3

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).

5

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.

7

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.

9

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

11

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.

13

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