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INTRODUCTION

The Stockholm Convention (2004) is an

international treaty that aims to eliminate or restrict the production and use of certain persistent organic

pollutants (POPs). Monitoring of the compounds on

the banned list is required in a variety of environmental matrices. Several pesticides banned

by the convention are difficult to analyze by

traditional EI+/GC/MS due to significant levels of fragmentation. Therefore, this makes selection of a

suitable precursor ion for MS/MS difficult. For

multiple reaction monitoring (MRM) analysis, the ability to obtain an intense and specific precursor

ion is critical in obtaining low detection limits.

Atmospheric Pressure GC (APGC) is presented here

as an alternative. Ionization in APGC is analogous to

atmospheric pressure chemical ionization (APCI) insomuch as molecular or quasi-molecular ions are

produced. APGC is a ‘soft’ ionization technique that

results in lower fragmentation. The presence of strong molecular or quasimolecular ions provides

ideal conditions for MS/MS analysis.

In this poster, an overview to the principle of the

technique will be shown: and how this can be

applied for both POPs analysis and other GC-MS applications. Some examples between EI-GC-MS and

APGC-MS ionisation.

APGC OVERVIEW

It is very easy to interchange between LC and GC on this

system as the MS does not require venting in order to switch

between the two chromatographic approaches. The standard

sample cone is replaced with the APGC ion chamber (Figure 1).

The ionisation that APGC provides is soft and can be compared

with APCI: this means that molecular ions are readily detected

using this approach.

A corona pin creates a nitrogen plasma which in the case of

charge transfer reacts directly with analyte molecules (Figure

2).

MECHANISMS OF IONISATION

There are two primary mechanisms of ionization that APGC can

undergo[1]:

1. Charge transfer (M+.) initiated by corona discharge

ionization of the nitrogen in the source to generate radical

cations of nitrogen which can then undergo charge transfer

with analyte molecules to generate radical cations of the

analyte molecules. This form of ionization is favoured by non-

polar compounds.

2. Protonation (M+H+) where the proton source can be water,

MeOH, etc… This form of ionization is favoured by relatively

polar compounds.

It is possible to select between proton transfer and charge

transfer in ASAP and APGC by altering source conditions

depending on the chemistry of the target analytes or the

system can be used in a mixed mode (whereby charge transfer

and protonation can occur).

ADVANCES IN ATMOSPHERIC PRESSURE GAS CHROMATOGRAPHY (APGC) FOR THE ANALYSIS OF PERSISTENT ORGANIC POLLUTANTS (POPS); BACKGROUND AND APPLICATIONS

Gerard Bondoux, Antonietta Gledhill, Jody Dunstan

Waters Corporation

References

1. Horning: Anal. Chem, 1973, 45, 936-943

2. NofaLab, The NL (http://www.nofalab.nl/)

RESULTS AND DISCUSSION

METHOD TRANSFER: GC-EI -> APGC (PDBEs)

BFRs/PDBEs can be challenging to analyze due to the thermal

labile nature of these compounds. Existing MRM transitions

from an EI-GC-MS method were used to produce an APGC-MS

MRM method (as in the case of PCBs—shown in Table 1). For

the analysis of the PDBEs the mechanism of ionization

observed was protonation.

APGC opens up several avenues of method development, as

the method is no longer limited by the pressure / flow

restrictions of EI: so it is expected that results can be

optimised and especially for those compounds containing a

higher number of brominated species.

With the Xevo TQ-S it is also possible to run, within one

analysis. both full scan spectra and MRM mode (known as

RADAR) and an example of this can be seen for the PDBEs in

Figure 4. This approach can be done with little or no impact on

the quality of the MRM data.

In RADAR you can accurately quantify target compounds while

at the same time track other sample matrix components,

arming you with a greater depth of knowledge about your

sample.

SENSITIVITY (Dioxins)

Much work has been done to look at dioxins with APGC-MS and

for these group of compounds, the source conditions need to

be dry in order to promote the charge transfer mechanism

(producing MRM transitions that are very similar to those used

for EI).

Figure 5 shows the MRM traces for the native dioxins and

furans in the main part of the diagram. It can be seen that

good chromatographic separation and signal-to-noise (top

middle picture) even at very low concentrations. Very good

linearity shown for several compounds over the required range

of concentrations (including down to the LOD) (linearity is

shown here for TCDD in the top right corner).

SEPARATION (PCBs)

Another application that has also been investigated using APGC

-Xevo TQ-S is the analysis of PCBs and the chromatographic

impact on separation that atmospheric pressure GC might have

on these group of compounds. For the PCBs, it is critical to

obtain chromatographic separation of the isobaric PCB isomers.

