Stable Isotope Internal Standards

Most of the analytical methods so far developed make of use triple quadmpoles or quadrapole-linear ion-trap instruments for the quantitative determination of analytes (Castighoni et al., 2006 van Nuijs et al., 2010). In these cases, the selected ion monitoring (SRM) acquisition mode provides the best sensitivity and selectivity, when at least two specific transitions are recorded. The use of isotope-stable internal standards, as already explained in the previous section, allows accurate quantitative determinations for each substance. 

Jemal, M., Schuster, A., and Whigan, D. B. (2003). Liquid chromatography/tandem mass spectrometry methods for quantitation of mevalonic acid in human plasma and urine method validation, demonstration of using a surrogate analyte, and demonstration of unacceptable matrix effect in spite of use of a stable isotope analog internal standard. Rapid Commun. Mass Spectrom. 17, 1723-1734. 

Liang, H.R. Foltz, R.L. Meng, M. Bennett, P. Ionization enhancement in atmospheric pressure chemical ionization and suppression in electrospray ionization between target drugs and stable-isotope-labeled internal standards in quantitative liquid chromatography/tandem mass spectrometry. Rapid Commun. Mass Spectrom. 2003, 17, 2815—2821. 

Following extractive deproteinization of the plasma, the amino acids (and their stable-isotope-labeled internal standards) are separated by HPLC and introduced into the mass spectrometer. Electrospray ionization results in the formation of electrically charged molecules, which are separated on the basis of their mass/charge (m/z) ratio in the first quadrupole. Following fragmentation in the collision cell, the characteristic fragment for each amino acid is selected in the second quadrupole. This process is named multiple reaction monitoring. 

Table 2.1.3 Multiple reaction monitoring of amino acids for their tandem mass spectrometry quantitation. In daily practise not all mentioned amino acids are measured in one run, but a set of ten dedicated evaluation programs has been developed, covering groups of amino acids associated with groups of disorders. Amino acids presented in italics indicate stable-isotope-labeled internal standards ... 

Reference values of this approach are not different from those for other amino acid analyses. An example of a mass chromatogram, representing the plasma of a PKU patient, is shown in Fig. 2.1.1. When evaluating the results of MS/MS amino acid analyses, one has to reahze that the hquid chromatographic separation is by far less efficient that the AAA separation. For this reason, any amino acid may (partly) coelute with other amino acid(s), which potentially interferes with its mass spectromet-ric behavior. This effect is known as quenching. In order to overcome this as much as possible, stable-isotope-labeled internal standards (as many as possible) should be used. However, this matrix effect of ion suppression is the major pitfall in the MS/MS analysis of amino acids. Consequently, the MS/MS analysis of amino acids cannot be regarded as a reference method, similar to all other amino acid analytical methods. 

Fig. 2.1.1 Tandem mass spectrometry analyses of plasma phenylalanine (phe) and tyrosine (tyr) in a patient with phenylketonuria (PKU left panel phe 793 pmol/1, tyr 70 pmol/1) and in a control (right panel phe 27 pmol/1, tyr 28 pmol/1). The stable-isotope-labelled internal standards are D4-tyrosine (D4-tyr), containing four deuterium atoms and D5-phenylalanine (D5-phe), which has five deuterium atoms.

SIM average detection limit (with stable-isotope-labeled internal standard 0.01 mmol/mol creatinine. Reproduced from reference [32], with permission. nd Not detected... 

Table 3.1.10 (continued) Reference ranges of organic acids and acylglycines in urine in children of different ages. Values are presented as mmol/mol creatinine. TIC average detection limit 0.1 mmol/mol creatinine SIM average detection limit (with stable-isotope-labeled internal standard 0.01 mmol/mol creatinine. Reproduced from reference [32], with permission. nd Not detected... 

Measured using a stable-isotope-labeled internal standard... 

