5546 905R80114
Measurement of Trihalomethanes in Drinking Water
with Gas Chromatography/Mass Spectrometry and
Selected Ion Monitoring
Method 501.3
1. Scope and Application *
1.1 This method (501.3) provides procedures for identification and *
measurement of the four regulated trihalomethanes (chloroform,
bromoform, bromodichloromethane, and chlorodibromomethane) in
finished drinking water, raw source water, or drinking water in 'any
2
treatment stage. Previously promulgated methods, , 501.1 and
501.2, involve gas chromatographic separation, identification, and
measurement of these specific trihalomethanes after they are
removed from the sample matrix. Method 501.2 is an extraction
procedure; Methods 501.1 and 501.3 involve removal of trihalo-
methanes with purge and trap procedures. In Method 501.3, selected
ion monitoring with a mass spectrometer is substituted for the
hali de-specific gas chromatographic detector specified in Method
501.1. Any one of these methods may be used to analyze drinking
water for these four trihalomethanes, whose total concentration is
called total trihalomethanes.
1.2 With Method 501.3, method detection limits (MDLs) for trihalo-
methanes are:
chloroform, 0.06 ug/L;
t
bromodichloromethane, 0.07 ug/L;
chlorodibromomethane, 0.05 ug/L; and
w .
bromoform, 0.04 ug/L; r :• ,: :
23 j sj .;.;„•
Chicago, iiimois 6u6v4
v
-------
sncy
-------
where MDL is the minimum amount that can be measured with 99%
confidence that the reported value is greater than zero.
2. Summary of Method
2.1 Trihalomethanes are removed (purged) from the sample matrix by
bubbling helium through the aqueous sample. Purged trihalomethanes
and other sufficiently volatile sample components with sufficiently
P
low water solubility are sorbed onto Tenax-GC (a porous polymer
based on 2,6-diphenyl-p_-phenylene oxide) contained in a stainless
steel tube. When purging is complete, the sorbent tube is heated
and backflushed with helium to desorb purged sample components into
a gas chromatograph (GC) interfaced to a mass spectrometer (MS).
Trihalomethanes eluting from the GC column are identified and
measured by acquiring mass spectral data for selected ions that are
characteristic of individual trihalomethanes.
3. Interferences and Contamination Sources
3.1 With selected ion monitoring, the mass spectrometer is essentially
a compound-selective detector, and interferences are minimal. GC
retention time and relative ion abundance data for the four
trihalomethanes provide reliable identifications. No known com-
pounds that are purged with the conditions used in this method have
the same GC retention times and also produce the same fragment ions
in the same relative abundances as the four trihalomethanes.
3.2 With selected-ion monitoring, interfering contamination is only
likely to occur when a sample containing low concentrations of
trihalomethanes is analyzed immediately after a sample containing
relatively high concentrations of trihalomethanes. A preventive
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technique is between-sample rinsing of the purging apparatus and
sample syringes with two portions of reagent water. After analysis
of a sample containing high concentrations of trihalomethanes, the
system should be baked for 10 min by passing helium through the
sample purging chamber into the heated (180°) sorbent trap. One
or .more method blanks should be analyzed to ensure that accurate
values are obtained for the next sample.
3.3 Samples may be contaminated during shipment or storage'by diffusion
of volatile organics through the sample bottle septum seal. Field
blanks must be analyzed to determine when sampling and storage
procedures have not prevented contamination.
3.4 During analysis, major contaminant sources are impurities in the
inert purging gas and in the sorbent trap. Analysis of field
blanks and method blanks provides information about the presence of
contaminants.
4. Safety
4.1 The toxicity or carcinogenicity of chemicals used in this method
has not been precisely defined; each chemical should be treated as
a potential health hazard, and exposure to these chemicals should
be minimized. Each laboratory is responsible for maintaining
awareness of OSHA regulations regarding safe handling of chemicals
used in this method. Additional references to laboratory safety
are cited.
4.2 Primary standards of trihalomethanes should be handled in a hood.
5. Equipment and Materials
5.1 Sample containers — 25-mL or larger glass bottles, equipped with a
screw cap with center hole (Pierce #13075 or equivalent) and a
Teflon** faced silicone septum.
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5.2 Purge and trap device (Figures 1-2) consisting of sample purging
chamber, sorbent trap and desorber. (Acceptable devices are
commercially available.)
5.2.1 The all glass sample purging chamber (Figure 1) holds 5-mL
samples with < 15 ml of gaseous headspace between the water
column and the trap. The helium purge gas passes through
the water column as finely divided bubbles (optimum diameter
of <3 mm at the origin). The purge gas must be introduced
at a point <5 mm from the base of the water column.
