21 Fig.2 26 24,25 22,23 14-16 12 11 13 10
8 9
1 2 3 45 1 6 7 1.5 18,19 32 17 33 36 39 46 47 2 2.5 3 Time (min) +EI TIC Scan SCAN Cal Salt 25-0b-1.D
Figure 2. Total ion chromatogram (TIC) from the scan analysis of the 25 µg/L standard. The numbers identifying the peaks correspond to the fi rst column in Table 2.
Fig.3 3.5 4 54 56 4.5 5 5.5 75 76 6 6.5 30,31 37,38 48 74 20,21 34,35 28,29 41-43 40 44,45 49,50 55 27 ISTD 51 52,53 57-60 61 62 63 64,65 67 77 79 78 80 66 68-70 71,73
The average calibration range was 0.07 to 24 µg/L, with an average R2 of 0.9990. If necessary, the relative standard error (RSE) value was used to guide removal of the lowest and highest calibration points, to achieve an RSE value of <20% and for choosing between a linear or quadratic fi t. For some compounds, a linear fi t would meet the <20% RSE criteria, but come close to the limit. However, use of a quadratic fi t would signifi cantly improve the RSE. For example, tert-butylbenzene had an RSE of 18.3 with a linear fi t, but changing to quadratic lowered the RSE to 8.1. Similar improvements were seen with some of the other substituted benzenes as well. As observed with the scan data calibration, the average response factor RSD was <20 for 76 analytes.
Figure 4 shows a typical example with the lowest calibrator and calibration curve for iodomethane. The improved signal-to-noise ratio provided by SIM, relative to that shown in Figure 3, is clear.
Method detection limits
An MDL study was performed after completion of the initial calibration. Eight trials were performed at the lowest level of calibration, 0.05 μg/L. The calculated MDLs were obtained by applying the formula shown in Equation 1. For compounds with higher reporting limits, eight trials were performed at the concentration of 0.1 μg/L. Table 3 lists the calculated MDLs for 80 VOCs. Six compounds had insuffi cient response, even at the 0.1 μg/L level, so the lowest calibration level used is listed instead in bold and square brackets. As noted in the scan results, acetone also had a contamination issue as observed in the blank, resulting in poor calibration results. The average MDL for the 80 compounds was 0.026 µg/L.
Equation 1: Formula for MDL calculations. MDL = s · t(n – 1, 1 – alpha = 99) = s · 2.998
Figure 3. (A) quantifi er EIC for iodomethane 0.05 µg/L calibration standard. (B) calibration curve for iodomethane from 0.05 µg/L to 25 µg/L. Fig.4
Scan
Where: t(n – 1, 1 – alpha) = t value for the 99% confi dence level with n – 1 degrees of freedom n = number of trials (8) s = standard deviation of the eight trials
Figure 4. SIM results for iodomethane. (A) quantifi er EIC for iodomethane 0.05 µg/L calibration standard. (B) calibration curve for iodomethane from 0.05 µg/L to 25 µg/L.
Scan
Initial calibration (ICAL) with scan data
The chromatographic parameters used in the method resulted in good separation of the 80 VOC compounds in less than 7 minutes, as shown in Figure 2. While there are overlapping peaks, their response was measured selectively with the quantifi er ions chosen. Most compounds had suffi cient response to be measured at or below 0.1 µg/L, and exhibit very good linearity. The average calibration range was 0.16 to 25 µg/L with an average R2 of 0.9978. If necessary, the relative standard error (RSE) value was used to guide removal of the lowest, and in one case highest, calibration points, to achieve an RSE value of <20% (except for acetone). The average Response Factor RSD was <20 for 76 analytes. As expected, polar compounds with higher solubility in water were the worst performers. Acetone is an example, where it also had a contamination issue as observed in the blank, resulting in poor calibration results. A typical example is shown in Figure 3, with the lowest calibrator and calibration curve for iodomethane.
Spectral fi delity
The 25 µg/L VOC standard was analysed with the software, where spectra of the compounds were deconvoluted and searched against the NIST20 library. As seen in Table 2, the library match scores (LMS) are excellent, with an average of 94. There were only six compounds with LMS scores below 90, and these were due to low response and/or interference from overlapping peaks not completely removed by deconvolution. Nitrobenzene (compound 76 in Table 2) gave a very good LMS value of 94. Nitrobenzene reacts readily with hydrogen in a conventional MS source to produce aniline [4], resulting in low LMS values typically in the 60s. The HydroInert source greatly reduces in-source reactions with hydrogen, resulting in the high LMS value for nitrobenzene.
Initial calibration with SIM data
The results of the SIM mode calibration are listed in Table 3. As expected, for most compounds, SIM provided excellent calibration linearity and measurement at or below 0.05 µg/L.
Figure 5. TIC (black) and deconvolution component (green) chromatograms of tap water samples. ISTD is shown in red. Top: Sample from Eastern Pennsylvania. Bottom: Sample from Southeastern Pennsylvania.
VOCs found in drinking water
Samples of municipal tap water from sources in the state of Pennsylvania were analysed using both the scan and SIM methods. Several VOCs were identifi ed by searching their deconvoluted spectra against the NIST20 library. The chromatograms from two of the samples are shown in Figure 5. The concentration of VOCs was determined using quantitative analysis, with both the scan and SIM calibrations. The results are presented in Table 4.
Trichloromethane, bromodichloromethane, dibromochloromethane, and tribromomethane (collectively known as the trihalomethanes) are very common in municipal water treated with chlorine for disinfection purposes.
WWW.LABMATE-ONLINE.COM
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72
