Injection systems large bore capillary columns

The split-flow injector is very similar to the packed column injector except that only part of the carrier flow passes to the column, the rest exits to waste. By varying the exit flow-impedance, the split-ratio can be adjusted over a wide range. Without this type of split injection system, the small bore capillary columns would be virtually impossible to use. However, because of the waste of sample and the relatively small mass range obtainable from small bore columns, the large bore capillary column was introduced. 

Injection Systems for Large Bore Capillary Columns... 

Large bore capillary columns can be considered as those that are sufficiently wide to permit a hypodermic needle to enter viz. 0.052 in. ID.) and thus allow on-column injection. A diagram of an injection system for large columns is shown in figure 4.5. 

Bonded-phase wide-bore capillary columns permit relatively large-volume septum injections and can offer advantages of improved efficiency, reproducibility, and reliability. A 60 m x 0.53 mm i.d. fused silica capillary coated with the dimethylpolysiloxane SPB-1 (5 pm film thickness) programmed from 40°C to 200°C offers many advantages over packed column and PLOT systems. Improved resolution of very volatile compounds is obtained and, even with an initial temperature of 40°C, the total analysis time can be reduced to 26 min, and good peakshapes can be obtained even for alcohols (Figure 1). Moreover, splitless septum injections of up to 300 pi headspace can be performed with no noticeable effect on column efficiency hence, sensitivity is at least as good as that attainable with a packed column. Indeed, nowadays packed and PLOT columns are only used in specific applications or if a capillary GC is not available. 

Bayliss and co-workers [10] combined ultra-high flow rates, parallel LC columns, a multiplex electrospray source, and mass spectrometric detection for the rapid determination of pharmaceuticals in plasma using four narrow bore (50 mm x 1 mm, 30 pm Oasis HLB) or capillary (50 mm x 0.18 mm, 25 pm Oasis HLB) HPLC columns with large particle sizes (to avoid high system back-pressure) in parallel with a multiple probe injector and a MUX MS interface. Small sample aliquots were injected directly into the system without sample pre-treatment procedure, obtaining very low limits of quantification (from 1 to 5 ng/mL). 

---