Spectrum manipulation

There are a number of techniques available to users of modern infrared spectrometers which help with both qualitative and quantitative interpretation of spectra. 

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Background and blank subtraction Since the OMA is a curve (spectrum) manipulator, it easily lends itself to subtraction of a background dark-charge (and "pattern") spectrum and/or a blank spectrum from each acquired analyte spectrum. The resultant analyte spectrum. The resultant analyte spectrum is thus free of any detector or blank e.g., solvent, distortions. Fig. 3. 

The use of data-processing equipment in conjunction with infra-red spectrophotometers, both dispersive and interferometric, produces three advantages, namely instrument control, spectrum manipulation, and spectrum identification and interpretation. Firstly, the control of the instrument is simplified and the reproducibility and accuracy of wavelength and transmission (or absorbance) are greatly enhanced. For example, one commercial Fourier transform instrument uses a helium-neon laser to obtain an accuracy of better than 0.01 cm at 4000cm, whereas older dispersive instruments could have an error of 50 cm The accuracy of... 

The second advantage, which is based upon the fact that spectra can be recorded in digital form, is the ability to accumulate spectra and to manipulate them. Solvent and impurity spectra can be recorded and subtracted from the sample spectrum, which can then be levelled, smoothed, and converted from transmittance to absorbance or vice versa. The absorbance scale can also be expanded considerably. The spectrum of a weak sample can be scanned repeatedly which, together with averaging of the signal, can reduce noise appreciably. This leads to a dramatic improvement in sensitivity. For example, there is little difference between the spectra of carbon disulphide and of benzocaine in carbon disulphide shown in Fig. 6, but with spectrum manipulation, a good spectrum of benzocaine is readily obtained (Fig. 7). The amount of benzocaine in the cell was approximately 4 Lig but only about one quarter of this was in the infra-red... 

Such problems are usually attacked by computer unfolding" of the overlapped peaks. This operation amounts to the subtraction of the counts in the assumed pulse distribution of one peak from those in the combined peak the result is two separate peaks which, if added together, would give the observed, overlapped peak. The computer that performs this operation and other kinds of spectrum manipulation, such as background subtraction, is part of the same small computer that acts as an MCA. 

Xemr is an (ESR) EPR (Electron Paramagnetic Resonance), ENDOR (Electron Nuclear DOuble Resonance), and TRIPLE (electron-nuclear-nuclear TRIPLE resonance) spectrum manipulation and simulation package written for Linux systems. It should be noted that Xemr does not run under Microsoft DOS/Windows environments and the porting would currently be a rather lengthy task. Xemr source and binary code is distributed under the GNU General Public License. 

Mathematical manipulation (Fourier transform) of the data to plot a spectrum... 

Apart from the actual acquisition of the mass spectrum and its subsequent display or printout, the raw mass spectral data can be processed in other ways, many of which have been touched on in other chapters in thi.s book. Some of the more important aspects of this sort of data manipulation are explained in greater detail below. 

Interestingly, if the original comparison of the spectrum from the reference compound with stored m/z data for that compound reveals discrepancies, the stored reference data are updated before the computer goes on to acquire data from the unknown compound. In this mode, the computer is not used simply to acquire and manipulate data but is also used to make decisions... 

Computers, often combined with transputers, are used for three main functions when connected to a mass spectrometer. The foremost requirements involve the acquisition and preprocessing of basic data and the control of the instrument s scanning operations. Additional software programs are available to manipulate the preprocessed data in a wide variety of ways depending on what is required, e.g., a mass spectrum or a total ion chromatogram. 

Several features of ISS quantitative analysis should be noted. First of all, the relative sensitivities for the elements increase monotonically with mass. Essentially none of the other surface spectroscopies exhibit this simplicity. Because of this simple relationship, it is possible to mathematically manipulate the entire ISS spectrum such that the signal intensity is a direct quantitative representation of the surface. This is illustrated in Figure 5, which shows a depth profile of clean electrical connector pins. Atomic concentration can be read roughly as atomic percent direcdy from the approximate scale at the left. 

Very rarely, however, will a single mass spectrum provide us with complete analytical information for a sample, particularly if mass spectral data from a chromatographic separation, taking perhaps up to an hour, is being acquired. The mass spectrometer is therefore set up to scan, repetitively, over a selected m jz range for an appropriate period of time. At the end of each scan, the mass spectrum obtained is stored for subsequent manipulation before a further spectrum is acquired. 

If preferred, the data system may be used to subtract the background spectrum (Figure 3.14) from that of the analyte (Figure 3.16). This manipulation yields the spectrum shown in Figure 3.17, in which the ions from the background are now totally absent. Care must be taken when adopting this procedure to ensure that any contribution from the analyte is not removed when ions at the same m/z value arise from both the background and the analyte of interest. 

Background-subtracted spectrum A mass spectrum from which ions arising from species other than the analyte have been removed by computer manipulation. 

ESI-MS has emerged as a powerful technique for the characterization of biomolecules, and is the most versatile ionization technique in existence today. This highly sensitive and soft ionization technique allows mass spectrometric analysis of thermolabile, non-volatile, and polar compounds and produces intact ions from large and complex species in solution. In addition, it has the ability to introduce liquid samples to a mass detector with minimum manipulation. Volatile acids (such as formic acid and acetic acid) are often added to the mobile phase as well to protonate anthocyanins. A chromatogram with only the base peak for every mass spectrum provides more readily interpretable data because of fewer interference peaks. Cleaner mass spectra are achieved if anthocyanins are isolated from other phenolics by the use of C18 solid phase purification. - ... 

There are many specific ways to generate equally spaced tags but they are all based on the same principle of manipulating the rf pulses to generate equally spaced bands of rf radiation in the frequency domain. It is well known that under ordinary conditions, meaning normal levels of nuclear spin excitation, the frequency spectrum of the rf excitation pulse(s) is approximately the Fourier transform of the pulses in the time domain. Thus, a single slice can be generated in the... 

We can manipulate the FID mathematically in various ways before Fourier transformation, in order to optimize the spectrum with respect to the line-width or the lineshape. 

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