5.2.2 The stainless steel sorbent trap (Figure 3) is 25 cm long by
2.5 mm ID and is packed with 1 cm of methyl-silicone coated
R
packing, 15 cm of Tenax-GC , and 8 cm of silica gel, in
that order with respect to the inlet end of the trap.
Silica gel is not necessary for efficient trapping of
trihalomethanes but does not hinder trapping; therefore,
P
silica gel may be replaced with additional Tenax-GC . A
trap with different dimensions can be used if it has been
evaluated and found to perform satisfactorily. Before
initial use, the trap should be conditioned overnight at
180°C by backflushing with helium flow of at least 20
mL/min. Each day the trap should be conditioned for 10 min
at 180°C with backflushing.
5.2.3 The desorber (Figure 3) should be capable of rapidly heating
the trap to 180°C. The trap section containing
Tenax-GC should not be heated to higher than 180°C, and
the temperature of the other sections should not exceed
200°C.
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5.3 Syringes and syringe valves
5.3.1 Two 5-mL glass hypodermic syringes with Luerlok tip (if
applicable to the purging device being used).
5.3.2 One 5-mL gas-tight syringe with shutoff valve.
5.3.3 Two two-way syringe valves with Luer ends (if applicable to
the purging device being used).
5.3.4 One 25-uL micro syringe with 0.006 in. ID needle.
5.3.5 One 100-uL micro syringe.
5.4 Miscellaneous
5.4.1 Standard solution storage containers — 10 ml bottles with
o
Teflon -lined screw caps.
5.4.2 Analytical balance capable of weighing 0.0001 g accurately.
5.4.3 Helium purge gas, as contaminant free as possible.
5.5 Sorfaent trap packing materials
5.5.1 Polymer based on 2,6-diphenyl-p_-phenylene oxide ~ 60/80
o
mesh Tenax-GC , chromatographic grade, or equivalent.
5.5.2 Methyl silicone coated packing — 3% OV-1 on 60/80 mesh
Chromosorb W, or equivalent.
5.5.3 Silica gel — 35/60 mesh, Oavison Chemical grade 15, or
equivalent.
5.6 Reagents
5.6.1 Sodium thiosulfate or sodium sulfite — granular, ACS
reagent grade.
5.6.2 Methanol — pesticide quality or equivalent.
5.6.3 Reagent water — water in which an interferent is not
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observed at the method detection limit of the compound of
interest. Reagent water may be prepared by passing tap water
through a filter bed containing about 0.5 kg of activated
carbon (Calgon Corp. Filtrasorb 300 or equivalent), by using
a water purification system (Millipore Super Q or equiva-
lent), or by boiling distilled water for 15 min followed by
a 1 h purge with inert gas while the water temperature is
held at 90°C. Reagent water should be stored in clean,
o
narrow-mouth bottles with Teflon -lined septa and screw
caps.
5.7 Stock standard solutions — These solutions may be purchased as
certified solutions or prepared from pure standard materials using
the following procedures:
5.7.1 Place about 9.8 ml of methanol in a 10-mL ground-glass
stoppered volumetric flask. Allow the flask to stand
unstoppered for about 10 min or until all alcohol-wetted
surfaces have dried. Weigh the flask to the nearest 0.1
mg. With a 100-uL syringe, immediately add two or more
drops of assayed reference compound to the flask. (The
liquid must fall directly into the alcohol without contact-
ing the flask.) Reweigh the flask, dilute to volume,
stopper, and mix by inverting several times.
5.7.2 From the net weight gain, calculate the concentration in
micrograms per microliter. When assayed compound purity is
> 96%, the uncorrected weight may be used to calculate
concentration.
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5.7.3 Stock standard solutions should be stored with minimal head-
is
space in Teflon -lined screw-capped bottles.
5.8 Secondary dilution standard — Stock standard solutions are used to
prepare a secondary dilution standard solution that contains the
four trihalomethanes in methanol. The secondary dilution standard
should be prepared at a concentration that can be easily diluted to
prepare aqueous calibration solutions (Section 8.2.3) at concentra-
tions that will bracket the working concentration range. (For this
method, the concentration of each trihalomethane in the secondary
dilution standard solution should be about 1 to 25 ug/mL.) The
solution should be stored with minimal headspace and should be
checked frequently for signs of deterioration or evaporation,
especially just before preparing calibration standards from it.
5.9 Internal standard spiking solution and surrogate compound spiking
solution — A spiking solution of fluorobenzene in methanol should
be prepared at a concentration of 0.5 ug/mL. When 10 uL of this
solution is added to 5 ml of sample or standard calibration
solution, the fluorobenzene concentration will be 1 ug/L. If the
internal standard technique is used, fluorobenzene serves as the
internal standard. If the external standard technique is used,
fluorobenzene is a surrogate compound added to each sample to
monitor method efficiency. The measured efficiency for fluoro-
benzene is considered to be indicative of method efficiency for the
four trihalomethanes being measured with this method.
