Process gases for analytical applications

The field of analytical application is extremelydiverse. Whether they are used to monitor thequality of foodstuffs, to test engines in theautomotive industry, to control processes in thechemical or pharmaceutical industries, in medicine,metallurgy or environmental monitoring: analyticalmethods are used everywhere for process control,quality assurance or even to prove compliance withstatutory regulations.

Analytical methods - the right gas for the rightpurposeThe methods employed and their uses are just asdiverse as their fields of application. The specializedfield of gas analysis in inorganic and organicanalytical chemistry has undergone phenomenaldevelopment during the last thirty years: in the early1970s, classic absorption/volumetric and titrimetricmethods were common industrial practice and gaschromatography was still a rarity; nowadays,preference is given to analytical methods involvinginstrumentation. Depending on the application,various methods have become established. Forexample, gas chromatography and analysis using 

NDIR methods are now preferred in environmentalmonitoring or process control, whereas sparkerosion spectrometry has become popular in themetal industry. Many of these methods require highpurity gases or gas mixtures to work as well as high-precision reference gases for calibration purposes.The detection limits, attainable analytical accuracyand the reliability of the results often depend on thequality of the gases used. Messer offers a widerange of high-purity gases, standard mixtures andcustomized gas mixtures that fulfill all the requiredperformance criteria. For the high-purity gases, thelevels of typical undesirable impurities (usually H2O,O2 and/or N2, hydrocarbons and CO + CO2) arespecified. The grade of purity is summarized bymeans of the commonly used point notation, e.g.Helium "6.0" if the sum of the specified impurities isless than 1 vpm (please refer to the separateinformation on "High-purity Gases"). When choosinga suitable gas, the "correct" gas quality as well asthe necessary gas supply system all requirementsconcerning the particular application have to betaken in account.

Analysis is the art of separating a substance into its constituents andmaking qualitative and quantitative determinations about it

The most widely used analytical methods andtheir specific requirements with respect to theirprocess gases are described below.

Gas chromatography

Gas chromatography is used to analyze mixturesof gaseous or volatile liquid substances. Theprepared sample is applied to a so-calledchromatography column by means of an injectoror a sample loop. The individual substancesinteract with the material in the column in acharacteristic manner. A carrier gas transportsthe individual substances through the column atdifferent speeds depending on the intensity ofthis interaction. Due to the relatively shortanalysis times, helium is often used as the carriergas; nitrogen or hydrogen are also used. Theneeded purity of the carrier gas depends on thenature and concentration of the substance beingdetected.

Gas chromatographyGas purities / measuring range

Detector Carrier gasesOperating

gasesUndesirableimpurities

< 100 < 10 > 10

ppb ppm ppm

TCD H2, He, Ar, N2 H2, O2 5.5 5.0

FID H2, He, N2

H2HC, CO

6.0 5.5 5.0

synth. air HC free

ECD H2, He, N2 N2, Ar/CH4 hal. HC, SF6 ECD grade

FPD H2, He, N2

H2HC, CO

6.0 5.5 5.0

synth. air HC free

HID He H2, O2 7.0 - 6.0 6.0

DID HeH2O, O2, HC

7.0 - 6.0 6.0 6.0CO, CO2, hal. HC

ED

He 6.0 6.0

N2 6.0 5.5

H2, O2 5.0 5.0

CH4 4.5 4.5

MS He H2O, O2 7.0 - 6.0 6.0

Downstream of the column, the individualconstituents of the sample are indicated by asuitable detector. Which kind of operating gasesare required in addition to the carrier gas directlydepends on the type of detector.

In principle, a thermal conductivity detector(TCD) can detect all substances. However, itslimits of detection are in the ppm to % range.There is no need of any other process gases thana carrier gas of purity level 5.0 or higher.A flame ionization detector (FID) can detect allcombustible substances except hydrogen. Itrequires a hydrogen purity of 5.0 to 6.0 andhydrocarbon-free air to feed the flame. In theautomotive industry, a mixture of helium andhydrogen (60:40) is often used instead of purehydrogen. The detection limit for hydrocarbonsgenerally is in the upper ppb range.An electron capture detector (ECD) is particularlysensitive for the specific detection ofhalogenated compounds with a detection limit inthe sub-ppb range. For this detector, we offerspecial gases of "ECD quality" with a specifiedlevel of halogenated hydrocarbon impurities ofless than 1 vpb. In addition to the carrier gas,usually helium "ECD" or nitrogen "ECD", a "make-up gas" is required to operate this detector. Thisgas is used to flush out of the detector anycontaminants that could stick to its cathode. Amixture of 5% or 10% methane in argon (ECD)as well as ECD grade nitrogen has proven to besuitable for this.

