Buffers and Baselines

LC•GC Europe - July 2001 LC troubleshooting 1 Buffers and Baselines Nan S. Wilson, Ryan Morrison, LC Resources Inc., McMinnville, Oregon, USA and John W. Dolan, LC Resources Inc., Walnut Creek, California, USA. Gradient baselines don’t always behave as desired. Drifting baselines are common with on scale and provide accurate integration, prepared and the formulation of the A and gradient elution liquid chromatography (LC) it can be ignored. Workers should be B solvents. separations. Drift is influenced by the mobile careful about the solubility of buffer salts Figure 2 illustrates the effect of a B phase’s organic solvents, the detection in the organic solvent, especially when solvent with higher absorbance than the A wavelength and the presence of additives. using phosphate buffers. For this reason, solvent. In this instance, the same This month’s “LC Troubleshooting” focuses our laboratory personnel avoid using phosphate buffer as in Figure 1 was used on the baseline characteristics of several phosphate buffers at concentrations with tetrahydrofuran instead of methanol common LC buffers and methods to greater than approximately 10 mM as the organic solvent. As Figure 2 shows, minimize drift. combined with organic concentrations tetrahydrofuran causes the baseline to drift When comparing figures in this column, greater than approximately 80%. to two absorbance units (2 AU) at 215 nm. readers should note the scale of the y axis, Methanol is less problematic in this regard This drift makes tetrahydrofuran unusable because it is not the same in all figures. For than acetonitrile. as an organic solvent for low-wavelength example, the absorbance scales for Figures It is best to maintain constant buffer gradients. Although many modern LC 1 and 2 differ by 10-fold. Also, don’t place concentration during the run with detectors are linear to an upper limit of too much emphasis on the peaks in the equimolar buffer in both the A and B 2 AU, remember that the 2 AU linear various runs — we chose the solvents. However, many workers take the range includes any background. For the chromatograms from our files to illustrate easy way out and run with buffer in the A present example, 50% tetrahydrofuran buffer and solvent drift and we made no bottle and organic solvent in the B bottle, generates a 1 AU background, so only special efforts to remove extraneous peaks. as is the situation for Figure 1. Although 1 AU is available as the maximum peak Extra peaks in a gradient run are often the this causes a gradient in buffer strength, it height to maintain detector linearity. Most result of contaminants in the buffer or seldom is a problem if the buffer older UV detectors and many older data water and they can be a challenge to concentration is greater than 10 mM, systems are limited to a 1 AU maximum eliminate. An earlier “LC Troubleshooting” organic concentrations are less than detector signal, which further limits the column has an example of the process to approximately 80%, and the buffer is used usefulness of tetrahydrofuran as a solvent. isolate extraneous peaks (1). within its normal buffering range. For reproducible separations, it is important to Phosphate — An Old Friend specify in the LC method how the buffer is 2.0 Phosphate buffers have long been a favourite of chromatographers. Some of phosphate’s endearing qualities include its 0.10 1.0 low cost, high purity, convenient preparation, useful pH ranges and good 0.00 0.0 chromatographic behaviour. Figure 1 Absorbance (AU) shows blank gradients for a 10 mM 0246810 12 Absorbance (AU) 0246810 phosphate buffer with methanol as the Time (min) Time (min) organic solvent. At a low wavelength of 215 nm, the baseline drift is minimal and a Figure 1: Baselines obtained using phosphate– Figure 2: Baselines obtained using phosphate– flat baseline is produced at 254 nm. We methanol gradients. Solvent A: 10 mM tetrahydrofuran gradients. Solvent A: 10 mM observe drift when the UV absorbance of potassium phosphate (pH 2.8); solvent B: potassium phosphate (pH 2.8); solvent B: the starting solvent does not match that of methanol; gradient: 5–80% solvent B in tetrahydrofuran; gradient: 5–80% solvent B the ending solvent; however, as long as the 10 min. Detection wavelength: 215 nm in 10 min. Detection wavelength: 215 nm drift is small enough to allow peaks to stay (upper trace), 254 nm (lower trace). (upper trace), 254 nm (lower trace). 