In rformassspectrometry/MetaboAnnotation: Utilities for Annotation of Metabolomics Data

library(BiocStyle)
BiocStyle::markdown()

Package: r BiocStyle::Biocpkg("MetaboAnnotation")
Authors: r packageDescription("MetaboAnnotation")[["Author"]]
Compiled: r date()

library(AnnotationHub)
library(MetaboAnnotation)
library(Spectra)

Introduction

The r Biocpkg("MetaboAnnotation") package defines high-level user functionality to support and facilitate annotation of MS-based metabolomics data [@rainer_modular_2022].

Installation

The package can be installed with the BiocManager package. To install BiocManager use install.packages("BiocManager") and, after that, BiocManager::install("MetaboAnnotation") to install this package.

General description

MetaboAnnotation provides a set of matching functions that allow comparison (and matching) between query and target entities. These entities can be chemical formulas, numeric values (e.g. m/z or retention times) or fragment spectra. The available matching functions are:

• matchFormula: to match chemical formulas.
• matchSpectra: to match fragment spectra.
• matchValues (formerly matchMz): to match numerical values (m/z, masses, retention times etc).

For each of these matching functions parameter objects are available that allow different types or matching algorithms. Refer to the help pages for a detailed listing of these (e.g. ?matchFormula, ?matchSpectra or ?matchValues). As a result, a Matched (or MatchedSpectra) object is returned which streamlines and simplifies handling of the potential one-to-many (or one-to-none) matching.

Example use cases

The following sections illustrate example use cases of the functionality provided by the MetaboAnnotation package.

library(MetaboAnnotation)

Matching of m/z values

In this section a simple matching of feature m/z values against theoretical m/z values is performed. This is the lowest level of confidence in metabolite annotation. However, it gives ideas about potential metabolites that can be analyzed in further downstream experiments and analyses.

The following example loads the feature table from a lipidomics experiments and matches the measured m/z values against reference masses from LipidMaps. Below we use a data.frame as reference database, but a CompDb compound database instance (as created by the r Biocpkg("CompoundDb") package) would also be supported.

ms1_features <- read.table(system.file("extdata", "MS1_example.txt",
                                       package = "MetaboAnnotation"),
                           header = TRUE, sep = "\t")
head(ms1_features)
target_df <- read.table(system.file("extdata", "LipidMaps_CompDB.txt",
                                    package = "MetaboAnnotation"),
                        header = TRUE, sep = "\t")
head(target_df)

For reference (target) compounds we have only the mass available. We need to convert this mass to m/z values in order to match the m/z values from the features (i.e. the query m/z values) against them. For this we need to define the most likely ions/adducts that would be generated from the compounds based on the ionization used in the experiment. We assume the most abundant adducts from the compounds being "[M+H]+" and "[M+Na]+. We next perform the matching with the matchValues function providing the query and target data as well as a parameter object (in our case a Mass2MzParam) with the settings for the matching. With the Mass2MzParam, the mass or target compounds get first converted to m/z values, based on the defined adducts, and these are then matched against the query m/z values (i.e. the m/z values for the features). To get a full list of supported adducts the MetaboCoreUtils::adductNames(polarity = "positive") or MetaboCoreUtils::adductNames(polarity = "negative") can be used). Note also, to keep the runtime of this vignette short, we match only the first 100 features.

parm <- Mass2MzParam(adducts = c("[M+H]+", "[M+Na]+"),
                           tolerance = 0.005, ppm = 0)

matched_features <- matchValues(ms1_features[1:100, ], target_df, parm)
matched_features

From the tested 100 features 55 were matched against at least one target compound (all matches are against a single compound). The result object (of type Matched) contains the full query data frame and target data frames as well as the matching information. We can access the original query data with query and the original target data with target function:

head(query(matched_features))
head(target(matched_features))

Functions whichQuery and whichTarget can be used to identify the rows in the query and target data that could be matched:

whichQuery(matched_features)
whichTarget(matched_features)

The colnames function can be used to evaluate which variables/columns are available in the Matched object.

colnames(matched_features)

These are all columns from the query, all columns from the target (the prefix "target_" is added to the original column names in target) and information on the matching result (in this case columns "adduct", "score" and "ppm_error").

