I am trying to validate our methamphetamine quantitation method on our new GC/MS. We have been using this method successfully for years on 7 other GC/MS instruments, but our new instrument does not produce a linear curve. Does anyone have any ideas on what could be causing our curve to look like this?
I looked up the highest split ratio available to use, 1038:1. The R^2 of the curve is now 0.99929. Besides decreasing the injection volume, are there any other settings I can change on the instrument? I don't want to use all of my helium. Thanks for any tips!
Please upload the file under your method subdirectory called acqmeth.txt . That file contains the method parameters. Any other information that will help would be appreciated - desired calibration range, sample preparation used, solvent injected, liner installed, etc.
What is the model number of your older GCMS systems? Your new one? What software and versions on the older and the newer?
INSTRUMENT CONTROL PARAMETERS: Havok
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D:\MassHunter\GCMS\1\methods\METH_QUANT2.M
Tue Sep 19 14:51:19 2023
Control Information
------- -----------
Sample Inlet : GC
Injection Source : GC ALS
Injection Location: Front
Mass Spectrometer : Enabled
No Sample Prep method has been assigned to this method.
GC
GC Summary
Run Time 4 min
Post Run Time 0 min
Oven
Temperature
Setpoint On
(Initial) 150 °C
Hold Time 0.5 min
Post Run 0 °C
Program
#1 Rate 25 °C/min
#1 Value 220 °C
#1 Hold Time 0.7 min
Equilibration Time 0.5 min
Max Temperature 325 °C
Maximum Temperature Override Disabled
Slow Fan Disabled
ALS
Front Injector
Syringe Size 5 μL
Injection Volume 0.5 μL
Solvent A Washes (PreInj) 1
Solvent A Washes (PostInj) 3
Solvent A Volume 4 μL
Solvent B Washes (PreInj) 1
Solvent B Washes (PostInj) 3
Solvent B Volume 4 μL
Sample Washes 1
Sample Wash Volume 4 μL
Sample Pumps 3
Dwell Time (PreInj) 0 min
Dwell Time (PostInj) 0 min
Solvent Wash Draw Speed 150 μL/min
Solvent Wash Dispense Speed 1000 μL/min
Sample Wash Draw Speed 150 μL/min
Sample Wash Dispense Speed 1000 μL/min
Injection Dispense Speed 3000 μL/min
Viscosity Delay 1 sec
Sample Depth 0 mm
Injection Type Standard
L1 Airgap 0.1 μL
Solvent Wash Mode A, B
Sample Overlap
Mode Sample overlap is not enabled
ALS Errors Pause for user interaction
Front SS Inlet He
Mode Split
Heater On 250 °C
Pressure On 16.008 psi
Total Flow On 1249.8 mL/min
Septum Purge Flow On 3 mL/min
Pre-Run Flow Test Off
Gas Saver On 15 mL/min after 0.5 min
Split Ratio 1038 :1
Split Flow 1245.6 mL/min
Liner A Liner has not been selected.
Column
Column #1
Flow
Setpoint On
(Initial) 1.2 mL/min
Post Run 0.57353 mL/min
Column Information Agilent 13323-1628655
Description Rxi-1ms
Temperature Range -60 °C—325 °C (350 °C)
Dimensions 30 m x 250 μm x 0.25 μm (Uncalibrated)
Heater Oven
Column lock Unlocked
In Front SS Inlet He
Out MSD
(Initial) 150 °C
Pressure 16.008 psi
Flow 1.2 mL/min
Average Velocity 41.683 cm/sec
Holdup Time 1.1995 min
Control Mode Constant Flow
Column Outlet Pressure 0 psi
Detector Evaluation
Perform Detector Evaluation Test Off
MSD Transfer Line
Temperature
Setpoint On
(Initial) 260 °C
Signals
Signal #1:
Description None
Signal #2:
Description None
Signal #3:
Description None
Signal #4:
Description None
Signal #5:
Description None
Signal #6:
Description None
Signal #7:
Description None
Signal #8:
Description None
MS Information
-- -----------
General Information
------- -----------
Acquisition Mode : SIM
Solvent Delay (minutes) : 1.5
Tune file : D:\MassHunter\GCMS\1\5977\etune.u
EM Setting mode Delta : 0.000000
Number of SIM Groups : 1
Run Time (if MS only) : 10 minutes
[SIM Parameters]
Group 1 Group ID : 1
Resolution : 1
Group Start Time : 1.5
Number of Ions : 4
Ions
Dwell In Group :( Mass, Dwell) ( Mass, Dwell) ( Mass, Dwell)
( 58.00,50 ) ( 91.00,50 ) ( 168.00,50 )
( 169.00,50 )
[MSZones]
MS Source : 230 C maximum 250 C
MS Quad : 150 C maximum 200 C
Timed Events
----- ------
Number Events= 0
END OF MS ACQUISITION PARAMETERS
TUNE PARAMETERS for SN: US2312MA29
---------------------------------
Trace Ion Detection is OFF.
