57 Food & Beverage Analysis
As shown in Figure 3, the abundance of TIC signal in full scan data acquisition mode was noticeably reduced for spinach, walnut, and cayenne pepper extracts after cleanup compared to crude extracts before cleanup.
rich in high-boiling components, with long retention times that exceed the retention times for the target pesticides. A common way to avoid ghost peaks in the subsequent runs was to use an extended column bake-out after the last target analyte eluted from the column.
However, this approach has several disadvantages, including the deposition of high boilers and GC column stationary phase into the EI source, contamination of the head of the GC column, a decrease of the column lifetime, and a longer cycle time due to the extended bake-out.
Midcolumn backfl ush allows the elution of the high-boiling matrix components from the column without the sacrifi ces encountered with the bake-out approach. After the MS data are collected, the oven is held at the fi nal temperature in postrun mode, and the carrier gas fl ow through the fi rst column is reversed. This reversed fl ow carries any high boilers that were in the column at the end of data collection. The high boilers are carried out of the head of the column and into the split vent trap (Figure 4A).
The ability to reverse the fl ow is provided by a tee that is inserted, in this case, between two identical 15 m columns. During the analysis, a small makeup fl ow of carrier gas is used to sweep the connection. During backfl ushing, the makeup fl ow is raised to a much higher value, sweeping high boilers backward out of the fi rst column while simultaneously providing forward fl ow in the second column. For the confi guration in this application, the backfl ushing time was 1.5 minutes.
The chromatograms shown in Figure 4B illustrate the effectiveness of the backfl ush technique in reducing cycle time sample carryover. The cycle time was reduced by 50%, and the columns did not have to be exposed to the higher bake-out temperatures for an extended time. Using backfl ush, excess column bleed and heavy residues are not introduced into the mass-spectrometer thereby reducing ion source contamination.
In addition, the midcolumn backfl ushing confi guration provides a signifi cant time-saving benefi t when coupled with a multimode inlet. Maintenance procedures such as column trimming, and septum and liner change can be performed without the need to cool down the MS transfer line and source. When the septum is removed, the system’s pneumatic switching device module provides the carrier gas fl owing backward through column 1. It also prevents air from entering the GC columns and the MS. Multimode inlet fast cooling capability enables more time savings. As a result, liner and septum replacement, which are the most common maintenance procedures, can be performed in a few minutes.
Figure 3. Scan TIC of the spinach (A), walnut (B), and cayenne pepper (C) extracts. The red trace corresponds to matrix sample with Captiva EMR cleanup and the black trace corresponds to matrix sample without clean up. The green trace corresponds to the acetonitrile solvent blank.
Matrix screening in full scan data acquisition mode
DE90315046
Screening samples in full scan data acquisition mode facilitates the evaluation of in-source matrix loading. To maintain optimal performance, every MS source limits the amount of material that can be present in the source at one time. If the electron ionisation (EI) source is overloaded with the matrix, quantitation accuracy of the analysis can be signifi cantly compromised. Therefore, it is essential to analyse the matrix in full scan mode to evaluate total ion chromatogram (TIC) and maintain the optimal GC/TQ performance.
The abundance of TIC in full scan mode is recommended not to exceed 7 ×107 counts when analysing with an electron multiplier (EM) gain set to 1. Out of the three analysed matrices, cayenne pepper featured the highest matrix background, although the background was noticeably reduced after the cleanup procedure. In this evaluation, pesticides that eluted between 11 and 12.5 minutes were expected to have sacrifi ced performance in the cayenne pepper matrix when evaluating sensitivity and the dynamic range. For example, Endosulfan I eluted at 11.273 minutes, and it could be quantitated only starting at 5 ppb in the cayenne pepper matrix with both GC/TQ systems. Spinach and walnut matrices had signifi cantly lower matrix levels coeluting with Endosulfan I, with 0.1 ppb limit of quantifi cation (LOQ) observed.
Some of the practices that can help lower the matrix background include adequate sample cleanup, sample dilution, and smaller injection volume. The latter two approaches often result in better LOQs, especially with the 7010C GC/TQ system equipped with a high effi ciency source (HES).
Midcolumn backfl ushing
Midcolumn backfl ushing is a technique in which the carrier gas fl ow is reversed after the last analyte has exited the column. The use of the midcolumn backfl ushing confi guration allows the analyst to limit the analysis time to the retention time of the last-eluting compound of interest. Challenging matrices – especially the oily ones, such as walnut - are
Figure 4. Midcolumn backflush configuration and gas flow during the GC run and the backflush cycle (A); TIC Scan chromatograms of a cayenne pepper extract followed by the analysis of an instrument blank with column bake-out, with backflush and without backflush or bake-out (B)
Figure 4. Midcolumn backfl ush confi guration and gas fl ow during the GC run and the backfl ush cycle (A); TIC Scan chromatograms of a cayenne pepper extract followed by the analysis of an instrument blank with column bake-out, with backfl ush and without backfl ush or bake-out (B)
Leak-free GC/TQ system
Preventing leaks in a GC/MS system is essential for the long-term performance of the instrument. Undesired leaks reduce the GC column lifetime and lead to oxidation of the EI source, degrading its performance. Some tools that enable tight connections and make installation easy and reproducible include self-tightening collared column nuts for GC and gold-plated flexible metal ferrules.
Leak-free GC/TQ system
Preventing leaks in a GC/MS system is essential for the long-term performance of the instrument. Undesired leaks reduce the GC column lifetime and lead to oxidation of the EI source, degrading its performance. Some tools that enable tight connections and make installation easy and reproducible include self-tightening collared column nuts for GC and gold-plated fl exible metal ferrules.
DE90315046
The self-tightening collared column nuts have an innovative spring-driven piston. The piston continuously presses against the short graphite/polyimide ferrule, maintaining a leak-free seal even after hundreds of temperature cycles of the oven. The addition of the collar makes column installation into the GC inlet and MS transfer line easy and reduces the possibility of variation. The locking collar allows locking the column in place, for accurate and repeatable installation results, time after time.
Figure 3. Scan TIC of the spinach (A), walnut (B), and cayenne pepper (C) extracts. The red trace corresponds to matrix sample with Captiva EMR cleanup and the black
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 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80
