Chromatography

GC troubleshooting

Prevention

Many GC problems can be prevented if the column is properly installed and GC is maintained routinely. For example, replacing septa or liner at regular intervals and keeping the injector and detector clean and well-maintained shoud solve many problems. Regular preventive maintenance depends on particular model of GC and you should consult required operations in the operator's and service manuals.

For the problem identification we recommend to use electronic flowmeter and leak detector.

Baseline problems

Baseline problems could be divided into 5 categories: drift, noise, offset, spiking and wander.

• Drift means slow baseline movement in one direction
• Noise is rapid and random movement of the baseline position
• Offset is sudden unexplained change of the baseline position
• Spiking is presented by peaks with no width, either positive or negative
• Wander is low frequency noise

Downward drift

Upward drift

Noise

Offset

Spiking

Wander

Distorted peak shapes

All peaks redused in size

Flattened top peaks

Fronting peaks

Fronting peaks are usually the result of column overloading. In this case, the effect should be a function of injection volume. Solutions include reducing the injection volume or using a column with greatr capacity. Columns with larger diameter or thicker stationary phase coatings generally have larger sample capacities, however, resolution may be reduced.

Ghost peaks

Irreproducible peak heights or areas

Negative peaks

No peaks at all

Selective sensitivity loss

Split peaks

Tailing peaks

Retention time shifts

Loss of resolution

Broad solvent front

Rapid column performance degradation

Column installation

Detailed information about GC column installation is available here.

Preventive maintenance

Injector cleaning

Detector cleaning

TOF vs. quadrupole

In this article, we will show you why GC/MS-TOF is a more suitable technique for modern GC/MS analyzes than the decades-old quadrupole analyzer technology.

1) Why does TOF provide better quality data?

The higher speed of the Time of Flight GC/MS system means a higher number of spectra per chromatographic peak. The analyst therefore has more data at his disposal and, thanks to the unique deconvolution algorithm, can identify compounds that have similar properties (isomers, etc.), when resolution by mass spectrum at the unit level is not enough for us.

TOF can work with MS spectrum analysis in the entire m/z range – thanks to good sensitivity, there is no need to use SIM mode and thus the analyst does not lose qualitative information about the structure of the compound. This can play a big role in inter-laboratory control or in cases where there are different results between laboratories.

2) Higher sensitivity

Time off Flight is not a scanning analyzer, but a pulse analyzer. Thanks to the high ejection rate of 30kHz, it enables the collection of spectra up to 1000/s, which a quadrupole analyzer cannot achieve. In addition, the quadrupole must return to the original value of the electric field after one scan, which takes additional time, the so-called "Interscan time". This significantly increases the average time of one scan. E.g. if PBDE analysis is performed in the range of 100-1000 amu, a GC/MS system with a scanning speed of 20000 amu/s, we will achieve a data collection rate of <20 spectra/s. With the TOF analyzer, we can easily achieve 200 spectra/s, i.e. 10x more. Higher speed enables not only Fast GC/MS, but better quality data. Fast GC/MS provides higher peaks, thus a better signal-to-noise ratio. In addition, in the analysis of compounds such as PBDE, it is necessary to analyze the entire mass spectra, and in the quadrupole analyzer, we thus lose a lot of sensitivity in EI.

3) Economy of operation

When using GC/MS-TOF, we achieve a high speed of collecting spectra, so we can shorten the analysis by 3x to 4x when using a suitable column (0.10 or 0.18 mm ID). This will significantly save on device operating time, carrier gas consumption and increase sample throughput. This can not only increase the capacity of the laboratory, but also speed up the delivery of analysis results.

GC liners

For most chromatographers poor sample reproducibility and mass transfer onto chromatographic column are generally the most critical issues in the method developement. Among others poor reproducibility and mass transfer can be caused by the degradation of the compounds by the glass inlet liner or by means of insufficient vaporization. Therefore selection of proper GC liner is very important and depends on the injection technique. Here you will find important information helping you with selectionof proper GC liner.

Issues surrounding the GC injector ports

New liner desing - FocusLiner™

Siltek deactivation

SGE liner selection guide

Restek TOPAZ™ liners

GC/MS-TOF Description

The most commonly used mass spectrometers in gas chromatography are systems based on quadrupole analyzers. Other analyzers used in GC/MS are ion traps. Both analyzers used the same principle. Four poles are connected into the electrical circuit that generates radiofrequency field. Change of this electrical field enables to generate scanning in time. Fragments comming from ion source through ion optics enters the analyzer. The ions separation is accomplished by applying alternating RF frequency and DC voltage to diagonally opposite ends of the quadrupole, which in turn allows aspecific mass fragment to pass through the quadrupole filter.

Picture 1: Scheme of quadrupole analyzer

SMB

The simulated counter-current process (Simulated Moving Bed - SMB) was developed in the early 60th by the Universal Oil Products Company. It was mainly applied to industrial-scale separations, like the xylene separation or the fructose-glucose separation.

There is a strong analogy between the SMB and the TMB process. Under the use of a suitable system of adsorbent and eluent, a feed stream is separated into two withdrawal streams containing the pure components of a binary or pseudo-binary mixture.In the SMB process a large column is divided into a finite number of small sections. A withdrawal tube is situated between two of such sections.These tubes are connected in a cyclic mode with the inlets and outlets via a special designed rotary valve (Knauer). An observer sitting on an in- or outlet remarks anapparent moving of solid counter-current the fluid flow at each switching although the solid is fixed in the column. Therefore, the process is called a simulated counter-current one. The SMB is equal to TMB for an infinite numberof columns.

General principle of SMB process

Thermo Fisher TriPlus GC AS syringes with fixed needles

Shimadzu GC AS syringes with removable needles

Shimadzu GC AS syringes with fixed needles

CTC GC AS syringes with fixed needles

Agilent GC AS syringes with removable needles