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278 LCGC NORTH AMERICA VOLUME 24 N UMBER 3 MARCH 2006 www.chromatographyonline.com

John V. HinshawGC Connections Editor

GC CONNECTIONSGC

Syringes for GasChromatography

A bewildering variety of

syringes is available for

gas chromatography.Volumes range from 1

L to as much as 2 mL,

with many different

needle styles and tips,

plunger arrangements,

and accessories. Syringe

styles include both

manual and autosampler

types. Choosing a suitable

syringe for a specific

application can be

difficult, especially if the

inlet system has special

requirements; choosing

the wrong syringe can

cause significant

problems.

n liquid chromatography (LC), the

syringe functions primarily as a

pipette or liquid-transfer device

that loads a sample loop. The syringe

generally does not take an active role in

injection, which occurs only after samplehas been displaced from the syringe. The

same is true of most gas chromatography

(GC) gas-sampling valves the time at

which the gaseous sample is injected into

the column is separate from the moment

that it is transferred into the injection

system. In GC analysis of liquid samples,

however, the syringe becomes an integral

part of the inlet during injection: sample,

in liquid or gaseous form, starts to enter

the column as soon as the syringe enters

the inlet.In GC inlet systems for liquids, the

injection technique, choice of syringe,

and inlet operating conditions all play a

crucial role in the injection process. Two

principal sample-transfer mechanisms

move sample from the syringe into the

inlet while the syringe is in the inlet.

First, liquid-sample transfer takes place as

the syringe plunger is depressed and liq-

uid is expelled from the syringe tip. In

cold injection, where the inlet tempera-

ture is not high enough to produce sig-nificant solvent vaporization, this

pipette-like action is the only major sam-

ple-transfer mechanism. A competing

process occurs in a hot injector, however.

Within a few tenths of a second after the

needle enters a hot inlet, sample begins

to evaporate inside the needle. Bubble

formation and concomitant increased

internal pressures force some liquid out

along with the vapor, so that part or all

of the sample contained in the syringe

needle volume is introduced into theinlet as a mixture of liquid and vapor. As

the plunger is depressed, additional

room-temperature liquid sample is forced

from the syringe through the needle,

which cools the needle and suppresses

but does not entirely stop in-needle evap-

oration. The needle heats up again oncesyringe plunger motion ceases, which

causes additional sample vaporization

from the needle into the inlet. All of

these processes take place in a matter of

seconds. The total amount of sample that

actually is injected into the inlet depends

strongly upon these two processes, their

timing, the volumes involved, and the

inlet conditions. Along with judicious

injection condition control, a good

understanding of the role of the syringe

in these processes will help gas chro-matographers obtain better injections.

Sample Distortion During

Injection

Syringe-needle effects influence not only

the injected sample volume; they also can

modify the relative amounts of individ-

ual sample components that enter the

inlet. To understand this secondary

effect, consider that not all sample com-

ponents have the same vapor pressures at

a given temperature. Lower molecularweight compounds have higher vapor

pressures, and conversely, heavier mole-

cules have lower vapor pressures. This

differentiation forms the basis for simple

thermal fractionation of a mixture of

compounds in a distillation column,

for example.

Unfortunately, for many gas chro-

matographers, the same kind of thermal

fractionation process can occur in the

syringe needle during injection. When

in-needle vaporization occurs, the lightercomponents vaporize first and leave the

I

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syringe needle quickly. Heavier com-

pounds take longer to evaporate and

leave the syringe needle more slowly. The

effect of initial needle heating during

injection is remediated largely by subse-

quent bulk liquid transfer through the

needle: straggling heavy compounds will

be rinsed out. At the end of injection,

however, if the needle is withdrawn fromthe inlet before complete transfer of all

compounds can occur, then the sample

that enters the inlet system will contain

more of the volatile components and less

of the heavier components than were

present in the sample before injection.

This effect is called mass discrimination

because it tilts the sample composition

according to components vapor pres-

sures, which are related largely to their

masses.

Two other thermal side-effects gohand-in-hand with needle fractionation.