2 LC troubleshooting LC•GC Europe - July 2001 At higher wavelengths, such as the the gradient at the detector by almost Negative drift is especially problematic 254 nm run shown in Figure 2, 3 min in this instance, as the baseline step because most data systems cannot process tetrahydrofuran is satisfactory, and it may for the 215 nm plot shows. Note that detector outputs less than roughly Ϫ100 mV be the solvent of choice because of its although the step looks large in Figure 3, (equivalent to Ϫ0.1 AU in this situation). separation characteristics. the actual offset is approximately 0.03 AU. Under these restrictions, analysts would The column dead time can be identified observe the baseline dropping to Ϫ0.1 AU Trifluoroacetic Acid — Another Favourite by the baseline disturbance immediately and then remaining flat, much as in the An alternative buffer for low-pH work is after 1 min. days of strip chart recorders when the pen trifluoroacetic acid. Trifluoroacetic acid is a would drift to the bottom of the paper. favourite buffer for the separation of Acetate for Intermediate pH This set of conditions is clearly biomolecules. Tetrahydrofuran–acetonitrile Trifluoroacetic acid is useful near pH 2, and unacceptable. To compensate for the gradients, such as those shown in Figure 3, phosphate is useful from approximately pH 2 negative drift, workers can add a UV- represent standard conditions for to pH 3.1 and from pH 6.2 to pH 8.2. For absorbing compound to the B solvent so its biomolecule separations. The volatility of intermediate pH values, acetate buffers are absorbance matches the A solvent. In the trifluoroacetic acid makes it compatible often the best choice. These buffers also present situation, we accomplished this with mass spectrometry (MS) detection or have the advantage of volatility, so they task by adding ammonium acetate to the suitable for sample recovery after mobile- can be used with LC–MS detection. methanol. For the best performance with phase evaporation. Because of the ease of However, using acetate can be a challenge ammonium acetate, chromatographers formulation (just add 1 mL/L) and its ability if chromatographers fail to take proper care should prepare the A and B solvents so to minimize absorbance differences during mobile-phase formulation. This they have the same buffer concentration between the A and B solvents at low problem is illustrated in Figure 4, in which — 25 mM in this instance. wavelengths, 0.1% trifluoroacetic acid can we took the buffer-in-A solvent and When we used equimolar buffer in both be added to water as the A solvent and organic-in-B solvent approach, as was the solvents, the drift was greatly reduced, acetonitrile as the B solvent. situation with phosphate in Figures 1 and 2. as Figure 5 shows (note the absorbance Figure 3 also illustrates the effect of the At 215 nm, ammonium acetate has much scale change). We observed a slight positive system dwell volume and the column greater background absorbance than drift, which is easy for the data system to volume on the baseline. The combination methanol and results in the severely handle. The magnitude of the drift of these two volumes delays the arrival of negative drift in Figure 4. (approximately 0.2 AU in this situation) is small enough so integration will be satisfactory and the chromatograms will be 0.10 0.2 visually pleasing. Note that when working at the higher wavelength of 254 nm, 0.1 matching the mobile-phase absorbance 0.00 0.0 is unnecessary. Ϫ0.1 Absorbance (AU) Absorbance (AU) High-pH Buffers 0246810 Ϫ0.2 Time (min) 0 10203040 Although phosphate is a useful buffer at Time (min) pH levels as high as pH 8.2, we recommend Figure 3: Baselines obtained using avoiding phosphate at levels higher than pH trifluoroacetic acid–acetonitrile gradients. Figure 5: Baselines obtained using equimolar 8 in favour of organic buffers that minimize Solvent A: 0.1% trifluoroacetic acid in water ammonium acetate–methanol gradients. the dissolution of the silica-based column (pH Ϸ 1.9); solvent B: 0.1% trifluoroacetic Solvent A: 25 mM ammonium acetate (pH 4) packing (2). One alternative is Tris acid in acetonitrile; gradient: 10–90% solvent in 5% methanol; solvent B: 25 mM ammonium (tris[hydroxymethyl]aminomethane), a buffer B in 10 min. Detection wavelength: 215 nm acetate in 80% methanol; gradient: 0–100% widely used by biochemists. Some workers (upper trace), 254 nm (lower trace). in 40 min. Detection wavelength: 215 nm have found ammonium bicarbonate to be (upper trace), 254 nm (lower trace). especially useful at high pH levels (3). 0.0 1.0 Ϫ0.2 0.1 0.0 Ϫ0.4 0.0 Ϫ0.6 Ϫ Ϫ Ϫ0.8 0.1 0.1 Ϫ1.0 Ϫ Absorbance (AU) 0.2 Absorbance (AU) Ϫ0.2 0 10203040 Absorbance (AU) Ϫ0.3 02468 10 Time (min) 0 5 10 15 20 Time (min) Time (min) Figure 4: Baselines obtained using ammonium Figure 7: Baselines obtained using ammonium acetate–methanol gradients. Solvent A: Figure 6: Baselines obtained using bicarbonate–methanol gradients. Solvent A: 25 mM ammonium acetate (pH 4); solvent B: Tris–methanol gradients. Solvent A: 50 mM ammonium bicarbonate (pH 9); solvent B: 80% methanol in water; gradient: 5–100% Tris; solvent B: methanol; gradient: 5–80% methanol; gradient: 5–60% solvent B in solvent B in 40 min.

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