Purge and trap (P&T) is a dynamic headspace compounds (VOC). · During the purge cycle, the purge...
Transcript of Purge and trap (P&T) is a dynamic headspace compounds (VOC). · During the purge cycle, the purge...
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Purge and trap (P&T) is a dynamic headspace
technique where a water sample (or solid sample
mixed with water) is purged or sparged with
an inert gas to strip out the volatile organic
compounds (VOC).
The VOCs are collected on a trap. Operationally, water
sits on a glass frit inside a glass sparging tube. The purge
gas, usually helium, is routed through the bottom of the
sparging tube and passes through the frit, which disperses
the gas and creates fine bubbles that carry the volatiles out
of the water onto the trap. Once purge is completed, the
trap is dried, and then heated rapidly. Analytes desorb off
the trap and are carried through the gas chromatographic
column and into the detector.
During the purge cycle, the purge gas carries water vapor along with
the volatile organic analytes through the chromatographic column
and to the detector.
The principal problem caused by P&T analysis is introduction of water into
the Mass Spectrometer (MS). Purge and trap manufacturers are aware of
the problem and include water management systems on all their newer P&T
systems; older systems may have a rudimentary water management system
or no water management at all. Older P&T systems should not be interfaced
to modern GCMS systems. The design of the water management system
varies considerably between the manufacturers. When they malfunction,
they can absorb analytes, usually water-soluble compounds (e.g. ketones).
Water in the GCMS creates:
• Variable area with early eluting compounds (Figure 1)
• Baseline rise from the water band eluting from the column (Figure 2)
The principal problem caused by Purge & Trap analysis is the introduction of water into the mass spectrometer (MS).
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1MINIMIZE WATER TO THE GCMS
Figure 1: Chromatogram of early eluting compounds that are most likely affected by water in the GCMS
Figure 2: Total Ion Chromatogram showing baseline rise from water. This baseline rise, between 1.5 and 2.5 minutes,
can cause irreproducibility of early eluting peaks.
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Shimadzu GCMS-QP2010 SE with EST Purge & Trap
SOLUTION
Get a new purge and trap and a modern GCMS to reduce water. The
combination of the Shimadzu GCMS-QP2010 SE and the EST P&T removes
most of the water during the purge process. Shimadzu applications
chemists validated EPA methods 524,
624, and 8260 and guarantee it will
meet or exceed all quality control
and detection limit requirements
of the EPA methods.
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Prior to analyzing any samples, standards or blanks, the
instrument must meet spectral and sensitivity requirements for
an injection of a method specified quantity (usually 50 or 25ng)
of 4-bromofluorobenzene (BFB).
See Table 1 for typical BFB requirements.
SOLUTION
For best performance, start the day out right. Each day, first thing in the
morning or before starting a batch:
• Make sure the system is leak free
• Cycle the P&T through at least two bake cycles.
• Simultaneously, bake the oven, injection port, Ion Source, and MS
interface at slightly elevated temperature for a minimum of one hour.
2TUNE TO THE RIGHT CRITERIA, AND KEEP SYSTEM FREE OF CONTAMINATION
Table 1: BFB Key m/z Abundance Criteria Method.
Relative Abundance Criteria
Mass (m/z)
Method 524.2
Method 524.3/
M524.4Method 624
Method 8260C
CLP-SOW
5015 to 40%
of 95NA
15 to 40% of 95
15 to 40% of 95
15 to 40% of 95
7530 to 80%
of 95NA
30 to 60% of 95
30 to 60% of 95
30 to 80% of 95
95Base Peak,
100%Base Peak,
100%Base Peak,
100%Base Peak,
100%Base Peak,
100%
965 to 9%
of 955 to 9%
of 955 to 9%
of 955 to 9%
of 955 to 9%
of 95
173 <2% of 174 <2% of 174 <2% of 174 <2% of 174 <2% of 174
174 >50% of 95 >50% of 95 >50% of 95 >50% of 9550 to 120%
of 95
1755 to 9% of 174
5 to 9% of 174
5 to 9% of 174
5 to 9% of 174
4 to 9% of 174
176>95 to
<101% of 174>95 to
< 105% of 174>95 to
<101% of 174>95 to
<101% of 17495 to 101%
of 174
1775 to 9% of 176
5 to 10% of 176
5 to 9% of 176
5 to 9% of 176
5 to 9% of 176
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Regular maintenance will hlep maintain peak system performance.
• Every day, check to be sure the system has no leaks. An air/water
check should be done at room temperature (in case there is a gross
leak), and at 200˚C after a 30 minute bake. The reason for doing the
air/water check at high temperature is because VOC columns adsorb
water and, to a certain extent, nitrogen. To get a true reading, it is
necessary to clear the column of water and keep it hot to prevent
reaccumulation.
• When the IS area drops, clean the source, lens stack, and pre-rods
thoroughly if they are in good condition. If they are scratched or
discolored, they should be replaced.
3KEEP THE SYSTEM PROPERLY MAINTAINED
• Water buildup is a function of the P&T, but you have control over
the amount of methanol added to standards, samples and blanks.
Minimize the amount of methanol you use to prepare standards.
Internal standard and surrogate additions should be kept to 5 µL
or less, if at all possible.
• Set a split ratio to get as much sample as needed for acceptable
detection limits, and no more.
• If the instrument is more than three or four years old, consider
refreshing the MS. Replace the repeller and ion box and the electron
multiplier. At a minimum, disassemble and inspect the lens stack.
Replace scratched or severely discolored (not just lightly stained)
lenses. Clean the quadrupole (not just the pre-rods) – especially if
there is a bromine ratio problem.
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Troubleshooting a GCMS, and particularly a P&T GCMS, is a broad topic
that takes a lot of time to cover. In this eBook, we have highlighted three
topics to help you keep your system running and produce high quality
data with little effort. Even with the greatest care, continued analysis will
cause something in your system to fail, but following these three steps
will help ensure that failures occur less often.
CONCLUSION
For more information on purge
and trap systems, please visit our
website www.LabTotalEnv.com.
7102 Riverwood Drive, Columbia, MD 21046, USA Phone: 800.477.1227 / 410.381.1227
www.ssi.shimadzu.com
To learn more about SSI instruments for drinking water analysis visit
www.LabTotalEnv.com