Many compounds are sensitive to ther-

mal stress, especially in the presence of a

hot steel syringe needle, and decompose

rapidly when subjected to high inlet tem-

peratures: high molecular weight glyc-

erols are one good example. Polar com-

pounds can become strongly adsorbed on

the needle surface. Many of the polar

polyaromatic hydrocarbons (PAHs) as

well as a host of chlorinated pesticides

suffer from this problem. In both cases,the use of nickel syringe needles, or nee-

dles that have been deactivated by

advanced surface treatments, can help

tremendously, as can deactivation of inlet

liners and injector inner surfaces exposed

to the sample. In these situations, ther-

mally mild injection techniques such as

cold on-column injection or pro-

grammed temperature vaporization usu-

ally produce results superior to what can

be obtained by careful deactivation of

needles and other hot injection materials,but the specialized inlets needed for these

somewhat more complex techniques

might not be available.

Judicious data-system calibration and

the use of multiple internal standards can

compensate for inaccuracies caused by

mass discrimination, decomposition, and

adsorption, as well as for a host of other

injection problems. However, the preci-

sion and repeatability of multiple analy-

ses are affected adversely when the rela-

tive amounts of individual componentsstrongly depend upon injection condi-

tions. Heavier components against

which injection strongly discriminates

or any other components that do not

come through the injection process at

100%, will exhibit degraded lower detec-

tion limits (LDLs) as well. Degradation

of precision, repeatability, and LDL are

significant effects even for samples that

do not span a wide range of volatilitiesand do not suffer from mass discrimina-

tion per se.

When this type of injection problem

does arise, its effects must be either sup-

pressed or controlled by applying special

syringe-handling techniques or by using

cold injection if possible. The sandwich

technique is a good example. The syringe

is first filled with 1 L or so of pure liq-

uid solvent followed by a small air plug.

The sample is pulled into the syringe,

then another plug of air, and finally somemore solvent. The entire liquid array is

pulled up into the syringe so that the

needle is largely empty. Upon injection,

only a little solvent is in the needle as it

enters the inlet. Sample passes through

the needle as the syringe plunger is

depressed, followed by a plug of solvent

that forces any remaining sample into

the inlet. Only pure solvent is left in the

needle at the end of injection, so needle

fractionation does not occur. The draw-

back of the sandwich technique is that itsignificantly increases the amount of sol-

vent that enters the inlet, which might

overload the inlet with solvent vapor or

broaden the solvent peak and interfere

with early-eluted analytes.

A high-speed autosampler also can

remediate needle fractionation. By inject-

ing the sample in a very short time, typi-

cally less than 500 ms and certainly

much faster than a human can achieve,

the syringe needle residence time in the

inlet is insufficient for significant needleheating to occur, and so needle fractiona-

tion is eliminated. Another benefit of a

fast injection is the suppression of sensi-

tive-component thermal decomposition

or adsorption on hot steel needle sur-

faces, again due to reduced contact

times. Most modern GC autosamplers

employ high-speed actions by default.

In-needle fractionation and other ther-

mal effects are some of the syringe-

related processes that interfere with injec-

tion accuracy and repeatability. There aremany other effects that occur before,

during, and after sample has entered the

inlet system: see the book by Grob (1)

for a comprehensive discussion.

Types of Syringes

The choice of syringe type plays an

important role in obtaining the best per-

formance from a gas chromatograph.

There are two basic kinds of manualsyringes: plunger-in-barrel (Figure 1a)

and plunger-in-needle (Figure 1b). In the

first type, sample is drawn up into a cali-

brated glass barrel; in the second, the

entire sample is contained in the needle,

and a thin wire plunger fitted into the

bore of the needle forces liquid out dur-

ing injection.

Plunger-in-Barrel Syringes

In this type of syringe, sample is con-

tained in the glass barrel (see Figure 1a).Because you can see the sample, it is easy

to observe the presence of liquid sample

and determine if the syringe needle is

obstructed. In use, the syringe usually is

overfilled with liquid, and the extra is

then ejected. A final clinging droplet can

be removed from the needle tip with a

clean laboratory wipe or by rinsing with

solvent. To gauge the injected volume,

consider that the amount displaced from

the markings on the syringe barrel does

not include sample vaporized from theneedle and is only an approximation.

The actual amount injected is closer to

the needle volume (about 0.7 L for a

10-L syringe) plus the amount dis-

placed. Another way to determine

injected volume is to load the syringe

with sample, pull the plunger back, and

read the total volume of the liquid plug.

After injection, pull the plunger back

slightly, read the remaining liquid vol-

ume, and subtract it from the original

amount. In this way, sample evaporatedfrom the needle is included in the total

amount. Plunger-in-barrel syringes can

be used for most routine applications.

