocarbons [29] and a super longopen tubular GC column (450 m long, 200 mm id) forthe separation of gasoline [30]. This column had 1.3million effective plates and was built up by connecting

9, 50 m columns in series and was able to resolve up to970 components in a standard gasoline.

A new porous layer open tubular (PLOT) columnfor the separation of gases and highly volatile com-pounds has been reported [31]. A comparison has beenmade between this column and a conventional dopedaluminium oxide capillary column, it was not adverselyaffected by water, carbon dioxide and sulphur gasesand appeared to be more inert than the aluminium ox-ide column since it did not cause decomposition of themost reactive analytes. Also a quartz-lined aluminiumcapillary GC column coated with graphitised carbonblack has been evaluated for the analysis of volatile or-

ganic compounds (VOCs) and oil products [32].The need for rapid and portable analytical measure-

ments has led to the development of GCs with fast sep-aration times [33]. Many portable high-speed gas chro-matographs (HSGC) are being used for on-site analysis[34]. A micro-GC coupled with a thermal conductivitydetector (TCD) was shown to separate carbon dioxideand C1C6 alkanes within 30 s. Petroleum industry ap-plications included the detection of hydrogen sulphideand carbonyl sulphide impurities [35] and rapid screen-ing for gasoline to diesel range organic compounds [36].Environmental problems are a major application forportable GC systems due to the complexity of the sam-

ples [37]. Recent applications have included air analysis[38] and the detection of polychlorinated biphenyls(PCBs) [39].

An important focus of research and development infield gas chromatography (FGC) during the last fewyears has been instrument miniaturisation. The ulti-mate miniature FGC system was designed and develop-ed using silicon micro-machining and integrated circuitprocessing techniques [40]. The chromatograph consist-ed of a 10-mm-long sampling loop, a 0.9-m rectangularshaped column, and an injection loop and column each

with a width of 300 mm and height of 10 mm. The co-lumn was coated with a 0.2 mm thickness of copperphthalocyanine as the stationary phase. Detection wasbased on a dual detection scheme using a coated chem-iresistor and thermal conductivity detection. The com-plete FGC system was packaged in less than 23 cm3 and

was 2.5 mm high. Although limited in scope to the de-tection of ammonia and nitrogen dioxide, this minia-ture chromatograph offers exciting possibilities for fu-ture field instruments.

The most common uses for FGC have hitherto beenin the determination of volatile and semi-volatile or-ganic compounds in the atmosphere. Target com-pounds for on-site screening by FGC include benzenein complex environments at the ppm level [41], dime-thyl sulphide and carbon disulphide [42], and polychlo-rinated biphenyl [43]. Indoor air pollutants such as tol-uene, a-pinene, and 1,4-dichlorobenzene were deter-mined, with detection limits in the low mg m31 level

[44] and long-lived species were identified in the uppertroposphere and lower stratosphere [45].

One major problem of FGC when compared to la-boratory based instrumentation is the reduced resolu-tion that most field instruments exhibit. This reducedresolution is partially compensated for by the utilisa-tion of selective detectors. Detectors such as the elec-tron capture detector [46]; the photoionisation detector[47, 48], and a miniature dual flame photometric detec-tor for phosphorus and sulphur compounds [49] haveall been interfaced to field gas chromatographs.

A versatile selective detector for GC is the ion mo-bility spectrometer or detector; recently this has been

coupled to chromatographs for field use [50, 51]. Ahand portable GC-ion mobility spectrometer has beenconstructed and called the environmental vapour moni-tor. This monitor has been used to separate and detectchemical warfare agents [52].

Numerous detectors are available for GC, broadlyfalling into two categories: ionisation detectors and op-tical detectors. Ionisation detectors include the flameionisation detector (FID), the helium ionisation detec-tor (HID), the nitrogen phosphorus detector (NPD),the electron capture detector (ECD), the surface ionis-ation detector (SID), the photo ionisation detector(PID), the ion mobility detector (IMD), and the glow

discharge detector (GDD). Other miscellaneous GCdetectors include acoustic flame detectors [53], andsemiconductor detectors which are possible replace-ments for FID [54] and for breathalysers [55]. Improve-ments in the sensitivity of thermal conductivity detec-tors have also been reported [56] along with the inter-facing with capillary column [57, 58].

