34
IdentifyingTarget Analytes
Qualitative identification of a target compound is based on retention time (± 0.06minutes) and
on comparison of the samplemass spectrum to a referencemass spectrum. Compounds are
identified from three ions of the greatest intensity. The quantification ion, usually the highest
m/z fragment, is used for determining the concentration of a particular analyte.When using
any column forGC/MS, attentionmust be given to coeluting compounds to determine if
acceptable quantification ions can be found. It is important that there be no coelution between
compounds sharing ions used for quantification.As long as unique ions can be selected for
quantifying compounds that share retention time, chromatographic coelution is acceptable.
Reducing analysis timewithout carefully checking for coelutions can lead to problems. For
example, internal standard chlorobenzene-d5 and analyte 1,1,1,2-tetrachloroethane, which
share quantification ion 117, can coelute from a “VMS,” “624,” or “VRX” stationary phase.
Many laboratories eliminate the need for chromatographic resolution by changing the quantifi-
cation ion for chlorobenzene-d5 from 117 to 82 (Figure 35).An alternative solution is to
replace chlorobenzene-d5with another internal standard that elutes in the same region of the
chromatogram, such as 4-bromofluorobenzene. In performance-basedmeasurement systems
(PBMS), surrogates and internal standardsmay be changed, as long as the analyst can show
that the performancewill be equivalent or better.Auditors for state or local regulatory agencies
might not allow these changes, however.An additional option, the suggested surrogates can be
used and the analysis performed using a slowerGC oven temperature program that resolves
the coelution (Figure 36). This option eliminates the need to change either the internal standard
or the quantification ion, but prolongs analysis time.An analysis time of less than 10minutes,
with chromatographic resolution of these compounds, is possiblewith a 20m x 0.18mm x
1.0µm df Rtx
®
-VMS column (seeApplication section, page 51).
IdentifyingNon-Target Analytes
For samples containing analytes that do not match retention time and/ormass spectra for the
target compounds, a library search can be used in an attempt tomatch the unknown spectra
with known spectra. Unknown compounds in the sample, referred to as
tentatively identified
compounds
(TICs), should be reported only as estimates.
Mass Spectral Clues for IdentifyingTICs:
TheMS provides three valuable clues to identi-
fyingTICs: parent ion, isotopic composition, and common fragmentation ions. The parent
ion (also known as themolecular weight ion) is the ionized form of the neutral compound,
but not all compounds are stable enough to produce amolecular weight ion.Most environ-
mental contaminants, except compounds that contain nitrogen, will have an even number
mass for a parent ion. The parent ion reveals information about the elemental composition
and the distribution of isotopes. The term “isotope” is used to describe atoms of an element
with differing numbers of neutrons.Most elements have isotopes in a particular distribution
to each other. For example, carbon occurs primarily in two forms,
12
C and
13
C.
13
C is at an
abundance of 1.1% relative to
12
C.
12
This information can contribute to determining the num-
ber of carbons present in the fragmented ion. Carbon is considered anA+1 element because
its isotopes vary by 1amu. Compounds such as oxygen, sulfur, silicon, chlorine, and bromine
areA+2 elements because their isotopes vary by 2amu ormore (Figure 37, page 35).
Fragmentation ions also can offer clues to compound composition (TableV).
Leak-FreeColumn/MS InstallationUsing an InjectionPort Connection
Themost common problem associatedwith volatiles analyses byGC/MS is the presence of
leaks. The following procedurewill help ensure optimum performance. Do not use this pro-
cedurewith columnswith IDs larger than 0.25mm, because the amount of oxygen that
would be introduced into theMS source during the last stepwill oxidize themetal parts and
reduce sensitivity.
Figure 36.
A slower oven temperature program
eliminates the need to change the internal
standard or the quantification ion.
chlorobenzene-d5
ethylbenzene
chlorobenzene
Figure 35.
The quantification ion for
chlorobenzene-d5 can be changed
from ion 117 to ion 82, to eliminate the
need for chromatographic resolution from
1,1,1,2-tetrachloroethane.
Ion 117
Ion 82
Ion 131
ion 117
ion 117
}
chlorobenzene-d5
Extracted ion chromatogram (black)
overlayed on total ion chromatogram (green)
1,1,1,2-tetrachloroethene
12. F.W. McLafferty andF. Turecek. Interpretation ofMass Spectra, University Science Books, Mill
Valley, 4th edition, 1993, pp. 283-291.