The common 10-L syringe can reliably

inject liquid volumes ranging from 1.0

L to 10 L.

Plunger-in-barrel syringes can be

cleaned in several ways. Pumping solvent

in and out of the syringe is effective and

can be a good idea between injections to

remove leftover sample before it evapo-

rates and leaves a film of residue behind.Eventually, a layer of nonvolatile residue

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fashion. These syringes tend to be more

tightly integrated with the autosampler

mechanics and are harder to replace. As

mentioned previously, and worth repeat-

ing, is the fact that autosampler syringes

need better care than manual syringes

because they are subjected to greater

mechanical stresses and more operating

cycles.Another familiar specialty syringe is

the cold on-column syringe. This is gen-

erally a plunger-in-barrel type with a spe-

cial removable needle made of narrow-

bore steel or fused-silica tubing. The

needle is designed to penetrate a narrow-

bore (0.20- or 0.25-mm i.d.) capillary

column to accomplish on-column injec-

tion. Such syringes are not interchange-

able across different injector designs; the

manufacturers specifications must be fol-

lowed. On-column syringes can be moresusceptible to needle blockage because of

their smaller inner diameters. Many on-

column injectors use a precolumn or

retention gap made of uncoated

0.53-mm i.d. tubing, which accepts a

standard 26-gauge syringe needle.

Syringe Repair

Many syringe manufacturers offer syringe

repair kits, needle replacements, and

repair service. Most removable needles

can be replaced in the field if the rest ofthe syringe is in good shape. For

plunger-in-needle syringes, needle dam-

age often is accompanied by plunger

damage, and in such cases, it is necessary

to replace both as a matched pair.

Syringe plungers generally are not

field-replaceable (or interchangeable)

because of the tight matching tolerances

between plunger and barrel. One excep-

tion is microliter gas-tight syringes with

PTFE-tipped plungers, which will accept

replacement plungers if the barrel is nototherwise damaged.

Is syringe repair worth the cost? In the

case of plunger-in-needle syringes, the

answer is a definite yes. Repair kits of

matched plungers and needles are one-

quarter to one-third the cost of a new

syringe. For the less-expensive plunger-

in-barrel types, replacement needles are

as much as half the cost of a new unit,

and it might be worthwhile instead to

attempt to reduce syringe damage and

extend syringe life through better han-dling and cleaning procedures.

John V. HinshawGC Connections edi-tor John V. Hinshaw issenior staff engineerat Serveron Corp.,Hillsboro, Oregon, anda member ofLCGCseditorial advisoryboard. Direct corre-spondence about thiscolumn to GC Con-

nections, LCGC, Woodbridge Corporate Plaza,485 Route 1 South, Building F, First Floor,Iselin, NJ 08830, e-mail lcgcedit@lcgcmag.com.

For an ongoing discussion of GC issues withJohn Hinshaw and other chromatographers,

visit the Chromatography Forum discussiongroup at http://www.chromforum.com.

Syringe Accessories

A few syringe and sampling accessories

are especially useful. First, a syringe rack

is a good way to keep syringes in one

place, rather than on top of the gas chro-

matograph or the bench, where they can

be pushed off and broken easily. Syringe

labels are also handy to keep track of

which syringe is used for which type ofsample.

Another useful item is one of the vari-

ous repeating adapters that permit the

plunger to be returned to the same posi-

tion each time, which can improve the

repeatability of multiple injections. Also,

repeating adapters act as guides and pre-

vent plunger bending or accidental with-

drawal from the barrel, a feature that is

important on plunger-in-needle syringes.

Several manufacturers also make syringes

with extended or reinforced barrels andplungers that prevent damage from a

heavy hand.

Conclusion

When viewed as an active part of an

inlet, the syringe takes on new impor-

tance. While injection technique is cru-

cial, the proper care and cleaning of the

syringe can contribute significantly to

improved accuracy and precision. Choos-

ing the correct syringe to meet sample

and injector requirements, and thenapplying and maintaining it correctly,

can go a long way toward producing bet-

ter and more reliable analytical results.

References(1) K. Grob, Split and Splitless Injection for Quan-

titative Gas Chromatography: Concepts,Processes, Practical Guidelines, Sources of Error,4th, Completely Revised Edition (Wiley,Hoboken, New Jersey, 2001).

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