We can extract the full matching table with matchedData. This returns a DataFrame with all rows in query the corresponding matches in target along with the matching adduct (column "adduct") and the difference in m/z (column "score" for absolute differences and "ppm_error" for the m/z relative differences). Note that if a row in query matches multiple elements in target, this row will be duplicated in the DataFrame returned by data. For rows that can not be matched NA values are reported.

matchedData(matched_features)

Individual columns can be simply extracted with the $ operator:

matched_features$target_name

NA is reported for query entries for which no match was found. See also the help page for ?Matched for more details and information. In addition to the matching of query m/z against target exact masses as described above it would also be possible to match directly query m/z against target m/z values by using the MzParam instead of the Mass2MzParam.

Matching of m/z and retention time values

If expected retention time values were available for the target compounds, an annotation with higher confidence could be performed with matchValues and a Mass2MzRtParam parameter object. To illustrate this we randomly assign retention times from query features to the target compounds adding also 2 seconds difference. In a real use case the target data.frame would contain masses (or m/z values) for standards along with the retention times when ions of these standards were measured on the same LC-MS setup from which the query data derives.

Below we subset our data table with the MS1 features to the first 100 rows (to keep the runtime of the vignette short).

ms1_subset <- ms1_features[1:100, ]
head(ms1_subset)

The table contains thus retention times of the features in a column named "rtime".

Next we randomly assign retention times of the features to compounds in our target data adding a deviation of 2 seconds. As described above, in a real use case retention times are supposed to be determined by measuring the compounds with the same LC-MS setup.

set.seed(123)
target_df$rtime <- sample(ms1_subset$rtime,
                          nrow(target_df), replace = TRUE) + 2

We have now retention times available for both the query and the target data and can thus perform a matching based on m/z and retention times. We use the Mass2MzRtParam which allows us to specify (as for the Mass2MzParam) the expected adducts, the maximal acceptable m/z relative and absolute deviation as well as the maximal acceptable (absolute) difference in retention times. We use the settings from the previous section and allow a difference of 10 seconds in retention times. The retention times are provided in columns named "rtime" which is different from the default ("rt"). We thus specify the name of the column containing the retention times with parameter rtColname.

parm <- Mass2MzRtParam(adducts = c("[M+H]+", "[M+Na]+"),
                       tolerance = 0.005, ppm = 0,
                       toleranceRt = 10)
matched_features <- matchValues(ms1_subset, target_df, param = parm,
                                rtColname = "rtime")
matched_features

Less features were matched based on m/z and retention times.

matchedData(matched_features)[whichQuery(matched_features), ]

Matching of SummarizedExperiment or QFeatures objects

Results from LC-MS preprocessing (e.g. by the r BiocStyle::Biocpkg("xcms") package) or generally metabolomics results might be best represented and bundled as SummarizedExperiment or QFeatures objects (from the same-named Bioconductor packages). A XCMSnExp preprocessing result from xcms can for example be converted to a SummarizedExperiment using the quantify method from the xcms package. The feature definitions (i.e. their m/z and retention time values) will then be stored in the object's rowData while the assay (the numerical matrix) will contain the feature abundances across all samples. Such SummarizedExperiment objects can be simply passed as query objects to the matchValues method. To illustrate this, we create below a simple SummarizedExperiment using the ms1_features data frame from the example above as rowData and adding a matrix with random values as assay.

library(SummarizedExperiment)

se <- SummarizedExperiment(
    assays = matrix(rnorm(nrow(ms1_features) * 4), ncol = 4,
                    dimnames = list(NULL, c("A", "B", "C", "D"))),
    rowData = ms1_features)

We can now use the same matchValues call as before to perform the matching. Matching will be performed on the object's rowData, i.e. each row/element of the SummarizedExperiment will be matched against the target using e.g. m/z values available in columns of the object's rowData:

parm <- Mass2MzParam(adducts = c("[M+H]+", "[M+Na]+"),
                     tolerance = 0.005, ppm = 0)
matched_features <- matchValues(se, target_df, param = parm)
matched_features

As query, the result contains the full SummarizedExperiment, but colnames and matchedData will access the respective information from the rowData of this SummarizedExperiment:

colnames(matched_features)
matchedData(matched_features)

