Optimizing the Analysis of Volatile Organic Compounds

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32 Figure 31. An open split interface or a jet separator will reduce the gas flow entering a mass spectrometer vacuum system. carrier gas make-up gas purge in open split interface jet separator vacuum www.restekcorp.com to MSD 1:10 split purge out to MSD MS Calibration and Tuning Calibration allows the correct identification of masses, whereas tuning adjusts the intensity and peakwidths for masses. The MS is calibrated by adjusting the DC/RF frequency so that mass axis points are aligned with expected mass fragments of known spectra. Tuning ensures that target compounds analyzed on the MS will have the same distribution (pattern) of ions, and peak widths for ions will be narrow enough that adjacent mass peaks will not overlap. A compound widely used for calibrating and tuning MS systems is perfluorotributylamine (PFTBA or FC43). Modern instruments introduce PFTBA into the ion source during the autotune procedure. The instrument software adjusts the MS parameters to match the known fragmentation pattern for PFTBA. The ion of greatest abundance in the spectrum is mass 69; the relative abundances of masses 131 and 219 are roughly 50% of that for mass 69 (Figure 32). In analyses of volatiles, mass 502 is less important because its relative abundance is 1% of the mass 69 value. Low peak heights or a loss of mass 502 generally indicate a cleanliness problem at the source. Specific Tuning Requirements: 4-Bromofluorobenzene: After the system is calibrated and tuned, using PFTBA, a 50ng solution of 4-bromofluorobenzene (BFB) is introduced. BFB usually is introduced by injection through the GC injection port but, alternatively, it can be purged from a water blank. Abundance criteria for BFB are listed in Table IV. If tuning with BFB fails under criteria acceptable for PFTBA, decrease the relative abundance of masses 131 and 219 to 30% of mass 69 by adjusting the entrance lens. If necessary, slightly decrease the repeller voltage. This procedure targets the ions from mass 173 through mass 177. A second tuning failure with BFB may dictate recalibration and tuning with PFTBA. Ion ratios for BFB should be checked every 12 hours. As long as results meet the specifications in Table IV, no further calibration or tuning is required. Poor tuning can significantly affect the sensitivity of the MS. Figure 33 shows spectra for a sample analyzed twice, first after a failing PFTBA tuning with mass 131 as the base peak (Figure 33, A), then after a passing tuning (Figure 33, B). The second analysis exhibits a three-fold increase in sensitivity. Figure 32. Perfluorotributylamine (PFTBA) fragmentation, using a quadrupole mass spectrometer. 69 100 80 60 40 20 0 131 219 50 100 150 200 250 300 350 400 450 500 550 600 Table IV. US EPA ion abundance criteria for 4-bromofluorobenzene (BFB). Mass/Charge Ratio Relative Abundance Criterion 50 15-50% of mass 95 75 30-80% of mass 95 95 Base peak, 100% relative abundance 96 5-9% of mass 95 173 50% of mass 95 175 5-9% of mass 174 176 >95% but
Abundance 320000 280000 240000 200000 160000 120000 80000 40000 Figure 33. Optimize MS tuning to increase sensitivity. A - Before tune B - After tune Time 1.00 1.40 1.80 2.20 2.60 3.00 3.40 3.80 4.20 4.60 5.00 5.40 5.80 Leak Checks: The MS is a powerful tool for determining the presence of leaks in the GC/MS system because it is able to detect air and water. Check for leaks by turning on the PFTBA valve and scanning for m/z 69, 18, and 28. By using the base peak for PFTBA (m/z 69), a relative concentration of water/nitrogen can be determined. The combined relative abundance of 18/28 to 69 should be between 0.1% and 3%. Figure 34 shows an air/water value of 0.14% (0.05+0.09). If the value is below 0.1%, compare the current total abundance of ion 69 with its abundance at the last leak check. Instruments with large leaks have reduced sensitivity for 69 and may show abundances of 0 for lower ions, suggesting there is no leak. This is due to saturation of the detector. If a leak is present, the instrument will not tune. An MS with a diffusion pump should be allowed more time to equilibrate because it is less efficient at removing low molecular weight contamination. After this preliminary air/water check, begin tuning the instrument. After the instrument passes tuning, check again for air and water. Figure 34. An initial air/water check on an Agilent 5971A MS, before tuning. 33 www.restekcorp.com
Page 1 and 2: Optimizing the Analysis of Volatile
Page 3 and 4: EPA Method Definitions Many EPA met
Page 5 and 6: Table I. State gasoline methods inc
Page 7 and 8: Purge and Trap Theory Concentration
Page 9 and 10: Step 5. Desorb Once the desorb preh
Page 11 and 12: Adsorbent Materials Tenax ® Adsorb
Page 13 and 14: Table II. Compositions and characte
Page 15 and 16: Troubleshooting Common Problems Ass
Page 17 and 18: Broad Peaks: Peak broadening is ano
Page 19 and 20: Figure 14. Configuring your GC is s
Page 21 and 22: Narrow-bore Systems (0.18mm ID - 0.
Page 23 and 24: Detection Systems VOCs can be analy
Page 25 and 26: FID Operation The flame ionization
Page 27 and 28: Figure 22. Cross-section of the The
Page 29 and 30: Similarly, the PTFE transfer line b
Page 31: Figure 29. Electromagnetic fields f
Page 35 and 36: Connect the capillary column, 0.25m
Page 37 and 38: Applications Using GC Detection Sys
Page 39 and 40: Figure 39B. An Rtx ® -VGC primary
Page 41 and 42: Rtx ® -502.2 75m, 0.45mm ID, 2.55
Page 43 and 44: Figure 41B. Analytes listed in EPA
Page 45 and 46: Figure 42B. An Rtx ® -VGC / Rtx ®
Page 47 and 48: Purge and Trap Applications Using P
Page 49 and 50: Gasoline Analysis with Oxygenates a
Page 51 and 52: Figure 47. Volatile organics by US
Page 53 and 54: 1. dichlorodifluoromethane 2. chlor
Page 55 and 56: 1. chloromethane 2. vinyl chloride
Page 57 and 58: Table IX Volatile organic compounds
Page 59 and 60: Table XI. Choosing a Volatiles GC C
Page 61 and 62: Ordering Information | Rtx ® Rtx -
Page 63 and 64: Siltek -Deactivated Guard Columns/
Page 65 and 66: Leak Detective II Leak Detector Af
Page 67 and 68: Method 8260 8260 Internal Standard
Page 69 and 70: 8021/502.2 Surrogate Mix #1 1-bromo
Page 71 and 72: 04.2 & 04.1 SOW CLP 04.1 VOA CAL200

Optimizing the Analysis of Volatile Organic Compounds

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