LC-MS combination

In an LC/MS combination, passage of the separated components (A, B, C, D) successively into the mass spectrometer yields their individual spectra. 

I have tried to make it clear that the LC-MS combination is usually more powerful that either of the individual techniques in isolation and that a holistic approach must be taken to the development of methodologies to provide data from which the required analytical information may be obtained. Data analysis is of crucial importance in this respect and for this reason the computer processing of LC-MS data is considered in some detail in both Chapters 3 and 5. 

What Problems, if Any, Have to be Addressed to Allow the LC-MS Combination to Function, and Function Effectively ... 

In the following chapters, the basic principles of HPLC and MS, in as far as they relate to the LC-MS combination, will be discussed and seven of the most important types of interface which have been made available commercially will be considered. Particular attention will be paid to the electrospray and atmospheric-pressure chemical ionization interfaces as these are the ones most widely used today. The use of LC-MS for identification and quantitation will be described and appropriate applications will be discussed. 

There are a number of specialist texts in which high performance liquid chromatography (HPLC) is described in varying amounts of detail (Lindsay [2] Robards et al. [3] Meyer [4]). It is not, therefore, the intention of this author to provide a comprehensive description of the technique but merely to discuss those aspects which are essential to the successful apphcation of the LC-MS combination. 

The LC/MS combination has been practiced for many years. Various types of interfaces have been developed for LC/MS, including the fast atom bombardment (FAB) source by Barber et al. in 1981 and thermospray by Blakely and VestaL in 1983. However, a broad acceptance of... 

LC-MS). LC-MS combines the power of MS detection with the versatility of LC, which allows chromatographic separation of thermally unstable analytes, as well as compounds with a wide range of polarities without performing derivatization processes. For this reason, LC-MS and LC-MS/MS applications for the analysis of antidepressants have signihcantly grown over the last decade. 

Higashi T (2006) Trace determination of steroids causing age-related diseases using LC/MS combined with detection-oriented derivatizafion. Chem Pharm Bull 54 1479-1485... 

Unfortunately, the LC-MS combination is less successful. In part, this may be due to technological interfacing problems, but even if these are solved, LC-MS is unlikely to provide the same degree of universality (large molecules will remain a problem), spectral information and reproducibility as the GC-MS combination. For the moment, the combination of LC with a multichannel UV absorption detector is a more realistic proposition. 

Another popular and efficient inlet system for the LC/MS combination is the atmospheric pressure chemical ionization process. This system has some similarity to the electrospray interface and can also cope with flow rates of up to 2 ml/min. and thus the total column eluent can be utilized without splitting the flow. 

Mass spectrometry may be characterized by successful results in the field of biochemistry this success is due above all to the development of GC/MS (and LC/MS), combined with a computer for rapid data processing. The use of mass fragmentometry (SIM, MID) has resulted in assays of very small quantities in mixtures (currently on the order of a pg, and even a fg, scale). 

Liquid chromatography-mass spectrometry (LC-MS) combines the separating power of LC with the uniquely powerful detection capabilities of MS. With this technique, it is now possible to separate, identify and quantify components in a mixture. As with GC-MS, there is a choice of mass spectrometric ionisation techniques and analysers available. LC-MS can also be taken a step further as LC-MS" (otherwise known as LC-tandem MS) where ions can be subjected to fragmentation so as to gain information on their structures. 

MS is becoming the detection system of choice for LC by virtue of its flexibility and high selectivity for individual solutesHowever, LC-MS is always less sensitive than GC-MS as a result of the need to transfer the analytes from the liquid phase into a high-vacuum gas phase. Other limitations of LC-MS combination include the inability to use nonvolatile buffers, the narrow optimum range for eluent flow rate influence of the proportion of organic modifier on the sensitivity, and the narrow choice of ionization methods.Nevertheless, LC-MS has been widely accepted as an advantageous choice for the determination of carbamate pesticides in water matrices, which is more robust and flexible in the absence of derivatization. Thermospray and particle-beam interfaces are probably most commonly used for offline and online determination of carbamates in Atmospheric pressure sources such as... 

Z. REALIZED ON-LINE COMBINATIONS OF LIQUID CHROMATOGRAPHY AND MASS SPECTROMETRY The realized on-line LC-MS combinations differ in respect of solvent handling ... 

In (A) and (B), ionization is achieved by electron impact, and in (C) the solvent is used as a reactant gas for chemical ionization or atmospheric pressure ionization. In Table I existing LC-MS combinations are classified according to this system and they are discussed below in more detail. 

Up to now, all realized LC-MS combinations have the limitation that they can be used only for substances with a reasonable degree of volatility. For ion-... 

S uzuki et al. report a hybrid triple quadrupole/linear ion trap MS/MS technique. The triple quad-rupole-linear ion trap LC/MS combines the scan speed and sensitivity of ion trap MS while retaining the selective scanning modes of triple quadrupole MS. The ability to perform both types of MS on a single instrument reduces the time required for analysis while eliminating the chance of errors due to analyzing different samples in multiple MS runs. The method, coupled with LC separation, was demonstrated to be usefuUy applied to the structure elucidation of YTX analogues. ... 

The results of PAH analysis with different types of interfaces (e.g. ESI, APCI, PBI and TSP - were reported by Clench et al. reviewing the state of the art of various mass spectral techniques [28]. For more polar PAHs pneumatically assisted ESI-LC-MS was used to determine mixtures of hydroxy polycyclic aromatic hydrocarbons. The abundance of ions dependent on flow rates was shown. ESI inonization was found to be less sensitive compared to APCI ionisation [304]. PAH analysis with ESI-LC-MS combined with RP-LC with post-column addition of silver nitrate was applied for the determination of 10 PAHs in river water. PAHs resulted in [Mj and [M-i-Ag]. The detection limits of different PAHs in spiked samples ranged from 0.001 to 0.03 pg L [442]. 

However, LC/MS, though specific, may not always be accurate in automatically determining the exact amount of compound present. Other factors have to be considered as well. For example, in these experiments LC/MS combined with electrospray ionization (ESI) generated a nonlinear response curve. As the concentrations in the two phases, octanol and aqueous, are often different, this nonlinearity makes it difficult to determine the logZ) values by simply comparing the ratios of the peak areas of the compound in two phases. 

---