ASTM E840-95(2005)
(Practice)Standard Practice for Using Flame Photometric Detectors in Gas Chromatography
Standard Practice for Using Flame Photometric Detectors in Gas Chromatography
ABSTRACT
This practice is intended as a guide for the use of a flame photometric detector (FPD) as the detection component of a gas chromatographic system. The different principles of flame photometric detectors, and detector construction are presented in details. The detector sensitivity, minimum detectability, dynamic range, power law of sulphur response, linear range-phosphorus mode, unipower response range, noise and drift, and specificity are presented in details. The photomultiplier dark current is the magnitude of the FPD output signal measured with the FPD flame off. Flame background current is the difference in FPD output signal with the flame on and with the flame off in the absence of phosphorus or sulfur compounds in the flame.
SCOPE
1.1 This practice is intended as a guide for the use of a flame photometric detector (FPD) as the detection component of a gas chromatographic system.
1.2 This practice is directly applicable to an FPD that employs a hydrogen-air flame burner, an optical filter for selective spectral viewing of light emitted by the flame, and a photomultiplier tube for measuring the intensity of light emitted.
1.3 This practice describes the most frequent use of the FPD which is as an element-specific detector for compounds containing sulfur (S) or phosphorus (P) atoms. However, nomenclature described in this practice are also applicable to uses of the FPD other than sulfur or phosphorus specific detection.
1.4 This practice is intended to describe the operation and performance of the FPD itself independently of the chromatographic column. However, the performance of the detector is described in terms which the analyst can use to predict overall system performance when the detector is coupled to the column and other chromatographic system components.
1.5 For general gas chromatographic procedures, Practice E 260 should be followed except where specific changes are recommended herein for use of an FPD.
1.6 &solely-SI-units;
This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. For specific safety information, see Section , Hazards.
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Designation: E840 − 95 (Reapproved2005)
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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope 2. Referenced Documents
2.1 ASTM Standards:
1.1 Thispracticeisintendedasaguidefortheuseofaflame
E260Practice for Packed Column Gas Chromatography
photometric detector (FPD) as the detection component of a
E355PracticeforGasChromatographyTermsandRelation-
gas chromatographic system.
ships
1.2 This practice is directly applicable to an FPD that
2.2 CGA Standards:
employs a hydrogen-air flame burner, an optical filter for
CGAP-1 Safe Handling of Compressed Gases in Contain-
selective spectral viewing of light emitted by the flame, and a
ers
photomultiplier tube for measuring the intensity of light
CGAG-5.4 Standard for Hydrogen Piping Systems at
emitted.
Consumer Locations
CGAP-9 The Inert Gases: Argon, Nitrogen and Helium
1.3 ThispracticedescribesthemostfrequentuseoftheFPD
CGAV-7 Standard Method of Determining Cylinder Valve
which is as an element-specific detector for compounds con-
Outlet Connections for Industrial Gas Mixtures
taining sulfur (S) or phosphorus (P) atoms. However, nomen-
CGAP-12Safe Handling of Cryogenic Liquids
clature described in this practice are also applicable to uses of
HB-3Handbook of Compressed Gases
the FPD other than sulfur or phosphorus specific detection.
3. Terminology
1.4 This practice is intended to describe the operation and
performance of the FPD itself independently of the chromato- 3.1 Definitions—For definitions relating to gas
graphic column. However, the performance of the detector is chromatography, refer to Practice E355.
described in terms which the analyst can use to predict overall 3.2 Descriptions of Terms—Descriptions of terms used in
system performance when the detector is coupled to the this practice are included in Sections 7-17.
column and other chromatographic system components.
3.3 Symbols—A list of symbols and associated units of
measurement is included in Annex A1.
1.5 For general gas chromatographic procedures, Practice
E260 should be followed except where specific changes are
4. Hazards
recommended herein for use of an FPD.
