ASTM E840-95(2000)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation:E840–95 (Reapproved 2000)
Standard Practice for
Using Flame Photometric Detectors in Gas
Chromatography
This standard is issued under the fixed designation E840; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope CGAP-1 Safe Handling of Compressed Gases in Contain-
ers
1.1 Thispracticeisintendedasaguidefortheuseofaflame
CGAG-5.4 Standard for Hydrogen Piping Systems at
photometric detector (FPD) as the detection component of a
Consumer Locations
gas chromatographic system.
CGAP-9 The Inert Gases: Argon, Nitrogen and Helium
1.2 This practice is directly applicable to an FPD that
CGAV-7 Standard Method of Determining CylinderValve
employs a hydrogen-air flame burner, an optical filter for
Outlet Connections for Industrial Gas Mixtures
selective spectral viewing of light emitted by the flame, and a
CGAP-12 Safe Handling of Cryogenic Liquids
photomultiplier tube for measuring the intensity of light
HB-3 Handbook of Compressed Gases
emitted.
1.3 ThispracticedescribesthemostfrequentuseoftheFPD
3. Terminology
which is as an element-specific detector for compounds con-
3.1 Definitions—For definitions relating to gas chromatog-
taining sulfur (S) or phosphorus (P) atoms. However, nomen-
raphy, refer to Practice E355.
clature described in this practice are also applicable to uses of
3.2 Descriptions of Terms—Descriptions of terms used in
the FPD other than sulfur or phosphorus specific detection.
this practice are included in Sections 7-17.
1.4 This practice is intended to describe the operation and
3.3 Symbols:Symbols—A list of symbols and associated
performance of the FPD itself independently of the chromato-
units of measurement is included in Annex A1.
graphic column. However, the performance of the detector is
described in terms which the analyst can use to predict overall
4. Hazards
system performance when the detector is coupled to the
4.1 Gas Handling Safety—The safe handling of com-
column and other chromatographic system components.
pressed gases and cryogenic liquids for use in chromatography
1.5 For general gas chromatographic procedures, Practice
is the responsibility of every laboratory. The Compressed Gas
E260 should be followed except where specific changes are
Association,(CGA),amembergroupofspecialtyandbulkgas
recommended herein for use of an FPD.
suppliers, publishes the following guidelines to assist the
1.6 This standard does not purport to address all of the
laboratory chemist to establish a safe work environment.
safety concerns, if any, associated with its use. It is the
Applicable CG publications include CGAP-1, CGAG-5.4,
responsibility of the user of this standard to establish appro-
CGAP-9, CGAV-7, CGAP-12, and HB-3.
priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use. For specific safety
5. Principles of Flame Photometric Detectors
information, see Section 4, Hazards.
5.1 The FPD detects compounds by burning those com-
pounds in a flame and sensing the increase of light emission
2. Referenced Documents
from the flame during that combustion process. Therefore, the
2.1 ASTM Standards:
2 FPD is a flame optical emission detector comprised of a
E260 Practice for Packed Column Gas Chromatography
hydrogen-air flame, an optical window for viewing emissions
E355 Practice for Gas Chromatography Terms and Rela-
2 generated in the flame, an optical filter for spectrally selecting
tionships
the wavelengths of light detected, a photomultiplier tube for
2.2 CGA Standards:
measuring the intensity of light emitted, and an electrometer
for measuring the current output of the photomultiplier.
5.2 The intensity and wavelength of light emitted from the
This practice is under the jurisdiction ofASTM Committee E13 on Molecular
FPDflamedependsonthegeometricconfigurationoftheflame
Spectroscopy and is the direct responsibility of Subcommittee E13.19 on Chroma-
tography.
Current edition approved May 15, 1995. Published July 1995. Originally
published as E840–81. Last previous edition E840–91. Available from Compressed Gas Association, Inc., 1725 Jefferson Davis
Annual Book of ASTM Standards, Vol 14.02. Highway, Arlington, VA 22202-4100.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E840–95 (2000)
burner and on the absolute and relative flow rates of gases the approximate square of the S-atom flow. Since the FPD
supplied to the detector. By judicious selection of burner response depends on the P-atom or S-atom mass flow per unit
geometryandgasflowrates,theFPDflameisusuallydesigned time into the detector, the FPD is a mass flow rate type of
to selectively enhance optical emissions from certain types of detector. The upper limit to the intensity of light emitted from
molecules while suppressing emissions from other molecules. boththeHPOandS moleculesisgenerallydeterminedbythe
5.3 Typical FPD flames are normally not hot enough to onset of self-absorption effects in the emitting flame. At high
promoteabundantopticalemissionsfromatomicspeciesinthe concentrations of S and P atoms in the flame, the concentra-
flame. Instead, the optical emissions from an FPD flame tionsofgroundstateS andHPOmoleculesbecomessufficient
usually are due to molecular band emissions or continuum to reabsorb light emitted from the radiating states of HPO and
emissionsresultingfromrecombinationofatomicormolecular S .