34.593 : EMISSION
70.007 : ENERGY
0.503 : REPELLER
89.822 : IONFOCUS
27.717 : ENTRANCE_LENS
928.951 : EMVOLTS
929 : Actual EMV
0.18 : GAIN FACTOR
1640.000 : AMUGAIN
133.938 : AMUOFFSET
1.000 : FILAMENT
0.000 : DCPOLARITY
6.495 : ENTLENSOFFSET
10.002 : Ion_Body
0.303 : EXTLENS
-470.000 : MASSGAIN
-23.000 : MASSOFFSET
END OF TUNE PARAMETERS
----------------------
END OF INSTRUMENT CONTROL PARAMETERS
------------------------------------
New model: 8890 / 5977C Column: Restek Rsi-1ms
Software: Mass Hunter Workstation 10.2.524.1 (G6845AA) and Chemstation G1701FA, but all data processing is in Excel
Old models: 8890 / 5977B, 6890/ 5973, 7890A / 5975C, 6890 / 5973N Same method in use for 10 years on multiple instruments, multiple labs. Varity of software versions, but all using Chemstation
4 mg/ml = 4,000 µg/ml.
One Agilent app notes calls out methamphetamine: "The 5 μg/mL original standards were directly used for injection, and were diluted 10 times with blank solvent to 500 ng/mL solution. " 5990-7596EN.pdf (agilent.com)
Another: "TIC of amphetamine mixture at 250 μg/mL (split 20:1 for on‑column concentration of 12.5 μg/mL)" application-drugs-of-abuse-gc-mass-spectrometry-5994-1012en-agilent.pdf
Since you are splitting so much, the highest I've ever seen, the easiest guess is inlet or source overload or both.
Try injecting even less sample. You're only injecting 0.5 µL now, but try 0.1 µL. The peaks will still be too big, so dilute 1:100 and change your split ratio to something like 100:1 or less.. Just because the split ratio can be entered does not mean that it is useful. The system may be able to get 1245.6 ml/min of split flow through, but the effectiveness, linearity, and reproducibility of that much flow in the liner are very suspect.
15 ml/min in gas saver mode may be entered, but that is not the safest for your column. That flow is so low that there is the possibility of oxygen migration into the column through diffusion in O rings and even back diffusion through the vents. Please increase this to 20 ml/min or even 30 ml/min.
That second app note says that the 9mm draw out lens was installed in the MS. This changes the dynamics in the ionization chamber and reduces linearity problems at very high concentrations/too much sample on column/ too much sample into the ion source. G3870-20449 | Agilent That would be worth a try. The tuning might be slightly different, and you do give up some ultimate sensitivity, but it does not look like sensitivity is a problem.
All of that stuff you inject has to go somewhere, so it's in the split vent line and the split vent trap. What goes into the column will be contaminating the ion source, source radiator, and manifold, and over time everything inside the vacuum chamber will need to be cleaned. You want to inject the smallest volume of the lowest concentration of the cleanest samples you can and still see the lowest standard concentration of your peaks of interest. The smallest peaks should be very small - in the low thousands of counts, maybe 3x to 4x the background noise. The cleaner you keep the system - the less you contaminate the system with samples - the longer the interval will be between maintenance and the lower the number of consumable parts replaced.