5.10 Gas Chromatograph/Mass Spectrometer/Data System (GC/MS/DS)
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5.10.1 The GC, which must be capable of temperature programming,
should be interfaced to the MS with an all-glass enrichment
device and an all-glass transfer line. Any enrichment
device or transfer line can be used, however, if performance
specifications described in this method can be demonstrated
with it. The recommended GC column is 1.3 m long by 2 mm ID
glass packed with 1% SP-1000 on 60/80 mesh Carbopack B.
Helium carrier gas flow rate is 30 mL/min. The column
temperature program is initial 3 min period at 45°C,
increased to 200°C at a rate of 8°C/min, and isothermal
at 200°C for 15 min. Other columns may be used if they
provide data with adequate accuracy and precision as
specified in this method. An alternative column is 1.8 m
long by 2 mm ID glass or stainless steel packed with 0.2%
Carbowax 1500 on 80/100 mesh Carbopack C.
5.10.2 Mass spectral data are to be obtained with electron-impact
iom'zation at a nominal electron energy of 70 eV. The mass
spectrometer must produce a mass spectrum that meets all
criteria in Table 1 when 50 ng or less of p_-bromofluoro-
benzene (8FB) is introduced into the GC.
5.10.3 An interfaced data system (OS) is required to acquire,
store, reduce and output mass spectral data. The data
system must be equipped with a program to acquire data for
only a few selected ions that are characteristic of the
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Table 1. Ion Abundance Criteria for _p-Bromofluorobenzene
Mass Ion Abundance Criteria
_15 to 40% of mass 95
75 30 to 60% of mass 95
95 Base Peak, 100% Relative Abundance
96 5 to 9% of mass 95
173 < 2% of mass 174
174 > 50% of mass 95
175 5 to 9% of mass 174
176 > 95% but < 101% of mass -174
177 5 to 9% of mass 176
internal standard and the trihalomethanes being analyzed.
As compounds elute from the GC, mass spectral data are
acquired continuously, but only for a few masses rather than
for a broad mass range that would provide a complete mass
spectrum of each sample component. This is known as
selected ion monitoring (SIM).
5.10.4 SIM is used because it provides lower detection limits than
does full-spectrum data acquisition. Although identifica-
tions based on SIM data are less reliable than those based
on full-spectrum data, identifications of the four trihalo-
methanes are more reliable with SIM than with the most
selective conventional chromatographic detectors. This
identification reliability is obtained by using relative
retention time information, by selecting appropriate
characteristic ions to be monitored, and by checking
relative abundances of naturally occurring isotopes of
chlorine and bromine.
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5.10.5 Increased sensitivity is achieved with SIM because more time
is dedicated to acquiring abundance data for each mass when
only a few masses (ions) are monitored rather than every
mass in the full-spectrum mass range. For example, if 4 s
are required for full-spectrum data acquisition, approxi-
mately 17 ms will be dedicated to each mass from 20 to 260
atomic mass units (amu). If only eight masses are
monitored, however, approximately 500 msec can be dedicated
to each mass during a 4 s data acquisition period. This
relatively long data acquisition time with SIM improves
detection limits by averaging random noise, which improves
the signal-to-noise ratio.
5.10.6 For SIM of trihalomethanes, the data system must be capable
of monitoring at least four ions; the capability to monitor
eight ions is highly desirable. When less than eight ions .
can be monitored simultaneously, this disadvantage can be
overcome if the ions being monitored can be changed as a
function of time. With knowledge of trihalomethane reten-
tion times, the operator can then change ions being
monitored as different compounds elute.
6. Selection of Ions
6.1 The four trihalomethanes are distinguished from one another and
from other sample components by acquiring abundance information
about characteristic ions and by using GC retention time data.
Mass spectra of the four trihalomethanes (Figures 4-7) contain
characteristic patterns caused by chlorine and bromine isotopes.
-------
For example, the chloroform mass spectrum contains an ion cluster
at
37,
at m/z 83, 85, and 87 with known relative abundances of Cl and
Cl isotopes. Similar isotope clusters are produced by fragmen-
tation of molecules containing bromine or chlorine and bromine.
6.2 Because two ions are monitored for each trihalomethane, relative
abundances of isotope cluster ions provide corroborating identifi-
cation information. To achieve maximum sensitivity, monitored ions
(Table 2) are the two most abundant ions in a trihalomethane mass
spectrum. Because chloroform and bromodichloromethane produce
identical isotope clusters at m/z 83, 85, and 87 (produced by loss
of chlorine and bromine, respectively), m/z 83 and 85 are used to
monitor both compounds. (They are distinguished from each other by
differences in retention time.) Two ions each are needed to
identify and measure dibromochloromethane and bromoform; one ion is
required for fluorobenzene. Therefore, the capability to monitor a
total of seven ions is needed to measure simultaneously all four
trihalomethanes and fluorobenzene.
