Agilent Single Quad 5975 GCMS Acquisition Overview Video
Introduction
This video will present an introduction to data acquisition. We will discussacquisition methods and principles, scan and sim principles, E M Saver and Trace ion detection.
Acquisition Methods
In this video we refer to a method. A method in the chemstation or acquisition software is a complete set of instructions for the instrument and computer to acquire and process a data file. We will refer in this video to the ACKwisition part of the method which consists primarily of the M S parameters and tune file.
In the chemstation, the methods can be viewed, saved or loaded in the method file menu shown here.
It's important to know how the different ACKwisition parameters can influence the data. For instance, proper ACKwisition parameters will help you collect valid data for your application. Improper settings of the ACKwisition parameters can cause mass misassignments, poor detection and false library search results.
GCMS Data
GC M. S data is three dimensional. It has both a response and a time axis as well as a mass axis as shown here. Each data point collected in GC M. S consists of a mass spectrum as in scan data or collection of masses as in sim data.
The T I C is the sumation of all masses in the scan or sim at a particular point of time. The software constructs the T I C by summing all the response of each mass at this point. This "re-constructed" chromatogram looks very similar to a FID GC data file.
For each point in the ion chromatogram there is a coresponding spectra consisting of mass intesity pairs.
The mass spectrum is collected by ramping the RF and DC over a short period of time. In the acquisition section of the method we designate a mass range. In addition to the mass range is the number of data samples collected over a point 1 A M U window. This is referred to in the method as the "N" value. A "N" value of 2 means two data points are collected over a .1 A M U window.
Data Collection and Processing
When collecting data, the real time the processor on the M S looks at five adjacent data points. From this, it centroids the data into the mass of highest intensity. If this intensity is greater then the threshold set in the method the data point is saved. If the value is less than the threshold, the value is not saved in the data file.
If the threshold is set too high as shown in the slide on the top, valid data can be "clipped" from the results. If the threshold is set too low, unwanted, extraneous noise can be collected in the data file.
During the scanning process the M. S scans from high mass to low mass. Given the value of N, the system will collect the requested number of data points. Since the goal is to detect peaks coming from the GC, it is necessary to set the N value so that there are enough data points across each peak. Ideally, 10 to 20 scans at half height are necessary to achieve a 99.9 % integration accuracy.
Spectral Tilting
A drawback if the N value is not set sufficiently high is that the spectrum does not remain consistent over the peak. In the first slide shown here, 20 scans were collected across the peak. Viewing the spectra on each side of the peak shows consistent values.
As we decrease the number of scans across the peak we can start to see that the spectra is not the same at the front edge of the peak as compared to the back end section of the peak.
As we continue to decrease the number of scans across the peak we eventually see a much different looking spectra. This is known as spectral tilting.
When choosing acquisition parameters it is necessary to look at the GC peak widths, the Signal to noise ratio required, and the peak shape and threshold.
SIM
Selected Ion Monitoring or sim is another data collection technique for the single quad. We will discuss how sim differs from Scan data, how the sim parameters affect your data,and how best to use sim in your application.
Sim monitors only one mass at a time and ignores the other masses. To do sim we program the mass spectrometer to only look at specific ions. THe benefits of sim are greater sensitivity, better peak shape and better accuracy and precision. sim can be useful in trace analysis of complex matrices where the use is looking for a target set of compounds.
in this picture we can see how scan mode differs from sim mode. In sim, the quad is set to only specific masses and "dwells" only on these masses.
To set up sim, we can choose one ion or multiple ions. The "dwell time is set so that we achieve 15 to 25 cycles across the peak. We can time program the sim or sim groups so that we can minimize the number of ions selected.
It is important to choose the correct mass when using sim. In the ACKwisition method, we can choose the mass to within .1 AMU. This is important since choosing the wrong mass can lead to improper ion ratios from tune to tune. From this data shown, it is important to choose the most intense mass shown. In cases where the autotune slightly shifts the mass axis by .1 AMU the selection of 370.9 and 472.9 would lead to improper mass ratio calculations.
CAreful selection of the exact mass in sim will help the overall accuracy of quantitation as shown in the slide here.
In sim we can choose 60 ions per time segment. We can have up to 100 time segments in our method. It is important to choose the minimum number of ions for correct compound identification. This will help minimize the overall dwell times and improve the sensitivity and precision. When choosing the sim ions we should try to select ions of higher mass, high intensity and uniqueness.
In the case shown here, the ideal mass would be 154. This is the highest mass in the mass spectrum, it has the greatest intensity and is unique. This also applies to the spectrum on the right.
EM Saver
EM saver is a parameter that is only used in sim methods. It imposes a limit to the number of counts obtained for the sim ion. This prevents "burn" of the multiplier and extends it's life. This is ideal for samples of high concentrations.
Shown in this slide is where the E M saver is set in the chemstation method.
SIM - SCAN
The single quad is also capable of collecting both sim and scan data simultaneously. This can provide good quantitation for target compounds and also provide library search capability. When running Sim scan, both sets of data are saved in the same data file. The software also contains a special data handling function to convert any full scan method to a sim method. Sim methods are automatically generated and no manual setup is required.
When using sim it is important to use the correct resolution mode. The low mode should not be used if co-eluting masses differ by < 3 AMU.
TID - Trace Ion Detection
Trace ion detection or TID is an ACKwisition parameter that optimizes the data collection parameters for very low level detection. The trace ion detection feature can help reduce noise and thereby give higher signal to noise values. It also improves peak shape and spectral fidelity as well as improves library matching.
This slide shows the baseline of the same sample with TID on and off. See the reduced noise levels.
SHown here are the improved peak shapes.
Here is a complex matrix where the target compounds are at very low level. See the improvement in signal with the TID on.
TID can be turned on in the method here.
ALthough trace level detection can help improve signal to noise and give better peak shape, there are some cases where it should not be used. Since TID will cause slightly broadened peaks and lower chromatographic resolution, it is recommended that TID only be used when there are at least 4 samples over the peak.
If more then 10 points across the peak exist, there is no appreciable peak height change. 7 points across the peak will lead to a 10% loss in peak height. 4 points will lead to a 35 percent reduction in peak height. It is recommended to turn off TID when using < 4 points across the peak.
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