Subsetting the result object, to e.g. just matched elements will also subset the SummarizedExperiment.

matched_sub <- matched_features[whichQuery(matched_features)]
MetaboAnnotation::query(matched_sub)

A QFeatures object is essentially a container for several SummarizedExperiment objects which rows (features) are related with each other. Such an object could thus for example contain the full feature data from an LC-MS experiment as one assay and a compounded feature data in which data from ions of the same compound are aggregated as an additional assay. Below we create such an object using our SummarizedExperiment as an assay of name "features". For now we don't add any additional assay to that QFeatures, thus, the object contains only this single data set.

library(QFeatures)
qf <- QFeatures(list(features = se))
qf

matchValues supports also matching of QFeatures objects but the user needs to define the assay which should be used for the matching with the queryAssay parameter.

matched_qf <- matchValues(qf, target_df, param = parm, queryAssay = "features")
matched_qf

colnames and matchedData allow to access the rowData of the SummarizedExperiment stored in the QFeatures' "features" assay:

colnames(matched_qf)
matchedData(matched_qf)

Matching of MS/MS spectra

In this section we match experimental MS/MS spectra against reference spectra. This can also be performed with functions from the r BiocStyle::Biocpkg("Spectra") package (see SpectraTutorials, but the functions and concepts used here are more suitable to the end user as they simplify the handling of the spectra matching results.

Below we load spectra from a file from a reversed-phase (DDA) LC-MS/MS run of the Agilent Pesticide mix. With filterMsLevel we subset the data set to only MS2 spectra. To reduce processing time of the example we further subset the Spectra to a small set of selected MS2 spectra. In addition we assign feature identifiers to each spectrum (again, for this example these are arbitrary IDs, but in a real data analysis such identifiers could indicate to which LC-MS feature these spectra belong).

library(Spectra)
library(msdata)
fl <- system.file("TripleTOF-SWATH", "PestMix1_DDA.mzML", package = "msdata")
pest_ms2 <- filterMsLevel(Spectra(fl), 2L)
## subset to selected spectra.
pest_ms2 <- pest_ms2[c(808, 809, 945:955)]
## assign arbitrary *feature IDs* to each spectrum.
pest_ms2$feature_id <- c("FT001", "FT001", "FT002", "FT003", "FT003", "FT003",
                         "FT004", "FT004", "FT004", "FT005", "FT005", "FT006",
                         "FT006")
## assign also *spectra IDs* to each
pest_ms2$spectrum_id <- paste0("sp_", seq_along(pest_ms2))
pest_ms2

This Spectra should now represent MS2 spectra associated with LC-MS features from an untargeted LC-MS/MS experiment that we would like to annotate by matching them against a spectral reference library.

We thus load below a Spectra object that represents MS2 data from a very small subset of MassBank release 2021.03. This small Spectra object is provided within this package but it would be possible to use any other Spectra object with reference fragment spectra instead (see also the SpectraTutorials workshop). As an alternative, it would also be possible to use a CompDb object representing a compound annotation database (defined in the r Biocpkg("CompoundDb") package) with parameter target. See the matchSpectra help page or section Query against multiple reference databases below for more details and options to retrieve such annotation resources from Bioconductor's r Biocpkg("AnnotationHub").

load(system.file("extdata", "minimb.RData", package = "MetaboAnnotation"))
minimb

We can now use the matchSpectra function to match each of our experimental query spectra against the target (reference) spectra. Settings for this matching can be defined with a dedicated param object. We use below the CompareSpectraParam that uses the compareSpectra function from the Spectra package to calculate similarities between each query spectrum and all target spectra. CompareSpectraParam allows to set all individual settings for the compareSpectra call with parameters MAPFUN, ppm, tolerance and FUN (see the help on compareSpectra in the r Biocpkg("Spectra") package for more details). In addition, we can pre-filter the target spectra for each individual query spectrum to speed-up the calculations. By setting requirePrecursor = TRUE we compare below each query spectrum only to target spectra with matching precursor m/z (accepting a deviation defined by parameters ppm and tolerance). By default, matchSpectra with CompareSpectraParam considers spectra with a similarity score higher than 0.7 as matching and these are thus reported.