Increasingly, discharge helium ionisation detectors(DID) are being used in process gas analysis applica-tions. An evaluation of the detector by Cufflin andSmith [59] has shown:

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Detector linearity five orders of magnitude Minimum detection limits for methane of less than

2.5 parts per billion by volume (ppbv), without pre-concentration of the sample

Standard deviations of less than 200 parts per trillionby volume (pptv).

A GC system has been developed for the directanalysis of atmospheric formaldehyde and other oxy-genated hydrocarbons [60]. This method utilises thetrapping of analytes in a loop cooled with liquid nitro-gen, separation by GC and subsequent detection usinga pulsed discharge helium ionisation detector (pHID).The detection limit of this instrument is estimated to be32 pptv for 0.2 l of gas sampled at a flow rate of 30 mlmin1.

At-Plant

A recent trend with IR spectrophotometers, GCs andother instrumentation, such as mass spectrometers, is tolocate them At-Plant. This movement of relatively so-phisticated analytical instrumentation out of the labora-tory and into the plant allows: Rapid data acquisition and increased operational ef-

ficiency, cf. central laboratories Continuous sampling and hence easier plant optimis-

ation The use of statistical process control techniques,

trend analysis, etc.Benefits derived from this approach include: Operating cost improvements

Increased production The reduction in out of specification product and

laboratory costs.The concept of At-Plant instrumentation, with its

attendant benefits, is possible largely due to increasedautomation and PC-based controls, which allow day today operation of the analysis equipment by plant tech-nicians. Software developments have been of crucialimportance, facilitating a simple but flexible approachat the user interface.

Specific sensors

Specific sensors and techniques for process gas analysisinclude: Paramagnetic detectors for oxygen purity and high

concentration measurements (%) Zirconia, fuel cells and electrolytic cells for trace oxy-

gen measurements (ppmv and ppbv) Chemiluminescence detectors for oxides of nitrogen

(NOX) (ppmv and ppbv) UV fluorescence detectors for sulphur dioxide (SO2)

(ppbv) determinations.

Different types of chemical sensors have been usedfor the electrochemical sensing of gases of medical in-terest [61]. In recent years the use of inhaled nitric ox-ide in neonates, paediatric and adult patients in inten-sive care units has become widespread. Of particularimportance has been the requirement to monitor the

concentration of inhaled nitric oxide [62] and nitrogendioxide impurities; comparisons of chemiluminescenceand electrochemical sensors have been made [63, 64].

A highly sensitive (10 ppb 1 ppm) ozone (O3) sen-sor using In2O3 as a sensing film has been developed[65], oxygen sensors based on Y2O3

stabilised ZrO2(YSZ) have demonstrated longer lifetimes [66] and thedetection of hydrocarbons in air and purification of ex-haust gas under lean-burn conditions using solid elec-trolytes demonstrated [67].

Odour sensing is in the forefront of analytical inter-est, and several optical sensors have been used in thisapplication. A chemiluminescence gas sensor made of

Al2O3 emits luminescence during the catalytic oxida-tion of a combustible odour vapour [68]. Novel types ofsensor are based on the immobilisation of a chemilumi-nescent reagent between a miniature photomultipliertube and a Teflon diffusion membrane [69]. Taste andflavour monitoring has been done using a fluorescentoptical probe detecting certain sulphur compounds inthe vapours of hams [70].

A wide variety of techniques and sensors are used tomeasure moisture in gases, including: Capacitive sensors, which include aluminium oxide

and other metal oxides, and silicon-based sensors Electrolytic (phosphorus pentoxide) sensors, which

consist of a film of the phosphorus pentoxide desic-cant which strongly absorbs water vapour from thesurrounding gas.