4.1 Gas Handling Safety—The safe handling of com-
1.6 The values stated in SI units are to be regarded as pressed gases and cryogenic liquids for use in chromatography
standard. No other units of measurement are included in this is the responsibility of every laboratory. The Compressed Gas
standard. Association,(CGA),amembergroupofspecialtyandbulkgas
suppliers, publishes the following guidelines to assist the
1.7 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
1 2
This practice is under the jurisdiction ofASTM Committee E13 on Molecular For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Spectroscopy and Separation Science and is the direct responsibility of Subcom- contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
mittee E13.19 on Separation Science. Standards volume information, refer to the standard’s Document Summary page on
Current edition approved Sept. 1, 2005. Published September 2005. Originally the ASTM website.
approved in 1981. Last previous edition approved in 2000 as E840–95(2000). Available from Compressed Gas Association, Inc., 1725 Jefferson Davis
DOI: 10.1520/E0840-95R05. Highway, Arlington, VA 22202-4100.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E840 − 95 (2005)
from the flame during that combustion process. Therefore, the intense emissions from the HPO and S molecules are the
FPD is a flame optical emission detector comprised of a result of chemiluminescent reactions in the flame rather than
hydrogen-air flame, an optical window for viewing emissions thermal excitation of these molecules (1). The intensity of
generated in the flame, an optical filter for spectrally selecting light radiated from the HPO molecule generally varies as a
the wavelengths of light detected, a photomultiplier tube for linearfunctionofP-atomflowintotheflame.Inthecaseofthe
measuring the intensity of light emitted, and an electrometer S emission,thelightintensityisgenerallyanonlinearfunction
for measuring the current output of the photomultiplier. of S-atom flow into the flame, and most often is found to vary
as the approximate square of the S-atom flow. Since the FPD
5.2 The intensity and wavelength of light emitted from the
response depends on the P-atom or S-atom mass flow per unit
FPDflamedependsonthegeometricconfigurationoftheflame
time into the detector, the FPD is a mass flow rate type of
burner and on the absolute and relative flow rates of gases
detector. The upper limit to the intensity of light emitted from
supplied to the detector. By judicious selection of burner
both the HPO and S molecules is generally determined by the
geometryandgasflowrates,theFPDflameisusuallydesigned
onset of self-absorption effects in the emitting flame. At high
to selectively enhance optical emissions from certain types of
concentrations of S and P atoms in the flame, the concentra-
molecules while suppressing emissions from other molecules.
tionsofgroundstateS andHPOmoleculesbecomessufficient
5.3 Typical FPD flames are normally not hot enough to to reabsorb light emitted from the radiating states of HPO and
promoteabundantopticalemissionsfromatomicspeciesinthe
S .
flame. Instead, the optical emissions from an FPD flame
5.6 InthepresenceofahydrocarbonbackgroundintheFPD
usually are due to molecular band emissions or continuum
flame, the light emissions from the phosphorus and sulfur
emissionsresultingfromrecombinationofatomicormolecular
compounds can be severely quenched (2). Such quenching can
speciesintheflame.Forsulfurdetection,lightemanatingfrom
occur in the gas chromatographic analysis of samples so
the S molecule is generally detected. For phosphorus
complex that the GC column does not completely separate the
detection,lightemanatingfromtheHPOmoleculeisgenerally
phosphorus or sulfur compounds from overlapping hydrocar-
detected. Interfering light emissions from general hydrocarbon
bon compounds. Quenching can also occur as the result of an
compounds are mainly comprised of CH and C molecular
underlying tail of a hydrocarbon solvent peak preceding
band emissions, and CO+O → CO +hγ continuum radia-
phosphorus or sulfur compounds in a chromatographic sepa-
tion.
ration. The fact that the phosphorus or sulfur response is
reducedbyquenchingisnotalwaysapparentfromachromato-
5.4 Hydrogen – air or hydrogen – oxygen diffusion flames
are normally employed for the FPD. In such diffusion flames, gram since the FPD generally gives little response to the
the hydrogen and oxygen do not mix instantaneously, so that hydrocarbon.Theexistenceofquenchingcanoftenberevealed
these flames are characterized by significant spatial variations by a systematic investigation of the variation of the FPD
in both temperature and chemical species. The important responseasafunctionofvariationsinsamplevolumewhilethe
chemical species in a hydrogen – air flame are the H, O, and analyte is held at a constant amount.