csp <- CompareSpectraParam(requirePrecursor = TRUE, ppm = 10)
mtches <- matchSpectra(pest_ms2, minimb, param = csp)
mtches

The results are reported as a MatchedSpectra object which represents the matching results for all query spectra. This type of object contains all query spectra, all target spectra, the matching information and the parameter object with the settings of the matching. The object can be subsetted to e.g. matching results for a specific query spectrum:

mtches[1]

In this case, for the first query spectrum, no match was found among the target spectra. Below we subset the MatchedSpectra to results for the second query spectrum:

mtches[2]

The second query spectrum could be matched to 4 target spectra. The matching between query and target spectra can be n:m, i.e. each query spectrum can match no or multiple target spectra and each target spectrum can be matched to none, one or multiple query spectra.

Data (spectra variables of either the query and/or the target spectra) can be extracted from the result object with the spectraData function or with $ (similar to a Spectra object). The spectraVariables function can be used to list all available spectra variables in the result object:

spectraVariables(mtches)

This lists the spectra variables from both the query and the target spectra, with the prefix "target_" being used for spectra variable names of the target spectra. Spectra variable "score" contains the similarity score.

We could thus use $target_compound_name to extract the compound name of the matching target spectra for the second query spectrum:

mtches[2]$target_compound_name

The same information can also be extracted on the full MatchedSpectra. Below we use $spectrum_id to extract the query spectra identifiers we added above from the full result object.

mtches$spectrum_id

Because of the n:m mapping between query and target spectra, the number of values returned by $ (or spectraData) can be larger than the total number of query spectra. Also in the example above, some of the spectra IDs are present more than once in the result returned by $spectrum_id. The respective spectra could be matched to more than one target spectrum (based on our settings) and hence their IDs are reported multiple times. Both spectraData and $ for MatchedSpectra use a left join strategy to report/return values: a value (row) is reported for each query spectrum (even if it does not match any target spectrum) with eventually duplicated values (rows) if the query spectrum matches more than one target spectrum (each value for a query spectrum is repeated as many times as it matches target spectra). To illustrate this we use below the spectraData function to extract specific data from our result object, i.e. the spectrum and feature IDs for the query spectra we defined above, the MS2 spectra similarity score, and the target spectra's ID and compound name.

mtches_df <- spectraData(mtches, columns = c("spectrum_id", "feature_id",
                                             "score", "target_spectrum_id",
                                             "target_compound_name"))
as.data.frame(mtches_df)

Using the plotSpectraMirror function we can visualize the matching results for one query spectrum. Note also that an interactive, shiny-based, validation of matching results is available with the validateMatchedSpectra function. Below we call this function to show all matches for the second spectrum.

plotSpectraMirror(mtches[2])

Not unexpectedly, the peak intensities of query and target spectra are on different scales. While this was no problem for the similarity calculation (the normalized dot-product which is used by default is independent of the absolute peak values) it is not ideal for visualization. Thus, we apply below a simple scaling function to both the query and target spectra and plot the spectra again afterwards (see the help for addProcessing in the Spectra package for more details on spectra data manipulations). This function will replace the absolute spectra intensities with intensities relative to the maximum intensity of each spectrum. Note that functions for addProcessing should include (like in the example below) the ... parameter.

scale_int <- function(x, ...) {
    x[, "intensity"] <- x[, "intensity"] / max(x[, "intensity"], na.rm = TRUE)
    x
}
mtches <- addProcessing(mtches, scale_int)
plotSpectraMirror(mtches[2])

The query spectrum seems to nicely match the identified target spectra. Below we extract the compound name of the target spectra for this second query spectrum.

mtches[2]$target_compound_name

As alternative to the CompareSpectraParam we could also use the MatchForwardReverseParam with matchSpectra. This has the same settings and performs the same spectra similarity search than CompareSpectraParam, but reports in addition (similar to MS-DIAL) to the (forward) similarity score also the reverse spectra similarity score as well as the presence ratio for matching spectra. While the default forward score is calculated considering all peaks from the query and the target spectrum (the peak mapping is performed using an outer join strategy), the reverse score is calculated only on peaks that are present in the target spectrum and the matching peaks from the query spectrum (the peak mapping is performed using a right join strategy). The presence ratio is the ratio between the number of mapped peaks between the query and the target spectrum and the total number of peaks in the target spectrum. These values are available as spectra variables "reverse_score" and "presence_ratio" in the result object). Below we perform the same spectra matching as above, but using the MatchForwardReverseParam.