Optical dew point hygrometers, where condensationas dew or frost is formed on a mirror within the in-strument and the onset of condensation is sensed op-tically by detecting changes in how the mirror reflectsor scatters light. Other non-optical condensation dewpoint sensors detect condensation electrically, or byother methods such as the change in frequency of aresonating quartz crystal [71].

Sampling systems

Analysers and their associated sampling systems areoften complex installations, which require careful de-sign, reliable utilities (water, electricity, carrier and fuelgases), protection against temperature extremes, rain,dusts and corrosion. Ease of access to all of the systemcomponents is essential to allow adequate preventivemaintenance and efficient operation.

The key requirements of a sampling system are thatthe system design includes all of the components re-quired for a safe and correct operation, e.g. probes,

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valves, filters, coolers, pressure regulators and reliefvalves, pumps, piping, etc., thus ensuring a samplewhich is representative of the overall process to be ad-mitted to the analyser for analysis [7274].

An important consideration is sample point location.The sample point should be chosen to:

Provide the most accurate information on the com-position of the process stream

Minimise time lags, particularly important when ana-lyser outputs are used for process control

Ensure that temperature, pressure, dryness, particu-late loading, etc. are analyser compatible whereverpossible to minimise the requirement for coolers, re-lief valves, filters, etc.

Ensure that sample take off points are readily acces-sible.Design and construction considerations include:

Materials selection and compatibility Requirements for purging, by-pass flow

Trace heating and insulation, requirements for intrin-sic safety and zoning of electrical equipment

Equipment location Labelling, tags, etc. Effluent and sample gas disposal Calibration facilities Automatic calibration Requirements for analyser communication, which in-

clude signal transmission, cables, alarms and systemvalidation and calibration.The complete analyser system should be designed,

installed and operated so that it does not present a haz-ard to either personnel or plant.

Calibration and traceability

Process gas analysers used for quantitative measure-ments require calibration. Calibrations are typicallyperformed using: Gas calibration cylinders of known composition,

either gravimetrically prepared or certified usingcomparison methods

Static or dynamic volumetric methods for the prepa-ration of standard gas mixtures

Permeation tubes and other blending devices such as

mass flow controllers, capillary based gas dividers,etc.The key attributes of a gas calibration cylinder are:

Stability the calibration standard shall remain atthe certified value throughout the specified shelf-lifeand as the contents are depleted in use. In order toachieve stability, cylinders may require the passiva-tion of the internal surfaces [75].

Accuracy the standard is fit-for-purpose and themeasurement uncertainty is consistent with identifiedrequirements for use and customer needs. Fig. 1 A traceability model

Traceability traceability depends upon a chain ofstandards linked back to an international primarystandard through a series of calibrations, i.e. inter-comparisons between two standards in the chain. Thevalue of each standard in the chain must have a de-fined measurement uncertainty.

Therefore, to ensure measurement accuracy, stability,traceable calibration and defined measurement uncer-tainty are required. See Fig. 1 :A traceability model.

The Guide to the Expression of Uncertainty in Meas-urement [76], Quantifying Uncertainty in AnalyticalMeasurement [77] and The Expression of Uncertaintyand Confidence in Measurement[78] are all documentswhich give detailed guidance to enable the calculationof expanded uncertainties. The International Organisa-tion for Standardisation (ISO) has defined measure-ment uncertainty as [79]: A parameter associated withthe result of a measurement, that characterises the dis-persion of the values that could reasonably be attri-

buted to the measurand. The stages in the evaluationof measurement uncertainty are illustrated in Fig. 2.

Sources of uncertainty in analytical procedures in-clude: Sampling homogeneity, the number of replicate

samples taken, variations in temperature and pres-sure during sampling

Sample preparation the presence of dilution errorsor contamination

The presentation of certified reference material(CRM) to the measuring system the uncertainty ofthe CRM

Calibration of the instrument

Analysis and data acquisition Data processing, for example control of the rounding

of results and the use of statistical treatments to de-rive results

The presentation and interpretation of results.

