22
Another type of column used forVOC analysis is based on cyanopropylphenyl/dimethyl
propyl polysiloxane phases, commonly known as the “624” phases. TheRtx
®
-624 column is
designed for EPAMethod 624, but also performswell forMethods 524.2, Revision IV, and
8260. Themain advantage of theRtx
®
-624 column is the complete separation of the highly
volatile gases, including dichlorodifluoromethane, chloromethane, vinyl chloride,
chloroethane, and bromomethane.
More recently, theRtx
®
-VRX columnwas developed, using computer-assisted stationary
phase design (CASPD), to address the expanded list of compounds inEPAMethods 8021
and 502.2. This unique column improves resolution and reduces overall analysis time com-
pared to traditional columns. Like theRtx
®
-624 column, theRtx
®
-VRX column provides
excellent separation of the highly volatile gases. Its only disadvantage is poor resolution of
themost common trihalomethanes (THMs), chloroform and bromodichloromethane, from
other target analytes. These analytes are frequently found in chlorinated drinkingwater sam-
ples.While theRtx
®
-VRX column has been used forMSmethodswith favorable results,
because of this poor resolution it is not recommended for drinkingwater analysis using
PID/ELCD detection.
Themost recent innovations forVOC analysis have been the development of theRtx
®
-VGC
andRtx
®
-VMS columns. These columns alsowere designed usingCASPD. Designed for
PID/ELCD analyses, theRtx
®
-VGC column resolves all compounds listed inEPAMethods
502.2 and 8021, with >80% resolution of each of the four trihalomethanes from the other
target compounds, >30% resolution between 2-chlorotoluene/1,1,2,2-tetrachloroethane, and
>60% resolution of all other volatile compounds in the twoEPAmethods. The column
resolves the gases and early-eluting compoundswell enough that theGC oven program can
be started at 50°C.
TheRtx
®
-VMS columnwas designed to address the increasing number of analytes listed in
EPAMethod 8260, and also is a good choice for separating compounds listed inEPA
Method 524.2, revision IV. Themajor difference between theRtx
®
-VMS phase and others
such as “502.2,” “624,” or “VRX” is its overall selectivity and the distance betweenmem-
bers of isomeric pairs, like 2-/4-chlorotoluene.A faster final oven ramp rate is possible
because these compounds elute farther apart on theRtx
®
-VMS phase, eliminating partial co-
elutions that would interferewith quantification. This column offers excellent separation of
EPAMethod 8260B compounds in less than 18minutes - the normal cycle time for a purge
and trap system. Using the EPA-suggested surrogates the analysis time can be less than 10
minuteswith a narrow bore column. Even faster analyses are possible if you replace the
internal standard chlorobenzene-d5with another compound, such as 4-bromofluorobenzene.
Sub-10-minute analysis times allow you to connect two purge& trap units to oneGC/MS
instrument, significantly increasing sample throughput (see page 37).
Metal Columns
In addition to the standard fused silica versions of the analytical columns discussed above,
metalMXT
®
columns coatedwith the same stationary phases also are available fromRestek.
To eliminate the activity problems associatedwithmetal tubing, wemake these columns
fromSilcosteel
®
-treated stainless steel tubing, assuring excellent inertness. Because these
columns aremuchmore durable than fused silica columns, and can be coiled to less than 5-
inch diameters, they are ideal for portableGC applications. Their durabilitymakes them a
popular choice for teaching laboratories at colleges and universities.Analyte resolution on
MXT
®
columns is similar to that on fused silica columns.