This is critical because these compound have identical

transitions and therefore must be chromatographically

separated in order to be correctly identified and quantified. An

example of the type of separation that can be observed using

APGC can bee seen in Figure 6 between PCB 118 and PCB 123.

Baseline separation is achieved for these compounds and

accurate peak integration is not an issue.

PRACTICAL DIFFERENCES BETWEEN APGC-MS AND EI-

GC-MS

As the two approaches are subtly different, so to are the

experimental experiences between AGPC and EI-GC-MS.

Feedback from a customer [2] who has been using APGC & the

Xevo TQ-S systems routinely for three years has been

summarized in Table 2.

As the instrument configuration is more sensitive, the primary

advantage is that fact the less matrix needs to be injected

directly onto the column. And this has an impact on how dirty

some of the instrument becomes ( e.g. liners, columns etc).

It takes a much longer time for a system using APGC-MS to get

dirty compared to a dedicated EI-GC-MS instrument and this

ultimately mean that the system downtime is less per year

(and more samples can be analyzed).

CONCLUSION

The set up of APGC with the Xevo TQ-S is very

easy and can be operated in both LC and GC

without much down time

For compounds like Dioxins and PCBs the

mechanism of ionization is charge transfer, but

for many other GC amenable compounds (e.g.

PDBEs and pesticides) the mode of action is

generally protonation.

A range of compound types have been

successfully ionized using APGC and gave LOD’s

in the low fg to low pg range.

Due to the ionisation used for APGC-MS being

much softer compared to EI-GC-MS, the system is

very sensitive and enables less matrix to be

injected on-column

As less matrix is injected directly on-column the

system down-time for routine maintenance and

cleaning is much less compared to EI-GC-MS.

Figure 1.

Top: Xevo TQ-S tandem quadrupole MS system with LC and GC

capability (set up shown in the GC mode of operation).

Bottom left: APGC attachment leading into the source.

Bottom right: APGC connection close up photographs (forward and side profile)

LC GC

Figure 2. Mode of operation for atmospheric pressure GC (APGC)

N2+●

N2e-

2e-

2N2

N4+● M● +

MCorona Pin

M● +

M

(I) Charge Transfer

“Dry” source conditions

Favoured by relatively non-polar

compounds

N2+●

N4+●

H2O

H2O+●

H2O

H3O+●

+OH●

[M+H]+

M(II) Protonation

Modified source conditions eg. with H2O / MeOH

present

Favoured by relatively polar compounds

Corona Pin

Figure 3. Two primary mechanisms of ionisation (I) Charge transfer and (II) Protonation.

Figure 5. 1/10 Dilution of CSL Dioxin standard (Wellington Labs)—10 fg TCDD on-column.

Congener Concentration (pg/µL)

Name1/10 CSL

(pg/µL)CSL CS0.5 CS1 CS2 CS3 CS4

TCDD 0.01 0.1 0.25 0.5 2 10 40

TCDF 0.01 0.1 0.25 0.5 2 10 40

PCDD 0.05 0.5 1.25 2.5 10 50 200

PCDF 0.05 0.5 1.25 2.5 10 50 200

HxCDD 0.05 0.5 1.25 2.5 10 50 200

HxCDF 0.05 0.5 1.25 2.5 10 50 200

HpCDD 0.05 0.5 1.25 2.5 10 50 200

HpCDF 0.05 0.5 1.25 2.5 10 50 200

OCDD 0.1 1 2.5 5 20 100 400

OCDF 0.1 1 2.5 5 20 100 400

Compound name: TCDD

Correlation coefficient: r = 0.999887, r^2 = 0.999774

Calibration curve: 1.02895 * x + -0.00220016

Response type: Internal Std ( Ref 1 ), Area * ( IS Conc. / IS Area )

Curve type: Linear, Origin: Exclude, Weighting: 1/x, Axis trans: None

pg/µL-0.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.5 25.0 27.5 30.0 32.5 35.0 37.5 40.0

Re

sp

on

se

-0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

Figure 6. Critical separation of PCB isomers PCB 118 and PCB 123.

PCB 123

PCB 118

Table 1. APGC optimised MRM transitions for the PDBEs.

Figure 4. PBDE MRM transitions alongside the MS Scan BPI from the RADAR analysis (example data of a computer keyboard).

MS Scan

BPI trace

PBDE MRM

transitions

Table 2. Comparison of APGC-MS approach versus EI-GC-MS. Several advantages to the technology include reduced source

cleaning due to less matrix injection on-column. (Table courtesy of NofaLab, NL)