Table 2. Ions Selected to Detect and
Measure Trihalomethanes
Theoretical
Compound
CMC 13
CHCl2Br
CHC18r2
C6H5F
CHBr3
Ion
CHCl2+
CHC12+
CHClBr+
C6H5F+
CHBr2+
m/z
83
85
83
85
127
129
96
171
173
Rel. Abun.
100
65
100
65
77
100
100
51
100
(X)
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6.3 If only four ions can be monitored simultaneously but the set of
ions can be changed with only a brief interruption of data
acquisition, different compounds can be monitored as they elute
from the GC. For example, an ion current profile similar to that
obtained when seven ions were monitored (Figure 8) can be obtained
by sequentially monitoring two sets of ions, one set of four ions
and one of three ions. Monitoring m/z 83, 85, 127 and 129 until
after chlorodibromomethane elutes would permit detection of chloro-
form, bromodichloromethane, and chlorodibromomethane. Changing the
set of monitored ions to m/z 96, 171 and 173 would then permit
detection of fluorobenzene and bromoform.
6.4 Although some purgeable organohalides have the same retention times
as trihalomethanes with the recommended columns, these compounds
are not observed because they do not produce ions being monitored.
A representative chromatogram of halogenated organic compounds
(Figure 9) shows no components coeluting with chloroform or bromo-
dichloromethane. Chlorodibromomethane, however, coelutes with
1,1,2-trichloroethane and cis-l,3-dichloropropene. Because neither
of the latter two compounds produces ions at m/z 127 or 129 (which
are the ions used to detect and measure chlorodibromomethane),
neither will be observed. A similar situation exists with the
coelution of bromoform (monitored with m/z 171 and 173) and
1,1,1,2-tetrachloroethane, which produces neither m/z 171 nor 173.
5.5 Non-halogenated compounds will not be falsely identified as
trihalomethanes, because two ions are monitored to detect isotope
clusters produced by trihalomethanes, but not by non-halogenated
comoounds.
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5.6 The length of time to be spent acquiring data for each ion must be
selected with consideration of several factors, such as the number
of ions monitored, total data acquisition time for the set of
monitored ions, GC resolution of sample components, ion counting
statistics, and dynamic range of ion detection and data storage
devices. Sufficient.time must be spent on each ion to acquire
reliable data about changes in ion abundance as a function of time.
If too much time is spent on any one ion, however, other sample
components will not be detected as they elute from the GC column.
At least five data points must be acquired for each GC peak. The
same data acquisition time need not be used for all ions monitored;
the data acquisition time used to monitor a sample component,
however, must be the same as the time used to prepare the cali-
bration curve for that trihalomethane or to calculate its relative
response factor.
7. Sample Collection, Preservation and Handling
7.1 All samples should be collected in duplicate. Sample bottles must
be filled to overflowing. No air bubbles should pass through the
sample as the bottle is filled, or be trapped in the sample when
the bottle is sealed. Samples must be kept sealed from collection
time until analysis; this storage period should not exceed 14 days,
because significant biodegradation may occur after this period.
7.1.1 When sampling from a water tap, open the tap and allow the
system to flush until the water temperature has stabilized.
Adjust the flow to about 500 mL/min and collect duplicate
samples from the flowing stream.
7.1.2 When sampling from an open body of water, fill a 1-qt
wide-mouth bottle with sample from a respresentative area,
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and carefully fill duplicate sample bottles from the 1-qt
bottle.
7.2 If a sample is expected to contain residual chlorine, a reducing
agent, sodium thiosulfate or sodium sulfite (10 mg per 40-mL sample
for up to 5 ppm chlorine) should be added to the empty sample
bottle before it is shipped to the sampling site. (The reducing
agent is not added to samples to be analyzed to determine maximum
trihalomethane potential.)
7.3 Duplicate field blanks must be collected along with each sample
set, which is composed of the samples collected from the same
general sample site at approximately the same time. Field blanks
are prepared by filling sample bottles with reagent water at the
laboratory and shipping the sealed bottles to the sampling site
along with empty sample bottles and back to the laboratory with
filled sample bottles. (If reducing agent is added to sample
bottles, it must also be added to blanks.)
8. Calibration
8.1 The analytical system is calibrated each 8 h period by analyzing
standard solutions with the same procedures that will be used to
analyze samples (Section 9). Either the external standard or
internal standard technique may be used. With either technique,
however, fluorobenzene must be added (Section 5.9) to all cali-
bration solutions, because it will be used either as an internal
standard or as a surrogate compound.
8.2 External Standard Technique
3.2.1 An external standard is a known amount of a pure compound
that is analyzed with the same procedures and conditions
that are used to analyze samples containing that compound.