Acylcarnitine analysis using stable-isotope-labeled internal standards provides quantitative data for acylcarnitine species [14]. However, to provide meaningful results to referring healthcare providers, it is critical to complement analytical proficiency with in-depth interpretation of results, as is true for many other examples of complex metabolic profiles [39]. 

Guo and co-workers [24,25] have spearheaded the development of MS/MS serum steroid profiles. Their most recent report describes profiling in 11 min of 12 steroids in 200 pi serum with minimal work-up, comprising acetonitrile protein precipitation. The steroids analyzed were as follows DHEA sulfate, DHEA, aldosterone, cortisol, corticosterone, 11-deoxycortisol, androstenedione, estradiol, testosterone, 17-hy-droxyprogesterone, progesterone, and 25-hydroxyvitamin D3. Stable-isotope-labeled internal standards were incorporated for each steroid. An API-5000 instrument was used with the APPI source in positive-ion mode, with the exception of aldosterone, which had greater sensitivity in negative-ion mode. Separation was carried out on a C8 column, which allowed more rapid separation than the more commonly utilized C18. The MRM transitions utilized are shown in Table 5.3.1. The lower level of sensitivity was between 1.5 and 10 pg/ml, dependent on the steroid. The authors were exhaustive in addressing issues of accuracy, recovery (90-110%) and reproducibility (< 12.2% for same-day and between-day). 

Palermo M, Gomez-Sanchez C, Roitman E, Shackleton CH (1996) Quantitation of cortisol and related - -4-ene steroids in urine using gas chromatography/mass spectrometry with stable isotope-labeled internal standards. Steroids 61 583-589... 

Perhaps no reagent is more important than the stable-isotope-labeled internal standard in any clinical assay utilizing mass spectrometry for quantification. Internal standards are important in many aspects of the analysis and are somewhat different than standards utilized in other clinical, non-mass-spectrometric assays. The ideal internal standard is an enriched isotopic version of the analyte being measure. For example, in the case of phenylalanine, a standard available may contain six 13C molecules rather than 12C in the aromatic ring. This has the net effect of shifting the mass of phenylalanine by six units while also maintaining nearly identical chemical... 

To put the discussion described above in a practical illustration, I have included a mass spectrum from a newborn blood spot of a patient confirmed to have medium-chain acyl coenzyme A dehydrogenase (MCAD) deficiency. Figure 8.1.3 is an acylcarnitine profile obtained from a methanol extract of a dried blood spot. Stable-isotope acylcarnitine internal standards were mixed with the methanol extracting solvent at a concentration that is equivalent to 1 or 2 pmol/1 of blood. The concentrations of each internal standard are marked on the illustration by the clear hexagons. 

Stokvis, E., Rosing, H., and Beijnen, J. H. (2005). Stable isotopically labeled internal standards in quantitative bioanalysis using liquid chromatography/mass spectrometry Necessity or not Rapid Commun. Mass Spectrom. 19 401-407. 

Lanckmans, K., Sane, S., Smolders, I., and Michotte, Y. (2007). Use of a structural analogue versus a stable isotope labeled internal standard for the quantification of angiotensin IV in rat brain dialysates using nano-liquid chromatography/tandem mass spectrometry. Rapid Commun. Mass Spectrom. 21 1187-1195. 

Wang, S., Cyronak, M., and Yang, E. (2007). Does a stable isotopically labeled internal standard always correct analyte response A matrix effect study on a LC/MS/MS method for the determination of carvedilol enantiomers in human plasma. J. Pharm. Biomed. Anal. 43 701-707. 

Abstract Internal standards play critical roles in ensuring the accuracy of reported concentrations in LC-MS bioanalysis. How do you find an appropriate internal standard so that analyte losses and experimental variations during sample preparation, chromatographic separation, and mass spectrometric detection could be corrected How is the concentration of an internal standard determined Should internal standard responses be monitored during the analysis of incurred samples What are the main causes for internal standard response variations How do they impact the quantitation Why are stable isotope labeled internal standards preferred And yet one should still have an open-mind in their usage for the analysis of incurred samples. All these questions are addressed in this chapter supported by theoretical considerations and practical examples. 

