High Source Temperatures and Meeting EPA Tuning Criteria15 Oct 2015
My first blog here at Restek was a demonstration of how increasing the source temperature could improve the raw response of 2,4-dinitrophenol while maintaining the relative response. Four years later I'm here with the promised follow-up on the elevated source temperature effects on tune evaluations (and making adjustments via target tuning). This blog is going to deal exclusively with meeting the EPA Method 8270D DFTPP tune evaluation criteria because fear of failing tunes is the primary feedback we receive when we ask customers why they don't try acquiring data at elevated source temperatures when performing semi-volatiles analysis.
Figure 1 - Table 3: DFTPP Ion Abundance Criteria, taken from EPA Method 8270D Rev. 4, Feb. 2007
Figure 1 lists the DFTPP ion abundance criteria for quadrupole instrumentation. It is important to note that it allows for m/z = 198 or m/z = 442 to be the base peak. The first step is to make sure that your software is configured to automatically switch between alternate base peaks (Figure 2 - Edit DFTPP tune evaluation criteria interface).
Figure 2 - EnviroQuant Tune Evaluation Criteria Edit Screen
N.B. The 8270D tune evaluation criteria are NOT the default DFTPP tune evaluation criteria in EnviroQuant. If you haven't updated them, be sure that you do.
The goal here is to kill two birds with one stone. If you've read last week's blog, Which Source Configuration is Right for your Application, you know that my conclusion was that a 6 mm ID or 9 mm ID extractor lens or drawout plate should be used for semivolatile analysis to minimize source tailing of low volatility compounds (while increasing signal to noise). Anticipating tune related hesitation, I performed all the tuning experiments detailed below with a 9 mm ID extractor lens installed and source and quadrupole temperatures set at 350⁰C and 200⁰C respectively.
Because I'm using an extractor source, I ran the etune.u with the default targets from the dftpp.u tune file. The tune wizard screens are Figures 3, 4, 5, and 6 below. Figure 7 is the detailed tune profile report.
Figure 3 - Tune Wizard Screen 1
Figure 4 - Tune Wizard Screen 2
Figure 5 - Tune Wizard Screen 3
Figure 6 - Tune Wizard Screen 4
Figure 8 is the tune evaluation report for a 10:1 split injection of the 50 ng/µL GC-MS tuning mix (cat# 31615) on a 30 m Rxi-5ms (cat# 13423), with an acquisition (scan) rate of 5.4 Hz and a gain factor of 3.0, using the etune.u file described above. The DFTPP tune evaluation met all criteria the first time it was evaluated for the peak apex scan (Figure 8), as well as the peak average (report not shown). Overall, the evaluation looks good, though the m/z = 51 relative to the base peak check (m/z = 198 in this case). 14.6% is a bit closer to the lower threshold of 10% relative response than I would like if I were setting this instrument up for routine work.
Figure 8 - EnviroQuant DFTPP Tune Evaluation Report
In fact, when run under alternate acquisition conditions, with the gain set to 0.3, the tune evaluation fails low for m/z = 51 relative to base peak response when evaluated on the peak apex scan (Figure 9) .
Figure 9 - EnviroQuant DFTPP Tune Evaluation Report
The mass selective detector can be tuned to elevate the relative response of low m/z fragments while leaving the higher m/z fragments largely unaffected. This is done by increasing the target response of PFTBA fragment m/z = 50 relative to m/z = 69 (the PFTBA base ion peak) while leaving the other target peaks the same. This is done on screen two of the tune wizard (Figure 10). For this exercise, I raised the m/z = 50 target from 1.0% to 1.1% of m/z = 69.
Figure 10 - Tune Wizard Screen 2 Showing Change in M/Z = 50 Tune Target
This 0.1% relative abundance target change for m/z = 50 resulted in an approximately 30% increase of the m/z = 51 response relative to the base peak in the DFTPP tune evaluation (Figure 11). This time the tune was evaluated for a 20:1 split injection of the 50 ng/µL GC-MS tuning mix (cat# 31615) on a 20 m Rxi-5ms (cat# 13402), with an acquisition (scan) rate of 9.7 Hz and a gain factor of 3.0, using the etune 350 target I.u file generated from the change above. Figure 12 is the detailed tune profile report.
Figure 11 - EnviroQuant DFTPP Tune Evaluation
Figure 12 - etune 350 target I.u Detailed Profile Report