Certain detectors can be used for the specificdetection of individual substances, e.g. a flamephotometric detector (FPD), a photo-ionizationdetector (PID), an atomic emission detector(AED) or a discharge ionization detector (DID).The table gives a summary of the requirementsfor the carrier and process gases required byvarious detectors depending on the concentrationrange.

Atomic emission spectrometryAtomic emission spectrometry (AES) can beused to analyze samples containing metals. Theabsorbed energy ionizes and excites the metallicconstituents of the sample. The excited ions re-emit the absorbed energy with a wavelength thatis characteristic for each metal. The intensity ofthis emission is directly related to theconcentration. The various methods aredifferentiated according to the type of excitation.

If excitation takes place in a flame, this is termedflame photometry. This is frequently used foralkali metals and alkaline earth metals. Propane2.5 or acetylene 2.6 is used as the fuel gas.

ICP spectroscopy (Inductively CoupledPlasma) is based on a similar principle; however,it is an essentially all-purpose method that can beused to detect nearly all substances. Highfrequency induction is used to generate an argonplasma that transfers the energy to theconstituents in the sample.

The emissions are characteristic of the substanceand are directly proportional to the concentration.The purity of the argon used for this is critical,because oxygen and moisture at a concentrationof a few ppm in the plasma may lead toundesirable secondary reactions. The individualconstituents of the sample may then be in theform of oxides or hydroxides instead of theirreduced form. Therefore, we recommend theuse of Argon 4.8 or, even better, "Argon forspectrometry".

Elements in metal alloys can also be determinedby spark erosion spectrometry, an analyticalprocedure used in steel production and castingprocesses. Similar to ICP spectroscopy, anelectrical gas discharge is used to generate anargon plasma that ionizes the constituent atomson the surface of the metal sample. Thisproduces a spark as the substance vaporizes. Theemitted radiation is characteristic for theindividual elements and its intensity is also adirect measure of the concentration. As in theICP method, the presence of oxygen andmoisture perturbs the sensitive measurements.Therefore, we have developed a special grade,"Argon for spectrometry" that has reduced levelsof oxygen and moisture impurities. The detectionlimits can be improved further if Oxisorb andHydrosorb purification cartridges are used (seethe chapter "Gas purification"). An argon/hydrogenmixture can also be used that acts as a reducingagent, allowing detection of any oxides which arepresent.

Spark erosion spectrometry:Spots on metallic samples

In flame AAS, the sample is atomized by aflame. This requires additional fuel and oxidationgases. A flame produced from acetylene (purity2.6) and air (at approx. 2,400 °C) is generallysufficient for the analysis of most metals. Formetals that form very stable oxides, such aschromium or vanadium, nitrous oxide (laughinggas) is often used as the oxidation gas. Thisproduces a very hot flame (approx. 2,800 °C) thatcan decompose these metal oxides. In the caseof the lighter alkaline metals or alkaline earthmetals, the best source of energy is often a"cooler" flame (approx. 2,100 °C) produced fromHydrogen 5.0 and air.

In a graphite tube furnace, the required energy(up to approx. 3,000 °C) is produced electrically.Argon (purity 4.8 or higher) or argon/hydrogenmixtures are used as an inert gas to preventoxidation of the graphite tube.

FTIR and NDIR spectroscopySpectroscopy from the UV to the IR range issometimes used to analyze gas mixtures.NDIR monitors are especially popular for theanalysis of carbon monoxide or carbon dioxidein automobile exhaust gases. In general, forinfrared spectroscopy, the FT-IR method has 

Important specialized cases include the chemo-luminescence method to determine nitrogenoxides NO/NOx and emission spectroscopy inthe UV/visible range with plasma excitation toanalyze the purity of reactive gases.

The use of mass spectrometry to analyze gasesis now routine, even in standard operations tomonitor e.g. tank farms and air separation unitsor even for filling of high-purity gases. The mostcommonly used ionization method is electronimpact ionization; examples for special cases areionization at atmospheric pressure (atmosphericpressure ionization mass spectrometry – APIMS)to measure ultra fine traces in high-purity gasesas well as ionization with inductively coupledplasmas, which, like emission spectrometry, canbe used for sensitive analysis of metals inreactive gases.