mp <- MatchForwardReverseParam(requirePrecursor = TRUE, ppm = 10)
mtches <- matchSpectra(pest_ms2, minimb, param = mp)
mtches

Below we extract the query and target spectra IDs, the compound name and all scores.

as.data.frame(
    spectraData(mtches, c("spectrum_id", "target_spectrum_id",
                          "target_compound_name", "score", "reverse_score",
                          "presence_ratio")))

In these examples we matched query spectra only to target spectra if their precursor m/z is ~ equal and reported only matches with a similarity higher than 0.7. CompareSpectraParam, through its parameter THRESHFUN would however also allow other types of analyses. We could for example also report the best matching target spectrum for each query spectrum, independently of whether the similarity score is higher than a certain threshold. Below we perform such an analysis defining a THRESHFUN that selects always the best match.

select_top_match <- function(x) {
    which.max(x)
}
csp2 <- CompareSpectraParam(ppm = 10, requirePrecursor = FALSE,
                            THRESHFUN = select_top_match)
mtches <- matchSpectra(pest_ms2, minimb, param = csp2)
res <- spectraData(mtches, columns = c("spectrum_id", "target_spectrum_id",
                                       "target_compound_name", "score"))
as.data.frame(res)

Note that this whole example would work on any Spectra object with MS2 spectra. Such objects could also be extracted from an xcms-based LC-MS/MS data analysis with the chromPeaksSpectra or featureSpectra functions from the r Biocpkg("xcms") package. Note also that retention times could in addition be considered in the matching by selecting a non-infinite value for the toleranceRt of any of the parameter classes. By default this uses the retention times provided by the query and target spectra (i.e. spectra variable "rtime") but it is also possible to specify any other spectra variable for the additional retention time matching (e.g. retention indices instead of times) using the rtColname parameter of the matchSpectra function (see ?matchSpectra help page for more information).

Matches can be also further validated using an interactive Shiny app by calling validateMatchedSpectra on the MatchedSpectra object. Individual matches can be set to TRUE or FALSE in this app. By closing the app via the Save & Close button a filtered MatchedSpectra is returned, containing only matches manually validated.

Query against multiple reference databases

Getting access to reference spectra can sometimes be a little cumbersome since it might involve lookup and download of specific resources or eventual conversion of these into a format suitable for import. MetaboAnnotation provides compound annotation sources to simplify this process. These annotation source objects represent references (links) to annotation resources and can be used in the matchSpectra call to define the targed/reference spectra. The annotation source object takes then care, upon request, of retrieving the annotation data or connecting to the annotation resources.

Also, compound annotation sources can be combined to allow matching query spectra against multiple reference libraries in a single call.

At present MetaboAnnotation supports the following types of compound annotation sources (i.e. objects extending CompAnnotationSource):

• Annotation resources that provide their data as a CompDb database (defined by the r BiocStyle::Biocpkg("CompoundDb")) package. These are supported by the CompDbSource class.

• Annotation resources for which a dedicated MsBackend backend is available hence supporting to access the data via a Spectra object. These are supported by the SpectraDbSource class.

Various helper functions, specific for the annotation resource, are available to create such annotation source objects:

• CompDbSource: creates a compound annotation source object from the provided CompDb SQLite data base file. This function can be used to integrate an existing (locally available) CompDb annotation database into an annotation workflow.

• MassBankSource: creates a annotation source object for a specific MassBank release. The desired release can be specified with the release parameter (e.g. release = "2021.03" or release = "2022.06"). The function will then download the respective annotation database from Bioconductor's r Biocpkg("AnnotationHub").