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From measured detector responses to known amounts of the
external standard, a concentration of that same compound can
be calculated from measured detector response to that
compound in a sample analyzed with the same procedures.
8.2.2 Calibration curves are prepared by analyzing at least three
calibration solutions, each containing a standard of each of
the four trihalomethanes. One solution should contain each
trihalomethane at a concentration approaching but greater
than the method detection limit (Table 4) for that compound;
the other two solutions should contain trihalomethanes at
concentrations that bracket the range expected in samples.
For example, if the detection limit for a particular
trihalomethane is 0.06 yg/L, and a 5-mL sample expected to
contain approximately 5 ug/L is analyzed, aqueous solutions
of standards should be prepared at concentrations of 0.1
ng/mL, 1 ng/mL, and 10 ng/mL.
8.2.3 Three calibration solutions are prepared by adding 20.0 uL
of the secondary dilution standard solution to 50 ml, 250
ml, and 500 ml aliquots of reagent water. (A 25-yL syringe
with a 0.006 in. ID needle is recommended for this
transfer.) Aqueous standard solutions may be stored for up
to 24 hr in sealed vials with zero headspace.
8.2.4 Because the surrogate, fluorobenzene, will be spiked into
all samples by adding 10 yL of the surrogate spiking solu-
tion, this technique should also be used to add the
surrogate to calibration solutions. The surrogate spiking
solution should be added to the syringe containing 5 ml of
calibration solution immediately before the syringe is
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attached to the purging device.
8.2.5 Each calibration solution is analyzed with the procedures to
be used to analyze samples. For each trihalomethane,
integrated abundances of the ion characteristic of that
compound are plotted as a function of the concentration.
The primary (most abundant) characteristic ion should be
used (Table 2). If the ratio of ion abundance to amount of
trihalomethane is constant (< 10% relative standard
deviation) throughout the concentration range, the average
ratio may be used instead of a calibration curve.
8.2.6 Calibration data must be checked each day by measurement of
one or more external standard calibration solutions. If the
absolute ion abundance measured for any trihalomethane
varies from expected abundance by more than 10%, a fresh
calibration solution must be prepared and analyzed. Prepar-
ation of a new calibration curve may be necessary, because
detector response may have changed.
8.3 Internal standard technique
8.3.1 An internal standard is a pure compound added to a sample in
known amounts and used to calibrate concentration measure-
ments of other compounds that are sample components. The
internal standard must be a compound that is not contained
in the sample. Fluorobenzene was selected as the internal
standard because it:
o is stable in aqueous solutions,
o is efficiently purged from aqueous solutions,
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o does not occur naturally;
o is not commercially produced in bulk quantities but is
available as a laboratory reagent chemical,
o does not coelute with any of the trihalomethanes being
monitored but elutes among them (Figure 8), and
o can be monitored with one ion.
8.3.2 A calibration curve should be prepared by analyzing at least
three aqueous solutions containing a known amount of each of
the four trihalomethanes and the internal standard, fluoro-
benzene. One of the solutions should contain trihalomethane
standards at concentrations near the limit of detection; in
the other solutions, trihalomethane concentrations should
bracket the range of concentrations expected in samples.
The internal standard concentration must be constant in all
calibration solutions.
8.3.3 Because the internal standard will be spiked into all
samples by adding 10 yL of the internal standard spiking
solution, this technique must also be used to add the
internal standard to calibration solutions. The internal
standard spiking solution is added to the syringe containing
5 ml of calibration solution immediately before the syringe
is attached to the purging device.
3.3.4 Trihalomethane measurements are calibrated by calculating
the mass spectrometer response to each compound relative to
fluorobenzene, the internal standard. The response factor
(RF) is calculated with the equation,
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RF = V QS .
V "x
where A = integrated abundance of the selected ion for
A
the trihalomethane standard;
A = integrated abundance of the selected ion for
the internal standard;
Q = quantity of internal standard; and
Q = quantity of trihalomethane standard.
^
RF is a unitless number; units used to express quantities of
trihalomethane and internal standard must be equivalent.
8.3.5 For each trihalomethane, the response factor should be
independent of trihalomethane quantity for the working range
of the calibration. Each day, one or more standards must be
analyzed to verify that response factors have not changed.
When changes occur (> 10% relative standard deviation), new
standard solutions must be prepared and analyzed to deter-
mine new response factors.
9. Sample Analysis
9.1 Analysis procedures
9.1.1 Initial conditions — Adjust the helium purge gas flow rate
to 40 ± 3 mL/min. Attach the sorbent trap to the purging
device, and set the device to the purge mode. Open the
syringe valve located on the sample introduction needle of
the purging chamber.