Atomic absorption spectroscopyAtomic absorption spectroscopy (AAS) is amodified form of flame photometry. Radiationfrom an element-specific spectral light source ispassed through a sample that has been thermallydissociated into atoms and the attenuation of theradiation intensity due to absorption is measured.The attenuation of the intensity is a measure ofthe concentration of the respective metal in thesample. Again, various methods aredifferentiated according to the type of excitation.

become established using long-path gas cellswith variable path lengths, if necessary. Newspectrometry techniques that use tunable lasersare becoming increasingly popular.

Mass spectrometer for purity analysis of gases

FT-IR spectrometer with long-path cell

Measurement of radioactivitySpecial gas mixtures for filling Geiger-Müllercounters allow nuclear radiation to be measured.Gas mixtures with 5 or 10 Vol% of methane inargon are generally used and are known as P5and P10 gas, respectively. The purity of the gas isalso important for reliable operation of themeasurement systems. In particular the contentof electro-negative impurities (e.g. halogenatedhydrocarbons) has to be very low.

Zero gasesAll analytical methods are affected in variousdegrees by undesirable impurities, such asoxygen or water. Also, other secondaryconstituents can raise the zero line or the noiseand thus shift the detection limit. Therefore, thegases have to have a minimum purity of 5.0 oreven better. If necessary, certain impurities canbe removed at the "point-of-use" by means ofsuitable purification processes (see the separateinformation on "Gas purification systems"). 

Method Use of gas Gas

Atomic emission spectrometry (AES)

Flame photometryFuel gas Propane 2.5, Acetylene 2.6

Oxidation gas Synth. air

ICP spectrometyPlasma gas/

Ar 4.8, Ar for spectrometrycarrier gas

Spark erosion spectroscopy Plasma gas Ar 4.8, Ar for spectrometry, Ar/H2mixtures

Atomic absorption spectrometry (AAS)

Flame AASFuel gas Acetylene 2.6, H2 5.0

Oxidation gas Ambient air, synth. air, O2, N2O 2.5

Graphite tube AAS Inert gas Ar higher than 4.8, Ar/H2 mixtures

Ionisation Chamber Filling gas 5 / 10 Vol.% CH4 in Ar (P5 or P10 gas)

Additionally required gasesIn addition to the pure gases and gas mixturesneeded for direct operation of the equipment orthe analyzers, a range of other gases are used inanalytical applications that are not discussed indetail here. For example, specific detectorsrequire cooling with liquid nitrogen or even liquidhelium (nuclear magnetic resonance, NMR),optical systems are often purged with purenitrogen and some gases are also used in samplepreparation. Messer offers not only competentadvice but also all the required gases inthe required purity. For example, we have aspecial grade for extraction with supercriticalcarbon dioxide (SFC), namely CO2 SFC, optionallyalso pressurised by gaseous helium. Calibration gasesMost of the methods of analysis currently used inpractice are, in principle, comparative methods.This means that the analyzer must be calibratedbefore quantitative measurements are possible.In the case of gas analysis, this is generallycarried out by measuring a zero gas as well asone or more calibration gases with a definedcomposition. We also produce the high-precisiongas mixtures used for this, customized accordingto the particular requirements of the analyticaltask with the desired tolerance and accuracy inour special gas facilities (see the separateinformation on "Gas mixtures").

Messer Group GmbHGahlingspfad 31

47803 KrefeldTel. +49 2151 7811-0

Fax +49 2151 7811-501info@messergroup.com

www.messergroup.com

Use of gasesHandling gases requires the user to exercisegreat care for reasons of safety and quality. Onlythe use of suitable gas supply systems andpipelines can ensure that the quality of the gas isnot affected during its passage from the storagecontainer to the point-of-use. Wherever possible,we recommend the installation of a central gassupply system that allows the requiredpressurized gas bottles to be set up outside thelaboratory. The gases can then be fed throughsuitable pipelines into the laboratory so that theyare available "on tap". Our customer advisoryservice is available for help and advice (see theseparate information on "gas supply systems andcomponents for special gases").

Analysis controls qualityToday, analytical procedures are an essentialcomponent of daily practice in very differentareas of business. The reliability and accuracy ofthe attainable results depend on many limitingconditions. The quality of the operating andcalibration gases often plays a decisive role.Messer offers a comprehensive range of high-purity gases, standard and individual gas mixturesas well as the required gas supply systems.

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