In the example below we create a annotation source for MassBank release 2022.06. This call will lookup the requested version in Biocondutor's (online) AnnotationHub and download the data. Subsequent requests for the same annotation resource will load the locally cached version instead. Upcoming MassBank database releases will be added to AnnotationHub after their official release and all previous releases will be available as well.

mbank <- MassBankSource("2022.06")
mbank

We can now use that annotation source object in the matchSpectra call to compare the experimental spectra from the previous examples against that release of MassBank.

res <- matchSpectra(
    pest_ms2, mbank,
    param = CompareSpectraParam(requirePrecursor = TRUE, ppm = 10))
res

The result object contains only the matching fragment spectra from the reference database.

target(res)

And the names of the compounds with matching fragment spectra.

matchedData(res)$target_name

Finding MS2 spectra for selected m/z and retention times

Sometimes it is needed to identify fragment spectra in a Spectra object for selected (precursor) m/z values and retention times. An example would be if compound quantification was performed with a LC-MS run and in a second LC-MS/MS run (with the same chromatographic setup) fragment spectra of the same samples were generated. From the first LC-MS data set features (or chromatographic peaks) would be identified for which it would be necessary to retrieve fragment spectra matching the m/z and retention times of these from the second, LC-MS/MS data set (assuming that no big retention time shifts between the measurement runs are expected). To illustrate this, we below first define a data.frame that should represent a feature table such as defined by an analysis with the r Biocpkg("xcms") package.

fts <- data.frame(
    feature_id = c("FT001", "FT002", "FT003", "FT004", "FT005"),
    mzmed = c(313.43, 256.11, 224.08, 159.22, 224.08),
    rtmed = c(38.5, 379.1, 168.2, 48.2, 381.1))

We next match the features from this data frame against the Spectra object using an MzRtParam to identify fragment spectra with their precursor m/z and retention times matching (with some tolerance) the values from the features.

fts_mtch <- matchValues(fts, pest_ms2, MzRtParam(ppm = 50, toleranceRt = 3),
                        mzColname = c("mzmed", "precursorMz"),
                        rtColname = c("rtmed", "rtime"))
fts_mtch
whichQuery(fts_mtch)

Thus, we found fragment spectra matching the m/z and retention times for the 2nd and 5th feature. We next extract the Spectra object with the matching fragment spectra and assign the IDs of the matching features to the respective spectra. We us the targetIndex and queryIndex function for this that extract the indices for the matching query-target pairs.

fts_ms2 <- target(fts_mtch)[targetIndex(fts_mtch)]
fts_ms2$feature_id <- query(fts_mtch)$feature_id[queryIndex(fts_mtch)]
fts_ms2$feature_id

This Spectra could next be used to match the fragment spectra from the experiment to e.g. a reference database and with the assigned spectra variable "feature_id" it would allow to map the results back to the quantified feature matrix from the LC-MS run.

Performance and parallel processing

Pre-filtering the target spectra based on similar precursor m/z (using requirePrecursor = TRUE generally speeds up the call because a spectra comparison needs only to be performed on subsets of target spectra. Performance of the matchSpectra function depends however also on the backend used for the query and target Spectra. For some backends the peaks data (i.e. m/z and intensity values) might not be already loaded into memory and hence spectra comparisons might be slower because that data needs to be first loaded. As an example, for Spectra objects, such as our pest_ms2 variable, that use the MsBackendMzRbackend, the peaks data needs to be loaded from the raw data files before the spectra similarity scores can be calculated. Changing the backend to an in-memory data representation before matchSpectra can thus improve the performance (at the cost of a higher memory demand).

Below we change the backends of the pest_ms2 and minimb objects to MsBackendMemory which keeps all data (spectra and peaks data) in memory and we compare the performance against the originally used MsBackendMzR (for pest_ms2) and MsBackendDataFrame (for minimb).

pest_ms2_mem <- setBackend(pest_ms2, MsBackendMemory())
minimb_mem <- setBackend(minimb, MsBackendMemory())
library(microbenchmark)
microbenchmark(compareSpectra(pest_ms2, minimb, param = csp),
               compareSpectra(pest_ms2_mem, minimb_mem, param = csp),
               times = 5)

There is a considerable performance gain by using the MsBackendMemory over the two other backends, that comes however at the cost of a higher memory demand. Thus, for large data sets (or reference libraries) this might not be an option. See also [issue

93](https://github.com/rformassspectrometry/MetaboAnnotation/issues/93) in the

MetaboAnnotation github repository for more benchmarks and information on performance of matchSpectra.

