v-xEPA
United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S7-82-033 August 1982
Project Summary
Interpretation of Low
Resolution Mass Spectra for
Level 1 Analysis of
Environmental Mixtures
James L. Stauffer
This report is a set of guidelines for
interpreting the low resolution mass
spectra (LRMS) of complex chemical
mixtures, within the context of EPA's
Level 1 Environmental Assessment
Program1. It discusses principles
underlying direct mass spectrometric
analysis of complex mixtures, tech-
niques for optimizing the analyses,
and interpreting and evaluating the
results. A chapter presents some
interpretive aids for LRMS analysis of
environmental mixtures. The guide-
lines are illustrated by a step-by-step
detailed analysis of the mass spectra
of four representative samples. The
final chapter gives direction for
reporting the results in the EPA Level
1 LRMS report format.
LRMS plays an important role in
determining the chemical composi-
tion of environmental mixtures. The
other components of the Level 1
organic analysis scheme, liquid
chromatography (LC) fractionation
and infrared analysis (IR), also con-
tribute significantly to the overall
analysis. The LC and IR procedures
are described elsewhere1. This report
is limited to a detailed discussion of
the LRMS component of the Level 1
scheme.
This Project Summary was devel-
oped by EPA's Industrial Environ-
mental Research Laboratory, Research
Triangle Park, NC, to announce key
findings of the research project that is
fully documented in a separate report
of the same title (see Project Report
ordering information at back).
Summary
Requirements for mass spectrometric
analysis of the environmental samples
obtained in the EPA Level 1 program are
for identification of chemical class, and
order of magnitude quantitation. These
standards are met through the com-
bination of complementary analysis
methods: (1) a liquid chromatographic
(LC) fractionation of the environmental
mixture, followed by (2) mass spec-
trometric and infrared spectrometric
(IR) analysis of each LC fraction. The LC
fractionation narrows the range of
chemical classes that can be present in
any of the individual fractions to a level
which is manageable by LRMS; the
mass spectrometric analysis of each LC
fraction establishes the molecular
weight range of the fraction, and the
chemical classes present. To eliminate
the chemical discrimination effects that
are likely to be encountered when gas
chromatography is used for LRMS
sample introduction, the samples in this
program are introduced in the LRMS via
the direct probe. This provides the most
representative aliquot of the sample to
the LRMS source, but also yields
spectra that, typically, contain the
superimposed spectra of several chem-
ical components. A batch inlet is used
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for samples that are too volatile for
successful introduction via the probe
inlet.
Complementary sources of informa-
tion available in the EPA Level 1
program that may be useful in interpret-
ing the mass spectra include the IR
spectra that are obtained on all of the LC
fractions. These support (confirm) the
assignments obtained via LRMS, and
signal the presence of chemical func-
tional groups that may not be recogniz-
able solely on the basis of the mass
spectra.
The LC f ractionation scheme provides
an initial polarity separation of the
mixture into seven fractions that range
from non-polar (fraction 1) to most
polar (fraction 7). Table 1 shows the
types of separations that are obtained.
The analysis results obtained by
LRMS are reported primarily as chemi-
cal classes and molecular weight
ranges of those classes, with subcate-
gory or specific compound or composi-
tion designation whenever possible.
The categories defined for reporting
LRMS data are shown in Table 2.
Direct Analysis of Mixtures
by LRMS
The detail that may be obtained from
the mass spectrum of multi-component
mixtures depends both on the complexity
of the spectrum itself, and on the
amount of supplemental information
that is available. The precision of the
identification that may be obtained will
vary accordingly, ranging from specific
compound or composition assignments
for all of the spectrum, to simply an
indication of the chemical classes that
are present. The task confronting the
analyst of the mass spectra of multi-
component mixtures is to discover the
correct combination of individual
spectra that will adequately account for
the experimentally observed spectrum.
The additive nature of superimposed
mass spectra ensures that this is
possible, and the multi-peak nature of
electron impact mass spectra makes it
practical in most cases. The combina-
tion of the two aspects ensures that, if
the observed mass spectrum is fully
accounted for by the combined indi-
vidual assignments, then those assign-
ments are an accurate indication of the
chemical class makeup of the sample.
Two principal techniques provide
clues to the analyst for tentative
individual chemical class or compound
Table 1. Representative Checmical class Distribution by LC Fraction*
LC Fraction
1. aliphatic hydrocar-
bons, sulfur
2. aromatic hydrocar-
bons
3. PAH species
4. carbazoles
5.
6.
7.
halogenated aliphatics
halogenated aromatics
aromatic hydrocarbons
nitroaromatics
carbolic acids
phenols
sulfonic acids
heterocyclic sulfur, PAH
heterocyclic N
heterocyclic O
ketones
esters
*This list represents the type of partitioning that may be expected to occur, but it
should not be taken as definitive.
Many conditions could cause compound class distribution to vary from that shown
here. The illustrated overlap between fractions similarly is representative, rather
than definitive.
assignments. The first and most im-
portant Off these is the fractional
distillation'of the sample that occurs as
the direct insertion probe is slowly
taken through its complete temperature
cycle from cool to hot. The second is the
use of both high (70 eV) and low (10 - 2O
eV) electron impacts or chemical
ionization modes, at or near the same
probe temperature. The thermal disti Na-
tion provides a separation into suc-
cessive molecular weight ranges, and
the change of ionization mode differen-
tiates between parent and fragment
ions. All of the data, taken in combina-
tion, provide enough information for
overall spectral interpretation.