There are two main types of internal standards. The first ones are stable isotope labeled (SIL) internal standards. They are compounds in which several atoms in the analytes are replaced by their respective stable isotopes, such as deuterium (2H, D or d), 13C, 15N, or 170. Labeling with the first three isotopes are most common, particularly labeling with deuterium (due to less difficulty in synthesis and therefore less expensive). For examples, raloxifene-d4-6-glucuronide was used as the internal standard for the determination of raloxifene-6-glucuronide [5] and 1, 2, 3, 4-13C4 estrone (PCJEl) was used as the internal standard for estrone (El) [6], The usage of stable isotope labeled internal standards in quantitative LC-MS or GC-MS analysis is often termed as isotope dilution mass spectrometry (IDMS) [7],... 

Stable Isotope Labeled Internal Standards vs. Structural Analogue Internal Standards... 

Internal standards play critical roles in ensuring the accuracy of final reported concentrations in quantitative LC-MS bioanalysis through the correction of variations during sample preparation, LC-separation, and MS detection. The physical-chemical properties of an internal standard, particularly hydrophobicity and ionization properties should be as close as possible to those of the corresponding analyte to better track the variations the analyte experiences. For this reason, stable isotope labeled internal standards should be used whenever possible. However, efforts should still be made to obtain clean extracts, adequate chromatographic separation, and optimized ionization mode and conditions. 

Stable isotope labeled internal standards may be the best, but they cannot always follow an analyte to compensate the variations of experimental conditions, particularly deuterated internal standards. In addition, low variation in internal standard responses may not be interpreted as good results, though it is favored. Stable internal standard response is good only when it is sure that the internal standard behaves the same way as the analyte does. 

Lindegardh N, Annerberg A, White NJ, Day NPJ (2008) Development and validation of a liquid chromatographic-tandem mass spectrometric method for determination of piperaquine in plasma stable isotope labeled internal standard does not always compensate for matrix effects. J Chromatogr B 862 227-236... 

Wang S, CyronakM, Yang E (2007) Does a stable isotopically labeled internal standard always... 

Unfortunately, for the majority of small molecule LC-MS/MS analyses, stable isotope labelled internal standards are not available so far. In such cases, compounds with a very similar molecular structure typically serve as internal standard ( homologues or analogues ). Since the ionization properties are substantially determined by functional groups of a molecule, ionization behaviour may differ significantly—even between compounds with very similar over-all molecular structure. Differential clustering, e.g. with sodium, ammonium or formate ions often present in mobile phases may as well impact the parity of ionization yield between analyte and internal standard. Hence the availability of an appropriate homologue is crucial and critical for the development of reliable LC-MS/MS methods in TDM [51]. 

Hewavitharana AK (2011) Matrix matching in liquid chromatography-mass spectrometry with stable isotope labelled internal standards-is it necessary J Chromatogr A 1218 359-361... 

The most important advantage of such an IDMS procedure is that it compensates in an ideal way both for losses during sample workup and for variations in mass spectrometric response. As demonstrated by Claeys et al. (1977), the use of a stable isotope-labeled internal standard, as opposed to the use of homologs, produces the lowest variance factors due to instrumental stability and sample manipulating errors. Moreover, IDMS can be used both with and without prior chromatographic separation, as for instance in... 

For HPLC MS/MS assays the use of stable isotope labeled internal standards is by far the best method to overcome any potential matrix effects and random variation in the MS/MS detector. If for any reason this stable isotope internal standard is not available, an analog compound with a mass different from the analyte can also be used. The chromatographic retention time of the internal standard, however, should be as close as possible to the retention time of the analyte. This ensures, that time dependent random variation in the ionization chamber, or whereever else in the MS/MS detector, are compensated by the internal standard. In a toxicokinetic assay described by Chi et al. (2003), for example, an internal standard was used which showed the same retention time as the analyte. 

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