If for target a Spectra using a SQL database-based backend is used (such as a MsBackendMassbankSql, MsBackendCompDb or MsBackendSql) and spectra matching is performed with requirePrecursorMz = TRUE, simply caching the precursor m/z values of all target spectra in memory improves the performance of matchSpectra considerably. This can be easily done with e.g. target_sps$precursorMz <- precursorMz(target_sps) where target_sps is the Spectra object that uses one of the above mentioned backends. With this call all precursor m/z values will be cached within target_sps and any precursorMz(target_sps) call (which is used by matchSpectra to select the candidate spectra against which to compare a query spectrum) will not require a separate SQL call.

Parallel processing can also improve performance, but might not be possible for all backends. In particular, backends based on SQL databases don't allow parallel processing because the database connection can not be shared across different processes.

Utility functions

MetaboAnnotation provides also other utility functions not directly related to the annotation process. These are presented in this section.

Creating mixes of standard compounds

The function createStandardMixes allows for grouping of standard compounds with a minimum difference in m/z based on user input.

library(MetaboCoreUtils)

Input format

As an example here I will extract a list of a 100 standard compounds with their formula from a tab delimited text file provided with the package. Such files could also be imported from an xlsx sheet using the readxl package.

standard <- read.table(system.file("extdata", "Standard_list_example.txt",
                               package = "MetaboAnnotation"),
                   header = TRUE, sep = "\t", quote = "")

We will use functions from the MetaboCoreUtil package to get the mass of each compounds and the m/z for the adducts wanted.

#' Calculate mass based on formula of compounds
standard$mass <- calculateMass(standard$formula)

#' Create input for function
#' Calculate charge for 2 adducts
standard_charged <- mass2mz(standard$mass, adduct = c("[M+H]+", "[M+Na]+"))

#' have compounds names as rownames
rownames(standard_charged) <- standard[ , 1]

#' ensure the input `x` is a matrix
if (!is.matrix(standard_charged))
    standard_charged <- as.matrix(standard_charged)

The input table for the createStandardMixes should thus look like the one shown below, i.e. should be a numeric matrix with each row representing one compound. Columns are expected to contain m/z values for different adducts of that compound. Importantly, the row names of the matrix should represent the (unique) compound names (or any other unique identifier for the compound).

standard_charged 

Using the function

The createStandardMixes function organizes given compounds in such a way that each compound is placed in a group where all ions (adducts) have a m/z difference exceeding a user-defined threshold (default: min_diff = 2). In this initial example, we aim to group only a subset of our compound list and execute the function with default parameters:

group_no_randomization <- createStandardMixes(standard_charged[1:20,])
group_no_randomization

Let's see the number of compounds per group:

table(group_no_randomization$group)

The grouping here worked perfectly, but let's now use the entire compound list and run with the default parameter again:

group_no_randomization <- createStandardMixes(standard_charged)
table(group_no_randomization$group)

This time we can see that the grouping is less ideal. In this case we can switch the iterativeRandomization = TRUE.

group_with_ramdomization <- createStandardMixes(standard_charged, 
                                                iterativeRandomization = TRUE)

table(group_with_ramdomization$group)

Changing iterativeRandomization = from the default FALSE to TRUE enables the randomization of input x rows until it fits the min_nstd parameter. If the list of compounds is very long or the requirement is hard to fit, this function can take a bit longer if iterativeRandomization = is set to TRUE.

What if we want groups of a maximum of 20 and a minimum of 15 compounds, and with a minimum difference of 2 m/z between compounds of the same group? If you want to know more about the parameters of this function, look at ?createStandardMixes.

set.seed(123)
group_with_ramdomization <- createStandardMixes(standard_charged, 
                                                max_nstd = 15,
                                                min_nstd = 10,
                                                min_diff = 2,
                                                iterativeRandomization = TRUE)

table(group_with_ramdomization$group)

Great ! these groups look good; we can now export. As the function already returns a data.frame, you can directly save is as an Excel file using write.xlsx or as below in text format that can also be open in Excel.

write.table(group_with_ramdomization, 
           file = "standard_mixes.txt", sep = "\t", quote = FALSE)

Session information {-}

sessionInfo()

References

rformassspectrometry/MetaboAnnotation documentation built on March 12, 2024, 4:09 a.m.