OH flame radicals. These highly reactive species play a major
5.7 The chromatographic detection of trace level phospho-
role in decomposing incoming samples and in the subsequent
rus or sulfur compounds can be complicated by the fact that
production of the desired optical emissions. Optical emissions
such compounds often tend to be highly reactive and adsorp-
from the HPO and S molecular systems are highly favored in
tive. Therefore, care must be taken to ensure that the entire
those regions of an FPD flame which are locally rich in
chromatographic system is properly free of active sites for
H-atoms, while CH and C light emissions from hydrocarbons
adsorption of phosphorus or sulfur compounds. The use of
originate mainly from those flame regions which are locally
silanized glass tubing as GC injector liners and GC column
rich in O-atoms. The highest sensitivity and specificity for
materials is a good general practice. At near ambient
sulfur and phosphorus detection are achieved only when the
temperatures, GC packed columns made of FEP TFE-
FPD flame is operated with hydrogen in excess of that
fluorocarbon, specially coated silica gel, or treated graphitized
stoichiometricamountrequiredforcompletecombustionofthe
carbon are often used for the analysis of sulfur gases.
oxygen supplied to the flame. This assures a large flame
volume that is locally abundant in H-atoms, and a minimal
6. Detector Construction
flame volume that is locally abundant in O-atoms. The sensi-
6.1 Burner Design:
tivity and specificity of the FPD are strongly dependent on the
6.1.1 Single Flame Burner (2, 3)—The most popular FPD
absolute and relative flow rates of hydrogen and air. The
burner uses a single flame to decompose sample compounds
optimum hydrogen and air flow rates depend on the detailed
and generate the optical emissions. In this burner, carrier gas
configuration of the flame burner. For some FPD designs, the
and sample compounds in the effluent of a GC column are
flows which are optimum for phosphorus detection are not the
mixed with air and conveyed to an orifice in the center of a
same as the flows which are optimum for sulfur detection.
flame tip. Excess hydrogen is introduced from the outer
Also, the flows which are optimum for one sample compound
perimeter of this flame tip so as to produce a relatively large,
may not necessarily be optimum for another sample com-
pound.
5.5 Although the detailed chemistry occurring in the FPD
The boldface numerals in parentheses refer to the list of references at the end
flame has not been firmly established, it is known that the of this practice.
E840 − 95 (2005)
diffuse hydrogen-rich flame. With this burner and flow
configuration, light emissions from hydrocarbon compounds
occur primarily in the locally oxygen-rich core of the flame in
close proximity to the flame tip orifice, while HPO and S
emissions occur primarily in the upper hydrogen-rich portions
of the flame. Improved specificity is therefore obtained by the
use of an optical shield at the base of the flame to prevent
hydrocarbon 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 burner are as follows:
6.1.1.1 Solvent peaks in the GC effluent can momentarily
starve the flame of oxygen and cause a flameout. This effect
can be avoided by interchanging the hydrogen and air inlets to
the burner (5) with a concomitant change in the flame gas flow
rates to achieve maximum signal-to-noise response. Whereas
FIG. 1 Spectral Distribution of Molecular Emissions from an FPD
interchanging the H and air inlets will eliminate flameout
Flame
problems, this procedure will often yield a corresponding
decreaseinthesignal-to-noiseratioandhencecompromisethe
FPD detectability.
6.3.1 The photomultiplier tube used in the FPD generally
6.1.1.2 Responsetosulfurcompoundsoftendeviatesfroma
has a spectral response extending throughout the visible
puresquarelawdependenceonsulfur-atomflowintotheflame.
spectrum with maximum response at approximately 400 nm.
Furthermore, the power law of sulfur response often depends
Some specific tubes that are used are an end-viewing EMI
on the molecular structure of the sample compound (4).
9524B, and side-viewing RCA 4552 or 1P21 tubes or their
6.1.1.3 The phosphorus or sulfur sensitivity often depends
equivalents. For FPD applications, the photomultiplier tube
on the molecular structure of the sample compound.
should have a relatively low dark current characteristic (for
6.1.1.4 Hydrocarbon quenching greatly reduces the re-
example,0.1to1.0nA)sothattheFPDbackgroundsignaland
sponse to phosphorus and sulfur compounds (2).
noiselevelsaredeterminedbytheFPDflameratherthanbythe
6.1.2 Dual Flame Burner (2, 5)—A second FPD burner
photomultiplier limitations. The photomultiplier dark current
design uses two hydrogen-rich flames in series.The first flame
anditsassociatednoise(seeSection15)dependstronglyonthe
is used to decompose samples from the GC and convert them
photomultiplier’s operating voltage and its ambient tempera-
into combustion products consisting of relatively simple mol-
ture.
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 illustr
...
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