9.1.2 Sample introduction and purging — Remove the plunger from a
5-mL syringe and attach a closed syringe valve. Open the
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sample or standard bottle, which has been allowed to come to
ambient temperature, and pour the sample into the syringe
barrel to just short of overflowing. Replace the syringe
plunger and compress the sample. Open the syringe valve and
vent any residual air while adjusting the sample volume to
5.0 ml. (Because this process of taking an aliquot impairs
the integrity of the remaining sample, a second syringe
should be filled at the same time, in case a second analysis
is required.) Add 10.0 uL of the spiking solution (Section
5.9) of fluorobenzene in methanol through the syringe valve
and close the valve. Attach the syringe and its valve
assembly to the syringe valve on the purging device. Open
the syringe valves and slowly inject the sample into the
purging chamber. Close both valves and purge the sample for
11.0 ± 0.1 min at ambient temperature.
9.1.3 Desorption and data acquisition — At the conclusion of
purging, attach the sorbent trap to the GC, adjust the
purging device to the desorb mode, and initiate the GC
temperature programming (Section 5.10.1), trap heating,
(Section 5.2) and MS data acquisition (Section 6). Trapped
sample components are transferred into the GC column by
heating the trap to 180°C rapidly (within 4 min) while it
is backflushed with helium flowing at 20 to 60 mL/min. (If
the trap cannot be heated rapidly, the GC column may be used
as a secondary trap by cooling the column to < 30°C during
desorption.)
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9.1.4 Sample chamber rinsing — During desorption empty the
purging chamber with the sample introduction syringe, and
rinse the chamber with two 5-mL portions of reagent water.
9.1.5 Trap reconditioning — After desorfaing the sample for 4 min,
reset the purging device to the purge mode. After 15 s,
close the syringe valve on the purging device to begin gas
flow through the trap. After approximately 7 min, turn off
the trap heater and open the syringe valve to stop gas flow
through the trap. When cool, the trap is ready for the next
sample.
9.1.6 Termination of data acquisition — When sample components
have eluted from the GC, terminate MS data acquisition and
store data files on the data system storage device. Use
appropriate data output software to display selected ion
abundance profiles. If any ion abundance exceeds the system
working range, dilute the sample aliquot in the second
syringe with reagent water and analyze the diluted aliquot.
9.2 Identification criteria
9.2.1 The GC retention time of the sample component trihalomethane
must be within £ s of the time observed for that same
compound when the standard solution was anaTyzed. The value
of Jt is calculated with the equation:
t =VRT ,
where RT = observed retention time (in seconds) of the
compound when standard solution was analyzed.
9.2.2 Relative abundances of naturally occurring isotopes must
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agree with theoretical values within ± 10% (Table 3).
Table 3. Relative Abundance Criteria for Trihalomethane Ions
Compound
CHC13
CHCl2Br
CHC1 Br2
CHBr3
Selected Ions
33
83
127
171
&
&
&
&
85
85
129
173
m/z
m/z
m/z
m/z
Abundance
85
85
127
171
= 58
= 58
* 69
= 46
to
to
to
to
Criteria
72%
72%
85%
56%
of
of
of
of
m/e
m/e
m/e
m/e
83
83
129
173
9.3 Concentration calculations
9.3.1 With either the internal or external standard technique,
concentrations are calculated with the equation:
Cx
AS . RF
where C = analyte concentration in micrograms per liter;
^
AX = integrated ion abundance of the primary charac-
teristic ion of the sample analyte;
AS = integrated ion abundance of the primary charac-
teristic ion of the standard (either internal
or external), in units consistent with those
used for the analyte ion abundance;
RF = response -factor (With an external standard,
RF = 1, because the standard is the same
compound as the measured analyte.);
Q = quantity of internal standard added or external
standard analyzed, in micrograms; and
V = purged sample volume in liters.
9,3.2 The concentration of total trihalomethanes is the sum of
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concentrations of the four individual trihalomethanes.
10. Quality Control
10.1 Minimum quality control requirements consist of initial demonstra-
tion of laboratory analytical capability (efficiency, accuracy and
precision procedures, Sections 10.2.7-10.2.9), analysis of labora-
tory control standards as a continuing performance check, quarterly
analysis of a quality control check sample (Section 10.2.11), and
maintenance of performance records to define the quality of gene-
rated data.
10.2 Quality control analyses
10.2.1 Field blanks — A field blank must be analyzed along with
each sample set. If a field blank contains trihalomethanes
at concentrations above the method detection limits, a
method blank must be analyzed. If trihalomethanes are not
detected in the method blank but are detected in the field
blank, sampling or storage procedures have not prevented
sample contamination, and the sample set must be discarded.
10.2.2 Method blanks — A method blank is a 5-mL portion of reagent
water placed in the purging apparatus and analyzed as if it
were a drinking water sample. A reagent water blank must be
analyzed each day to demonstrate acceptable levels of inter-
ferences and contaminants in the analytical system. No
sample is to be analyzed until no trihalomethanes are
detected in method blanks at concentrations above method
detection limits.