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Fig. 2 The uncertainty estimation process [80]

Data collection and processing

Commercially produced software is widely available forchemometrics, experimental design and statistical proc-ess control. Software is typically used to control: Instrument set up and optimisation Calibration and sampling, including external timed

events such as valve switching, column switching,etc.

Collection and quantification of analytical data The statistical treatment of results.As with other aspects of method development andprocess gas analysis the use of software requires valida-tion.

Chemometrics

Chemometrics is an approach to analytical and meas-urement science based on the idea of indirect observa-tion. Measurements related to the chemical composi-tion of a substance are taken, and the value of a prop-erty of interest is inferred from them through a mathe-matical relation. Chemometrics works because theproperties of substances, such as, say, gasoline, areuniquely defined by their chemical composition. The

indirect observation of a property is the objective forreasons of speed, simplicity and economy. Texts on theuse of chemometrics in environmental chemistry havebeen published [81] and experimental design [82] andmethod validation [83], particularly in the pharmaceuti-cal industry, are important to the analytical scientist.

Procedures for chromatographic method validationhave been reviewed by Jenke [8486] and the impor-tance of method validation as a part of total qualitymanagement is discussed by Christensen et al. [87], whohave illustrated the importance of correcting measure-ment errors by the use of reference materials. The Ana-lytical Methods Committee [88] of the Royal Society ofChemistry has reviewed the concepts and practices ofdata quality in analytical chemistry in relation to meas-urement uncertainty. The use of statistics for the assess-ment of laboratory performance during interlaboratorytesting has been reported and Schrantz et al. [89] havenoted that the testing laboratories generally perform

well in interlaboratory comparisons when referencematerials are used to validate their analytical proce-dures. A review of the statistical methods used to ana-lyse data from interlaboratory comparisons has beenmade by Feinberg [90].

Conclusions

In summary, the analysis of process gases is necessaryto monitor and control processes, thus enabling com-pliance with legislation and international standards. Italso ensures operational safety and enables the produc-

tion of a wide variety of items of consistent productquality in a cost-effective manner. In short, the analysisof process gases impinges upon virtually every type ofindustrial, medical and environmental activity and theaccurate quantification of components present in aprocess stream, including trace constituents present asimpurities, is essential and, in certain cases, this knowl-edge leads to competitive advantage.

Future technical developments in process gas analy-sis are likely to focus on: The increased miniaturisation of equipment, with

further developments in HSGC The wider use of broad-based techniques, such as

mass spectrometry and IR spectroscopy, allowing therapid acquisition of data from process streams via asingle analyser system

The continuation of the trend of locating instrumen-tation At-Plant with further software developmentssimplifying the user interface and allowing a greateruse of statistical packages to control and optimiseprocesses

A greater emphasis on the benefits of accreditationand traceability with respect to gas calibration cylin-ders used to quantify components in process streams,

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driven by requirements for robust data to enable costsavings and compliance with legislation, etc.In order to achieve these objectives consistently it is

essential that a formal quality system is adopted. Thereare a variety of quality standards to which organisationsand laboratories may become accredited. These include

ISO EN 9000, ISO Guide 25 and ISO EN 45000. A newstandard ISO/IEC 17025: 1999(E), General require-ments for the competence of testing and calibration labo-ratories will shortly replace the ISO Guide 25 and EN45000. The accreditation to an external quality standardensures that, in addition to the purely technical issuesof method development and selection, instrument se-lection, sampling and calibration organisations alsoconsider and document their approach to: The quality management system The organisation and responsibilities for quality The qualifications and training of staff

System review The use of calibration, testing equipment and refer-

ence materials Measurement traceability and calibration Record keeping, document and data control Handling complaints, anomalies and departures from

documented procedures The participation in internal and external audit pro-

grammes, which may also include proficiency testingschemes and correlation exercises.The adoption of a formalised quality system and ex-

ternal accreditation to a quality standard by an inde-pendent agency will ensure a consistent approach toprocess gas analysis and importantly should thereforelead to comparability of measurements and hence re-sults, which is a vital attribute in industrial, medical andcommercial areas.

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