Tentative assignments made on the
basis of the above information are
confirmed or modified in the confirma-
tion phase of the analysis. In the first
phase of confirmation, standard spectra
obtained either from the literature2"3 or
from reference compounds are used to
evaluate how completely the experi-
mentally observed mass spectrum is
accounted for by the combined tentative
individual assignments. The second
phase is to make a similar comparison
against the data obtained in the IR
analysis, to see if all fractional groups
and IR spectral features have been
recognized and accounted for in the
mass spectral analysis. An accurate set
of assignments will account for all
spectral features of both the MS and the
IR data.
Several interpretation aids can be
helpful in analyzing the LRMS data. The
first of these is a table of mass numbers
and associated Z values, where the Z
value is given by the relationship:
mW = CnH(2n + Zl
A Z value for any ion in the spectrum
can be correlated with a limited range of
possible chemical classes, and a very
limited range of possible chemical
compositions. Mass values for PAH
species can be correlated to specific
chemical compositions and numbers of
rings, although not to specific isomers.
In most cases, similarily specific chemi-
cal composition assignments can be
made to individual mass values for aza-
arenes, and for oxygen- or sulfur-
containing polycyclic species.
Several step-by-step examples of
analyses are included in the report. The
samples used for examples are:
(1) A synthetic mixture of polar
materials (LC fractions 5, 6, 7).
(2) A simple PAH mixture (LC fraction
3).
(3) A complex hydrocarbon mixture
(without LC chromatographic
separation).
(4) A set of potentially confusing
spectra, obtained from cigarette
tar (LC fractions 3 and 4).
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Table 2. Categories for Reporting LRMS Data
Category
(Subcategory)
Most probable
LC fraction*
Category
(Subcategory}
Most probable
LC fraction*
Aliphatic hydrocarbons
(Alkanes)
(Alkenes)
(Alkynes)
Halogenated aliphatics
(Saturated)
(Unsaturated)
Aromatic hydrocarbons
(Benzenes}
Halogenated aromatic hydrocarbons
Nitro aromatic hydrocarbons
Fused alternate, nonalternate hydrocarbons
MW<216 (methyl pyrene)
MW<216
Ethers
(Halogenated ethers)
Epoxides
Aldehydes
Heterocyclic oxygen compounds
Nitrites
(Aliphatic)
(Aromatic)
Alcohols
(Primary, secondary, tertiary)
(Glycols)
1
1
1
1
1,2
1,2
1,2
2,3
2,3
2,3
4,5
2.3
2,3
2,3
4
4
4
4
3.4
4
4
4
6
6
6
Phenols
(Alkyl, etc.)
(Halogenated phenols)
(Nitrophenols)
Esters
(Phthalates)
Ketones
Amines
(Primary, secondary, tertiary)
(Hydrazines, azo compounds)
(Nitrosoamines)
Heterocyclic nitrogen compounds
(Indoles, carbazoles)
(Quinolines, acridines)
Alkyl sulfur compounds
(Mercaptans)
(Sulfides. disulfides)
Heterocyclic sulfur compounds
(Benzothiophenes)
Sulfonic acids, sulfoxides
Amides
Carboxylic acids
Silicones
Phosphates
6
6
6
6
6
6
6
6
6
6
6
4
6
6
6
6
7
6
6,7
2,3,4
5.6.7
*Possible assignments. Fractions 4-5, 5-6, 6-7 generally overlap to a considerable extent. Also, additional components of a
particular molecule may cause it to elute in an LC fraction other than that expected. For example, a short-chain ester would
probably elute in LC Fraction 5 or 6, whereas a long-chain ester would elute in Fraction 3 or 4.
The several examples are designed to
illustrate:
(1) The establishment of mass
spectral peak associations.
(2) The use of reference spectra for
identification assignments in a
complex spectral matrix.
(3) The principle of total spectrum
accounting.
(4) Utilization of the LC fractionation
scheme for interpreting LRMS
spectra.
(5) Chemical class assignments in a
complex spectral matrix.
The spectra utilized in the analysis
examples are then used as reporting
examples to illustrate the use of the
standard EPA Level 1 Environmental
Assessment Program LRMS report
form.
References
1. Lentzen, D.E., Wagoner, D.E., Estes,
E.D., and Gutknecht, W.F., "IERL-
RTP Procedures Manual: Level 1
Environmental Assessment (2nd
edition)," EPA-600/7-78-201, NTIS
PB 293-795.(1978).
2. "Eight Peak Index of Mass Spectra,"
4 volumes, 2nd edition, published by
Mass Spectrometry Data Centre,
AWRE, Aldermaston, Reading, RG7
4PR, United Kingdom, 1974.
3. Heller, S.R., and Milne, G.W.A.,
"EPA/NIH Mass Spectral Data
Base," 5 volumes, U.S. Department
of Commerce/National Bureau of
Standards, NSRDS-NBS 63 (1978).
* U.8.30VEHNMEWTPHIKT1NQOFFICE:HtS-559-017/0787
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James L. Stauffer is with Arthur D. Little, Inc.. Cambridge, MA 02140.
Larry D. Johnson is the EPA Project Officer (see below).
The complete report, entitled "Interpretation of Low Resolution Mass Spectra for
Level 1 Analysis of Environmental Mixtures, "(Order No. PB 82-232 455; Cost:
$15.00, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Postage and
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
CHICAGO
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