10.2.3 Laboratory duplicates — To determine precision associated
-------
with laboratory techniques, analyze two aliquots (Section
9.1.2) of at least 5% of the samples in which trihalo-
methanes were observed at concentrations above method
detection limits. Calculate percent deviation (D) of
duplicate analyses using the formula:
D = —^— . 100
C
where R = range of concentrations observed, and
C~ = mean concentration observed.
If D is greater than 30%, precision is inadequate, and
laboratory techniques must be improved.
10.2.4 Field duplicates — At least 10% of samples should be
analyzed in duplicate to determine precision limitations
imposed by sampling, transport and storage techniques.
10.2.5 MS performance standard — Near the beginning of each 8 h
work period day that trihalomethanes are to be measured, the
mass spectrum produced by £ 50 ng of £-bromofluorobenzene
(BFB) must be measured to ensure that it meets performance
criteria (Table 1). BFB may be introduced into the MS
either by syringe injection or through the purge and trap
system. The entire mass spectrum (mass range 35 to 260
amu) should be obtained at an MS scan rate that produces at
least five spectra for the BFB GC peak but does not exceed
7 s per spectrum. If the BFB spectrum is unacceptable,
GC/MS operating parameters must be adjusted until an accept-
able spectrum is produced before samples are analyzed.
-------
10.2.5 Laboratory control standard — To demonstrate ability to
produce data within acceptable accuracy and precision
limits, a laboratory control standard must be analyzed
during each 8 h work period. A laboratory control standard
is reagent water spiked with known amounts of trihalo-
methanes.
10.2.S.I For each trihalomethane to be measured, select a
concentration representative of its occurrence in
drinking water samples. From stock standard
solutions, prepare a laboratory control standard
concentrate in methanol. This solution should
contain all four trihalomethanes at concentrations
500 times those selected as representative
concentrations. (Laboratory control standard
concentrates, which are also called QC check
sample concentrates, are available from the U. S.
Environmental Protection Agency, Environmental
Monitoring and Support Laboratory, Quality
Assurance Branch, Cincinnati, Ohio 45268.)
10.2.6.2 Add 10 uL of the laboratory control standard
concentrate to a 5-mL aliquot of reagent water,
and analyze according to procedures in Section 9.
10.2.7 Method efficiency - For each trihalomethane, method
efficiency is calculated by comparing the detector response
when the compound is introduced by syringe injection with
-------
the detector response when the same amount is introduced by
purging, trapping, and desorption. Because of the calibra-
tion technique used in this method, high efficiency is not
required for acceptable precision and accuracy but is
required for acceptable sensitivity. Low method efficiency
will cause unacceptably high detection limits. Method
efficiency for each trihalomethane must be recalculated when
the analytical system undergoes major modification, such as
replacement of trap packing.
10.2.7.1 At least five laboratory control standards are
analyzed with the purge, trap, desorption and
GC/MS SIM detection procedures. Interspersed
among these five or more analyses, three or more
aliquots of the secondary dilution standard
solution (Section 5.8) are injected directly into
the GC to introduce each trihalomethane in an
amount equivalent to that introduced by purge and
trap procedures. The recommended amount is 5 ng
of each trihalomethane. The same MS data acquisi-
tion parameters are used for SIM of injected
trihalomethanes as are used for those introduced
with the purge and trap procedures.
10.2.7.2 Calculate the method efficiency (E) for each
trihalomethane in each aliquot of the laboratory
control standard with the equation:
-------
100
where A = ion abundance of compound introduced
with purge and trap techniques, and
A.. = ion abundance produced by an equal
amount of the same compound when
injected.
For this calculation, use data obtained from an
injection either closely preceding or following
the purge and trap analysis from which data are
used.
10.2.7.3 Calculate the mean method efficiency for each
compound and the mean of the mean method efficien-
cies for all four trihalomethanes. Acceptable
detection limits can be achieved if the mean of
the mean method efficiencies is j> 60%; the minimum
required effficiency for any individual trihalo-
methane is 30 %.
10.2.8 Accuracy — Accuracy can be calculated from the same set of
data acquired to determine efficiency. One aliquot of the
laboratory control standard analyzed with purge and trap
techniques is selected to be treated as a standard with
known component concentrations, and the other aliquots are
treated as samples. (Data obtained from direct injections
are not used in accuracy calculations.) Data acquired for
the aliquot chosen to be the standard may be treated as an
-------
external standard or may be used to calculate response
factors relative to fluorobenzene used as an internal
standard.