speciesintheflame.Forsulfurdetection,lightemanatingfrom
5.6 InthepresenceofahydrocarbonbackgroundintheFPD
the S molecule is generally detected. For phosphorus detec-
flame, the light emissions from the phosphorus and sulfur
tion, light emanating from the HPO molecule is generally
compounds can be severely quenched (2). Such quenching can
detected. Interfering light emissions from general hydrocarbon
occur in the gas chromatographic analysis of samples so
compounds are mainly comprised of CH and C molecular
complex that the GC column does not completely separate the
band emissions, and CO+O→ CO +hg continuum radia-
phosphorus or sulfur compounds from overlapping hydrocar-
tion.
bon compounds. Quenching can also occur as the result of an
5.4 Hydrogen – air or hydrogen – oxygen diffusion flames
underlying tail of a hydrocarbon solvent peak preceding
are normally employed for the FPD. In such diffusion flames,
phosphorus or sulfur compounds in a chromatographic sepa-
the hydrogen and oxygen do not mix instantaneously, so that
ration. The fact that the phosphorus or sulfur response is
these flames are characterized by significant spatial variations
reducedbyquenchingisnotalwaysapparentfromachromato-
in both temperature and chemical species. The important
gram since the FPD generally gives little response to the
chemical species in a hydrogen – air flame are the H, O, and
hydrocarbon.Theexistenceofquenchingcanoftenberevealed
OH flame radicals. These highly reactive species play a major
by a systematic investigation of the variation of the FPD
role in decomposing incoming samples and in the subsequent
responseasafunctionofvariationsinsamplevolumewhilethe
production of the desired optical emissions. Optical emissions
analyte is held at a constant amount.
from the HPO and S molecular systems are highly favored in
5.7 The chromatographic detection of trace level phospho-
those regions of an FPD flame which are locally rich in
rus or sulfur compounds can be complicated by the fact that
H-atoms, while CH and C light emissions from hydrocarbons
such compounds often tend to be highly reactive and adsorp-
originate mainly from those flame regions which are locally
tive. Therefore, care must be taken to ensure that the entire
rich in O-atoms. The highest sensitivity and specificity for
chromatographic system is properly free of active sites for
sulfur and phosphorus detection are achieved only when the
adsorption of phosphorus or sulfur compounds. The use of
FPD flame is operated with hydrogen in excess of that
silanized glass tubing as GC injector liners and GC column
stoichiometricamountrequiredforcompletecombustionofthe
materials is a good general practice.At near ambient tempera-
oxygen supplied to the flame. This assures a large flame
tures, GC packed columns made of FEP TFE-fluorocarbon,
volume that is locally abundant in H-atoms, and a minimal
specially coated silica gel, or treated graphitized carbon are
flame volume that is locally abundant in O-atoms. The sensi-
often used for the analysis of sulfur gases.
tivity and specificity of the FPD are strongly dependent on the
absolute and relative flow rates of hydrogen and air. The
6. Detector Construction
optimum hydrogen and air flow rates depend on the detailed
6.1 Burner Design:
configuration of the flame burner. For some FPD designs, the
6.1.1 Single Flame Burner (2,3)—The most popular FPD
flows which are optimum for phosphorus detection are not the
burner uses a single flame to decompose sample compounds
same as the flows which are optimum for sulfur detection.
and generate the optical emissions. In this burner, carrier gas
Also, the flows which are optimum for one sample compound
and sample compounds in the effluent of a GC column are
may not necessarily be optimum for another sample com-
mixed with air and conveyed to an orifice in the center of a
pound.