10.2.8.1 When using the external standard procedure, data
obtained from the solution selected as a standard
are assumed to be true values, and accuracy is the
ion abundance found in the sample solution
expressed as a percentage (P) of the ion abundance
found in the external standard solution:
A..
As
. 100
where A = abundance of ion used to monitor
A
trihalomethane treated as an unknown,
and
A = abundance of ion used to monitor the
same trihalomethane treated as an
external standard.
10.2.8.2 When using the internal standard procedure,
fluorobenzene in the solution of trihalomethane
standards is selected as an internal standard, and
response factors are calculated (Section 8.3.4)
for each trihalomethane relative to fluorobenzene.
With these response factors, SIM data acquired for
the other solutions analyzed are used to calculate
accuracy:
-------
Ax . 100
where A = abundance of ion used to monitor a
/\
trihalomethane in the laboratoary
control standard solution,
A = abundance of ion used to monitor
fluorobenzene in the same solution, and
RF * response factor of the particular
trihalomethane relative to fluorobenzene.
10.2.8.3 For each of the four trihalomethanes, the mean
accuracy is calculated; the mean of these four
means is the method accuracy and must be in the
range of 90 to 110%.
10.2.9 Precision
10.2.9.1 For each trihalomethane, method precision is
expressed as the standard deviation(s) of the
percentages of the true values (?) obtained in the
accuracy calculations:
'J2
n (n-1)
where n = number of measurements for each trihalo-
methane.
10.2.9.2 The dispersion of the set of means for each
trihalomethane is expressed as the relative
standard deviation (RSD):
-------
RSD = _s_ . 100
P
where s - standard deviation, and
P - mean percentage of true value.
10.2.9.3 Adequate precision is obtained when the relative
standard deviation is _< 15%.
10.2.10 Monitoring the surrogate compound ^- If the external
standard technique is used, f1uorobenzene is a surrogate
compound used to monitor method performance. Each day
method efficiency is determined for fluorobenzene by
analyzing a laboratory control standard and comparing
results obtained with purge and trap procedures to those
obtained with direct injection. If for any sample,
method efficiency (Section 10.2.7) and accuracy (Section
10.2.8) values obtained for fluorobenzene fall below
acceptable values, trihalomethane values obtained for
that sample should not be reported.
10.2.11 At least quarterly, a quality control check sample
obtained from the U.S. Environmental Protection Agency,
Environmental Monitoring and Support Laboratory, Quality
Assurance Branch, Cincinnati, must be analyzed. If
measured trihalomethane concentrations are not within
±20% of true values, the entire analytical procedure must
be checked to locate and correct the problem source.
11. Method Performance
11.1 The method detection limit is defined as the minimum
concentration of analyte that can be measured and reported with 99%
-------
confidence that the value is above zero. Method detection limits
and single laboratory accuracy and precision data (Table 4) were
obtained from seven replicate analyses (using the external standard
technique) of reagent water spiked with trihalomethanes. For the
four trihalomethanes, individual mean method accuracies were
calculated, and a mean method accuracy for all four was calculated
to be 102.4%.
-------
REFERENCES
1. "National Interim Primary Drinking Water Regulations; Control of
Trihalomethanes in Drinking Water," Federal Register, Volume 44, No.
231, p. 68624 (November 29, 1979).
2. "Appendix C. Analysis of Trihalomethanes in Drinking Water," Federal
Register, Volume 44, No. 231, p. 68672-68690 (November 29, 1979).
3. "Definition and Procedure for the Determination of the Method Detection
Limit," U. S. Environmental Protection Agency, Office of Research and
Development, Environmental Monitoring and Support Laboratory,
Cincinnati, OH, July 1981, Revision 1.12.
4. "Carcinogens - Working With Carcinogens," Department of Health,
Education, and Welfare, Public Health Service, Center for Disease
Control, National Institute for Occupational Safety and Health ,
Publication No. 77-206, Aug. 1977.
5. "OSHA Safety and Health Standards, General Industry," (29CFR1910),
Occupational Safety and Health Administration, OSHA 2206, (Revised,
January 1976).
6. "Safety in Academic Chemistry Laboratories," American Chemical Society
Publication, Committee on Chemical Safety, 3rd Edition, 1979.
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100-
80-
60-
40-
20-
li
50
100
Figure 4 Mass Spectrum of Chloroform
100-
80-
60-
20
li__ L '-:!
il
30
100
150
Figure 5 Mass Spectrum of Bromodichioromethane
-------
100-r
|
30-
20-i
100
ISO
200
Figure 6 Mass Spectrum of Chlorodibromomethane
.COT
bu-
40 -j
20-i
I .!«.
1!
100
150
200
250
Figure 7 Mass Spectrum of Bromoform
-------
50
100
150
Figure 8 Selected Ion Current Profile of Trihalomethanes
and Internal Standard
-------
U.S. r t^ Protection Agency
60604
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