flame tip. Excess hydrogen is introduced from the outer
5.5 Although the detailed chemistry occurring in the FPD
perimeter of this flame tip so as to produce a relatively large,
flame has not been firmly established, it is known that the
diffuse hydrogen-rich flame. With this burner and flow con-
intenseemissionsfromtheHPOandS moleculesaretheresult
figuration,lightemissionsfromhydrocarboncompoundsoccur
of chemiluminescent reactions in the flame rather than thermal
primarily in the locally oxygen-rich core of the flame in close
excitation of these molecules (1). The intensity of light
proximity to the flame tip orifice, while HPO and S emissions
radiated from the HPO molecule generally varies as a linear 2
occur primarily in the upper hydrogen-rich portions of the
function of P-atom flow into the flame. In the case of the S
flame. Improved specificity is therefore obtained by the use of
emission,thelightintensityisgenerallyanonlinearfunctionof
an optical shield at the base of the flame to prevent hydrocar-
S-atom flow into the flame, and most often is found to vary as
bon emissions from being in the direct field of view. The light
emissions generated in this flame are generally viewed from
the side of the flame. Some of the known limitations of this
The boldface numerals in parentheses refer to the list of references at the end
of this practice. burner are as follows:
E840–95 (2000)
6.1.1.1 Solvent peaks in the GC effluent can momentarily narrow-bandpassopticalfilterwithpeaktransmissionat525to
starve the flame of oxygen and cause a flameout. This effect 530 nm is generally used. For sulfur detection, a filter with
can be avoided by interchanging the hydrogen and air inlets to
peak transmission at 394 nm is most often used although the
the burner (5) with a concomitant change in the flame gas flow
optical region between 350 to 380 nm can also be employed.
rates to achieve maximum signal-to-noise response. Whereas
Typically, the filters used have an optical bandpass of approxi-
interchanging the H and air inlets will eliminate flameout
2 mately 10 nm.
problems, this procedure will often yield a corresponding
6.3 Photomultiplier Tube:
decreaseinthesignal-to-noiseratioandhencecompromisethe
6.3.1 The photomultiplier tube used in the FPD generally
FPD detectability.
has a spectral response extending throughout the visible
6.1.1.2 Responsetosulfurcompoundsoftendeviatesfroma
spectrum with maximum response at approximately 400 nm.
puresquarelawdependenceonsulfur-atomflowintotheflame.
Some specific tubes that are used are an end-viewing EMI
Furthermore, the power law of sulfur response often depends
9524B, and side-viewing RCA 4552 or 1P21 tubes or their
on the molecular structure of the sample compound (4).
equivalents. For FPD applications, the photomultiplier tube
6.1.1.3 The phosphorus or sulfur sensitivity often depends
should have a relatively low dark current characteristic (for
on the molecular structure of the sample compound.
example,0.1to1.0nA)sothattheFPDbackgroundsignaland
6.1.1.4 Hydrocarbon quenching greatly reduces the re-
noiselevelsaredeterminedbytheFPDflameratherthanbythe
sponse to phosphorus and sulfur compounds (2).
photomultiplier limitations. The photomultiplier dark current
6.1.2 Dual Flame Burner (2, 5)—A second FPD burner
anditsassociatednoise(seeSection15)dependstronglyonthe
design uses two hydrogen-rich flames in series.The first flame
photomultiplier’s operating voltage and its ambient tempera-
is used to decompose samples from the GC and convert them
ture.
into combustion products consisting of relatively simple mol-
ecules.Thesecondflamereburnstheproductsofthefirstflame
6.3.2 Operating voltages are typically in the range of 400 to
in order to generate the light emissions that are detected. A
900V,dependingonthetubetype.Generally,itisunlikelythat
principal advantage of the dual flame burner is that it greatly
two photomultiplier tubes of the same type have exactly the
reduces the hydrocarbon quenching effect on the phosphorus
samecurrentamplificationatagivenvoltage.Also,thecurrent
and sulfur emissions (6). Other advantages of the dual flame
amplificationofagivenphotomultipliertubeoftendecreasesas
burner compared to a single flame burner are that sulfur
the tube ages. Therefore, it is generally necessary to periodi-
responses more uniformly obey a pure square law response,
cally adjust the tube operating voltage in order to maintain the
and more uniform responses to phosphorus and sulfur com-
same FPD sensitivity.
pounds are obtained irrespective of the molecular structure of
6.3.3 Since the FPD burner housing generally operates at
thesamplecompound.Adisadvantageofthedualflameburner
elevatedtemperatures,acriticaldesignconstraintintheFPDis
is that it generally provides lower sensitivity to sulfur com-
the coupling of the maximum amount of light from the flame
pounds than a single flame burner in those analyses where
to the photomultiplier with minimum thermal coupling. In
hydrocarbon quenching is not a problem.
someFPDdesigns,opticallensesorfiberopticlightguidesare
6.2 Optical Filter—Fig. 1 illustrates the spectral distribu-
used to allow the photomultiplier to be operated in as cool an
tions of emissions from the S , HPO
...